Distribution map of natural gamma-ray dose rates for studies of the additional annual exposure dose after the Fukushima Dai-ichi Nuclear Power Station accident

The information on the absorbed dose rate which is derived from natural radionuclides is needed to evaluate additional exposure dose. However, there is inadequate positional resolution and precision for such data around Fukushima Dai-ichi Nuclear Power Station (FDNPS). In this study, we created a map of the absorbed dose rate that is derived from natural radionuclides based on several airborne radiation monitoring data. The reliability and accuracy of the created map was verified by comparison with the many in-situ measurements on the ground. To evaluate the effectiveness of this study, the effective half-lives of the ambient dose rate at residential areas of Fukushima Prefecture were assessed by discriminating these absorbed dose rate of the natural background from the results of a periodic dose rate survey by local government. The results of the distribution of natural background absorbed dose rates are expected to contribute to the evaluation of the additional exposure dose after the FDNPS accident.