抄録
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, OIL4 that is the screening level for decontamination against surface contaminations on the skin, clothes and others was adopted, and the default OIL4 value is 13,000 cpm. The guideline mentions that conversion of count rate is necessary when using different models of GM tube survey meters. Thus, count rate measurements using a surface radiation source of 10 cm square were performed with four typical GM tube survey meters in this work, and the instrument efficiencies were compared. In addition, the dependences of the distance from the detector window of the GM tube survey meter to the source are also evaluated between 1 cm and 5 cm. The resulting count rates for OIL4 of the tested GM tubes survey meters varied between 7,000 and 9,000 cpm. Count rates were decreasing as distance from the source increased with the different ratios depending on the model of GM tube survey meter. The screening levels showed between 3,000 and 6,000 cpm when distances between the source and the detector windows were 5 cm. This study suggested the importance to evaluate the intrinsic characteristics of the model and to derive the appropriate screening level at a few centimeters from the screening object in order to detect contamination reliably.
