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Finally, Libby had a method to put his concept into practice. The circular arrangement of Geiger counters (center) detected radiation in samples while the thick metal shields on all sides were designed to reduce background radiation.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.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.Top of page Carbon-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.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.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.
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.
Living organisms from today would have the same amount of carbon-14 as the atmosphere, whereas extremely ancient sources that were once alive, such as coal beds or petroleum, would have none left.
For organic objects of intermediate ages—between a few centuries and several millennia—an age could be estimated by measuring the amount of carbon-14 present in the sample and comparing this against the known half-life of carbon-14.
They also sampled artifacts from museums such as a piece of timber from Egyptian pharaoh Senusret III’s funerary boat, an object whose age was known by the record of its owner’s death.
In 1949, Libby and Arnold published their findings in the journal Science, introducing the “Curve of Knowns.” This graph compared the known age of artifacts with the estimated age as determined by the radiocarbon dating method.
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Willard Libby (1908–1980), a professor of chemistry at the University of Chicago, began the research that led him to radiocarbon dating in 1945.