Abstract
We converted the measured values to the measurement value of 1 m height obtained by the calibrated survey meter by using the regression of the car-borne survey measurements on existing survey meter measurements of the absorbed dose rate in air. Detailed investigation was conducted for all factors considered to potentially influence the measured value. The equation made by these factors was nearly accordance with this regression. The result of converting the measurement results into the survey meter measurement value was 22-111 nGy h-1. Comparing the distribution map of environmental gamma-ray dose rate by car-borne survey with a geological map shows that the area with high gamma-ray dose rate coincides well with areas of Granitoid, Paleogene system and Tertiary Volcanic Rocks. The area with low gamma-ray dose rate coincides with areas of Quaternary Volcanic Rocks and their surroundings. The source of these rocks is considered to be volcanic ejecta. The annual effective dose from the geometric mean value in Aomori prefecture was estimated to be about 0.21 mSv y-1. This value is lower than the annual external dose that would be obtained by exposure to the average terrestrial gamma-ray dose rate in Japan.
