Development of cross-section models for liquid H 2 and solid CH 4

In order to clarify neutronic performance of a pulsed cold-neutron source, scattering properties of some typical hydrogenous moderators are studied. Namely, cross-section models for liquid ortho- and para-H2 at temperatures between 14 and 20.4 K and solid and liquid CH4 at temperatures down to 20 K are developed. A set of group constants for liquid H2 is also generated: 80 groups between 0.1 μeV and 10 eV in a P3 approximation. The major results are as follows. (a) Cold-neutron production due to a para-to-ortho transition and up-scattering of lower-energy neutrons by ortho-H2. (b) Neutron slowing-down by rotational excitation of a CH4 molecule, especially at 20 K. And (c) an optimum condition of a liquid-H2 source model for cold-neutron production.