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 4He, H2H2, D2D2, CH4CH4, H2OH2O and D2OD2O and solid CH4CH4 at widely different temperatures. The neutron energy range from 10 MeV down to 0.1μeV 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 (∼25meV), cold (∼2meV) and ultracold (<< about 0.3μeV) 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- 4He sphere surrounded with liquid D2D2 and D2OD2O 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-4He 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 4He.