Plants and animals assimilate carbon 14 from carbon dioxide throughout their lifetimes.
When they die, they stop exchanging carbon with the biosphere and their carbon 14 content then starts to decrease at a rate determined by the law of radioactive decay.
Measurements have shown that in recent history, radiocarbon levels have remained relatively constant in most of the biosphere due to the metabolic processes in living organisms and the relatively rapid turnover of carbonates in surface ocean waters.
Gas proportional counting is a conventional radiometric dating technique that counts the beta particles emitted by a given sample. In this method, the carbon sample is first converted to carbon dioxide gas before measurement in gas proportional counters takes place.
Liquid scintillation counting is another radiocarbon dating technique that was popular in the 1960s.
Radiocarbon dating is essentially a method designed to measure residual radioactivity.
By knowing how much carbon 14 is left in a sample, the age of the organism when it died can be known.