Abstract
In skyshine dose evaluation for gamma-rays, simple calculation methods are still used in many radiation facilities since various computer codes for the skyshine calculation require considerable experience and expertise to obtain reliable results. The effect of scattered radiations in ceilings needs to be considered in order to obtain reasonable results using simple calculation methods. Therefore, dose distributions on the ceiling were calculated using MCNP-4C in the geometry of the KSU experiment. The results showed that gamma-rays of 0.3MeV or less scattered inside the facility made a contribution to the dose at the ceiling position, and the dose distribution on the concrete ceiling was strongly affected by the oblique incidence on the ceiling in the case of a point source. Additional calculations confirmed that the effect of the oblique incidence depended on the thickness of the ceiling as a function of mean-free-path. It was found that the effect was expressed as a simple formula considering gamma-ray attenuation and build-up, which would be useful in the simple skyshine calculation.
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