A First Last Collision Model Of The Air Ground Interface Effects On Fission Neutron Distributions

A method has been developed for predicting the effect of an air/ground interface on the fast neutron flux or dose at large distances from a point isotropic source of neutrons in air. The method is called the "First-Last Collision Model" because it is based on the influence of the ground upon the distribution of "first" collisions of neutrons about the source and of "last" collisions about the receiver. Generalized numerical results have been computed and means have been developed for applying these results to those derived from Monte Carlo calculations and from experiments performed at the ORNL Tower Shielding Facility and the Nevada Test Site indicate that the first-last collision model predicts the fraction of the infinite air medium intensity within 5 per cent in almost all cases.