,nuclear ,clocks ,the ,understanding the ,atom ,series ,nuclear ,energy is playing a vital role in the life of every man1 woman1 and child in the ,united ,states today4 ,in the years ahead it will affect increasingly all the peoples of the earth4 ,it is essential that all ,americans gain an understanding of this vital force if they are to discharge thoughtfully their responsibilities as citizens and if they are to realize fully the myriad benefits that nuclear energy offers them4 ,the ,united ,states ,atomic ,energy ,commission provides this booklet to help you achieve such understanding4 ,illustration3 ,signature of ,edward ,j4 ,brunenkant ,edward ,j4 ,brunenkant1 ,director ,division of ,technical ,information ,,united ,,states ,,atomic ,,energy ,,commission ,dr4 ,glenn ,t4 ,seaborg1 ,chairman ,james ,t4 ,ramey ,wilfrid ,e4 ,johnson ,francesco ,costagliola ,illustration3 ,,nuclear ,,clocks by ,henry ,faul ,,contents ,,introduction #a ,,theory ,,of ,,nuclear ,,age ,,determination #e ,,the ,,carbon-#ad ,,clock #i ,carbon-#ad ,counting #ab ,carbon-#ad ,results #ae ,,the ,,long-,,lived ,,clocks #ai ,the ,rubidium-,strontium ,clock #bj ,the ,uranium ,fission ,clock #bd ,plumbology #bg ,,the ,,age ,,of ,,the ,,earth #bg ,analytical ,techniques #ca ,minerals ,that ,can ,be ,dated #cd ,,some ,,interesting ,,results #dj ,the ,old ,man ,from ,olduvai #dj ,the ,geologic ,time ,scale #da ,precambrian ,stratigraphy #dg ,,and ,,where ,,do ,,we ,,go ,,from ,,here8 #dh ,,glossary #di ,,appendix #eb ,,suggested ,,references #eh ,united ,states ,atomic ,energy ,commission ,division of ,technical ,information ,library of ,congress ,catalog ,card ,number3 #fg-fjaie #aiff2 #aifh7,rev47 ,illustration3 ,a #ad1jjj-year-old burial site being uncovered in the area of the ,aswan ,reservoir in ,sudan4 ,to determine the age of such ancient remains1 archaeologists search for every scrap of associated wood or charcoal that could be used for age measurement of carbon-#ad1 one of the 8nuclear clocks0 described in this booklet4 ,illustration3 ,,nuclear ,,clocks ,by ,,henry ,,faul ,,introduction ,how old is a rock8 ,how old is man8 ,how old is the earth8 ,it would be difficult to find a reason why anyone would really have to know the answers to these questions1 yet they have been asked over and over again since the dawn of human society4 ,the records of every civilization disclose attempts to delve into the past beyond the memory of the oldest man1 beyond recorded history1 beyond earliest legend4 ,curiosity about the remote past may be very ancient1 but the only reliable method of measuring very long intervals of time is new4 ,the possibility of doing so became apparent only after the discovery of ,,radioactivity#a in #ahif by ,henri ,becquerel1 when ,marie and ,pierre ,curie in #ahih recognized that some atoms are radioactive and change by themselves into other atoms at regular and constant rates4 ,if something gradually transforms itself into something else1 if this transformation goes on at a known pace1 and if all the products of the activity are preserved in some kind of a ,,closed ,,system1 then it is theoretically possible to calculate the time that has elapsed since the process started4 ,the theory was clear for years2 the only problem was how to satisfy all those ifs4 ,by #aiaj it was well established that the earth must be extremely ancient4 ,analyses of some minerals containing uranium showed them to be hundreds of millions of years old1 even though the uranium came from rocks that were known to be relatively young among geologic ,,strata4 ,measurements still were inaccurate1 however1 and only a few rare and unusually rich radioactive minerals contained enough of the products of ,,radioactive ,,decay#b to allow analysis of their age by the crude methods then available4 ,not much progress was made for about #cj years until ,a4 ,o4 ,c4 ,nier1 a ,harvard ,university physicist1 perfected an instrument called a ,,mass ,,spectrometer 7to be described later7 just before ,world ,war ,,ii4 ,the rapid technological advances of the war years followed4 ,the ,manhattan ,project#c made the atomic bombs that ended the fighting2 it also developed new scientific techniques that could be applied1 when peace returned1 to the measurement of geologic time4 ,the next important advance was contributed in #aidf by ,arthur ,holmes in ,england and by ,f4 ,g4 ,houtermans in ,germany4 ,each of these scientists had seen ,nier's reports before the war and had realized that ,nier's mass-spectrometer analyses of lead made it possible1 for the first time1 to make rational calculations about the age of the earth4 ,the two scientists independently calculated that age at about #b to #c billion years1 using the handful of data available to them from ,nier's measurements4 ,it is interesting that today1 thousands of analyses later1 our planet's age usually is given as #d4e billion years4 ,the early estimates were not far off4 ,development of various methods for measuring the age of minerals followed rapidly1 and by #aiee many fundamental studies needed for measuring the age of very old substances were complete4 ,the basic techniques are summarized in ,table ,i4 ,they will be explained later4 ,the new methods produced broad confirmation of the early rough estimates and they also brought a few surprises4 ,before we go into these discoveries1 let us look at some theoretical foundations4 ,table ,i ,,basic ,,measurement ,,methods ,method ,material ,time ,dated ,useful ,time ,span 7years7 ,carbon-#ad ,wood1 peat1 ,when plant died #ajjj-ej1jjj charcoal ,bone1 shell ,slightly before #bjjj-ce1jjj animal died ,potassium-argon ,mica1 some ,when rock last #ajj1jjj and up whole rocks cooled to about #cjj;,c ,hornblende ,when rock last #aj1jjj1jjj and up ,sanidine cooled to about #ejj;,c ,rubidium-strontium ,mica ,when rock last #e1jjj1jjj and up cooled to about #cjj;,c ,potash ,when rock last #ej1jjj1jjj and up feldspar cooled to about #ejj;,c ,whole rock ,time of #ajj1jjj1jjj and up separation of the rock as a closed unit ,uranium-lead ,zircon ,when crystals #bjj1jjj1jjj and up formed ,uranium-#bch ,many ,when rock last #ajj-a1jjj1jjj1jjj fission cooled 7,depending on material7 ,illustration3 ,time scale of the ,earth1 drawn to scale4 ,see also the ,holmes time scale on page #df4 ,event ,geologic ,time ,period ,era ,man appears#d 7#b million years ago7 ,quaternary ,,cenozoic ,tertiary ,extinction of the dinosaurs 7#gj million years ago7 ,cretaceous ,,mesozoic ,mammals appear 7#acj million years ago7 ,jurassic ,triassic ,oldest known reptiles 7#cjj million years ,permian ,,paleozoic ago7 ,pennsylvanian ,mississippian ,devonian ,silurian ,ordovician ,cambrian ,first abundant life in the sea 7animals without backbones7 7#eej million years ago7 ,algae and other microorganisms 7#a1ijj ,proterozoic ,,precambrian million years ago7 ,archean ,oldest rocks in ,north ,america 7#b1hjj million years ago7 ,first hint of life 7bacteria87 7#c1ajj million years ago7 ,oldest rocks 7#c1cjj million years old7 ,formation of ,earth's core 7#d1ejj million years ago7 ,,theory ,,of ,,nuclear ,,age ,,determination ,we can think of the nucleus of an atom as a sort of drop--a bunch of ,,neutrons and ,,protons held together by very strong short-range forces4 ,these elementary particles within a nucleus are not arranged in any fixed or rigid array1 but are free to move about within the grip of these forces4 ,these motions may be quite violent1 but for most ,,nuclides found in nature1 the nuclear forces are powerful enough to keep everything confined2 thus the nuclei of these atoms hold together1 and are said to be stable4 ,if any one nucleus of a given ,,isotope is stable1 then all others are also stable1 because what is true for one atom of a given kind is true for all others of the same kind4#e ,some nuclides1 both man-made and natural1 are unstable1 however4 ,their nuclei are in such violent turmoil that the nuclear forces cannot always hold them together1 and various bits and pieces fly off4 ,if we were to try to predict when one particular unstable nucleus would thus disintegrate1 however1 we could not succeed1 because the instant any specific decay 7or disintegration7 event will occur is a matter of chance4 ,only if a large number of unstable nuclei of one kind are collected together can we say with certainty that1 out of that number1 a certain proportion will decay in a given time4 ,it turns out that this proportion is the same regardless of any external conditions4 ,this property of nuclei to decay by themselves is called radioactivity4 ,radioactive nuclei decay at constant rates regardless of temperature1 pressure1 chemical combination1 or physical state4 ,the process goes on no matter what happens to the atom4 ,in other words1 the activity inside the nucleus is in no way affected by what happens to the ,,electrons circling around it4 7,only in very special cases can outside disturbances affect the radioactivity of a nucleus and then only slightly4 ,for all practical purposes1 rates of radioactive decay are constant47 ,most radioactive nuclides have rapid rates of decay 7and lose their radioactivity in a few days1 or a few years1 at most72 most of these are known today only because they are produced artificially4 ,some of them may have been present at the time the solar system was formed1 but they have since decayed to such insignificant fractions of their original amounts that they can no longer be detected4 ,only a few radioactive nuclides decay slowly enough to have been preserved to this day1 and so are present in nature4 ,they are listed in ,table ,,ii4 ,table ,,ii ,,radioactive ,,nuclides ,,with ,,half-,,lives ,,large ,,enough ,,to ,,be ,,still ,,present ,,in ,,useful ,,amounts ,,on ,,the ,,earth#f ,,parent ,element ,,daughter ,,half-,,life 7years7 ,type of ,decay ,product ,potassium-#dj ,argon-#dj #a4c #aj 7total7 ,,electron ,,capture ,calcium-#dj ,,beta ,,decay ,vanadium-#ej ,titanium-#ej #f #aj 7total7 ,electron capture ,chromium-#ej ,beta decay ,rubidium-#hg ,strontium-#hg #d4g #aj ,beta decay ,indium-#aae ,tin-#aae #e #aj ,beta decay ,tellurium-#abc ,antimony-#abc #a4b #aj ,electron capture ,lanthanum-#ach ,barium-#ach #a4a #aj 7total7 ,electron capture ,cerium-#ach ,beta decay ,cerium-#adb ,barium-#ach #e #aj ,,alpha ,,decay ,neodymium-#add ,cerium-#adj #b4d #aj ,alpha decay ,samarium-#adg ,neodymium-#adc #a4jf #aj ,alpha decay ,samarium-#adh ,neodymium-#add #a4b #aj ,alpha decay ,samarium-#adi ,neodymium-#ade #d #aj8 ,alpha decay ,gadolinium-#aeb ,samarium-#adh #a4a #aj ,alpha decay ,dysprosium-#aef ,gadolinium-#aeb #b #aj ,alpha decay ,hafnium-#agd ,ytterbium-#agj #d4c #aj ,alpha decay ,lutetium-#agf ,hafnium-#agf #b4b #aj ,beta decay ,rhenium-#ahg ,osmium-#ahg #d #aj ,beta decay ,platinum-#aij ,osmium-#ahf #g #aj ,alpha decay ,lead-#bjd ,mercury-#bjj #a4d #aj ,alpha decay ,thorium-#bcb ,lead-#bjh #a4da #aj #f ,alpha #d beta#g ,uranium-#bce ,lead-#bjg #g4ac #aj #g ,alpha #d beta ,uranium-#bch ,lead-#bjf #d4ea #aj #h ,alpha #f beta ,in a large number of radioactive nuclei of a given kind1 a certain fraction will decay in a specific length of time4 ,let's take this fraction as one-half and measure the time it takes for half the nuclei to decay4 ,this time it is called the ,,half-,,life of that particular nucleus and there are various accurate physical ways of measuring it4 ,during the interval of one half-life1 one-half of the nuclei will decay1 during the next half-life half of what's left will decay1 and so on4 ,we may tabulate it like this3 ,elapsed time ,amount left of 7,number of what was half-lives7 originally present #a #b #c #d _/ #e _/ #f _/ #g _/ 444 444 ,in other words1 after seven half-lives1 less than #a of the original amount of material will still be radioactive and the remaining #ii of its atoms will have been converted to atoms of another nuclide4 ,this kind of process can be made the basis of a clock4 ,it works1 in effect1 like the upper chamber of an hourglass4 ,mathematically it is written3 ,n ,ne-t where ,n the number of radioactive atoms present in the system now1 ,n the number that was present when t #j1 7in other words1 at the time the clock started71 e the base of natural 7or ,napierian#h7 logarithms 7the numerical value of e #b4gah 44471 7lambda7 the decay rate of the radioactive material1 expressed in atoms decaying per atom per unit of time1 t the time that has elapsed since the origin of system1 expressed in the same units4 ,obviously1 in ordinary computations that would not be enough information to calculate the time1 because there still are two unknowns1 ,n and t4 ,in a closed system1 however1 the atoms that have decayed do not disappear into thin air4 ,they merely change into other atoms1 called daughter atoms1 and remain in the system4 ,and at any point in time1 there will be both ,,parent and ,,daughter atoms mixed together in the material4 ,the older the material1 the more daughters and the fewer parents4 ,some daughters are also radioactive1 but this does not change the basic situation4 ,thus it follows that ,n ,n ,d where ,d the number of daughter 7decayed7 atoms4 ,we may then substitute into the first equation ,n 7,n ,d7 e-t and solve t #a_/ ln7#a ,d_/,n7 where ln the natural logarithm1 the logarithm to base e4 ,this kind of system can be represented crudely by an old-fashioned hourglass1 as shown in the figure1 which has the parameters of these equations marked4 7,keep in mind1 however1 that this is only a gross analogy4 ,nuclear clocks run at logarithmically decreasing rates1 but the speed of a good hourglass is roughly constant47 ,illustration3 ,an hourglass illustrates an ideal closed system4 ,nothing is added and nothing is removed--the sand just runs from the top bulb to the bottom4 ,remember that the decaying nucleus does not disappear4 ,it changes into another nucleus1 and this new nucleus forms an atom that may be captured and held fixed by natural processes4 ,the decayed nuclei are thus collected1 so that here we have the bottom chamber of the hourglass4 ,but sometimes we need only the top chamber of an hourglass4 ,,the ,,carbon-#ad ,,clock ,carbon-#ad decay is the best example of a top-only hourglass4 ,carbon-#ad is constantly being produced in the upper atmosphere from atoms of nitrogen-#ad being struck by neutrons that had their origin in ,,cosmic ,,rays4 ,the reaction is written3 ,n neutron ,c proton ,radioactive decay then follows1 with a half-life of #ehjj years#i for the ,c4 ,c ,n electron 7,,beta ,,particle7 ,the radiocarbon emits an electron and changes back into nitrogen4 ,as far as anyone can tell1 ,c was produced at a constant rate above the earth for at least #ej1jjj years before the first atomic bomb was exploded4 ,in other words1 the ,c cycle is like an hourglass in which the sand in the upper part is replenished as fast as it runs out through the hole in the waist4 ,a process of this sort1 where production equals decay1 is called a ,,secular ,,equilibrium4 ,the newly produced ,c soon is evenly mixed with the carbon dioxide in the air1 is taken up by all living plants1 and then finds its way into all living animals4 ,in effect1 all carbon in living organisms contains a constant proportion of ,c4 ,if any of this carbon is taken out of circulation--when a tree branch is broken off1 for instance1 or when a shellfish dies in the ocean--no more new ,c is added to that particular system1 but the old ,c continues to run out4 ,in effect it now starts measuring time as an hourglass should4 ,illustration3 ,to illustrate secular equilibrium1 one must imagine an hourglass in which the sand in the top bulb is continuously replenished--as fast as it runs out through the hole in the waist and disappears4 ,when we find a piece of charcoal in a cave or a piece of wood in some ancient structure1 for example1 we can measure the amount of carbon in it1 determine how much of it is ,c1 and then calculate back to the time when the radioactivity from the ,c was the same as we now find in living wood4 ,in other words1 if we assume that we know from the observed secular equilibrium how much ,c originally was present in living material1 then we can calculate the time of death of any similar but ancient material4 ,that is the basis of the ,c method of age determination4 ,illustration3 ,dinosaur tracks imprinted in rock in ,navajo ,canyon1 ,arizona1 arouse the professional interest of this scientist4 ,fossil traces of extinct prehistoric creatures were for a long time the best clues to the age of rock formations4 ,illustration3 ,a scientist using liquid nitrogen to freeze carbon dioxide gas made from a sample of ancient material that he is preparing for age determination by the carbon-#ad technique4 ,for example1 a bit of a rafter from a prehistoric cliff-dwelling or a remnant of charcoal from an ancient fire may be analyzed for its remaining ,c content1 and its age determined accurately within the margin of a few hundred years4 ,this fixes the time at which the wood for the rafter or the firewood was broken or cut from the living tree1 and hence the period in which the men lived who used the wood4 ,illustration3 ,the most useful samples for carbon-#ad age determination are charcoal1 wood1 and shells4 ,carbon-#ad ,counting ,carbon-#ad measurements are made by taking a known amount of carbon1 reducing it to a gas1 and then counting the ,c disintegrations in the gas4 ,this may sound simple1 but in reality the measurement process is a formidable undertaking1 because the amount of the ,c isotope in the carbon is so extremely small4 7,the remainder of the carbon1 of course1 consists of other isotopes--,c or ,c1 which are stable47 ,there are two basic techniques4 ,the carbon can be3 #a4 ,burned with oxygen to form carbon dioxide1 or #b4 ,reduced chemically to methane or ethane1 or to a carbide from which acetylene can be evolved by adding water4 7,see booklet cover and description on page #ei47 ,the first technique is the simpler1 but carbon dioxide 7,,co7 contains only one atom of carbon per molecule1 whereas acetylene 7,c,h7 and ethane 7,c,h7 each contain two4 ,consequently1 the ,,specific ,,activity of acetylene or ethane is twice that of carbon dioxide1 other things being equal4 ,for that reason acetylene or ethane are the preferred gases in some laboratories4 ,on the other hand1 they are explosive1 and that cautions other scientists into using the carbon dioxide method4 ,whichever gas is used1 it is first purified and then stored in a bottle for a month or so4 ,this storage allows for decay 7disappearance7 of any radon1 the gaseous radioactive product of uranium decay4 ,uranium contamination is difficult to avoid at the low radioactivity levels of ,c1 but the half-life of radon is only #c4hb days1 so that it will decay to an insignificant level in a month4 ,after the storage period1 the gas is pumped into an array of instruments known as a low-background ,,proportional ,,counter1 and its radioactivity is determined4 ,this is an involved process4 ,first there is the matter of the ,,background ,,count4 ,the background count of an instrument is the number of pulses 7counts7 it will give per unit time when there is no radioactive sample in it4 ,these counts are caused by cosmic rays1 by radioactive contamination always found in the vicinity of the counter1 or by any contamination inside the instrument4 ,in ,c counting1 all these sources of background must be reduced to negligible levels4 ,this is done in a number of ways4 ,for one thing1 the whole assembly is constructed with surrounding walls of lead or iron more than a foot thick4 ,such a shield will stop all the ,,gamma ,,rays coming from radioactive contamination in the laboratory and much of the cosmic radiation2 high-energy cosmic rays and all neutrons still will get through4 ,therefore1 there is an ,,anticoincidence ,,ring inside the lead shield4 ,this is a cylindrical space completely surrounded by ,,geiger ,,counters that are connected to each other and to the ,,sample ,,counter in the middle4 ,with this arrangement1 when the sample counter and any ring counter discharge simultaneously1 it is a signal that the pulse triggering this response was caused by some energetic particles1 such as a cosmic ray1 passing through the whole assembly4 ,a pulse recorded simultaneously on two counters is automatically rejected from the counting mechanisms4 ,some instruments have been designed with a cylinder of paraffin immediately inside the anticoincidence ring1 to slow down neutrons so that they can be captured1 and with a final shield of highly purified mercury between two cylinders of selected steel to hold out even more unwanted radiation4 ,illustration3 ,illustration3 ,a carbon-#ad counter4 ,sketch 7above7 shows arrangement of components in photo 7left74 ,finally1 the sample counters are made of specially selected metal tubing that is extremely low in radioactive content1 with a fine wire stretched down the middle4 7,in some recent designs1 the anticoincidence ring and sample counter are combined in a single cylindrical housing with a thin foil of metallized plastic between them47 ,a thin glass filling tube connects the sample counter with the outside world4 ,illustration3 ,detail of one type of sample-counting tube for carbon-#ad work4 ,carbon-#ad in benzene gas molecules is placed in the central cell and mixed with a fluid that scintillates1 or emits light flashes1 when exposed to radiation4 ,photomultiplier tubes convert the flashes to electric signals4 ,the radiocarbon-bearing gas is pumped into the sample counter through the filling tube1 and all the counts resulting from its disintegrations are recorded electronically4 ,the age of the sample is calculated from the ,,net ,,counting ,,rate 7the sample counting rate minus the background72 the lower the counting rate the higher the age4 ,the upper limit of the age that can be measured is determined by the ,,statistical ,,error 7that is1 by the measure of the instrument accuracy7 in the net count4 ,in very old samples this error may be great enough so that the calculated age of the sample may have little or no meaning4 ,carbon-#ad ,results ,,objects ,,dated ,,by ,,radiocarbon ,illustration3 ,hair of an ,egyptian woman4 #ejbj #bij years old4 ,illustration3 ,linen wrapping from the ,dead ,sea ,scroll containing the ,book of ,isaiah4 #aiag #bjj years old4 ,illustration3 ,peruvian rope4 #bfcb #bjj years old4 ,illustration3 ,preglacial wood found in ,ohio4 ,more than #bj1jjj years old4 ,illustration3 ,rope sandal found in an eastern ,oregon cave4 ,one of a pair of #cjj pairs found in this cave4 #ijce #cbe years old4 ,carbon-#ad is by far the most widely used method of measuring geologic time4 ,it has become the mainstay of archeology and geology for studies of events of the past #ej1jjj years or so1 and also has wide applications in climatology1 ecology1 and geography4 ,it would be difficult to pick out the most significant example of the use of this method1 but one important contribution has been in study of the early inhabitants of ,north ,america4 ,with the aid of ,c it has been possible to date human living sites from many points in the western ,united ,states4 ,the first appearance of these sites1 about #aa1ejj years ago1 apparently coincided with the time when a land bridge was open from ,asia to ,america over what is now the ,bering ,strait4 ,an ice-free passage extended from this bridge through present-day ,alaska and western ,canada to the ,united ,states4 ,this may have been the route taken by the first immigrants to ,america--a population of mammoth-hunters1 who made the characteristic flint ,clovis arrow and spear points4 ,illustration3 ,a ,clovis arrow point chipped from flint by the earliest men on the ,american continent4 ,the photograph is actual size4 ,by about #aa1jjj years ago1 these ,clovis people had spread across the area of the ,united ,states and into ,mexico4 ,it may have been they who killed off the mammoths and then gradually assumed the characteristics of the ,folsom culture4 ,the ,folsom people were bison-hunters1 and long were thought to have been the first population in ,america4 ,it was with the use of ,c that it finally was possible to place these two cultures in proper sequence--the ,clovis first--and to correlate them with major natural changes1 especially the advance and retreat of glaciers across the continent4 ,,the ,,long-,,lived ,,clocks ,all other practical age-determination schemes are based on a few long-lived isotopes1 with half-lives relatively near the age of the earth 7#d4e ,,aeons74 ,they are3 ,table ,,iii ,isotope ,emits ,decays to ,half-life ,uranium-#bch #h ,,alpha ,,particles#aj ,lead-#bjf #d4ea aeons ,uranium-#bch ,spontaneous fission #b ,fragments #aj million aeons#aa ,uranium-#bce #g ,alpha particles ,lead-#bjg #j4gac aeons ,thorium-#bcb #f ,alpha particles ,lead-#bjh #ad4a aeons ,rubidium-#hg ,beta particle ,strontium-#hg #d4g aeons ,potassium-#dj ,electron capture ,argon-#dj #a4c aeons 444444 444444 444444 444444 ,rhenium-#ahgab ,beta particle ,osmium-#ahg #dj aeons ,it is apparent that ,table ,,ii on page #f1 showing the long-lived radioactive nuclides1 is much longer than the list of the seven shown here that are actually useful in practice4 ,some of the nuclides that are theoretically available are useless on a practical basis1 because they are so rare in nature4 ,many others cannot be used for reasons that are fundamental to the whole process of nuclear age determination by 8whole hourglass0 7that is1 parent-daughter7 methods4 ,let's look at these reasons4 ,these methods are based on closed systems in which the daughter products of the radioactive decay are locked with the parent material from the beginning of the system1 and nothing is added or removed thereafter4 ,to state it in terms of our analogy1 the hourglass must be in perfect working order--no leaks or cracks permitted4 ,there is another fundamental requirement3 ,at the beginning1 the bottom part of the hourglass must be empty4 ,if some sand were already in the bottom at the start1 we would mistakenly be led to conclude that the time elapsed was longer than it actually was4 ,that necessity places a severe limitation on the type of system we can use4 ,consider1 for example1 the decay of potassium-#dj into calcium-#dj4 ,measuring this process is perfectly suitable from the point of view of half-life1 but the daughter product is identical with the most common isotope of ordinary calcium4 ,and calcium is present everywhere in nature6 ,even the purest mineral of potassium1 sylvite 7the salt1 potassium chloride71 contains so much calcium impurity that the ,,radiogenic daughter calcium1 produced by the decay of potassium in geologic time1 is negligible in comparison4 ,we can say that the bottom of this potassium-#dj hourglass has been stuffed with so much sand from the very beginning that the few grains that fall through the waist are lost in the overall mass4 ,this demonstrates that schemes involving the decay of a relatively rare nuclide into a relatively common one are not usable4 ,natural geochemical separations of elements are never perfect1 anyway4 ,similarly1 the decay of any of the ,,rare ,,earth elements into other rare earth elements is not particularly helpful1 because the rare earths are so similar chemically they tend to travel together when they move in nature4#ac ,wherever the parent isotope goes1 the daughter tags along4 ,the ,rubidium-,strontium ,clock ,the decay of rubidium-#hg 7,rb7 into strontium-#hg 7,sr7 is perhaps the most useful scheme for geologic age determination4 ,the same problem shows up here1 but at least there is a way out of the wilderness4 ,it is not exactly simple1 but a consideration of it is fundamental to understanding the process of nuclear dating4 ,the figure shows patterns from mass spectrometer charts2 each peak represents an isotope of strontium1 and the height of every peak is proportional to the relative abundance of that isotope4 ,in the figure1 ,a shows the mass-spectrum of a rock or mineral containing ,,common strontium 7which is a mixture of several isotopes74 ,the peak of ,sr is small compared to the others4 ,b shows the mass-spectrum of strontium from an old rubidium-rich mineral ,,crystal1 drawn to the same scale1 as far as the nonradiogenic isotopes1 ,sr1 ,sr1 and ,sr1 are concerned4 ,the ,sr peak in this spectrum is obviously larger than in the common strontium in ,a4 ,this is because this isotope is radiogenic and has been accumulating from the decay of rubidium since this crystal was formed4 ,the question we must answer is3 ,how much of this ,sr was formed from ,rb decay and how much originally was present in the crystal as an impurity8 ,if the amount of this ,,original strontium is not too large1 the problem can be solved by simple arithmetic4 ,first1 we must find a good sample of common strontium--that is1 ordinary strontium1 the kind shown at left in the figure4 ,we cannot require that this strontium be entirely uncontaminated by radiogenic strontium1 because all strontium is more or less contaminated4 ,what we need is strontium contaminated to just the same extent as the strontium that was taken as an impurity into the closed system when it first formed4 ,in geological specimens such a material is usually available4 ,illustration3 ,drawings of mass-spectrometer charts showing the isotopic spectra of two kinds of strontium3 7,a7 common strontium and 7,b7 strontium from an old mineral rich in rubidium4 7,see page #cb for photo of a mass spectrometer47 ,illustration3 ,rubidium-#hg and strontium-#hg fall on the same spot in the mass spectrum4 ,therefore1 rubidium must be separated chemically from strontium before the strontium can be analyzed in a mass spectrometer4 ,it is done with ion-exchange columns4 ,four of them are shown in this photograph4 ,the author of this booklet is adding a sample1 dissolved in a few drops of hydrochloric acid1 to the second column4 ,let us take as our closed system a mica crystal in a mass of granite4 ,mica contains a fair amount of rubidium1 and it retains its radiogenic strontium very well4 ,furthermore1 mica crystals are often associated or even intergrown with the slender1 rod-shaped crystals of a mineral called apatite--a phosphate of calcium4 ,it is justifiable1 on the basis of geological knowledge1 to say that the mica and the apatite grew at roughly the same time and thus presumably from the same liquid medium that became granite when it later solidified4 ,now strontium is geochemically similar to calcium1 and some strontium will have gone into the apatite crystal in place of calcium4 ,apatite contains no alkalis--hence apatite will have virtually no rubidium 7which is an alkali7 in it to contaminate the ,sr4 ,consequently1 when we find apatite in an old granite1 we know the apatite will still contain the kind of common strontium that was taken into the mica crystal when it grew originally4 ,we can separate the apatite from the granite by standard mineralogical techniques1 extract the strontium from the apatite chemically1 and analyze it on a mass spectrometer to obtain the isotopic spectrum--the relative amount of each isotope that is present4 ,we can then perform the same isotopic analysis on the strontium extracted from the mica1 and subtract the original 7apatite7 strontium from the total 7mica7 strontium1 to obtain the radiogenic component or daughter product4 7,see page #cb for details of this method47 ,obviously1 there is a certain error associated with every isotopic analysis1 so such a calculation is meaningful only when the radiogenic component is large compared with the error in the measurement of isotopic abundance4 ,when one large quantity must be subtracted from another large quantity to obtain a small difference1 there is an obvious limit to how much one can trust the result4 ,the absolute accuracy in measuring strontium isotope abundance is a few tenths of #a1 using the best mass spectrometers now available4 ,in practice1 one can trust a calculated age for a specimen only when the ,sr is as little as about #e radiogenic4 ,the results do not mean much when only #a or #b is radiogenic4 ,illustration3 ,a sample of granite being made ready for crushing and mineral separation4 ,the ,uranium ,fission ,clock ,when a neutron strikes the nucleus of uranium-#bce 7,u7 or plutonium-#bci 7,pu71 it may cause the nucleus to split into two roughly equal fragments1 releasing neutrons and energy4 ,this is the well-known process of neutron-induced fission1 the method in which nuclear energy is produced in both reactors and bombs4#ad ,the most common uranium isotope1 ,u1 also breaks up by fission1 but does so all by itself1 without the need for any external neutrons4 ,that process is spontaneous fission and it goes on at random1 very much like radioactive decay4 ,it is a relatively rare process and the fission half-life is long--about #aj million aeons 7#aj years74 ,that means that only about one spontaneous fission occurs in uranium-#bch for every #b million alpha decays4 ,that is enough to make a useful clock1 however1 because ,u is present almost everywhere4 7,see ,table ,,iii on page #ai47 ,imagine an atom of ,u in some mineral4 ,when the atom suddenly fissions1 it breaks in two with considerable energy1 and the two fission fragments rip like cannon balls through the surrounding crystalline structure in opposite directions1 creating havoc along the way4 ,they travel a distance something like #aj microns 7#d millionths of an inch7 before they are finally slowed down and stopped by all their collisions with other atoms4 ,each fragment's path remains behind as an intensely damaged tube through the crystal4 ,the process was known for a long time before anyone was able to find these fission tracks 7the damaged tubes7 in the crystals4 ,finally1 about #aifj1 three young physicists1 ,r4 ,l4 ,fleischer1 ,p4 ,b4 ,price1 and ,r4 ,m4 ,walker1 working at the ,general ,electric ,research ,laboratory1 fell upon the idea of etching freshly broken surfaces of crystals with acid4 ,they reasoned that a region so intensely disturbed by the passage of a fission fragment should be etched more easily and deeply than the undisturbed surrounding crystal4 ,that idea turned out to be correct1 and fission tracks have now been found in almost every common mineral 7since almost all minerals contain small amounts of uranium74 ,illustration3 ,tracks of uranium fission from a fossil antelope bone fragment from ,hopefield1 ,cape ,province1 ,south ,africa4 ,the fission clock method works this way3 ,a cleavage face or a polished surface of a crystal or glass fragment is etched with a suitable solvent4 ,different acids work best for different materials1 and a suitable procedure must be developed especially for each substance4 ,the etching brings out the fission tracks so they can be seen 7usually as little conical pits7 and counted under a microscope4 ,after this1 the sample is exposed to a known amount of slow neutrons#ae in a nuclear reactor4 ,new fissions are produced1 but this time only in ,u 7which is present in all natural uranium in the proportion of #a atom of ,u to #acg4g atoms of ,u71 because slow neutrons do not produce fissions in ,u4 ,after the neutron irradiation1 the same surface is etched again1 and the new tracks counted4 ,the old tracks1 having been etched twice1 now appear larger and thus can be distinguished from the new ones that were caused by ,u fission4 ,the rate at which ,u decays by fission1 f1 is known1 as are the rate it decays by alpha decay1 1 and the total number of slow neutrons1 n1 to which the sample was exposed in the reactor4 ,the age of the crystal or glass can then be calculated3 t 7#a7_/77 ln 7#a 7n,ns7_/7,ni7 constant7 where ln the natural logarithm 7log to the base e71 ,ns the number of atoms in the sample4 ,ni the number of atoms of ,u in the irradiated sample1 and the constant has the value3 7 #ehb #aj7_/7;f #acg4g7 #d4be #aj ,fission-track dating is a brand new technique1 still only partly developed4 ,it has enormous range and is applicable to numerous minerals2 these advantages imply that it is likely to become very useful4 ,illustration3 ,an atomic absorption spectrophotometer is used to measure the amount of potassium in samples of mica dissolved in acid4 ,plumbology ,the most complicated and therefore probably the most interesting decay scheme of all is the decay of uranium to lead1 discovered well over half a century ago and still intensively studied4 ,there are several reasons for the interest4 ,first1 uranium and lead are geochemically separated to a high degree1 not only on the small scale of an ore deposit but also on the scale of the earth as a whole4 ,second1 natural uranium has two isotopes with half-lives that are neither too long nor too short to be useful 7the greater half-life almost exactly equaling the age of the earth71 and these half-lives differ from each other by a factor of about #f4c4 ,that leads to very important consequences1 as we shall see4 ,third1 uranium and lead are both common1 and techniques are available for extracting them in measurable quantities from almost any natural material4 ,as a consequence of these happy circumstances1 the study of uranium and lead has contributed a great deal to understanding the earth's history and the processes that go on inside it4 ,f4 ,g4 ,houtermans1 one of the great pioneers in this study1 jokingly called the method ,,plumbology1 and it seems a useful name4 ,,the ,,age ,,of ,,the ,,earth ,the greatest achievement of the 8plumbologists0 has been the calculation of the age of the earth1 first proposed by ,houtermans1 a ,german physicist1 and independently by ,arthur ,holmes1 a ,british geologist1 in #aidf and finally perfected by ,c4 ,c4 ,patterson in #aiec4 ,it is actually a rather simple calculation1 although the way to discovering it was far from easy4 ,before we look at it in detail1 however1 let's consider some basic assumptions and explain what is meant by 8the age of the earth04 ,from studying the mechanics of the solar system1 scientists have become reasonably certain that the earth and the other planets and their satellites all were formed in a common process in a relatively short period of time1 geologically speaking4 ,perhaps it took a dozen million years or so1 but compared to the time that has elapsed since1 that is a twinkling4 ,at some time soon afterwards1 the earth became molten1 or at any rate fluid enough to allow much of its iron to settle toward the center to form the earth's core4 ,similar cores presumably formed in other planets4 ,as the iron went down1 it took some lead with it1 and as the silica went up1 uranium followed it toward the surface1 because of the chemical affinity between these kinds of elements4 ,in the present earth1 we have found1 almost all the uranium is concentrated in the top layer1 or crust1 which is only about #be miles thick under the continents and even thinner under the oceans4 ,illustration3 ,internal structure of the earth4 ,the central core is probably an alloy of iron and nickel1 surrounded by a mantle of less dense silicate material1 with a thin crust of still lighter silicates4 ,the time of this early and relatively rapid separation of uranium and lead on a worldwide scale is the event that plumbologists can determine1 and the period since then is what they mean by 8the age of the earth04 ,when ,houtermans first wrote about it1 he called it 8the age of uranium04 ,how is this done8 ,we have said that one of the isotopes of uranium1 ,u1 decays faster--about #f4c times faster--than the other1 ,u4 ,they decay into two different isotopes of lead4 ,therefore1 if we can determine the isotopic composition of average ordinary lead in the earth's crust today1 and if we can somehow obtain a sample of the kind of lead that is locked in the earth's core1 we can calculate how long it took to change the ,,primordial lead 7like that in the core7 into present-day lead in the crust by the gradual addition of radiogenic lead--lead that has resulted from the decay of uranium4 ,now1 someone might logically ask1 8,isn't it necessary to know also the actual amount of uranium involved in the process1 and isn't this difficult to determine80 ,it turns out to be a remarkable aspect of the ,holmes-,houtermans calculation that the uranium-concentration terms cancel out in the equations and only the ratio of the isotopes and their decay constants need be considered4 ,these are all known accurately4 ,next1 we must decide just what is average present-day lead8 ,it isn't enough to go to a lead mine and get a sample1 because1 unfortunately1 leads from different mines have widely varied isotopic composition--that is1 a different mixture of four natural isotopes1 ,pb1 ,pb1 ,pb1 and ,pb--as a result of their geologic histories4 ,no1 lead samples from a mine won't do4 ,however1 geologists have been able to separate lead from recent marine sediments1 obtained from the ocean bottom1 far from land4 ,these are of uniform composition1 and are good samples of what the world's rivers bring into the ocean4 ,other useful samples can be found in plateau basalts1 which are enormous bodies of dark volcanic rock that make up the bedrock in many parts of the world4 ,the lead from these basalts is isotopically very much like the lead in the oceans4 ,very well1 but how about the lead from the core8 ,where can we hope to find a sample of it8 ,it turns out to be easier than you might think4 ,astronomers believe it highly probable that most meteorites are fragments of a former planet that broke up for reasons that are not entirely clear4 ,it is pretty definite1 however1 that this protoplanet 7or these protoplanets1 for there may have been more than one7 had an iron core1 and this core 7or these cores7 is the source of the iron meteorites sailing around in space4 ,a large meteorite hit the earth not too long ago 7geologically speaking7 and caused the ,meteor ,crater near ,canyon ,diablo in ,arizona4 ,illustration3 ,examining ocean-bottom sediments obtained by lowering a tube-like instrument that brings up a long rod-shaped 8core01 prior to nuclear age determination of the sample4 ,many fragments of the meteorite iron have been found around the crater1 and it is reasonable to assume that this is the kind of iron we would expect to find in the core of the earth4 ,like the core iron1 it is mixed with a little lead1 which can be isolated and analyzed in a mass spectrometer for its isotopic composition4 ,this lead is found to be much less contaminated with radiogenic lead1 and hence is much more primitive than the oldest leads found on earth4 ,thus1 meteorites presumably are as close as we can get to true primordial lead--the lead of the time when the earth 7and the protoplanet7 first formed4 ,once these measurements were available1 it was easy to write the ,houtermans equation for present-day and primordial leads in this way3 77,pb7_/7,pb77 present - 77,pb7_/7,pb77 primordial ------------------------ -------------------------- 77,pb7_/7,pb77 present - 77,pb7_/7,pb77 primordial #acg4g 77e#bcht - #a77_/77;e#bcet - #a77 ,the present ratio of ,u to ,u is #acg4g4 e the base of natural logarithms the ,,decay ,,constant of each isotope of uranium t the age of the earth4 ,substituting the best experimental lead isotope ratios into the equation and solving for t1 ,patterson was able to calculate that the earth is #deej million years 7#d4ee aeons7 old4 ,subsequent calculations based on other procedures generally have confirmed that result4 ,analytical ,techniques ,each method of nuclear age determination involves a different sequence of sample preparation4 ,wood1 peat1 charcoal1 bones1 or shells are cleaned for carbon-#ad dating in order to remove every trace of possible contamination by modern carbon as well as extraneous old carbon4 ,rocks are crushed and ground1 minerals are separated according to what is needed in any particular study1 and the desired elements are extracted and separated by chemical procedures4 ,often there may be several different ways of doing the same thing2 different laboratories use different procedures4 ,in every case1 however1 long and complicated procedures must be followed before results are obtained from which an age can be calculated4 ,there is no such thing as a black box into which you can throw a rock and read its age on a dial6 ,of all the elements that are part of the useful parent-daughter systems1 only potassium is common enough to be analyzed by conventional chemical techniques4 ,all the other elements1 especially the radiogenic ones1 are present in such small quantities that special processes had to be developed to measure them4 ,the most valuable and generally used process is called ,,isotope ,,dilution4 ,isotope ,dilution ,this is a process for analyzing an unknown material by incorporating uniformly into it a small amount of a radioactive test substance and determining how much the tracer radioactivity is altered by dilution in the original material4 ,it works like this3 ,let's say that we have an unknown number of atoms1 x1 of a given element4 ,the normal isotopic composition of this element is accurately known1 as it is for most elements1 and the ratio of two of its isotopes can be expressed as ,a_/,b4 ,we now add to x a known 7but usually smaller7 amount1 c1 of the same element4 ,this quantity has a drastically different isotopic ratio1 ,a_/,b4 ,we mix x and c thoroughly together4 ,the ratio ,a_/,b can have almost any value1 but must be different from ,a_/,b and we must know exactly what it is4 7,there are many ways of determining this chemically1 or we can use a sample isotope of known composition obtained from the ,u4 ,s4 ,atomic ,energy ,commission's ,oak ,ridge ,national ,laboratory at ,oak ,ridge1 ,tennessee47 ,the substance added is known colloquially as the ,,spike4 ,after the original material and the spike are thoroughly mixed we have3 x7,a_/,b7 c7,a_/,b7 7x c7 7,a,b7 in which ,a_/,b will be the ratio of the two isotopes in the mixture4 ,with this information in hand1 we can perform any chemical purification or transfer process with the material 7see photo on page #bb71 without having to worry about loss4 7,even if #ij of the material should be lost in some operation1 the isotopic composition would not be changed1 and that is all we are interested in47 ,now we can place the material containing the isotopic mixture in a ,,mass ,,spectrometer1 which will determine the ratio ,a_/,b4 ,when we have that1 we may substitute the value of ,a_/,b in the equation and quickly calculate x1 the unknown concentration of atoms in the original sample4 ,illustration3 ,a large 7#ab-;inch7 mass spectrometer 7at left7 in use4 ,electronic equipment 7right7 charts results 7see page #ba74 ,illustration3 ,essential parts of a mass spectrometer4 ,atoms to be analyzed are changed to ions in the source4 ,then the ions are accelerated by high voltage1 deflected in a magnetic field according to their mass1 and the intensity of the separated beams is measured in the collector4 ,mass ,spectrometry ,the mass spectrometer measures isotopic abundances using a magnetic field to sort electrically charged particles into groups according to their masses4 ,it works this way3 ,a small drop of material to be analyzed is placed on a metal filament and dried4 ,the filament1 in its holder1 is placed inside the mass spectrometer1 and heated electrically in a vacuum1 like the filament in a light bulb4 ,as the wire begins to glow1 some of the sample begins to radiate1 or 8boil off01 losing an electron or two in the process4 ,in other words1 some of the atoms will be changed into positive ,,ions4 ,an alternative method is to introduce the sample material into the vacuum chamber in the form of a gas 7like argon1 for example71 and then bombard the gas with electrons streaming from a hot filament4 ,the electron stream will knock some electrons off the gas molecules and this also will produce positive ions4 ,either process of ion production is satisfactory1 depending on the problem to be tackled1 but the mass spectrometers for the two methods are naturally quite different4 ,whichever way the ions were produced1 they are next exposed to a strong electric field1 accelerated1 and electrostatically focused into a beam4 ,these charged particles are directed into a magnetic field between the pole faces of an electromagnet4 ,the magnet does the analyzing by the principle of magnetic deflection that was known to ,andr ,ampere and ,michael ,faraday more than a century ago4 ,any moving electric charge has a magnetic field associated with it4 ,this field interacts with the field of the analyzing magnet to impress a deflecting force on the charge4 ,the force acts at right angles to the direction the charge travels and also at right angles to the direction of the impressed magnetic field4 ,the pull of this force depends only on the electric charge and the speed of each particle3 ,a light single-charged particle will be deflected more than a heavier particle with the same charge4 ,in this way1 the ions in the beam are sorted out into a number of separate beams1 each made up of particles of the same charge_/mass ratio4 ,each beam contains one isotope of the original material1 because isotopes differ on the basis of their mass4 ,by adjusting the current in the electromagnet we can direct these separate beams into a 8collector0 and electrically measure their intensity one by one4 ,this gives the relative abundance of the separate isotopes in the sample4 ,minerals ,that ,can ,be ,dated ,measuring age by one of the long-lived radioisotopes requires a closed system4 ,usually this is some kind of crystal formed in a period of time that is short1 compared to the time that has elapsed since1 and that has remained unchanged since it formed4 ,specifically1 neither the parent isotopes can have been added nor the daughter isotopes removed by any process other than radioactive decay4 ,the earth is a dynamic system1 however4 ,things are always changing and moving--not very rapidly1 perhaps1 but fast enough1 in geologic time1 to raise mountains and shift oceans4 ,solutions are moving around1 dissolving something here and depositing it again somewhere else4 ,temperatures are changing as one place is denuded by erosion and another area buried under layers of sediment4 ,under such conditions1 few systems remain closed4 ,it is perhaps surprising that we find any closed systems at all4 ,let us look at a few that are known to be reliable4 7,they are listed in ,table ,i on page #d47 ,potash ,feldspar ,in the early #aiejs1 when the potassium-argon 7parent-daughter7 method was being developed by scientists at the ,university of ,chicago1 it was thought that the potash-bearing variety of the mineral feldspar would be an ideal closed system1 because it was usually optically clear and free of flaws4 ,this widely shared1 logical1 and perfectly scientific deduction soon turned out to be quite wrong4 ,the scientific workers discovered that when feldspar and mica from the same rock 7and thus of the same age7 were analyzed side by side1 the mica always came out older6 ,investigation showed that feldspar 8leaked0 argon 7lost some of its radiogenic argon7 even at room temperature1 but the mica retained all or nearly all of the argon that had been generated in it4 ,mica ,with the development of the rubidium-strontium 7parent-daughter7 method by ,l4 ,t4 ,aldrich and his co-workers at the ,carnegie ,institution of ,washington1 came the realization that mica was also very useful for this analysis1 for it usually contains ample rubidium and not much original strontium that would mask the presence of the radiogenic strontium4 ,as a result1 mica1 especially black mica 7the mineral biotite71 has enjoyed great popularity as a good and easy-to-find closed system4 ,illustration3 ,a scientist making adjustments on an 8argon train01 a maze of glass tubing in which argon is released from minerals and purified for analysis4 ,everything has its limits1 and mica is no exception3 ,even mica tends to leak argon at elevated1 but still relatively low 7geologically speaking71 temperatures4 ,these effects also depend on pressure and other factors1 not all of which are well known2 these elevated temperatures1 pressures and other conditions of course act to some extent on all rocks buried in the earth's crust4 ,it is known that at only about #cjj;,c at moderate pressures argon is leaked from mica faster than it is being generated in it by the decay of radioactive potassium4 ,the temperature needed to cause the rapid loss of strontium from mica is not much higher4 ,mica1 especially biotite1 will recrystallize and lose all its radiogenic constituents 7argon and strontium7 at temperatures where many other minerals show little or no change4 ,that means that we cannot always rely on mica to give the date of the original crystallization of a rock--the time when it cooled from a molten state4 ,instead1 mica will tell us when the rock last cooled from1 say1 several hundred degrees centigrade1 regardless of what may have happened to the rock before that4 ,the mica may have been reheated as a result of being buried under a few miles of sediment1 for example4 ,the mica will show when the rock last cooled--in other words1 when it came up again4 ,low-,strontium ,feldspar ,in spite of early disappointments with potash feldspar for argon dating1 some of it is useful for rubidium-strontium procedures4 ,it all depends on how much original strontium the potash feldspar contains4 ,most feldspars1 unfortunately1 contain far too much1 but rapid screening by ,x-ray fluorescence or flame photometry methods can weed these out and identify specimens low enough in original strontium to be useful4 ,otherwise1 feldspar is an excellent closed system for rubidium and strontium2 it remains closed even at temperatures high enough to melt many other minerals4 ,it is not affected at all by the same degree of heating that will drive argon out of biotite4 ,the rubidium-strontium age of feldspar usually comes close to the time of original crystallization of the rock4 ,obviously1 here is a geologically important tool4 ,if we find feldspar and biotite in one rock1 and if feldspar1 tested by the rubidium-strontium method gives the same age as biotite tested by potassium-argon decay1 then we can say with confidence that the rock has not been reheated since shortly after it crystallized4 ,conversely1 if the biotite comes out much younger than the feldspar1 we can be sure that something has happened to this rock long after it first crystallized4 ,such information is not only valuable to pure science--it can also be useful in locating areas favorable for ore prospecting and in other practical ways4 ,zircon ,another very interesting mineral is zircon 7a silicate of zirconium71 one of the accessory minerals found in small quantities in many crystalline rocks4 ,zircon usually occurs in very small grains and is heavy and hard1 so that it can be separated from the other rock without much difficulty1 even though it may take #ajj pounds of rock to supply a gram of zircon4 ,zircon usually contains a fair amount of uranium and very little lead4 ,it holds radiogenically produced lead well1 even at relatively high temperatures4 ,but that is not all4 ,even if some of the lead is lost1 there is a mathematical way of correcting for it4 ,this technique is called ,,concordia ,,analysis and was developed by ,g4 ,w4 ,wetherill1 a physicist then at the ,carnegie ,institution of ,washington4 ,it is based1 again1 on the fact that natural uranium has two long-lived isotopes--,u and ,u--and that the lighter one1 ,u1 decays faster than the heavier4 ,the daughter products of both uranium decay processes are isotopes of the same element1 lead--,pb and ,pb1 respectively4 ,heavy isotopes are not separated to any significant degree by chemical processes1 so that if radiogenic lead has been lost from a system for any reason1 the other lead isotopes also will have been lost in whatever proportion they were present originally4 ,if we plot a graph of the radiogenic ,pb_/,u ratio against the radiogenic ,pb_/,u ratio for concordant 7closed7 systems of all ages1 we obtain the curved line shown in the figure below4 ,the curve is the locus of all concordant ,u-,pb ages and is called ,concordia4 ,then if we test two or more particular zircons of the same age that have lost different amounts of lead1 at about the same time1 the plot of their ,pb_/,u ratios against their ,pb_/,u ratios will fall on a straight line that is a chord of the ,concordia curve4 ,the upper intersection of this chord with the curve then will mark the true age of the zircons4 ,this is an elaborate technique utilizing difficult chemical procedures1 but it has proved invaluable in solving some important geologic problems4 ,illustration3 ,the ,concordia curve offers a useful way of analyzing results of age determinations on the mineral zircon4 ,hornblende ,the mineral hornblende provides another useful system4 ,hornblende is a complex silicate of sodium1 calcium1 iron1 magnesium1 and aluminum1 and usually contains a few tenths of #a of potassium4 ,it is unusual in that it tenaciously retains its radiogenic argon1 even at relatively high temperatures4 ,sanidine ,still another good system is the rare feldspar1 sanidine1 which is excellent for both potassium-argon and rubidium-strontium age determination4 ,sanidine usually is found in volcanic ash falls and has been important in the establishment of the geologic time scale1 as we shall see4 ,whole ,rocks ,finally there is still another way of obtaining a closed system by using the whole rock1 not just a crystal of a single mineral within it4 ,a large body of granite or similar rock may contain a number of minerals1 some or none of which may be closed systems4 ,yet as long as this body of rock remains impermeable to solutions 7which in nature means mostly to water71 no substance will be able to move very far in it because diffusion in solids is so slow4 ,consequently it will remain a closed system1 as a whole1 regardless of what happens to the individual mineral grains4 ,if we take a piece from near the middle of this body of rock and if this piece is much larger than the largest constituent grain in it1 then we have a fair sample of a closed system--the whole rock4 ,the only difficulty arises from the fact that few rocks are sufficiently impermeable to solutions to retain argon1 and many rocks contain so much common strontium that rubidium-strontium analysis is impractical4 ,still1 we can use the rapid survey methods as for feldspar1 selecting the few rocks that would be useful4 ,this work has been done frequently1 and the results have been fruitful for rubidium-strontium analysis4 ,the whole-rock rubidium-strontium age dates the time when the rock became impermeable4 ,,some ,,interesting ,,results ,the ,old ,man from ,olduvai ,illustration3 ,fossil skull of ,zinjanthropus1 nearly #b1jjj1jjj years old1 discovered in #aiei by ,dr4 ,l4 ,s4 ,b4 ,leakey in ,olduvai ,gorge4 ,accurate dating of this earliest human ancestor was possible by using the potassium-argon method4 ,one of the most talked-about age measurements in recent years was the determination of the unexpectedly great age of fossil ancestors of man1 found by the ,british anthropologist1 ,dr4 ,l4 ,s4 ,b4 ,leakey1 in ,olduvai ,gorge in ,tanzania4 ,the measurements were made by ,garniss ,h4 ,curtis and ,jack ,f4 ,evernden at the ,university of ,california in ,berkeley by the potassium-argon method4 ,the age came out a little less than #b million years1 about twice as old as it 8should be0 in the view of many scientists4 ,human remains of such great antiquity had never been found before1 and much doubt was raised about the validity of the figures4 ,time periods as short as two million years are not easy to measure by potassium-argon4 ,the amount of argon produced in that time is extremely small1 and contamination by argon from the air is a serious problem4 ,still1 the measurements were repeated1 the rocks were studied again1 and the result did not change3 ,the fossils were still about #b million years old4 ,in cases like this1 one tries to find some other method to check the results in an independent way4 ,after many attempts it was discovered that the same rock strata dated by potassium-argon also contained some pumice--a porous volcanic glass--and that this glass was suitable for uranium fission-track dating4 ,the measurements were made in the ,general ,electric ,research ,laboratory4 ,what was the result8 ,just about #b million years6 ,when such altogether different techniques give the same number1 one can have some confidence that the number is exact4 ,it would be difficult to imagine a disturbance in nature that would cause these unrelated methods to give the same wrong number--in both cases by a factor of two4 ,the double check simply means the ,olduvai man is #b million years old4 ,there is not much doubt about it4 ,the ,geologic ,time ,scale ,much of historical geology is based on a relationship called the ,,law ,,of ,,superposition4 ,this simply means that when some rock formation was placed on top of some other formation by natural processes 7sedimentation or volcanic eruption1 for example71 the layer on top must be younger than the one on the bottom4 ,such a conclusion may now seem obvious1 but the concept was not even expressed until the very end of the eighteenth century and was still a matter of scientific controversy when ,abraham ,lincoln was a boy4 ,it was the law of superposition1 however1 that led the early geologists to establish the first geologic time scales and to realize the enormous extent of geologic time4 ,illustration3 ,diagram illustrating the law of superposition4 ,each rock bed is younger than the strata under it4 ,in essence the system of establishing age by this concept is this3 ,somewhere a large and easily recognized layer of sedimentary rock was known4 ,it had a characteristic color1 texture1 gross composition1 and overall appearance4 ,let us call it bed ,m4 ,this bed could be traced across the countryside until a place was reached where it could be seen that bed ,m rested on another1 different layer of rock1 which we might call bed ,l4 ,bed ,l could also be traced some distance and ultimately could be observed resting on a still different stratum1 which we shall call bed ,k4 ,some of these beds had fossils in them1 and it was eventually realized that rocks with the same kinds of fossils are of the same age1 even though they may differ in other respects--in color or composition1 for instance4 ,if bed ,m was followed in another direction1 perhaps a point was reached where it dipped down a little1 and here1 was found still another layer--call it bed ,n--on top of ,m4 ,obviously1 the sequence of beds from oldest to youngest was ,k-,l-,m-,n4 ,their relative ages were now established4 ,over the years1 hundreds of geologists described various rock layers and identified the fossils in them4 ,by the middle of the nineteenth century1 this 8layer-cake0 structure of sedimentary rocks was well-known in western ,europe4 7,one may say1 parenthetically1 that ,america was geologically a vast unknown at that time4 ,today we know much more about the geology of ,antarctica than anyone a hundred years ago knew about the geology of the ,united ,states47 ,a system of nomenclature for the 8layer-cake0 was developed and refined4 ,gradually this nomenclature was accepted internationally4 ,long-range correlations between beds of the same age1 distant halfway around the globe from each other1 were made possible as the science of ,,paleontology developed4 ,the relative age of almost any rock containing even poorly preserved fossils could be determined anywhere in the world with precision4 ,that is1 the age of rock layers in relation to one another was known4 ,but the real age--the absolute age--remained unknown until knowledge of radioactivity provided the necessary clocks4 ,illustration3 ,a geologist carefully maps the layers of soil in which ancient flint tools have been found4 ,exact correlation of the tool-bearing strata with material datable by carbon-#ad analysis must be accomplished before the age of the tools can be determined4 ,even this process wasn't entirely without problems4 ,the difficulty lay in the fact that ordinary sedimentary rocks 7shale1 sandstone1 and limestone7 cannot be dated by the usual nuclear methods because they present no suitable closed systems4 ,only some volcanic sediments can be reliably dated by the mica1 feldspar1 and zircon they contain2 but these ancient ashfalls are rare1 usually are only a few inches thick1 and are not easy to identify at the surface because they weather quickly to clay4 ,only about a dozen volcanic beds have been accurately dated in ,north ,america4 ,the time points they established are mainstays of the present geologic time scale1 but they have had to be supplemented by indirect information4 ,illustration3 ,steeply dipping sandstone strata from the ,cretaceous ,period near ,gallup1 ,new ,mexico4 ,this photo1 taken in #aija 7note the horse-drawn wagon71 was made on an early government survey of ,western lands4 ,illustration3 ,fossils of bird tracks in sandstone in ,death ,valley1 ,california4 ,the most important of these indirect time points are furnished by what geologists call ,,bracketed ,,intrusives4 ,it often happens in geologic history that a mass of rock becomes molten at a great depth and forces its way up through several layers of sedimentary rocks4 ,the sedimentary layers are usually bent and twisted 7folded7 by the upthrust1 and where the cooler rock comes in contact with the molten mass1 cooler material is burned 7recrystallized74 ,geologists call that process ,,contact ,,metamorphism because the sedimentary rock forms are changed1 or metamorphosed1 to have another form or composition4 ,contact-metamorphosed rocks1 in spite of the damage they have suffered1 may contain recognizable and accurately datable fossils4 ,thereby the metamorphic rock establishes a lower limit for the age of the intrusion3 ,it must be younger than the fossils in the youngest of the metamorphic rocks it touched4 ,now on top of the intrusive rock1 we may find another sedimentary rock1 deposited on top of the intrusive after it cooled and was exposed by the erosion of overlying materials1 perhaps millions of years later4 ,this new sediment may also contain fossils and thus furnish an upper limit for the age of the intrusion4 ,the measured age of the intrusive rock thus can be used to set upper and lower limits on the absolute ages of the two sediments4 ,if we are lucky1 the two sediments will bracket a relatively short interval1 making our measurement quite precise4 ,illustration3 ,idealized sketch of a bracketed intrusive4 ,the igneous 7molten7 rock must be younger than the sedimentary rock 7,a7 it intrudes1 and older than the rock 7,b7 that overlies it4 ,the relative age of the sedimentary beds is known from their fossils4 ,illustration3 ,facsimile reprint of the famous time scale proposed by ,arthur ,holmes in #aiei4 ,time-scale in millions of years ,,periods ,since beginning of period ,duration of period ,,pleistocene ca #a 4444 ca #a 4444 ,,pliocene #aj 4444 #aa 4444 ,,miocene #ad 4444 #be 4444 ,,oligocene #ae 4444 #dj 4444 ,,eocene #bj 4444 #fj 4444 ,,paleocene #aj 4444 #gj #b 4444 ,upper ,,cretaceous #fe ,lower 4444 #ace #e 4444 ,upper ,,jurassic #de ,mid4 ,lower 4444 #ahj #e 4444 ,,triassic #de 4444 #bbe #e 4444 ,,permian #de 4444 #bgj #e 4444 ,upper ,,carboniferous #hj ,lower 4444 #cej #aj 4444 ,upper ,,devonian #ej ,lower 4444 #djj #aj 4444 ,,silurian #dj 4444 #ddj #aj 4444 ,,ordovician #fj 4444 #ejj #ae 4444 ,,cambrian #ajj 4444 #fjj #bj 4444 ,using measurements made in many laboratories1 and interpolating between them by using the relative thicknesses of sediments1 the great ,british geologist1 ,arthur ,holmes1 established the time scale that is in general use today4 ,his original scale is shown on the preceding page4 ,small changes can be expected to be made in this scale from time to time1 but major alterations are not likely1 except perhaps in the ,cambrian ,epoch where the present data are unreliable because they are not complete4 7,a scale showing the epochs or periods as often given now is on page #d47 ,precambrian ,stratigraphy ,so far we have talked only about rocks that are of the ,cambrian ,epoch or younger--rocks that may contain fossils4 ,yet there are vast areas 7most of ,canada1 for example7 that are covered with rocks older than the ,cambrian formation4 ,some ,precambrian fossils have been found1 but they are so rare that they are useless for dating the strata containing them4 ,long-range correlation of ,precambrian rocks must rely on nuclear measurements4 ,therefore it has been only in the last dozen or so years that some order could be established for the ,precambrian rock sequences4 ,the elaborate ,precambrian stratigraphies 7arrangements of strata in sequence7 proposed in the past1 most of them based on superficial similarities of the rocks in one place to those in another place1 now have been drastically altered and in some cases completely overturned by nuclear measurements4 ,we are still far from understanding the sequence of all the events in that vast span of time we call the ,precambrian4 ,many thousands of nuclear age determinations will have to be made to lighten the dark corners of our ignorance4 ,illustration3 ,folded strata of ,precambrian rocks1 including limestones and shales1 in ,glacier ,national ,park1 ,montana4 ,,and ,,where ,,do ,,we ,,go ,,from ,,here8 ,perhaps we must first realize that we really haven't come very far yet4 ,granted that the age of rocks in many parts of the world is now suddenly known--and that this was a total mystery some dozen years ago4 ,granted that enormous strides forward have been made4 ,it's only a beginning4 ,vast areas of the world are still geologically unexplored4 ,the geologic time scale is still fragmentary and crude4 ,thousands of important geologic questions remain to be defined1 explored1 and answered by nuclear age determination4 ,and--as is inevitable in science--many of them will lead to new questions4 ,it is apparent that dating techniques have barely begun to be used and understood by geologists4 ,but apart from geologic work1 what else is in store8 ,it is difficult to predict1 but probably the most important advance in the next decade or two will come when we obtain samples of rock from the moon4 ,will there be young rocks there or will they all be #deej million years old8 ,or will they perhaps be some other age8 ,the chemical composition and nuclear ages of the first moon samples will probably be the most important information we can hope to obtain from them4 ,these results from the moon will contribute enormously to our understanding of the processes that formed the earth1 made the continents1 and determined the major features of our world4 ,we have a long way to go4 ,,glossary ,,aeon ,one billion 7#aj7 years4 ,,alpha ,,decay ,radioactive decay with emission of an alpha particle4 ,,alpha ,,particle ,essentially the nucleus of helium1 composed of two neutrons and two protons with double positive charge4 ,,anticoincidence ,,ring ,a ring of counters connected to exclude outside radiation4 ,,background ,,count ,the number of impulses per unit time registered on a counting instrument when no sample is present4 ,,beta ,,decay ,radioactive decay with emission of a beta particle4 ,,beta ,,particle ,an electron emitted by a nucleus4 ,,bracketed ,,intrusive ,igneous rock extending into sedimentary rocks that are datable by their fossils4 ,,closed ,,system ,a system in which the parent material radioactively decays into its daughter products and nothing is added or removed4 ,,common 7strontium1 lead1 etc47 ,the ordinary element present in nature at any one time as distinguished from that produced by radioactive decay4 ,,concordia ,,analysis ,a mathematical technique to determine graphically the age of a material containing radiogenic lead by comparing its uranium-to-lead ratio with the similar ratio in a closed uranium-lead system4 ,,contact ,,metamorphism ,a metamorphism genetically related to the intrusion of molten masses of rock and taking place at or near the contact4 ,,cosmic ,,rays ,high-energy particles moving in our galaxy4 ,,crystal ,a periodic or regularly repeating arrangement of atoms1 formed from a single element or compound4 ,,daughter ,a nuclide formed from the radioactive decay of another nuclide4 ,,decay ,,constant ,the number of atoms decaying per atom per unit of time 7#j4fic_/;half-life74 ,,electron ,,capture ,a nuclear process in which the nucleus of an atom captures an electron from one of the inner shells4 ,,electrons ,elementary particles with a unit negative electrical charge and a mass #a_/ahcg that of the proton1 or #i4ab #aj gram4 ,electrons surround the atom's positively charged nucleus and determine the atom's chemical properties4 ,,gamma ,,rays ,electromagnetic radiation from an atomic nucleus4 ,,geiger ,,counters ,instruments that count pulses produced by radioactivity1 consisting of a counting tube with a central wire anode1 usually filled with a mixture of argon and organic vapor4 ,,half-,,life ,the time it takes for half the atoms in a radioactive substance to decay4 ,,ion ,an atom or molecule that has lost or gained one or more electrons and is thus electrically charged4 ,,isotope ,,dilution ,an analytical technique involving addition of a known amount of an isotopic mixture of abnormal composition to the unknown amount of an element of normal or known isotopic composition4 ,,isotopes ,nuclides of the same atomic number but different atomic weight4 ,isotopes of a given element have an identical number of protons but different numbers of neutrons in their nuclei4 ,,law ,,of ,,superposition ,statement that overlying strata must be younger than underlying strata if there has been no inversion4 ,,mass ,,spectrometer ,an instrument for separation and measurement of isotopes by their mass4 ,,net ,,counting ,,rate ,sample counting rate minus background counting rate4 ,,neutrons ,elementary particles in the nucleus having no electric charge and the mass of one atomic mass unit4 ,,nuclide ,a species of atom characterized by the constitution of its nucleus4 ,,original 7strontium1 lead1 etc47 ,common strontium1 lead1 etc41 taken into a system at the time of its formation4 ,,paleontology ,the study of fossil remains4 ,,parent ,the radioactive element from which a daughter nuclide is produced by radioactive decay4 ,,plumbology ,the study of the uranium and thorium-lead decay systems4 ,the name is derived from the ,latin name for lead1 plumbum4 ,,primordial ,present at the time of the formation of the earth4 ,,proportional ,,counter ,an instrument for detecting radiation by producing pulses of electrical charge that are proportional to the energy of the radiation being measured4 ,the design permits use of radiation of a desired energy level 7within limits71 and discrimination against other radiation1 especially background radiation4 ,,protons ,elementary particles with a single positive electrical charge and a mass approximately #ahcg times that of the electron4 ,the atomic number of an atom is equal to the number of protons in its nucleus4 ,,radioactive ,,decay ,the change of one nuclide to another by the emission of charged particles from the nucleus of its atom4 ,,radioactivity ,the property of some nuclides to decay by themselves into others4 ,,radiogenic ,formed as the result of radioactive decay4 ,,rare ,,earth ,any of the elements from atomic number #eg 7lanthanum7 to #ga 7lutetium74 ,,sample ,,counter ,an instrument into which a sample of material can be placed to have its radiation measured4 ,,secular ,,equilibrium ,the production of a radioactive substance at a rate equal to its decay4 ,,specific ,,activity ,the number of atoms decaying per unit time per unit weight of the total amount being tested4 ,,spike ,a known amount of an element of unusual isotopic composition used in isotope-dilution analysis4 ,,statistical ,,error ,the error associated with nuclear measurements and arising from the random distribution of nuclear events4 ,,strata ,plural of stratum4 ,a sheet or mass of sedimentary rock 7formed by deposits of sediments1 as from ancient seas7 of one kind1 usually in layers between beds or layers of other kinds4 ,,appendix ,radioactive ,decay ,when a radioactive nucleus disintegrates or decays1 the resultant remaining nucleus may still be radioactive1 and sooner or later it also will disintegrate and become still another kind of atom4 ,this process continues through a series of distinct steps until a stable atom--one that is not radioactive--is formed4 ,all natural radioactivity in the heavy elements proceeds by such a series of steps1 and the series finally ends with a stable form of lead as its end product4 ,in other words1 any naturally radioactive heavy element eventually becomes nonradioactive lead4 ,the nucleus of every atom 7except hydrogen7 contains one or more neutrons and one or more protons4 ,the instability of the nuclei of the heavy atoms is related to the ratio of the number of neutrons to the number of protons in the nuclei4 ,radioactive decay is1 in fact1 a way of adjusting these ratios4 ,the adjustment can occur in various ways4 ,the most common is the emission of alpha particles or beta particles4 ,an alpha particle is identical with the nucleus of a helium atom and has two neutrons and two protons bundled together4 ,loss of an alpha particle from a nucleus lowers the mass number 7the total of protons and neutrons7 of the parent nucleus by four and the atomic number 7the number of protons7 by two2 the number of neutrons also is reduced by two4 ,a beta particle is an electron and has a negative electric charge4 ,when a beta particle is emitted from a nucleus1 the nucleus is changed so that it has one more proton 7which has a positive charge7 and one less neutron 7which has no charge72 in effect1 a neutron has changed into a proton as the nucleus lost a negative charge4 ,beta decay occurs in nuclei with a greater proportion of neutrons than is normal for the number of protons4 ,since beta emission increases the proportion of protons1 the process raises the atomic number of the parent nucleus by one and leaves the mass number the same4 ,gamma rays are a form of electromagnetic radiation4 ,they are emitted when a nucleus shifts from one energy state to a lower energy state--the energy difference emerging as the gamma radiation4 ,gamma emission often accompanies alpha or beta emission1 but the production of gamma rays does not itself alter the atomic number nor the mass number of the parent4 ,nuclei also can decay by emission of a positron1 which is a positively charged electron4 ,when this occurs1 the new nucleus has one more neutron and one less proton than its parent2 in effect a proton has become a neutron as the nucleus loses a positive charge4 ,positrons usually are emitted by nuclei that have a greater proportion of protons than is normal for the number of neutrons4 ,another process--internal electron conversion--sometimes occurs in connection with gamma-ray emission1 usually in heavy elements when the gamma-ray energy is low4 ,instead of being emitted directly1 the gamma ray strikes an orbital electron1 knocking the electron out of the atom2 the gamma ray then disappears4 ,another electron jumps into the 8hole0 in the orbit from which the first electron was emitted1 and this jump--from a higher to a lower energy level--results in the emission of an ,x ray 7which is similar to a gamma ray1 but originates in the electron orbit region of the atom1 not in the nucleus74 ,finally1 a nucleus may be altered by electron capture4 ,in a nucleus with a low ratio of neutrons to protons1 the nucleus captures one of its own orbital electrons4 ,this immediately combines with a proton to form a new neutron and emit a neutrino 7a high-energy particle with neither mass nor charge74 ,the process increases the neutron-to-proton ratio of the nucleus2 the daughter has the same mass number as the parent1 but has an atomic number one less than the parent4 ,there are three series by which naturally radioactive nuclei decay to stable ones3 ,the ,uranium ,series1 the ,thorium ,series1 and the ,actinium ,series4 ,man-made radioactive nuclei decay similarly1 with bismuth as the end product1 via the ,neptunium ,series4 ,these can be illustrated in tabular form and diagrammatically4 ,the ,actinium ,series 7,uranium-#bce ,series71 for example1 proceeds like this3 ,,the ,,uranium-#bce ,,series ,element ,symbol ,radiation ,emitted ,half-life ,uranium ,u #g4ac #aj years ,thorium ,th #be4f hours ,protactinium ,pa #c4be #aj years ,actinium#af ,ac 7#ih4h7 and 7#a4b7 #ba4b years ,thorium ,th #ah4ag days ,francium ,fr #bb minutes ,radium ,ra #aa4g days ,radon ,rn #d4j seconds ,polonium#af ,po 7#ajj7 and 7#e #a4hc #aj second #aj7 ,lead ,pb #cf4a minutes ,astatine ,at #aj second ,bismuth#af ,bi 7#ii4g7 and 7#j4c7 #b4ae minutes ,polonium ,po #j4be second ,thallium ,tl #d4gh minutes ,lead ,pb -- ,stable ,illustration3 ,the ,uranium-#bce ,series ,the ,uranium 7,uranium-#bch7 ,series proceeds like this3 ,,the ,,uranium-#bch ,,series ,element ,symbol ,radiation ,half-life ,uranium ,u #d4ea #aj years ,thorium ,th #bd4a days ,protactinium#ag ,pa #a4ah minutes ,uranium ,u #b4dh #aj years ,thorium ,th #g4f #aj years ,radium ,ra #a4fb #aj years ,radon ,rn #c4hb days ,polonium#ah ,po 7#ii4ih7 and #c4je minutes 7#j4jb7 ,lead ,pb #bf4h minutes ,astatine ,at #a4c seconds ,bismuth#ah ,bi 7#ii4if7 and #ai4g minutes 7#j4jd7 ,polonium ,po #a4f #aj second ,thallium ,tl #a4cb minutes ,lead ,pb #bb years ,bismuth#ah ,bi 7#ajj7 and 7#b #e4j days #aj7 ,polonium ,po #ach4d days ,thallium ,tl #d4cj minutes ,lead ,pb 444 ,stable ,illustration3 ,the ,uranium-#bch ,series ,,suggested ,,references ,books ,how ,old ,is the ,earth81 ,patrick ,m4 ,hurley1 ,doubleday ,company1 ,inc41 ,garden ,city1 ,new ,york1 #aiei1 #afj pp41 #a4be4 ,radiocarbon ,dating 7second edition71 ,willard ,f4 ,libby1 ,university of ,chicago ,press1 ,chicago1 ,illinois1 #aiee1 #age pp41 #e4jj 7hardback72 #a4ie 7paperback74 ,the ,birth and ,death of the ,sun1 ,george ,gamow1 ,the ,viking ,press1 ,new ,york1 #aidi1 #bch pp41 #d4ge 7out of print but available through libraries71 #j4fj 7paperback7 from the ,new ,american ,library of ,world ,literature1 ,inc41 ,new ,york4 ,inside the ,nucleus1 ,irving ,adler1 ,the ,john ,day ,company1 ,inc41 ,new ,york1 #aifc1 #aia pp41 #d4ie 7hardback72 #j4fj 7paperback7 from the ,new ,american ,library of ,world ,literature1 ,inc41 ,new ,york4 ,ages of ,rocks1 ,planets1 and ,stars1 ,henry ,faul1 ,mc,graw-,hill ,book ,company1 ,new ,york1 #aiff1 #aji pp41 #b4de4 ,principles of ,geochemistry 7second edition71 ,brian ,mason1 ,john ,wiley ,sons1 ,inc41 ,new ,york1 #aieh1 #caj pp41 #g4ej4 7,new edition due in #aifg47 ,potassium ,argon ,dating1 ,j4 ,h4 ,zahringer and ,o4 ,a4 ,schaeffer1 ,springer-,verlag ,new ,york1 ,inc41 ,new ,york1 #aiff1 #bej pp41 #ab4ej4 ,articles ,a ,clock for the ,ages3 ,potassium ,argon1 ,garniss ,h4 ,curtis1 ,national ,geographic ,magazine1 #abj3 #eij 7,october #aifa74 ,exploring #a1gej1jjj ,years into ,man's ,past1 ,l4 ,s4 ,b4 ,leakey1 ,national ,geographic ,magazine1 #abj3 #efd 7,october #aifa74 ,five-,billion-,year ,clock1 ,patrick ,m4 ,hurley1 ,saturday ,evening ,post1 #bcd3 #bf 7,march #ah1 #aifa74 ,geologic ,time ,scale1 ,j4 ,laurence ,kulp1 ,science1 #acc3 #aaje 7,april #ad1 #aifa74 ,geology1 ,reginald ,a4 ,daly1 ,scientific ,american1 #ahc3 #cf 7,september #aiej74 ,tracks of ,charged ,particles in ,solids1 ,r4 ,l4 ,fleischer1 ,p4 ,b4 ,price1 and ,r4 ,m4 ,walker1 ,science1 #adi3 #chc 7,july #bc1 #aife74 ,how ,old ,is ,it81 ,lyman ,j4 ,briggs and ,kenneth ,f4 ,weaver1 ,national ,geographic ,magazine1 #aad3 #bcd 7,august #aieh74 ,moving ,picture of the ,last ,ice ,age1 ,richard ,foster ,flint1 ,natural ,history1 #ff3 #ahh 7,april #aieg74 ,modern ,methods for ,measurement of ,geologic ,time1 ,e4 ,j4 ,zeller1 ,mineral ,information ,service1 #ah3 #i 7,january #aife74 ,single copies are #j4aj from ,division of ,mines and ,geology1 ,ferry-,building1 ,san ,francisco1 ,california #idaaa4 ,fluted ,projectile ,points3 ,their ,age and ,dispersion1 ,c4 ,vance ,haynes1 ,jr41 ,science1 #ade3 #adjh 7,september #be1 #aifd74 ,unraveling the ,age of ,earth and ,man1 ,e4 ,l4 ,simons1 ,natural ,history1 #gf3 #eb 7,february #aifg74 ,radiocarbon ,dating and ,archeology in ,north ,america1 ,f4 ,johnson1 ,science1 #aee3 #afe 7,january #ac1 #aifg74 ,fission-track ,dating of ,bed ,i1 ,olduvai ,gorge1 ,r4 ,l4 ,fleischer and others1 ,science1 #adf3 #gb 7,april #b1 #aife74 ,lead ,isotopes and the ,age of the ,earth1 ,g4 ,r4 ,tilton and ,r4 ,h4 ,steiger1 ,science1 #aej3 #ahje 7,december #ca1 #aife74 ,strontium-,rubidium ,age of an ,iron ,meteorite1 ,g4 ,j4 ,wasserburg and others1 ,science1 #aej3 #ahad 7,december #ca1 #aife74 ,illustration3 ,,the ,,cover ,u4 ,s4 ,geological ,survey scientists prepare acetylene gas1 made from the carbon-#ad in a geological specimen1 in a vacuum line4 ,this gas will be fed into a proportional counter to determine the age of the specimen by its ,c count4 7,see 8,carbon-#ad ,counting0 beginning on page #ab47 ,illustration3 ,,the ,,author ,geophysicist ,henry ,faul1 an authority on nuclear dating1 is professor of ,geophysics and chairman of the ,department of ,geology at the ,university of ,pennsylvania1 ,philadelphia4 ,he holds a doctoral degree from the ,massachusetts ,institute of ,technology1 and has taught at the ,universities of ,strasbourg and ,bern in ,europe4 ,dr4 ,faul was a geophysicist with the ,manhattan ,project during ,world ,war ,,ii1 and was formerly chief of the ,radiation ,laboratory1 ,u4 ,s4 ,geological ,survey1 ,denver1 ,colorado4 ,for many years he was engaged in geological age determination work with the ,geological ,survey and the ,carnegie ,institution of ,washington in ,washington1 ,d4 ,c4 ,,photo ,,credits ,frontispiece courtesy ,anthropology ,department1 ,southern ,methodist ,university 7,,smu7 ,cover photo courtesy ,u4 ,s4 ,geological ,survey 7,,usgs7 ,page #aa ,,usgs #ab ,,usgs #ad1 #af1 #ag ,willard ,f4 ,libby #ah ,university of ,arizona #ba ,massachusetts ,institute of ,technology #bb ,graduate ,research ,center of the ,southwest #bc ,graduate ,research ,center of the ,southwest #be ,robert ,l4 ,fleischer #bf ,perkin-,elmer ,corporation #cj ,university of ,texas #cb ,graduate ,research ,center of the ,southwest #cf ,massachusetts ,institute of ,technology #dj ,des ,bartlett1 ,armand ,denis ,productions #dc ,anthropology ,department1 ,,smu #dd ,,usgs #dg ,,usgs #ei ,author's photo courtesy ,graduate ,research ,center of the ,southwest ,footnotes #a,words appearing in ,,small ,,capital ,,letters are defined in the ,glossary beginning on page #di4 #b,the process of natural radioactive decay is described in the ,appendix beginning on page #eb4 #c,the ,u4 ,s4 ,war ,department program during ,world ,war ,,ii that developed the first nuclear weapons4 #d;,drawn to scale1 the whole age of man is represented by less than the width of the line4 #e;,for more information on the structure of atoms1 see ,our ,atomic ,world1 a companion booklet in this series4 #f;,from the ,chart of the ,nuclides1 prepared by ,david ,t4 ,goldman1 ,knolls ,atomic ,power ,laboratory1 ,august #aifd4 #g,this decay process proceeds in a series of steps1 during which #f alpha particles and #d beta particles are emitted4 7,see ,appendix47 #h,named after their creator1 ,john ,napier1 a ,scottish mathematician 7#aeej-afag71 who also invented the decimal point4 #i;,it is difficult to determine the half-life of ,c exactly4 ,in the early days of ,c dating1 in order not to delay continued work1 an arbitrary value of #eefh years was chosen and this value is still used in calculations4 #aj,this means that uranium decays through successive steps in which the entire series emits eight alpha particles4 7,see ,appendix47 #aa,remember1 this enormous period of time is a measure of the rate of spontaneous fission1 not of the age of ,u4 #ab,the rhenium-osmium scheme is shown below the dotted line because the method is still in an early experimental stage and its general utility is not yet established4 #ac;,for more on this family of elements1 see ,rare ,earths1 ,the ,fraternal ,fifteen1 a companion booklet in this series4 #ad;,for a fuller explanation of the fission process1 see ,our ,atomic ,world1 another booklet in this series4 #ae,neutrons that have had their speed reduced by passing through a moderator 7graphite1 for example7 which is built into every reactor to accomplish this very thing4 ,for more about how this is done1 see ,nuclear ,reactors and ,research ,reactors1 companion booklets in this series4 #af,note that some radionuclides sometimes decay by one method1 sometimes by another4 ,for example1 #ih4h of the nuclei of actinium-#bbg emit a beta particle to form thorium-#bbg2 the remaining #a4b emit an alpha particle to form francium-#bbc2 both of these daughter products decay to radium-#bbc4 #ag,some of the protactinium 7#j4ab7 changes by an intermediate step1 known as isomeric transition1 in which its nucleus shifts to a lower energy state4 ,the process does not alter the remaining parent-daughter progression in the series4 #ah,undergoes both alpha and beta decay1 in definite proportion of decay events1 as shown4 ,this booklet is one of the 8,understanding the ,atom0 ,series4 ,comments are invited on this booklet and others in the series2 please send them to the ,division of ,technical ,information1 ,u4 ,s4 ,atomic ,energy ,commission1 ,washington1 ,d4 ,c4 #bjede4 ,published as part of the ,,aec's educational assistance program1 the series includes these titles3 ,accelerators ,animals in ,atomic ,research ,atomic ,fuel ,atomic ,power ,safety ,atoms at the ,science ,fair ,atoms in ,agriculture ,atoms1 ,nature1 and ,man ,careers in ,atomic ,energy ,computers ,controlled ,nuclear ,fusion ,cryogenics1 ,the ,uncommon ,cold ,direct ,conversion of ,energy ,fallout ,from ,nuclear ,tests ,food ,preservation by ,irradiation ,genetic ,effects of ,radiation ,index to the ,,uas ,series ,lasers ,microstructure of ,matter ,neutron ,activation ,analysis ,nondestructive ,testing ,nuclear ,clocks ,nuclear ,energy for ,desalting ,nuclear ,power and ,merchant ,shipping ,nuclear ,power ,plants ,nuclear ,propulsion for ,space ,nuclear ,reactors ,nuclear ,terms1 ,a ,brief ,glossary ,our ,atomic ,world ,plowshare ,plutonium ,power from ,radioisotopes ,power ,reactors in ,small ,packages ,radioactive ,wastes ,radioisotopes and ,life ,processes ,radioisotopes in ,industry ,radioisotopes in ,medicine ,rare ,earths ,reading ,resources in ,atomic ,energy ,research ,reactors ,,snap1 ,nuclear ,space ,reactors ,sources of ,nuclear ,fuel ,space ,radiation ,synthetic ,transuranium ,elements ,the ,atom and the ,ocean ,the ,chemistry of the ,noble ,gases ,the ,first ,reactor ,whole ,body ,counters ,your ,body and ,radiation ,a single copy of any one booklet1 or of no more than three different booklets1 may be obtained free by writing to3 ,,usaec1 ,p4,o4 ,,box #fb1 ,,oak ,,ridge1 ,,tennessee #cghcj ,complete sets of the series are available to school and public librarians1 and to teachers who can make them available for reference or for use by groups4 ,requests should be made on school or library letterheads and indicate the proposed use4 ,students and teachers who need other material on specific aspects of nuclear science1 or references to other reading material1 may also write to the ,oak ,ridge address4 ,requests should state the topic of interest exactly1 and the use intended4 ,in all requests1 include 8,zip ,code0 in return address4 ,printed in the ,united ,states of ,america ,,usaec ,division of ,technical ,infection ,extension1 ,oak ,ridge1 ,tennessee ,transcriber's ,notes --,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 --,in the text versions only1 superscript text is preceded by caret4 --,in the text versions only1 subscript text is preceded by underscore4