Last-Collision Model of the Air-Ground Interface Effect on Fast-Neutron Angle Distributions

The “last-collision” method for computing the air-ground interface effect on scattered neutron intensity is extended to give the effect on the intensity within individual polar angle groups at a detector near the ground. The method yields angle-dependent perturbation factors which can be used to adjust infinite-air angle distributions to account for the presence of an air-ground interface, or to adjust angle distributions from one detector height to another. To determine the factors, a uniform scattering distribution in the air about the detector is assumed, and the fractional contribution from each last-collision center in the air is calculated. In addition, the fraction scattered directly to the detector from the ground surface is calculated using a simplified albedo model. An evaluation of the angle-dependent last-collision model indicated that the results of discrete ordinate calculations for infinite air could be modified to give relative polar angle distributions of the scattered neutron dose near the air-ground interface within 10 to 20% of those calculated directly for the air-over-ground case by the discrete ordinate method.