Calculation of dose conversion coefficients for external exposure to radioactive cesium distributed in soil

In order to estimate effective dose for the public for external exposure to radioactive cesium distributed in a soil due to the accident of the Fukushima Daiichi Nuclear Power Station, dose conversion coefficients (DCC) have been calculated by using a three-dimensional radiation transport code PHITS. The DCC were evaluated for different ages constituting the public, that is 6-day-old babies, 1-, 5-, 10-, and 15-year-old children, and adults, under the condition which the radioactive cesiums, Cs-134 and Cs-137, are distributed uniformly in the soil with a specific depth of 0.0 g/cm², 0.5 g/cm², 2.5 g/cm², 5.0 g/cm², or 10.0 g/cm². In addition, we evaluate the DCC for cumulative effective dose for external exposure by remaining on ground with radioactive cesium for a period of concern, which is 1 month, 1 year, and 50 years. As a result of the calculation, it was found that the effective dose for any ages does not exceed the value of ambient dose equivalent H^*(10), while the effective dose increases with decreasing the age, and the difference of dosimetry system between the ICRP 1990 and 2007 Recommendations does not make a significant difference for the external dose calculation on contaminated ground. We also developed a method to evaluate the DCC for volume source with an arbitrary distribution in a soil, and derived an equation to be applicable to dose estimation in contaminated areas around Fukushima Daiichi Nuclear Power Station considering initial condition of radioactivity of Cs-134 and Cs-137. (author)