Self-Shielding of Fallout Gamma Rays by Terrain Roughness

摘要

The study presents a first order calculation of the self-shielding of nuclearweapon fission fragment gamma rays by surface roughness (microscopic terrain irregularities). To simulate fallout particles, polymer microspheres were deposited on slides containing samples of soil, concrete and asphalt roofing shingles. Scanning electron microscopy was used to photographically map the surfaces. The thickness of surface irregularities at various angles was measured by converting the photographs to digital images. Measurements showed a dependence upon the surface type, but not upon particle size. Average values of the measurements were modelled using a homogeneous buried source. With this geometry, dose rates were calculated using the Monte Carlo transport code, MORSE. The MORSE calculations showed self-shielding due to roughness of not more than five percent for the soil samples and eight percent for the shingles. The 30 percent self-shielding used in The Effects of Nuclear Weapons for level terrain is six times as great. It is postulated the large attenuation formerly attributed to roughness may actually be an artifact of an incorrect global/local fallout partition.