During the early 1940s at Columbia Libby worked on the separation of uranium isotopes for producing fissionable uranium-238 for the atomic bomb. Back in Chicago after the war he turned his attention to carbon-14, a radioactive isotope of carbon that had been discovered in 1940 by Serge Korff. It occurs as a small constant percentage of the carbon in the carbon dioxide in the atmosphere – resulting from cosmic-ray bombardment – and in the carbon in the tissues of all living plants and animals. Carbon-14 has an extremely long half-life (5,730 years) but when the plant or animal dies, it accumulates no more of the radioactive isotope, which steadily decays and changes into nitrogen. Libby reasoned that a determination of the carbon-14 content of anything derived from plant or animal tissue – such as wood, bones, cotton or woollen cloth, hair, or leather – gives a measure of its age (or the time that has elapsed since the plant or animal died). He and his co-workers accurately dated ancient Egyptian relics by measuring the amount of radiocarbon they contained using a sensitive Geiger counter. By 1947 they had developed the technique so that it could date objects up to 50,000 years old. It has proved to be extremely useful in geology, anthropology, and archaeology. In 1946 Libby showed that tritium (a radioactive isotope of hydrogen of mass 3) is formed by the action of cosmic rays and devised a method of dating based on the amount of tritium in the water in an archaeological specimen. Later workers extended the method using other isotopes, such as potassium-40.

Categories: biography; chemistry

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