Calculation of neutron scattering libraries for liquid ortho-deuterium and hydrogen deuteride

Abstract: The European Spallation Source (ESS) has launched a project called HighNESS, with the task of investigating the potential of adding a second moderator system to the facility, with ortho-deuterium (o-D2) being the main material of interest. Another moderator material which could be of interest is hydrogen deuteride (HD). The purpose of this second moderator system would be to provide a high intensity source of neutrons, compared to the high brightness of the first moderator system, as well as providing a colder neutron spectrum. In order to accurately simulate the low-energy neutron production from moderators, the thermal scattering law (TSL) of the materials in question is needed, typically in the form of scattering libraries. Previous scattering libraries for ortho-deuterium were created based on input derived from experimental data and early theoretical models, and no scattering library exists for hydrogen deuteride. This work used a type of quantum molecular dynamics (QMD), namely ring polymer molecular dynamics (RPMD) to obtain inputs for the NJOY software wherein the scattering libraries and the cross-section were calculated for ortho-deuterium and hydrogen deuteride. Seeing as NJOY was not developed for heterogeneous molecules such as hydrogen deuteride, it had to be extended to include models for such molecules. The quantities from the molecular dynamics simulations and the cross section of both materials were benchmarked against experimental data and simulated data from other research groups. Possible improvements and continuations beyond this work are also discussed and presented in this report.