carbon dating There are two isotopes of carbon, namely C14 and C12. C14 is radioactive and has half life of 5600 years. C14 is formed in the atmosphere by cosmic rays. In atmosphere there is a radioactive equilibrium between the production of C14, and its depletion by radioactive disintegration. Therefore the percentage of C14, in the atmosphere remains nearly constant.
In the tissues of plants and animals, the amount of C14 is in equilibrium with C14 in the atmosphere. Its concentration is 10-6 % of C12 in the living tissue. The intake of C14 stops after a living organism dies. Hence its concentration will continuously fall in dead tissue. By measuring the concentration of C14 in the fossil of animals and plants, the time since the death occurred, can be computed.