An Analysis of a Shielding Experiment Performed in a Real Fallout Field

Four simple enclosure shields were exposed to fallout from Shot Small Boy of Operation Sunbeam. Gamma-ray measurements were made both inside and outside the enclosures to determine time-dependent and time-integrated dose transmission factors (dose inside divided by dose outside). The analysis of the experiment consisted of calculating similar factors and comparing with the measured data. Monte Carlo procedures were used to determine the radiation distribution incident upon the shields and to compute the radiation penetration into the shields. Fallout gamma-ray spectra resulting from several different theoretical and experimental investigations were used in calculating the dose transmission factors. The calculated dose transmission factors were found to be consistently higher than the measured factors by as much as a factor of 2, depending upon the particular source term. After investigating several possible causes, it was concluded that the discrepancy was probably the cumulative effect of a deficiency of low-energy photons in the calculated source terms, omission of support structure inside the enclosure shields in the penetration calculations, and of neglecting the effects of ground roughness in the calculations.