Dating artefacts

Large sample sizes were needed for both counting methods, which limited their usefulness in such applications as studies of artwork, where only small samples could be taken.Accurate dating also had to wait for a good calibration of the radiocarbon time-scale in the 1960s, using an absolute chronology based on tree rings.

Accelerator mass spectrometry (AMS) is a technique for direct measurement of the concentration of radioisotopes.By the end of 1997, some two dozen AMS laboratories were in operation around the world, with more in the planning stages.C produced in the atmosphere were always the same, then we could calculate a "radiocarbon age" using the equation we have discussed directly as an estimate of sample age. This was recognized soon after Libby published his first Curve of Knowns (Arnold and Libby, 1949).When a C 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 fairly large samples of several grams of carbon.

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