,worlds ,within ,worlds3 ,the ,story of ,nuclear ,energy ,volume #b ,mass and ,energy ,the ,neutron ,the ,structure of the ,nucleus by ,isaac ,asimov ,u4 ,s4 ,energy ,research and ,development ,administration ,office of ,public ,affairs ,washington1 ,d4,c4 #bjede ,library of ,congress ,catalog ,card ,number3 #ge-ahidgg #aigb ,nothing in the history of mankind has opened our eyes to the possibilities of science as has the development of atomic power4 ,in the last #bjj years1 people have seen the coming of the steam engine1 the steamboat1 the railroad locomotive1 the automobile1 the airplane1 radio1 motion pictures1 television1 the machine age in general4 ,yet none of it seemed quite so fantastic1 quite so unbelievable1 as what man has done since #aici with the atom 444 there seem to be almost no limits to what may lie ahead3 inexhaustible energy1 new worlds1 ever-widening knowledge of the physical universe4 ,isaac ,asimov ,illustration3 ,photograph of night sky ,the ,u4 ,s4 ,energy ,research and ,development ,administration publishes a series of booklets for the general public4 ,please write to the following address for a title list or for information on a specific subject3 ,,userda--,technical ,information ,center ,p4 ,o4 ,box #fb ,oak ,ridge1 ,tennessee #cghcj ,illustration3 ,isaac ,asimov ,,isaac ,,asimov received his academic degrees from ,columbia ,university and is ,associate ,professor of ,biochemistry at the ,boston ,university ,school of ,medicine4 ,he is a prolific author who has written over #aej books in the past #bj years1 including about #bj science fiction works1 and books for children4 ,his many excellent science books for the public cover subjects in mathematics1 physics1 astronomy1 chemistry1 and biology1 such as ,the ,genetic ,code1 ,inside the ,atom1 ,building ,blocks of the ,universe1 ,understanding ,physics1 ,the ,new ,intelligent ,man's ,guide to ,science1 and ,asimov's ,biographical ,encyclopedia of ,science and ,technology4 ,in #aife ,dr4 ,asimov received the ,james ,t4 ,grady ,award of the ,american ,chemical ,society for his major contribution in reporting science progress to the public4 ,illustration3 ,photograph of night sky ,,contents ,,volume #a ,introduction #e ,atomic ,weights #f ,electricity #aa ,units of ,electricity #aa ,cathode ,rays #ac ,radioactivity #ag ,the ,structure of the ,atom #be ,atomic ,numbers #cj ,isotopes #ce ,energy #dg ,the ,law of ,conservation of ,energy #dg ,chemical ,energy #ej ,electrons and ,energy #ed ,the ,energy of the ,sun #ee ,the ,energy of ,radioactivity #eg ,,volume #b ,mass and ,energy #fi ,the ,structure of the ,nucleus #ge ,the ,proton #ge ,the ,proton-,electron ,theory #gf ,protons in ,nuclei #hj ,nuclear ,bombardment #hb ,particle ,accelerators #hf ,the ,neutron #ib ,nuclear ,spin #ib ,discovery of the ,neutron #ie ,the ,proton-,neutron ,theory #ih ,the ,nuclear ,interaction #aja ,neutron ,bombardment #ajg ,,volume #c ,nuclear ,fission #aag ,new ,elements #aag ,the ,discovery of ,fission #abb ,the ,nuclear ,chain ,reaction #abg ,the ,nuclear ,bomb #aca ,nuclear ,reactors #ada ,nuclear ,fusion #adg ,the ,energy of the ,sun #adg ,thermonuclear ,bombs #adi ,controlled ,fusion #aea ,beyond ,fusion #aei ,antimatter #aei ,the ,unknown #afd ,reading ,list #aff ,illustration3 ,a field-ion microscope view of atoms in a crystal4 ,each tiny white dot is a single atom1 and each ring system is a crystal facet or plane4 ,the picture is magnified #a1ejj1jjj times4 ,,mass ,,and ,,energy ,in #aijj it began to dawn on physicists that there was a vast store of energy within the atom2 a store no one earlier had imagined existed4 ,the sheer size of the energy store in the atom--millions of times that known to exist in the form of chemical energy--seemed unbelievable at first4 ,yet that size quickly came to make sense as a result of a line of research that seemed1 at the beginning1 to have nothing to do with energy4 ,suppose a ball were thrown forward at a velocity of #bj kilometers per hour by a man on top of a flatcar that is moving forward at #bj kilometers an hour4 ,to someone watching from the roadside the ball would appear to be travelling at #dj kilometers an hour4 ,the velocity of the thrower is added to the velocity of the ball4 ,if the ball were thrown forward at #bj kilometers an hour by a man on top of a flatcar that is moving backward at #bj kilometers an hour1 then the ball 7to someone watching from the roadside7 would seem to be not moving at all after it left the hand of the thrower4 ,it would just drop to the ground4 ,there seemed no reason in the #aith century to suppose that light didn't behave in the same fashion4 ,it was known to travel at the enormous speed of just a trifle under #cjj1jjj kilometers per second1 while earth moved in its orbit about the sun at a speed of about #cj kilometers per second4 ,surely if a beam of light beginning at some earth-bound source shone in the direction of earth's travel1 it ought to move at a speed of #cjj1jcj kilometers per second4 ,if it shone in the opposite direction1 against earth's motion1 it ought to move at a speed of #bii1igj kilometers per second4 ,could such a small difference in an enormous speed be detected8 ,illustration3 ,albert ,a4 ,michelson ,the ,german-,american physicist ,albert ,abraham ,michelson 7#aheb-aica7 had invented a delicate instrument1 the interferometer1 that could compare the velocities of different beams of light with great precision4 ,in #ahhg he and a co-worker1 the ,american chemist ,edward ,williams ,morley 7#ahch-aibc71 tried to measure the comparative speeds of light1 using beams headed in different directions4 ,some of this work was performed at the ,u4 ,s4 ,naval ,academy and some at the ,case ,institute4 ,the results of the ,michelson-,morley experiment were unexpected4 ,it showed no difference in the measured speed of light4 ,no matter what the direction of the beam--whether it went in the direction of the earth's movement1 or against it1 or at any angle to it--the speed of light always appeared to be exactly the same4 ,to explain this1 the ,german-,swiss-,american scientist ,albert ,einstein 7#ahgi-aiee7 advanced his 8special theory of relativity0 in #aije4 ,according to ,einstein's view1 speeds could not merely be added4 ,a ball thrown forward at #bj kilometers an hour by a man moving at #bj kilometers an hour in the same direction would not seem to be going #dj kilometers an hour to an observer at the roadside4 ,it would seem to be going very slightly less than #dj kilometers an hour2 so slightly less that the difference couldn't be measured4 ,however1 as speeds grew higher and higher1 the discrepancy in the addition grew greater and greater 7according to a formula ,einstein derived7 until1 at velocities of tens of thousands of kilometers per hour1 that discrepancy could be easily measured4 ,at the speed of light1 which ,einstein showed was a limiting velocity that an observer would never reach1 the discrepancy became so great that the speed of the light source1 however great1 added or subtracted zero to or from the speed of light4 ,accompanying this were all sorts of other effects4 ,it could be shown by ,einstein's reasoning that no object possessing mass could move faster than the speed of light4 ,what's more1 as an object moved faster and faster1 its length in the direction of motion 7as measured by a stationary observer7 grew shorter and shorter1 while its mass grew greater and greater4 ,at #bfj1jjj kilometers per second1 its length in the direction of movement was only half what it was at rest1 and its mass was twice what it was4 ,as the speed of light was approached1 its length would approach zero in the direction of motion1 while its mass would approach the infinite4 ,could this really be so8 ,ordinary objects never moved so fast as to make their lengths and masses show any measurable change4 ,what about subatomic particles1 however1 which moved at tens of thousands of kilometers per second8 ,the ,german physicist ,alfred ,heinrich ,bucherer 7#ahfc-aibg7 reported in #aijh that speeding electrons did gain in mass just the amount predicted by ,einstein's theory4 ,the increased mass with energy has been confirmed with great precision in recent years4 ,einstein's special theory of relativity has met many experimental tests exactly ever since and it is generally accepted by physicists today4 ,einstein's theory gave rise to something else as well4 ,einstein deduced that mass was a form of energy4 ,he worked out a relationship 7the 8mass-energy equivalence07 that is expressed as follows3 ,e mc where ,e represents energy1 m is mass1 and c is the speed of light4 ,if mass is measured in grams and the speed of light is measured in centimeters per second1 then the equation will yield the energy in a unit called 8ergs04 ,it turns out that #a gram of mass is equal to #ijj1jjj1jjj1jjj1jjj1jjj1jjj 7#ijj billion billion7 ergs of energy4 ,the erg is a very small unit of energy1 but #ijj billion billion of them mount up4 ,the energy equivalent of #a gram of mass 7and remember that a gram1 in ordinary units1 is only _/ of an ounce7 would keep a #ajj-watt light bulb burning for #ce1jjj years4 ,illustration3 ,,energy ,,created compared to ,,matter 7,,or ,,mass7 ,,destroyed ,it is this vast difference between the tiny quantity of mass and the huge amount of energy to which it is equivalent that obscured the relationship over the years4 ,when a chemical reaction liberates energy1 the mass of the materials undergoing the reaction decreases slightly--but very slightly4 ,suppose1 for instance1 a gallon of gasoline is burned4 ,the gallon of gasoline has a mass of #bhjj grams and combines with about #aj1jjj grams of oxygen to form carbon dioxide and water1 yielding #a4ce million billion ergs4 ,that's a lot of energy and it will drive an automobile for some #be to #cj kilometers4 ,but by ,einstein's equation all that energy is equivalent to only a little over a millionth of a gram4 ,you start with #ab1hjj grams of reacting materials and you end with #ab1hjj grams minus a millionth of a gram or so that was given off as energy4 ,no instrument known to the chemists of the #aith century could have detected so tiny a loss of mass in such a large total4 ,no wonder1 then1 that from ,lavoisier on1 scientists thought that the law of conservation of mass held exactly4 ,radioactive changes gave off much more energy per atom than chemical changes did1 and the percentage loss in mass was correspondingly greater4 ,the loss of mass in radioactive changes was found to match the production of energy in just the way ,einstein predicted4 ,it was no longer quite accurate to talk about the conservation of mass after #aije 7even though mass was just about conserved in ordinary chemical reactions so that the law could continue to be used by chemists without trouble74 ,instead1 it is more proper to speak of the conservation of energy1 and to remember that mass was one form of energy and a very concentrated form4 ,the mass-energy equivalence fully explained why the atom should contain so great a store of energy4 ,indeed1 the surprise was that radioactive changes gave off as little energy as they did4 ,when a uranium atom broke down through a series of steps to a lead atom1 it produced a million times as much energy as that same atom would release if it were involved in even the most violent of chemical changes4 ,nevertheless1 that enormous energy change in the radioactive breakdown represented only about one-half of #a of the total energy to which the mass of the uranium atom was equivalent4 ,once ,rutherford worked out the nuclear theory of the atom1 it became clear from the mass-energy equivalence that the source of the energy of radioactivity was likely to be in the atomic nucleus where almost all the mass of the atom was to be found4 ,the attention of physicists therefore turned to the nucleus4 ,,the ,,structure ,,of ,,the ,,nucleus ,the ,proton ,as early as #ahhf ,eugen ,goldstein1 who was working with cathode rays1 also studied rays that moved in the opposite direction4 ,since the cathode rays 7electrons7 were negatively charged1 rays moving in the opposite direction would have to be positively charged4 ,in #aijg ,j4 ,j4 ,thomson called them 8positive rays04 ,once ,rutherford worked out the nuclear structure of the atom1 it seemed clear that the positive rays were atomic nuclei from which a number of electrons had been knocked away4 ,these nuclei came in different sizes4 ,were the nuclei single particles--a different one for every isotope of every element8 ,or were they all built up out of numbers of still smaller particles of a very limited number of varieties8 ,might it be that the nuclei owed their positive electrical charge to the fact that they contained particles just like the electron1 but ones that carried a positive charge rather than a negative one8 ,all attempts to discover this 8positive electron0 in the nuclei failed1 however4 ,the smallest nucleus found was that produced by knocking the single electron off a hydrogen atom in one way or another4 ,this hydrogen nucleus had a single positive charge1 one that was exactly equal in size to the negative charge on the electron4 ,the hydrogen nucleus1 however1 was much more massive than an electron4 ,the hydrogen nucleus with its single positive charge was approximately #ahcg times as massive as the electron with its single negative charge4 ,was it possible to knock the positive charge loose from the mass of the hydrogen nucleus8 ,nothing physicists did could manage to do that4 ,in #aiad ,rutherford decided the attempt should be given up4 ,he suggested that the hydrogen nucleus1 for all its high mass1 should be considered the unit of positive electrical charge1 just as the electron was the unit of negative electrical charge4 ,he called the hydrogen nucleus a 8proton0 from the ,greek word for 8first0 because it was the nucleus of the first element4 ,illustration3 ,one proton balances #ahcg electrons4 ,why the proton should be so much more massive than the electron is still one of the unanswered mysteries of physics4 ,the ,proton-,electron ,theory ,what about the nuclei of elements other than hydrogen8 ,all the other elements had nuclei more massive than that of hydrogen and the natural first guess was that these were made up of some appropriate number of protons closely packed together4 ,the helium nucleus1 which had a mass four times as great as that of hydrogen1 might be made up of #d protons2 the oxygen nucleus with a mass number of #af might be made up of #af protons and so on4 ,this guess1 however1 ran into immediate difficulties4 ,a helium nucleus might have a mass number of #d but it had an electric charge of #b4 ,if it were made up of #d protons1 it ought to have an electric charge of #d4 ,in the same way1 an oxygen nucleus made up of #af protons ought to have a charge of #af1 but in actual fact it had one of #h4 ,could it be that something was cancelling part of the positive electric charge8 ,the only thing that could do so would be a negative electric charge#a and these were to be found only on electrons as far as anyone knew in #aiad4 ,it seemed reasonable1 then1 to suppose that a nucleus would contain about half as many electrons in addition to the protons4 ,the electrons were so light1 they wouldn't affect the mass much1 and they would succeed in cancelling some of the positive charge4 ,thus1 according to this early theory1 now known to be incorrect1 the helium nucleus contained not only #d protons1 but #b electrons in addition4 ,the helium nucleus would then have a mass number of #d and an electric charge 7atomic number7 of #d - #b1 or #b4 ,this was in accordance with observation4 ,this 8proton-electron theory0 of nuclear structure accounted for isotopes very nicely4 ,while oxygen-#af had a nucleus made up of #af protons and #h electrons1 oxygen-#ag had one of #ag protons and #i electrons1 and oxygen-#ah had one of #ah protons and #aj electrons4 ,the mass numbers were #af1 #ag1 and #ah1 respectively1 but the atomic number was #af - #h1 #ag - #i1 and #ah - #aj1 or #h in each case4 ,again1 uranium-#bch has a nucleus built up1 according to this theory1 of #bch protons and #adf electrons1 while uranium-#bce has one built up of #bce protons and #adc electrons4 ,in these cases the atomic number is1 respectively1 #bch - #adf and #bce - #adc1 or #ib in each case4 ,the nucleus of the #b isotopes is1 however1 of different structure and it is not surprising therefore that the radioactive properties of the two--properties that involve the nucleus--should be different and that the half-life of uranium-#bch should be six times as long as that of uranium-#bce4 ,the presence of electrons in the nucleus not only explained the existence of isotopes1 but seemed justified by two further considerations4 ,first1 it is well known that similar charges repel each other and that the repulsion is stronger the closer together the similar charges are forced4 ,dozens of positively charged particles squeezed into the tiny volume of an atomic nucleus couldn't possibly remain together for more than a tiny fraction of a second4 ,electrical repulsion would send them flying apart at once4 ,on the other hand1 opposite charges attract1 and a proton and an electron would attract each other as strongly as #b protons 7or #b electrons7 would repel each other4 ,it was thought possible that the presence of electrons in a collection of protons might somehow limit the repulsive force and stabilize the nucleus4 ,second1 there are radioactive decays in which beta particles are sent flying out of the atom4 ,from the energy involved they could come only out of the nucleus4 ,since beta particles are electrons and since they come from the nucleus1 it seemed to follow that there must be electrons within the nucleus to begin with4 ,the proton-electron theory of nuclear structure also seemed to account neatly for many of the facts of radioactivity4 ,why radioactivity at all1 for instance8 ,the more complex a nucleus is1 the more protons must be squeezed together and the harder1 it would seem1 it must be to keep them together4 ,more and more electrons seemed to be required4 ,finally1 when the total number of protons was #hd or more1 no amount of electrons seemed sufficient to stabilize the nucleus4 ,the manner of breakup fits the theory1 too4 ,suppose a nucleus gives off an alpha particle4 ,the alpha particle is a helium nucleus made up1 by this theory1 of #d protons and #b electrons4 ,if a nucleus loses an alpha particle1 its mass number should decline by #d and its atomic number by #d - #b1 or #b4 ,and1 indeed1 when uranium-#bch 7atomic number #ib7 gives off an alpha particle1 it becomes thorium-#bcd 7atomic number #ij74 ,suppose a beta particle is emitted4 ,a beta particle is an electron and if a nucleus loses an electron1 its mass number is almost unchanged4 7,an electron is so light that in comparison with the nucleus1 we can ignore its mass47 ,on the other hand1 a unit negative charge is gone4 ,one of the protons in the nucleus1 which had previously been masked by an electron1 is now unmasked4 ,its positive charge is added to the rest and the atomic number goes up by one4 ,thus1 thorium-#bcd 7atomic number #ij7 gives up a beta particle and becomes protactinium-#bcd 7atomic number #ia74 ,if a gamma ray is given off1 that gamma ray has no charge and the equivalent of very little mass4 ,that means that neither the mass number nor the atomic number of the nucleus is changed1 although its energy content is altered4 ,even more elaborate changes can be taken into account4 ,in the long run1 uranium-#bch1 having gone through many changes1 becomes lead-#bjf4 ,those changes include the emission of #h alpha particles and #f beta particles4 ,the #h alpha particles involve a loss of #h #d1 or #cb in mass number1 while the #f beta particles contribute nothing in this respect4 ,and1 indeed1 the mass number of uranium-#bch declines by #cb in reaching lead-#bjf4 ,on the other hand the #h alpha particles involve a decrease in atomic number of #h #b1 or #af1 while the #f beta particles involve an increase in atomic number of #f #a1 or #f4 ,the total change is a decrease of #af - #f1 or #aj4 ,and indeed1 uranium 7atomic number #ib7 changes to lead 7atomic number #hb74 ,it is useful to go into such detail concerning the proton-electron theory of nuclear structure and to describe how attractive it seemed4 ,the theory appeared solid and unshakable and1 indeed1 physicists used it with considerable satisfaction for #ae years4 --,and yet1 as we shall see1 it was wrong2 and that should point a moral4 ,even the best seeming of theories may be wrong in some details and require an overhaul4 ,protons in ,nuclei ,let us1 nevertheless1 go on to describe some of the progress made in the #aibjs in terms of the proton-electron theory that was then accepted4 ,since a nucleus is made up of a whole number of protons1 its mass ought to be a whole number if the mass of a single proton is considered #a4 7,the presence of electrons would add some mass but in order to simplify matters1 let us ignore that47 ,when isotopes were first discovered this indeed seemed to be so4 ,however1 ,aston and his mass spectrometer kept measuring the mass of different nuclei more and more closely during the #aibjs and found that they differed very slightly from whole numbers4 ,yet a fixed number of protons turned out to have different masses if they were first considered separately and then as part of a nucleus4 ,using modern standards1 the mass of a proton is #a4jjghbe4 ,twelve separate protons would have a total mass of twelve times that1 or #ab4jici4 ,on the other hand1 if the #ab protons are packed together into a carbon-#ab nucleus1 the mass is #ab so that the mass of the individual protons is #a4jjjjjj apiece4 ,what happens to this difference of #j4jjghbe between the proton in isolation and the proton as part of a carbon-#ab nucleus8 ,according to ,einstein's special theory of relativity1 the missing mass would have to appear in the form of energy4 ,if #ab hydrogen nuclei 7protons7 plus #f electrons are packed together to form a carbon nucleus1 a considerable quantity of energy would have to be given off4 ,in general1 ,aston found that as one went on to more and more complicated nuclei1 a larger fraction of the mass would have to appear as energy 7though not in a perfectly regular way7 until it reached a maximum in the neighborhood of iron4 ,iron-#ef1 the most common of the iron isotopes1 has a mass number of #ee4icdi4 ,each of its #ef protons1 therefore1 has a mass of #j4iihh4 ,for nuclei more complicated than those of iron1 the protons in the nucleus begin to grow more massive again4 ,uranium-#bch nuclei1 for instance1 have a mass of #bch4jejf1 so that each of the #bch protons they contain has a mass of #a4jjjb4 ,by #aibg ,aston had made it clear that it is the middle elements in the neighborhood of iron that are most closely and economically packed4 ,if a very massive nucleus is broken up into somewhat lighter nuclei1 the proton packing would be tighter and some mass would be converted into energy4 ,similarly1 if very light nuclei were joined together into somewhat more massive nuclei1 some mass would be converted into energy4 ,this demonstration that energy was released in any shift away from either extreme of the list of atoms according to atomic number fits the case of radioactivity1 where very massive nuclei break down to somewhat less massive ones4 ,consider that uranium-#bch gives up #h alpha particles and #f beta particles to become lead-#bjf4 ,the uranium-#bch nucleus has a mass of #bch4jejf2 each alpha particle has one of #d4jjbf for a total of #cb4jbjh2 each beta particle has a mass of #j4jjaed for a total of #j4jjibd2 and the lead-#bjf nucleus has one of #bje4igde4 ,this means that the uranium-#bch nucleus 7mass3 #bch4jejf7 changes into #h alpha particles1 #f beta particles1 and a lead-#bjf nucleus 7total mass3 #bch4jjde74 ,the starting mass is #j4jdfa greater than the final mass and it is this missing mass that has been converted into energy and is responsible for the gamma rays and for the velocity with which alpha particles and beta particles are discharged4 ,nuclear ,bombardment ,once scientists realized that there was energy which became available when one kind of nucleus was changed into another1 an important question arose as to whether such a change could be brought about and regulated by man and whether this might not be made the source of useful power of a kind and amount undreamed of earlier4 ,chemical energy was easy to initiate and control1 since that involved the shifts of electrons on the outskirts of the atoms4 ,raising the temperature of a system1 for instance1 caused atoms to move more quickly and smash against each other harder1 and that in itself was sufficient to force electrons to shift and to initiate a chemical reaction that would not take place at lower temperatures4 ,to shift the protons within the nucleus 78nuclear reactions07 and make nuclear energy available was a harder problem by far4 ,the particles involved were much more massive than electrons and correspondingly harder to move4 ,what's more1 they were buried deep within the atom4 ,no temperatures available to the physicists of the #aibjs could force atoms to smash together hard enough to reach and shake the nucleus4 ,in fact1 the only objects that were known to reach the nucleus were speeding subatomic particles4 ,as early as #aijf1 for instance1 ,rutherford had used the speeding alpha particles given off by a radioactive substance to bombard matter and to show that sometimes these alpha particles were deflected by atomic nuclei4 ,it was1 in fact1 by such an experiment that he first demonstrated the existence of such nuclei4 ,rutherford had continued his experiments with bombardment4 ,an alpha particle striking a nucleus would knock it free of the atom to which it belonged and send it shooting forward 7like one billiard ball hitting another74 ,the nucleus that shot ahead would strike a film of chemical that scintillated 7sparkled7 under the impact4 ,in a rough way1 one could tell the kind of nucleus that struck from the nature of the sparkling4 ,in #aiai ,rutherford bombarded nitrogen gas with alpha particles and found that he obtained the kind of sparkling he associated with the bombardment of hydrogen gas4 ,when he bombarded hydrogen1 the alpha particles struck hydrogen nuclei 7protons7 and shot them forward4 ,to get hydrogen-sparkling out of the bombardment of nitrogen1 ,rutherford felt1 he must have knocked protons out of the nitrogen nuclei4 ,indeed1 as was later found1 he had converted nitrogen nuclei into oxygen nuclei4 ,this was the first time in history that the atomic nucleus was altered by deliberate human act4 ,rutherford continued his experiments and by #aibd had shown that alpha particles could be used to knock protons out of the nuclei of almost all elements up to potassium 7atomic number #ai74 ,there were1 however1 limitations to the use of natural alpha particles as the bombarding agent4 ,first1 the alpha particles used in bombardment were positively charged and so were the atomic nuclei4 ,this meant that the alpha particles and the atomic nuclei repelled each other and much of the energy of the alpha particles was used in overcoming the repulsion4 ,for more and more massive nuclei1 the positive charge grew higher and the repulsion stronger until for elements beyond potassium1 no collision could be forced1 even with the most energetic naturally occurring alpha particles4 ,illustration3 ,man-made transmutation4 ,nitrogen-#ad 7#g,n1#g,p7 ,helium-#d 7#b,n1#b,p7 7,alpha particle7 ,oxygen-#ag 7#i,n1#h,p7 ,hydrogen-#a 7#a,p7 ,neutron,n1 ,proton,p ,second1 the alpha particles that are sprayed toward the target cannot be aimed directly at the nuclei4 ,an alpha particle strikes a nucleus only if1 by chance1 they come together4 ,the nuclei that serve as their targets are so unimaginably small that most of the bombarding particles are sure to miss4 ,in ,rutherford's first bombardment of nitrogen1 it was calculated that only #a alpha particle out of #cjj1jjj managed to strike a nitrogen nucleus4 ,the result of these considerations is clear4 ,there is energy to be gained out of nuclear reactions1 but there is also energy that must be expended to cause these nuclear reactions4 ,in the case of nuclear bombardment by subatomic particles 7the only way1 apparently1 in which nuclear reactions can be brought about71 the energy expended seems to be many times the energy to be extracted4 ,this is because so many subatomic particles use up their energy in ionizing atoms1 knocking electrons away1 and never initiate nuclear reactions at all4 ,it was as though the only way you could light a candle would be to strike #cjj1jjj matches1 one after the other4 ,if that were so1 candles would be impractical4 ,in fact1 the most dramatic result of alpha particle bombardment had nothing to do with energy production1 but rather the reverse4 ,new nuclei were produced that had more energy than the starting nuclei1 so that energy was absorbed by the nuclear reaction rather than given off4 ,this came about first in #aicd1 when a ,french husband-and-wife team of physicists1 ,frdric ,joliot-,curie 7#aijj-aieh7 and ,irne ,joliot-,curie 7#ahig-aief7 were bombarding aluminum-#bg 7atomic number #ac7 with alpha particles4 ,the result was to combine part of the alpha particle with the aluminum-#bg nucleus to form a new nucleus with an atomic number two units higher--#ae--;and a mass number three units higher--#cj4 ,the element with atomic number #ae is phosphorus so that phosphorus-#cj was formed4 ,the only isotope of phosphorus that occurs in nature1 however1 is phosphorus-#ca4 ,phosphorus-#cj was the first man-made nucleus--the first to be manufactured by nuclear reactions in the laboratory4 ,illustration3 ,frdric and ,irne ,joliot-,curie ,the reason phosphorus-#cj did not occur in nature was that its energy content was too high to allow it to be stable4 ,its energy content drained away through the emission of particles that allowed the nucleus to change over into a stable one1 silicon-#cj 7atomic number #ad74 ,this was an example of 8artificial radioactivity04 ,since #aicd1 over a thousand kinds of nuclei that do not occur in nature have been formed in the laboratory through various kinds of bombardment-induced nuclear reactions4 ,every single one of them proved to be radioactive4 ,particle ,accelerators ,was there nothing that could be done to make nuclear bombardment more efficient and increase the chance of obtaining useful energy out of nuclear reactions8 ,in #aibh the ,russian-,american physicist ,george ,gamow 7#aijd-aifh7 suggested that protons might be used as bombarding agents in place of alpha particles4 ,protons were only one-fourth as massive as alpha particles and the collision might be correspondingly less effective2 on the other hand1 protons had only half the positive charge of alpha particles and would not be as strongly repelled by the nuclei4 ,then1 too1 protons were much more easily available than alpha particles4 ,to get a supply of protons one only had to ionize the very common hydrogen atoms1 i4e41 get rid of the single electron of the hydrogen atom1 and a single proton is left4 ,illustration3 ,artificial radioactivity4 ,aluminum-#bg 7#ad,n1#ac,p7 ,helium-#d 7#b,n1#b,p7 7,alpha particle7 7#af,n1#ae,p7 ,n ,phosphorus-#cj 7,radioactive7 7#ae,n1#ae,p7 ,positron ,silicon-#cj ,neutron,n1 ,proton,p ,of course1 protons obtained by the ionization of hydrogen atoms have very little energy1 but could energy be imparted to them8 ,protons carry a positive charge and a force can therefore be exerted upon them by an electric or magnetic field4 ,in a device that makes use of such fields1 protons can be accelerated 7made to go faster and faster71 and thus gain more and more energy4 ,in the end1 if enough energy is gained1 the proton could do more damage than the alpha particle1 despite the former's smaller mass4 ,combine that with the smaller repulsion involved and the greater ease of obtaining protons--and the weight of convenience and usefulness would swing far in the direction of the proton4 ,physicists began to try to design 8particle accelerators0 and the first practical device of this sort was produced in #aibi by the two ,british physicists ,john ,douglas ,cockcroft 7#ahig-aifg7 and ,ernest ,thomas ,sinton ,walton 7#aijc- 74 ,their device1 called an 8electrostatic accelerator01 produced protons that were sufficiently energetic to initiate nuclear reactions4 ,in #aica they used their accelerated protons to disrupt the nucleus of lithium-#g4 ,it was the first nuclear reaction to be brought about by man-made bombarding particles4 ,other types of particle accelerators were also being developed at this time4 ,the most famous was the one built in #aicj by the ,american physicist ,ernest ,orlando ,lawrence 7#aija-aieh74 ,in this device a magnet was used to make the protons move in gradually expanding circles1 gaining energy with each lap until they finally moved out beyond the influence of the magnet and then hurtled out of the instrument in a straight line at maximum energy4 ,this instrument was called a 8cyclotron04 ,illustration3 ,inventors of one of the first accelerators1 ,ernest ,t4 ,s4 ,walton1 left1 and ,john ,d4 ,cockcroft1 right1 with ,lord ,ernest ,rutherford at ,cambridge ,university in the early #aicjs4 ,illustration3 ,the bombardment of lithium-#g with protons was the first nuclear reaction caused by man-made particles4 ,lithium-#g 7#d,n1#c,p7 ,hydrogen-#a 7,proton7 ,helium-#d 7#b,n1#b,p7 7,alpha particle7 ,helium-#d 7#b,n1#b,p7 7,alpha particle7 ,neutron,n1 ,proton,p ,the cyclotron was rapidly improved1 using larger magnets and increasingly sophisticated design4 ,there are now1 at this time of writing1 8proton synchrotrons0 7descendants of that first cyclotron7 that produce particles with over a million times the energy of those produced by ,lawrence's first cyclotron4 ,of course1 the first cyclotron was only a quarter of a meter wide1 while the largest today has a diameter of some #bjjj meters4 ,as particle accelerators grew larger1 more efficient1 and more powerful1 they became ever more useful in studying the structure of the nucleus and the nature of the subatomic particles themselves4 ,they did not serve1 however1 to bring the dream of useful nuclear energy any closer4 ,though they brought about the liberation of vastly more nuclear energy than ,rutherford's initial bombardments could1 they also consumed a great deal more energy in the process4 ,it is not surprising that ,rutherford1 the pioneer in nuclear bombardment1 was pessimistic4 ,to the end of his days 7he died in #aicg7 he maintained that it would be forever impossible to tap the energy of the nucleus for use by man4 ,hopes that 8nuclear power0 might some day run the world's industries were1 in his view1 an idle dream4 ,illustration3 ,ernest ,o4 ,lawrence holds a model of the first cyclotron in #aicj1 a year after its conception4 ,,the ,,neutron ,nuclear ,spin ,what ,rutherford did not 7and could not7 take into account were the consequences of a completely new type of nuclear bombardment involving a type of particle unknown in the #aibjs 7though ,rutherford speculated about the possibility of its existence74 ,the beginnings of the new path came about through the reluctant realization that there was a flaw in the apparently firmly grounded proton-electron picture of nuclear structure4 ,the flaw involved the 8nuclear spin04 ,in #aibd the ,austrian physicist ,wolfgang ,pauli 7#aijj-aieh7 worked out a theory that treated protons and electrons as though they were spinning on their axes4 ,this spin could be in either direction 7or1 as we would say in earthly terms1 from west-to-east1 or from east-to-west74 ,quantum theory has shown that a natural unit exists for what is called the angular momentum of this spin4 ,measured in terms of this natural unit of spin1 the proton and the electron have spin 4 ,if the particle spun in one direction it was 1 if in the other it was -4 ,when subatomic particles came together to form an atomic nucleus1 each kept its original spin1 and the nuclear spin was then equal to the total angular momentum of the individual particles that made it up4 ,for instance1 suppose the helium nucleus is made up of #d protons and #b electrons1 as was thought in the #aibjs4 ,of the #d protons1 suppose that two had a spin of and two of -4 ,suppose also that of the #b electrons1 one had a spin of and one of -4 ,all the spins would cancel each other4 ,the total angular momentum would be zero4 ,of course1 it is also possible that all #f particles were spinning in the same direction2 all or all -4 ,in that case the nuclear spin would be #c1 either in one direction or the other4 ,if #e particles were spinning in one direction and #a in the other1 then the total spin would be #b1 in one direction or the other4 ,illustration3 ,wolfgang ,pauli lecturing in ,copenhagen in ,april #aibi4 ,in short if you have an even number of particles in a nucleus1 each with a spin of or -1 then the total spin is either zero or a whole number1 no matter what combination of positive and negative spins you choose4 7,the total spin is always written as a positive number47 ,on the other hand1 suppose you have lithium-#g1 which was thought to be made up of #g protons and #d electrons4 ,if the #g protons were all and the #d electrons were all - in their spins1 the nuclear spin would be _/ - _/ _/4 ,if you have an odd number of particles in the nucleus1 you will find that any combination of positive and negative spins will never give you either zero or a whole number as a sum4 ,the sum will always include a fraction4 ,consequently1 if one measures the spin of a particular atomic nucleus one can tell at once whether that nucleus contains an even number of particles or an odd number4 ,this quickly raised a problem4 ,the nuclear spin of the common isotope1 nitrogen-#ad1 was measured accurately over and over again and turned out to be #a4 ,there seemed no doubt about that and it could therefore be concluded that there were an even number of particles in the nitrogen-#ad nucleus4 ,and yet1 by the proton-electron theory of nuclear structure1 the nitrogen-#ad nucleus1 with a mass number of #ad and an atomic number of #g1 had to be made up of #ad protons and #g electrons for a total of #ba particles altogether--an odd number4 ,the nuclear spin of nitrogen-#ad indicated 8even number0 and the proton-electron theory indicated 8odd number04 ,one or the other had to be wrong1 but which8 ,the nuclear spin was a matter of actual measurement1 which could be repeated over and over and on which all agreed4 ,the proton-electron theory was only a theory4 ,it was therefore the latter that was questioned4 ,what was to be done8 ,suppose it is wrong to count protons and electrons inside the nucleus as separate particles4 ,was it possible that an electron and a proton1 forced into the close confinement of the atomic nucleus might1 by the force of mutual attraction1 become so intimately connected as to count as a single particle4 ,one of the first to suggest this1 as far back as #aibj1 was ,rutherford4 ,such a proton-electron combination would be electrically neutral and in #aiba the ,american chemist ,william ,draper ,harkins 7#ahgc-aiea7 used the term 8neutron0 as a name for it4 ,if we look at the nitrogen-#ad nucleus in this way then it is made up1 not of #ad protons and #g electrons1 but of #g protons and #g proton-electron combinations4 ,instead of a total of #ba particles1 there would be a total of #ad2 instead of an odd number1 there would be an even number4 ,the structure would now account for the nuclear spin4 ,but could such a revised theory of nuclear structure be made to seem plausible8 ,the proton-electron theory seemed to make sense because both protons and electrons were known to exist separately and could be detected4 ,if an intimate proton-electron combination could also exist1 ought it not exist 7or be made to exist7 outside the nucleus and ought it not be detected as an isolated particle8 ,discovery of the ,neutron ,throughout the #aibjs scientists searched for the neutron but without success4 ,one of the troubles was that the particle was electrically neutral4 ,subatomic particles could be detected in a variety of ways1 but every single way 7right down to the present time7 makes use of their electric charge4 ,the electric charge of a speeding subatomic particle either repels electrons or attracts them4 ,in either case1 electrons are knocked off atoms that are encountered by the speeding subatomic particle4 ,the atoms with electrons knocked off are now positively charged ions4 ,droplets of water vapor can form about these ions1 or a bubble of gas can form1 or a spark of light can be seen4 ,the droplets1 the bubbles1 and the light can all be detected one way or another and the path of the subatomic particle could be followed by the trail of ions it left behind4 ,gamma rays1 though they carry no charge1 are a wave form capable of ionizing atoms4 ,all the particles and rays that can leave a detectable track of ions behind are called 8ionizing radiation0 and these are easy to detect4 ,the hypothetical proton-electron combination1 however1 which was neither a wave form nor a charged particle was not expected to be able to ionize atoms4 ,it would wander among the atoms without either attracting or repelling electrons and would therefore leave the atomic structure intact4 ,its pathway could not be followed4 ,in short1 then1 the neutron was1 so to speak1 invisible1 and the search for it seemed a lost cause4 ,and until it was found1 the proton-electron theory of nuclear structure1 whatever its obvious deficiencies with respect to nuclear spin1 remained the only one to work with4 ,then came #aicj4 ,the ,german physicist ,walther ,wilhelm ,georg ,bothe 7#ahia-aieg7 and a co-worker1 ,h4 ,becker1 were bombarding the light metal1 beryllium1 with alpha particles4 ,ordinarily1 they might expect protons to be knocked out of it1 but in this case no protons appeared4 ,they detected some sort of radiation because something was creating certain effects while the alpha particles were bombarding the beryllium but not after the bombardment ceased4 ,illustration3 ,walther ,w4 ,g4 ,bothe ,to try to determine something about the properties of this radiation1 ,bothe and ,becker tried putting objects in the way of the radiation4 ,they found the radiation to be remarkably penetrating4 ,it even passed through several centimeters of lead4 ,the only form of radiation that was known at that time to come out of bombarded matter with the capacity of penetrating a thick layer of lead was gamma rays4 ,bothe and ,becker1 therefore1 decided they had produced gamma rays and reported this4 ,in #aicb the ,joliot-,curies repeated the ,bothe-,becker work and got the same results4 ,however1 among the objects they placed in the path of the new radiation1 they included paraffin1 which is made up of the light atoms of carbon and hydrogen4 ,to their surprise1 protons were knocked out of the paraffin4 ,gamma rays had never been observed to do this1 but the ,joliot-,curies could not think what else the radiation might be4 ,they simply reported that they had discovered gamma rays to be capable of a new kind of action4 ,illustration3 ,james ,chadwick ,not so the ,english physicist ,james ,chadwick 7#ahia- 74 ,in that same year he maintained that a gamma ray1 which possessed no mass1 simply lacked the momentum to hurl a proton out of its place in the atom4 ,even an electron was too light to do so4 7,it would be like trying to knock a baseball off the ground and into the air by hitting it with a ping-pong ball47 ,any radiation capable of knocking a proton out of an atom had to consist of particles that were themselves pretty massive4 ,and if one argued like that1 then it seemed that the radiation first observed by ,bothe and ,becker had to be the long-sought-for proton-electron combination4 ,chadwick used ,harkins' term1 neutron1 for it and made it official4 ,he gets the credit for the discovery of the neutron4 ,chadwick managed to work out the mass of the neutron from his experiments and by #aicd it was quite clear that the neutron was more massive than the proton4 ,the best modern data have the mass of the proton set at #a4jjghbe1 and that of the neutron just a trifle greater at #a4jjhffe4 ,the fact that the neutron was just about as massive as the proton was to be expected if the neutron were a proton-electron combination4 ,it was also not surprising that the isolated neutron eventually breaks up1 giving up an electron and becoming a proton4 ,out of any large number of neutrons1 half have turned into protons in about #ab minutes4 ,nevertheless1 although in some ways we can explain the neutron by speaking of it as though it were a proton-electron combination1 it really is not4 ,a neutron has a spin of while a proton-electron combination would have a spin of either #j or #a4 ,the neutron1 therefore1 must be treated as a single uncharged particle4 ,the ,proton-,neutron ,theory ,as soon as the neutron was discovered1 the ,german physicist ,werner ,karl ,heisenberg 7#aija- 7 revived the notion that the nucleus must be made up of protons and neutrons1 rather than protons and electrons4 ,it was very easy to switch from the latter theory to the former1 if one simply remembered to pair the electrons thought to be in the nucleus with protons and give the name neutrons to these combinations4 ,thus1 the helium-#d nucleus1 rather than being made up of #d protons and #b electrons1 was made up of #b protons and #b proton-electron combinations2 or #b protons and #b neutrons4 ,in the same way the oxygen-#af nucleus instead of being made up of #af protons and #h electrons1 would be made up of #h protons and #h neutrons4 ,the proton-neutron theory would account for mass numbers and atomic numbers perfectly well4 ,if a nucleus was made up of x protons and y neutrons1 then the atomic number was equal to x and the mass number to x y4 7,it is now possible to define the mass number of a nucleus in modern terms4 ,it is the number of protons plus neutrons in the nucleus47 ,illustration3 ,werner ,heisenberg ,the proton-neutron theory of nuclear structure could account for isotopes perfectly well1 too4 ,consider the #c oxygen isotopes1 oxygen-#af1 oxygen-#ag1 and oxygen-#ah4 ,the first would have a nucleus made up of #h protons and #h neutrons2 the second1 one of #h protons and #i neutrons2 and the third1 one of #h protons and #aj neutrons4 ,in each case the atomic number is #h4 ,the mass numbers however would be #af1 #ag1 and #ah1 respectively4 ,in the same way uranium-#bch would have a nucleus built of #ib protons and #adf neutrons1 while uranium-#bce would have one of #ib protons and #adc neutrons4 ,by the new theory1 can we suppose that it is neutrons rather than electrons that somehow hold the protons together against their mutual repulsion1 and that more and more neutrons are required to do this as the nucleus grows more massive8 ,at first the number of neutrons required is roughly equal to the number of protons4 ,the helium-#d nucleus contains #b protons and #b neutrons1 the carbon-#ab nucleus contains #f protons and #f neutrons1 the oxygen-#af nucleus contains #h protons and #h neutrons1 and so on4 ,for more complicated nuclei1 additional neutrons are needed4 ,in vanadium-#ea1 the nucleus contains #bc protons and #bh neutrons1 five more than an equal amount4 ,in bismuth-#bji1 it is #hc protons and #abf neutrons1 #dc more than an equal amount4 ,for still more massive nuclei containing a larger number of protons1 no amount of neutrons is sufficient to keep the assembly stable4 ,the more massive nuclei are all radioactive4 ,the manner of radioactive breakdown fits the theory1 too4 ,suppose a nucleus gives off an alpha particle4 ,the alpha particle is a helium nucleus made up of #b protons and #b neutrons4 ,if a nucleus loses an alpha particle1 its mass number should decline by #d and its atomic number by #b1 and that is what happens4 ,suppose a nucleus gives off a beta particle4 ,for a moment1 that might seem puzzling4 ,if the nucleus contains only protons and neutrons and no electrons1 where does the beta particle come from8 ,suppose we consider the neutrons as proton-electron combinations4 ,within many nuclei1 the neutrons are quite stable and do not break up as they do in isolation4 ,in the case of certain nuclei1 however1 they do break up4 ,thus the thorium-#bcd nucleus is made up of #ij protons and #add neutrons4 ,one of these neutrons might be viewed as breaking up to liberate an electron and leaving behind an unbound proton4 ,if a beta particle leaves then1 the number of neutrons decreases by one and the number of protons increases by one4 ,the thorium-#bcd nucleus 7#ij protons1 #add neutrons7 becomes a protactinium-#bcd nucleus 7#ia protons1 #adc neutrons74 ,in short1 the proton-neutron theory of nuclear structure could explain all the observed facts just as well as the proton-electron theory1 and could explain the nuclear spins1 which the proton-electron theory could not4 ,what's more1 the isolated neutron had been discovered4 ,the proton-neutron theory was therefore accepted and remains accepted to this day4 ,the ,nuclear ,interaction ,in one place1 and only one1 did the proton-neutron theory seem a little weaker than the proton-electron theory4 ,the electrons in the nucleus were thought to act as a kind of glue holding together the protons4 ,but the electrons were gone4 ,there were no negative charges at all inside the nucleus1 only the positive charges of the proton1 plus the uncharged neutron4 ,as many as #hc positive charges were to be found 7in the bismuth-#bji nucleus7 squeezed together and yet not breaking apart4 ,in the absence of electrons1 what kept the protons clinging together8 ,was it possible that the electrical repulsion between #b protons is replaced by an attraction if those protons were pushed together closely enough8 ,can there be both an attraction and a repulsion1 with the former the more important at very short range8 ,if this were so1 that hypothetical attraction would have to have two properties4 ,first1 it would have to be extremely strong--strong enough to overcome the repulsion of two positive charges at very close quarters4 ,secondly1 it would have to be short-range1 for no attractive force between protons of any kind was ever detected outside the nucleus4 ,in addition1 this short-range attraction would have to involve the neutron4 ,the hydrogen-#a nucleus was made up of a single proton1 but all nuclei containing more than #a proton had to contain neutrons also to be stable1 and only certain numbers of neutrons4 ,until the discovery of the neutron1 only two kinds of forces1 or 8interactions01 were known in the universe4 ,these were the 8gravitational interaction0 and the 8electromagnetic interaction04 ,the electromagnetic interaction was much the stronger of the two--trillions and trillions and trillions of times as strong as the gravitational attraction4 ,the electromagnetic attraction1 however1 includes both attraction 7between opposite electric charges or between opposite magnetic poles7 and repulsion 7between like electric charges or magnetic poles74 ,in ordinary bodies1 the attractions and repulsions usually cancel each other entirely or nearly entirely1 leaving very little of one or the other to be detected as surplus4 ,the gravitational interaction1 however1 includes only attraction and this increases with mass4 ,by the time you have gigantic masses such as the earth or the sun1 the gravitational interaction between them and other bodies is also gigantic4 ,both the gravitational and electromagnetic interactions are long-range4 ,the intensity of each interaction declines with distance but only as the square of the distance4 ,if the distance between earth and sun were doubled1 the gravitational interaction would still be one-fourth what it is now4 ,if the distance were increased ten times1 the interaction would still be #a_/7aj #aj7 or #a_/ajj what it is now4 ,it is for this reason that gravitational and electromagnetic interactions can make themselves felt over millions of miles of space4 ,but now1 with the acceptance of the proton-neutron theory of nuclear structure1 physicists began to suspect the existence of a third interaction--a 8nuclear interaction0--much stronger than the electromagnetic interaction1 perhaps #acj times as strong4 ,furthermore1 the nuclear interaction had to decline very rapidly with distance much more rapidly than the electromagnetic interaction did4 ,in that case1 protons in virtual contact1 as within the nucleus1 would attract each other1 but if the distance between them was increased sufficiently to place one outside the nucleus1 the nuclear interaction would decrease in intensity to less than the electromagnetic repulsion4 ,the proton would now be repelled by the positive charge of the nucleus and would go flying away4 ,that is why atomic nuclei have to be so small2 it is only when they are so tiny that the nuclear interaction can hold them together4 ,in #aicb ,heisenberg tried to work out how these interactions might come into being4 ,he suggested that attractions and repulsions were the result of particles being constantly and rapidly exchanged by the bodies experiencing the attractions and repulsions4 ,under some conditions1 these 8exchange particles0 moving back and forth very rapidly between #b bodies might force those bodies apart2 under other conditions they might pull those bodies together4 ,in the case of the electromagnetic interaction1 the exchange particles seemed to be 8photons01 wave packets that made up gamma rays1 ,x rays1 or even ordinary light 7all of which are examples of 8electromagnetic radiation074 ,the gravitational interaction would be the result of exchange particles called 8gravitons04 7,in #aifi1 there were reports that gravitons had actually been detected47 ,both the photon and the graviton have zero mass and there is a connection between that and the fact that electromagnetic interaction and gravitational interaction decline only slowly with distance4 ,for a nuclear interaction1 which declines very rapidly with distance1 the exchange particle 7if any7 would have to have mass4 ,in #aice the ,japanese physicist ,hideki ,yukawa 7#aijg- 7 worked out in considerable detail the theory of such exchange particles in order to decide what kind of properties the one involved in the nuclear interaction would have4 ,he decided it ought to have a mass about #bej times that of an electron1 which would make it about _/ as massive as a proton4 ,since this mass is intermediate between that of an electron and proton1 such particles eventually came to be called 8mesons0 from a ,greek word meaning 8intermediate04 ,once ,yukawa published his theory1 the search was on for the hypothetical mesons4 ,ideally1 if they existed within the nucleus1 shooting back and forth between protons and neutrons1 there ought to be some way of knocking them out of the nucleus and studying them in isolation4 ,unfortunately1 the bombarding particles at the disposal of physicists in the #aicjs possessed far too little energy to knock mesons out of nuclei1 assuming they were there in the first place4 ,there was one way out4 ,in #aiaa the ,austrian physicist ,victor ,francis ,hess 7#ahhc-aifd7 had discovered that earth was bombarded from every side by 8cosmic rays04 ,these consisted of speeding atomic nuclei 78cosmic particles07 of enormous energies--in some cases1 billions of times as intense as any energies available through particles produced by mankind4 ,if a cosmic particle of sufficient energy struck an atomic nucleus in the atmosphere1 it might knock mesons out of it4 ,in #aicf the ,american physicists ,carl ,david ,anderson 7#aije- 7 and ,seth ,henry ,neddermeyer 7#aijg- 71 studying the results of cosmic-particle bombardment of matter1 detected the existence of particles of intermediate mass4 ,this particle turned out to be lighter than ,yukawa had predicted2 it was only about #bjg times as massive as an electron4 ,much worse1 it lacked other properties that ,yukawa had predicted4 ,it did not interact with the nucleus in the manner expected4 ,illustration3 ,hideki ,yukawa ,illustration3 ,victor ,f4 ,hess ,illustration3 ,c4 ,d4 ,anderson ,in #aidg1 however1 the ,english physicist ,cecil ,frank ,powell 7#aijc-aifi7 and his co-workers1 also studying cosmic-particle bombardment1 located another intermediate-sized body1 which had the right mass and all the other appropriate properties to fit ,yukawa's theories4 ,anderson's particle was called a 8mu-meson01 soon abbreviated to 8muon04 ,powell's particle was called a 8pi-meson01 soon abbreviated to 8pion04 ,with the discovery of the pion1 ,yukawa's theory was nailed down and any lingering doubt as to the validity of the proton-neutron theory vanished4 ,illustration3 ,c4 ,f4 ,powell 7,actually1 it turns out that there are two forces4 ,the one with the pion as exchange particle is the 8strong nuclear interaction04 ,another1 involved in beta particle emission1 for instance1 is a 8weak interaction01 much weaker than the electromagnetic but stronger than the gravitational47 ,the working out of the details of the strong nuclear interaction explains further the vast energies to be found resulting from nuclear reactions4 ,ordinary chemical reactions1 with the electron shifts that accompany them1 involve the electromagnetic interaction only4 ,nuclear energy1 with the shifts of the particles inside the nucleus1 involves the much stronger nuclear interaction4 ,neutron ,bombardment ,as soon as neutrons were discovered1 it seemed to physicists that they had another possible bombarding particle of extraordinary properties4 ,since the neutron lacked any electric charge1 it could not be repelled by either electrons on the outside of the atoms or by the nuclei at the center4 ,the neutron was completely indifferent to the electromagnetic attraction and it just moved along in a straight line4 ,if it happened to be headed toward a nucleus it would strike it no matter how heavy a charge that nucleus might have and very often it would1 as a result1 induce a nuclear reaction where a proton would not have been able to4 ,illustration3 ,j4 ,robert ,oppenheimer ,to be sure1 it seemed just at first that there was a disadvantage to the neutron's lack of charge4 ,it could not be accelerated directly by any device since that always depended on electromagnetic interaction to which the neutron was impervious4 ,there was one way of getting around this and this was explained in #aice by the ,american physicist ,j4 ,robert ,oppenheimer 7#aijd-aifg7 and by his student ,melba ,phillips4 ,use is made here of the nucleus of the hydrogen-#b 7deuterium7 nucleus4 ,that nucleus1 often called a 8deuteron01 is made up of #a proton plus #a neutron and has a mass number of #b and an atomic number of #a4 ,since it has a unit positive charge1 it can be accelerated just as an isolated proton can be4 ,suppose1 then1 that a deuteron is accelerated to a high energy and is aimed right at a positively charged nucleus4 ,that nucleus repels the deuteron1 and it particularly repels the proton part4 ,the nuclear interaction that holds together a single proton and a single neutron is comparatively weak as nuclear interactions go1 and the repulsion of the nucleus that the deuteron is approaching may force the proton out of the deuteron altogether4 ,the proton veers off1 but the neutron1 unaffected1 keeps right on going and1 with all the energy it had gained as part of the deuteron acceleration1 smashes into the nucleus4 ,within a few months of their discovery1 energetic neutrons were being used to bring about nuclear reactions4 ,actually1 though1 physicists didn't have to worry about making neutrons energetic4 ,this was a hangover from their work with positively charged particles such as protons and alpha particles4 ,these charged particles had to be energetic to overcome the repulsion of the nucleus and to smash into it with enough force to break it up4 ,neutrons1 however1 didn't have to overcome any repulsion4 ,no matter how little energy they had1 if they were correctly aimed 7and some always were1 through sheer chance7 they would approach and strike the nucleus4 ,in fact1 the more slowly they travelled1 the longer they would stay in the vicinity of a nucleus and the more likely they were to be captured by some nearby nucleus through the attraction of the nuclear interaction4 ,the influence of the nucleus in capturing the neutron was greater the slower the neutron1 so that it was almost as though the nucleus were larger and easier to hit for a slow neutron than a fast one4 ,eventually1 physicists began to speak of 8nuclear cross sections0 and to say that particular nuclei had a cross section of such and such a size for this bombarding particle or that4 ,the effectiveness of slow neutrons was discovered in #aicd by the ,italian-,american physicist ,enrico ,fermi 7#aija-aied74 ,of course1 there was the difficulty that neutrons couldn't be slowed down once they were formed1 and as formed they generally had too much energy 7according to the new way of looking at things74 ,at least they couldn't be slowed down by electromagnetic methods--but there were other ways4 ,a neutron didn't always enter a nucleus that it encountered4 ,sometimes1 if it struck the nucleus a hard1 glancing blow1 it bounced off4 ,if the nucleus struck by the neutron is many times as massive as the neutron1 the neutron bounced off with all its speed practically intact4 ,on the other hand1 if the neutron hits a nucleus not very much more massive than itself1 the nucleus rebounds and absorbs some of the energy1 so that the neutron bounces away with less energy than it had4 ,if the neutron rebounds from a number of comparatively light nuclei1 it eventually loses virtually all its energy and finally moves about quite slowly1 possessing no more energy than the atoms that surround it4 7,you can encounter this situation in ordinary life in the case of billiard balls4 ,a billiard ball1 colliding with a cannon ball1 will just bounce1 moving just as rapidly afterward as before1 though in a different direction4 ,if a billiard ball strikes another billiard ball1 it will set the target ball moving and bounce off itself with less speed47 ,the energy of the molecules in the atmosphere depends on temperature4 ,neutrons that match that energy and have the ordinary quantity to be expected at room temperature are called 8thermal0 7from a ,greek word meaning 8heat07 neutrons4 ,the comparatively light nuclei against which the neutrons bounce and slow down are 8moderators0 because they moderate the neutron's energy4 ,fermi and his co-workers were the first to moderate neutrons1 produce thermal neutrons1 and use them1 in #aice1 to bombard nuclei4 ,he quickly noted how large nuclear cross sections became when thermal neutrons were the bombarding particles4 ,it might seem that hope could now rise in connection with the practical use of energy derived from nuclear reactions4 ,neutrons could bring about nuclear reactions1 even when they themselves possessed very little energy1 so output might conceivably be more than input for each neutron that struck4 ,furthermore because of the large cross sections involved1 thermal neutrons missed far less frequently than high-energy charged particles did4 ,but there was a catch4 ,before neutrons could be used1 however low-energy and however sure to hit1 they had to be produced2 and in order to produce neutrons they had to be knocked out of nuclei by bombardment with high-energy protons or some other such method4 ,the energy formed by the neutrons was at first never more than the tiniest fraction of the energies that went into forming the neutrons in the first place4 ,it was as though you could indeed light a candle with a single match1 but you still had to look through #cjj1jjj useless pieces of wood before you found a match4 ,the candle would still be impractical4 ,even with the existence of neutron bombardment1 involving low energy and high cross section1 ,rutherford could1 with justice1 feel right down to the time of his death that nuclear energy would never be made available for practical use4 ,and yet1 among the experiments that ,fermi was trying in #aicd was that of sending his neutrons crashing into uranium atoms4 ,rutherford had no way of telling 7and neither had ,fermi7 that this1 finally1 was the route to the unimaginable4 ,,footnotes #a,the attempt to work out the structure of the nucleus resulted in a false1 but useful1 theory that persisted throughout the #aibjs4 ,the great advances in nuclear science in this decade were made in the light of this false theory and1 for the sake of historical accuracy1 they are so presented here4 ,the theory now believed correct will be presented shortly1 and you will see how matters can be changed from the earlier concept to the later one4 ,quotation ,credit ,inside front cover ,copyright by ,abelard-,shuman1 ,ltd41 ,new ,york4 ,reprinted by permission from ,inside the ,atom1 ,isaac ,asimov1 #aiff4 ,photo ,credits ,cover ,thorne ,films ,page facing inside ,the 8,horsehead0 ,nebula in ,orion1 ,hale front cover ,observatories4 ,author's ,photo ,jay ,k4 ,klein ,contents pages ,lick ,observatory #fh ,dr4 ,erwin ,w4 ,mueller1 ,the ,pennsylvania ,state ,university #gj ,yerkes ,observatory #hf ,from ,discovery of the ,elements1 ,mary ,e4 ,weeks1 ,chemical ,education ,publishing ,company1 #aifh4 #hi ,the ,central ,press ,photos1 ,ltd41 and ,sir ,john ,cockcroft #ia ,ernest ,orlando ,lawrence ,livermore ,laboratory #ic ,samuel ,a4 ,goudsmit #if #ig ,nobel ,institute #ii ,copyright #aife by ,barbara ,lovett ,cline1 reprinted from her volume ,the ,questioners3 ,physicists and the ,quantum ,theory by permission of ,thomas ,y4 ,crowell1 ,inc41 ,new ,york4 #aje #ajf ,nobel ,institute #ajg ,alan ,w4 ,richards ,u4,s4 ,,government ,,printing ,,office3 #aige----fdj--bhe_/ad ,a word about ,,erda4444 ,the mission of the ,u4 ,s4 ,energy ,research ,development ,administration 7,,erda7 is to develop all energy sources1 to make the ,nation basically self-sufficient in energy1 and to protect public health and welfare and the environment4 ,,erda programs are divided into six major categories3 ,,conservation ,,of ,,energy--,more efficient use of existing energy sources1 development of alternate fuels and engines for automobiles to reduce dependence on petroleum1 and elimination of wasteful habits of energy consumption4 ,,fossil ,,energy--,expansion of coal production and the development of technologies for converting coal to synthetic gas and liquid fuels1 improvement of oil drilling methods and of techniques for converting shale deposits to usable oil4 ,,solar1 ,,geothermal1 ,,and ,,advanced ,,energy ,,systems--,research on solar energy to heat1 cool1 and eventually electrify buildings1 on conversion of underground heat sources to gas and electricity1 and on fusion reactors for the generation of electricity4 ,,environment ,,and ,,safety--,investigation of health1 safety1 and environmental effects of the development of energy technologies1 and research on management of wastes from energy production4 ,,nuclear ,,energy--,expanding medical1 industrial and research applications and upgrading reactor technologies for the generation of electricity1 particularly using the breeder concept4 ,,national ,,security--,production and administration of nuclear materials serving both civilian and military needs4 ,,erda programs are carried out by contract and cooperation with industry1 university communities1 and other government agencies4 ,for more information1 write to ,,userda--,technical ,information ,center1 ,p4 ,o4 ,box #fb1 ,oak ,ridge1 ,tennessee #cghcj4 ,illustration3 ,,energy ,,research ,,development ,,administration ,,usa ,united ,states ,energy ,research and ,development ,administration ,office of ,public ,affairs ,washington1 ,d4,c4 #bjede ,transcriber's ,notes --,retained publication information from the printed edition3 this e,book is public-domain in the country of publication4 --,in the text version only1 underlined or italicized text is delimited by underscores4 --,where possible1 ,,utf superscript and subscript numbers are used2 some e-reader fonts may not support these characters4 --,in the text versions only1 other superscript text is preceded by caret and delimited by brackets4 --,in the text versions only1 other subscripted text is preceded by underscore and delimited by brackets4