A method used to determine the age of organic materials up to 70 000 years old. It relies on the fact that the
14C isotope of carbon is unstable and decays, emitting beta rays, to
14N, with a half-life of about 5700 years. Plants incorporate
14C0
2 into their tissues during photosynthesis but when they die the concentration of
14C starts to fall at a rate related to the half-life. By comparing a specimen of unknown age with a sample of zero age, the age of the specimen may be calculated by measuring the amount of
14C using a mass spectrometer. The method assumes that the
14C:
12C ratio in the atmosphere has always remained constant. Certain discrepancies between age determinations based on radiocarbon dating and *
dendrochronology show there have been systematic variations in this ratio.
