Neutron Flux Distribution in a Cavity with a Point Source

Measurements of neutron flux in the laboratory are known to show significant departure from the inverse square law due to reflection of neutrons from the walls, floor, and ceiling of the laboratory. A simple model is developed to describe the flux distribution due to a point isotropic source in such a situation by treating the room as a cavity with reflecting walls. The model is exactly solvable for a spherical cavity and leads to a simple formula for the flux distribution. The formula thus derived shows good agreement with Monte Carlo computations. Small deviations of the formula from the computed results, particularly for thin walls, are explained as being caused by the anisotropy of the incoming angular distribution of the reflected flux.