The concept of radiocarbon dating relied on the ready assumption that once an organism died, it would be cut off from the carbon cycle, thus creating a time-capsule with a steadily diminishing carbon-14 count. Finally, Libby had a method to put his concept into practice. 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. 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. Fortunately, Libby’s group developed an alternative. This method worked, but it was slow and costly. 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. Libby reached out to Aristid von Grosse (1905–1985) of the Houdry Process Corporation who was able to provide a methane sample that had been enriched in carbon-14 and which could be detected by existing tools. In order to prove his concept of radiocarbon dating, Libby needed to confirm the existence of natural carbon-14, a major challenge given the tools then available.Īt the time, no radiation-detecting instrument (such as a Geiger counter) was sensitive enough to detect the small amount of carbon-14 that Libby’s experiments required.
But no one had yet detected carbon-14 in nature- at this point, Korff and Libby’s predictions about radiocarbon were entirely theoretical. Further research by Libby and others established its half-life as 5,568 years (later revised to 5,730 ± 40 years), providing another essential factor in Libby’s concept. Their results predicted the distribution of carbon-14 across features of the carbon cycle and gave Libby encouragement that radiocarbon dating would be successful.Ĭarbon-14 was first discovered in 1940 by Martin Kamen (1913–2002) and Samuel Ruben (1913–1943), who created it artificially using a cyclotron accelerator at the University of California Radiation Laboratory in Berkeley. Libby and graduate student Ernest Anderson (1920–2013) calculated the mixing of carbon across these different reservoirs, particularly in the oceans, which constitute the largest reservoir. However, the rates of movement of carbon throughout the cycle were not then known. 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. Libby’s next task was to study the movement of carbon through the carbon cycle. Based on Korff’s estimation that just two neutrons were produced per second per square centimeter of earth’s surface, each forming a carbon-14 atom, Libby calculated a ratio of just one carbon-14 atom per every 10 12 carbon atoms on earth. (Fortunately for him, this was later proven to be generally true.)įor the second factor, it would be necessary to estimate the overall amount carbon-14 and compare this against all other isotopes of carbon. 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. In the absence of any historical data concerning the intensity of cosmic radiation, Libby simply assumed that it had been constant. Radiocarbon dating would be most successful if two important factors were true: that the concentration of carbon-14 in the atmosphere had been constant for thousands of years, and that carbon-14 moved readily through the atmosphere, biosphere, oceans and other reservoirs-in a process known as the carbon cycle. The concept of radiocarbon dating focused on measuring the carbon content of discreet organic objects, but in order to prove the idea Libby would have to understand the earth’s carbon system.
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