It has been occasionally found from an area monitoring data obtained at an atomic power station that the daily periodical change of the environmental radiation dose rate appeared near the ground surface. It is suggested that the variation has resulted from the change of atmospheric radon concentration and that it has also depended on the geographical features of the observation point. The change component has been estimated to be up to nearly 30 percent of the environmental radiation dose rate from monitoring data. To investigate precisely the phenomena, observation was performed near a monitoring station and at other area in Fukui Prefecture during the summer. The dose rate from terrestrial radiation was 42nGy/h at the Natasho in Ooi Town and was 30nGy/h at the Kaminaka in Wakasa Town. The concentration of radon decay products in the air was 0.5-30Bq/m³ at the Natasho site and was 1-10Bq/m³ at the Kaminaka site. From the concentration of radon decay products in the atmosphere, the dose rate was respectively estimated to be 0-7.0nGy/h at the Natasho site, and to be 0-5.5nGy/h at the Kaminaka site. Furthermore, 0.26 (nGy/h)/(Bq/m³) and 0.56(nGy/h)/(Bq/m³) were estimated as the dose rate conversion factor of radon concentration in the air at the Natasho site and at the Kaminaka site, respectively.

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