The radiocarbon dating method is based on the fact that cosmic radiation produces neutrons that enter the earth's atmosphere and react with nitrogen. They produce carbon-14, a carbon isotope with eight rather than the usual six neutrons in the nucleus. With these additional neutrons, the nucleus is unstable and is subject to gradual radioactive decay. Willard Libby calculated that it took 5,568 years for half the carbon-14 in any sample to decay, its so-called half-life. (The half-life is now more accurately measured to be 5,730 years.) He found that the neutrons emitted radioactive particles when they left the nucleus, and he arrived at a method for counting the number of emissions in a gram carbon. Carbon-14 is believed to behave exactly lik tik ordinary carton from a chemical standpoint, and fogether with ondgery carbon it enters into the carbon dioxide of the atmosphere. The tempo of the process corresponds to the rates of supply and dis-integration. Because living vegetation builds up its own organic matter through photosynthesis and by using atmospheric carbon dioxide, the proportion of radiocarbon present in it is equal to that in the atmosphere. As soon as an organism dies, no further radiocarbon is incorporated into it. Radiocarbon dating is the most widely used dating method for dating the
Past from 40,000 years ago until 1500 AD.