GM管式サーベイメータの計数値とOIL4の初期設定値との対応関係

Correspondence between the count rates of the various GM tube survey meters and the initial setting value based on OIL4

中西 千佳; 平山 悠介; 秋山 聖光

Nakanishi, Chika; Hirayama, Yusuke; Akiyama, Kiyomitsu

原子力災害対策指針では、不注意な経口摂取、皮膚汚染からの外部被ばくを防止するため、除染を講じるための基準として、OIL4を設定した。OIL4の初期設定値は線で13000cpmであるが、計測器の仕様によっては、計数率の換算が必要である。そこで代表的なGMサーベイメータ4機種について、各計測器におけるOIL4の初期設定値に相当する計数率の目安値について検討した。その結果、OIL4の初期設定値に相当する計数率は、計測器により7,000-9,000cpmと異なることがわかった。また、スクリーニング時に検出器が身体表面等から離れた場合として、検出窓と線源間の距離が5cmの場合のOIL4の初期設定値に相当する計数率を算出したところ、3,000-6,000cpmとなった。各防災関係機関においては、予め所有機器の計数率と表面密度との関係及び検出器または検出窓と線源間の距離と計数率との関係を求めておき、それぞれOIL4の初期設定値に相当する計数率を把握しておくことが肝要である。

After the Fukushima Daiichi Nuclear Power Station accident, the Nuclear Emergency Response Guidelines developed by the Nuclear Regulatory Authority of Japan has introduced Operational Intervention Levels (OILs). Particularly, the screening level defined as OIL4 of decontamination against surface contaminations on the skin, clothes and others has adopted, and the default OIL4 value is 13,000 cpm. The count rate value indicated by GM survey meters varies according to the model. Thus, count rates measurements using the same radiation source were performed with four typical GM survey meters in this work, and the instrument efficiencies were compared. In addition, the dependences of the distance between the detector and the source are also evaluated between 1 cm and 5 cm. The screening levels corresponding to OIL4 for each GM survey meter were derived, and the value has 7,000-9,000 cpm from the instrument efficiencies. Count rates were decreased with the increasing distance from the source at the different ratios depending on the model. Then the screening levels at 5 cm between the source and the detector windows showed 3,000-6,000 cpm. This study suggested the importance to evaluate the intrinsic characteristics of the model and to derivate the appropriate screening level at a few centimeters from the screening object to detect contamination reliably.