Extensive field survey of natural radiation in Japan.

Abstract

The exposure rate due to natural radiation composed of terrestrial radiation, cosmic ray as well as a slight contribution from nuclear tests was estimated at 1,115 sites by an ionization chamber and sodium iodide scintillation surveymeters. The distribution of the exposure rates along with the population-weighted annual collective dose in each area were obtained. The mean exposure rate of all Japan was calculated to be 9.0 μR/h. The population-weighted mean dose for an individual person was 68.7 mrad/yr. Uncertainties involved in the field measurements and calibrations were estimated to be satisfactorily low. However, the result cannot be interpreted as to its very small variation because the sampling error could have been significant. Nevertheless the gross feature is meaningful. There is a general trend that South-West Japan has higher level than North- East Japan. It was also revealed that the population size is a more important factor than the radiation level in determining the distribution of the population-weighted mean dose in Japan.