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Libby, Willard Frank

carbon technique atmosphere dating

(1908–80) US chemist: developed radiocarbon dating technique.

Libby taught at the University of California at Berkeley (where he had graduated) until 1941, when he joined the Manhattan Project developing the atom bomb. After the war he moved to the Institute of Nuclear Studies at the University of Chicago, returning to California in 1959.

In 1939 Serge Korff (1906–) discovered carbon-14, a radioactive isotope of carbon with a half-life of 5730 years, and showed that it is produced in the upper atmosphere by the action of cosmic rays on nitrogen atoms. In 1947 Libby and his colleagues used this discovery to develop their radiocarbon dating technique, which has proved to be invaluable in archaeology and Quaternary geology. The technique is based on the fact that living biological material contains carbon-14 and carbon-12 in equilibrium with the atmosphere (which contains a very small but approximately constant proportion of carbon-14 to carbon-12). However, when the organism dies it stops taking up carbon dioxide from the atmosphere and so the proportion of carbon-14 to carbon-12 starts to diminish as the carbon-14 undergoes radioactive decay. By measuring the proportion of carbon-14 to carbon-12, therefore, the time since death may be determined. The technique is applicable with reasonable accuracy in dating organic objects up to about 40 000 years old, but greater accuracy can be achieved by calibrating the technique with objects of known age, and this has been done back to about 5000 years ago. This calibration is desirable because the rate of production of carbon-14 in the atmosphere varies slightly with time. Libby was awarded the 1960 Nobel Prize for chemistry for his work.

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