Comparison of Some Monte Carlo Models for Bound Hydrogen Scattering

Three approximate models for incoherent inelastic scattering of epithermal neutrons from 1 H bound in water are examined. The theoretical relationships between them are outlined, and the effects of using one versus another in Monte Carlo calculations are determined. A scattering law model based on the use of the short collision time approximation for downscattering, which simultaneously preserves detailed balance, gives the best results for the distribution of postcollision energies. For some criticality benchmark models, the differences in keff between the models can be as much as 50 pcm.