選択性吸着ディスクとNalシンチレーションカウンターを用いた表流水中放射性セシウ厶の迅速微量分析方法の確立と特性評価

摘要

We developed a novel rapid analysis method to measure trace levels of radiocaesium in surface water by using absorbent disks for radioactive caesium with a Nal scintillation counter (ADiCS). ADiCS involves 1) the selective absorption of dissolved radiocaesium onto the disk and the collection of particulate radiocaesium on the PTFE membranes by surface water filtration and then 2) directly measuring the radioactivity on PTFE membranes and the disk by a NaI scintillation counter. Evaluation of the accuracy and rapidity of ADiCS for 20 L samples with less than 20 mg·L~(-1) suspended solids showed that ADiCS needed six times less concentration time than the evaporation method and that the limit of detection was approximately 10 mBq·L~(-1) . Moreover, radioactivity data obtained by ADiCS had less than 30 % of the error of data obtained by the evaporation method. ADiCS also revealed a significant increase in radiocaesium in the Kanto region after the nuclear power plant accident. Especially, a radiocaesium concentration of over 100 mBq·L~(-1) was observed in surface water around high radiocaesium deposition areas. These results indicated ADiCS can evaluate current and future trace levels of radiocaesium in surface water, including bioaccumulation characteristics, more easily than the evaporation method.%表流水中微量放射性セシウム放射能測定法(ADiCS法)を確立した。ADiCS法は前段のPTFE膜による懸濁態セシウムの分離，後段の選択性ディスクによる溶存態セシウムの吸着を連続的に行い，その後PTFE膜とディスクの放射能をNaIシンチレーションカウンターで測定する。作業効率性およぴ分析精度を検討した結果，SS濃度が20mg·L~(-1)以下の表流水20Lでは蒸発乾固法より約6倍以上迅速に濃縮でき，検出下限値は約10mBq·L~(-1)であった。また，ADiCS法測定値は蒸発乾固法測定値の30%の誤差範囲内で一致した。ADiCS法による関東地方の表流水測定の結果，高沈着量地域の表流水から100mBq·L~(-1)以上の放射性セシウムが検出され，事故後の有意な増加が確認できた。以上より，従来法に比較して本手法は容易に現在および将来の水中放射能や水生生物中蓄積特性を評価しうると考えられた。