Carbon 14 is continually being created in the Earth's atmosphere by the interaction of nitrogen and gamma rays from outer space.
Since atmospheric carbon 14 arises at about the same rate that the atom decays, the Earth's levels of carbon 14 have remained constant.
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Research has been ongoing since the 1960s to determine what the proportion of in the atmosphere has been over the past fifty thousand years.
The resulting data, in the form of a calibration curve, is now used to convert a given measurement of radiocarbon in a sample into an estimate of the sample's calendar age.
This technique works well for materials up to around 50,000 years old.
Each radioactive isotope decays by a fixed amount, and this amount is called the half-life.
The older a sample is, the less (the period of time after which half of a given sample will have decayed) is about 5,730 years, the oldest dates that can be reliably measured by this process date to around 50,000 years ago, although special preparation methods occasionally permit accurate analysis of older samples.
The idea behind radiocarbon dating is straightforward, but years of work were required to develop the technique to the point where accurate dates could be obtained.
He first noted that the cells of all living things contain atoms taken in from the organism's environment, including carbon; all organic compounds contain carbon.
Most carbon consists of the isotopes carbon 12 and carbon 13, which are very stable.
In living organisms, which are always taking in carbon, the levels of carbon 14 likewise stay constant.