A Study of Buildup Factors, Angular and Energy Distributions at Small Distances from Three Source Geometries — Plane Isotropic, Point Isotropic, and Plane Normal — for Low-Energy Gamma-Ray Incidence on Water

Exposure buildup factors for plane isotropic, point isotropic, and plane normal sources have been calculated using a discrete ordinates direct integration code, PALLAS-PL, SP-Br, in infinite and finite water shields in the 0.06- to 0.1-MeV range. The values of the attenuation kernel, Be^-µr, are greater than unity at distances up to a few mean-free-paths in an infinite medium. The maximum value of Be^-µr depends on the incident energy, and this effect reaches a maximum for a 0.08-MeV source. The implication that the dose rate with a shield is greater than without a shield should be noticed. Results of this study show, however, that the large degree of scattering in a low-z material, such as water, produces this effect. Buildup factors, energy spectra, and angular distributions were analyzed for three source geometries in the comparisons of scattered gamma-ray transport in infinite and finite water shields.