A Tandetron accelerator mass spectrometer (AMS), based on mainly a tandem electrostatic accelerator and an apparatus to analyze charge state, energy, mass number, and atomic number of accelerated ions, has been used to measure
14C ages of geological and archeological samples, as well as
14C concentrations of natural samples, since 1983 at Nagoya University.
Traditional methods of radioactivity measurement have been applied to determine
14C ages of natural samples since 1947. However, their applications have been restricted to samples younger than 30000 to 40000y.B.P., as well as to ones from which 0.1 to 5 grams of carbon can be recovered. Usage of the Tandetron spectrometer has overcome these limitations. The amount of carbon necessary for the spectrometer is 0.2 to 5mg, being more than three orders of magnitude less than for traditional measurement. In addition, the oldest age measurable with the spectrometer has been extended to c.a. 60000y.B.P., as discussed in detail here.
The
14C background level of the spectrometer has been estimated by measuring
14C content of commercial graphite and mineral graphite from an ore deposit in Sri Lanka (Ceylon), which are too old to contain
14C. The background level corresponds to the apparent
14C age of 66340y.B.P. for commercial graphite and 64440y.B.P. for Ceylon graphite, as shown in Fig. 2. Amorphous carbon reduced by magnesium and graphite-like carbon deposited on iron-powder by hydrogen, from CO
2 prepared from “dead” carbon containing no
14C, show the apparent
14C ages of 37430 and 54000y.B.P., respectively. The solid carbon prepared from CO
2 gives a younger limit of the measurable oldest age than natural graphite, owing to the rather complicated preparation procedures which may introduce
14C contamination. Thus the method of producing amorphous carbon by reducing CO
2 with magnesium is not applicable to date such samples older than 40000y.B.P. The natural wood samples show
14C ages ranging from 59000 to 67860y.B.P., as shown in Fig. 3. These values are considered to indicate the oldest age measurable for woody materials.
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