Also known as radiocarbon dating, it is a method of dating – measuring the age of (usually archaeological) materials that contain matter of living origin. It is based on the fact that ¹⁴C, a beta emitter of half-life approximately 5730 years, is being formed continuously in the atmosphere as a result of cosmic-ray action.
The ¹⁴C becomes incorporated into living organisms. After death of the organism the amount of radioactive carbon decreases exponentially by radioactive decay. The ratio of ¹²C to ¹⁴C is thus a measure of the time elapsed since the death of the organic material.
The method is most valuable for specimens of up to 20 000 years old, though it has been modified to measure ages up to 70 000 years. For ages of up to about 8000 years the carbon time scale has been calibrated by dendrochronology; i.e. by measuring the ¹²C:¹⁴C ratio in tree rings of known antiquity.