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

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 in an infinite cold moderator at 20 K and the number density of hydrogen NH 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 and high NH. 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 due to the internal rotation or libration of the methyl group with a small rotational potential barrier. Although NH for the above molecules is low as compared with methane, this drawback would be largely compensated by use of the pre-moderator containing high NH such as polyethylene, light water and ammonia.