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

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 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 concentrations. Our experiment was performed at the Hokkaido University 45 meV electron linac facility. The neutron energy spectra were measured by the time-of-flight method. Pulse shapes were measured by the Bragg scattering of a mica crystal. The neutron energy spectra change within 20%, depending on the para hydrogen concentrations. With increasing para hydrogen concentration, the pulse peak intensity increases and the pulse width becomes narrower. Furthermore, for a decoupled moderator, the pulse decay becomes faster with increasing para hydrogen concentration. From a viewpoint of the figure of merit (FOM=I/FWHM2) the para hydrogen moderator showed almost the same performance as that of the solid methane moderator, which is considered to be a high performance moderator for pulsed neutron source.