降水時におけるガンマ線線量率上昇事象の再現期間の推定(第2部:和文論文)

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In order to contribute for the establishment of criteria whether the high level of gamma (γ) ray dose rate is an unusual value, statistical analysis for its frequency distribution was carried out. The γ ray dose rates monitored at six sites (Minamihoroni, Kariwa, Wajima, Asahikawa, Ohno, and Tokai-mura Oshinobe) in Japan during the periods from 2000 to 2004 were subjected to the analysis. The γ ray dose rates were not normally distributed but the distribution distorted to higher side. Our statistical analysis suggests that the γ ray dose rates above "average +3σ" or "100nGy/h", which are the conventional definition, occurred frequently. Here we propose the use of more suitable method to detect the unusual value of the enhanced γ ray dose rates on the basis of the "return period". It is the recurrence frequency of high γ ray dose rates. Estimate of the return periods, which is usually used for the extreme event of meteorology, is given for the first time in this field. In this study, we compared two methods to calculate the return periods: empirical return period (T_H) based on Hazen plot by using the enhanced γ ray dose rates and theoretical estimated return period (T_G) based on Gumbel distribution by using average and standard deviation of the enhanced γ ray dose rates. There were significant differences of T_H as well as T_G between monitoring sites. It turned out that the judgment of an unusual value must be considered based on T_H and T_G at each monitoring site.
日本保健物理学会の論文

降水時におけるガンマ線線量率上昇事象の再現期間の推定(第2部:和文論文)

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