Radiocarbon Dating
Radiocarbon dating is a technique for determining the age of very old objects consisting of organic (carbon-based) materials, such as wood, paper, cloth, and bone. The technique is based on the fact that both stable and radioactive isotopes of carbon exist. These isotopes behave almost identically in biological, chemical, and physical processes.
Carbon-12, a stable isotope, makes up about 99% of all carbon found in nature. Radioactive carbon-14 is formed in the atmosphere when neutrons produced in cosmic ray showers react with nitrogen atoms.
Despite the fact that it makes up no more than 0.08% of the earth's crust, carbon is an exceedingly important element. It occurs in all living materials and is found in many important rocks and minerals, including limestone and marble, as well as in carbon dioxide. Carbon moves through the atmosphere, hydrosphere, lithosphere, and biosphere in a series of reactions known as the carbon cycle. Stable and radioactive isotopes of the element take part in identical reactions in the cycle. Thus, when green plants convert carbon dioxide to carbohydrates through the process of photosynthesis, they use both stable carbon-12 and radioactive carbon-14 in exactly the same way. Any living material consists, therefore, of a constant ration of carbon-14 to carbon-12.
In the mid-1940s, Willard F. Libby realized that this fact could be used to date organic material. As long as that material was alive, he pointed out, it should continue to take in both carbon-12 and carbon-14 in a constant ratio. At its death, the material would no longer incorporate carbon in any form into its structure. From that point on, the amount of stable carbon-12 would remain constant. The amount of carbon-14, however, would continuously decrease as it decayed by beta emission to form nitrogen. Over time, the ratio of carbon-14 to carbon-12 would grow smaller and smaller. That ratio would provide an indication of the length of time since the material had ceased being alive.
Radiocarbon dating has been used to estimate the age of a wide variety of objects ranging from charcoal taken from tombs to wood found in Egyptian and Roman ships. One of its most famous applications was in the dating of the Shroud of Turin. Some religious leaders had claimed that the Shroud was the burial cloth in which Jesus was wrapped after his crucifixion. If so, the material of which it was made would have to be nearly 2,000 years old. Radiocarbon dating of the material showed, however, that the cloth could not be more than about 700 years old.
Radiocarbon dating can be used for objects up to 30,000 years of age, but it is highly reliable only for objects less than 7,000 years old. These limits result from the fact that eventually carbon-14 has decayed to such an extent that it can no longer be detected well or, eventually, at all in a sample. For older specimens, radioisotopes with longer half-lives can be used for age determination.
Half-Life; Radioactive Decay
Resources
Books
Taylor, R. E., et al., eds. Radiocarbon Dating: An Archaeological Perspective. Orlando: Academic Press, 1987.
——. Radiocarbon After Four Decades: An Interdisciplinary Perspective. New York: Springer Verlag, 1992.
This is the complete article, containing 516 words
(approx. 2 pages at 300 words per page).
