Evaluation of Ambient Dose Equivalent Rates Owing to Natural Radioactive Nuclides in Eastern Japan by Car-Borne Surveys Using KURAMA–II

Abstract

　In order to determine the contribution of radioactive cesium due to the Fukushima Daiichi nuclear power plant accident to the ambient dose equivalent rates measured by car-borne surveys, natural background radiation was evaluated for eastern Japan as municipality-averaged values. The window count method for the distinction between natural and artificial radioactive nuclides was applied to car-borne surveys using KURAMA–II. The distribution of the evaluated natural background radiation reflected geological features, and it was found that the radiation measured along paved roads reflected the distribution of terrestrial gamma rays. The contribution of the radioactive cesium as of 2014 for the municipalities designated as the Intensive Contamination Survey Area was beyond the uncertainty of the natural background radiation. That for the other municipalities, however, was found to be almost negligible.