Modeling of Low-Energy Neutron Scattering in Liquid Hydrogen Using NCrystal

Sammanfattning: The European Spallation Source ESS, under construction in Lund, Sweden, aims to produce high-brightness and intensity neutrons for a multitude of experiments upon completion. An efficient production of neutrons requires elaborate understanding of the moderation process which slows the neutrons from relativistic speed to a kinetic energy of meV. The moderator used at the ESS will be partially made of water and liquid para-hydrogen, which enables simultaneous production of thermal and cold neutrons. The interaction between neutrons and the moderator is best described by the total neutron scattering cross section which in turn is computationally derived from the scattering kernel. The liquid hydrogen kernel itself includes models of a self- and distinct scattering function which describe the interaction of individual molecules with neutrons as well as interference effects of scattered matter waves from neighboring molecules. Scattering kernels are commonly produced by a software called NJOY, however, an effort at ESS has been made to develop and transition to a new software called NCrystal. Its more flexible design allows for easier implementation of physics models beyond what is currently included in NJOY for more moderator materials. Thus far, NCrystal is primarily aimed at generating scattering kernels for poly-crystalline materials in solid phase, a capability which was now expanded to liquid water and liquid hydrogen. The resulting cross sections were bench-marked against proven software and show good agreement.