DISCRIMINATION BETWEEN NATURAL AND OTHER GAMMA RAY SOURCES FROM ENVIRONMENTAL GAMMA RAY DOSE RATE MONITORING DATA

In this study, a method to discriminate between natural and other g-ray sources from environmental g-ray dose rate monitoring data was developed, and it was successfully applied to actual monitoring data around nuclear facilities. The environmental dose rate is generally monitored by NaI( Tl) detector systems in the low dose rate range. The background dose rate varies mainly as a result of the deposition of Rn-222 progeny in precipitation and shielding of the ground by snow cover. Increments in the environmental dose rate due to radionuclides released from nuclear facilities must be separated from these background variations. The method in the present study corrects for the dose rate variations from natural sources by multiple regression analysis based on the gamma-ray counting rates of single-channel analysers opened in the energy ranges of gamma-rays emitted by Bi-214 and Tl-208. Assuming a normal distribution of the results and using the one-sided type I error of 0.01 while ignoring the type II error, the detection limit of the gamma-ray dose rate from artificial sources was 0.77 nGy h(-1).