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Analysis of $$^{90}$$Sr in environmental samples at the attogram-level by accelerator mass spectrometry

加速器質量分析による環境試料中$$^{90}$$Srのアトグラムレベルでの分析

Martschini, M.*; 本多 真紀   ; Merchel, S.*; Winkler, S.*; Golser, R.*

Martschini, M.*; Honda, Maki; Merchel, S.*; Winkler, S.*; Golser, R.*

$$beta$$線放出核種であるために$$^{90}$$Srの定量分析は煩雑で時間がかかる。従来の加速器質量分析(AMS)法による$$^{90}$$Srの検出限界は、同重体$$^{90}$$Zrの干渉が主な原因で、$$beta$$線検出法の一般的な検出限界3mBqと同程度であった。ウィーン大学のAMS施設で実施している、世界的にユニークなイオンレーザー相互作用質量分析法(ILIAMS)は$$^{90}$$Zrをイオンレーザーとリアクションガスで効果的に除去するため、このイオンレーザー相互作用による同重体除去システムを装備していない従来のAMSシステムよりも$$^{90}$$Srの検出限界は優れている($$<$$0.1mBq)。本研究では極限条件(例えば$$mu$$Bq/gの低濃度、グラムオーダーの試料量)の環境試料であるサンゴ等の$$^{90}$$Sr分析を試みた。検出限界$$<$$0.1mBqを下げるために、大気圏内核実験由来の$$^{90}$$Sr汚染がほとんど無い古い年代のサンゴからSrを精製し(Sr担体)、環境試料の化学分離を実施し、AMSで$$^{90}$$Srを測定した。分析の結果、検出限界$$<$$0.03mBq($$^{90}$$Sr/Sr$$<$$5$$times$$10$$^{-16}$$$$beta$$線検出法の1/100)を達成した。本研究で達成した検出限界は、1mgのSrターゲットに含まれる$$^{90}$$Sr量2agに相当する。本発表では少量の一般環境試料中$$^{90}$$Srの高感度分析に成功した最新の成果を主に報告する。

The quantitative analysis of $$^{90}$$Sr is cumbersome and time-consuming because it is a pure $$beta$$-ray emitter. The detection limit of $$^{90}$$Sr by conventional accelerator mass spectrometry (AMS) methods is comparable to the typical detection limit of 3 mBq for $$beta$$-ray detection methods, mainly due to the interference of the isobaric $$^{90}$$Zr. The worldwide unique Ion Laser Interaction Mass Spectrometry (ILIAMS), performed at the University of Vienna, effectively removes $$^{90}$$Zr with a laser and reaction gases. Therefore, the detection limits of $$^{90}$$Sr is superior ($$<$$0.1 mBq) to conventional AMS systems not equipped with such an ion-laser interaction isobaric removal system. In this study, $$^{90}$$Sr analysis in coral samples etc., an environmental sample under extreme conditions (e.g. low concentrations of $$mu$$Bq/g and limited sample volumes of grams), was tried. To lower the detection limit $$<$$0.1 mBq, Sr was purified (Sr carrier) from old age coral with almost no $$^{90}$$Sr contamination from atmospheric nuclear tests, chemical separation of environmental samples were conducted, and $$^{90}$$Sr was measured by AMS. The results showed that the detection limit of $$^{90}$$Sr achieved in the analysis was $$<$$0.03 mBq ($$^{90}$$Sr/Sr $$<$$5$$times$$10$$^{-16}$$, 1/100 of the $$beta$$-ray detection methods). The detection limit achieved in this study corresponds to 2 ag of $$^{90}$$Sr in 1 mg of Sr target. The latest results of highly sensitive $$^{90}$$Sr analysis in small amounts of general environmental samples (coral, seawater, etc.) are mainly presented.

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