Neutron multigroup cross-sections of moderator materials for cold and ultracold neutron production

A total of 32 sets of multigroup (energy-averaged) cross-sections are developed for liquid He-4, H-2, D-2, CH4, H2O and D2O and solid CH4 at widely different temperatures. The neutron energy range from 10 MeV down to 0.1 mu eV is covered. It is intended to treat both the moderation of fission and spallation neutrons and the production of low-energy neutrons such as thermal (similar to 25 meV), cold (similar to 2 meV) and ultracold (< about 0.3 mu eV) neutrons. The entire energy range is divided into 140 intervals (groups) and the angular distribution of scattering is represented by the expansion in Legendre polynomials up to order 3. The cross-sections characterizing low-energy neutron scattering are calculated using the physical models of molecular dynamics and structure for each moderator material. To demonstrate availability of the cross-section sets, a study is made of ultracold neutron production and storage in a liquid-He-4 sphere surrounded with liquid D-2 and D2O as reflectors. It is shown that the relevant cross-sections are successful in describing neutron energy spectra for fast and low-energy neutrons and that the study of a liquid-He-4 source clarifies neutronic conditions to be required on the high-density storage of ultracold neutrons. The latter results serve physical and quantitative interpretation of the recent experimental measurement of ultracold neutron production in superfluid He-4. (c) 2007 Elsevier B.V. All rights reserved.