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

Abstract

A total of 32 sets of multigroup (energy-averaged) cross-sections are developed for liquid 4He, H 2 , D 2 , CH 4 , H 2 O and D 2 O and solid CH 4 at widely different temperatures. The neutron energy range from 10 MeV down to 0.1 μ 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 ( ∼ 25 meV ) , cold ( ∼ 2 meV ) and ultracold ( < about 0.3 μ 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- 4He sphere surrounded with liquid D 2 and D 2 O 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.