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Taking the First Measure of Prehistory

On December 17, 1908, American physical chemist Willard Frank Libby was born. He specialized in radiochemistry, the chemistry of radioactive materials. Radiochemistry includes the study of both natural and man-made radioisotopes. Isotopes are variants of a specific chemical element that differ from each other in the number of neutrons each contains.

Libby investigated radiocarbon dating, also known as Carbon-14 or C-14 dating. Like other elements, carbon can exist in several versions or isotopes. Some isotopes never seem to decay. Other isotopes, like C-14, deteriorate over time and are considered unstable. However, unstable isotopes decay at a predictable, measurable rate until they return to a stable form. This makes an unstable isotope a very interesting tool for scientists.

Libby knew that constant cosmic radiation pounding our planet breaks down nitrogen in Earth’s upper atmosphere that reacts to produce quantities of the C-14 isotope. Since all living things are mostly made of carbon, all living things assimilate C-14. It’s in our food, too, of course, and the food of our food. At the end of plant or animal life, C-14 is no longer assimilated. Libby also knew that it took C-14 about 5700 years to deteriorate to the point it reached its own half-life on its way to the final point at which it would not decay further. Good in theory.

What Libby wanted was a test and equipment sensitive enough to measure C-14 as it decayed over its entire life so he could use it to calculate the age of ancient life. Before they began to calculate ages of pre-historic material, he and his colleagues tested ages of the oldest known living things around the world – ancient tree rings, charcoal from Pompeii, First Dynasty tombs in Egypt. Libby and the project scientists came up with a useable scientific formula for measuring decaying amounts of C-14 to determine the age of a sample to approximately 50,000 years of age.

Libby described his theory and how it worked in his book Radiocarbon Dating. The C-14 technique provided a powerful tool that continues to be refined for researchers in archaeology, oceanography, paleontology, climatology, meteorology, and biomedicine. Among the recognition and many honours for his work, he received the Elliott Cresson Medal from the Franklin Institute, for the radiocarbon dating technique, and the Nobel Prize for Chemistry for leading the radiocarbon dating team.

B Bondar / Real World Content Advantage