加速器質量分析(AMS)による宇宙線生成放射性同位体の測定と若い地質年代測定への応用  [in Japanese] AMS measurement of cosmogenic-radioisotopes and its application to age determination of young geological samples  [in Japanese]

1977年に開発された加速器質量分析(AMS)は, タンデム加速器と加速されたイオンの電荷, エネルギー, 質量数および原子番号を検知するための付属装置を用いて, 天然に存在するごく微量の放射性同位体である^<10>Be, ^<14>C, ^<26>Al, ^<36>Cl, ^<41>Ca, ^<53>Mn, ^<129>Iなどを直接検出し定量する技術である。AMSの主たる応用は^<14>C年代測定である。1981-1982年にタンデトロンAMS装置が名古屋大学に設置され^<14>C年代測定が開始された。定常的に年間700〜800個の試料が, 約1.5mgの試料炭素を用いて行われており, 1997年末の測定総数は7300個にのぼる。1996年に新たに最新型のタンデトロンAMSが設置された。その性能は, 炭素試料は0.1〜1mgでよく, 測定誤差が±20〜±30年, 測定時間は40分程度で, 年間当たり3,000個の試料の測定ができる。さまざまな若い地質試料の年代測定の利用が期待されている。

Techniques of accelerator mass spectrometry (AMS), developed since 1977, based mainly on a tandem accelerator and associated apparatus used to analyze charge state, energy, mass number, and atomic number of accelerated ions, enabled us to measure extremely-low-abundance radioisotopes such as ^<10>Be, ^<14>C, ^<26>Al,^<36>Cl, ^<41>Ca, ^<53>Mn, ^<129>I, etc., in. natural samples. One of the main AMS application is ^<14>C measurement, and in particular, ^<14>C dating. A Tandetron AMS system, an apparatus dedicated to ^<14>C measurements with high sensitivity, manufactured by General Ionex Corporation, USA, has been used since 1983, to measure ^<14>C concentrations of environmental samples as well as ^<14>C dates of geological and archeological materials, at the Dating and Materials Research Center, Nagoya University. Routinely, about 700-800 samples have been measured a year, and totally 7,300 samples in various research fields have been measured since the installation of the machine. Recently, we have introduced a second Tandetron AMS system which was manufactured by High voltage Engineering Europe (HVEE), BV, the Netherlands. Two sets of the similar HVEE AMS system have previously been installed successfully at the University of Groningen, the Netherlands, and at the University of Christian-Albrechts, Kiel, Germany. They have already proved excellent performances in carbon-isotope-ratio measurements for graphite targets prepared by themselves from natural samples. The highest performances in the ^<14>C measurements with the new Tandetron AMS system are: (1) a measurement error of ^<14>C age may be as small as ±20 years with a measurement time of a few tens of minutes for a carbon sample of about 1 mg; (2) a full automatic measurement can be routinely performed; (3) more than 3,000 samples can be measured annually. Some promising and potential applications in various fields of geological researchare discussed here, with AMS measurement of ^<14>C as well as ^<10>Be, ^<26>Al, ^<129>I and other cosmogenic radioisotopes.