Measurements of radiation from the Fukushima Dai-ichi Nuclear Power Plant by the Compton Camera: Estimation of Plant-derived Radioactivity

The dominant interaction of γ-rays (662 keV) from 137Cs in the air is the Compton scattering. The mean-free-path is approximately 10 g/cm2 (～100 m at NTP). The γ-rays lose their energies and change directions in this distance. Moreover, there are many obstacles around the ground level in nuclear power plants. The effects of the scattered γ-rays from the upper air, therefore, become dominant at the long distance (skyshine) from the radiation source. We observed the two-dimensional images of those scattered γ-rays (the scattering angle between 18 and 34 degrees) in the site of the Fukushima Dai-ichi Nuclear Power Plant using the Compton camera. The detail study of those images enables us to estimate radioactivity derived from the Unit-2.