Calculation of Thermal Neutron Flux Spectra in a Polyethylene Moderated Medium

A model is proposed for calculating the scattering of thermal energy neutrons by polyethylene. It is seen that a reasonable fit to the experimental total cross section results. This model is then used to calculate infinite medium spectra and the results compared with calculations using water and perfect gas scattering kernels, with a harder predicted spectrum apparent. The results are compared with experimental data for two amounts of absorption, and agreement between experiment and theory using an appropriate scattering model is observed. The model is then used to calculate reactor quantities for a particular one-dimensional finite lattice. The sensitivity of the flux spectra and criticality to the choice of scattering kernel is presented. The former quantity is more sensitively dependent than the latter.