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環境放射能学の発展を目指したAMSにおけるアプリケーションの開拓; $$^{90}$$Srと$$^{135}$$Cs AMSの実用化に向けた研究

Pioneering applications in AMS for the development of environmental radiology; Studies towards the implementation of $$^{90}$$Sr and $$^{135}$$Cs AMS

本多 真紀   ; Martschini, M.*; Lachner, J.*; Marchhart, O.*; Wieser, A.*; Priller, A.*; Steier, P.*; Golser, R.*; 坂口 綾*

Honda, Maki; Martschini, M.*; Lachner, J.*; Marchhart, O.*; Wieser, A.*; Priller, A.*; Steier, P.*; Golser, R.*; Sakaguchi, Aya*

人工放射性核種である$$^{90}$$Sr(28.79年)及び$$^{137}$$Cs(30.1年)の環境動態調査において、更なる研究の発展を目指して、廃棄物・環境安全研究グループでは加速器質量分析(AMS)による新規の$$^{90}$$Sr, $$^{135}$$Cs(230万年)分析法の開発を進めてきた(長半減期の$$^{135}$$Csは$$^{137}$$Csのプロキシとして利用する)。AMS法の利点は簡略な化学分離で微量の$$^{90}$$Srと$$^{135}$$Csを分析可能なことである。$$^{90}$$Srに関しては、IAEAが頒布している$$^{90}$$Sr濃度既知の放射能環境標準物質等から、2日程度で完了する化学分離でSrF$$_{2}$$ターゲットを調製し、ウィーン大学VERAで$$^{90}$$Srを測定した。その結果、$$beta$$線測定に匹敵する検出限界$$<$$0.1mBqを得た。更に環境試料中$$^{90}$$Srの検出にも成功し、環境試料へ適応できることが示された。一方で$$^{135}$$Cs AMSは測定においていくつか課題があるため、試験測定を進めている。

We have developed new analytical methods for$$^{90}$$Sr (28.79 yr) and $$^{135}$$Cs (2.3x10$$^{6}$$ yr) by accelerator mass spectrometry (AMS) for the further development of the research on the environmental fate of artificial radionuclides$$^{90}$$Sr and $$^{137}$$Cs (30.1 yr). The AMS method enables the analysis of trace amounts of$$^{90}$$Sr and $$^{135}$$Cs with a simple chemical separation procedure. Having a longer half-life $$^{135}$$Cs applies as a proxy for $$^{137}$$Cs. For $$^{90}$$Sr, SrF$$_{2}$$ targets were prepared from radioactive environmental reference materials with known $$^{90}$$Sr concentrations (IAEA) by a chemical separation procedure that takes about two days to complete. $$^{90}$$Sr measured by AMS at VERA, University of Vienna. As a result, a limit of detection of $$<$$0.1 mBq was obtained, which is comparable to the $$beta$$ ray measurement. The detection of $$^{90}$$Sr in environmental samples was also successful, indicating that the $$^{90}$$Sr AMS can apply to environmental samples. On the other hand, there are still some technical challenges in the $$^{135}$$Cs AMS. Therefore, the experimental measurements are in progress.

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