A method used to determine the age of organic materials up to 70 000 years old. It relies on the fact that the 14
C isotope of carbon is unstable and decays, emitting beta rays, to 14
N, with a half-life of about 5700 years. Plants incorporate 14
into their tissues during photosynthesis but when they die the concentration of 14
C 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 14
C using a mass spectrometer. The method assumes that the 14
C 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.