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By measuring the remaining 14C in a sample of excavated material, a calendar date could therefore be ascribed to it.
Libby worked out a way to do this that involved measuring the emissions of beta particles using a Geiger counter.
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In living organisms, which are always taking in carbon, the levels of carbon 14 likewise stay constant.
Most carbon atoms have six protons and six neutrons in their nuclei and are called carbon 12. But a tiny percentage of carbon is made of carbon 14, or radiocarbon, which has six protons and eight neutrons and is not stable: half of any sample of it decays into other atoms after 5,700 years.
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.
So by measuring carbon 14 levels in an organism that died long ago, researchers can figure out when it died.
The procedure of radiocarbon dating can be used for remains that are up to 50,000 years old.That is why radiocarbon dates are now ‘calibrated’ using a tree-ring calibration curve.