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Carbon-14, 14C, or radiocarbon, is a radioactive isotope of carbon discovered on February 27, 1940, by Martin Kamen and Sam Ruben. Its nucleus contains 6 protons and 8 neutrons. Its presence in organic materials is used extensively as basis of the radiocarbon dating method to date archaeological, geological, and hydrogeological samples. It occurs naturally and has a relative abundance up to 1 part per trillion (0.0000000001%) of all naturally-occurring carbon on Earth. The half-life of carbon-14 is 5730±40 years. It decays into nitrogen-14 through beta-decay.* The activity of modern standard radiocarbon is about 14 disintegrations per minute (dpm) per gram carbon (ca. 230 mBq/g). 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 participate in the following reaction: n + 14N → 14C + 1H The highest rate of carbon-14 production takes place at altitudes of 9 to 15 km (30,000 to 50,000 feet), 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. Most of man-made chemicals are made of fossil fuels, such as petroleum or coal, in which the carbon-14 has long since decayed. Presence of carbon-14 in the isotopic signature of a sample of carbonaceous material therefore indicates its possible biogenic origin and relatively recent geologic age.
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