Effect of Anisotropic Scatter on Atmospheric Neutron Transport.

摘要

Anisotropy is present in the angular distributions of neutrons departing from a nuclear scattering event. This anisotropy cannot be defined in a closed-form solution, and it is dependent on the incident neutron energy, elastic or inelastic scatter, along with the inelastic level, and the species struck. The underlying question is, if anisotropic behavior is worth the computational cost to be included in certain simulations, and if so, what level of precision is effected by the inclusion of anisotropic scatter. A Watt spectrum of U235 fission neutrons was examined as it collided with species in a nitrogen-oxygen atmosphere. In a stochastic manner, 10 the the 8th power collision samples were taken, utilizing cross section-based weighting for random sampling of collision types and cross section weighting along with concentration weighting to determine the species struck. The collective anisotropy of the resultant angular distribution was apparent, with a definite average forward bias across the spectrum and a bias toward scattering angles less than 30 degrees. Additionally, with the elimination of lower energies, on average, the higher the energy, the greater the apparent magnitude of the forward bias of the anisotropic scatter. Using 1-D slab geometry, two studies exploring the relative effect of anisotropy were conducted. The maximum relative error of 0.24% was observed in energy and 0.23% in time. In short, if precision is required past the second decimal place in long-distance high-altitude transport, anisotropy deserves consideration.