An Angle-Dependent Albedo for Fast-Neutron-Reflection Calculations

An albedo model for calculating the dose due to fast neutrons reflected from materials of low to moderate hydrogen content has been developed through analysis of extensive Monte Carlo data. The model, which was developed from reflection data for iron, concrete and three types of soil, is for reflection to a unit non-directional receiver and is of the form α(Ε₀)cos^2-3θ₀cosθ where α(Ε0) is a coefficient tabulated as a function of incident energy, Ε₀, for the various materials, θ₀ is the angle of incidence and θ is the angle of reflection (both measured from the normal). The differential albedo, in units of reflected dose/steradian per unit dose incident at angle θ₀, may be converted to a total albedo by multiplying by π. The total dose albedo for normally incident fission neutron was found to be closely approximated by 0.435(Σ_ΤΣ_Η)/Σ_Τ where Σ_Τ is the macroscopic total cross section of all elements of the material, and Σ_Η is the macroscopic cross section of the hydrogen of the material, both weighted by the fission spectrum.