Experimental studies on neutronics of CH_3D and HD cold neutron moderators

摘要

Liquid and solid CH_4 have been used as high efficiency cooled and cold moderators in spallation neutron sources, since CH_4 has a rotational level of about 1 meV, which is very effective at thermalizing the neutrons to low temperatures. Moreover, CH_4 also has high hydrogen number density. Heavy hydrogen-substituted methane, CH_3D, has lower rotational energy levels than CH_4 but has lower number density of light hydrogen, that is the macroscopic scattering cross section is lower than that of CH_4. Liquid H_2 is the most promising candidate as a cold moderator for high power spallation neutron sources, since it is stable against radiation damage effects, but it lacks effective low energy levels to thermalize neutrons.