Research for the spatial diffusion of radionuclide from Cs/Cs ratio in indoor dust from near the Fukushima Daiichi Nuclear Power Plant

Minowa, Haruka*; Yoshikawa, Hideki; Nakama, Shigeo ; Satou, Yukihiko   ; Sueki, Keisuke*

In this study, we aimed to estimate the diffusion distribution of radioactive Cs from each reactor source using the environmental samples. First, the influence of the sum-peak effect on the counting efficiency of Cs in the Ge detector was investigated for the accurately radioactive measurement, and applied it to the actual sample. To measure the variation of the counting efficiency depends on the distance between the Ge detector and a sample, a disk-shaped standard sample (25 mm of filter paper) and a particle standard sample (200 m in size) were used. When the sample was placed close to the detector (distance 0.1 mm), the counting efficiency [cps/Bq] of Cs was about 20% lower than that of Cs in both the disk-shaped and the particle standard sample. It seemed to be caused by the sum-peak effect. "Smear paper" samples with indoor dust collected from about 40 points within 10 km from Fukushima Daiichi Nuclear Power Plant in 2014-2019 were measured, and Cs and Cs were quantified using the counting efficiency with the disk-shaped standard sample. From the Cs/Cs ratio at the time of the accident, the mixing ratio of materials derived from Reactors 1, 2 and 3 was estimated, and the spatial diffusion distribution of radioactive materials for each reactor was indicated.