,,our ,,nuclear ,,future 444 ,,facts ,,dangers ,,and ,,opportunities ,,by ,edward ,teller ,,and ,albert ,l4 ,latter ,,criterion ,,books ,,new ,,york ,copyright #aieh1 by ,criterion ,books1 ,inc4 ,library of ,congress ,catalog ,card ,number #eh-hghc ,designed by ,sidney ,feinberg ,,manufactured ,,in ,,the ,,united ,,states ,,of ,,america ,,by ,,american ,,book-,,stratford ,,press1 ,,inc41 ,,new ,,york ,preface ,this book has been written for the layman who has no knowledge about atoms1 bombs and radioactivity4 ,he knows that the world is made of atoms1 that bombs might destroy it and that radioactivity could make it a place much less agreeable to live in4 ,we should like to give some advice about the use of the book3 ,each chapter can be read by itself4 ,the chapters need not be taken in the order in which they are printed4 ,to read them all will give a more complete understanding--and if you have time it is best to read them in the order they are arranged4 ,some of the earlier chapters perhaps overflow with facts4 ,in some later chapters we wish that more facts were available4 ,these latter the reader will probably understand and remember quite easily4 ,he may not agree with all of their contents4 ,on the other hand the more scientific chapters 7,,ii to ,,viii7 will not be questioned but may be harder to read and to remember4 ,it will be a help to keep this in mind3 ,no chapter follows from another but most chapters are related and support some other part of the book4 ,our knowledge about fallout is increasing rapidly4 ,some questions which are raised in the book may already have been answered4 ,with this added knowledge we might have been more quantitative in some of our statements4 ,but we believe the main conclusions would not be altered4 ,this book was completed before the ,sputniks4 ,in their present form these have little to do with the subject of nuclear energy4 ,however1 to our mind1 the urgency has become greater for the non-scientist to understand those parts of science and technology which may affect his safety and well-being1 and the safety and well-being of his country4 ,we hope that this book will contribute in some measure to such understanding4 ,contents ,preface #e ,i4 ,the ,need to ,know #ac ,,ii4 ,atoms #ah ,,iii4 ,nuclei #bf ,,iv4 ,the ,law of ,radioactive ,decay #cg ,v4 ,breakup of the ,nucleus #da ,,vi4 ,reactions ,between ,nuclei #di ,,vii4 ,fission and the ,chain ,reaction #eh ,,viii4 ,action of ,radiation on ,matter #fh ,,ix4 ,the ,test #hj ,x4 ,the ,radioactive ,cloud #hg ,,xi4 ,from the ,soil to ,man #ajd ,,xii4 ,danger to the ,individual #aaf ,,xiii4 ,danger to the ,race #abg ,,xiv4 ,the ,cobalt ,bomb #acd ,,xv4 ,what ,about ,future ,tests8 #acg ,,xvi4 ,has ,something ,happened to the ,weather8 #adf ,,xvii4 ,safety of ,nuclear ,reactors #aeb ,,xviii4 ,by-products of ,nuclear ,reactors #afj ,,xix4 ,the ,nuclear ,age #afh ,glossary #age ,list of ,illustrations ,a section of photographs will be found between pages #if and #ig4 #a4 ,a shallow underground explosion4 #b4 ,an atomic test tower4 #c4 ,a tower shot4 #d4 ,an air shot4 #e4 ,leg bone of a rabbit after injection of ,sr4 #f4 ,leg bone of a woman dead of radium poisoning4 #g4 ,capsules of cobalt4 #h4 ,cobalt irradiation4 #i4 ,smoke-ring cloud from the air-defense atomic weapon4 #aj4 ,wilful exposure--an experiment4 #aa4 ,condensation trails produced in a ,wilson ,cloud ,chamber4 #ab4 ,closely-spaced tracks form a cloud4 #ac4 ,cutaway section of a nuclear reactor4 ,,our ,,nuclear ,,future ,,chapter ,i ,the ,need to ,know ,our world is changing1 and the change is becoming more rapid4 ,the moving force behind this change is scientific discovery4 ,all of us are deeply affected by the consequences of science4 ,at the same time1 very few understand the highly technical foundations of our civilization4 ,in this situation it is natural that scientific and technical progress should create uneasiness and alarm4 ,fear of what we do not know or do not understand has been with us in all ages4 ,man1 knowing that his life will end1 has often been prey to an even more terrible nightmare--the end of his whole world4 ,in a scientific age most of the past terrors have turned out to be senseless chimeras4 ,but one menace remains4 ,it is the great and permanent unknown3 what will we humans do to each other and to ourselves8 ,the worry about our own actions will continue4 ,it may grow as our power over nature increases4 ,against this worry there exist two weapons3 understanding and courage4 ,of the two1 courage is more important but understanding must come first4 ,we are frequently alarmed by imaginary dangers1 while disregarding risks which are much more real4 ,there should exist a close interaction between public opinion on the one hand and technical progress on the other4 ,for this end an understanding of modern scientific developments is required4 ,there is an increasingly urgent need to know4 ,little is done to satisfy this need4 ,the opinion has gained ground that this need can in fact not be satisfied4 ,at the same time1 more and more people believe that the scientists and technical people themselves are responsible for the changes which their ideas and inventions have brought about4 ,the scientist is put in the position where his voice is heard1 not only in the highly specialized fields in which he is an expert1 but also in the much more general matters which are affected by his discoveries4 ,the real source of important decisions in our country is the people4 ,we believe that this is rightly so1 and we believe that it is not proper if scientists take over any essential part of these decisions4 ,the responsibility of a technical man certainly includes two important functions4 ,one is to explore nature and to find out the possible limits of our power over nature4 ,the other is to explain what he has found in clear1 simple1 and straightforward terms1 so that essential decisions can be made by all the people of our country--to whom the power of decision properly belongs1 and whom the consequences of these decisions will ultimately affect4 ,to explain scientific and technical matters is not easy1 and to become familiar with all science might actually be impossible4 ,in the specialized field of physics there have been revolutionary developments in the twentieth century like the theory of relativity discovered by ,einstein and the theory of the atom originated by ,niels ,bohr4 ,these new discoveries are not easy to understand1 and every good physicist has spent years of his life trying to get thoroughly acquainted with their meaning4 ,all of us who have done so feel that we are well rewarded by the better understanding of nature which we have acquired4 ,but it is not necessary to talk of these matters here4 ,what we have to discuss in this book is connected with parts of atomic and nuclear physics which are much more elementary4 ,the facts which we shall present in a simple fashion are sufficient to give the reader an orientation in the seemingly bewildering fields of nuclear energy and atomic explosions4 ,we shall have to start by describing atoms and nuclei4 ,these are rather small objects1 but this circumstance need not particularly bother us2 and it is not necessary to frighten ourselves with the idea that we are talking about 8unimaginably0 small objects4 ,our minds adapt themselves quite readily to new dimensions2 and while we are talking about nuclei1 we can temporarily forget that any bigger objects exist4 ,real difficulties arise only when science discovers laws which seem to contradict common sense4 ,this does not happen frequently1 and we shall not need to dwell upon such subjects4 ,the difficulties of explaining science are increased by the fact that scientists have developed a language of their own which they practice and perfect by talking to each other4 ,one sometimes has the impression that they talk to each other exclusively4 ,the authors feel that their own 8native tongue0 is this scientific language2 this book is an effort at a translation4 ,a further difficulty is connected with the special subject3 radioactivity4 ,the great practical importance of this subject has dawned upon the public in connection with the explosion at ,hiroshima4 ,this was a frightening occasion1 and the subsequent developments and prospects are no less frightening4 ,it is not necessary that everything connected with nuclear explosions should be equally frightening2 and it is important that we should approach the subject with an open mind and with as few emotions as is humanly possible4 ,the emotions have their necessary place when we get to the stage in which we want to decide our actions4 ,we suggest to the reader that he should delay this stage until the time when he has finished reading the book4 ,the greatest difficulty in discussing the radiation hazards arises because the working of living organisms is involved4 ,basically1 we are in the dark about the question how such an organism works4 ,we are equally in the dark about the question how such an organism is affected by radiation4 ,it would therefore seem that we must remain in doubt whether or not radioactivity is dangerous1 except for those cases where obvious damage has been done4 ,since the immediate effects of radioactivity are not perceived by our senses1 we are faced with the thought of an invisible menace of unknown extent4 ,some of the harmful consequences may show up years later1 and therefore even the absence of any observed damage will not reassure people4 ,fortunately1 our practical knowledge is by no means as deficient as these statements would suggest4 ,radioactivity1 and processes similar to radioactivity1 surround us and have surrounded our ancestors for as long as life has existed on earth4 ,we do not know what life is1 and we do not know in what detailed manner life is affected by radioactivity2 but we have broadly based and certain knowledge that artificial radioactivity will produce similar effects to those produced by the natural background of radioactivity4 ,this background1 therefore1 provides us with a yardstick to which all man-made contaminations can be compared4 ,there is a final obstacle to the explanation of matters connected with radioactivity4 ,this is the secrecy which has been associated with the development of nuclear energy1 and in particular with the military applications of nuclear energy4 ,the arguments for keeping information concerning weapons secret are strong1 proper1 and generally understood4 ,there is1 however1 no such strong argument1 and in fact no possibility for secrecy connected with the widely dispersed radioactivity which originates from the weapons4 ,in recognition of this fact1 secrecy has been completely and properly removed from this field4 ,it is not surprising that it took some time to do so4 ,administrative decisions have been involved1 and these are never taken in a very great hurry4 ,even though world-wide radioactive contamination has been since #aiee open to general scientific discussion1 the time does not seem to have been sufficient to insure a wide dissemination and explanation of the results4 ,there may also remain some lingering doubts whether all relevant information has been made available4 ,in actual fact1 the scientific information on this important topic is completely and freely available at the present time4 ,information concerning the peaceful applications of nuclear energy is also completely and freely available4 ,even in the field of military applications1 much of the essential information has been published4 ,we are therefore in a position to put before the reader the most important facts about the peaceful and military applications of nuclear energy--of the possible dangers and of the eventual benefits4 ,if we do not succeed1 we cannot blame either secrecy or the difficulty of the subject4 ,it is true that the subject is involved1 but only in the same way as are those subjects of everyday experience with which all of us have to struggle once in a while4 ,no greater intellectual effort is needed than is involved in the understanding of the income tax form or the racing form1 to mention two analogies of rather diverse emotional content4 ,many of the ideas will be unfamiliar1 but they are not complex4 ,furthermore1 their bearing on our safety1 well-being1 and the possible improvement of our lives is great4 ,therefore we hope that the reader will give as much of his attention to this matter as he is accustomed to devote to other subjects which are connected with his necessities or his amusement4 ,,chapter ,,ii ,atoms ,all matter is composed of atoms1 which are very tiny objects4 ,we cannot see them because waves of light wash over them like ocean waves over a pebble4 ,an atom is about as big in comparison to a human cell1 which can be clearly seen under an ordinary microscope1 as a human cell is in comparison to a billiard ball4 ,somewhat more precisely1 a hundred million atoms laid side by side would be about an inch in length4 ,despite its ,greek name1 which means indivisible1 the atom is made up of parts4 ,it consists of a central nucleus1 which carries a positive electrical charge1 around which one or more negatively charged electrons are distributed4 ,one frequently hears of the electrons revolving in orbits around the nucleus1 somewhat as the planets revolve around the sun in our own solar system4 ,this is not quite a correct picture1 however4 ,for one thing the electrons are more elusive than the planets4 ,they do not revolve in definite orbits as the planets do4 ,also the orbits are more delicate4 ,one would destroy the atom by the attempt to find out precisely what the electron orbits are4 ,illustration3 ,this is how an atom does not look4 ,the electrons do not move along well-defined paths4 ,it is more difficult to convey the idea of an atom by a picture than it is to make a drawing of last night's dream4 ,the planets do not fly away from the sun because of the gravitational attraction which the sun exerts4 ,the electrons and the nucleus1 however1 are held together because positive and negative electrical charges attract each other4 ,the gravitational attraction between the electrons and the nucleus is incredibly weak compared to the electrical attraction4 ,most of the atom's weight comes from its nucleus4 ,even the lightest known nucleus weighs about #ahdj times as much as an electron4 ,in spite of this1 the nucleus occupies only a tiny portion of the total volume of the atom4 ,in fact1 the nucleus is about as big in comparison to the whole atom as the atom is in comparison to the human cell4 ,twenty thousand nuclei laid side by side would be about equal in length to the diameter of the atom4 ,if matter were composed of nothing but nuclei densely packed together1 an object the size of a penny would weigh approximately forty million tons4 ,later we are going to see that the size of the nucleus has a great effect upon the ways in which nuclei react with each other4 ,for that very reason the size of the nucleus is a well-defined measurable quantity4 ,it is much harder to say precisely what one means by the size of the electron4 ,it seems acceptable to say that it is somewhat less than the size of the average nucleus4 ,in any case it is certain that both the electrons and the nucleus are small compared to the size of the whole atom4 ,consequently1 the atom must consist mostly of empty space4 ,this means1 of course1 that when you look at solid matter1 what is before your eyes is empty space with a slight addition of real substance4 ,what lends strength to solids is the interplay of electric attractions and repulsions inside atoms and between atoms4 ,when a charged particle1 such as an electron or a nucleus1 happens to move through solid matter1 it is constantly acted on by large electric forces4 ,to such a particle matter does not seem to be very transparent4 ,but if there were such a thing as an electrically neutral particle1 comparable in size to the nucleus1 it would be able to move around freely inside matter1 without experiencing electric forces1 and only now and again bumping into a nucleus or maybe an electron4 ,as a matter of fact1 there is such a particle and it can pass right through an inch or two of solid matter without bumping into anything4 ,later on in this book we shall be very interested in this particle1 which is called a neutron4 ,although the electrons and the nucleus are charged particles1 the atom as a whole is electrically neutral2 this means that the positive charge of the nucleus must be equal in magnitude to the total charge of all the negative electrons4 ,all electrons have precisely the same charge1 which is the smallest charge that has ever been observed4 ,what is particularly strange and not yet explained is the fact that all other charges are as big as the electron charge1 or twice as big1 or three times as big1 or a million times as big4 ,but we never find a charge which1 expressed in terms of the electron charge1 is fractional4 ,no object ever carries three and a half electron charges4 ,the electron charge therefore may be used conveniently as a standard unit of charge4 ,every atom can be distinguished by the charge of its nucleus4 ,the simplest atom one can imagine would clearly be one with a single electron revolving around a nucleus having one unit of positive charge4 ,such an atom exists and is called hydrogen4 ,an atom with a nucleus of charge two and two electrons revolving around it1 is called helium2 three1 lithium 444 six1 seven1 eight2 carbon1 nitrogen1 oxygen 444 #ib1 uranium4 ,atoms with almost all charges from one to #ib are found in nature1 and practically none above #ib are found4 ,some odd charges--#dc1 #fa1 #he1 and #hg--;are missing4 ,the reason for these missing atoms is connected with the properties of the nucleus4 ,the nucleus will soon become our main object of interest4 ,the most surprising fact about atoms is their similarity1 indeed their identical behavior4 ,if two atoms have the same kind of nucleus and have the same number of electrons revolving around these nuclei1 then these two atoms are apt to be encountered in a condition which is most precisely the same for the two4 ,one could imagine that the various component parts of the atom would be arranged in different ways and found in different states of motion1 in a variety without limit4 ,whence the complete similarity8 ,the answer to this question is not only most surprising1 but it is even in apparent contradiction to common sense4 ,for this very reason it is difficult to explain4 ,the hardest things to understand are not those which are complicated but those which are unexpected4 ,fortunately for our purpose we need not go into this more intricate portion of atomic physics4 ,it is sufficient to say that there is one arrangement or pattern of motion of the electrons which is preferred and which leads to the greatest stability of the atom4 ,if the electrons are in this particular state of motion1 which is called the ground state1 they have less energy than they would have if they were in any other state of motion4 ,there are other less stable1 but not less sharply defined1 states of atoms which we call 8excited0 states4 ,when an atom is in such an excited state1 it tends to be unstable and tries to get into the ground state as soon as possible4 ,since the ground state contains less energy than any other state1 the atom must release energy in the process of adjustment4 ,the released energy manifests itself in the form of electromagnetic radiation--often as a little pulse of visible light4 ,the color of this light depends upon the amount of energy released1 going progressively through the rainbow from red toward blue as the amount of energy increases4 ,there are very few states in which the excitation energy is small4 ,but of strongly excited states there is a great abundance4 ,in the region of this high excitation small additional changes are possible4 ,thus we approach a situation more in accordance with experience and common sense3 the pattern of motion can be changed by any small amount4 ,the description we have just given is of course incomplete4 ,we must avoid here the crucial questions why only some patterns of motion are possible1 why one lowest level is stable and why the electrons never descend into decreasing states of energy1 following the attraction of the nucleus4 ,at the same time one should emphasize that a complete explanation of these facts has been given4 ,this explanation makes precise predictions about many of the properties of matter1 and we can have complete confidence that1 but for the involved mathematical procedure1 all ordinary properties of materials could be precisely predicted4 ,the atom has been explained as completely as ,newton has explained the motion of planets4 ,to form an idea what an atom is or why two atoms of1 let us say1 hydrogen are precisely the same1 it is not necessary to search for intricate reasons or deep meanings4 ,two atoms of the same kind are alike as two pawns are for the chess player1 except for one little point3 in the case of the pawns we do not care about the difference2 in the case of the atoms there is no difference4 ,this is a simple statement and it honestly describes a simple situation4 ,the beauty of science is due to the fact that the correct answers to our most interesting questions have turned out to be surprising by their simplicity4 ,in order to understand an atom one must consider the distribution of electrons around one nucleus4 ,in order to understand a molecule one has to consider the distribution of electrons around two or more nuclei4 ,the chemical behavior of an atom is the manner in which it interacts with other atoms1 and that means the precise way in which the electrons rearrange themselves when two or more atoms approach each other4 ,the interaction between atoms occurs mainly between their outermost electrons4 ,it may happen that two quite different atoms1 containing nuclei of different charges and different numbers of electrons1 may nevertheless be similar in the structure of their outermost electrons4 ,in this case the two atoms exhibit similar chemical properties4 ,examples are lithium with charge #c and sodium with charge #aa2 also helium1 charge #b and neon1 charge #aj4 ,a most important example for our purpose is the set of three chemically similar atoms3 calcium1 charge #bj1 strontium1 charge #ch2 and radium1 charge #hh4 ,when two or more atoms approach each other1 whether they are similar or different1 their electrons--particularly the outermost ones--find new states of motion instead of those that were available to them when there was only one nucleus in the vicinity4 ,it may now happen that amongst these new states of motion there are some that are even more stable than the state of the separated atoms4 ,in this event the atoms will tend to stick together1 and the electrons will adopt whatever new state of motion now corresponds to maximum stability4 ,the composite system of the atoms is called a molecule1 and its state of maximum stability1 the ground state of the molecule4 ,there are atoms of particularly great stability which cannot increase their stability by combining with other atoms4 ,examples are helium1 neon1 and argon4 ,these atoms tend to remain single1 retain their independent motion in a rather 8permanent0 gaseous state1 and are generally unsociable4 ,they are called therefore the noble gases4#a ,an especially simple example of the formation of a molecule is the combining of sodium and chlorine to form ordinary table salt4 ,the sodium atom happens to have a rather loosely bound outer electron4 ,the chlorine atom possesses a convenient niche for an extra electron4 ,consequently the energy spent in prying the outer electron loose from the sodium atom is largely repaid by adding it to the chlorine atom4 ,the remaining sodium 8atom10 deprived of one of its electrons1 now has a net positive charge4#b ,the chlorine 8atom0 with its extra electron has a net negative charge4 ,the two 8atoms0 therefore attract each other to make a molecule of sodium chloride4 ,actually matter will continue to aggregate4 ,a great number of positive sodium 8atoms0 and negative chlorine 8atoms0 will arrange themselves into a beautiful and regular lattice which is the sodium chloride crystal4 ,the simplest molecule which does not tend to grow into a bigger aggregate is made up of two hydrogen atoms4 ,around two hydrogen nuclei a particularly stable pattern of two electrons can be formed4 ,because of this fact hydrogen atoms associate pairwise so that this pattern should become possible4 ,the ways in which atoms can be joined are incredibly manifold4 ,they can form metals in which the outer electrons roam freely and carry electric currents with the greatest of ease4 ,they can form liquids in which atoms or molecules are tied together in a loose and disorderly fashion4 ,they can move independently making occasional encounters1 which is what happens in a gas4 ,and they can form long spiraling molecules where groups of atoms are strung together without an apparent simple order1 but in a way which is somehow related to the processes of life4 ,illustration3 ,arrangement of sodium and chlorine 8atoms0 in a crystal of common salt4 ,we all know in how many forms matter can appear and how changeable these forms are4 ,that the stone and the spray1 the air and an insect1 and even the human brain should be composed of the same few kinds of atoms1 and that these atoms should be subject to laws which are subtle and simple and precisely described--this certainly is the most remarkable fact that we have learned since ,newton proved that the same science applies to the earth and in the heavens4 ,,chapter ,,iii ,nuclei ,up to now we have regarded atoms as being divisible into electrons and nuclei4 ,electrons and nuclei1 however1 we have regarded as indivisible entities4 ,this point of view is perfectly adequate to account for all the facts of chemistry and most of the facts of physics4 ,even in physics1 it has not been necessary to ascribe an internal structure to the electron4#c ,the electron is a truly elementary particle in this sense4 ,however1 to understand some physical phenomena1 and radioactivity is one of these1 it is necessary to recognize that the nucleus is not indivisible but consists of parts4 ,the parts of the nucleus are called protons and neutrons4 ,the simple statements of the previous chapter apply to these smaller particles also4 ,all electrons are equal--precisely equal4 ,all protons are equal and all neutrons are equal4 ,there are methods which would have shown up exceedingly small differences between these particles4 ,no such differences have been discovered4 ,as far as we know these particles are always the same4 ,we cannot pour energy into them and excite them as was the case with the atoms4 ,when we come to consider these small particles1 the complex structure of the world has an end4 ,instead what we find is simple4 ,a proton and a neutron have almost exactly the same weight4 ,the proton has one unit of positive charge1 which means that its charge is the same as that of the electron except that it is opposite in sign4 ,the neutron1 as its name implies1 is an electrically neutral particle4 ,hence the charge of the nucleus is equal to the number of protons it contains1 and is independent of the number of neutrons4 ,the weight of the nucleus1 however1 taking the proton 7or the neutron7 as a unit of weight1 is equal to the number of protons plus the number of neutrons4 ,imagine that we have two atoms whose nuclei have the same number of protons but a different number of neutrons4 ,such atoms exist in nature and are called isotopes4 ,the point about these isotopes is that since they have the same number of protons1 they have the same nuclear charge1 the same electron structures1 and hence they have almost the same chemical properties4 ,their nuclei have somewhat different volumes4 ,but the nucleus is small in any case4 ,it is almost as though we tried to look for the difference between nothing and twice-nothing4 ,the difference in the weights of isotopes due to the difference in their numbers of neutrons1 has only a negligible influence on their chemical behavior4 ,an important consequence of this fact is that molecules which differ only in that one isotope has been substituted for another are biologically indistinguishable4 ,they taste the same and smell the same4 ,they are ingested in our bodies in the same way1 and they are deposited or excreted in the same way4 ,the simplest isotopes are the isotopes of hydrogen4 ,most of the hydrogen atoms we find in nature have a nucleus which is a single proton4 ,this is the common hydrogen or light hydrogen4 ,a few hydrogen atoms1 however1 have nuclei which consist of a proton and a neutron4 ,this is the heavy hydrogen1 found in heavy water4 ,in all natural sources of water these two kinds of hydrogen are mixed in a ratio which is practically the same for every sample4 ,the electron circulating around the nucleus behaves almost exactly the same way whether the extra neutron is present or not4 ,on the state of that electron depend most properties of the atom and the molecules which contain it4 ,of course1 heavy hydrogen has twice the weight of common hydrogen1 and heavy water is somewhat more dense than light water4 ,but otherwise there is little difference4 ,the story of the discovery of the hydrogen isotopes is amusing4 ,about half a century ago--before the discovery of any isotope--two scientists tried to measure the density of water4 ,they purified the water by boiling it and condensing the vapor4 ,but the more they purified1 the lighter it became--slightly but perceptibly4 ,finally they gave up3 water seemed to have no density6 ,what really happened was this3 ,light water boils a little bit more easily than heavy water4 ,without realizing it1 these scientists had started to separate isotopes4 ,many years later ,harold ,urey--on the basis of some mistaken experiments of other people--concluded that heavy hydrogen must exist4 ,he looked for it and found it1 but found much less than he had expected4 ,there was so little heavy hydrogen that on the basis of correct experiments ,urey never would have guessed its presence4 ,it seems that an unfounded idea is much more fruitful than the absence of an idea4 ,almost all naturally occurring elements are found to consist of more than one isotope4 ,uranium1 for example1 is composed mainly of two1 one having #adc neutrons and the other having #adf4 ,since both of these isotopes have #ib protons1 their weights are #ib #adc #bce and #ib #adf #bch respectively4 ,it is customary to refer to these isotopes as ,u and ,u4 ,the ,u1 which is valuable in atomic reactors and in the manufacture of atomic bombs1 is comparatively rare1 occurring as only one part in #adj of natural uranium4 ,the separation of this rare isotope from the common #bch was one of the major undertakings of the two billion dollar ,manhattan ,project during ,world ,war ,,ii4 ,we come now to a most important question1 one that will lead us to the idea of radioactivity3 ,what is it that determines which isotopes a given element will have8 ,for example1 uranium has isotopes weighing #bce and #bch4 ,small amounts of ,u and ,u are also found in nature4 ,why do we not find ,u1 ,u1 ,u or ,u8 ,evidently only certain numbers of neutrons will hang together with #ib protons4 ,consider another example1 this time of the lightest known element1 hydrogen4 ,we have already mentioned two isotopes of hydrogen3 light hydrogen with weight #a 7symbolized ,h71 having a nucleus consisting of a single proton and no neutrons1 and heavy hydrogen 7also called deuterium7 of weight #b 7,h71 having one proton and one neutron4 ,the latter isotope occurs as only about one part in #e1jjj of natural hydrogen4 ,there is also a slight trace of tritium 7,h71 having one proton and two neutrons4 ,but here the sequence stops4 ,what has happened to ,h1 ,h1 ,h1 etc8 ,this question is related to the earlier one3 why there are no atoms in nature of charge #dc1 #fa1 #he1 and #hg1 and why there are none with charges greater than #ib4 ,to answer these questions requires a little knowledge about the laws which govern the motion of neutrons and protons within the nucleus1 and the nature of the forces which are exerted by a neutron on a neutron1 a neutron on a proton1 and a proton on a proton4 ,the motion of neutrons and protons within the nucleus is governed by the same laws which govern the motion of electrons within the atom4 ,for both the nucleus and the atom there is a ground state of motion which has more stability 7less energy7 than any other state4 ,of course the arrangement and motion of electrons in the atom depend not only on this general rule but also on the specifically electrical nature of the forces which act between the electrons and the nucleus4 ,in the same way the arrangement and motion of the neutrons and protons within the nucleus depend upon the nature of the forces which act between neutrons and protons4 ,these forces are definitely not of gravitational origin4 ,gravitational attraction is extremely weak compared to the attraction between neutrons and protons1 and is utterly negligible in the realm of nuclear phenomena4 ,neither can the nuclear forces be electrical in origin4 ,the neutrons are electrically neutral2 and the protons actually repel each other by virtue of their electrical charge4 ,the nuclear forces are something entirely new4 ,they are the strongest forces yet encountered1 and they are without a counterpart in the macroscopic world4 ,nuclear forces are not yet completely understood4 ,but to understand nuclear stability we need to know only one peculiar fact governing the behavior of neutrons and protons 7and incidentally also electrons73 ,they want to be different4 ,to each particle a state or pattern of motion can be assigned4 ,when any two neutrons are compared1 their pattern of motion must be essentially different4 ,the same holds for any two protons4 ,a neutron and a proton1 however1 may be found in similar patterns since they differ anyway in their charge4 ,now among the possible patterns of motion some have lower and some have higher energies4 ,individual neutrons and protons will first occupy the lowest energy states1 in accordance with the rule of least energy for maximum stability4 ,then the demand for a difference will force subsequent particles into patterns of higher and higher energies4 ,since a neutron does not exclude a proton from being in the same pattern1 the lowest energy state may be occupied simultaneously by one neutron and one proton4#d ,if another neutron or proton is added1 it must be put into the next state of higher energy4 ,for this reason we would expect that nuclei are most stable when they contain an equal or nearly equal number of neutrons and protons4 ,for nuclei which are not too heavy1 this is indeed the case4 ,for example1 nitrogen1 which has seven protons1 has two stable isotopes1 ,n and ,n1 with seven and eight neutrons respectively4 ,for heavy nuclei1 however1 the situation is a little different4 ,the nuclear force between neutrons and protons acts only over a very short range--the particles must almost be in contact with each other in order to experience a sizeable attraction4 ,consequently a neutron or a proton interacts only with its immediate neighbors in the nucleus4 ,the electrical repulsion between the protons1 however1 acts over a much longer range4 ,a proton is repelled by all the other protons in the nucleus4 ,for heavy nuclei this repulsion is sufficient to reduce the number of protons relative to the number of neutrons4 ,lead1 for example1 with #hb protons1 has four stable isotopes1 with #abb1 #abd1 #abe1 and #abf neutrons4 ,we have said that seven protons will combine stably with seven or eight neutrons4 ,what happens if seven protons are combined with six or nine neutrons 7to make ,n or ,n78 ,our rule does not prevent them from sticking together2 it says only that these combinations would be more stable if a proton could be converted into a neutron 7in the case of six7 or a neutron into a proton 7in the case of nine74 ,actually seven protons and nine neutrons do stick together1 but such a nucleus is not stable and does not continue to exist indefinitely4 ,the reason is quite simple and a little surprising3 ,the conversion of a neutron into a proton is actually a physically realizable process1 and furthermore it releases some energy4 ,similarly a nucleus containing seven protons and six neutrons will have an existence of only finite duration because the conversion of a proton into a neutron can also occur4 ,of course the proton is charged and the neutron is not4 ,what happens to the charge during these transformations8 ,actually the neutron is transformed1 not into a proton1 but into a proton plus an electron4 ,the proton is transformed likewise into a neutron plus something else4 ,this something else is called a positron and is identical with the electron in every respect except in having a positive instead of a negative charge4 ,the changes just described occur spontaneously4 ,they are examples of radioactivity4 ,more specifically they are called 8beta decay0 processes because an electron 7or a positron7 when emitted by a nucleus is called a beta ray4 ,such beta-radioactive substances are produced whenever nuclear energy is used in an explosion or in a power plant4 ,many of the difficulties and worries concerning nuclear energy are connected with these beta activities4 ,we shall be concerned with them often as harmful1 sometimes as helpful agents4 ,when a neutron is converted into a proton and an electron inside a nucleus1 the electron escapes immediately1 but the proton remains in the nucleus4 ,similarly1 when a proton is converted into a neutron and a positron1 the positron escapes and the neutron remains in the nucleus4 ,since the electron and the positron have a negligible weight compared to a proton or a neutron1 the process of beta decay leaves the weight of the nucleus nearly unchanged4 ,since the electron and the positron are charged1 the process of beta decay increases or decreases the charge of the nucleus by one unit4 ,after beta decay a nitrogen nucleus with seven protons and six neutrons 7,n7 becomes a nucleus with six protons and seven neutrons--carbon with weight #ac 7,c71 which is a stable combination4 ,similarly a nitrogen nucleus with seven protons and nine neutrons 7,n7 becomes a nucleus with eight protons and eight neutrons1 oxygen with weight #af 7,o71 which is ordinary stable oxygen4 ,sometimes after a beta decay the residual nucleus finds itself with a 8correct0 number of neutrons and protons but with an excess of energy4 ,that is1 the residual nucleus is not in its ground state but is excited4 ,this happens in about two thirds of the known cases of beta decay4 ,it happens1 for instance1 when ,n decays to ,o4 ,in this situation the excited nucleus will behave like an excited atom4 ,an excited atom1 the reader will recall1 gets rid of its excess energy by emitting electromagnetic radiation1 usually visible or near-visible light4 ,the excited nucleus will get rid of its excess energy in exactly the same way4 ,the only difference is that the amount of energy carried by the electromagnetic radiation from the nucleus is approximately a million times greater than that carried by the electromagnetic radiation from the atom--an indication of the large quantity of energy stored up inside the nucleus4 ,such energetic electromagnetic radiation emanating from a nucleus is usually called a gamma ray4 ,gamma-ray emission1 or gamma decay1 like beta decay1 is an energy-releasing process which changes an unstable nucleus into a stable one1 or at least into a more stable one4 ,more generally1 any spontaneous energy-releasing process 7which tends to stabilize the nucleus7 is called radioactivity4 ,beta and gamma decay are two examples4 ,later on we shall consider a third example called alpha decay4 ,an alpha particle is the nucleus of the helium atom and consists of two neutrons and two protons4 ,the decay of a neutron and the decay of a proton appear to be quite analogous processes4 ,actually there is an important difference between the two4 ,a free neutron--one not confined inside a nucleus--will decay into a proton and an electron2 but a free proton will not decay into a neutron and a positron4 ,this difference is due to the fact that the proton has a slightly lower weight than the neutron and therefore has less energy4 ,for the proton to decay1 it must be inside a nucleus where it can absorb some energy from the other protons and neutrons4 ,one sometimes finds pairs of nuclei which could transform into each other by a proton-neutron 7or neutron-proton7 conversion2 nevertheless neither of these conversions can occur in the way we have just described4 ,the reason is that in a proton-neutron or neutron-proton conversion an additional electron or positron has to be emitted4 ,now according to ,einstein the mass of the electron or positron corresponds to some energy 7,e mc71 and it may happen that neither the neutron-proton transformation or the proton-neutron transformation releases enough energy to make an electron or a positron4 ,in such cases one of the innermost electrons of the atom may combine with a proton to make a neutron4 ,such an electron-capture process will always release energy provided that the reverse process--the transformation of a neutron into a proton and an electron--is connected with an energy deficit4 ,thus1 excluding the possibility of a really exact coincidence of two energies1 one of the two transformations from neutron to proton or proton to neutron will always be possible4 ,it is one of the most firmly established laws of nature that energy is always conserved4 ,one would therefore expect that the energy of a beta ray would be exactly equal to the difference between the energy of the nucleus before the beta decay and the energy of the nucleus after the beta decay4 ,as a matter of fact the energy of a beta ray is found never to be as great as this amount4 ,frequently it is much less4 ,some of the energy has apparently disappeared and the suspicion has been voiced that energy may not be conserved after all4 ,it has turned out1 however1 that the missing energy is smuggled out of the nucleus1 and the smuggler 7who has only recently been caught7 is called the neutrino4 ,the neutrino is an electrically neutral particle1 like the neutron1 but its weight1 like the weight of a ray of light1 is equal to zero4 ,like such a ray1 it moves with the velocity of light4 ,the energy released by the nucleus in the beta-decay process is shared more or less equally between the neutrino and the beta ray4 ,we shall see later that the electron gives rise to a number of effects4 ,some of these are harmful4 ,the neutrino1 however1 is not in the least dangerous4 ,like an ideal smuggler it passes unnoticed and practically without a trace4 ,it interacts so slightly with matter that several billion of them may go right through the whole sphere of our earth before a single collision occurs4 ,very recently this strange little particle has upset one of our most unquestioned concepts about symmetry4 ,we have always believed that nature made no distinction between her right hand and her left hand2 that for every natural process that exists1 there exists also the mirror image of this process4 ,the neutrino1 however1 is an exception4 ,it has a definite symmetry1 like a screw4#e ,this fact may turn out to be most important in the development of science4 ,it has no bearing1 however1 on the questions to be discussed in this book4 ,neutrinos reach us from some distant and hidden places like the interior of our sun and of exploding stars4 ,it may become possible to use neutrinos as messengers to reveal the kind of nuclear reactions from which the energy of the stars is derived4 ,neutrinos are also emitted every time we release some nuclear energy4 ,among all the remarkable practical consequences of nuclear energy1 the neutrinos have a unique distinction3 they are never useful1 and they are never harmful4 ,they have not even been suspected of any mischief4 ,,chapter ,,iv ,the ,law of ,radioactive ,decay ,a radioactive nucleus is one that will eventually disintegrate and release some energy4 ,but when8 ,one might imagine that a radioactive nucleus would begin to 8age0 from the moment of its birth1 and that after the passage of a predetermined time1 the disintegration process would take place4 ,this is how radioactivity might work in a deterministic universe4 ,what actually happens to a radioactive nucleus1 however1 is much more interesting4 ,at any instant of its life1 the radioactive nucleus has some probability of disintegrating in the next second4 ,this probability is unaffected by its age4 ,no matter how long the nucleus has lived1 its chance of disintegrating in the next second is always the same4 ,it is as if a game of roulette were being played4 ,the wheel spins1 and if its number comes up1 the nucleus disintegrates in the first second4 ,if not1 the wheel spins again4 ,each time the wheel spins there is some probability of its number coming up4 ,the precise value of this probability is a characteristic of each particular radioactive species4 ,the higher the probability1 the more rapidly the nucleus may be expected to disintegrate4 ,but a given nucleus need not do at any particular time what is expected of it4 ,the notion of probability 7or chance7 has meaning only when applied to a large number of cases4 ,to say that a given nucleus has one chance in a hundred of decaying in the next second means that out of some large number 7say #ajj million7 of such radioactive nuclei1 one per cent 7one million7 will decay in the next second4 ,but it is absolutely impossible to say beforehand which nuclei will be the ones to decay4 ,a particular nucleus may decay immediately or only after some very long time4 ,the collection as a whole1 however1 will always do the expected thing4 7,this is the principle on which insurance companies operate47 ,the situation is best described in terms of a time span which is called the half-life of the radioactive species4 ,the half-life is defined as the amount of time which is required for one half of a large number of identical radioactive nuclei to disintegrate4 ,it makes no difference what this large number is1 provided only that it is large enough4 ,if the number is not large enough1 fluctuations will occur1 and instead of #ej per cent of the nuclei decaying during the period of a half-life1 it may be #dj per cent or #fj per cent4 ,as a matter of fact the #dj per cent to #fj per cent limits correspond to a sample size of about #ajj nuclei4 ,for #aj1jjj nuclei1 the limits will be #di per cent to #ea per cent4 ,the number of radioactive nuclei with which we customarily deal1 is about #aj 7#ajj1jjj1jjj1jjj1jjj1jjj1jjj1jjj74 ,this is the number1 for example1 of radioactive nuclei in about an ounce of radium4 ,for such a large number of nuclei the deviation from #ej-per-cent decay during a half-life will be utterly negligible4 ,thus we live in a universe which1 on a macroscopic scale1 appears ordered and subject to exact laws2 while underlying these laws1 on a microscopic scale1 nature plays out a game of chance1 full of randomness and uncertainty in the individual case4 ,we may draw a graph showing how ,n1 the number of the remaining radioactive nuclei1 varies with the time t4 ,the graph shows that3 in the first half-life ,t1 half of the original number ,n of radioactive nuclei decay4 ,in the second half-life1 half of those remaining decay1 and so on4 ,after the time ,t1 one half of the original radioactive nuclei still remain2 after #b,t1 one quarter remain2 and so forth4 ,illustration3 uncaptioned ,different radioactive species have different half-lives4 ,many are only a small fraction of a second2 some are billions of years4 ,n decays to ,o 7plus an electron and a neutrino7 with a half-life of about eight seconds4 ,a free neutron decays into a proton1 an electron1 and a neutrino with a half-life of #ac minutes4 ,strontium with weight #ij 7,sr7 undergoes a beta decay with a half-life of #bh years4 7,this is an isotope that is not found anywhere in nature1 but is made in fairly large quantities in the fission process47 ,potassium with weight #dj 7,k71 which is present in the amount of #j4ja per cent in ordinary potassium1 has a half-life of one billion years4 ,it has presumably been left over from the time when the primordial elements were formed4 ,half-lives for gamma decay are extremely short by comparison to those for beta decay4 ,they usually amount to a small fraction of a second4 ,radioactivity is characterized by the kind of particle emitted from the nucleus 7our examples1 so far1 have been of beta and gamma particles71 by the energy possessed by this particle1 and by the half-life in which the radioactive decay takes place4 ,the biological hazard from radioactivity depends on all three of these characteristics4 ,no matter whether the radioactive nuclei are produced in an atomic explosion or in an atomic reactor1 some time will in general elapse before a human population can become exposed4 ,if this time is long compared to the half-life of the radioactive species1 most of the nuclei will have disintegrated1 and the hazard will thereby be reduced4 ,if1 on the other hand1 the half-life is long compared to this time1 as well as to the life-span of a human being1 the rate at which disintegrations occur will be low1 and again the hazard will be reduced4 ,in short the dangerous half-lives are the intermediate ones1 not too long1 not too short4 ,sr is an example4 ,,chapter ,v ,breakup of the ,nucleus ,the positive electric charges within an atomic nucleus repel one another4 ,in the most heavily charged nuclei this repulsion becomes so great that the nucleus can break into two parts1 simultaneously releasing a considerable amount of energy4 ,in the case of spontaneous nuclear fission the two parts are more or less equal in size4 ,in the process of alpha decay one of the parts 7the alpha particle7 is much smaller than the other4 ,an alpha particle consists of two neutrons and two protons and is identical with the nucleus of the helium atom4 7,the symbol for this nucleus is ,he47 ,since two neutrons and two protons can simultaneously occupy the lowest energy state1 the alpha particle is an especially stable nuclear unit4 ,as a result1 from time to time in heavy nuclei1 two neutrons and two protons will coalesce into an alpha particle1 which may then attempt to escape4 ,in attempting to escape from the nucleus1 however1 an alpha particle encounters considerable resistance because of the short-range nuclear attraction of the other neutrons and protons4 ,this resistance which an alpha particle experiences in trying to leave the nucleus is usually referred to as an 8energy barrier40 ,if the alpha particle could acquire a little additional energy1 it would be able to overcome the barrier and get away from the nuclear attraction4 ,once outside the nucleus1 just beyond the reach of the nuclear attraction1 the alpha particle would be accelerated violently outward by the large electrical repulsion between its two protons and the other protons in the residual nucleus4 ,illustration3 ,how an alpha particle escapes from the nucleus4 ,from ,a to ,b it goes 8uphill10 losing speed4 ,at ,b its speed is zero and it almost always turns around4 ,with a small probability it may sneak through the energy barrier ,b to ,c4 ,beyond ,c1 it is repelled and emerges with increasing speed4 ,the alpha particle needs some extra energy to escape4 ,according to the laws of older physics there is no possibility for it to obtain this extra energy and therefore escape is impossible4 ,but the more newly discovered laws governing the motion of neutrons and protons 7the laws of quantum mechanics7 are not so stringent2 they permit the alpha particle to use 8borrowed0 energy to overcome the energy barrier4 ,of course the alpha particle must always repay the loan--which it can easily do out of the large fund of electric energy that is released when it gets out of the repulsive range of the residual nucleus4 ,there is no interest on the loan4 ,such energy loans are not automatically granted in nature4 ,there are two factors which make the loan improbable3 if the amount is big or if the term is long4 ,these restrictions effectively limit the particles which may apply for an energy loan4 ,objects of great size and weight are unable to qualify1 but the small particles of the atomic world often do4 ,the more energy carried off by the alpha particle after the alpha decay1 the less energy must be borrowed in order to overcome the barrier1 and the more rapidly the decay may be expected to occur4 ,so sensitive is the decay to the energy of the alpha particle1 that an alpha particle carrying twice the energy is emitted a hundred trillion times fester4 ,half-lives for alpha decay vary from a fraction of a second to billions of years4 ,but even the shortest half-life for alpha decay is remarkably long compared to the time required for the alpha particle to cross the nucleus4 ,this means that the alpha particle makes a tremendous number of attempts to escape from the nucleus before it actually succeeds4 ,according to the older classical theory the alpha process should never occur1 and in fact it occurs with a very small probability4 ,a single alpha decay is not usually a sufficient process to bring about stability of the daughter nucleus4 ,a whole chain of radioactive decays is usually required before stability is achieved4 ,most nuclei which emit alpha particles belong to one of these radioactive decay chains4 ,the heavy nuclei for which alpha decay occurs all contain a large excess of neutrons4 ,since the alpha particle carries off exactly two neutrons and two protons1 the ratio of the number of neutrons to the number of protons is increased in the daughter nucleus4 ,this has an unstabilizing influence4 7,actually1 in lighter nuclei stability requires that the ratio of neutrons to protons be closer to unity47 ,the daughter nucleus is thus apt to be beta-active1 converting a neutron into a proton 7plus an electron and a neutrino7 in order to decrease its ratio of neutrons to protons4 ,in this way a chain of radioactive decays may occur1 more or less alternating between alpha and beta emissions1 with gamma rays being occasionally emitted also4 ,there are four radioactive chains4 ,one of them starts with the abundant isotope of uranium1 ,u4 ,this isotope undergoes a few alpha decays and a couple of beta decays to become radium1 which has a charge of #hh and a weight of #bbf4 ,all the radium in the world is produced in this manner as a daughter product in the fifth decay of the chain4 ,after a number of further decays1 stable lead 7weight #bjf7 is produced and the chain terminates4 ,the other chains are similar to the ,u chain1 though not quite as long4 ,one chain starts with the rare isotope of uranium1 ,u2 another starts with the isotope of thorium that weighs #bcb4 ,both of these terminate in stable isotopes of lead4 ,in all cases the first decay of the chain has a very long half-life4 ,the half-life of ,u is #d4e billion years2 of ,u1 #gaj million years2 and of thorium1 #ad billion years4 ,the fourth radioactive chain has been made in the laboratory but is not found in nature because its first isotope1 neptunium with weight #bcg1 has too short a half-life4 ,it decays in two million years and all the other members of the chain live for even shorter periods4 ,thus the neptunium chain decayed long ago1 whereas the three other chains have survived from the time when the elements were made4 ,it is interesting to notice that the lesser abundance of ,u1 as compared with ,u1 is connected with its shorter half-life4 ,assuming that comparable amounts of both isotopes were present at the beginning of the universe 7and there is good reason to believe that this was the case71 one would expect to find significantly less ,u than ,u after a period of a few hundred million years4 ,after #gaj million years 7the half-life for ,u7 only one half of the original number of ,u nuclei would still exist4 ,but #ij per cent of the original ,u nuclei 7half-life #d4e billion years7 would remain4 ,from the presently observed ratio of ,u to ,u nuclei 7#a to #aci71 it may be calculated1 using the law of radioactive decay1 that #f billion years ago natural uranium consisted of equal parts of ,u and ,u4 ,the age of the universe is hotly debated4 ,with each passing year the universe seems to be a billion years older4 ,right now six billion years does not seem widely off the mark4 ,natural radioactivity occurs mainly among the heavy elements1 but there are a few light elements that are naturally radioactive4 ,of these1 potassium is an especially interesting one because it can decay either by electron emission or by electron capture4 ,the processes are3 potassium calcium electron neutrino1 7#a4a billion years7 and potassium electron argon neutrino4 7#aa billion years7 ,calcium and argon are both stable nuclei4 ,the second reaction is followed immediately by a gamma ray emission from the argon4 ,the one per cent of argon found in the earth's atmosphere comes almost entirely from the second reaction4 ,these radioactivities are also interesting because appreciable amounts of potassium are always present in human tissue4 ,all nuclei at the heavy end of the periodic system are radioactive alpha emitters4 ,uranium1 for example1 has no stable isotopes2 they all undergo alpha decay4 ,but there is another mode of spontaneous decay of uranium1 which is much less frequent than alpha decay but is of much greater practical importance4 ,this is the fission process4 ,the fission process is just like alpha decay in that the nucleus breaks up into two fragments4 ,the main difference between these processes is in the relative weights of the fragments4 ,in the alpha decay of ,u1 for instance1 one fragment has a weight of four and the other #bcd4 ,in the fission process the fragments tend to be more nearly equal in weight4 ,for example1 one may weigh #ij and the other #adh4f ,other weight combinations are also possible4 ,the explanation of spontaneous fission is in essence the same as that of alpha decay4 ,spontaneous fission1 however1 is a less probable process because the two fragments are more strongly bound to each other by the nuclear forces than they are in alpha decay4 ,more energy must be borrowed1 and it must be borrowed for a longer term in order to penetrate the energy barrier4 ,the relative likelihoods of spontaneous fission and alpha decay can be appreciated from the following fact4 ,in one hour in a gram of ,u there occur about #de million alpha decays but only about #be spontaneous fissions4 ,once the energy barrier has been overcome1 the energy released in alpha decay or spontaneous fission is proportional to the charges on the two fragments4 ,for alpha decay1 the product of the charges is #b #ij #ahj2 for spontaneous fission1 this product will typically be about #dj #eb #b1jhj4 ,hence one might expect the fission energy release to be #aj to #ae times greater than the alpha energy release4 ,as a matter of fact the fission energy release is even greater than this estimate indicates1 being about #cj to #ej times greater than the alpha energy release4 ,that so large an amount of energy is released1 is a very important feature of the fission process from the point of view of practical utilization of atomic energy4 ,being at the end of the periodic system1 uranium requires a large ratio of neutrons to protons for its greatest stability4 ,the fission fragments1 however1 lie in the middle of the system of elements1 requiring a much smaller ratio of neutrons to protons for stability4 ,this has two consequences4 ,one is that the fragments themselves may be expected to be unstable4 ,they will undergo beta decay 7electron emission7 several times consecutively before a stable combination of neutrons and protons is reached4 ,this radioactivity of the fission products constitutes a potential hazard in any practical application of fission atomic energy4 ,in later chapters of this book we shall consider particularly the possible hazard from the fallout of radioactive fission products created in atomic explosions1 and also the hazard associated with the operation and maintenance of atomic reactors4 ,the second consequence of the neutron excess is that neutrons may boil off from the fragments immediately after the fission process has occurred4 ,this can happen because a lot of disorderly internal motion is generated by the fission process within the fragments1 and these fragments do not have a particularly strong hold on their neutrons4 ,the practical value of the released neutrons is something we shall discuss at length in a later chapter4 ,for the present we mention only that these neutrons provide the mechanism whereby a chain reaction is made possible4 ,spontaneous fission and alpha decay are responsible for the fact that elements with charge greater than #ib are not found in nature4 ,there is little doubt that these elements were made in the beginning4 ,but they have long since decayed4 ,an interesting case of spontaneous nuclear fission is californium 7charge #ih71 with a half-life of #ee days4 ,this isotope is formed in large quantities in certain stellar explosions called super-novae4 ,once in a millennium one of a collection of a billion stars flares into incredible brilliance4 ,for a few weeks this single star shines with the combined energy and luster of a billion ordinary stars--then it fades away gradually4 ,such a 8new0 star 7nova71 with the greatest power of radiation1 is called a 8super-nova40 ,we believe that many nuclear reactions take place in a super-nova4 ,it has been observed that a few weeks after the initial outburst of light1 the intensity of light is reduced almost exactly by a factor of two every #ee days for a year or so4 ,this is precisely what would be expected if the energy generated in the star during this time were due to the spontaneous fission of californium4 ,here we see a model of what happens to naturally radioactive elements4 ,of these we have retained on earth only the ones with the longest half-lives1 like uranium1 thorium1 and potassium4 ,,chapter ,,vi ,reactions ,between ,nuclei ,the alchemists tried to transform one element into another artificially4 ,they used heat1 they used chemicals2 they even used witchcraft4 ,they failed4 ,their simplest method--to heat the substance in order to transform it--was really correct4 ,the trouble was that their temperatures were too low by a factor of more than #aj1jjj4 ,what is needed1 is a temperature of the order of tens of millions of degrees4 ,at such high temperatures two nuclei may occasionally approach each other in spite of the electrical repulsion between them4 ,sometimes they may even get close enough to each other to undergo a nuclear reaction4 ,this1 of course1 happens with least difficulty if the nuclear charge is small4 ,hydrogen nuclei1 which carry charge #a1 participate in such reactions most easily4 ,in the interior of stars temperatures range from about #aj to #ajj million degrees1 and nuclear reactions do occur4 ,the reaction responsible for the production of energy in the stars is3 #d;,h ,he energy ,four protons combine to make an alpha particle with a release of energy4 ,actually this reaction does not take place all at once but several steps are required4 ,that energy should be released1 one expects from the fact that the alpha particle is very stable4 ,any process in which light nuclei combine to form a heavier nucleus with a release of energy is known as 8fusion40 ,the particular fusion process that goes on in the stars releases its energy in many forms3 as positrons1 neutrinos1 electromagnetic radiation1 and motion of the reacting particles4 ,the positrons also carry off the excess charge of the reaction4 ,the neutrinos fly through the star without interacting1 carrying their energy away into outer space1 probably never again to make contact with the material universe4 ,the remainder of the fusion energy is deposited within the star's interior1 which is thus kept hot enough so that the fusion reaction can keep going4 ,the name 8thermonuclear0 is appropriately applied to this type of reaction4 ,a lot of effort and imagination is being devoted to the problem of making a controlled thermonuclear reaction4 ,the motivation for this project comes from the fact that good thermonuclear fuels1 such as deuterium 7,h71 are abundant and cheap4 ,there is enough deuterium in the oceans of the world to supply man's energy needs for many millions of years4 ,one difficulty1 of course1 is to find a container for the reaction4 ,even under stellar conditions the rate of fusion reactions is not very great4 ,it takes approximately a billion years for only one per cent of the nuclei to react4 ,consequently even higher temperatures than those found in stars are required to produce large amounts of energy in a short time4 ,but no known materials can withstand temperatures of more than a few thousand degrees centigrade4 ,one idea is to keep the 8burning0 fuel away from material walls by means of magnetic fields4 ,is there a way to make nuclei react without the extreme temperatures needed in the thermonuclear reactions8 ,what one is really trying to do is bring two nuclear particles into intimate enough contact so that the nuclear forces can act between them4 ,there is no reason why one should not use a cold target material1 which is bombarded from the outside by energetic nuclear projectiles1 for example protons or alpha particles4 ,the projectiles1 if they are energetic enough1 can overcome the electrical repulsion of the target nuclei1 and they actually can penetrate4 ,the resulting 8compound0 nuclei would either be unstable and disintegrate instantaneously1 or else be almost stable 7i4e41 radioactive7 and disintegrate after some period of time4 ,in either case nuclei of new elements would probably be formed in the reaction4 ,this procedure sounds simple1 but it has its difficulties4 ,illustration3 ,interior of the sun4 ,the thermonuclear reactions take place mainly in the very hot1 very dense central region 7shaded74 ,this region is about #bj1jjj miles in radius and has a density approximately #hj times the density of water4 ,the main difficulty is that the nucleus is a very tiny target4 ,its area is about #ajj million times smaller than the area of the atom as a whole4 ,if a piece of matter is bombarded by an energetic particle1 chance alone will determine whether the particle is directed toward a nucleus4 ,to be sure1 if the particle misses the nucleus of one atom1 it still has the opportunity of hitting the nuclei of other atoms which may lie in its path4 ,it does not have many such opportunities1 however1 because1 being charged1 it constantly interacts with the atomic electrons1 which gradually absorb energy causing the particle to slow down4 ,as the particle slows down1 its chance of hitting a nucleus decreases1 even if it is heading directly toward one1 because of the repulsion between its charge and that of the nucleus4 ,unless the particle has sufficient speed1 it cannot overcome this repulsion4 ,charged particles may be given the required speeds by accelerating them through large electric fields4 ,if a unit charge is accelerated through a potential difference of one volt1 it acquires an energy of one electron-volt4 ,the energies required for nuclear bombardment are of the order of several million electron-volts1 which can be provided by atom-smashing machines such as the cyclotron4 ,even at such high energies very few of the nuclear projectiles actually find their way to a target nucleus4 ,most of them are slowed down by the electrons1 wasting their energy in heating up the target material4 ,perhaps one particle out of a million will be lucky enough to induce a nuclear reaction4 ,if the purpose of the nuclear accelerating machines were to produce cheap energy1 they would not be of much value4 ,a nuclear reaction may typically release five to #bj million electron-volts of energy4 ,but to obtain this reaction1 a million particles had to be accelerated to energies of several million electron-volts4 ,the recoverable and useable energy will be only a minute fraction of the total invested4 ,on the other hand1 as a tool for scientific discovery1 the atom-smashers have been of great importance4 ,that one event in a million has given us much of our knowledge of nuclear physics4 ,the achievement of nuclear reactions by particle bombardment did not actually wait on the invention of man-made accelerating machines4 ,energetic alpha particles are available from the radioactive decay of heavy elements4 ,in #aiai ,ernest ,rutherford used such radioactive elements as a source of alpha particles4 ,the alpha particles were made to bombard ordinary nitrogen1 causing the reaction3 ,he ,n ,o proton 7#b protons7 7#g protons7 7#h protons7 7#b neutrons7 7#g neutrons7 7#i neutrons7 ,that is1 an alpha particle plus a nitrogen nucleus react to produce a nucleus of 7stable7 oxygen plus a proton4 ,oxygen is a nucleus with #h protons and #i neutrons4 ,the ordinary abundant form of oxygen has #h protons and #h neutrons4 ,natural oxygen contains a small amount of oxygen4 ,later1 in #aicd1 ,irene ,curie ,joliot 7the daughter of the discoverer of radium1 ,madame ,curie7 and her husband1 ,frederic ,joliot1 used naturally available alpha particles to make artificial radioactive nuclei for the first time4 ,the reaction was3 ,he aluminum phosphorus neutron 7#b protons7 7#ac protons7 7#ae protons7 7#b neutrons7 7#ad neutrons7 7#ae neutrons7 ,phosphorus is an unstable nucleus and emits a beta ray 7a positron7 to become silicon 7which is stable74 ,the half-life for this decay is about #b4e minutes4 ,the ,joliots' reaction was the first instance in which man had produced radioactivity and known it4 ,actually cyclotrons had been producing radioactivity in good abundance for the preceding two years--but physicists had been unaware of this fact4 ,it is amusing that nature has also provided us with an atom-smashing machine and indeed one that produces far greater energies than any apparatus yet devised by man4 ,this machine operates on the principle of fluctuating1 turbulent magnetic fields in interstellar space4 ,cosmic particles--mainly protons1 but also some alpha particles and even heavier nuclei--are accelerated by these changing magnetic fields and hurled occasionally into the earth's atmosphere4 ,the energies of these cosmic particles are enormous1 ranging from billions of electron-volts to values a million times higher4 ,when a cosmic particle gets inside the earth's atmosphere1 it does not go far before colliding with a nucleus of nitrogen or oxygen4 ,out of this nuclear event emerge all the fundamental particles mentioned so far1 and some others known as mesons4 ,mesons are particles which may be charged or neutral1 and which have a weight a few hundred times that of the electron4 ,some of these particles are believed to be connected with the forces that hold the nucleus together4 ,the nuclear debris from the collision will itself be very energetic and will further disrupt other nitrogen and oxygen nuclei4 ,there soon develops a cascade of electrons1 positrons1 mesons1 neutrons1 protons1 and electromagnetic radiation moving toward the surface of the earth4 ,about once a second every square inch of the earth's atmosphere receives such an energetic particle from outer space4 ,the cascade that results carries penetrating radiations to the surface of the earth4 ,all living organisms are constantly subjected to this radiation background4 ,it is an important fact that the intensity of this radiation is reduced in its passage through the air1 and inhabitants of ,denver or ,lima receive more cosmic radiation than the inhabitants of ,los ,angeles or ,new ,york4 ,some neutrons made by collisions of the primary cosmic particles in the atmosphere may collide with nuclei of nitrogen4 ,when this happens1 the following reaction occurs3 nitrogen neutron carbon proton 7#g protons7 7#f protons7 7#g neutrons7 7#h neutrons7 ,carbon is a radioactive electron emitter with a half-life of #e1fjj years4 ,this half-life is long enough so that much of the carbon in the world today was probably made ten to twenty thousand years ago4 ,willard ,libby studied this process in a careful and quantitative way1 traced the history of the radioactive carbon from the atmosphere into living beings1 and1 by measuring the carbon content in historical remains1 opened up a whole new branch of archeology4 ,living organisms breathe in carbon 7in the form of carbon dioxide7 from the air4 ,most of this carbon is ordinary stable carbon2 a tiny fraction is radioactive carbon4 ,the organism is unable to distinguish between the two isotopes1 and takes in carbon in the same ratio to carbon as exists in the atmosphere4 ,this ratio persists throughout the organism's lifetime1 but when the organism dies and no more carbon is assimilated1 the ratio begins to decrease as the carbon nuclei gradually disintegrate4 ,by observing the ratio of carbon to carbon in fossil remains and other archeological objects1 the date at which death occurred can be calculated4 ,in this way the age of ancient ,egyptian mummies has been found1 and it has been shown that some sequoia wood is more than #a1ejj years old4 ,by measuring the carbon in trees that were killed by the last advance of glaciation1 and looking into other remains of life from the last ice age1 it has been possible to show that this last ice age occurred only #aj1jjj years ago--instead of #bj1jjj years1 as had been previously believed4 ,carbon-dating has therefore thoroughly revised our ideas about the rapidity with which the empires known to history have emerged from the most primitive conditions4 ,a crucial part of the argument is that isotopes of the same element are chemically indistinguishable4 ,an alternative reaction which may occur when neutrons strike nitrogen1 is ,n neutron carbon ,h 7#g protons7 7#f protons7 7#a proton7 7#g neutrons7 7#f neutrons7 7#b neutrons7 ,h1 triton1 is also radioactive1 undergoing a beta decay to become ,he 7#b protons and #a neutron7 with a half-life of #ab4be years4 ,tritons too can be used for dating old objects--for example1 old wine4 ,the water in the wine cannot be replenished with cosmic-ray tritons after the wine has been bottled4 ,thus fifty per cent of the tritons disappear every #ab4be years4 ,we have here two examples of nuclear reactions induced by neutron bombardment4 ,recalling the disadvantages of charged particles as nuclear projectiles for alchemists1 it must surely seem that neutrons would be ideal for this purpose4 ,being chargeless1 they are neither electrically repelled by the nuclei nor constantly slowed down by energy-losing collisions with the electrons4 ,the fate of almost every neutron moving in a large piece of matter is eventual collision with a nucleus4#g ,neutrons are ideal nuclear projectiles1 except for one thing3 they are hard to get4 ,protons and alpha particles are found abundantly in nature as the nuclei of hydrogen and helium atoms4 ,neutrons1 however1 are not found in nature1 and in the past have been made in nuclear reactions that were themselves initiated by charged particles4 ,for example1 ,he beryllium ,c neutron 7#b protons7 7#d protons7 7#f protons7 7#b neutrons7 7#e neutrons7 7#f neutrons7 ,but now we encounter again the difficulty associated with charged particles4 ,only one alpha particle in a million undergoes a nuclear reaction to produce a neutron4 ,the neutron1 of course1 makes a nuclear reaction every time4 ,over-all1 then1 we obtain two nuclear reactions per million nuclear projectiles1 instead of one per million4 ,with such methods we are not so much better off than the old alchemists4 ,a cheap and plentiful source of neutrons would1 however1 put the alchemist in business4 ,in this way one could make rare elements and radioactive isotopes1 and what is more important1 he would be able to utilize concentrated nuclear energy4 ,,chapter ,,vii ,fission and the ,chain ,reaction ,neutrons are ideal projectiles for nuclear bombardment because they carry no charge1 can approach nuclei easily1 and interact with them strongly4 ,these neutral particles1 discovered by ,james ,chadwick in #aicb1 were used soon afterward by ,enrico ,fermi and his collaborators to bombard most of the elements of the periodic table4 ,very often in these experiments a nucleus would capture a neutron and become unstable with too much weight for its charge4 ,stability would then be restored by a beta decay1 leaving the nucleus with one more unit of charge than it had to begin with4 ,in #aicd ,fermi tried this experiment with uranium1 charge #ib1 the most highly charged element known at that time4 ,he hoped to make a transuranic element with charge #ic4 ,throughout the experiments the uranium was observed with radioactive counters and found to become far more radioactive than uranium ordinarily is in its natural state4 ,there was no way to account for all this radioactivity except to assume that new elements had been formed in the process of neutron bombardment4 ,a chemical analysis revealed no elements with charges between #hf and #ia4 ,from this evidence ,fermi concluded that no elements of charge less than #ib had been made and therefore the radioactivity must be due to charges greater than #ib4 ,he concluded that transuranic elements had been made in the laboratory4 ,neither ,fermi nor anyone else1 however1 was happy with this conclusion4 ,there was far too great a variety of radioactivity for comfort4 ,it had to be assumed that not only was the element with charge #ic being made1 but also elements with charges #id1 #ie1 and many more4 ,this was very hard to understand4 ,ida ,noddack1#h a chemist1 published a paper proposing an alternative explanation of the experiment3 that a nucleus of uranium1 when it captures a neutron1 might break up into two fragments that could have any of various weights and charges4 ,in other words1 she suggested that ,fermi had produced nuclear fission4 ,fermi1 however1 believed that the fission process was an impossibility4 ,he had a convincing proof1 based on the measured values of the weights of nuclei and the formula of ,einstein1 ,e mc4 ,from this formula ,fermi calculated the energy liberated when uranium breaks into two pieces2 then he took into account the energy of electric repulsion between the pieces and found that the energy barrier was so large that the fission process could not take place4 ,this proof was absolutely correct4 ,the only trouble was that the measured values of the weights of nuclei happened to be inaccurate at that time6 ,but for this accident1 fission would have been discovered in #aicd instead of #aich4 ,if it had been1 ,nazi ,germany might easily have been the first country to make the atomic bomb4 ,at that time some ,german scientists were active in the field of military applications4 ,the ,american physicists had not yet turned much attention to the subject4 ,an important feature of ,fermi's experiment is the large amount and variety of radioactivity that he found4 ,the reason for this variety1 as we now know1 is that the fission process does not take place in a unique manner4 ,the two primary fission fragments are very rarely of equal weight and charge4 ,on the average the lighter fragment weighs about #ij1 and the heavier one about #adj4 ,sometimes the lighter fragment will weigh as little as #ge1 and the heavier one as much as #afj4 ,as the weight varies1 of course1 so also does the charge4 ,the charge of the lighter fragment averages #ch1 which is strontium1 and the heavier one #ed1 which is xenon4 ,all in all there are more than a hundred different species of nuclei represented among the primary fission fragments4 ,practically all of these nuclei are radioactive and undergo three or four disintegrations before reaching stability4 ,overall therefore1 several hundred distinct radioactive species are created by the fission process in uranium4 ,elements with charges #dc and #fa 7which are not found in nature7 have been identified as fission products in fairly appreciable quantities4 ,most of the fission products are short-lived electron and gamma emitters that can contribute only to the local and immediate radioactive hazard4 ,two of the long-lived products are abundant and important4 ,these are cesium and strontium4 ,cesium has a half-life of #cj years and emits a gamma ray with an energy of #j4f million electron-volts4 ,strontium has a half-life of #bh years and emits an electron with an average energy of #j4bb million electron-volts4 ,the daughter nucleus in this process is yttrium1 which emits another electron with an average energy of one million electron-volts4 ,the half-life of yttrium is #fd hours4 ,in effect1 therefore1 strontium emits two electrons1 each with an average energy of #j4f million electron-volts4 ,for the long-term radioactive hazard1 particularly the world-wide fallout associated with atomic explosions1 the two isotopes cesium and strontium are the most significant4 ,strontium is the more dangerous to living organisms because it is deposited in the bones and retained in the body for long periods4 ,besides radioactivity there is another feature of the fission process which is so conspicuous that it may seem hard to understand how ,fermi failed to notice it--namely the large amount of energy released4 ,the fission of a single nucleus of uranium releases an energy of #bjj million electron-volts as contrasted with ordinary radioactive decay energies of #e to #aj million electron-volts4 7,the energy released from the burning of one atom of coal is only #d electron-volts47 ,of the #bjj million electron-volts released in fission1 about #aj million go into gamma rays and neutrons created in the fission process itself4 ,this energy contributes to the immediate and local radiation danger4 ,another #bd million electron-volts go into radioactivity of the fission products1 and of this1 about half go into neutrinos1 which are neither dangerous nor useful2 the other half is carried by electrons and gives rise to the delayed radioactive hazard4 ,but the bulk of the energy1 over #afj million electron-volts1 goes into kinetic energy of the two primary fission fragments4 ,of this amount1 #ajj million1 on the average1 go to the lighter fragment4 ,one hundred million electron-volt fission fragments should certainly have been noticed by ,fermi's radioactive counters--if they had been able to reach the counters4 ,the fragments were not able to reach the counters1 however4 ,the reason is that ,fermi was a careful worker4 ,he knew that his sample of uranium would emit some radioactive particles even before neutron bombardment4 ,this natural radioactivity he did not want to get mixed up with the radioactivity that would be produced in the experiment4 ,so he put an absorbing foil between the uranium sample and the radioactive counters4 ,the fission fragments could not get through the foil4 ,it is amusing that shortly afterward another noted physicist repeated ,fermi's experiment1 but this time without the foil4 ,he reported that he was unable to get any significant results because his counter1 for reasons unknown1 started to spark4 ,thus fission remained a secret4 ,but in ,england ,leo ,szilard obtained patent papers on the nuclear chain reaction4 ,he pointed out that in some nuclear reactions free neutrons might be released4 ,these neutrons might then succeed in producing further reactions which would produce more neutrons4 ,provided that at least one neutron made in each reaction were able to induce a reaction in another nucleus1 a chain reaction would take place4 ,the main problem1 of course1 was to avoid excessive neutron losses4 ,there are two ways in which the losses mainly occur4 ,one is by wasteful1 nonreproductive capture in the nuclei2 the other1 by neutron leakage from the material surface4 ,this second loss1 ,szilard showed1 could be minimized by using a sufficiently large amount of chain-reacting material4 ,the point is that a neutron born in a nuclear reaction must travel on the average a certain distance before it can produce another reaction4 ,if the size of the chain-reacting material is much less than this distance1 practically all of the neutrons produced will be able to escape through the material surface1 and no chain reaction will be possible4 ,if the size of the material is large compared to this distance1 the leakage loss becomes negligible1 and the possibility of a chain reaction depends entirely on the magnitude of the first kind of loss1 the wasteful captures in nuclei4 ,if this loss is not too great1 and a chain reaction is possible1 there will be a critical size of the material at which on the average exactly one neutron per reaction will be able to induce another reaction4 ,a just critical chain reaction of this kind is what is needed for an atomic reactor4 ,if the size of the material is greater than the critical size1 on the average more than one neutron per reaction will cause another reaction and the chain reaction will run away4 ,if1 for example1 two neutrons can cause another reaction1 there will be two neutrons after the first generation1 four after the second1 eight after the third1 and so forth4 ,this is the principle of the atomic bomb4 ,after about #hj generations1 an appreciable fraction of all the nuclei in the material will have undergone a nuclear transformation and so much energy will have been released that the material will not stay together even for the short time needed to produce the next generation4 ,the whole material begins to fly apart1 the system becomes sub-critical1 and the chain reaction stops4 ,the entire process lasts only a fraction of a microsecond4 ,thus even before fission was discovered1 ,szilard laid the basis for constructing the atomic bomb and the nuclear chain reactor4 ,as materials in which a chain reaction might conceivably be made to occur he named thorium1 uranium and beryllium4 ,on beryllium he was wrong because the mass of this atom was incorrectly known4 ,on thorium1 his guess was good4 ,on uranium1 he hit the bull's eye4 ,finally in ,december #aich the secret broke4 ,hahn and ,strassmann in ,germany made a chemical analysis of a uranium target that had been exposed to neutrons4 ,they were far more thorough than previous investigators had been1 and they found barium1 charge #ef1 which had not been present in the target material before the experiment4 ,the only possible explanation was the fission process4 ,within a few weeks the violent kicks caused by the fission products in counters were found1 and in the following days this experiment was repeated around the world4 ,there was no doubt that neutrons could induce fission in uranium nuclei4 ,a few more weeks1 and it was ascertained that the fission process released neutrons which might lead to more fissions4 ,the chain reaction1 however1 was still far from a reality4 ,niels ,bohr and ,john ,wheeler proved that a neutron could not cause fission in ,u unless its energy were greater than about one million electron-volts4 ,when the neutrons are first made in the fission process1 many of them do have energies greater than one million electron-volts4 ,but before they can cause a fission1 they usually make a few nonfission collisions with uranium nuclei1 giving part of their energy to the nuclei and escaping with the remainder4 ,the nuclei are then left with too little energy to undergo fission and the neutrons with too little energy to cause fissions in their next encounters4 ,thus too few neutrons reproduce themselves and no chain is possible4 ,bohr and ,wheeler suggested1 however1 that the rare isotope of uranium1 ,u1 can undergo fission when any neutron1 even a slow neutron1 hits it4 ,thus a chain reaction is possible in ,u4 ,this was confirmed experimentally shortly afterwards by ,john ,dunning and ,alfred ,drier and their co-workers at ,columbia ,university4 ,why the isotopes #bce and #bch behave so differently1 is not difficult to understand4 ,the #bce is more explosive and more prone to undergo fission than #bch because it is smaller and therefore its protons repel each other more strongly4 ,more important still1 when a neutron is captured by #bce1 it acquires a greater kinetic energy by virtue of the short-range nuclear attraction than a neutron acquires when it is captured by #bch4 ,this happens for the simple reason that nuclei tend to be more stable when they have an even number of neutrons 7or protons7 than when they have an odd number4 ,u1 having an odd number of neutrons1 is more eager to receive an additional neutron than #bch1 which already has an even number of neutrons4 ,consequently1 the capture of a slow neutron by #bce almost always eventuates in the fission process2 while in #bch1 the excess energy1 introduced by the neutron1 is merely ejected from the nucleus in the form of a gamma ray1 and ,u becomes ,u4 ,a chain reaction is possible in ,u 1 but it is necessary to separate this rare isotope from the abundant ,u4 ,the separation process is anything but simple since isotopes of the same element are chemically indistinguishable4 ,even the weight difference in this case1 is little more than one per cent4 ,bohr rejected the idea of a large-scale separation with the remark3 8,you would have to turn the whole country into a factory40 ,of course it is now a matter of history that the job was actually done under the ,manhattan project during ,world ,war ,,ii4 ,during the war ,bohr 7alias ,nicholas ,baker7 again visited the ,united ,states and was shown the separation plants4 ,he said3 8,you see ,i was right4 ,you did turn the country into a factory40 ,natural uranium contains ,u in the ratio of #a part to #aci of ,u4 ,it was hoped at first that this concentration would be sufficient to make a chain reaction1 and that the expensive enrichment processes could be avoided4 ,this seemed possible because at energies of a fraction of an electron-volt the neutrons are much more easily caught by ,u than by ,u1 which compensates for the low concentration4 ,actually neutrons are slowed down until their energy is as low as the energy of all other particles participating in the general agitation caused by the temperature4 ,this energy is low enough for the purpose4 ,however1 the neutrons are made in the fission process with an energy of about a million electron-volts4 ,before they slow down sufficiently1 they must pass through a stage in which their energy is about #g electron-volts4 ,in the neighborhood of this energy1 it happens that the ,u has an extremely high probability for capturing a neutron and changing into ,u4 ,near some other energies1 similar though smaller absorption hurdles must be passed4 ,therefore natural uranium by itself cannot be used to make a chain reaction4 ,in #aidj1 ,fermi and ,szilard1 working now in the ,united ,states1 found a way around this difficulty4 ,their trick was to mix the natural uranium with a material whose nuclei are so lightweight that they suffer a big recoil when struck by a neutron and thus absorb a large fraction of the neutron energy4 ,the neutron is thus moderated down to a low energy1 rapidly and in big energy jumps1 so that either it does not spend much time at the unfavorable energies where it can be caught by ,u or else it misses these energies altogether4 ,by imbedding the uranium in lumps in the moderating material instead of making a homogeneous mixture of the two1 the absorption can be circumvented even better4 ,for the purpose of making a controlled chain reaction1 one may use the method of enrichment1 or the method of moderation1 or both4 ,but to produce a violent chain reaction1 an atomic bomb1 only the enrichment method will work4 ,the reason is that all the energy of the bomb must be generated in a time that is as short as the time it takes the bomb to fly apart1 which is a fraction of a microsecond4 ,if natural uranium were used1 the reaction would be slow and sluggish and would be extinguished before a substantial fraction of the nuclei could have reacted4 ,it is interesting to consider that chain-reacting substances could have been obtained easily six billion years ago1 before the ,u had time to decay and become a rare isotope4 7,the ,u was then about as abundant as ,u47 ,a chemical separation would still have been necessary and so we do not need to imagine that chain-reacting mixtures accumulated spontaneously on the young earth4 ,on the other hand1 six billion years from now ,u will have become so rare that it will be impossible to get a reactor going by moderation4 ,at the same time the isotope separation will have become most expensive since the isotope to be separated will be present in an abundance of less than #ajj parts in a million4 ,for those who like to worry about the distant future we should hasten to add that other methods of obtaining atomic energy will remain possible4 ,and in any case there is good reason to believe that some stellar explosions produce fresh supplies of ,u which space merchants could undoubtedly make available4 ,as to our present terrestrial supplies3 uranium1 like other heavy elements1 is quite rare4 ,but the earth is divided into layers of which the topmost #aj miles1 forming something of a slag or scum1 contain quite a few rare compounds4 ,in particular almost all of the uranium in our planet is conveniently collected right under our feet1 for us to use as we see fit4 ,,chapter ,,viii ,action of ,radiation on ,matter ,when an energetic particle moves through matter 7living or nonliving71 what happens is a question of chemistry4 ,chemistry is the subject that deals with the arrangement and rearrangement of electrons in atoms and molecules4 ,a chemical rearrangement generally requires an energy in the neighborhood of a few electron-volts4 7,as we have seen1 an electron-volt is the energy released when an electron moves through a potential of one volt1 i4e41 a little less than one per cent of the driving force in a standard electric outlet47 ,an energetic particle1 such as might be emitted in a radioactive decay1 typically has an energy of a few million electron-volts4 ,thus a single such particle has the potentiality of about a million chemical rearrangements4 ,energetic particles may be charged or neutral1 light or heavy1 or electromagnetic in nature4 ,because of this diversity one might think there would be no common grounds for comparing the action on matter of different particles4 ,each particle might conceivably make its own inimitable variety of chemical rearrangements4 ,actually this is not the case4 ,unlike some chemical poisons1 which seek out specific molecules in our body1 the energetic particles strike at whatever atoms or molecules happen to get in their way4 ,they act1 in this sense1 like a sledge hammer4 ,their effects can be measured directly from the strength 7or energy7 of the blow4 ,which particle delivers the blow is of little consequence provided the same amount of energy is delivered and provided the same tissues are affected 7in the case of living matter74 ,after the blow1 however1 some specific chemical effects may occur4 ,when water or some other molecule in the body is broken up by radiation1 the fragments produced may themselves be chemical poisons and attack the biologically important large molecules in a secondary way4 ,in fact1 it seems probable that a considerable part of the radiation damage caused in living systems1 both healthwise and genetically1 occurs in this manner4 ,although the energetic particles are all similar in their ultimate action on matter1 namely in producing wholesale destruction of atoms and molecules1 they differ somewhat in the way in which they bring about this destruction4 ,charged particles act in one way1 gamma rays in another1 and neutrons in still another4 ,it is simplest to begin our discussion with the charged particles4 ,the most important charged particles are those connected with the natural background of radioactivity and cosmic rays1 and the fission process4 ,these include alpha rays1 beta rays1 mesons1 and fission fragments4 ,for review1 a table of the weights and charges of these particles1 as well as a few others1 is shown4 ,as usual1 we have used the weight and charge of the proton as units4 ,particle ,weight ,charge proton #a #a alpha #d #b electron beta rays #a_/ahdj -#a positron 0 #a_/ahdj #a deuteron #b #a triton #c #a meson #a_/h #a1 -#a average light fission fragment #ig #bj average heavy fission fragment #ach #bb ,if the fission fragments were completely stripped of their orbital electrons1 they would have charges even greater than the values indicated in the table4 ,the reader will recall that the average charge of the nucleus of the light fission fragment is #ch1 and of the heavy1 #ed4 ,but such highly positively charged particles exert an enormous attraction on electrons4 ,some of these remain attached even during the fission process itself4 ,as the fission products lose their speed during passage through matter1 they pick up more electrons and gradually lose their charge4 ,when any of these energetic charged particles moves through matter1 it interacts with electrons in the atoms4 ,as a result of this interaction1 the electrons may be dislodged from their usual states of motion4 ,if the interaction is gentle--either because the charged particle passes the atom at a considerable distance or else because the particle is moving so rapidly that the interaction lasts for only a short time--the electron may be left undisturbed4 ,if the interaction is more violent1 however1 the electron may be excited to a more energetic state of motion while still remaining in the same atom or molecule2 or it may actually be ejected1 ending up at some other atomic site4 ,in this latter event the original atom is left with a residual positive charge and is said to be ionized4 ,at the same time the displaced electron is apt to unite with whatever atom or molecule happens to be nearby1 creating in this way a negative ion4 ,the whole process may be described as forming an ion pair4 ,in the wake of the charged particle one finds1 therefore1 ionized and excited atoms and molecules4 ,a rearrangement of atoms will now ensue which leads to new chemical compounds4 ,the important thing for us is1 however1 that these chemical changes do not depend very much on the type of particle which produced the ionization2 the proportion between ionization1 excitation1 and eventual chemical reaction remains more or less the same4 ,roughly speaking1 the more ion pairs that are formed in living cells1 the greater is the extent of biological damage4 ,to make an ion pair requires the expenditure of a certain amount of energy4 ,it might seem as though this amount should depend crucially on the weight1 charge1 and energy of the particle1 and also on the medium through which the particle is moving4 ,this is not so4 ,there is some dependence1 of course1 but only slight4 ,any charged particle1 irrespective of its energy1 moving in any medium--air1 water1 soil1 or living tissue--creates ion pairs at the rate of about one per #cb electron-volts4 ,a one-million-electron-volt particle produces about #cj1jjj ion pairs before losing all of its energy4 7,when it does lose its energy1 if it is a positively charged particle1 it will pick up enough electrons to become neutral4 ,an alpha particle1 for example1 will become an ordinary helium atom2 a proton will become an atom of hydrogen47 ,we have said that two charged particles having the same energy1 produce the same total number of ionizations4 ,there is an important respect1 however1 in which charged particles of the same energy may differ4 ,that is1 in the density of ionization along their paths4 ,in particular1 the more slowly the particle is moving and the greater its charge1 the more ionization and damage it will produce in a given distance4 ,at the same time it will lose energy at a greater rate4 ,if we compare two charged particles of the same energy plowing into matter1 the one which leaves the deeper furrow will be stopped more quickly4 ,for a greater charge it is easy to understand that the electrical interaction is increased and hence each atomic electron is more strongly disturbed4 ,if1 on the other hand1 the particle moves more slowly 7which is usually the case if it is heavy7 it spends a longer time in the neighborhood of the atomic electrons4 ,the electrical interaction thus has a longer duration and is more effective in ejecting an electron4 ,for this same reason the density of ionization along the path of a particular charged particle should tend to become greater and greater as the particle slows down4 ,actually this tendency is opposed in the case of a fission fragment by the increased likelihood of the particle's picking up electrons and reducing its charge4 ,as a result1 the ionization density for these fragments is rather uniform4 ,if a heavily charged1 slow particle moves through matter it leaves so many disturbed and disrupted molecules behind that now these molecules may react with each other4 ,therefore heavy ionization may lead to peculiar effects4 ,nevertheless all ionizing particles give rise to roughly similar chemical change and destruction4 ,except for the beta rays1 all the charged particles are very heavy compared to the electron4 ,consequently1 as they move through matter and interact with the atomic electrons1 their paths are not perceptibly deflected from the original direction4 ,the beta rays1 on the other hand1 having the same weight as the atomic electrons1 are appreciably affected by their encounters and are frequently forced to change direction4 ,their paths are thus winding and random4 ,because the beta ray does not travel in a straight line1 its ability to penetrate matter must not be measured by its total path length4 ,as a rule of thumb1 the range of a beta particle1 being the distance it travels along the line of its original direction1 is about one half of its total path length4 ,for heavier charged particles1 however1 no distinction need be made between range and actual distance traveled4 ,the most important fact about the ranges of charged particles is that they are small4 ,an alpha particle1 for instance1 with a typical radioactive energy of a few million electron-volts1 has a range in water 7or living tissue7 of a few thousandths of an inch4 ,such a particle could not penetrate a sheet of paper4 ,a fission fragment1 despite its great energy1 is even less penetrating than the alpha particle4 ,the proton has a somewhat greater range than the alpha particle4 ,but the beta ray1 because of its low weight1 has by far the greatest range of any of the charged particles4 ,even it1 however1 goes only a fraction of an inch in solid or liquid materials4 ,the following table shows the ranges 7in inches7 of some of the charged particles in air and water as a function of energy 7in millions of electron-volts73 ,range ,air ,water 7,same as living tissue7 ,energy #e #a #b #e #e #a #b #e alpha #j4a #j4b #j4d #a4d #j4jjja #j4jjjb #j4jjjd #j4jjad proton #j4c #j4i #b4h #ac4d #j4jjje #j4jja #j4jjc #j4jad beta #di4 #acj4 #cjj4 #ggj4 #j4jfc #j4af #j4ch #a4j ,the table shows that charged particles travel only short distances in matter4 ,for this reason these particles are not a serious external radiation hazard4 ,the protons and the alpha rays are usually stopped by less than a foot of air4 ,ordinary clothing or even the outer layer of our skin 7which is composed of nonliving cells7 will stop them completely4 ,beta rays are stopped by less than seventy feet of air or an inch or less of solid material4 7,actually most of the beta rays produced in the fission process have energies less than a million electron-volts or so1 and hence their ranges are even smaller47 ,radioactive contamination of beta emitters directly on one's clothes or body could cause trouble2 but a good scrubbing soon after exposure will eliminate this problem4 ,the interior of a house or building should be quite safe from any outside source of charged particles emitted by radioactive substances except possibly the most energetic beta rays4 ,only if the source of charged particles is inside the body so that in spite of their limited ranges the particles can find their way to sensitive tissues1 is there any danger4 ,in this case1 as we shall see in a later chapter1 the danger may be considerable4 ,charged particles of one type stand pretty much by themselves4 ,these are the mesons found in cosmic rays4 ,these particles move as fast as energetic beta rays and1 like the beta rays1 carry unit charge4 ,their biological effects are therefore the same as the biological effects of beta radiation1 with one important difference4 ,the cosmic ray mesons carry much more energy and therefore have a much greater range4 ,whereas the beta rays are stopped in the skin1 the mesons can cause damage throughout the entire body4 ,the mesons produce the same effects as a substance which emits beta radiation uniformly in the whole body4 ,this fact is important4 ,it puts us in the position to compare effects of man-made radioactivity with effects of the cosmic rays to which we are constantly exposed4 ,not all the energy in cosmic rays is carried by mesons4 ,we also find showers of electrons4 ,these are almost the same as beta rays except that they have more energy and arrive frequently in fairly sizeable numbers traveling along nearly parallel tracks4 ,their effects1 however1 are the same as the effects of the mesons4 ,we have been talking now about the interactions between charged particles and the atomic electrons4 ,no mention has been made of interactions between the charged particles and nuclei4 ,nuclear interactions do occur sometimes1 but by and large they have only a negligible influence in slowing down the charged particle4 ,they do affect1 however1 beta rays4 ,when a beta ray collides with a highly charged nucleus1 the beta particle is violently deflected4 ,the violence of this process is due to the heavy charge of the nucleus and the small mass of the beta particle4 ,in the sudden change of velocity which occurs1 part of the electric force field which surrounds the electron breaks loose2 the result is high-frequency radiation called ,x-rays4 ,the importance of such electromagnetic radiation is that it can penetrate more deeply into matter4 ,in our bodies1 for typical beta-ray energies1 only a small part of the beta-ray energy is converted into ,x-rays4 ,but in many radioactive processes gamma rays 7which are physically the same as ,x-rays7 are produced quite abundantly4 ,these rays may carry as much or more energy than the beta rays4 ,unlike charged particles1 which constantly interact as they move through matter1 gamma rays can go for long distances without having a single encounter4 ,the actual distance depends on the energy of the gamma ray1 the medium in which it moves1 and pure chance4 ,on the average1 a one-million-volt gamma ray goes about six inches in water before anything at all happens to it4 ,a four-million-volt gamma ray goes about a foot4 ,in living matter the distances are approximately the same4 ,thus gamma rays from an external source can find their way deep inside the body4 ,of course living matter is not injured by the mere presence of a gamma ray4 ,there is a small probability that the gamma ray could go right through the body without a single encounter4 ,if so1 there would be no biological effect4 ,an effect is produced only when the gamma ray interacts with the matter4 ,there are three most important ways in which such an interaction may occur4 ,one way is simple absorption of the gamma ray by one of the atomic electrons4 ,the gamma ray disappears in this process1 and the electron acquires all of its energy4 ,a tiny bit of this energy is used for the electron to break its bond with the atom4 ,the remainder goes into kinetic motion of the electron4 ,the electron is now on the loose and can cause biological damage by exciting and ionizing other atomic electrons4 ,in fact it is now the same thing which we used to call a beta ray4 ,a second way in which the gamma ray may interact with matter is by scattering4 ,in this case the gamma ray does not disappear but merely loses a part of its energy to the atomic electron4 ,again the electron is free to cause biological damage1 while the gamma ray goes on to its next encounter4 ,the third way requires that the gamma ray be near a nucleus and have an energy greater than a million electron-volts4 7,ordinary ,x-rays such as are used in medical practice are not energetic enough for this process to occur47 ,under these conditions the gamma ray may disappear1 with the simultaneous appearance of an electron and a positron4 ,this is an example of the creation of matter out of pure energy4 ,in accordance with the formula ,e mc1 a part of the gamma-ray energy is consumed in producing particles with definite masses4 ,this amounts to about one million electron-volts4 ,the remainder of the gamma-ray energy goes into kinetic motion of the two particles4 ,again biological damage results from the subsequent ionization due to the charged particles4 ,after the positron has expended its kinetic energy in the ionization process1 it will join with an electron in a disappearing act4 ,the energy reappears in the form of two or three gamma rays 7each having less energy than the original gamma ray74 ,in no case is the gamma ray directly responsible for any biological damage4 ,the damage is always made by electrons 7or positrons7 to which the gamma ray has transferred some or all of its energy4 ,but this only makes gamma rays the more dangerous4 ,they can first penetrate to the sensitive tissues of the body1 and then cause ionization4 ,we have already mentioned that ,x-rays are the same as gamma rays4 ,the latter are produced by an excited nucleus1 the former in the collision of an electron 7or a beta ray7 with a nucleus4 ,the man-made ,x-rays are obtained by first accelerating a stream of electrons and then letting them impinge on a target containing highly charged nuclei4 ,the usefulness of ,x-rays is1 of course1 due to their power of penetration2 that is the same property which renders ,x-rays dangerous4 ,one can use ,x-rays to find out what happens to be inside the human body4 ,but this cannot be done without producing some disruption and rearrangement in the tissues which lie in the path of the ,x-rays4 ,the damage is of the same kind as that caused by radioactivity or cosmic rays4 ,the effects of neutrons on matter are rather similar to the effects of gamma rays4 ,like gamma rays1 neutrons can travel long distances in matter without interacting4 ,on the average1 a million-volt neutron goes a few inches in water before having a collision of any kind4 ,also like the gamma rays1 the neutrons are not themselves directly responsible for any biological damage4 ,being neutral1 they interact only with the atomic nuclei to which they are strongly attracted4 ,by far the most important of these interactions is with the nuclei of hydrogen4 ,there are a great number of these in living tissue in the form of protein and water molecules4 ,the collisions with hydrogen nuclei 7i4e41 protons7 are important because a large fraction of the neutron energy is transferred in the process4 ,this happens because the neutron and the proton have very nearly the same weight4 ,if the neutron hits a heavy nucleus1 it loses only a small fraction of its energy in the impact4#i ,after colliding with hydrogen or a heavier nucleus1 the neutron continues on to other such collisions4 ,the nucleus1 however1 being charged and energetic1 now causes excitation and ionization of atomic electrons4 ,thus1 like gamma rays1 energetic neutrons are exceedingly dangerous1 because they can first penetrate and then cause ionization4 ,neutrons are dangerous even when they are not energetic4 ,a nonenergetic neutron may react with nuclei of living matter in a number of ways of which two are particularly probable4 ,either the neutron may be captured by a proton to form a deuteron1 in which case the excess energy will be emitted in the form of a two-million-volt gamma ray that will cause further damage4 ,or the neutron may react with a nucleus of nitrogen 7abundantly present in living matter7 to produce a nucleus of carbon and an energetic proton4 ,thus a nonenergetic neutron will have a biological effect equivalent to an energetic gamma ray1 or to an energetic proton plus an energetic carbon ion4 ,in summary1 all particles1 charged or not1 have a similar action on matter4 ,directly or indirectly1 they produce excited atoms1 molecules1 and ion pairs4 ,these processes always occur in practically the same proportions1 and therefore the number of ion pairs formed can be used as a measure of the radiation effects4 ,the more ion pairs produced in living matter1 the greater the extent of biological damage4 ,for this reason it is customary to describe radiation effects in terms of the number of ion pairs created per gram of living tissue in various parts of the body4 ,since each ion pair corresponds to an energy transfer of about #cb electron-volts1 an alternative description may be given in terms of the amount of energy deposited4 ,the unit in common usage for this purpose is the roentgen1 which means specifically an energy equivalent to lifting the body 7in which the radiation is deposited7 by one twenty-fifth of an inch4 ,this is equivalent to about #fj million million ion pairs in each ounce4 ,it is less exact but more significant to say that one roentgen deposits in a cell of our body a few thousand ion pairs4 ,of course the amount of ionization within individual cells is not a quantity that is easily measured4 ,what one usually knows instead1 is the roentgen dosage to a piece of tissue1 which consists of many cells4 ,if the charged particles inducing the ionization are electrons 7as they are when the primary radiation is a beta ray or a gamma ray71 the ionization will be distributed more or less uniformly among the cells in the affected neighborhood4 ,if the charged particle is heavy--a proton or an alpha ray--the density of ionization which it produces is much greater1 so that some cells receive a good many more ion pairs1 while others nearby may receive none4 ,for this reason it is sometimes important to specify not just how many roentgens the tissue has been exposed to1 but also which kind of radiation has been responsible4 ,in a later chapter we shall discuss the biological effects of various amounts of radiation4 ,we may mention here1 however1 that #ajjj roentgens of ,x-rays or gamma rays delivered more or less uniformly over the whole body of a human being in a time less than a few hours or so1 will lead to almost certain death4 ,and it is a remarkable fact that nature has not provided us with a warning4 ,radiation does not hurt4 ,the greater is the need that we understand this process which affects our well-being but not our senses4 ,,chapter ,,ix ,the ,test ,testing of atomic explosives is usually carried out in beautiful surroundings4 ,there is a good reason for this3 the radioactive fallout4 ,because of the fallout1 the test site must be isolated4 ,the presence of human population does not improve nature 7with exceptions which are quite rare and the more notable74 ,also1 to keep the site clean1 tests must be carried out in the absence of rain4 ,therefore1 at the site one usually finds sunshine and solitude4 ,for the participants the beauty of nature forms the back-drop to preparations of experiments which are difficult and exciting to everyone involved4 ,at the end1 the atomic explosion is always dwarfed by its setting4 ,but the work that culminates in the detonation is rewarded by something quite different from a flash and a bang4 ,the really important results of a test consist in marks on photographic plates4 ,most of the apparatus that produced the plates has been destroyed in the explosion4 ,but enough is saved so that one can conclude what has happened in the short fractions of a second that pass between the pressing of the button and the knowledge in the observer3 this was it4 ,in those fractions of a second another stone was added to the structure which we may call astrophysical engineering4 ,what happens and what is observed in nuclear explosions are closely related to the behavior of matter in the interiors of the stars4 ,the details of the nuclear explosion cannot be described here for three reasons4 ,first1 the details are secret4 ,second1 the size of this book and the forbearance of the reader set limitations4 ,and third1 we understand only a small part of the process4 ,within these limitations1 this is what happens3 ,the actual nuclear reaction takes only a fraction of a microsecond 7one microsecond one millionth of a second74 ,all the energy of the bomb is released in this short period4 ,at the end of this period1 the main body of the nuclear material is moving apart at a rapid rate and by this motion further nuclear reactions are stopped4 ,in addition to the more or less orderly outward motion1 considerable portions of the energy are found in the disorderly temperature motion1 which has stripped most of the electrons off the nuclei and has transformed the atoms into a freely and chaotically moving assembly of charged particles4 ,by this time many of the original nuclei have been transformed into nuclei of radioactive species1 partly by the fission process and partly by the capture of neutrons in all sorts of atoms which had been originally present in the bomb materials4 ,still another portion of the energy is present as electromagnetic radiation4 ,this radiation closely resembles light except that it is of shorter wave length and is therefore not actually visible2 but it can be absorbed and re-emitted by all sorts of materials1 and is in a violent exchange of energy with the exploded bomb fragments4 ,all this perturbation spreads outward from the region where the nuclear reaction has taken place into the surrounding components of the bomb4 ,during the outward spread1 more atoms and more space get engulfed4 ,the agitation and the radiation become somewhat less hot4 ,this hot region tends to be limited by a sharply defined boundary which is called a shock front and which is moving outward at a speed of several hundred miles per second4 ,this front finally reaches the limits of the more or less dense material in which the whole bomb structure was originally encased4 ,it then breaks through into the surrounding air4 ,the air heats up in the immediate vicinity1 and this is the beginning of the fireball4 ,from this point on1 the energy spreads due to the push of the high-temperature air4 ,a sharp shock front forms and keeps moving outward at a speed greatly surpassing ordinary sound speed4 ,the radioactive material is contained within this hot and expanding sphere4 ,as the fireball expands and the temperature falls1 more and more visible radiation is emitted4 ,actually1 the surface is growing less brilliant as the structure expands and cools1 but its greater size and the longer time that is available for the emission of radiation overcome this disadvantage4 ,finally1 at a radius of perhaps a few hundred feet for a small bomb and a mile for a big one1 the fireball expansion halts4 ,this happens because the shock front is no longer strong enough to make the air luminous4 ,the luminosity not only stops advancing but is actually partly dimmed by absorbing substances formed by the badly mistreated air molecules4 ,the time which has elapsed to reach this stage of the explosion depends on the bomb energy4 ,if two explosions are compared1 and the bigger one has a thousand times the explosive power of the smaller one1 then the time needed to reach the extreme expansion of the fireball will be approximately ten times greater for the more violent event4 ,in any case1 a reasonably close observer has to use strongly absorbing glasses during this time if he is not to be blinded4 ,for small bombs1 the expansion of the fireball is too short to register4 ,for the really big ones1 you can see the expansion developing and you wonder when it will stop4 ,to the unprotected eye the small bombs are almost as dangerous as the big ones1 because there is not enough time to blink4 ,in the meantime1 the shock wave1 now separated from the fireball1 travels through the air and carries with it a considerable fraction of the original explosive power4 ,an important part of the damage which a bomb can cause is due to this invisible pressure wave which spreads with a speed close to that of sound1 over a distance of miles1 before it settles down into harmless rumbling4 ,the rest of the energy is still sitting in the fireball near the point where the explosion occurred and the hot air now commences to ascend1 breaking up into a turbulent mushroom as it goes4 ,the hot interior portions get occasionally exposed and the object gives the appearance of an enormous flaming mass1 at least when seen in a motion picture which slows down the action and reduces the size4 ,the radiant tongues are too big and too fast for any ordinary flames4 ,during this stage the display gradually pales sufficiently so that it can be viewed with the naked eye4 ,the originally hot masses have now emitted enough energy in the form of light and mixed with a sufficiently great mass of cool air that they no longer glow violently4 ,this mass of central and rising gas contains practically all the radioactivity1 not only that originally formed in the explosion but also some produced by neutrons which leaked out of the bomb and got captured by a variety of nuclei in the air1 water1 or ground within the neighborhood4 ,and now the aftermath of the explosion is turning into a display growing rapidly and yet in a measured manner so that not only the eye of the observer but his mind and his feelings can follow the events4 ,the mushroom which has been formed by the first updraft develops into a column with more and more agitated boiling masses added on the top and with slanting skirts of a snowy appearance descending toward the sides4 ,what is this white mass that looks just like a cloud of peculiar shape and that has grown up to the high heavens 7or as the meteorologists call it3 the stratosphere7 in a few minutes before our eyes8 ,it is actually a cloud3 a collection of droplets of water too small to turn into rain but big enough to reflect the white light of the sun4 ,and it is formed in a similar way to the cumulus clouds of a thunderstorm4 ,indeed it is a beautiful example of a many-storied castle of cumulus upon cumulus4 ,but strangely enough what makes this cloud is not the heat of the bomb4 ,it is the cooling of the air masses that have been sucked in as the remnants of the fireball rush upward like a giant balloon4 ,under this balloon air is drawn upward4 ,as this air rises1 it cools and water vapor contained in it condenses into droplets3 precisely the same mechanism which gives rise to thunderheads on a hot summer day4 ,the white skirts 7which are not always present7 do not consist of any material that is falling out of the cloud4 ,on the contrary1 a moist layer of air is sucked up into the cloud from the side and the droplets which form in this layer give rise to a cloud-sheet with the appearance of a skirt4 ,in big bombs near the top a particularly smooth and white cap is seen4 ,this is again condensation1 not into droplets but into fine crystals of ice4 ,in some explosions more than one of these caps are present4 ,finally the cloud has gained its full height4 ,depending on the size of the bomb it may have grown to #bj1jjj feet1 to #ajj1jjj feet or more4 ,then the wind blowing at various levels in various directions tears the structure apart sweeping some of it to the east1 some to the west4 ,the radioactive debris in the cloud has started on its travel4 ,what this radioactivity will do1 how it can affect living beings1 how dangerous it actually is1 we shall discuss in succeeding chapters4 ,but one thing is clear and remains present in the minds of all participants in an atomic test3 ,the danger of the test is nothing compared to the catastrophe that may occur if great numbers of these weapons should be used in an unrestricted nuclear war4 ,it has been frequently asserted that our present atomic explosives can wipe out the cities and industries of the greatest countries4 ,why continue with further development and testing8 ,the answer is simple3 ,the main purpose of a war is not to destroy the enemy's civilian centers but rather to defeat his armed forces1 and for this purpose we need flexible refined weapons of all kinds and sizes4 ,we also need weapons with which to defend our own cities4 ,we need weapons with which to defend our allies and in particular we need weapons which will do their job against an aggressor and will do the least possible damage to the innocent bystander4 ,in this last respect1 in particular1 notable progress has been made4 ,we are developing clean weapons which are effective by their blast and their heat1 but which produce little radioactivity4 ,of course1 blast and heat will do damage only near the point of detonation4 ,radioactivity may be carried by the winds and escape the control of man to a considerable extent4 ,it is clear that war is and always has been terrible4 ,we refuse to believe that wars will always be with us but we cannot disregard the danger of war as long as the world is half free and half slave4 ,an atomic war1 limited or even unlimited1 need not be connected with more suffering than past wars4 ,however1 such a war would probably be more violent and it would be shorter4 ,the story is told that a war which turned out to be perhaps the most dreadful in the history of mankind was started with this message3 8,thou hast chosen war4 ,that will happen which will happen and what is to be we know not4 ,god alone knows40 ,perhaps the only possible path for a free people is to be well prepared for war but never to choose war as long as the choice is free4 ,but what will happen ,god alone knows4 ,,chapter ,x ,the ,radioactive ,cloud ,in ,february #aied preparations were made on ,bikini ,atoll for the explosion of a hydrogen bomb4 ,march #a was the 8ready0 date4 ,it did not seem probable that the shot would actually be fired on that date because the shot could be fired only under quite favorable wind conditions4 ,large amounts of radioactivity1 especially fission products1 were expected from the explosion4 ,the shot could be fired only if no inhabited places lay in the downwind direction4 ,bikini is an oval-shaped coral reef1 an atoll4 ,it is one of several such atolls belonging to the group called the ,marshall ,islands4 ,if you look at the map1 you will see that west of ,bikini at a distance of #bjj miles lies ,eniwetok1 on which our people were making preparations for further tests4 ,to the east of ,bikini1 a hundred miles or so1 is ,rongelap ,atoll4 ,at that time #fd people were living there4 ,they lived primitively in palm houses on the southern part of the atoll4 ,the northern part was uninhabited4 ,on nearby ,ailinginae ,atoll #ah of the ,marshallese islanders were on a fishing expedition1 while farther to the east on ,rongerik #bh ,american servicemen were stationed4 ,the servicemen lived and worked in aluminum huts4 ,their main job was to collect weather data4 ,illustration3 ,map of the ,marshall ,islands ,much farther to the east1 #cjj miles from ,bikini1 is ,utirik4 ,one hundred and fifty-seven ,marshallese people lived on this atoll4 ,early on the morning of ,march #a1 a ,japanese fishing boat lay somewhere to the north of ,rongelap4 ,her name was ,fukuryu ,maru1 which means in ,english the ,fortunate ,dragon4 ,there were #bc men on board4 ,actually she was in a patrolled zone but had not been sighted by the patrol aircraft4 ,operations for the test were being directed from ships of ,joint ,task ,force #g4 ,for several days prior to the morning of ,march #a1 the weathermen had been mapping the winds4 ,a wind to the west would be bad for ,eniwetok4 ,a wind to the east might hurt ,rongelap and ,rongerik4 ,a wind to the south could affect ,kwajalein4 ,the ideal direction would have been due north1 but this probably would not happen for months4 ,on 8shot0 morning the wind was blowing to the northeast4 ,the meteorologists gave their 8,o4,k40 ,it was at dawn1 the first of ,march1 #aied4 ,the firing crew of nine people led by a man of considerable experience1 ,jack ,clark1 were responsible for the final arrangements4 ,they were in a blockhouse on the south side of the atoll #bj miles from the bomb4 ,others1 more than #ajjj people1 watched from shipboard under the direction of ,al ,graves1 who was responsible for the technical phases of the operation4 ,the ships lay south and a little east of ,bikini4 ,the firing mechanism was set into operation in the blockhouse4 ,one after another signals indicated that the various experiments and observations were set to work4 ,finally a red light went off and a green light appeared on the panel4 ,this meant that the bomb had been detonated4 ,the men on shipboard watched the enormous fireball through darkened glasses4 ,the firing crew1 sealed off in the blockhouse1 saw nothing4 ,a couple of long seconds and ,graves' voice announced over their radio3 8,it was a good shot40 ,a quick estimate indicated #ae megatons4 ,some more slow seconds and the expected ground shock arrived4 ,it was like a big earthquake4 ,a bad moment passed4 ,the blockhouse rocked but held4 ,another minute or so and the air shock passed over4 ,one could hear the hinges groan--but this was no longer frightening4 ,would the water wave pour over the blockhouse8 ,everything was watertight4 ,after fifteen minutes a porthole was opened--no water came in4 ,the men in the blockhouse emerged to look at the drifting atomic cloud4 ,while they watched1 ,jack ,clark's radiation instrument began to show a reading4 ,the firing crew was called back into the blockhouse4 ,there1 in the lowest corner shielded by a considerable amount of sand1 they were safe4 ,outside1 the evaporated and condensing coral came down in pellets carrying more and more radioactivity4 ,in the meantime there was fallout on the ships too4 ,the wind had definitely veered after shot time4 ,quickly the activity was washed down4 ,no one got a dangerous exposure4 ,but it was wiser to sail away4 ,a message was sent to the blockhouse3 8,we will come back for you in the evening40 ,after a little more than an hour the activity around the blockhouse started slowly to decrease4 ,the firing crew waited patiently inside without communication1 without light for the rest of the day4 ,finally the ships came back4 ,at sundown a helicopter went out to the island using the last of daylight and allowing as much time as possible for the activity to decay4 ,clark and his friends rushed out of the blockhouse wrapped in sheets to stop the beta rays and keep off the radioactive dust4 ,they moved as fast as possible to avoid unnecessary exposure4 ,it was a hard experience but they got no more than two roentgens--no more reason to worry than if they had had a medical ,x-ray4 ,toward the east1 however1 some people were in real trouble4 ,six or seven hours after the shot the ,american servicemen on ,rongerik noticed a mistlike fallout of highly radioactive dust4 ,the wind had veered enough to carry the atomic cloud over the occupied islands of ,ailinginae1 ,rongelap1 and ,rongerik4 ,in the anxious hours which followed no one could say how much damage had been done4 ,the ,americans on ,rongerik had had some education in the dangers of radioactivity4 ,they washed themselves1 put on extra clothes1 and remained inside of the aluminum huts as much as possible4 ,these actions helped to protect them against beta ray burns on the skin4 ,the ,marshallese on ,rongelap and ,ailinginae knew nothing of the danger and took no precautions4 ,many of them suffered quite severe skin burns4 ,all of the exposed persons were evacuated to ,kwajalein as soon as the ,task ,force facilities would permit4 ,but it was not until a week or so after the explosion that arrangements could be made for men with radiation measuring instruments to tour the atolls and determine what the levels of exposure had been4 ,on the southern tip of ,rongerik they measured the activity and calculated that the ,american servicemen had received approximately #gh roentgens4 ,this was good news because a dosage of #ej to #ajj roentgens is not lethal and only in rare cases leads to any sickness4 ,in any event full recovery could be expected within a few days4 ,as they prowled around ,rongerik atoll1 the measuring crew found places where the radiation levels had been much higher4 ,at the northern end a person would have received more than #bjj roentgens4 ,on ,ailinginae the measured values were comparable to those on ,rongerik4 ,the estimated dosage to the ,ailinginae people was #fi roentgens4 ,on ,rongelap the situation was much worse4 ,measurements in the southern part of the atoll showed that the ,rongelap people had gotten a dose of about #age roentgens4 ,such a dose would not be fatal1 but at least some of the people would probably be sick4 ,the crew then went on to explore the rest of the atoll4 ,as they moved north1 the dose levels rose higher and higher4 ,in the middle of the atoll1 only ten or fifteen miles from the inhabited part1 a person would have received #djj roentgens of radiation4 ,at this level he would have a fifty-fifty chance of surviving4 ,on the northern tip of the atoll1 about thirty miles away1 the dose would have been over a thousand roentgens4 ,such a dose means certain death in less than a month4 ,the following table contains a summary of what happened3 ,number ,time of ,time of ,dose of fallout evacuation 7roentgens7 persons after after shot shot 7hours7 7hours7 ,rongelap #fd #d to #f #ea #age ,ailinginae #ah #d to #f #eh #fi ,rongerik #bh #g #cb #gh ,utirik #aeg #bb #fe #ad ,fortunate ,dragon #bc #d #bjj ,on ,kwajalein the ,marshallese were cared for and underwent medical observation4 ,as soon as possible their skin and hair were scrubbed with soap and water4 ,the coconut oil in their hair made decontamination difficult4 ,during all this time the presence of the ,japanese fishing boat in the area was not even suspected4 ,not until two weeks after the explosion1 when the little boat returned to ,yaizu harbor1 did the world find out4 ,by this time the #bc fishermen were pretty sick4 ,we do not know precisely what dose the fishermen received1 but the best guess is about #bjj roentgens4 ,unhappily1 one of the fishermen died1 presumably from complications associated with the exposure to radiation4#aj ,the other #bb1 however1 are in good health and back at work4 ,our medical information on the ,marshallese islanders is complete4 ,after staying three months on ,kwajalein they were removed to ,majuro atoll1 where homes were built for them and where they have been cared for and under continuous surveillance since the incident4 ,frequent and thorough medical examinations have been conducted1 handicapped somewhat by the problem of communicating through an interpreter4 ,in the first twenty-four hours some of the victims complained of nausea1 fever1 and stomach-ache4 ,but these symptoms abated promptly in every case without treatment4 ,there was also some complaint of skin itching and a burning sensation1 but these symptoms also lasted only a couple of days4 ,then followed a week or so of comfort and no complaint4 ,after that skin lesions and loss of hair began to occur4 ,fifty to eighty per cent of the beta rays during the exposure period had an average energy of #j4c million electron-volts4 ,much of this energy was stopped in the outer layer of skin1 which is two thousandths of an inch thick4 ,the remainder of the beta rays had an average energy of #j4f million electron-volts2 these beta rays could easily penetrate into the deeper layer of live skin4 ,the most important fact1 however1 was that clothing of any kind1 even a thin cotton fabric1 provided protection against all the beta rays4 ,lesions developed only on the exposed parts of the body and in a few other places such as the armpits and the creases of the neck where material tends to accumulate4 ,bare feet were especially bad4 ,during the acute period some of the people walked on their heels4 ,at the end of six months lost hair had grown out again unaltered in texture and color1 and the skin lesions had healed4 ,everyone appeared healthy and normal with no apparent after effects4 ,there had been four pregnancies amongst ,rongelap women at the time of the exposure4 ,one baby was born dead1 but the other three were quite normal4 ,there was no evidence that the stillbirth had been due to radiation effects4 ,in fact the percentage of stillbirths amongst the ,rongelapese is normally high4 ,statistically1 one in four is not an unusual ratio4 ,today1 more than three years since the accident1 all of the ,marshallese and ,american victims seem to be fully recovered4 ,no malignancies or leukemias have shown up1 but these long-term effects are still being carefully watched for by an ,,aec medical group4 ,all in all some serious but limited harm has been done4 ,it was a close shave4 ,to see how close1 one only needs to glance at the map below1 which shows the roentgen dosage for #dh hours of exposure4 ,at the southern tip of ,rongelap1 where the inhabitants lived1 the dosage was #age roentgens4 ,but at the northern tip1 less than thirty miles away1 the dosage was more than a thousand roentgens4 ,if the wind had veered just a little bit farther to the south1 probably all of the people on ,ailinginae1 ,rongelap1 and ,rongerik would have been killed4 ,illustration3 ,dosage in ,first #dh ,hours ,after ,fallout ,began ,this shot proved what had been argued for many years3 that radioactivity is not just an incidental part of an atomic explosion4 ,the people on ,rongelap were far outside the area of danger from blast and thermal effects4 ,but they received a sizeable dose of radiation4 ,in fact1 a person could have stood unprotected at a distance of thirty miles from the explosion and been perfectly safe from the blast and thermal radiation4 ,but at that same distance in a downwind direction he would have accumulated a lethal dose of radiation within a matter of minutes after the fallout began4 ,because of the radioactive fallout1 the test sites must be located in remote parts of the world4 ,it would be desirable if sites could be found which are so remote from populous areas that the tests could be conducted without regard to the direction of the winds4 ,unfortunately the bombs are too big and the planet is too small4 ,as a result the winds must be watched before every test2 and the tests must be delayed until the winds are favorable4 ,what happened to the ,marshallese was an accident which might have been avoided if the winds had been blowing more directly toward the north at shot time4 ,since this accident the wind requirements for the tests have become far more stringent1 our knowledge of the danger has increased1 and the rules of safety have in all respects improved4 ,many large yield weapons have been tested since ,march #a1 #aied1 but no other accidents have occurred4 ,we can be confident that accidents of this kind are now very improbable4 ,at the ,u4 ,s4 test site in ,nevada there has been no instance of a major fallout on a populated area4 ,probably the most worrisome situation which has occurred there was in the spring of #aiec during the ,upshot-,knothole test series4 ,after the ninth shot of the series the cloud drifted eastward over ,st4 ,george1 ,utah1 a town of about #ejjj people4 ,some fallout occurred shortly before nine o'clock in the morning4 ,about nine-thirty ,,aec officials issued a warning advising the residents to stay indoors4 ,by noon the warning was withdrawn and people were allowed to continue with their normal affairs4 ,the incident left everyone a little bit scared1 but no one had received a radiation dose greater than two or three roentgens4 ,we have been talking about the local fallout which occurs within a few hundred miles of the test site4 ,not all the radioactivity which is made in the explosion goes into this fallout4 ,some of it travels for really long distances1 not hundreds but actually thousands of miles from ground zero4 ,this part of the radioactivity is disseminated world-wide and completely escapes the control of man4 ,to be sure1 by the time this radioactivity is distributed over a large fraction of the earth's surface1 the dosage levels of radiation are very tiny1 less than a ten thousandth of a roentgen for a megaton explosion4 ,there is no danger whatever that a person would die or even become mildly sick from this amount of radiation4 ,there is1 however1 the possibility of long-range effects such as bone cancer1 leukemia1 and genetic mutation4 ,the world-wide danger is1 of course1 primarily due to the big bombs4 ,the little ones1 such as are tested in ,nevada1 release about ten kilotons 7,,tnt equivalent7 of fission energy4 ,some of the big ones in the ,pacific release a few megatons of fission energy4 ,since the amount of radioactivity is proportional to the fission energy released1 one big bomb is equivalent to several hundred or possibly a thousand little ones4 ,altogether in ,nevada1 to date1 there have been only sixty or seventy shots4 ,it may be desirable to minimize the world-wide fallout from the big shots in the ,pacific4 ,but for the little shots in ,nevada1 it is probably more important to minimize the local fallout4 ,how much radioactivity goes into the local fallout1 how much into the world-wide1 and how these relative amounts can be controlled1 are the main topics for the remainder of this chapter4 ,not all the radioactivity which is made in the explosion contributes to the fallout1 either local or world-wide4 ,some of the radioactive fission fragments 7gamma emitters7 have such short half-lives#aa that they actually disintegrate before the bomb has disassembled4 ,a great many others disintegrate in the first few minutes while the atomic cloud is rising4 ,the energetic beta and gamma rays released in these early1 rapid disintegrations are stopped in short distances and merely add to the havoc at the scene of the explosion4 ,illustration3 ,,usaec--,joint ,office of ,test ,information #a4 ,a shallow underground explosion4 ,the radioactivity and the ground dirt are thoroughly mixed4 ,illustration3 ,,usaec--,lookout ,mountain ,laboratory1 ,,usaf #b4 ,an atomic test tower--five hundred feet high4 ,illustration3 ,,usaec #c4 ,a tower shot4 ,ground dirt rises along the stem1 but very little actually mixes with the fireball4 ,illustration3 ,elton ,p4 ,lord--,,usaec #d4 ,an air shot--#c1ejj feet above ground4 ,no dirt4 ,illustration3 #e4 ,leg bone of a three-month-old rabbit killed ten minutes after injection of ,sr4 ,the darkened areas show where the strontium has been deposited4 ,sr and normal ,sr would be deposited in the same places4 ,it is an important fact that the deposition is fairly uniform in the calcified portions of the bone4 ,from a chapter by ,vaughan1 ,tutt1 and ,kidman in the book ,biological ,hazards of ,atomic ,energy1 edited by ,haddow1 published by ,oxford ,university ,press1 #aieb ,illustration3 #f4 ,leg bone of a woman who died of radium poisoning4 ,the bright regions show where the radium has been deposited4 ,hot spots are clearly visible4 ,from an article1 8,the ,late ,effects of ,internally ,deposited ,radioactive ,materials in ,man10 by ,aub et al41 in ,medicine--a professional journal1 ,vol4 #ca1 ,no4 #c1 ,september1 #aieb ,illustration3 ,,usaec--,knolls ,atomic ,power ,laboratory #g4 ,capsules of cobalt1 shielded in a water tank4 ,one hundred and thirty million dollars' worth of radium1 twice the world's present supply1 would be needed to equal the rays from this powerful gamma source4 ,illustration3 ,,usaec #h4 ,cobalt irradiation4 ,illustration3 #a4 ,the metallic element cobalt is machined into wafers slightly larger than a dime4 ,illustration3 #b4 ,the wafers are placed edge to edge in aluminum containers1 then inserted into an atomic furnace1 or reactor4 ,illustration3 #c4 ,under bombardment of neutrons1 the nuclei of the cobalt atoms become excited and emit radiation1 or rays4 ,illustration3 #d4 ,after 8cooking0 in the reactor a certain time1 the cobalt is removed and placed in shielded containers for shipment4 ,illustration3 #e4 ,the now radioactive cobalt goes from the ,savannah ,river ,plant to ,oak ,ridge for re-shipment to medical centers all over the country4 ,illustration3 #f4 ,at medical centers1 it is placed in tele-therapy machines4 ,its powerful rays aid medical specialists in the fight against cancer4 ,illustration3 ,,nto--,lookout ,mountain ,laboratory ,photo #i4 ,the smoke-ring cloud from the air-defense atomic weapon4 ,illustration3 ,wide ,world ,photo #aj4 ,illustration3 ,university of ,california ,radiation ,laboratory #aa4 ,the streaks are condensation trails produced by charged particles in a ,wilson ,cloud ,chamber4 ,they appear bright because the chamber is illuminated and the condensation trails reflect light just as an ordinary cloud does4 ,illustration3 ,university of ,california ,radiation ,laboratory #ab4 ,another picture in the ,wilson ,cloud ,chamber4 ,a large number of closely-spaced tracks form a cloud4 7,the tracks are curved because of the presence of a magnetic field47 ,illustration3 ,,usaec--,argonne ,national ,laboratory #ac4 ,cutaway section of a nuclear reactor4 ,the heart of the reactor is a small region at the center where the fission energy is generated4 ,most of the weight and volume are needed for cooling apparatus and shielding material to keep in nuclear radiation4 ,for the radioactivity to affect areas at a large distance from the point of the explosion1 considerable time must elapse while the atomic cloud rises and drifts in the horizontal winds4 ,during this time more disintegrations occur1 due mainly to the short-lived nuclei4 ,the rate at which they occur keeps diminishing as the short-lived nuclei disappear4 ,roughly speaking1 the rate diminishes simply in proportion to the time4 ,more precisely1 the rate drops somewhat faster1 decreasing by a factor of ten when the time increases by a factor of seven4 ,a minute after the explosion the activity is less than one per cent of what it is at a second4 ,after an hour it is less than one per cent of its value at a minute4 ,this law for the decrease in activity of fission products is1 of course1 quite different from the simple law of radioactive decay4 ,the latter law applies to a single radioactive species4 ,the fission products consist at any instant of many different radioactive species4 ,each one obeys the simple law of radioactive decay1 but the totality follows a different law4 ,it should be kept in mind that the product nucleus of a radioactive disintegration may itself be radioactive with a different half-life4 ,for example1 there is strontium4 ,only a small amount of this isotope is made directly in the fission process4 ,the fission process yields large quantities of krypton1 which decays with a half-life of one-half minute into rubidium4 ,the latter has a half-life of three minutes and decays into strontium4 ,this is how practically all of the strontium is made in the explosion4 ,thus both the intensity and the nature of the radioactivity keep changing with time4 ,these facts are important because they determine the magnitude and the character of the danger when the radioactivity finally falls out of the cloud and is deposited on the surface of the earth4 ,those radioactive particles which disintegrate while still in the cloud need not worry us since this radiation can have no effect on living organisms that may be underneath4 ,provided that the cloud is more than a few hundred feet above the ground1 the beta and gamma rays released in these disintegrations merely dissipate their energy in ionizing the air4 ,the time which the radioactive debris spends in the cloud depends most critically on one factor3 the proximity of the explosion to the ground surface4 ,the nature of the surface1 whether it is soil or water1 also plays a role4 ,if the explosion has taken place right on the ground1 on a soil surface1 a lot of big1 heavy dirt particles become incorporated into the fireball and begin to fall under the action of gravity even before the cloud stops rising4 ,this fallout continues for a period of several hours to perhaps a half day4 ,at the same time some of the radioactive fission products which have adhered to these dirt particles also fall out4 ,this is the origin of the so-called close-in or local fallout1 which extends for a distance downwind of the explosion of a few miles to a few hundred miles1 according to the energy of the bomb and the strength of the winds4 ,approximately eighty per cent or so of all the fission products are accounted for by this close-in fallout in the case of a surface explosion4 ,the shot on ,march #a1 #aied was of this variety4 ,there are several possibilities for influencing the amount of close-in fallout4 ,one is to explode the bomb over deep water4 ,in this case the close-in fallout amounts to between thirty and fifty per cent4 ,this is because many of the water drops to which radioactive particles have adhered evaporate before they hit the ground4 ,over shallow water1 however1 if the fireball actually touches the bottom1 the close-in fallout resembles the case of a land explosion and is again about eighty per cent or so4 ,the close-in fallout for underground or underwater explosions will be even higher than for the surface explosions4 ,in fact a really deep underground or underwater explosion would be completely contained and no activity would be spread around4 ,another possibility for reducing the close-in fallout is to detonate the bomb on a tower so tall that the fireball cannot touch the surface4 ,in this case the amount of close-in fallout is reduced from eighty per cent to approximately five per cent4 ,of course1 it is not feasible to build towers for really big bombs whose fireballs may be a mile or so in diameter4 ,in this case the bomb might be dropped from an airplane to produce the same effect4 ,the ,hiroshima explosion was an example of an air burst of a small bomb4 ,the close-in fallout in that case was very small4 ,such radiation sickness as occurred there was due to the direct gamma rays and neutrons released in the explosion itself4 ,in the case of a near-surface explosion1 where the fireball almost touches the ground1 the close-in fallout is also only about five per cent4 ,this is a somewhat surprising fact since in this case photographs show large quantities of surface material being sucked up into the cloud1 just as they are in a true surface explosion4 ,this material certainly consists of large1 heavy dirt particles which subsequently fall out of the cloud4 ,yet most of them somehow fail to come in contact with the radioactive fission products4 ,this peculiar phenomenon can be understood by looking at the details of how the fireball rises4 ,at first the central part of the fireball is much hotter than the outer part and thus rises more rapidly4 ,as it rises1 however1 it cools and falls back around the outer part1 creating in this way a doughnut-shaped structure4 ,the whole process is analogous to the formation of an ordinary smoke ring4 ,in most of the photographs one sees1 the doughnut is obscured by the cloud of water that forms1 but sometimes when the weather is particularly dry1 it becomes perfectly visible4 ,during the rather orderly circulation of air through the hole1 the bomb debris and the dirt that has been sucked up remain separated4 7,see pictures #a-d47 ,the close-in fallout accounts for only a portion of the radioactivity1 ranging from less than a per cent for a high altitude shot to almost complete deposition for some ground shots4 ,for the world-wide fallout we are interested in what happens to the remainder4 ,this depends on how the atomic cloud is carried by the upper winds for long distances4 ,in this connection it is important to distinguish between a big bomb and a little bomb4 ,it is also important to distinguish between the lower and higher portions of the atmosphere called1 respectively1 the troposphere and the stratosphere4 ,the atmosphere is heated by the sun in an indirect way4 ,the sun's rays pass through air without warming it4 ,they heat up instead the bottom of the atmosphere1 that is1 the solid ground4 ,the atmosphere is heated in the same manner in which a boiling pot is heated on the kitchen range4 ,the heat is delivered from below and is carried in rising currents to the top4 ,only in the case of the atmosphere there is no sharp upper limit4 ,the currents rise to an altitude of thirty to fifty thousand feet1 then turn and descend4 ,this boiling part of the atmosphere is called the troposphere or region of heat4 ,above it there is less vertical motion4 ,the upper region is called the stratosphere or stratified region4 ,for a little bomb the atomic cloud stops rising before it reaches the stratosphere4 ,for a big bomb1 above about a megaton of energy 7a million tons of ,,tnt equivalent71 the cloud pokes right into the stratosphere and keeps going to a height of a hundred thousand feet or so4 ,the most important fact about the stratosphere is this3 ,it has very little weather4 ,most of the weather phenomena such as clouds1 rain1 snow1 fog1 mist1 etc41 are confined to the lower portion of the atmosphere1 the troposphere4 ,the stratosphere1 however1 contains practically no water4 ,now suppose a little bomb whose cloud will remain in the troposphere has been exploded at one of the ,united ,states test sites4 ,the ,nevada test site is at a latitude of #cg,n and the ,pacific test site at #ab,n4 ,in these middle latitudes1 in the troposphere1 the winds blow mainly from west to east with an average speed of approximately #bj miles an hour4 ,there will be a slight southerly or northerly motion on top of this4 ,but by and large the radioactive cloud will stay in a pretty narrow band around the latitude at which the explosion took place4 ,after the first few hours1 when the close-in fallout has dwindled1 the radioactive particles remaining in the cloud are too light and too fine to fall any more under the action of gravity4 ,at this point the weather becomes important4 ,rain1 fog1 or mist captures the radioactive particles1 and returns them to the ground in the rainfall4 ,this results in the so-called tropospheric fallout4#ab ,the average time for this fallout to occur is approximately two weeks to a month4 ,during this time1 while staying more or less in the latitude of the explosion1 the radioactive particles may actually have encircled the earth4 ,the clouds of the big bombs rise high into the stratosphere4 ,the winds in the stratosphere do not blow so predominantly in a latitudinal direction4 ,what is more important1 they stay in the stratosphere for years1 in which time the radioactivity is distributed to all areas of the globe4 ,the fallout from the big bombs is thus really world-wide4 ,the tropospheric fallout takes about a month4 ,the stratospheric fallout takes #e to #aj years4 ,the reason for this difference is the weather1 or rather the lack of it4 ,in the stratosphere there is no rain or fog to catch the radioactive particles and hence no effective mechanism for producing the fallout4 ,in fact1 since the radioactive particles are too fine to fall by gravity1 they must simply wait until some turbulent motions impel them downward back into the troposphere4 ,this process requires a long time4 ,that rainfall is the most important mechanism for producing the world-wide fallout has been shown by examining the fallout in certain dry regions of southern ,california and ,south ,america4 ,in every case the fallout was found to be considerably sub-normal4 ,in one place in ,chile1 where there is never any rain1 the fallout was found to be only one per cent of what might be expected on the basis of the average fallout at the same latitude4 ,in regions having at least a few inches of rain per year1 the fallout tends to be proportional to the rainfall on the average4 ,however1 the proportionality to rainfall depends on the nature of the weather so that1 say1 twenty inches of rain in one part of the world may not give as much fallout as the same amount of rain in other weather zones4 ,we are rapidly learning about this4 ,having said what the age is of the various kinds of fallout1 we are in a position to say which radioactive species are still present when the radioactivity is deposited on the ground4 ,the close-in fallout1 being only a few hours old1 still includes many short-lived isotopes1 which disintegrate before there is a possibility of ingestion or inhalation into the body4 ,consequently the danger from the close-in fallout results from external exposure1 mainly to gamma radiation on the whole body1 and to a lesser extent to energetic beta rays on the skin4 ,clothes and ordinary housing provide relatively little shielding against gamma rays4 ,special protective shelters are needed4 ,during a war if the enemy were to bomb our cities with super-megaton weapons surface-burst1 the close-in fallout would be a far greater agent of destruction against an unsheltered populace than either blast or thermal radiation4 ,in the stratospheric world-wide fallout1 however1 all of the short-lived radioactivity has disappeared1 since a period of many years has elapsed since the explosion4 ,after a year or so the only gamma emitter which is left in appreciable quantity is cesium1 with a half-life of #cj years4 ,its gamma ray1 however1 is not very penetrating4 ,in spite of this fact cesium is considered to be the second most important hazard for the long term fallout4 ,the first is strontium1 which is a beta emitter with a half-life of #bh years4 ,this is long enough so that most of these nuclei will still be present even after spending a long time in the stratosphere4 ,since strontium is chemically similar to calcium1 it contaminates our foodstuffs and is easily incorporated into our bodies4 ,once inside it stays for long periods of time1 deposited in our bones4 ,we shall see in a later chapter how serious this danger may be4 ,the tropospheric fallout1 and to a lesser extent1 the stratospheric1 includes some other radioactive species besides cesium and strontium1 and we shall discuss these in the next chapter4 ,but by and large they are of little consequence 7with the possible exception of iodine7 either because they are not easily absorbed in the body or else because their radiation is not very energetic4 ,the world-wide hazard is thus narrowed down to just two isotopes1 an internal beta emitter and a weak gamma emitter4 ,,chapter ,,xi ,from the ,soil to ,man ,there is a bewildering variety of radioactive products deposited in the fallout4 ,given certain conditions all of them could be dangerous to man4 ,actually1 very few are4 ,an example of a radioactive isotope which is produced in large quantity by the fission process and about which there is some reason to worry1 but actually is not dangerous to man1 is iodine4 ,this isotope in the fallout is not dangerous because it has a rather short half-life3 eight days4 ,during the first weeks after a nuclear explosion some radioactive iodine may fall out of the cloud and contaminate grazing land4 ,a cow eats hundreds of pounds of grass in a few days time4 ,now iodine is found in the cow's body or in the body of any mammal mainly in one spot4 ,this is the thyroid gland located in man near the ,adam's apple4 ,the thyroid gland is important because it secretes a chemical which regulates many of the body functions4 ,in man1 these include how we burn up our food and in what mood we are4 ,about twenty per cent of all the iodine which is taken up1 whether radioactive or natural1 is concentrated in this one rather small gland4 ,such a concentration is precisely the kind of danger for which we must watch4 ,shortly after nuclear tests1 cows that graze on range land have been found with abnormally large amounts of radioactive iodine1 although not so large as to be harmful4 ,in human beings1 however1 the measured levels of radioactive iodine are less than a hundredth of what they are in the cows because by the time this radioactive isotope has reached man1 it has mostly decayed into a stable1 harmless variety of xenon gas4 ,there are many potentially dangerous isotopes in the radioactive debris of a nuclear explosion4 ,but most of them decay too soon to affect man4 ,isotopes which live an extremely long time compared to the human life-span are also not dangerous to man4 ,a radioactive particle in the body is not harmful unless it disintegrates and releases its energy while the individual is still alive4 ,two examples of long-lived radioactive isotopes1 which are used as fuel in the bombs and which may be left over from the explosion in large quantities1 are3 uranium and plutonium4 ,uranium has a half-life of #gaj million years1 which is much too long to be dangerous4 ,plutonium has a half-life of #bd1jjj years and is somewhat more dangerous4 ,the danger from plutonium arises because it emits an energetic alpha ray4 ,the danger from radioactivity depends on the kind of particle emitted--alpha1 beta1 or gamma rays--and whether these rays attack the body from the inside or the outside4 ,from the outside the gamma rays are the most dangerous and the alpha rays the least dangerous4 ,from the inside the order is just reversed4 ,to cause damage from the outside the radiation must be very penetrating4 ,gamma rays can go through the whole body4 ,beta rays are stopped in the skin tissue4 ,alpha rays cannot even penetrate the outer layer of non-living1 protective skin4 ,on the inside1 however1 in the sensitive organs1 the short range of the alpha rays makes them exceedingly dangerous4 ,their energy is concentrated in a small amount of tissue to which damage is severe4 ,the beta rays cause a slightly less concentrated damage1 and the gamma rays the least concentrated of all4 ,radioactivity may enter the body as contamination in the food we eat or in the air we breathe4 ,to be dangerous1 however1 it must remain in the body1 either in the intestines or the lungs or in other vital organs1 long enough for disintegrations to occur1 which will ionize and injure the living cells4 ,fortunately1 plutonium in our food is easily excreted from the body4 ,only a few thousandths of a per cent of what is eaten1 is actually absorbed4 ,if inhaled1 large particles are stopped in the nasal passages4 ,small particles get into the lungs but are quickly exhaled4 ,only intermediate sized particles are absorbed4 ,however1 the plutonium which is absorbed generally gets laid down in the bones1 where it stays for a long period of time4 ,altogether1 plutonium in the small amounts we usually deal with is not one of the greater dangers to human beings4 ,perhaps its most disagreeable property is that1 being an alpha emitter1 it is not very easy to detect4 ,since alpha particles do not penetrate through the surface of most radiation meters1 special instruments are needed to find them4 ,two fission products which are readily absorbed upon ingestion are3 strontium 7,sr7 and cesium 7,cs74 ,depending somewhat on their chemical form1 approximately thirty-five per cent of the ,sr is absorbed1 and all of the ,cs is absorbed4 ,both of these isotopes are plentifully made in the fission process4 ,moreover they have very 8dangerous0 half-lives--about #cj years--which is long enough so that decay is negligible between the explosion and contact with man1 but short enough so that decay is probable after contact4 ,from such arguments as these one concludes that ,sr and ,cs are the most important isotopes for the internal hazard from the world-wide fallout4 ,one can be reasonably sure that there are no others of importance1 because careful and extensive research has not found significant amounts of any in our bodies4 ,we need not fear that one has been overlooked1 because the beta activity of the fission products is always easy to detect4 ,the two main questions which we have to answer are these3 ,in what precise way will the dangerous elements ,sr and ,cs be distributed in the body8 ,and after they are distributed1 what kind of damage will they produce8 ,we know too little about the chemistry of the living body to obtain a complete answer to the second question4 ,hence it has to be admitted that the actual danger cannot be stated in a precise way4 ,fortunately1 enough is known from direct experience to obtain a good value for the greatest damage that might be produced4 ,in the present chapter we shall describe what is known about the uptake of the dangerous elements into the body4 ,in following chapters we shall turn to the question of the biological consequences4 ,we may begin by comparing the danger from ,cs with that from ,sr4 ,both of these isotopes are made in the fission process in about equal numbers4 7,roughly #b or #b per cent of all the fission products are ,sr1 and #c per cent ,cs47 ,they have approximately the same radioactive half-lives4 ,but they differ in an important respect3 ,the ,cs is deposited more or less uniformly throughout the body2 the ,sr is concentrated in the bones4 ,cs emits a large part of its radioactive energy in the form of a gamma ray1 which causes ionization uniformly in the body4 ,sr1 on the other hand1 emits all of its energy in the form of two beta rays1 which have ranges of only a small fraction of an inch in the bone4 ,thus in the one case the radioactive disintegration energy is distributed in the whole body2 in the other1 the energy is deposited in the bones only4 ,since the bones comprise about ten per cent of the total body weight1 they are subjected to ten times the radiation dosage4 ,the bones are quite sensitive to radiation1 and an overdosage can cause bone cancer and interfere with the production of blood cells that goes on in the marrow4 ,thus we are led to the conclusion that ,sr is a far greater potential hazard than ,cs4 ,a further point1 which leads to the same conclusion1 is that ,cs1 after being absorbed1 is retained in the body less than six months and then excreted4 ,sr is retained for many years4 ,on the other hand1 ,cs can cause a type of damage which ,sr cannot cause3 namely1 damage to the reproductive cells4 ,the effect of ,sr is indeed limited to the bones and adjacent or nearby bone marrow1 and does not reach the reproductive organs4 ,in a later chapter we shall take up the question of genetic danger1 and then we shall be very interested in ,cs4 ,for the remainder of this chapter1 however1 we may focus our attention on ,sr4 ,since a large fraction of the ,sr which enters the body stays there1 the most important questions which remain are3 how it gets there and how much gets there4 ,the essential fact in this connection is that the ,sr generally occurs in the fallout in a chemical form which is easily dissolved in water4 ,the water is taken up by plants1 by absorption through the leaves and the roots4 ,animals graze on the plants4 ,human beings eat the plants and drink the milk from the grazing animals1 and thus become exposed to ,sr4 7,see pictures #e and #f47 ,one might worry because ,sr is not a naturally occurring isotope but has been made for the first time by man in the fission process4 ,here is an unfamiliar poison being scattered over the earth4 ,can we have any idea how much will be taken up by human beings8 ,the answer depends on a fact which we have emphasized throughout this book3 that isotopes of the same element are chemically and biologically indistinguishable4 ,the radioactive variety of strontium will behave exactly like the stable natural variety4 ,in particular1 the ratio of ,sr to stable strontium in the human body must be the same as this ratio is in our food4 ,from this premise we can predict how much ,sr will reach the human body4 ,from the total yield of fission energy released in all nuclear tests to date1 one can calculate exactly how much ,sr has been produced4 ,this amount turns out to be about #ajj pounds4 ,approximately one half of this amount has been deposited in and near the test sites in the close-in fallout4 7,most of the radioactivity comes from the big bombs1 and most of these have been burst on the ground or over shallow water47 ,a small portion of the #ajj pounds has disintegrated in the cloud4 ,the remainder1 roughly #ej pounds1 is partially still in the stratosphere and partially has been disseminated around the world in the tropospheric and stratospheric fallout4 ,at the present time measurements show that #be or #cj pounds have actually been returned to the surface of the earth4 ,local values vary from about one third to more than twice the average world-wide value4 ,in the northern part of the ,united ,states1 in the regions of frequent rainfall1 the measured values are about twice the world-wide average4 ,in the latitudes between #aj,s and #ej,n the average value is about #ej per cent greater than the world-wide average4 ,for the rest of the world one finds1 with some variations1 about one third the world-wide average4 ,most of the ,sr fallout is caught in the top two or three inches of the soil4 ,it exists there in a water-soluble form that is readily assimilated by plants4 ,also in the soil1 chemically inseparable from the ,sr1 is stable natural strontium4 ,plants1 animals1 and human beings have no way of distinguishing between the two4 ,it is not easy to determine how much natural strontium is in a form which is available to the plants4 ,some of the natural strontium is insoluble2 and some is below the root depth4 ,our best estimate is that there are about #fj pounds per acre actually available for uptake by the plants4 ,this is1 of course1 an average4 ,the amount of natural strontium in the human body is a quantity we know rather well4 ,it has been carefully measured and is about #j4g gram in the average adult1 with proportionately less in children4 ,now since we know how greatly ,sr has been diluted in the soil and how much natural strontium there is in our bodies1 we can calculate the expected quantity of ,sr in our bones4 ,considering the many uncertainties in the calculation one should not expect too good an agreement4 ,the remarkable fact is that the quantity of ,sr measured in small children does agree with the calculated amount4 ,for adults the measured value is quite a bit less than the calculated amount because adult bones have been made for the most part before there was any ,sr in the environment4 ,the fact that we can calculate how much ,sr is at present in the body is most important because it gives us confidence that we understand what is happening4 ,it is especially important for us to understand what is happening so that we can predict how nuclear tests which are carried out today will affect future levels of ,sr in the body4 ,from arguments such as we have given1 plus a record of the ,sr content of bones over the last several years1 it seems unlikely that the level of ,sr will increase by more than a factor of two or so due to tests already conducted4 ,actually this factor may be even smaller both because of the mixing of the strontium with the deeper layers of the soil1 and because the radioactive strontium which stays in the ground for a long time tends to become chemically less soluble and mixed more thoroughly with that part of the natural strontium which is chemically unavailable4 ,this latter process is called 8chemical aging40 ,to follow radioactive strontium and normal strontium from the soil into the food and the bones is not an easy matter4 ,we must worry about the question of the strontium depth in the soil and the chemical form of the strontium4 ,the complete identity of ,sr and normal strontium holds only if both are near the same place and in the same chemical form4 ,a further difficulty is that until recently little was known about the behavior of normal strontium and knowledge is accumulating slowly4 ,much more is known about calcium4 ,now calcium and strontium do not behave in an identical way1 but they do behave similarly4 ,in passing from soil to man the ratio of calcium to strontium does not remain the same but at least it changes in a more or less definite manner4 ,actually most work on ,sr uptake has been done by comparing ,sr with calcium4 ,in order to use the data on calcium one has to find out how the calcium to strontium ratio is changed when the material is taken up into the human body4 ,in the soil there is1 on the average1 about #a part of strontium to #ajj parts of calcium4 ,in the human body the ratio is about #a to #adjj4 ,thus the strontium is discriminated against relative to calcium in going from the soil to man by a factor of about #ad4 ,this is a factor of protection4 ,it is good to double-check this conclusion and to find out how the calcium to strontium ratio changes step by step in going from the soil to man4 ,one finds a factor of #a4d in going from the soil to the plant1 a factor of #g in going from the plant to the milk1 and a factor of about #b in going from the milk to man4 ,actually1 if we put all these factors together we should expect that on the way from the soil to man the calcium to strontium ratio increases by a factor #bj4 ,this is in reasonable but not in excellent agreement with the ratio #ad given above4 ,once the factor of protection is established we can get a value of the expected strontium uptake from the way in which the radioactive material is diluted by calcium rather than by normal strontium4 ,this is a less straightforward but1 for the time being1 a more practical method than the direct ,sr--normal strontium comparison4 ,it is particularly important when one compares soils of rather different calcium content4 ,plants and animals require calcium4 ,when they do not get it1 they develop a calcium-hunger4 ,since strontium is chemically similar to calcium1 a lack of calcium in the soil is readily substituted by available strontium4 ,one would expect that plants grown on calcium-poor soil and animals raised on such land would exhibit abnormally high natural strontium content and also a proportionately high ,sr content4 ,the high ,sr content has in fact been verified4 ,some sheep in ,wales1 for example1 appear to have about ten times the average amount of ,sr in their bodies4 ,fortunately most people derive their food from many areas widely separated from each other4 ,soil that is deficient in calcium is not likely to supply more than a small part of an individual's sustenance4 ,however1 the possibility of a large fluctuation cannot be ignored4 ,in this event corrective measures would be needed4 ,one simple measure would be to fertilize deficient soil with additional calcium4 ,that soil can be successfully treated in this way is illustrated by the present situation in ,wales4 ,the sheep with the abnormally high ,sr content all come from the steep1 poor pastures which are not limed4 ,the sheep from the lower pastures1 which are limed 7not because of the fallout but for economic reasons71 show an activity of only one third the value mentioned above4 ,the point we have tried to make in this chapter is that the present human levels of ,sr can be satisfactorily accounted for by simple arguments based on the chemical similarity of elements and the identity of isotopes4 ,these arguments give us confidence that we correctly understand how ,sr and how much ,sr is getting from the soil to the human body4 ,at the same time we have seen how many factors influence the eventual uptake into the human body3 geographical latitude1 frequency of rainfall1 the chemical form in which strontium is found1 the calcium content of the soil1 the method of agriculture4 ,even though the ,united ,states has pushed this investigation vigorously since #aieb the bulk of the work is still ahead of us4 ,for instance1 in the ,united ,states1 dairy products provide most of the calcium and strontium in our diets4 ,in ,japan1 however1 the situation is somewhat different4 ,there the main source of calcium and strontium is rice4 ,as a result1 the ratio of strontium to calcium may be passing differently from the soil to man4 ,also the fallout strontium might be washed deeper into the soil and the soluble to non-soluble ratio might be different4 ,considering the complex nature of the ,sr uptake into man1 it is important to keep close track of the actual ,sr levels in the soil1 in our food1 and in our own bodies4 ,the following graphs show how these levels have risen in the last several years due to the bomb tests3 ,illustration3 ,sr in the soil--measured in thousandths of a gram per square mile4 ,illustration3 ,average ,sr in ,u4,s4 milk--measured in trillionths of a gram per quart4 ,illustration3 ,average radiation doses from ,sr in bones of young children 7,u4,s47--measured in roentgens per year4 ,the actual amounts of ,sr in the soil1 in the milk1 and in the bones of young children are only approximately known4 ,but the main point that we are trying to illustrate1 is that since #aied the buildup of ,sr has gone on at a rather steady rate4 ,how far will this buildup continue8 ,more radioactivity was released in tests in the year #aied than in all other years put together4 ,probably more than one-half of that activity has already been deposited4 ,since that time the fission energy produced in ,u4,s4 tests has steadily decreased4 ,furthermore1 we have learned how to minimize the world-wide fallout by employing ground bursts which deposit most of their activity in the close-in fallout near the test site4 ,it is also possible to place chemical additives near the bomb in order to convert the strontium into a more insoluble form or else into a form which will more readily fall out in the immediate neighborhood of the explosion4 ,and what is most important--we are developing clean nuclear weapons1 which produce blast and heat but greatly reduced radioactivity4 ,in the future these clean weapons may eliminate the additional radioactivity altogether4 ,it is hard to make predictions about the plans of all nations4 ,if we find--and others also find--that clean weapons are the most desirable1 the total strontium contamination is not likely to become more than perhaps two to four times the present value4 ,we believe that all reasons--respect for human life1 military considerations and simple sanity--lead to one conclusion4 ,in the development of nuclear explosives we must endeavor to make them clean4 ,but the real reason for this does not lie in the small contamination due to tests4 ,the real reason is that war could turn contamination into a danger to countless people4 ,,chapter ,,xii ,danger to the ,individual ,how much harm is being done by the atomic tests8 ,some scientists have claimed that from past tests alone about #ej1jjj persons throughout the world will die prematurely4 ,there is no general agreement on this point4 ,some think the number should be smaller4 ,it is possible that radioactivity produces some effects which prolong life rather than shorten it4 ,but even if all the biological consequences of radiation were known many questions would still demand answers4 ,can tests be justified if they actually shorten some human lives8 ,even the possibility of a health hazard must be taken most seriously4 ,on the other hand3 ,are there any reasons which make continued testing necessary8 ,we shall return to these questions in a later chapter4 ,first1 however1 we shall try to put before the reader the known facts about the fallout danger to the individual4 ,we shall try to put this danger into perspective by relating it to other more familiar dangers to which all of us are exposed4 ,in the following chapter we shall discuss how the fallout may affect future generations4 ,the dangers from big doses of radiation are well known4 ,exposure to a thousand roentgens over our whole body causes almost certain death in less than thirty days4 ,four or five hundred roentgens give a fifty-fifty chance of survival4 ,at less than a hundred roentgens1 there is no danger of immediate death4 ,three years ago the ,marshallese got a dose of #age roentgens4 ,none died4 ,apparently all are in good health4 ,over longer periods of time even bigger radiation doses can be tolerated4 ,a thousand roentgens spread over a lifetime produce no apparent biological consequences in individual cases4 ,a rough rule 7which is not too well-established7 is that five times as much radiation can be tolerated if one is exposed to only a little radiation at any one time4 ,a hundred roentgens all at once1 or several times this amount over a protracted time period1 will not cause sickness or death that can be directly blamed on the radiation4 ,however1 such a dose of radiation may have harmful biological consequences which are more subtle4 ,an exposed individual may develop an increased susceptibility to certain diseases1 notably bone cancer and leukemia4 ,leukemia is a fatal disease in which the white blood cells multiply too rapidly4 ,a person who receives a hundred roentgens does not necessarily contract bone cancer or leukemia4 ,rather1 his chance of contracting these diseases during his lifetime may have been increased4 ,knowledge of this kind can be obtained only with the help of statistics4 ,if1 for example1 a large number of mice receive a heavy dosage of radiation over a long period of time1 one finds that the incidence of tumors and leukemia is higher amongst such irradiated animals than the natural incidence of these diseases4 ,direct evidence with human beings--fortunately--is rather scarce4 ,statistics exist on the survivors of ,hiroshima and ,nagasaki1 and also on radiologists4 ,the latter group probably receive several hundred roentgens during their professional lifetimes4 ,in addition1 some statistics exist on children who have been treated with large doses of radiation for enlarged thymuses4 ,persons suffering from ankylosing spondylitis1 which is a painful disease of the spinal joints1 have also been treated with large ,x-ray doses4 ,the statistics in all these cases lead to the same conclusion3 that large doses of radiation increase the likelihood that an individual's life will be shortened by leukemia and possibly also other cancers4 ,furthermore1 it appears 7mainly from the experiments on animals7 that the increased likelihood is simply proportional to the amount of radiation received1 at least for doses in the neighborhood of several hundred roentgens or so4 ,this of course sounds frightening4 ,but the radiation doses from the world-wide fallout are in a completely different class from those we have been discussing4 ,they are very much smaller4 ,on the average human bones are getting about #j4jjb roentgens per year from the ,sr in the fallout4 ,in addition the whole body is receiving a roughly equal amount in gamma rays1 mainly from ,cs4 ,these figures apply to new bone in young children who have grown up in an environment of ,sr in the northern part of the ,united ,states4 ,this is a region of maximum fallout4 ,adults whose bones were made for the most part before the atomic testing started are getting about #j4jjjc roentgens per year from ,sr4 ,none of these figures appears to be alarming4 ,at this present rate a lifetime dosage in northern ,u4,s4 is only a small fraction of a roentgen4 ,a rare individual might get several times this amount4 ,if tests continue at the present rate1 radiation levels could increase by as much as five-fold4 ,however1 even in this situation it is difficult to imagine anyone receiving a lifetime dose of more than five or ten roentgens from the world-wide fallout4 ,a more reasonable estimate for the average lifetime dose would be a few roentgens or less4 ,one might conclude from these figures that there is no danger whatsoever from the fallout4 ,this conclusion1 however1 may not be correct4 ,the danger from such small doses of radiation is not easy to define4 ,even the best statistical methods are insufficient4 ,one is looking for small effects which show up only after millions of cases have been studied4 ,animal experiments are extremely difficult to carry out under these conditions4 ,direct controlled experience with human beings is1 of course1 impossible4 ,as a result1 one is forced to draw conclusions from the effects at higher dose levels1 where experimental data have been obtained4 ,this may be done in many ways4 ,one way is to assume that the law of proportionality holds down to the smallest doses4 ,this means that one roentgen produces one hundredth as many cases of bone cancer and leukemia as #ajj roentgens produce4 ,this law is plausible4 ,it is by no means proven4 ,by arguing in this way one finds that for each megaton of fission energy which escapes from the test site in the world-wide fallout the lives of approximately four hundred persons would be shortened by leukemia or bone cancer4 ,under present conditions of testing1 roughly one half of the fission products are deposited as close-in fallout in and near the test site4 ,per megaton of fission energy exploded1 therefore1 perhaps #bjj persons may get leukemia or bone cancer4 ,this figure could actually be higher1 possibly even a thousand persons or more per megaton4 ,it could also be lower4 ,it could be zero4 ,it is possible that radiation of less than a certain intensity does not cause bone cancer or leukemia at all4 ,in the past small doses of radiation have often been regarded as beneficial4 ,this was not supported by any scientific evidence4 ,today many well-informed people believe that radiation is harmful even in the smallest amounts4 ,this statement has been repeated in an authoritative manner4 ,actually there can be little doubt that radiation hurts the individual cell4 ,but a living being is a most complex thing4 ,damage to a small fraction of the cells might be beneficial to the whole organism4 ,some experiments on mice seem to show that exposure to a little radiation increases the life expectancy of the animals4 ,scientific truth is firm--when it is complete4 ,the evidence of what a little radiation will do to a complex animal like a human being is in an early and uncertain state4 ,in any event the number of additional cases of leukemia and bone cancer due to the fallout radiation is certainly too small to be noticed against the natural incidence of these disorders4 ,in the next thirty years about #f1jjj1jjj people throughout the world will die from leukemia and bone cancer4 ,from past tests1 which have involved the explosion of about fifty megatons of fission energy1 the possibility exists that another #ej #bjj1 i4e41 #aj1jjj cases may occur4 ,statistical methods are not able to find the difference between #f1jjj1jjj and #f1jaj1jjj4 ,there is no way to differentiate between the fallout-induced cases of leukemia and bone cancer1 and those which occur naturally4 ,the possible shortening of ten thousand lives may seem rather ominous4 ,but mere figures can be misleading4 ,a better way to appreciate the danger from fallout is to compare it with other more familiar dangers4 ,such a comparison can be made with the natural background of cosmic rays and radioactivity in the earth and in our own bodies4 ,we are constantly and inescapably exposed to this radiation4 ,our ancestors have been exposed to it4 ,the human race has evolved in such a radioactive environment4 ,moreover1 the biological effects from different kinds of radiation can be compared in a meaningful way in terms of roentgens4 ,therefore the danger from ,sr is not unknown in every respect4 ,in some ways it is very well-known because we and all living beings have spent our days in a similarly dangerous surrounding4 ,we live on an earth which has radioactivity in its rocks1 which carries a similar activity in its waters1 and which is exposed from all sides1 to a rain of particles which produce effects identical with the effects of radioactive materials4 ,not all radiations which have the same intensity 7the same number of roentgens7 have precisely the same effect4 ,the damage produced also depends somewhat on the spacing of the ionized and disrupted molecules4 ,the cosmic rays and the ,sr1 however1 are quite similar even in this respect4 ,the reader will recall that the spacing of the ionization depends only on the charge and the speed of the ionizing particle4 ,the ionizing particle from the ,sr is an energetic beta ray1 which has a charge of one and a speed close to that of light4 ,a large part of the background radiation which reaches our bones comes from the cosmic rays4 ,the main portion of the cosmic rays is due to the mesons4 ,the meson1 like the beta ray1 has a unit charge and a speed close to that of light4 ,the two particles may therefore be expected to produce identical biological effects4 ,the only difference between their effects is that the beta ray does not have enough energy to leave the bones1 while the meson is so energetic that it deposits its energy both in our bones and throughout our whole body4 ,thus if we compare a ,sr dose with the same dose of cosmic rays the same effect to the bones must be expected4 ,but the cosmic rays give rise to additional effects in our bodies4 ,the total background dose to the bones is about #j4ae roentgens per year for the average person living at sea level in the ,united ,states4 ,of this amount1 about #j4jce roentgens is due to cosmic rays4 ,at higher altitudes the cosmic ray dosage increases4 ,in ,denver1 at an altitude of #ejjj feet1 the cosmic rays contribute #j4je roentgens per year4 ,the above numbers should be compared with the present level of world-wide fallout radiation to the bones3 about #j4jjc roentgens per year 7from ,sr and other sources74 ,the fallout radiation is thus only a few per cent of the natural cosmic radiation4 ,it is small even when compared to the variation of cosmic ray intensity between sea level and #ejjj feet4 ,a correlation between the frequency of leukemia and bone cancer1 and the intensity of natural radiation has been looked for4 ,some statistics for the year #aidg1 before weapons testing began1 are available4 ,they show the number of cases of these diseases occurring in that year per #ajj1jjj population4 ,bone ,cancer ,leukemia ,denver #b4d #f4d ,new ,orleans #b4h #f4i ,san ,francisco #b4i #aj4c ,the extra radiation that one gets in ,denver from cosmic rays is many times greater than the fallout radiation4 ,but the table shows no increased incidence of bone cancer or leukemia4 ,on the contrary--the incidence of these diseases is actually lower in ,denver4 ,not all of the natural background radiation is due to cosmic rays4 ,part of the background comes from natural radioactive elements in the soil and in the drinking water4 ,these include uranium1 potassium1 thorium and radium4 ,radium behaves like calcium and strontium1 and gets deposited in our bones4 ,all these effects are1 to the best of our knowledge1 at least as intensive in the ,denver area as in ,san ,francisco or ,new ,orleans4 ,one possible explanation for the lower incidence of bone cancer and leukemia in ,denver is that disruptive processes like radiation are not necessarily harmful in small enough doses4 ,cell deterioration and regrowth go on all the time in living creatures4 ,a slight acceleration of these processes could conceivably be beneficial to the organism4 ,one should not forget that while radiation can cause cancer1 it has been used in massive doses to retard and sometimes even to cure cancer4 ,the reason is that some cancer cells are more strongly damaged by radiation than the normal cells4 ,in spite of the table1 however1 there may actually be an increased tendency toward bone cancer and leukemia that results from living in ,denver4 ,if so--and this is the main point--the effect is too small to be noticed compared to other effects4 ,we must remember that ,denver differs from ,new ,orleans and ,san ,francisco in many ways 7besides altitude71 and these differences may also influence the statistics4 ,a more thorough consideration of the background radiation gives further evidence that this radiation is more important than the present or expected effects of ,sr4 ,the radium deposited in our bones from drinking water has been observed to reach values as high as #j4ee roentgens per year4 ,furthermore the heavier and slower alpha particles emitted by radium cause ionization processes which occur in closer spacing and are therefore more damaging than the ionization due to ,sr4 ,to make things worse radium is deposited in our bones in little nodules 7hot spots74 ,thus the possibility of local damage is enhanced4 ,the background radiation to which we are exposed varies for some unexpected reasons4 ,it has been pointed out recently that brick may contain more natural radioactivity than wood4 ,the difference between living in a brick house and living in a wood house could give rise to ten times as much radiation as we are currently getting from fallout4 7,the additional radiation from the brick might be as much as #j4jc roentgens per year47 ,human beings are subject to radiation not only from natural sources1 but also from man-made sources4 ,one of these is wearing a wrist watch with a luminous dial4 ,another is having ,x-rays for medical purposes4 ,both of these sources give much more radiation than the fallout4 ,of all ionizing radiation to which we are exposed the ,x-rays are most important4 ,in some cases medical ,x-rays have intensities which are noticeably harmful4 ,yet this damage is practically always of little consequence compared to the advantage from correct recognition of any trouble that the ,x-ray discloses4 ,we may summarize in this way4 ,our knowledge of the effects from the fallout is deficient4 ,we cannot say exactly how many lives may be impaired or shortened4 ,on the other hand1 our knowledge is sufficient to state that the fallout effect is below the statistically observable limit4 ,it is also considerably less than the effect produced by moving from sea level to an elevated location like ,denver1 where cosmic radiation has a greater intensity4 ,it is also less than having a chest ,x-ray every year4 ,in other words1 we know enough to state positively that the danger from the world-wide fallout is less than many other radiation effects which have not worried people and do not worry them now4 ,we have compared radiation from the fallout with radiation from other sources4 ,it is also possible and helpful to compare the fallout danger with different kinds of dangers4 ,for this purpose it is convenient to express all dangers in terms of a reduced life-expectancy4 ,for example1 smoking one pack of cigarettes a day seems to cut one's life-expectancy by about #i years4 ,this is equivalent to #ae minutes per cigarette4 ,that cigarettes are this harmful is1 of course1 not known with certainty4 ,it is a 8best guess10 due to ,dr4 ,hardin ,jones1 based on an analysis of statistical data4 ,a number of ,dr4 ,jones' statistical findings are listed in the following table3#ac ,reduced ,life ,expectancy ,being #aj per cent overweight #a4e years ,smoking one pack of cigarettes a day #i years ,living in the city instead of the country #e years ,remaining unmarried #e years ,having a sedentary job instead of one involving exercise #e years ,being of the male sex #c years ,automobile accidents #a year ,one roentgen of radiation #e to #aj days ,the world-wide fallout 7lifetime dose at present level7 #a to #b days ,the reader will see that the world-wide fallout is as dangerous as being an ounce overweight or smoking one cigarette every two months4 ,illustration3 ,how people get radiation ,average dose in roentgens per year ,the objection may be raised that the fallout1 while not yet dangerous1 may become so as more nations develop and test atomic weapons4 ,on this point we can only say that the future is not easy to predict4 ,some factors1 however1 justify optimism4 ,we are learning how to regulate the fallout by exploding bombs under proper surroundings4 ,development of clean bombs will greatly reduce the radioactivity produced4 ,deep underground tests will eliminate fallout altogether4 ,the activity put into the atmosphere in #aied was considerably greater than the activity released in any other year4 ,it is highly probable that the activity produced by ,united ,states tests will continue to decline4 ,finally1 we may remark that radiation is unspecific in its effects4 ,chemicals are specific4 ,about the effects of a new ingredient in our diet1 in our medicine1 or in the air we breathe1 we know much less than we know about radiation4 ,if we should worry about our ignorance concerning our chemical surroundings as we worry about the possible effects of radiation1 we would be condemned to a conservatism that would stop all change and stifle all progress4 ,such conservatism would be more immobile than the empire of the ,pharaohs4 ,it has been claimed that it is wrong to endanger any human life4 ,is it not more realistic and in fact more in keeping with the ideals of humanitarianism to strive toward a better life for all mankind8 ,,chapter ,,xiii ,danger to the ,race ,radiation may hurt the individual4 ,it may also be harmful for our children and hurt the race4 ,we have seen that the danger from the radiation due to testing is small compared to many risks which we habitually take and almost always ignore1 which in fact we have to ignore to continue to live in this civilized world4 ,in addition we are not even quite sure that the danger to the individual is real4 ,there can be little doubt1 however1 that radiation does produce some harmful changes in our children4 ,what seems even more frightening1 is that these changes may not show up in our children but only in their children or further progeny4 ,a danger which may lie hidden for generations might seem more terrifying1 especially as it has often been repeated that all such radiation effects are harmful4 ,we transmit our properties to coming generations in a most curious and concentrated fashion4 ,from the mother and the father a child inherits a number of chromosomes1 twenty-four from each4#ad ,these are structures along which the actual carriers of the properties--the genes--are strung up4 ,we are beginning to understand something about the nature of the genes4 ,they seem to be very big spiral molecules4 ,they carry the master plan of our body and even of our character in a strange chemical code4 ,the laws of heredity are complicated because of the fact that the same property is influenced by a gene from each parent4 ,frequently these two genes dictate different behavior and then the result is a compromise1 sometimes evenhanded1 sometimes unbalanced4 ,but of the two genes only one will find its way to the child of the next generation4 ,the compromise is temporary and original properties may emerge again4 ,which one of any pair of chromosomes 7or of the two assemblies of genes7 carries on is a matter of chance4 ,in the world of the cells as in the world of atoms it is chance that determines the future--not fate4 ,of all these facts we need be particularly interested in one4 ,the units of inheritance are rather constant but not quite immutable4 ,there is a small possibility that any gene may suffer a mutation4 ,that is1 it may turn into a new chemical1 carrying a new code and new properties4 ,a gene is an extremely finely and precisely constituted object4 ,it must be so in order to carry all the racial past in so little material4 ,a mutation due to chance will spoil this order in almost every instance4 ,the great majority of mutations are detrimental4 ,many are lethal4 ,it is an incredible fact that these random mutations1 almost always harmful and never proceeding according to any plan1 should have been responsible in the very long run for all the many beautiful and perfect living creatures that nature has produced 7and this includes the human race74 ,the thread leading from single cells to cell colonies1 worms1 fishes1 vertebrates1 mammals and human beings does certainly not seem to be the work of chance4 ,much less does it seem to be the work of a gamble taking one chance of a small improvement against a thousand chances of deformity or death4 ,nevertheless it is such a terrible game of chance which has produced both the human body and in some manner also the human spirit4 ,big numbers are strange things and when each member of a huge assembly must be given individual attention then the numbers are even harder to appreciate4 ,billions of contemporary lives in billions of distinct generations have led to the incredible outcome3 the harmony of life produced by gambling4 ,radiation is surely disruptive4 ,it does cause mutations4 ,since the genes appear to be single molecules1 a single process of ionization or excitation is likely to result in a change4 ,as has been said before there is doubt whether or not cancer and leukemia can be caused by exceedingly little radiation4 ,there is little doubt1 however1 that mutations can be caused by any small amount of radiation4 ,the less radiation the less the chance4 ,but the chance will always be there4 ,a very great increase in the natural rate of mutations could indeed have terrifying effects4 ,we can be quite certain1 however1 that radiation from atomic tests will increase the chance of mutations by only a very small amount4 ,the argument is essentially the same as the one concerning the danger to the individual4 ,the tests are responsible for #j4jja or #j4jjb roentgens per year to the human reproductive cells4 ,this is equivalent to approximately #j4je roentgen per generation4 ,most of this radiation is due to gamma rays from ,cs which has been deposited on the ground or absorbed in the body4 ,the number of mutations caused by this radiation is to be compared with the number of natural mutations4 ,some of the natural mutations are caused by heat and chemicals4 ,some are due to background radiation1 to cosmic rays or to gamma and beta rays emitted by natural radioactive substances in or near our bodies4 ,our best estimate is that #aj per cent of the natural mutations are due to the background radiation4 ,over a period of one generation the background radiation dosage to the human reproductive cells is approximately five roentgens4 ,assuming a simple proportionality between dosage and the number of mutations1 it follows that fifty roentgens would be required to induce a number of mutations equal to the total number of natural mutations 7from background radiation and all other causes74 ,that is1 fifty roentgens is a 8doubling dose40 ,the atomic tests are therefore increasing the number of mutations by about #j4je #ej1 which is #j4a per cent4 ,this kind of increase in the rate of mutations would certainly not seem to be a serious reason for worry4 ,actually the number of mutations from the tests is very small even compared to geographical and altitude variations in the natural radioactivity4 ,the ,inca empire existed for many generations in the high country of ,peru4 ,the people of ,tibet have been exposed for generation after generation to the greater cosmic ray intensity which bombards them through a thinner layer of atmosphere4 ,these people have been exposed to much greater additional radiation than anything which is caused by atomic tests4 ,yet genetic differences have not been noticed in the human race or for that matter in any other living species in ,peru or ,tibet4 ,we are certainly talking here about questions which may strike hard on some individuals but which from the point of view of the community or race are not serious4 ,it has been often repeated that all mutations due to radiation are harmful4 ,there is every reason to believe that mutations due to radiation are not different in kind from other mutations4 ,should we then seriously believe that all mutations are harmful8 ,that most of them are is admitted4 ,if all of them were indeed always harmful1 we must deny the simplest facts of evolution4 ,there will be some who maintain that the human race is not capable of improvement4 ,such an argument is irrefutable4 ,it is also unreasonable4 ,what cannot be further improved is perfect1 and not many people will maintain that our species can claim perfection4 ,another and much more plausible argument has been advanced3 ,in the wild state living species do perfect themselves by means of natural selection4 ,human society by caring for the imperfect and defective individual has eliminated natural selection4 ,therefore further mutations will not improve mankind4 ,it is very hard to discuss this question for the simple reason that the argument involves the interaction of two processes extremely different in magnitude and in fact different in kind4 ,on the one hand it concerns itself with evolution which proceeds in the slow deliberate way of a glacier4 ,on the other hand it focuses attention upon the process of human civilization with its technical and social changes which has gained momentum like an avalanche4 ,the momentum is still there and it is still increasing and where we shall land we do not know4 ,to consider the motion of the glacier while being carried along by the avalanche puts things completely out of proportion4 ,long before the present rates of mutation could have any effect upon the human species we shall live in a very different world and we shall have started to influence our own behavior including those of selection1 natural or otherwise1 in ways which today we cannot foresee4 ,if we discuss the question how civilization will influence natural selection1 we shall not do it with the hope of arriving at a firm answer4 ,we shall do it rather in order to illustrate how doubtful all the arguments are which concern the interplay of two processes which cannot be measured in the same scale4 ,it is true that we can and do preserve the lives of children who1 because of inherited weaknesses1 would perish under natural conditions4 ,it is true too that we do this for reasons and for feelings concerning the individual and we do it without regard to the consequences to the race4 ,however1 under our present condition of civilization a disease which can be corrected by administering chemicals or using the surgeon's knife is no longer effectively a disease4 ,in our present condition such a life can be as valuable to society and to the race as a life which does not have these superficial shortcomings4 ,that we can and do preserve more life in this manner only emphasizes that under present conditions biological differences which used to be important no longer matter4 ,on the other hand1 in social living many properties which used to be indifferent for a wild being have become of great significance4 ,ability to communicate and to get along with our fellows is not the only one1 but is perhaps the most obvious one of such properties4 ,the struggle for existence has become more gentle1 and the chance of any individual to live on in his children is governed by new ways of behavior4 ,nevertheless the difference between the individual adapted to civilized living and the one who is not adapted is of great importance and will become of greater importance4 ,it is likely that civilization will not eliminate evolution of the race4 ,rather it will direct it into new paths4 ,but the greatest change might be expected from an entirely different direction4 ,we are going to understand in real detail the intricacies of human inheritance4 ,then we shall be faced with problems and shall find possibilities of an entirely new and different kind4 ,the interest of a person in his children is not a superficial one4 ,it is one of the most strong and lasting forces in biology1 sociology and history4 ,a clear understanding of the details of inheritance may bring about some grave difficulties because a new situation is never fitted easily into existing patterns of living4 ,in the end more understanding may bring about improvements of a kind beside which all the worthwhile things that have been so far accomplished1 might look unimportant4 ,the real importance of radioactivity for heredity does not lie in the fact that we may speed up the glacier by one inch in a millennium4 ,the real importance of nuclear radiation is rather that it is helping us to understand the strange processes of life and the curious substances which connect one generation to the next4 ,,chapter ,,xiv ,the ,cobalt ,bomb ,nuclear explosions seem horrible for many reasons4 ,they were presented to an unprepared world as a dramatic surprise--as the climax to the slaughter of the ,second ,world ,war4 ,their power of destruction is fantastic4 ,before we had adjusted our thinking to atomic bombs1 an even more potent tool of warfare--the hydrogen bomb--was invented4 ,worst of all3 ,to the fear of destruction there was added the dread of the unknown4 ,it is not surprising that discussion of nuclear weapons has not proceeded on a purely rational level4 ,to the nightmare of the atomic and hydrogen bombs has been added--not as a reality but as a further threat--the cobalt bomb4 ,the idea of such a bomb is to intensify the most terrifying aspect of nuclear explosions3 the radioactivity4 ,this radioactivity could be used to poison the enemy4 ,it could get out of hand and poison everyone4 ,cobalt is a radioactive isotope of the fairly common metal cobalt4 ,it can be easily produced by absorbing slow neutrons in the natural and stable cobalt4 ,it has a half-life of five years and it emits penetrating gamma rays4 ,these properties make it useful in cancer therapy4 ,many cancerous growths are more sensitive to radiation than healthy tissue4 ,therefore radiation can be used to reduce--sometimes even to destroy--dangerous tumors4 ,the penetrating rays of cobalt can reach the cancer even deep inside the human body4 ,the lifetime of cobalt is long enough so that this substance is easily installed in hospitals4 ,but the same properties which make cobalt useful also make it potentially dangerous4 ,a nuclear explosion produces many neutrons and these could be absorbed in ordinary cobalt4 ,the radioactivity produced in this way lives long enough to become widely distributed4 ,its ray can easily penetrate a foot of masonry and several hundred feet of air4 ,a cobalt bomb would indeed be a most unpleasant object4 7,see pictures #g and #h47 ,one widely discussed possibility is that future nuclear tests will be used to develop a cobalt bomb or other bombs for radiological warfare4 ,actually tests have little to do with the cobalt bomb4 ,once one has a powerful nuclear weapon1 such as a hydrogen bomb1 it is relatively easy to make a radiological bomb4 ,further tests are not necessarily required4 ,to the extent that any testing need be carried out1 it is only necessary to activate a moderate amount of substance to find out in what way a certain bomb would function as a tool of radiological warfare4 ,tests of this kind would add only a negligible amount of radioactivity to the atmosphere4 ,therefore1 in connection with the test program we need not worry about the cobalt bomb or any related experiment4 ,the question of the cobalt bomb or radiological warfare in general is not whether it is feasible--it is--but rather whether it serves a useful military purpose4 ,it is not impossible that situations might arise in which radiological warfare could be militarily advantageous4 ,instead of cobalt1 other materials may be placed near the nuclear bombs4 ,in this way other radioactive substances can be produced4 ,by an appropriate choice of such a substance one can get a radioactive material which1 when deposited near the point of explosion1 will contaminate the site for a time which can be adjusted to the military requirements4 ,the lifetime of the radioactive material may be long enough to give an opportunity to the people to escape from the contaminated area4 ,at the same time1 one may precipitate almost all the activity near the explosion so that distant localities would not be seriously affected4 ,it is conceivable1 therefore1 that radiological warfare could be used in a humane manner4 ,by exploding a weapon of this kind near an island one might be able to force evacuation without loss of human life4 ,no instrument1 not even a weapon1 is evil in itself4 ,everything depends on the way in which it is used4 ,public opinion has all but persuaded itself that nuclear weapons will be used not for a military objective but to terrorize and kill the greatest number of people4 ,this is technically feasible4 ,in fact1 it does not even require the atomic bomb4 ,for the last hundred years this possibility has been with us4 ,bacteriological warfare may cause widespread destruction4 ,yet no one has resorted to this horrible way of making war4 ,we do not believe that anyone will expose his enemy and ultimately himself to indiscriminate bacteriological or radiological destruction4 ,our guarantee against this danger is not that it cannot be done4 ,our guarantee is the better and saner part of human nature3 the will to survive and the feeling of common decency4 ,,chapter ,,xv ,what ,about ,future ,tests8 ,many people feel that tests should be discontinued4 ,this feeling is widespread and strong4 ,the question of tests is obviously important4 ,it may influence our security as individuals4 ,it certainly will influence our security as a nation4 ,if in a free1 democratic country the majority believes that something should be done--it will be done4 ,the sovereign power in a democracy is 8the people40 ,it is of the greatest importance that the people should be honestly and completely informed about all relevant facts4 ,in no other way can a sound decision be reached4 ,the basic and relevant facts are simple4 ,the story can be presented without unnecessary frills or undue emotion4 ,when this has been done1 the right decision will be reached by common sense rather than by exceptional cleverness4 ,unfortunately much of the discussion about continued experimentation with nuclear explosives has been carried out in a most emotional and confused manner4 ,one argument concerning tests is so fantastic that it deserves to be mentioned for that very reason3 ,it has been claimed that nuclear explosions may change the axis of the earth4 ,of course1 nuclear explosions do produce such changes4 ,only the changes are so small that they are impossible to observe and even difficult to estimate4 ,searching for effects connected with past tests that may displace the axis of the earth1 or the position of the ,north ,pole1 we could find no effect that would have caused a change of position even as great as the size of an atom4 ,one could design tests with the specific purpose to produce such a change1 but these man-made effects could not be compared even remotely with the forces of nature4 ,the motion of the ,gulf ,stream has a small effect on the ,north ,pole2 but this effect is incomparably greater than what any nuclear explosion could accomplish4 ,it is good to know that the old top on which we live does have some stability4 ,the argument about world-wide radioactive fallout is more serious4 ,it is asserted that fallout is dangerous and that we are ignorant of the extent of the danger4 ,in a narrow1 literal sense both these statements are correct4 ,but in the preceding chapters we have seen that the danger is limited4 ,we do not know precisely how great it is4 ,we do know1 however1 that the danger is considerably smaller than the danger from other radiations to which we continue to expose ourselves without worry4 ,the danger from the tests is quite small compared with the effects of ,x-rays used in medical practice4 ,the fallout produces only a fraction of the increase in cosmic ray effect to which a person subjects himself when he moves from the seashore to a place of higher altitude like ,colorado4 ,people may or may not be damaged by the fallout4 ,but it is quite certain that the damage is far below a level of which we usually take notice4 ,fallout in the vicinity of the test sites did cause damage4 ,in the past this damage was not great although in one ,pacific test it was serious4 ,precautions have been increased and we may hope that future accidents will be avoided altogether4 ,the safety record of the ,atomic ,energy ,commission compares favorably with other enterprises of similar scale4 ,it seems probable that the root of the opposition to further tests is not connected with fallout4 ,the root is deeper4 ,the real reason against further tests is connected with our desire for disarmament and for peace4 ,there can be no doubt that the desire for peace is most deep1 and this desire is felt by all thinking and honest people on our earth4 ,all of us certainly hope that the catastrophe of war can be avoided4 ,this great and universal wish for peace is the driving force behind the desire for disarmament4 ,in the minds of most people it would be an important step toward disarmament if the testing of nuclear weapons were stopped by all nations4 ,this belief is widely held1 but it is not necessarily well-founded4 ,in fact1 there are arguments on the other side which should be considered carefully4 ,it is generally believed that the ,first ,world ,war was caused by an arms race4 ,for some strange reason most people forget that the ,second ,world ,war was brought about by a situation which could be called a race in disarmament4 ,the peace-loving and powerful nations divested themselves of their military power4 ,when the ,nazi regime in ,germany adopted a program of rapid preparation for war1 the rest of the world was caught unawares4 ,at first they did not want to accept the fact of this menace4 ,when the danger was unmistakable1 it was too late to avert a most cruel war1 and almost too late to stop ,hitler short of world conquest4 ,unfortunately1 disarmament is safe only when no one wants to impose his will by force of arms upon his neighbors4 ,in the uneasy world in which we live today no reasonable person will advocate unilateral disarmament4 ,what people hope is that all sides will agree to reduce their military power and thereby contribute to a more peaceful atmosphere4 ,the elimination of tests has appeared possible and proper for two reasons4 ,one is that tests are conspicuous1 and therefore it is believed that we can check whether or not testing has actually been stopped by everyone4 ,the second reason is that nuclear explosives already represent such terrifying power that further tests appear useless and irrational4 ,these arguments are simple and almost universally accepted4 ,they are based on misconceptions4 ,a nuclear explosion is a violent event1 but in the great expanses of our globe such tests can be effectively hidden if appropriate care is taken to hide them4 ,there can be no doubt that this is possible4 ,the question is only how much it costs to hide a test and how big is the explosion that can be carried out in secret for a certain amount of expenditure4 ,if an agreement were made to discontinue the tests1 the ,united ,states would surely keep such an agreement4 ,the very social and political structure of our country excludes the possibility that many people would collaborate in breaking an international undertaking4 ,whether ,russia would or would not keep such an agreement would depend on the ingenuity of the ,russians1 on their willingness to make economic sacrifices1 and on their honesty4 ,of these three factors we can have a firm opinion about the first4 ,the ,russians are certainly ingenious enough to devise secret methods of testing4 ,as to the other questions1 whether the ,russians will want to invest the effort and whether they will be bound by their word1 we feel that each man is entitled to his own opinion4 ,according to past experience1 an agreement to stop tests may well be followed by secret and successful tests behind the iron curtain4 ,in a more general way we may ask the question3 ,is it wise to make agreements which honesty will respect1 but dishonesty can circumvent8 ,shall we put a free1 democratic government at a disadvantage compared to the absolute power of a dictatorship8 ,shall we introduce prohibition in a new form1 just to give rise to bootlegging on a much greater scale8 ,it is almost certain that in the competition between prohibition and bootlegging1 the bootlegger will win4 ,all of these arguments1 however1 would become irrelevant if it were true that further testing would not accomplish any further desirable result4 ,it has been said and often repeated that we now possess adequate nuclear explosives to wipe out the cities of any enemy4 ,what more do we need8 ,our main purpose in further experimentation with nuclear bombs is not1 of course1 to make city-busters more horrible4 ,we would prefer not to have to use our nuclear weapons at all4 ,we keep them as a counterthreat against the danger that we ourselves should be subjected to a devastating attack4 ,to understand what we are actually trying to do in the tests1 we have to take a closer look at some military problems4 ,in the ,second ,world ,war strategic bombing was used for the first time on a really massive scale4 ,it may well be and1 in fact1 it is probable that such strategic bombing will not be repeated in the future4 ,there are two military reasons for the bombing of cities4 ,one is that factories are located in cities1 and these factories support the war effort4 ,the other reason is that cities are centers of transportation through which the supplies of war materials pass4 ,by destroying these centers the flow of the war supplies can be interrupted4 ,nuclear warfare is likely to be quite different from past conflicts4 ,the great concentration of firepower which a nuclear weapon represents makes it possible to attack on enemy anywhere1 at very short notice4 ,this is true no matter what the particular target is1 whether one is trying to attack the planes1 ships1 tanks1 or troop concentrations of an enemy4 ,the great mobility of nuclear firepower makes it highly probable that the nuclear conflict will be short4 ,what the factory produces during this conflict will not affect the outcome of the fighting4 ,the only weapons on which anyone can rely are the weapons which are already stockpiled4 ,therefore1 it will be militarily useless to bomb factories4 ,the same fact of mobility also implies that no great flow of war material will need to be maintained4 ,practically all movement can be executed by light and fast methods1 by planes1 submarines1 and small battle groups4 ,under these conditions the cities will lose their importance as centers of transportation4 ,the only purpose in bombing cities will be to spread terror among the enemy4 ,this was rarely done in past wars4 ,in fact1 terror is self-defeating because it provokes retaliation from the other side4 ,we believe that the role of nuclear weapons in a future war is by no means the killing of millions of civilians4 ,it is rather to stop the armed forces of an aggressor4 ,this is not easy to do because it requires not only nuclear weapons1 but very special kinds of nuclear weapons which are hard to develop and harder to perfect4 ,but with proper experimentation and proper planning the defensive use of nuclear weapons is possible4 ,the idea of tactical nuclear weapons is not new4 ,the possibility of using nuclear explosives in small wars has been frequently discussed4 ,what kind of weapons do we need in order to fight these small wars and to defend the freedom of people wherever such defense becomes necessary8 ,it has often been suggested that in small wars1 small weapons will be used1 while big weapons are appropriate for big wars4 ,such a statement is much too simple and has no relation to reality4 ,in every case the right kind of weapon is the one which performs the job of stopping the enemy's armed forces without inflicting unnecessary loss on the innocent bystander4 ,for this purpose we need a great number of weapons which are adaptable to specific purposes1 which are easy to transport and easy to deliver1 and give rise to the kind of effect which the situation requires4 ,for instance1 a nuclear weapon may be carried by a fighter plane and used to shoot down an attacking bomber4 ,since the carrying capacity of the fighter plane is severely limited1 the weapon for this purpose must be small and light4 ,a major objective of the test program is to develop such purely defensive weapons4 ,the encounter between the fighter plane and the bomber may well take place in our own country over populated areas4 ,this possibility would fill most people with alarm lest the population underneath the explosion should be hurt4 ,fortunately1 in a recent nuclear test in ,nevada1 five well-informed and courageous ,air ,force officers demonstrated that there is complete safety to people on the ground4 ,they did this by standing directly beneath the explosion at ground zero4 ,this important test took place only a few months ago--on ,july #ai1 #aieg4 ,an ,f-#hi jet fighter plane flying at #ai1jjj feet above sea level delivered an air-to-air atomic rocket to a preassigned point in the sky4 ,the ground zero men were #ae1jjj feet immediately below4 ,they wore no helmets1 no sun-glasses1 and no protective clothing4 ,at the instant of the explosion the men looked up1 saw the fireball and felt the heat4 ,there was no discomfort1 only a gentle warmth4 ,then they waited for the shock wave to arrive--approximately ten seconds4 ,when the shock came1 it was actually just a loud noise4 ,however1 one of the men ducked his head instinctively4 7,see pictures #i and #aj47 ,the blast and the thermal pulse were over4 ,but the ,air ,force men stood their ground4 ,one question still remained3 ,would there be any fallout8 ,they checked their radiation instruments and waited while the cloud drifted slowly away4 ,there was no significant rise in the radiation level4 ,the test had been a complete success4 ,the effects of the explosion were utterly insignificant on the ground4 ,but high in the air an enemy plane could have been demolished even if the nuclear explosion had missed it by a considerable distance4 ,in order that nuclear weapons should be effective against armed invaders1 it is clear that great numbers of these weapons are needed4 ,such great numbers of weapons1 some of which must be ground-burst1 will produce a considerable amount of radioactive contamination1 and this contamination will endanger friend and foe alike4 ,in particular1 the radioactivity is likely to kill people in the very country whose liberty we are trying to defend4 ,for this reason it is most important that we should be able to use nuclear weapons which cause the least possible contamination4 ,in recent nuclear tests more and more attention has been paid to the development of such clean weapons1 and most fortunately these efforts are well on the way toward success4 ,the radioactive fallout from nuclear testing gives rise to a possible danger which is quite limited in size4 ,the danger from the fallout in a nuclear war1 however1 would be real and great4 ,if we stop testing now1 and if we should fail to develop to the fullest possible extent these clean weapons1 we should unnecessarily kill a great number of noncombatants4 ,not to develop the explosives with the smallest radioactive fallout would1 indeed1 be completely inexcusable4 ,the only alternative is that nuclear weapons should not be used at all4 ,since these weapons have been presented as purely evil instruments1 most people hope that they will never be used1 and indeed one should hope that wars1 and therefore the use of these weapons1 can be avoided4 ,but in our conflict with the powerful communistic countries which strive for world domination1 it may be too much to hope for uninterrupted peace4 ,if we abandon our light and mobile weapons1 we shall enable the ,red bloc to take over one country after another1 close to their borders1 as opportunities arise4 ,the free nations cannot maintain the massive armies throughout the world which would be required to resist such piecemeal aggression4 ,on the other hand1 the flexible power of clean nuclear explosives would put us in a position where we could resist aggression in any part of the world1 practically at a moment's notice4 ,the announced policy of our country is to maintain peace and stability in the world4 ,by being patient and prepared we are trying to arrive at a world order based on law and justice for all peoples4 ,there is no doubt that this policy is supported by the overwhelming majority of ,americans4 ,our armed forces need the greatest possible flexibility in order to give strength to this policy4 ,such flexibility we can possess only if we have in our possession the strongest1 best developed weapons which are also the cleanest1 so that they may be used for defense rather than for random destruction4 ,if we renounce nuclear weapons1 we open the door to aggression4 ,if we fail to develop clean explosives1 we expose people to disaster from radioactive fallout in any serious military conflict4 ,to our way of thinking these are weighty arguments in favor of continued experimentation and development of nuclear weapons4 ,but still another1 more general1 point of view should be considered4 ,the spectacular developments of the last centuries1 in science1 in technology1 and in our everyday life1 have been based on one important premise3 to explore fearlessly any consequences to which greater knowledge and improved skills can lead us4 ,when we talk about nuclear tests1 we have in mind not only military preparedness but also the execution of experiments which will give us more insight and more ability to control the forces of nature4 ,there are many specific political and military reasons why such experiments should not be abandoned4 ,there also exists this very general reason--the tradition of exploring the unknown4 ,we can follow this tradition1 and we can at the same time be increasingly careful that radioactivity1 carelessly dispersed1 should not interfere with human life4 ,,chapter ,,xvi ,has ,something ,happened to the ,weather8 ,the weather is no longer quite as unpredictable as it used to be4 ,yet we are hardly ever sure of it even a few hours in advance4 ,one week is about the limit of the period of any prediction4 ,where the best men lack knowledge untrammeled fantasy has a field day4 ,weather has so far remained a safe topic of conversation and of speculation4 ,nuclear explosions have1 of course1 been made responsible for the weather--for any kind of unusual weather4 ,be it rain or drought or a hard season of hurricanes--the nuclear tests are dragged in4 ,the weather bureau says3 no4 ,but then--the weather bureau has not always been correct4 ,indeed it would be a miracle if the popular talk and the popular press would not have seen some connection between atomic explosions and the wayward behavior of the seasons4 ,in one case--and to our knowledge only in one case--there has occurred a chain of events starting with a nuclear test and ending in a copious and unusual downpour4 ,in the spring of #aiee a test shot of moderate size was fired in ,nevada4 ,at the same time the last storm of the season was blowing itself out in ,california4 ,according to the usual rules of meteorology the radioactive cloud should have been carried east by the steady westerly winds which blow over the temperate zone4 ,but this time the cloud was caught up by the swirl of the dying ,california storm and some of the radioactivity was carried to the west coast4 ,hours after the explosion radioactive rain began to fall in ,california4 ,the activity was weak enough and did not give rise to any worry4 ,but a remarkable thing happened4 ,as the active cloud arrived over ,california the storm revived4 ,it developed into an abundant rain which is not usual at that place and time4 ,did we--quite unintentionally--do something about the weather8 ,the weather bureau said3 no4 ,one must certainly admit that this single case proves nothing4 ,only greatly improved methods of weather observation and weather prediction would make it possible to decide if such a chain of events consists of the strong links of cause and effect or else of a simple sequence of haphazard occurrences4 ,even though our knowledge is incomplete there is at least one simple fact which should be borne in mind4 ,all the energy in that ,nevada explosion was not quite sufficient to evaporate the water droplets in a cloud one mile broad1 one mile wide1 and one mile deep4 ,this is not a very big rain cloud4 ,such a cloud would give about one third of an inch of rain water over one square mile--not an impressive amount4 ,even the biggest hydrogen bomb would give only energy enough to evaporate a cloud ten miles by ten miles and towering to the top of the 8boiling0 portion of our air1 which we call the troposphere4 ,this would give roughly three inches of rain over a hundred square miles--a more impressive amount but vanishing in the vastness of the ,pacific ,ocean4 ,nuclear explosions are violent enough4 ,but compared to the forces of nature--compared even with the daily release of energy from not particularly stormy weather--all our bombs are puny4 ,offhand one might guess that our nuclear fireworks could not swing the scales in the massive energy changes that we see around us in the common occurrences of wind and rain4 ,but the interplay of clouds and sunshine1 of water evaporating1 freezing1 dropping and thawing--in short the vagaries of weather--are both involved and tricky4 ,small causes can give rise to big effects4 ,some processes of air masses sweeping over oceans and continents are irresistible and predictable4 ,others1 like the first upsurge of hot air from the overheated ground1 may be a question of close competition and trigger action4 ,this is what makes it so difficult to predict the weather4 ,one of the most delicate processes we must think about is the formation of water droplets4 ,when some water molecules are mixed with air molecules1 we have moist air4 ,if such air rises1 expands and cools1 the water molecules lose some of their agitated motion and have a greater tendency to stick together to form droplets4 ,but it is not easy to get them started on this joint enterprise4 ,if two or three molecules stick together1 they soon are shaken apart4 ,if1 however1 two or three dozen are collected1 this is enough to start a growth which ends in a droplet of water4 ,if moist air is cooled1 droplets will form1 provided there is a meeting place from which the growth can start4 ,if there is no such meeting place1 there are no droplets and we get no cloud4 ,if there are few meeting places1 each will collect a rather great amount of water1 we will get big drops1 and we may get rain4 ,if there is an abundance of meeting places many tiny droplets are formed which will remain suspended as a cloud4 ,the present attempts at rain-making are connected with a birth-control of droplets4 ,we have seen earlier that in each radioactive decay charged particles are emitted4 ,as these move along their paths1 they tear up more atoms and leave in their wake an assembly of charged particles4 ,these charged particles strongly attract the molecules of water4 ,they attract the molecules of air much less4 ,the reason is that in a water molecule positive and negative charges are separated to a considerable extent whereas in the nitrogen and oxygen molecules of air the charges are distributed more evenly4 ,as a result the track of each particle emitted in a radioactive decay provides many meeting places for the formation of water droplets4 ,actually1 cooled moist air has been used for many decades to make the tracks of fast charged particles visible4 ,in one of the photographs you can see a picture of such 8vapor trails40 ,it is a photograph through an apparatus called the ,wilson ,cloud ,chamber4 ,the myriads of radioactive disintegrations in the debris of a nuclear explosion can give vapor trails which coalesce into a real cloud4 ,in this way weather might be influenced4 7,see pictures #aa and #ab47 ,in spite of all this it remains highly probable that testing of nuclear explosions1 as practiced at present1 does not influence the weather4 ,radioactivity does furnish an opportunity for droplets to form4 ,but other abundant sources are also available for droplet formation4 ,dust1 smoke and many forms of air pollution will do the trick4 ,foam scattered from ocean waves evaporates and leaves a speck of salt behind4 ,this particle of salt may be carried by the winds for many miles and may eventually become the germ around which a new drop will condense4 ,the cosmic rays by which we are bombarded give rise to vapor trails similar to those produced by the radioactive decay products4 ,among the many processes of nature and the usual by-products of civilization the few atomic tests do not play an important role4 ,this statement can stand1 not as a certainty1 but as a very good guess4 ,among the many surprises that the future holds one may be closely connected with the weather4 ,in the age of the airplane we are getting more and more information about the air masses around us4 ,air travel demands this information and also furnishes it4 ,new techniques1 such as radar1 can detect the formation of a cloud and can measure the size of droplets at a great distance4 ,in fact the information received is so plentiful that one may doubt whether we can properly understand it and utilize it4 ,fortunately we no longer need to rely exclusively on our own brains4 ,human thought is a remarkable thing but it is slow4 ,the modern computing machines1 the 8electronic brains10 are simpletons as compared to the apparatus which each of us wears in his skull4 ,but the electronic computers have one advantage3 they are fast4 ,soon they will be a million times as fast as our mental processes4 ,the expression 8fast as thought0 is dated--it is a contemporary of the horse-and-buggy4 ,the electronic machines can digest weather information as fast as it is received4 ,some progress has already been made4 ,in a few years all weather predictions may be machine-made4 ,this need not mean that weather can be predicted with certainty or for a long time ahead4 ,the trigger processes which1 starting from an insignificant and unnoticed spot of turbulence1 can grow into the dimensions of a cyclone will set a limit to any art of prediction4 ,but to the extent that weather cannot be predicted it may be influenced4 ,if small causes may have big effects then even the puny means available to man may change the weather--provided we know how and where to apply the lever4 ,first we shall have to acquire a better understanding of the weather-science of meteorology4 ,then we shall have to look for the appropriate trigger mechanism4 ,this may be a cloud of dust of the right kind--or else a chemical--or perhaps a great number of radioactive particles4 ,in one way or another atomic explosions may be used as the trigger but the trigger will not be effective until and unless the rest of the machinery is understood4 ,of course atomic explosions cannot be used in really significant numbers unless we learn how to avoid those radioactive by-products which are really dangerous4 ,fortunately the use of nuclear fusion1 best known from the hydrogen bomb1 makes it possible to regulate the kind of radioactivity one obtains4 ,we may make only such kinds of activity which decay before they have a chance to get into the human body4 ,experience has proved that to talk about weather is not dangerous4 ,to do something about the weather will be more risky4 ,shall the weather become a ward of the government8 ,shall we have ,republican ,rainstorms and ,democratic ,droughts8 ,in this way we shall certainly lose the last safe topic of conversation4 ,in the narrower confines of ,europe where sovereign nation is a few hours from sovereign nation 7as the wind blows7 the situation will be much more serious4 ,but even the whole planet may prove too small for fiercely conflicting interests when more knowing fingers are placed on more sensitive triggers4 ,to govern the weather can be most useful4 ,it could give ample livelihood to all the people of the earth and to many more billions4 ,such endeavor is surely good and it would appear peaceful4 ,but in this case as in many other cases knowledge will lead to power and power will lead to disaster if it is not tempered by wisdom4 ,yet this knowledge or some similarly dangerous knowledge will come to us in our lifetimes4 ,nuclear explosions do not stand alone as a potential source of mischief4 ,,chapter ,,xvii ,safety of ,nuclear ,reactors ,at the beginning of the scientific and industrial revolution two old ambitions were found to be impossible dreams4 ,one was the transmutation of elements1 the other the machine of perpetual motion4 ,modern nuclear physicists had to retract one of these statements3 elements can be transmuted4 ,but the product is expensive1 for the time being much more expensive than gold4 ,the perpetual motion machine remains impossible in principle but the problem may be considered solved in practice4 ,it can be proved1 of course1 that a machine can do useful work only if it burns up some fuel4 ,but the price of fuel is quite often less than the cost to operate and maintain the machine4 ,nuclear fuel even today is no more expensive than conventional fuel in many parts of the ,united ,states4 ,nuclear fuel is neither heavy nor bulky and can be therefore transported easily4 ,in those parts of the world where ordinary fuel is expensive1 nuclear energy will soon become of great importance4 ,furthermore1 we shall learn to use most of the energy in uranium rather than just the part contained in its rare and valuable isotope1 ,u4 ,one only has to add a neutron to common ,u to get radioactive ,u4 ,in the course of time this decays into plutonium4 ,this element can be used like ,u3 ,it produces fission1 a great amount of energy and enough neutrons to keep the process going4 ,we shall also learn to extract energy from other nuclear fuels4 ,thorium acts like uranium1 while deuterium can give energy by building up bigger nuclei rather than breaking them into smaller pieces4 ,therefore the source of energy will be universally available and quite inexpensive4 ,this really means that we are as well off as though we had a machine of perpetual motion4 ,but1 of course1 all this does not mean that the machine will do its job free of charge4 ,even a perpetual motion machine would need servicing and maintenance4 ,unfortunately our nuclear machines need a lot of such servicing and therefore for the time being1 nuclear energy is not the cheapest4 ,the main reason why a nuclear energy source1 or a nuclear reactor is difficult and expensive to run is that the reactor after a short time of operation becomes strongly radioactive4 ,therefore it cannot be approached and it has to be handled by remote control4 ,we can hardly expect that energy will be free like air or water4 ,but when we learn how to handle inexpensively our nuclear machines1 we shall be able to obtain energy for a reasonable price at any place on the earth4 ,sooner or later conventional fuel will become scarce4 ,but nuclear energy will allow the industrial revolution to continue and to expand into every corner of the earth4 ,there can be little doubt that during the next decades nuclear reactors will greatly multiply and by the beginning of the next century they will be found everywhere4 ,it is therefore of the greatest importance that these reactors should be operated safely4 ,on the face of it1 a nuclear reactor is a sluggish instrument which can be made to run itself4 ,but the ease of operation is deceptive4 7,see picture #ac47 ,one need not fear that a nuclear reactor might explode like an atomic bomb4 ,nuclear explosives are very carefully constructed so that they can release a lot of energy in a short time4 ,nuclear reactors on the other hand are put together so as to make it possible that energy will be released only at a moderate rate4 ,some reactors if improperly handled may explode1 but the violence of the explosion cannot greatly exceed that of a similar weight of high explosive4 ,nevertheless a reactor accident could become exceedingly dangerous4 ,the reactor is charged with radioactive fission products and some other radioactive substances produced by neutron absorption4 ,any accident which will allow even a portion of these products to escape into the air will endanger people at a considerable distance in the downwind direction4 ,one reason why reactors can be dangerous is that in protracted operation of the reactor1 fission products which have longer lives accumulate4 ,it is precisely these longer-lived products which are more dangerous because they have a better chance to find their way into the human body4 ,reactors are now planned which will produce #cjj1jjj kilowatts of electricity4 ,if such a reactor operates for half a year and then explodes and releases its radioactive content into the atmosphere1 its radioactivity will be comparable to that of a hydrogen bomb4 ,in one important respect such an accident would be worse than a hydrogen explosion4 ,the nuclear explosive lifts most of its radioactive products to a high altitude and the poisonous activity gets dispersed and diluted before it descends4 ,the activity from a reactor on the other hand will remain close to the ground and might endanger the lives of the people in an area of hundreds of square miles4 ,it will contaminate an even greater territory4 ,in the extensive operation of many reactors in the ,united ,states no one has yet been killed by the radioactivity4 ,this has been due to extremely careful operation and also to good luck4 ,we must be prepared that sooner or later accidents will occur4 ,on the other hand we must try to take sufficient precautions to avoid the kind of catastrophic accident which we have mentioned above4 ,with great care such accidents can indeed be avoided4 ,in thinking of all kinds of man-made machines we find some which move fast and seem dangerous like1 for instance1 airplanes2 others which are stationary and apparently harmless1 like the bath tub4 ,yet more accidents happen in bath tubs than in air travel4 ,the most dangerous element in all operations is the human element4 ,we ourselves constitute the greatest safety hazard4 ,this is a situation no different in nuclear technology than in any other kind of technology4 ,what is new in nuclear technology is that a reactor is usually very safe but may become extremely dangerous when something unexpected happens to it4 ,also we dare not use the method of trial and error4 ,an error in the reactor business could exact a far heavier toll of lives than an error in the testing of ,h-bombs4 ,we cannot wait to learn by experience2 we must forestall accidents4 ,an especially difficult safety problem is connected with the use of reactors in small countries4 ,a serious accident could endanger the lives of people in adjacent countries4 ,thus modern technology may force cooperation across national boundaries4 ,there is only one way to avoid traffic accidents and that is care exercised by everyone1 particularly the drivers4 ,similarly reactor safety will depend on the people who operate the reactors4 ,at the same time a lot of help can be obtained by careful construction and scrutiny of each new reactor4 ,one of the first acts of the ,atomic ,energy ,commission was to establish a ,committee for ,reactor ,safeguards4 ,with the passing of years this committee had to take on more heavy responsibilities4 ,at first it had to operate under secrecy4 ,with the wider and more public use of reactors the safety considerations are becoming more available to the public4 ,the question of safe operation of a machine cannot be separated from a thorough understanding of the working of the machine4 ,we cannot attempt to give an adequate description of a reactor or of the safety rules4 ,a few general statements have to suffice4 ,a working reactor is full of neutrons4 ,in a small fraction of a second these neutrons produce fission and a new generation of neutrons comes into being4 ,in slow reactors which contain lots of light elements like hydrogen or carbon1 the neutrons move with speeds little greater than that of sound and a generation may last as long as a millisecond 7one thousandth of a second74 ,in fast reactors which contain almost exclusively heavier elements like uranium or iron1 neutrons move with a great speed which is about three per cent of the speed of light4 ,in this case one generation replaces another in less than a microsecond 7one millionth of a second74 ,fortunately not all the neutrons get reproduced so rapidly4 ,some fissions produce delayed neutrons which are emitted usually with a delay of several seconds4 ,in a steadily working reactor each generation should have the same number of neutrons as the previous one4 ,if each succeeding generation has even a slight surplus1 the reactor will become hot and may explode in a small fraction of a second4 ,the main reason why safe operation is possible is the fact that fast multiplication can occur only if each generation becomes more populous even when one does not count the delayed neutrons4 ,a slightly overactive reactor is easily governed1 but there comes a point when the dormant dragon begins to stir4 ,this happens when there are enough neutrons produced so that multiplication can occur without waiting for the delayed neutrons4 ,at that point a well behaved dragon will perform a harmless action4 ,for instance it may blow a fuse4 ,but a vicious dragon will spit radioactive fire4 ,it is not easy to predict whether the dragon will be always well behaved4 ,but with careful analysis one can make such a prediction4 ,for instance one must look into the question of whether the reactor is stable4 ,if it gets hotter1 does this make the reactor proceed even faster so that the rate of heating increases and the reactor runs away8 ,in a stable reactor excess heat should tend to stop the energy production and thus the reactor cools and returns to its normal operating temperature4 ,but too great a stability may also be dangerous4 ,heating may be overcompensated by the cooling mechanism2 after the reactor has become too cold it may then heat up too fast and overshoot again4 ,we must guard not only against a simple run-away1 but also against increasing oscillations4 ,in many reactors unusual chemical compounds are used4 ,a reactor accident may start with nothing worse than an ordinary chemical reaction between strange compounds under strange conditions4 ,but if this chemical reaction destroys the reactor sufficiently to allow some fission products to escape1 then such a chemical accident can be as bad as one of nuclear origin4 ,in the interior of the reactor materials are exposed to unusually strong radiation4 ,under this effect some materials can change their chemical properties so that what has been inert as a construction material may become dangerous during the operation of the reactor4 ,perhaps the most important single item is the arrangement of mechanical controls4 ,the reactor is adjusted by a system of sheets or rods made of a material which absorbs neutrons4 ,this arrangement must be so constructed that the control rods can be withdrawn only at a very slow rate4 ,but it must be possible to put them back quite fast4 ,any danger signal should shove the absorbers in at maximum speed4 ,the technical expression is 8scram40 ,the main point1 however1 is that all the dangers and safety devices can be studied and after careful study a nuclear accident can be avoided4 ,some reactors are now so thoroughly understood that they can be safely used for training of future nuclear engineers4 ,other reactors which are more powerful or less well studied have to be used more carefully4 ,some reactors should be1 and are being1 enclosed in gas-tight containers4 ,if an explosion occurs the fission products will be harmlessly confined inside the container4 ,of course1 one must be quite sure that the reactor is of such a type that it cannot produce an explosion great enough to burst the container and what is even more important one should be quite sure that the container is closed except when the reactor is shut down and completely safe4 ,often it may be best to build the reactor underground4 ,the safety of a reactor1 of course1 depends to a great extent on the use to which the reactor is put4 ,in general a power station is less likely to give trouble than a moving power source4 ,it is not probable that nuclear locomotives will ever be safe4 ,in nuclear ships more room is available and more room permits more safety measures4 ,but even so the safety of nuclear motors in ships will have to be considered particularly carefully because ships will have accidents in harbors4 ,between the urgent need for progress and the absolute necessity of safety it is difficult to keep a sense of balance and one can easily make the mistake of being unnecessarily cautious4 ,such unnecessary caution was probably exercised when the ,committee on ,reactor ,safeguards considered the earthquake hazard of the ,brookhaven reactor on ,long ,island4 ,a seismologist1 who is a ,jesuit ,father1 was asked to tell the committee#ae of the possibilities and probabilities of an earthquake on ,long ,island4 ,the chairman#af of the committee subjected the expert to a long and detailed questioning4 ,after half an hour the ,committee on ,reactor ,safeguards ran out of questions4 ,but the ,jesuit ,father had not given any signs of running out of answers4 ,the session being at an end the expert1 looking the chairman of the committee firmly in the eye and in a more authoritative voice than he had yet used1 said1 8,mr4 ,chairman1 ,i can assure you on the highest authority that there will be no major earthquakes on ,long ,island in the next fifty years40 ,,chapter ,,xviii ,by-products of ,nuclear ,reactors ,nuclear reactors generate energy with the help of nuclear fission4 ,every time a fission occurs we are left with radioactive by-products4 ,it is most important to prevent the uncontrolled escape of these fission products from the reactor4 ,fortunately the dangerous products can be retained in the reactor--if the machine has been constructed and operated with reasonable care4 ,in the end1 however1 the burnt or partly burnt uranium charge will have to be removed from the reactor and fresh charge1 fresh fuel will have to be added4 ,what will become of the fission products at this time8 ,during protracted operation of a reactor most of the short-lived fission products decay4 ,those with longer lives accumulate4 ,the discharge of the reactor is strongly radioactive1 and it will remain radioactive for many years4 ,one certainly must not dispose of this radioactive waste in a careless manner4 ,there are1 however1 many ways in which one can store such waste with reasonable safety4 ,one can deposit the radioactive material in well-built underground tanks4 ,one can concentrate the activity1 imprison it in concrete blocks1 and deposit it at the bottom of the ocean4 ,if one is very much worried he might even put the radioactivity in rockets and let it decay harmlessly in outer space4 ,these procedures will cost money and will add to the expense of nuclear energy4 ,it would be far better if we could find a way in which the radioactive by-products could be made to serve a useful and safe purpose4 ,some of the by-products can be used and have been used4 ,these uses are connected with some hazards4 ,furthermore1 only a small fraction of the fission products have found good employment up to the present4 ,but the importance of fission products is growing4 ,we are using them in research4 ,a radioactive isotope imitates the behavior of its non-active brother in all chemical reactions and in all the intricate processes in which matter changes its form inside a living body4 ,furthermore a radioactive substance can be detected with the greatest ease4 ,it can be found in a concentration which is less than a millionth of a safe radiation dose4 ,what the microscope has been in the exploration of the structure of organisms1 the radioactive elements may become in the understanding of the chemical functioning of living matter4 ,with better understanding there comes the possibility of using radioactive by-products for diagnosis4 ,as with the medical use of ,x-rays the possible small damage due to radiation exposure should be regarded as the price for the help we can get from early and correct recognition of diseases4 ,in the treatment of patients1 particularly in the case of persons stricken by cancer1 radioactive destruction of the diseased tissue is often preferable to the use of the surgeon's knife4 ,such radioactive treatment is new4 ,there is much room for improvement4 ,appropriate use of radioactive substances for this purpose may become a far more powerful tool and much more widespread than it is at present4 ,but all these applications will use up only a vanishing fraction of the fission products4 ,moreover1 most of the biologically important elements are not produced in the fission of uranium4 ,many useful activities can be produced by neutron absorption in reactors4 ,but among the fragments of uranium perhaps only radio-iodine has been put so far to direct physiological use4 ,industry deals with less sensitive objects than living tissue4 ,therefore greater amounts of radioactive materials can be used here4 ,and indeed radioactivity has done a great variety of jobs4 ,the penetrating power of ,x-rays has been used to control the thickness of sheets in an easy and automatic manner4 ,radioactivity has been incorporated into surfaces which are exposed to mechanical wear or corrosion1 to check the rate at which the surface is worn away by the appearance of activity in the lubricant or other fluids which have been in contact with the surface4 ,by such methods industry has accumulated savings which are rapidly approaching the billion dollar mark4 ,these savings will increase as people learn how to use the new materials4 ,but in all these cases it is important to make sure that the activity will not hurt anyone while it is used and after it has served its purpose4 ,possibly the greatest amount of radioactivity will be needed in food sterilization and preservation4 ,one may incorporate the activities into rods which will safely retain the materials but which will allow a considerable fraction of the penetrating gamma rays to escape4 ,to sterilize food means to destroy all microorganisms4 ,many of these are radiation-resistant and may have to be exposed to #ej1jjj or more roentgens--that is one hundred times as much as would kill a mammal4#ag ,such massive irradiation begins to affect the foodstuff itself4 ,in some cases sterilization by irradiation changes the food more than would be the case by boiling it or freezing it4 ,in other cases irradiation produces less undesirable side effects than any other methods4 ,another way to use radiation is the preservation of agricultural products4 ,this need not be done by the difficult procedure of sterilization4 ,it is enough to control pests and to prevent germination of the seeds which one is trying to preserve4 ,thus we need here approximately one per cent of the radiation that would be required for sterilization4 ,by so little radiation the food is not altered to a noticeable extent4 ,it is precisely in such processes1 where great amounts of materials will have to be irradiated1 that a substantial fraction of the fission products might find employment4 ,in all applications care has to be exercised lest radioactive materials should inadvertently be scattered around4 ,where great amounts are needed as in food sterilization and preservation1 caution has to be redoubled4 ,that trouble may arise has been illustrated by an occurrence in ,houston1 ,texas4 ,radioactive iridium1 which is a beta and a gamma emitter1 was being used by a certain industrial concern to take ,x-ray pictures of metal parts4 ,a shipment of this radioactive material in the form of powder pellets was being opened by remote control when compressed gas in the container exploded and scattered some radioactivity around4 ,the area was shielded but some of the radioactive dust escaped to the rest of the building4 ,the two men who were operating the remote control apparatus became contaminated4 ,they washed themselves and cleaned up the area but did not report the incident4 ,a few weeks later a standard radiation check showed that the plant was still radioactive4 ,company officials became worried and called in experts4 ,at this late stage the plant was thoroughly decontaminated4 ,the homes of the two men were also examined and were found to be slightly radioactive4 ,the men and their families were temporarily moved out while their homes were being cleaned up4 ,when they returned1 neighbors and friends shunned them4 ,the four year old son of one of the men lost his playmates4 ,people were afraid to enter the houses4 ,one of the houses was put up for sale but no one wanted to buy it4 ,the fact that the houses had been checked by radiation meters and found to be clean1 and the fact that the half-life of iridium is only #ge days so that any trace of activity would disappear in a reasonably short time1 did not dispel people's fears4 ,it is fortunate that no one was seriously hurt in this incident4 ,but there is an important lesson we can learn from it3 ,ignorance may hurt more than radioactivity4 ,that a house should lose its value in spite of the fact that its radioactive contamination has been removed1 that a little boy should be shunned as though radioactivity were infectious like the plague--these are examples of suffering caused by one of the greatest sources of human misery3 unreasoning fear4 ,the greatest potentialities of fission products for the future might lie in still a different direction4 ,radioactivity can induce mutations4 ,to what extent this is a danger we have discussed in an earlier chapter4 ,in the hands of a breeder who tries to bring about changes in animals or plants radioactivity could become exceedingly useful4 ,of course it is true that most mutations are harmful4 ,it is also true that artificial mutations have been produced for many decades4 ,but now it is possible to place simple and cheap tools in the hands of many more people4 ,therefore the chances will increase to find among the many wrong mutations the few and decisive changes which lead to improvement4 ,do we dare to place dangerous materials in so many hands8 ,we should not do so without making certain that only competent and responsible individuals will get radioactive materials4 ,this can be done4 ,druggists have dispensed poison2 doctors and biologists have bred in their laboratories the multiplying menace of germs4 ,all this was done and is being done with safety and to the great benefit of all people4 ,the use of radioactivity should be even more safe because this material is easy to detect4 ,if poisons or germs become lost1 they may be hard to find4 ,radioactive materials1 however1 give unmistakable evidence of their presence4 ,it is1 of course1 never easy to find a needle in a haystack4 ,but the chance to find it is much better if it is a radioactive needle4 ,radioactive by-products need not remain what they seem to be today3 dirt and danger to be disposed of and hidden4 ,but in the immediate future we shall incur some expense to keep radioactivity in a safe place4 ,some gaseous by-products like the long-lived krypton 7half-life3 #aj4d years7 might continue to give rise to real difficulties and to considerable expense4 ,the trouble is1 of course1 that a noble gas like krypton will not be bound to any material by strong bonds4 ,it may be inadvisable to let long-lived gases escape4 ,on the other hand1 their adsorption or their storage at low temperature or high pressure may prove to cost a considerable amount of money4 ,we have been talking about the problem of handling the by-products of nuclear power4 ,this problem will not appear in proper proportion unless we also give some thought to the by-products of the kind of power we are using at present4 ,that we do not like smoke and smog is obvious4 ,to what extent these residues of incomplete burning can cause cancer or other damage we do not know4 ,chemistry is more tricky than radiation4 ,our lack of knowledge about the slow biological effects of chemicals is much greater than our remaining uncertainties about radiation4 ,in addition to the obvious annoyance and worry caused by the products of incomplete combustion there exists an interesting question connected with the result of complete combustion4 ,the carbon that has been deposited through the geologic ages as coal and as oil is being used up gradually and converted to a colorless1 odorless1 harmless gas--carbon dioxide4 ,there is always some carbon dioxide in our atmosphere4 ,the amount is approximately #cjj parts per million of common air4 ,all the carbon that has been burned since the beginning of the industrial revolution could have increased the carbon dioxide in the atmosphere by ten per cent to the value of #ccj parts per million4 ,this increase could be significant4 ,carbon dioxide acts like a blanket or a valve for some kinds of radiation4 ,in the daytime we receive energy in the form of visible light from the sun4 ,this form of radiation has no difficulty in penetrating the carbon dioxide gas4 ,however1 the incoming radiation is balanced by invisible heat radiation1 which flows out from the earth into space day and night4 ,this infrared radiation is quite similar in nature to light1 only our eyes are not sensitive to it4 ,now the carbon dioxide gas acts like a barrier1 though only a partially effective barrier1 to this outgoing heat radiation4 ,if the carbon dioxide content of our atmosphere were to increase too greatly1 it would act like the glass in a greenhouse and our climate would grow warmer4 ,a ten per cent increase in the carbon dioxide content of the atmosphere should have produced an observable rise in temperature4 ,such a temperature rise has not1 in fact1 been observed4 ,the reason is that not all the carbon dioxide which has been generated in the processes of combustion has actually remained in the atmosphere4 ,most of it has found its way into the great reservoir of our oceans4 ,some of it is deposited as lime at the bottom of the oceans4 ,however1 some time is required for the carbon dioxide to be removed from the atmosphere and to reach the oceans4 ,one would expect1 therefore1 that there would have been at least a slight increase in the carbon dioxide content of the atmosphere4 ,measurements show that this is the case and that the increase is about two per cent--which is too small to have changed our climate4 ,if we continue to consume fuel at an increasing rate1 however1 it appears probable that the carbon dioxide content of the atmosphere will become high enough to raise the average temperature of the earth by a few degrees4 ,if this were to happen1 the ice caps would melt and the general level of the oceans would rise4 ,coastal cities like ,new ,york and ,seattle might be inundated4 ,thus the industrial revolution using ordinary chemical fuel could be forced to end before the advantages of civilization have spread all over the earth4 ,however1 it might still be possible to use nuclear fuel4 ,with nuclear fuel the industrial revolution and its countless benefits for man could continue to every part of the globe4 ,the by-products of the nuclear age are less bulky and therefore are more easily handled than the by-products of our coal- and oil-economy4 ,the main advantage of nuclear energy may yet turn out to be this3 ,with proper care nuclear energy may turn out to be the cleanest among the available sources of power4 ,,chapter ,,xix ,the ,nuclear ,age ,the future depends on people4 ,people are unpredictable4 ,therefore1 the future is unpredictable4 ,however1 some general conditions of mankind depend on things like the development of technology1 the control won by man over nature and the limitations of natural resources4 ,these can be predicted with a little greater confidence4 ,the future is unknown but in some respects its general outline can be guessed4 ,such guesses are important4 ,they influence our present outlook and our present actions4 ,the nuclear age has not yet started4 ,our sources of energy are not yet nuclear sources4 ,even in the military field1 where development has been most rapid1 the structure of the armed forces has not yet adjusted itself to the facts of the nuclear age in a realistic manner4 ,in politics the atomic nucleus has entered as a promise and as a menace--not as a fact on which we can build and with which we can reckon4 ,some technical predictions seem safe3 ,nuclear energy will not render our older power plants obsolete in the near future4 ,but nuclear energy will make it possible to maintain the pace--even the acceleration--of the industrial revolution4 ,it will be possible to produce all the energy we need at a moderate cost4 ,furthermore--and this is the important point--this energy will be available at any place on the globe at a cost which is fairly uniform4 ,the greater the need for power1 the sooner will it be feasible to satisfy the need with the help of nuclear reactors4 ,nuclear energy can be made available at the most outlandish places4 ,it can be used on the ,antarctic continent4 ,it can be made to work on the bottom of the ocean4 ,the expanding front of industrialization has been called the 8revolution of rising expectations40 ,that nuclear energy should be involved in the current and in the turbulence of this expanding front1 is inevitable4 ,one can say a little more about the effects of scientific and technological discoveries on the relations among the people of the globe4 ,with added discoveries raw materials will no longer be needed with the old urgency4 ,for most substances substitutes are being found4 ,this may make for greater economic independence4 ,on the other hand1 new possibilities will present themselves4 ,we shall learn how to control the air and how to cultivate the oceans4 ,this will call for cooperation and more interdependence4 ,the dangers from radioactive by-products will act in a similar direction4 ,the radioactive cloud released from a reactor accident may be more dangerous than a nuclear explosion4 ,such a cloud will not stop at national boundaries4 ,some proper form of international responsibility will have to be developed4 ,what effect the existence of nuclear weapons will have upon the coexistence of nations is a question less understood and less explored than any other affecting our future4 ,most people turn away from it with a feeling of terror4 ,it is not easy to look at the question with calm reason and with little emotion4 ,a few predictions seem disturbing but are highly probable3 ,nuclear secrets will not keep4 ,knowledge of nuclear weapons will spread among nations--at least as long as independent nations exist4 ,prohibition will not work4 ,laws or agreements which start with the word 8don't0 can be broken and will always be broken4 ,if there is hope1 it must lie in the direction of agreements which start with the word 8do40 ,the idea of 8,atoms for ,peace0 succeeded because it resulted in concrete action4 ,an all-out nuclear war between the major powers could occur but we may have good hope that it will not occur if we remain prepared to strike back4 ,no one will want to provoke the devastation of his own country4 ,atomic bombs may be used against cities4 ,but there will be no military advantage in destroying cities4 ,in a short and highly mobile war neither centers of supply and communication nor massive means of production will count4 ,if cities are bombed1 this will be done primarily for reasons of psychological warfare4 ,we must be and we are prepared for this kind of war but only as a measure of retaliation4 ,there is good reason to believe that as long as we are prepared for all-out war1 our civilian population will not suffer from a nuclear attack4 ,the certainty of a counterblow gives real protection against all-out war4 ,no such protection exists against wars limited in territory and in aims4 ,in the history of mankind such wars have been most frequent4 ,there is no indication that these limited wars have ended4 ,we must be prepared for these conflicts with effective and mobile units1 and this requires the use of nuclear firepower4 ,nuclear weapons will certainly have a profound effect upon such limited warfare4 ,not all of this effect need be and indeed it must not be in the direction of greater devastation4 ,in a nuclear war it will not make sense to use massed manpower4 ,any such concentration will provide too good a target for atomic weapons4 ,to use big1 costly and conspicuous machines of war will be unwise4 ,such machines will be defeated by nuclear explosions in the same way as the mailed knight went down before firearms4 ,any fighting unit in a nuclear war will have to be small1 mobile1 inconspicuous and capable of independent action4 ,such units whether on sea1 land or in the air cannot rely and will not rely on fixed lines of supply4 ,there will be no possibility and no need to occupy territory and to fight at fixed and definite fronts4 ,if a war should be fought for military reasons and for military advantage1 it will consist of short and sharp local engagements involving skill and advanced techniques and not involving masses that slaughter and are being slaughtered4 ,if an invader adopts extreme dispersion1 it will become impossible to defeat him with atomic weapons4 ,but a very highly dispersed army can be defeated by a determined local population4 ,therefore the main role of nuclear weapons might well be to disperse any striking force so that the resistance of people defending their homes can become decisive4 ,nuclear weapons may well become the answer to massed armies and may put back the power into the hands where we believe it belongs3 the hands of the people4 ,at this point we are brought back to the main topic of this book3 radioactivity4 ,in a limited nuclear war the radioactive fallout will probably kill many of the innocent bystanders4 ,we have seen that the testing program gives rise to a danger which is much smaller than many risks which we take in our stride without any worry4 ,in a nuclear war1 even in a limited one1 the situation will probably be quite different4 ,that noncombatants suffer in wars is not new4 ,in a nuclear war1 this suffering may well be increased further due to the radioactive poisons which kill friend and foe1 soldier and civilian alike4 ,fortunately there exists a way out4 ,our early nuclear explosives have used fission4 ,in the fission process a great array of radioactive products are formed1 some of them intensely poisonous4 ,more recently we have learned how to produce energy by fusion4 ,fusion produces fewer and very much less dangerous radioactivities4 ,actually the neutrons which are a by-product of the fusion reaction may be absorbed in almost any material and may again produce an assortment of radioactive nuclei4 ,however1 by placing only certain materials near the thermonuclear explosion1 one may obtain a weapon in which the radioactivity is harmless4 ,thus the possibility of clean nuclear explosions lies before us4 ,clean1 flexible and easily delivered weapons of all sizes would make it possible to use these bombs as we want to use them3 as tools of defense4 ,when stopping an aggressor we would not let loose great quantities of radioactive atoms which would spread death where we wanted to defend freedom4 ,clean nuclear weapons would be the same as conveniently packaged high explosives4 ,they would be nothing more4 ,the possibility of clean explosions opens up another development3 the use of nuclear explosives for the purposes of peace4 ,conventional high explosives have been used in peace fully as much as in war4 ,from mining to the building of dams there is a great variety of important jobs that dynamite has performed4 ,nuclear explosives have not been used in a similar way4 ,the reason is the danger from radioactivity4 ,once we fully master the art of clean explosions peaceful applications will follow and another step will be made in controlling the forces of nature4 ,all this is of course only a small part in the process of the increasing power of man and the increasing responsibility of man4 ,as the impossible of yesterday becomes the accomplished fact of today we have to be more and more aware of our neighbors on this shrinking planet4 ,the arts of peace may lead to conflicting interest as easily as they may lead to fruitful cooperation4 ,if we ever learn to control the climate of the world1 a nation may find itself in the same relation to another nation as two farmers who have to use the waters of the same river4 ,rivals are men who fight over the control of a river4 ,when the same word 8rivals0 comes to mean cooperation for the best common use of the river or any other resource--that will be the time of law and of peace4 ,surely this sounds like ,utopia and no one sees the way4 ,but the general direction in which we should go is not to consider atomic explosives and radioactivity as the inventions of the devil4 ,on the contrary1 we must more fully explore all the consequences and possibilities that lie in nature1 even when these possibilities seem frightening at first4 ,in the end this is the way toward a better life4 ,it may sound unusually optimistic in the atomic age1 but we believe that the human race is tough and in the long run the human race is reasonable4 ,,glossary ,activity3 ,short for radioactivity4 ,also the strength of a radioactive source measured in disintegrations per second4 ,air burst3 ,a nuclear explosion at such an altitude that the fireball does not touch the earth's surface4 ,an air burst produces very little local fallout4 ,alpha ray 7particle73 ,energetic but non-penetrating radiation emitted by heavy radioactive nuclei4 ,an alpha particle consists of two neutrons and two protons1 and is identical with the nucleus of the ordinary helium atom4 ,atom3 ,a positively charged nucleus surrounded by negatively charged electrons4 ,atomic bomb3 ,a fission bomb4 ,atomic cloud3 ,the cloud remaining after the energy of the explosion has been carried off by the shock wave and the thermal radiation4 ,it consists of condensed water vapor1 ground material1 and bomb debris including the radioactivity4 ,atomic energy3 ,energy released in nuclear reactions1 for example in fission4 ,atomic energy and nuclear energy mean the same thing1 but the latter name is more appropriate4 ,atomic reactor3 ,same as nuclear reactor4 ,background radiation3 ,natural radiation due to cosmic rays1 and due to radioactive substances in the earth1 in the atmosphere1 and in our own bodies4 ,beta ray 7particle73 ,an energetic electron or positron emitted by some radioactive nuclei4 ,practically all of the fission products are beta 7electron7 emitters4 ,blast wave3 ,same as shock wave4 ,cesium3 ,a radioactive fission product4 ,it emits a #j4e million volt beta ray and a #j4g million volt gamma ray with a half-life of #cj years4 ,the daughter nucleus is stable barium4 ,chain reaction3 ,self-maintained sequence of fissions4 ,neutrons released by the fission of one nucleus are used to induce fission in another nucleus4 ,chromosome3 ,a small irregularly shaped body found in cells4 ,chromosomes carry the genes1 which are responsible for heredity4 ,clean bomb3 ,a nuclear bomb which produces heat and blast1 but only a negligible amount of radioactivity4 ,the energy of such a bomb is derived almost entirely from the fusion process4 ,cobalt3 ,radioisotope--decays into nickel with the emission of a weak beta ray4 ,the half-life for this decay is #e4c years4 ,the nickel immediately ejects two gamma rays with a total energy of #b4e million electron-volts4 ,cobalt bomb3 ,a radiological bomb which produces a large quantity of cobalt4 ,control rod3 ,a rod of neutron-absorbing material used to control the power level of a nuclear reactor4 ,cosmic rays3 ,energetic particles from outer space4 ,they induce nuclear reactions in the earth's atmosphere and thus contribute to the background radiation4 ,this cosmic radiation is more intense at high altitudes than at sea level4 ,counter3 ,a device which detects nuclear radiation4 ,critical mass3 ,the amount of fissionable material required to sustain a steady chain reaction4 ,with less than the critical amount1 the reaction stops because too many neutrons are lost4 ,cyclotron3 ,a machine that accelerates charged particles to high energy4 ,energetic charged particles can be used to induce nuclear reactions4 ,daughter3 ,the nucleus which remains after decay of a radioisotope4 ,decay3 ,spontaneous process in which a radioactive nucleus emits an alpha1 beta1 or gamma ray4 ,delayed neutrons3 ,those released after a fraction of a second to a half-minute or so by the fission products4 ,they comprise less than one per cent of the total number of neutrons released in the fission process but are useful for the purpose of control in reactors4 ,deuterium3 ,stable hydrogen isotope4 ,its nucleus 7called a deuteron7 consists of one proton and one neutron4 ,disintegration3 ,same as decay4 ,dose3 ,a quantity of radiation--usually measured in roentgens4 ,e mc3 ,einstein's equation relating mass 7m7 and energy 7,e74 ,the speed of light 7c7 enters as a proportionality constant4 ,the equation asserts that one pound of mass is equivalent to ten megatons of energy4 ,in the fission process only one-tenth of one per cent of the mass is converted4 ,therefore1 to produce ten megatons of energy by fission #ajjj pounds of uranium would be required4 ,electromagnetic radiation3 ,includes radio waves1 visible1 infrared1 and ultraviolet waves2 also ,x-rays and gamma rays4 ,the latter two are energetic1 penetrating forms of radiation4 ,electron3 ,a particle having a unit negative charge and a weight equal to #a_/ahdj of the weight of the lightest atom 7hydrogen74 ,electron capture3 process in which an atomic electron unites with a proton in the nucleus producing a neutron and a neutrino4 ,electron-volt3 ,the amount of energy acquired by an electron which is accelerated through an electric potential of one volt4 ,typically1 the energy required to 8knock0 an electron out of an atom is a few electron-volts or so2 particles ejected from radioactive nuclei have energies between a few hundred thousand and a few million electron-volts4 ,element3 ,a collection of atoms whose nuclei all have the same charge4 ,an element may consist of many isotopes4 ,enriched material3 ,uranium which contains a greater proportion of the #bce-;isotope than is found in the natural ore4 ,excited state3 ,a state of an atom1 molecule1 or nucleus having excess energy4 ,as soon as possible this excess energy is released and the system goes to the ground state4 ,fallout3 ,radioactive particles from an atomic explosion4 ,they may be carried in the atomic cloud to large distances from ground zero and then 8rained down0 to the earth's surface4 ,fireball3 ,the luminous ball of hot air and bomb material which expands and cools as the shock wave races out4 ,fission3 ,the breaking-up of a heavy nucleus into two or more fragments4 ,a large amount of energy and some free neutrons are released in the process4 ,fissionable material3 ,isotopes which undergo fission when bombarded by slow neutrons3 uranium and plutonium4 ,fission products3 ,fission fragments and their daughters1 including hundreds of different radioactive species1 among them strontium and cesium4 ,fusion3 ,the combining of light nuclei into heavier ones with a release of energy4 ,for example1 deuteron triton alpha neutron4 ,about #ah million electron-volts are released in this process4 ,gamma ray3 ,energetic1 penetrating electro-magnetic radiation emitted by certain radioactive nuclei1 frequently after a beta emission4 ,genes3 ,parts of the chromosomes4 ,they are big molecules that determine heredity4 ,ground state3 ,the state of least energy and greatest stability of atoms1 molecules1 and nuclei4 ,ground zero3 ,the point on the surface of the earth directly above or below a nuclear explosion4 ,half-life3 ,the time required for one half of a large number of identical radioactive nuclei to disintegrate4 ,h-bomb3 ,same as hydrogen bomb4 ,heavy hydrogen3 ,same as deuterium4 ,heavy water3 ,water with heavy hydrogen substituted for ordinary hydrogen4 ,hydrogen bomb3 ,a high-yield thermonuclear bomb4 ,iodine3 ,a radioactive fission product with a half-life of #h days4 ,it emits an electron of average energy #j4b million electron-volts and a gamma ray of energy #j4d million electron-volts4 ,ion3 ,a charged atom or molecule4 ,ions are produced in abundance when energetic charged particles pass through matter4 ,ionization3 ,the process of removing electrons from neutral atoms or molecules4 ,neutrons and gamma rays as well as energetic charged particles are very effective in producing ionization4 ,iridium3 #ge day radioisotope4 ,it emits an electron of average energy #j4b million volts and a #j4c million volt gamma ray4 ,isotopes3 ,atoms whose nuclei have the same number of protons but a different number of neutrons4 ,such atoms have the same chemical behavior4 ,kiloton3 ,the amount of energy released by a thousand tons of ,,tnt4 ,krypton3 ,a radioactive fission product4 ,it has a ten year half-life and emits an electron of average energy #j4b million volts and a #j4e million volt gamma ray4 ,leukemia3 ,a usually fatal disease in which white blood cells are overproduced4 ,local fallout3 ,radioactive fallout in the neighborhood of a nuclear explosion4 ,megaton3 ,the amount of energy released by a million tons of ,,tnt4 ,meson3 ,a particle intermediate in weight between the electron and the proton4 ,actually1 there are two kinds of mesons1 called pi and mu4 ,the pi meson weighs #bgf times as much as the electron and is connected with the forces that hold the nucleus together4 ,the mu meson weighs #bab times as much as the electron and contributes appreciably to the cosmic radiation4 ,microsecond3 ,one millionth of a second4 ,it takes light #e microseconds to go a mile4 ,million volt particle3 ,short for million electron-volt particle4 ,moderator3 ,a material used in nuclear reactors to reduce the speed of neutrons4 ,molecule3 ,a combination of atoms held together chemically4 ,mutation3 ,a genetic change1 which is transmitted to offspring and affects hereditary characteristics4 ,such changes in genes may be caused by radiation as well as chemical and thermal agents4 ,neutrino3 ,a weightless1 uncharged particle which carries off energy in the process of beta decay4 ,neutron3 ,a neutral particle1 one of the basic constituents of the nucleus4 ,a neutron weighs slightly more than a proton1 and when free1 decays into a proton plus an electron and a neutrino4 ,noble gases3 ,helium1 neon1 argon1 krypton1 and xenon4 ,they do not combine chemically with any elements including themselves4 ,nuclear bomb3 ,a bomb which derives its energy from nuclear fission or fusion4 ,nuclear reactor3 ,a machine for maintaining a controlled chain reaction4 ,nucleus3 ,the core of an atom1 consisting of neutrons and protons4 ,its charge is equal to the number of protons4 ,its weight is equal to the number of protons plus the number of neutrons4 ,periodic system3 ,the chemical elements arranged in order of increasing atomic charge4 ,elements with similar chemical properties occur periodically4 ,plutonium3 ,element with charge #id1 produced by capturing a neutron in uranium followed by two beta emissions4 ,like uranium1 plutonium is valuable as an atomic fuel4 ,positron3 ,the positive counterpart of the electron4 ,potassium3 ,a natural radioactive isotope4 ,it has a half-life of one billion years and emits beta and gamma rays4 ,proton3 ,a constituent of the nucleus4 ,it has one unit of positive charge and weighs slightly less than a neutron4 ,radiation3 ,energetic charged particles1 neutrons and gamma rays which cause ionization in matter4 ,radiation is produced in nuclear explosions but also occurs naturally from cosmic rays and from the decay of radioactive substances in our surroundings4 ,radioactivity3 ,spontaneous nuclear decay1 releasing an alpha1 beta1 or gamma ray4 ,radioisotope3 ,short for radioactive isotope4 ,radiological bomb3 ,a bomb designed to create radioactive contamination4 ,radium3 ,element with charge #hh4 ,the principal isotope has a weight of #bbf and emits an alpha particle with a half-life of #afbj years4 ,range3 ,distance traveled by an energetic charged particle in matter before it stops4 ,heavy charged particles move in a straight line inside matter1 but electrons frequently change their course4 ,for this reason the range of electrons is only about one-half the total distance traveled4 ,reactor3 ,same as nuclear reactor4 ,roentgen3 ,a measure of radiation dose--defined in terms of the amount of energy deposited per unit weight of irradiated material4 ,a dose of #djj1jjj roentgens in living tissue deposits enough energy to raise the temperature by #a;,c4 ,a dose of only #djj roentgens in a human being will cause death fifty per cent of the time4 ,shock wave3 ,expanding front of high pressure and strong winds produced by an explosion4 ,spontaneous fission3 ,natural fission1 not induced by a neutron4 ,the half-life for this process in uranium is #h #aj years4 ,stratosphere3 ,the atmosphere above the weather zone4 ,the altitude of the stratosphere varies from thirty to fifty thousand feet depending on latitude and season4 ,stratospheric fallout3 ,world-wide fallout from big bombs whose clouds rise into the stratosphere4 ,on the average the radioactivity remains in the stratosphere for about ten years and is then deposited more or less uniformly over the surface of the earth4 ,strontium3 ,a radioactive fission product4 ,it has a half-life of #bh years and emits two electrons of average total energy #a4b million electron-volts4 ,strontium is chemically similar to calcium and gets deposited in bones4 ,thermal radiation3 ,electromagnetic radiation1 mainly visible1 but also ultraviolet and infrared1 emitted from the fireball of a nuclear explosion and transmitted long distances in the surrounding cold air4 ,thermonuclear bomb3 ,a bomb which derives a significant fraction of its energy from the fusion of hydrogen isotopes4 ,thermonuclear reaction3 ,a fusion reaction induced by high temperature4 ,thorium3 ,element with charge #ij4 ,the principal isotope has a weight of #bcb and emits an alpha particle with a half-life of #ad billion years4 ,trigger process3 ,a small cause which leads to a big effect4 ,tritium3 ,an isotope of hydrogen4 ,its nucleus 7called a triton7 consists of one proton and two neutrons4 ,tritons are radioactive beta emitters having a half-life of #ab4be years4 ,troposphere3 ,the weather portion of the atmosphere1 from sea level to about forty thousand feet4 ,tropospheric fallout3 ,world-wide fallout1 mainly from small bombs 7less than a megaton7 whose clouds remain in the troposphere4 ,this fallout occurs on the average two weeks to a month after the explosion and stays in a latitude close to the latitude of the explosion4 ,uranium3 ,element with charge #ib4 ,natural uranium contains #a part of ,u to #aci parts of ,u4 ,u is a fissionable material and ,u can be converted to plutonium1 which is fissionable4 ,x-ray3 ,penetrating electromagnetic radiation1 usually made by bombarding a metal target with energetic electrons4 ,x-rays and gamma rays are really the same thing4 ,,footnotes #a,the word 8noble0 is perhaps a misnomer--these atoms do not even seek the company of each other4 #b,quotes are put around the word atom because1 having lost one of its electrons1 it is no longer an ordinary neutral atom in its ground state4 #c,yet4 #d;,actually the same state may be occupied by two neutrons and two protons4 ,the reason is that neutrons and protons are magnetic particles with a north and a south pole4 ,consequently the demand for a difference can be satisfied by having one neutron 7or proton7 with its north pole pointing up and another with its north pole pointing down4 #e;,it seems that neutrinos emitted in the company of electrons have the symmetry of a right screw2 those emitted together with a positron have the symmetry of a left screw4 #f;,actually the weights rarely add up to the original #bch because1 as a rule1 one or more neutrons are emitted which carry off some of the original mass4 #g,only a very few unlucky ones are overtaken by beta decay first4 #h,she and her husband were the discoverers of two elements1 rhenium and masurium4 ,one of these exists4 #i;,a great portion of the energy might be lost if the neutron is quite fast4 ,in this case the neutron can cause internal excitation of the nucleus4 #aj,there seems to be a good possibility that he died from a hepatitis entirely unrelated to the initial radiation exposure4 #aa;,half-lives of radioactive nuclei are uninfluenced by the extreme temperatures or pressures of the explosion1 or by the state of motion of the particles or where they happen to be4 #ab;,a small amount may drift down to the ground in the winds4 ,this may get deposited on leaves and grass4 #ac,the last line of the table is based on our own estimates4 #ad,recent evidence suggests this number is sometimes twenty-three4 #ae;,dubbed by its friends 8,committee for ,reactor ,prevention40 #af,one of the authors4 #ag,this difference is not surprising4 ,when we sterilize1 we have to kill all germs1 even those which are most resistant to radiation4 ,furthermore small organisms may escape the radiation effects by mere chance4 ,on the other hand a big and complicated organism will cease to function when the most sensitive among its essential tissues have been destroyed4 ,transcriber's ,notes --,silently corrected a few typos4 --,retained publication information from the printed edition3 this e,book is public-domain in the country of publication4 --,in the text versions only1 text in italics is delimited by underscores4