Application of return period to evaluation of extreme events of gamma ray dose rate due to precipitation

The concept of the return period, which is a recurrence interval for the establishment of criteria whether the high level of gamma (γ) ray dose rate is an unusual value, is very effective. In this study, estimation method of the return period is described with using two probability functions, the Hazen plot and the Gumbel distribution. The practical application and its procedures of return period how to judge an extremely high γ ray dose rate as an unusual value are plainly described. (author)