J.K. Shultis and R.E. Faw, Extensions to the Integral Line-Beam Method for Gamma-Ray Skyshine Analyses, SAND94-2019, 161 pp, Sandia National Lab., Albuquerque, NM, 1995.

Abstract

A computationally simple method for estimating gamma-ray skyshine dose rates has been developed on the basis of the line-beam response function. Both Monte Carlo and point-kernel calculations that account for both annihilation and bremsstrahlung were used in the generation of line beam response functions (LBRF) for gamma-ray energies between 10 and 100 MeV. The LBRF is approximated by a three-parameter formula. By combining results with those obtained in an earlier study for gamma energies below 10~MeV, LBRF values are readily and accurately evaluated for source energies between 0.02 and 100~MeV, for source-to-detector distances between 1 and 3000~m, and beam angles as great as 180 degrees. Tables of the parameters for the approximate LBRF are presented.
The new response functions are then applied to three simple skyshine geometries, an open silo geometry, an infinite wall, and a rectangular four-wall building. Results are compared to those of previous calculations and to benchmark measurements. A new approach is introduced to account for overhead shielding of the skyshine source and compared to the simplistic exponential- attenuation method used in earlier studies. The effect of the air-ground interface, usually neglected in gamma skyshine studies, is also examined and an empirical correction factor is introduced. Finally, a revised code based on the improved LBRF approximations and the treatment of the overhead shielding is presented, and results shown for several benchmark problems.

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