From AMS Glossary
Name given to the isotope 14C, which has a half-life of about 5500 years and is formed in the upper atmosphere by the reaction of neutrons on nitrogen.
It has also been formed locally by nuclear detonations in the upper atmosphere. Thus, it is useful as a radioactive tracer for distinguishing stratospheric transport and for dating organic matter. As a result of the gradual decay of this isotope, carbon-containing compounds that have been deposited at the earth's surface have a different isotopic signature than the carbon in the atmosphere.