In situ Measurement of Distribution of Radioactive Substances in the Houses Using Imaging Plates

A large amount of radiocesium spread into the environment during the accident at the Fukushima Dai-ichi Nuclear Power Plant in Japan. Radiocesium entered into houses around the plant and formed radiocesium-containing indoor dust. Physical and chemical properties of radiocesium-bearing particles, such as solubility, size distribution and their radioactivity, are important for the estimation and reduction of inhalation doses from indoor dust. It is not possible to obtain samples such as ceiling boards or floor panels from inhabited houses which were potential sources of radiocesium-containing dust. Therefore, on-site measurement is required instead of sample collection. An imaging plate (IP) can be used to visualize the distribution information of radioactive substances on ceiling boards or floor panels. Information on the number density and radioactivity of substances containing radiocesium can be obtained by using IPs. The analytical conditions (selection of geometry of radiation sources, correction of signal attenuation during measurement and transport, evaluation of background radiation and so on) were optimized for the quantitative measurement using IPs in the houses under relatively high air doses. Consequently, it was shown that particles with radioactivity of about 0.1-0.7 Bq were distributed on the ceiling boards of houses under evacuation.