Decrease of per capita natural radiation dose in Japan in the last three decades.

Abstract

The long term variation of the mean dose due to natural radiation in Japan received by an individual person was investigated. The mean exposure rate in each prefecture obtained by in-situ measurements was adopted as a basis of the radiation level. Population data were taken from both the resident registration and the census taking into account the essential difference of their meanings. It was revealed that the per capita dose due to outdoor natural radiaiton in Japan has been decreasing in the last approximate 30 yr. It will be due to the population movement from countrysides towards urban areas. That is because countrysides generally consist of stable mountainlands where granitic rocks distribute though urban areas generally consist of alluvial plains where volcanic ash or humus dominates. Although the contribution of low radiation level (below 9.0μR/h) areas to the nation-wide collective dose still remains under 50%, its relative importance has certainly been increasing in the last three decades. It is...