Carbon-14 - Origin and Radioactive Decay

Origin and Radioactive Decay

Carbon-14 is produced in the upper layers of the troposphere and the stratosphere by thermal neutrons absorbed by nitrogen atoms. When cosmic rays enter the atmosphere, they undergo various transformations, including the production of neutrons. The resulting neutrons (1n) participate in the following reaction:

1n + 14N → 14C + 1p

The highest rate of carbon-14 production takes place at altitudes of 9 to 15 km (30,000 to 50,000 ft) and at high geomagnetic latitudes, but the carbon-14 readily mixes and becomes evenly distributed throughout the atmosphere and reacts with oxygen to form radioactive carbon dioxide. Carbon dioxide also dissolves in water and thus permeates the oceans.

Carbon-14 then goes through radioactive beta decay.

By emitting an electron and an electron antineutrino, carbon-14 (half-life of 5730 years) decays into the stable (non-radioactive) isotope nitrogen-14.

The inventory of carbon-14 in Earth's biosphere is about 300 megacuries (11 EBq), of which most is in the oceans.

As of 2008, the rate of carbon-14 production was not known - while the reaction can be modelled or the current concentrations and the global carbon budget can be used to backtrack, attempts to measure production had not agreed with these models. Production rates vary because of changes to the cosmic ray flux incident, such as supernovae, and due to variations in the Earth's magnetic field. The latter can create significant variations in carbon-14 production rates, although the changes of the carbon cycle can make these effects difficult to tease out.