Thermal Neutron Scattering Analysis for Crystalline and Amorphous Light Water Ice

Light water ice is an important material for cold neutron moderator applications or future criticality safety evaluations in low temperature systems. Depending on temperature and pressure conditions, crystalline and amorphous phases may coexist in the same light water ice. The purpose of this study is to elucidate the differences between crystalline and amorphous phases in light water ice in terms of thermal neutron scattering. To achieve this purpose, molecular dynamics simulations are performed to evaluated velocity auto correlation function of light water ice for the crystalline and amorphous models with SCIGRESS, and from the simulation results, thermal neutron scattering cross section data are evaluated with an analysis code called KUNSCA. The total cross section in the crystalline model shows good agreement with experimental values of light water ice at 115 K, and this suggest that the analysis in the present study is generally valid. The thermal neutron scattering cross sections in the amorphous model shows different behavior from that in the crystalline model. In the crystalline model, the inelastic scattering is dominant and higher in the cold neutron energy region (< 5 meV) than that in the amorphous model. This suggests that it may be preferable to minimize the formation of amorphous ice when using light water ice as a cold neutron source.