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There is a quantitative relationship between the decay of 14C and the production of a beta particle. That is, the probability of decay for an atom of 14C in a discrete sample is constant, thereby requiring the application of statistical methods for the analysis of counting data.
It follows from this that any material which is composed of carbon may be dated.
Libby and his team intially tested the radiocarbon method on samples from prehistoric Egypt.
They chose samples whose age could be independently determined.
There is a useful diagrammatic representation of this process given here Libby, Anderson and Arnold (1949) were the first to measure the rate of this decay.
They found that after 5568 years, half the C14 in the original sample will have decayed and after another 5568 years, half of that remaining material will have decayed, and so on (see figure 1 below).
Plants and animals which utilise carbon in biological foodchains take up 14C during their lifetimes.
They exist in equilibrium with the C14 concentration of the atmosphere, that is, the numbers of C14 atoms and non-radioactive carbon atoms stays approximately the same over time.
As soon as a plant or animal dies, they cease the metabolic function of carbon uptake; there is no replenishment of radioactive carbon, only decay.
The radiocarbon method was developed by a team of scientists led by the late Professor Willard F.
Libby of the University of Chicago in immediate post-WW2 years.