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Measurements of Ir concentration in geological standard samples using neutron activation analysis with multiple $$gamma$$-ray coincidence method

多重$$gamma$$線放射化分析法による標準地質学試料中のイリジウムの分析

初川 雄一; 大澤 崇人; 大島 真澄; 藤 暢輔; 木村 敦; 小泉 光生; 古高 和禎

Hatsukawa, Yuichi; Osawa, Takahito; Oshima, Masumi; Toh, Yosuke; Kimura, Atsushi; Koizumi, Mitsuo; Furutaka, Kazuyoshi

多重$$gamma$$線放射化分析法により地球化学試料中のイリジウムの超高感度分析を行った。イリジウムは地殻中に含まれる元素のなかで最も存在量が少なく、1ppb程度しか存在しない。一方、始原的隕石中には豊富に存在しておりイリジウムの異常濃度は過去の地球への隕石衝突を探るうえで重要な証拠となっている。本研究では南アフリカ産のSARM-76、デンマーク産FC-1, FC-2を測定試料としてそれぞれの値付を行った。またそれぞれの試料の生成過程に違いから得られた測定結果について議論を加えた。

The Ir concentrations in some standard rock samples were determined by using the multiple $$gamma$$ ray detection method. The use of the multiple $$gamma$$ ray detection method, which was developed for nuclide quantification, yielded better resolution and sensitivity than the ordinary singles $$gamma$$ ray detection method. Iridium is one of the least abundant elements in Earth's crust, with an average mass fraction of 0.001 ppm in a crust rock. However, iridium is relatively more abundant (concentration: 0.5 ppm or more) in undifferentiated meteorites. Therefore iridium abundance anomalies in geological samples provide important information about meteorite impact. The standard rock samples used in this study were SARM-76, FC-1 and FC-2. The SARM-76 was prepared from a platinum ore, and issued by the South African Bureau of Standards. FC-1 and FC-2 were obtained from fish clay sediment samples from Stevens Klint, Denmark.

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