A study in <14>^C dating of old samples with a tandetron accelerator mass spectrometer

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  • タンデトロン加速器質量分析計による古い試料の<14>^C年代測定について

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Abstract

A Tandetron accelerator mass spectrometer (Tandetron AMS), based on mainly a tendem electrostatic accelerator and an apparatus to analyze charge state, energy, mass number, and atomic number of accelerated ions, has been used to measure ^<14>C ages of geological and archeological samples, as well as ^<14>C concentrations of natural samples, since 1983 at Nagoya University. Traditional methods of radioactivity measurement have been applied to determine ^<14>C ages of natural samples by Libby and researchers following him since 1947. However, their applications have been restricted to samples younger than 30,000 to 40,000 y.B.P., as well as ones from which a few grams of carbon can be recovered. Usage of the AMS has overcome these limitations. The amount of carbon necessary for the AMS is 100μg to 5mg, being more than three orders of magnitude less than for traditional measurement. In addition, the oldest age measurable with the AMS has been extended to c.a. 60,000 y.B.P. The ^<14>C background level of the Tandetron AMS at Nagoya University has been estimated by measuring ^<14>C counts for commercial graphite and mineral graphite from an ore deposit in Ceylon, which are too old to contain ^<14>C. The background level corresponds to the apparent age of 66,340 y.B.P. for the former and 64,440 y.B.P. for the latter. The natural wood samples show ^<14>C ages ranging from 59,000 to 67,860 y.B.P. These values are considered to indicate the oldest age measurable at present for natural woody materials.

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