Carbon-14 is a radioactive isotope of carbon with a half-life of 5,730 years. Carbon-14 can be used to find the age of formerly living things through a process known as radiocarbon dating. The principle behind this form of dating is as follows.
It is known that a small amount of naturally-occurring carbon is carbon-14. Although carbon-14 decays into nitrogen-14 through beta decay, the amount of carbon-14 in the environment remains constant because new carbon-14 is always being created in the upper atmosphere by cosmic rays. Living things tend to ingest materials that contain carbon, so the percentage of carbon-14 within living things is the same as the percentage of carbon-14 in the environment. Once an organism dies, the carbon-14 within it is no longer replaced and the percentage of carbon-14 begins to decrease as it decays. By measuring the percentage of carbon-14 in the remains of an organism, and by assuming that the natural abundance of carbon-14 has remained constant over time, scientists can estimate when that organism died. For example, if the concentration of carbon-14 in the remains of an organism is quarter of the natural concentration of carbon-14, a scientist would estimate that the organism died about 10,500 years ago – twice the half-life of carbon-14.