Computational comparison of hydrogenous molecules as a cold moderator for compact accelerator-based neutron sources

Abstract

For searching potential candidates of a cold moderator for compact accelerator-based neutron sources, neutronics properties of various hydrogenous molecules are evaluated. As performance measures of a cold moderator, the neutron temperature $T_n$ in an infinite cold moderator at 20 K and the number density of hydrogen $N_H$ are analyzed by combining molecular dynamics simulation with our developed code, KUNSCA. Among examined hydrogenous molecules, methane is found to have the most favorite property i.e., low $T_n$ and high $N_H$. Apart from gaseous molecules at the normal temperature and pressure (293 K, 1 atm), 2-butyne, 2,4-hexadiyne, mesitylene, m-xylene and p-xylene have low $T_n$ due to the internal rotation or libration of the methyl group with a small rotational potential barrier. Although $N_H$ for the above molecules is low as compared with methane, this drawback would be largely compensated by use of the pre-moderator containing high $N_H$ such as polyethylene, light water and ammonia.