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:
“It seems to me that your doctor [Tronchin] is more of a philosopher than a physician. As for me, I much prefer a doctor who is an optimist and who gives me remedies that will improve my health. Philosophical consolations are, after all, useless against real ailments. I know only two kinds of sickness—physical and moral: all the others are purely in the imagination.”
—Philip Dormer Stanhope, 4th Earl Chesterfield (1694–1773)
“Women generally should be taught that the rough life men must needs lead, in order to be healthy, useful and manly men, would preclude the possibility of a great degree of physical perfection, especially in color. It is not a bad reflection to know that in all probability the human animal has endowments enough without aspiring to be the beauty of all creation as well as the ruler.”
—Caroline Nichols Churchill (1833–?)
“We are close to dead. There are faces and bodies like gorged maggots on the dance floor, on the highway, in the city, in the stadium; they are a host of chemical machines who swallow the product of chemical factories, aspirin, preservatives, stimulant, relaxant, and breathe out their chemical wastes into a polluted air. The sense of a long last night over civilization is back again.”
—Norman Mailer (b. 1923)
“Pilate with his question “What is truth?” is gladly trotted out these days as an advocate of Christ, so as to arouse the suspicion that everything known and knowable is an illusion and to erect the cross upon that gruesome background of the impossibility of knowledge.”
—Friedrich Nietzsche (1844–1900)