水素還元法によるAMS法炭素-14測定のためのグラファイトターゲットの作成法

概要

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Carbon-14 has been used extensively as a tracer in environmental science and a dating method for geological and archaeological samples. The recent developement in ^<14>C measurements by accelerator mass spectrome-tory (AMS) has been reduced amounts of carbon and its application has been expanded largely to many new fields. However, several difficulties have been remained in dealing with small amounts of carbon. We have improved the H_2 reduction method with iron powder (Vorgel, et al., 1984,1987) and developed a simple and reliable method of graphite target preparation applicable to samples of sub-milligram carbon. The targets have intense and long-lived ion beams and extremely low ^<14>C background and have allowed us to achieve : (1) For samples in the range of 0.2-3mg carbon, ^<14>C background of targets from coal is 0.11% modern on average (equivalent age 55kyr) and do not vary with the amount of carbon. (2) The isotopic fractionation during the preparation is within 3〓. (3) The beam intensity of produced targets is typically 75% of that of natural graphite. The required time to accumulate 10,000 counts of ^<14>C for a modern sample is 10-15 minutes. (4) Some samples for duration tests of beam currents yield stable currents that decrease gradually and extreamly stable isotope ratio (^<14>C/^<13>C). The AMS ^<14>C measurements for milligram to sub-milligram carbon are made possible with high accuracies.
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