Experimental studies of the effect of the ortho/para ratio on the neutronic performance of a liquid hydrogen moderator for a pulsed neutron source

Oi, Motoki ; Ogawa, Hirotatsu*; Kamiyama, Takashi*; Kiyanagi, Yoshiaki*

Liquid hydrogen is a realistic cold moderator material for high-power spallation neutron sources. The neutronic performance of a hydrogen moderator depends on the ortho/para ratio of the hydrogen, and thus experimental data are needed that will clarify the ortho/para ratio effects on neutronic performance. In this study, we measured the neutronic performance of a liquid hydrogen moderator at several para-hydrogen ratios. The neutron energy spectra changes within 20%, depending on the para-hydrogen ratios. With increasing para-hydrogen ratio, the pulse peak intensity increases and the pulse width becomes narrower. With a decoupled moderator, the pulse decay becomes faster. In comparison with the figure of merit (FOM), the para-hydrogen moderator showed almost the same performance as that of the methane moderator considered to be the best moderator for the pulsed neutron source.