The Energy Spectrum of Fast Neutrons Slowing Down from a Fission Source in a Homogeneous Medium

A method is presented for calculating the energy spectrum of fission neutrons slowing down in a homogeneous medium of arbitrary composition. The integral equation for ø(E) is solved by an iterative method, yielding a sum of partial spectra which are then summed to obtain a very simple expression for the slowing down spectrum. The effects of absorption and inelastic scattering are accounted for in an approximate manner. The fundamental-mode spectrum for a typical fast-reactor composition was calculated by this method, and the results agree favorably with the central spectrum from a 26-group diffusion calculation. A further application is given for using this method to generate weighting spectra for computing average multigroup cross sections.