Radiocarbon Dating - Physical and Chemical Background

Physical and Chemical Background

Carbon has two stable, nonradioactive isotopes: carbon-12 (12C), and carbon-13 (13C). In addition, there are trace amounts of the unstable isotope carbon-14 (14C) on Earth. Carbon-14 has a relatively short half-life of 5,730 years, meaning that the amount of carbon-14 in a sample is halved over the course of 5,730 years due to radioactive decay. Carbon-14 would have long ago vanished from Earth were it not for the unremitting cosmic ray flux interactions with the Earth's atmosphere, which create more of the isotope. The neutrons resulting from the cosmic ray interactions participate in the following nuclear reaction on the atoms of nitrogen molecules (N2) in the atmosphere:

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 spreads evenly throughout the atmosphere and reacts with oxygen to form carbon dioxide. Carbon dioxide also permeates the oceans, dissolving in the water. For approximate analysis it is assumed that the cosmic ray flux is constant over long periods of time; thus carbon-14 is produced at a constant rate and the proportion of radioactive to non-radioactive carbon is constant: ca. 1 part per trillion (600 billion atoms/mole). In 1958 Hessel de Vries showed that the concentration of carbon-14 in the atmosphere varies with time and locality. For the most accurate work, these variations are compensated by means of calibration curves. When these curves are used, their accuracy and shape are the factors that determine the accuracy of the age obtained for a given sample. 14C can also be produced at ground level at a rate of 1 x 10−4 atoms per gram per second, which is not considered significant enough to impact on dating without a known other source of neutrons.

Plants take up atmospheric carbon dioxide by photosynthesis, and are ingested by animals, so every living thing is constantly exchanging carbon-14 with its environment as long as it lives. Once it dies, however, this exchange stops, and the amount of carbon-14 gradually decreases through radioactive beta decay with a half-life of 5,730 ± 40 years. Carbon-14 is stored in different amounts in different reservoirs because there is a dynamic equilibrium between 14C production and decay.

Carbon-14 was discovered on February 27, 1940, by Martin Kamen and Sam Ruben at the University of California Radiation Laboratory at Berkeley.

Read more about this topic:  Radiocarbon Dating

Famous quotes containing the words physical and, physical, chemical and/or background:

    Happy is that mother whose ability to help her children continues on from babyhood and manhood into maturity. Blessed is the son who need not leave his mother at the threshold of the world’s activities, but may always and everywhere have her blessing and her help. Thrice blessed are the son and the mother between whom there exists an association not only physical and affectional, but spiritual and intellectual, and broad and wise as is the scope of each being.
    Lydia Hoyt Farmer (1842–1903)

    Unfortunately, moral beauty in art—like physical beauty in a person—is extremely perishable. It is nowhere so durable as artistic or intellectual beauty. Moral beauty has a tendency to decay very rapidly into sententiousness or untimeliness.
    Susan Sontag (b. 1933)

    We do not want actions, but men; not a chemical drop of water, but rain; the spirit that sheds and showers actions, countless, endless actions.
    Ralph Waldo Emerson (1803–1882)

    I had many problems in my conduct of the office being contrasted with President Kennedy’s conduct in the office, with my manner of dealing with things and his manner, with my accent and his accent, with my background and his background. He was a great public hero, and anything I did that someone didn’t approve of, they would always feel that President Kennedy wouldn’t have done that.
    Lyndon Baines Johnson (1908–1973)