carbon-14

Carbon-14, 14C, or radiocarbon, is a radioactive isotope of carbon discovered on February 27, 1940, by Martin Kamen and Sam Ruben at the University of California Radiation Laboratory in Berkeley.[2] Its nucleus contains 6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon dating method to date archaeological, geological, and hydrogeological samples.

There are three naturally occurring isotopes of carbon on Earth: 99% of the carbon is carbon-12, 1% is carbon-13, and carbon-14 occurs in trace amounts, e.g. making up as much as 1 part per trillion (0.0000000001%) of the carbon in the atmosphere. The half-life of carbon-14 is 5,730±40 years. It decays into nitrogen-14 through beta-decay.[3] The activity of the modern radiocarbon standard[4] is about 14 disintegrations per minute (dpm) per gram carbon.[5]

The atomic mass of carbon-14 is about 14.003241 amu. The different isotopes of carbon do not differ appreciably in their chemical properties. This is used in chemical research in a technique called carbon labeling: some carbon-12 atoms of a given compound are replaced with carbon-14 atoms (or some carbon-13 atoms) in order to trace them along chemical reactions involving the given compound.
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 + 1H

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 can also be produced in ice by fast neutrons causing spallation reactions in oxygen.

Carbon-14 then goes through radioactive beta decay.

\mathrm{~^{14}_{6}C}\rightarrow\mathrm{~^{14}_{7}N}+ e^- + \bar{\nu}_e

By emitting an electron and an anti-neutrino, 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 million Curies, of which most is in the oceans.[6]