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When an organism, eg a tree, dies it stops taking in carbon dioxide.The amount of carbon-14 in the wood decreases with time as it decays into nitrogen with a half-life of about 5700 years.The starting ratio of radiocarbon to stable carbon is locked in at that point. The purpose in each of these methods is to determine the ratio of radiocarbon to stable carbon in the sample.From then on, the ratio of radiocarbon to stable carbon will decrease, because the unstable radiocarbon atoms will slowly decay. From this measurement the age in radiocarbon years is calculated. Modern radiocarbon dates are calibrated using long tree-ring chronologies.By cross-matching tree-ring sequences in individual specimens a long, continuous tree-ring chronology is constructed with very little dating uncertainty. for more information on tree-ring chronologies.) By measuring radiocarbon concentrations in these tree-rings of known age a calibration table is constructed giving the true date of a sample versus its raw radiocarbon date.The raw radiocarbon date of any sample can then be converted to true date by using this calibration table.This is because the amount and strength of cosmic radiation entering the earth's atmosphere has varied over time.(This, in turn, is caused by variations in the magnetic fields of the earth and sun, for example.) Although the ratio of radiocarbon to stable carbon in the atmosphere has varied over time, it is quite uniform around the globe at any given time because the atmosphere mixes very quickly and constantly.
Libby and a team of scientists at the University of Chicago.After about 50,000 years, the radiocarbon concentration remaining is too small to be measured for the purpose of radiocarbon dating. This is necessary to remove errors in raw radiocarbon dates caused by fluctuations in the amount of radiocarbon in the atmosphere in the past.Radiocarbon dating works by precisely measuring the ratio of radiocarbon to stable carbon in a sample. The tree-ring chronologies have been constructed by counting the annual rings in living trees and matching patterns in these rings to older wood and dead trees.This ratio is the same for all organisms across the globe at a given time due to the mixing of the atmosphere mentioned above.When an organism dies (whether plant or animal) its intake of carbon atoms ceases.