Scattering Model Approximations for Neutron Thermalization Problems

A variety of scattering model approximations have been devised and evaluated. One such scattering model, designated the balanced single collision thermalization (BSCT) approximation, has proven to be very effective. It assumes that neutrons attain a thermalized distribution with only a single collision within the moderating material, independent of incident energy. This approximation leads to separability of the incident and outscattering energies and to significant simplification of the neutron scattering kernel for thermalization problems. The BSCT approximation is particularly useful in thermalization problems involving cold neutron sources, for which it yields flux predictions to within a few percent of exact solutions of theoretical problems. The BSCT approximation also predicts cold neutron fractions to within 10% of measured values for a cold neutron thermalization experiment done at Argonne National Laboratory.