1 Nuclear Dating An Historical Perspective
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W. F. LIBBY University of California—Los Angeles, Institute of Geophysics and Planetary Physics, Environmental Science and Engineering, Los Angeles, CA 90024 I want to thank the symposium members and the Chairman, Dr. Lloyd Currie for the honor you do me. During my work with nuclear dating in 1930, I built a Geiger Counter, and using air as a gas which is undoubtedly one of the worst possible gases, by brute force got i t going using a string electrometer to detect the pulses. In a few months, however, we built a simple electronic circuit which detected and registered them on moving photographic paper using a tiny mirror glued to the needle of a microammeter. Our voltage supply was a motorgenerator set and, needless to say, I several times found myself on the floor of the laboratory as a result of having touched the output electrodes. We take a great deal for granted today in electronics and voltage supplies and detection equipment. All this had to be developed, and development was initiated in this period of the early 1930's. My professor, Wendell Latimer and I, decided that we would use the Geiger Counters to test for natural radioactivity in ordinary elements. We thought i t would be a good idea to begin with the Rare Earths since as far as we could tell no one had looked at them. It had been known for many years that potassium was radioactive, as was rubidium. So the idea of radioactivity in the ordinary elements was hardly new. But a systematic search with as sensitive an instrument as the Geiger Counter seemed to be a good idea. We looked at the rare earths through the kindness of Herbert N. McCoy, who had been in the rare earth business in Chicago. Through him, we obtained samples of considerable purity of the various rare earths. So, in the fall of 1931, spreading into 1932, we finally got going with the samples of the rare earth oxides. We built a counter with a screen instead of a solid cylinder as its wall to permit soft radiation to enter the counting volume. The first sample we tested was samarium oxide, and we found it to be very radioactive. Now we know today that the r a d i o a c t i v e isotope i s samarium-147 w i t h a h a l f l i f e o f 105,000,000,000 and t h a t the r a d i a t i o n i s an emission o f helium ions
0097-6156/82/0176-0001$05.00/0 © 1982 American Chemical Society Currie; Nuclear and Chemical Dating Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
years, (alpha
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N U C L E A R AND
C H E M I C A L DATING TECHNIQUES
particles). I t i s a most remarkable r a d i o a c t i v i t y and now seems to be a u s e f u l t o o l i n d a t i n g the e a r t h . We were q u i t e e l a t e d , and i t appeared t h a t i t was a r i c h f i e l d . Now, f i f t y years l a t e r , I must say t h a t i t wasn't as r i c h as we thought. But we have over the years discovered h a l f a dozen natural r a d i o a c t i v e elements, and two of these, the samarium-147 w i t h i t s decay t o neodymium-143 and rhenium-187 w i t h i t s decay t o osmium-187, prove to be of use i n Nuclear Dating. The importance of rhenium i s t h a t i t i s i r o n s o l u b l e w h i l e the other r a d i o a c t i v i t i e s are i n s o l u b l e i n m e t a l l i c i r o n . In f a c t , the best h a l f l i f e we have f o r rhenium-187 was obtained by measuring the osmium-187 to rhenium-187 r a t i o i n i r o n meteorites which had been dated by other methods. This work was s t a r t e d many years ago by Dr. Herr and others i n Germany. The h a l f l i f e i s 43,000,000,000 years. The other n a t u r a l a c t i v i t i e s such as indium-115, which has a h a l f l i f e of 1 0 y e a r s , are i n t e r e s t i n g i n t h e i r nuclear prop e r t i e s but are too l o n g - l i v e d t o be u s e f u l i n Nuclear Dating. Rhenium-187 i n r a d i o a c t i v e decay has the l e a s t e n e r g e t i c of a l l known nuclear transformations - i t s t o t a l d i s i n t e g r a t i o n energy i s not known but i t i s probably i n the range of 2 t o 3 k i l o v o l t s . I f the e l e c t r o n i c b i n d i n g energies were not i n c l u d e d the t r a n s f o r mation might not occur, h i n t i n g s t r o n g l y t h a t there i s a c l o s e connection between nuclear transformations and the e x t e r n a l e l e c trons i n the atom. In the 1930's we began a search f o r carbon-14 t o f i l l an obvious blank i n the isotope t a b l e , f o r the reason t h a t carbon i s so very important i n b i o l o g y - a r a d i o a c t i v e carbon isotope longer l i v e d than a few minutes would be very v a l u a b l e as a t r a c e r . Due to an e r r o r i n t h e o r e t i c a l judgment we f a i l e d to f i n d carbon-14. I t was l a t e r discovered by Samuel Ruben working w i t h M a r t i n Kamen. An i n t e r e s t i n g p o i n t about the f a i l u r e was t h a t we, Kamen, Ruben, and I , guessed the h a l f l i f e to be about three months. As you a l l know, i t i s 5730 years. This meant t h a t we d i d not make enough of i t t o detect i t . Kamen and Ruben bombarded g r a p h i t e w i t h a deuteron beam - a sledge hammer approach - and found i t . This development was i n t e r r u p t e d by World War I I . We resumed work on i t i n about 1945 a t the end of the war when we went to the U n i v e r s i t y of Chicago. Again, we used the screen w a l l counter together w i t h a new t r i c k to s h i e l d the mesons. We surrounded the d a t i n g counter w i t h a c y l i n d r i c a l s h i e l d c o n s i s t i n g of Geiger counters, perhaps a dozen, some two inches i n diameter; t h i s arrangement i s e l e c t r o n i c a l l y connected so t h a t i f a cosmic ray meson t r i g g e r s one of the s h i e l d i n g counters, the d a t i n g counter i s turned o f f . The carbon r a d i a t i o n i s so very weak there i s no p o s s i b i l i t y t h a t i t c o u l d i t s e l f t r i g g e r the s h i e l d i n g counters. This whole bundle was put i n s i d e a massive i r o n s h i e l d , and i n t h i s way we were able to measure the n a t u r a l radiocarbon and to measure the radiocarbon age. 1 5
Currie; Nuclear and Chemical Dating Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
1.
LIBBY
Historical Perspective of Nuclear Dating
The generation process f o r radiocarbon i n the atmosphere makes C 0 which enters the biosphere; because o f the long l i f e t i m e the mixing i s e s s e n t i a l l y p e r f e c t . We assumed the r a t e o f produc t i o n t o be constant which turns out t o be somewhat i n c o r r e c t . V a r i a t i o n s o f about 10 percent can be seen back i n time t o e a r l y Egyptian periods and before. The earth's magnetic f i e l d was apparently weaker then as the cosmic rays d e l i v e r e d t o the surface and the atmosphere were more intense. Now we have many tens o f thousands o f radiocarbon dates from many l a b o r a t o r i e s throughout the world and the r e s u l t s continue t o proliferate. One o f the most i n t e r e s t i n g o f the geophysics r e s u l t s from radiocarbon dates i s the h i s t o r y o f the sun. Apparently, i t i s r e g i s t e r e d i n f l u c t u a t i o n s o f the cosmic ray i n t e n s i t y . These are f l u c t u a t i o n s o f r a t h e r s h o r t d u r a t i o n i n terms o f the radiocarbon l i f e t i m e , perhaps a century o r so, and apparently they are caused by v a r i a t i o n s i n the s o l a r wind due t o long-term changes i n the s o l a r emissions. This idea has been developed i n some d e t a i l r e c e n t l y by Dr. L a i and h i s c o l l a b o r a t o r s . I t promises t o g i v e us a way o f watching the h i s t o r y o f the sun over tens o f thousands of years. This f i n e s t r u c t u r e on the curve o f c a l i b r a t i o n was discovered by Dr. Suess and others. In archaeology there are many a p p l i c a t i o n s . They are very g r a t i f y i n g and s u c c e s s f u l . We have seen the development r e c e n t l y o f a new method o f measuring r a d i o a c t i v e isotopes which promises t o evaluate s m a l l e r samples than we needed before. The present method r e q u i r e s per haps 10 g o f wood, o i l , o r charcoal o r whatever the m a t e r i a l i s . The newer method o f measuring the carbon-14 i s by d i r e c t counting of the carbon-14 atoms i n s t e a d o f i t s decays. This should a l l o w us t o use only a few m i l l i g r a m s . This i s a wonderful development which may a l l o w us t o make major advances i n many important areas where the a v a i l a b l e samples were p r e v i o u s l y too s m a l l . One important case i s the organic matter i n sea cores. (Many i n v e s t i gators t h i n k the organic matter i s more r e l i a b l e f o r d a t i n g than s h e l l . ) Measurements o f v a r i a t i o n s i n the carbon-14 c o n c e n t r a t i o n i n t h i s organic matter may a l l o w e v a l u a t i o n o f the h i s t o r y o f the s o l a r f l u c t u a t i o n s . Other small samples o f s p e c i a l i n t e r e s t are works o f a r t and r e l i g i o u s a r t i f a c t s which are too v a l u a b l e t o date by the conventional method. They may be datable now. Nuclear d a t i n g has been most h e l p f u l i n e s t a b l i s h i n g the h i s t o r y o f the e a r t h and o f the moon and o f the meteorites. The f a c t i s , there i s no other way o f measuring t h e i r ages. P r i o r t o the d i s c o v e r y o f n a t u r a l r a d i o a c t i v i t y i n the l a t e 19th century, i n d i r e c t methods were used t o estimate the age o f the e a r t h , but there were no r e a l answers u n t i l the r a d i o a c t i v i t y o f thorium, uranium, and potassium were discovered and we began t o understand atomic s t r u c t u r e and t o r e a l i z e t h a t nuclear t r a n s f o r m a t i o n was e s s e n t i a l l y independent o f the chemical form. 2
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Currie; Nuclear and Chemical Dating Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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NUCLEAR AND CHEMICAL DATING
TECHNIQUES
In a d d i t i o n , other e x c i t i n g and i n t e r e s t i n g approaches such as f i s s i o n t r a c k d a t i n g and d a t i n g by means o f chemical r e a c t i o n s o c c u r r i n g under proper c o n d i t i o n s a r e r a p i d l y developing. Of course, there may be as y e t undiscovered techniques o f great importance. Once a g a i n , I want to thank the members of t h i s symposium and the Chairman, D r . L l o y d C u r r i e . 1981.
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RECEIVED July 7,
Currie; Nuclear and Chemical Dating Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
