For organic material or artifacts composed of organic material, a different parent/daughter pair is used. Organic materials are based on hydrocarbon compounds. In 1955 a scientist at Caltech, W. F. Libby, proposed that dating of such material could be done by measuring the ratio of C-12, the most abundant isotope of carbon, to C-14, a radioactive isotope of carbon with a half-life of about 5730 yrs.

Carbon-14 occurs in nature but not in the same way as carbon-12. The supply of carbon-14 is the result of nuclear reactions in the upper atmosphere. In the stream of particles and EMR that constantly bombard the planet, neutrons occasionally collide with nitrogen atoms:

The carbon-14 formed by this mechanism gradually becomes incorporated into atmospheric carbon dioxide.