The physics behind 14C dating
Three carbon isotopes
In nature, carbon is found in the form of three different isotopes:12C, 13C und 14C. The greatest part by far (approx. 99 %) is made up of the stable isotope, followed by 13C (approx. 1 %) and the radioactive isotope 14C (10-10 %). Statistically speaking, every trillionth carbon atom is radioactive, so that in modern atmospheric CO2 samples the proportion of 14C to 12C is approximately 1 to 1012. This corresponds to 0.23 Bq per gram of carbon. Radioactive 14C is also known as radiocarbon.
Formation of 14C
14C is formed in the upper atmosphere when neutrons of the cosmic radiation collide with nitrogen atoms (14N). The following nuclear reaction can occur:
During this reaction, a 14N nucleus is converted to a 14C nucleus:
Radioactive decay of 14C
Contrary to the stable isotopes 12C and 13C, radiocarbon atoms are subject to β−-decay with a half-life of 5730 ± 40 years. Each 14C atom decays to form one 14N atom, one electron, and one antineutrino:
By measuring the current rate of decay and comparing that rate with a calibration curve, the age of organic materials can be determined. The calibration process also takes into account factors such as fractionation, Suess effect, nuclear weapons effect, and variations of the14C to 12C ratio over time.
Thanks to its half-life of 5730 years, radiocarbon is ideally suited for the dating of historical samples (wood, peat, corals, etc.) up to an age of approx. 50,000 years. Our department is also renowned for dating groundwater in the age range of 500 to 20,000 years.