Relation between spatial and personal doses of environmental radioactivity in Iitate village

Iitate village, which used to be known for its recycling industry promotion based on forestry, dairy, and agricultural farming, has been seriously affected by the TEPCO Fukushima Daiichi Nuclear Power Plant accident in March 2011 and all villagers are still under the inexcusable situations. In forming the evacuation order and radiation-removal standards, spatial doses of the contaminated area were at-first used as a criterion, but it is now considered personal doses should be used. Personal dosimeters are used as personal-dose monitors, although it is not certified how much degree the personal dosimeters, which are calibrated to show 1-cm dose-equivalents for parallel X- and γ-ray fluence from front normal, are accurate in the environment where radioactivity exists on the ground around, or the personal dosimeters, which are designed for adult body, are valid for children. Since results of several research tackling with the problem were rather scattered, we measured spatial and personal doses in the present environment in Iitate village and derived ratios between the personal doses measured with several personal dosimeters mounted on human-body phantoms and the spatial doses measured with energy-compensated NaI(Tl) scintillation survey meter. It has been shown that the ratios match within experimental errors in Nagadoro and Hiso districts, but is rather high in Sasu, suggesting that the ratios depend on energy spectra of projecting γ-rays and that considerations about the radiation sources and their energy spectra are essential in evaluating personal dose equivalents in Fukushima. (author)