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IGERT program in Archeological Science

Principles of AMS

Radiocarbon dating





Age Calculation


Cosmogenic Radioisotopes



History of Radiocarbon Dating
Photograph circa 1968, courtesy of University Archives, The Bancroft Library. 1900:24.

    The first use of radiocarbon dating was the work of Libby and his co-workers [Anderson et al. 1947]. Libby's original measurements on 14C were done by counting the decays of 14C, using samples of several grams of carbon-black powder. Unfortunately, in the 1950's, due to atmospheric nuclear testing, this method was subject to errors due to the absorption of nuclear contaminant.

     More accurate methods were developed using gas-proportional counters and liquid-scintillation counters. These

methods relied on the observation of a decay of the radioactive carbon atoms. When a 14C atom decays, it emits a beta-particle, which can be counted in a gas by the electrical pulse it generates. In a liquid-scintillation counter, the beta-particle excites the emission of light from a complex organic molecule or "scintillant". Because only about 13.5 decays per minute occur in one gram of modern carbon, it was necessary to use fairy large samples of several grams of carbon.

    It was recognized that direct measurement of the number of 14C atoms in the sample would greatly enhance the sensitivity, and some unsuccessful attempts were made in this direction using conventional mass spectrometry. In 1977, two papers [Nelson et al. 1977; Bennett et al. 1977] were published simultaneously in Science, reporting on a development which added a particle accelerator into a mass spectrometer to produce an accelerator mass spectrometer (AMS). This technique has allowed the measurement of radiocarbon in samples of much less than a milligram, or over a thousand times less material than is needed from the older counting methods.

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