Carbon and Cosmic Rays
The atom of Carbon constitutes a substantial part of living animal and vegetal tissue and has a molecular weight of 12.
Cosmic rays cause Carbon-12 to decay gradually into its unstable and weakly radioactive isotope Carbon-14 (Radiocarbon) at a known rate, but it is replaced by Carbon-12 atoms at a constant rate and therefore it is nearly constant at any given time - about 1 in a trillion Carbon atoms are Carbon-14.
However, when life of the hosting organism ceases its decay continues but the above replacement can no longer occur. Therefore, measuring the radioactivity of a sample of dead organic tissue gives an indication of how long ago its death occurred.
Carbon-14 decays with its half-life of 5,700 years - i.e. its percentage is halved - while the amount of Carbon-12 remains constant in the sample.
By determining the ratio of carbon-12 to carbon-14 in the sample and comparing it to that in a currently alive organism, it is possible to determine the age of a formerly living thing fairly precisely using the following formula:
Age = [ logn (Nf/No) / (-0.693) ] x 5,700 years
where Nf/No is the percentage of Carbon-14 in the sample to the amount in living tissue.