Radiocarbon dating

Carbon has three naturally occurring isotopes, i.e. carbon atoms that have equal numbers of protons but different numbers of neutrons and thus different masses. The most abundant isotope is 12C making up approximately 98.89 % of all carbon in nature. The abundance of the stable isotope 13C is about 1.11 %, whereas the radioactive 14C is present in only very small quantities of 1.176 x 10-12 atoms per atom 12C.

The existence of 14C in nature and its use for dating was first demonstrated by Willard Libby (1946) who received the Nobel Prize in chemistry for his investigations. The radiocarbon dating method is generally suitable for age determination up to 50,000 years.

14C is mainly produced in the upper atmosphere by collision of low-energy cosmic ray neutrons with nitrogen atoms (14N) of the air. The produced 14C is oxidised rapidly to 14CO2 which enters the plant biomass by assimilation of CO2 during photosynthesis and subsequently the heterotrophic organisms via the food chain. Various exchange processes between the atmosphere, the biosphere, and the hydrosphere result in a dynamic equilibrium between 14C production and decay, displayed by a constant14C/12C ratio. Exchange rates between the different reservoirs differ greatly depending on the respective carbon dynamics.

After death of an organism the exchange of 14C with the atmosphere is stopped and the radiocarbon concentration begins to decrease through radioactive decay with a half-life of 5730 years. The residual 14C concentration of a sample at a certain time, which is defined by the radioactive decay equation, can be determined by counting the decay of 14C atoms or by determining the 14C/12C abundance ratio in comparison to standard materials (accelerator) mass spectrometrically.

Conventional beta-decay counting methods count 14C atoms which decay within the measurement period of several days. In contrast, accelerator mass spectrometry (AMS) determines the 14C/12C abundance ratio by counting the isotopes themselves. This makes it possible to reduce the necessary sample size from over 1 gram to less than 1 milligram of carbon and the measuring time from days to hours.