An unstable (radioactive) isotope of carbon with atomic mass 14 that is produced in the atmosphere by cosmic radiation. It is the basis for radiocarbon dating, the method most frequently used in archaeology. It acts like C12, being taken into the organic compounds of all living matter. The proportions of radioactive and inert carbon are identical throughout the vegetable and animal kingdoms' carbon cycle. When organic matter dies it ceases to exchange its carbon, as carbon dioxide, with the atmosphere, so its C14 dwindles by decay and is not replenished. Determination of the radioactivity of carbon from a sample will reveal the proportion of C14 to C12, and this will in turn, through the known rate of decay of C14, give the age of, or more accurately the time elapsed since the death of, the sample.