II-3. Development of Cryogenic Moderators Using a Small Proton Accelerator

Liquid para-hydrogen has smaller scattering cross section than those of normal hydrogen at an energy region below about 15 meV, and the para-rich-hydrogen moderators might decrease the width of the cold neutron pulses emitted from the moderators due to low up-scattering cross-section in para-hydrogen. We developed a hydrogen gas supply system to convert ortho-hydrogen to para-hydrogen using an iron hydroxide (Fe (OH) 3) catalyst and an ortho/para ratio measuring apparatus using the Pirani gauges. The para-rich-hydrogen gas was cooled and condensed into the moderator chamber equipped with the pulsed fast neutron source at the Hokkaido University 45MeV Linac. We obtained the flight time spectra of neutrons from the moderator chamber. It turned out that in the case of the decoupled moderator the momentary maxima of the cold neutron pulses from the para-rich-hydrogen moderator were larger than those of the normal hydrogen moderator in the cold neutron energy region. This result on the para-rich-hydrogen moderator shows the excellence as a short pulsed neutron moderator when we cannot use the methane moderator.