Carbon dating for students

08 Oct

To test the technique, Libby’s group applied the anti-coincidence counter to samples whose ages were already known.

Among the first objects tested were samples of redwood and fir trees, the age of which were known by counting their annual growth rings.

Willard Libby (1908–1980), a professor of chemistry at the University of Chicago, began the research that led him to radiocarbon dating in 1945.

He was inspired by physicist Serge Korff (1906–1989) of New York University, who in 1939 discovered that neutrons were produced during the bombardment of the atmosphere by cosmic rays.

He reasoned that a state of equilibrium must exist wherein the rate of carbon-14 production was equal to its rate of decay, dating back millennia.

(Fortunately for him, this was later proven to be generally true.) For the second factor, it would be necessary to estimate the overall amount carbon-14 and compare this against all other isotopes of carbon.

In a system where carbon-14 is readily exchanged throughout the cycle, the ratio of carbon-14 to other carbon isotopes should be the same in a living organism as in the atmosphere.

However, the rates of movement of carbon throughout the cycle were not then known.

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In 1946, Willard Libby proposed an innovative method for dating organic materials by measuring their content of carbon-14, a newly discovered radioactive isotope of carbon.

Using this sample and an ordinary Geiger counter, Libby and Anderson established the existence of naturally occurring carbon-14, matching the concentration predicted by Korff. Fortunately, Libby’s group developed an alternative. They surrounded the sample chamber with a system of Geiger counters that were calibrated to detect and eliminate the background radiation that exists throughout the environment.

The assembly was called an “anti-coincidence counter.” When it was combined with a thick shield that further reduced background radiation and a novel method for reducing samples to pure carbon for testing, the system proved to be suitably sensitive.

Known as radiocarbon dating, this method provides objective age estimates for carbon-based objects that originated from living organisms.

The “radiocarbon revolution” made possible by Libby’s discovery greatly benefitted the fields of archaeology and geology by allowing practitioners to develop more precise historical chronologies across geography and cultures.