Differences between Radiation Dose Values under Wearing and Non-wearing Geometries of Personal Dosimeters during Sleep

With the return of residents after the Fukushima Daiichi Nuclear Power Station accident, the measurement results of radiation doses to residents can be used to assess the risk or safety of remaining in their homes. However, personal radiation doses vary depending on the behavior and residential environment of each subject, even for a group of subjects living in the same region at the same time. In past studies, subjects were required to wear a personal dosimeter on their chests outdoors, but they were not required to wear the device indoors. This study investigated the difference between the dose values indicated for the wearing and non-wearing geometries of personal dosimeters during sleep. In particular, an adult human phantom was used to compare the indicated dose values when personal dosimeters were placed on the chest (reproducing the wearing geometry) and near the head (reproducing the non-wearing geometry) in two houses with a high measured radiation dose. Furthermore, to understand the reason for the difference in the indicated dose values, the radiation dose rate during sleep was calculated using a radiation transport calculation code. The dose values for the wearing geometry were approximately 4% lower on the first floor and approximately 15% lower on the second floor than those for the non-wearing geometry. In addition, the radiation dose rates and radiation dose rate ratios (head/chest) differed by approximately 30% and 20%, respectively, depending on the distance from the nearest window (1 to 3 m).