Experimental validation of the brightness distribution on the surfaces of coupled and decoupled moderators composed of 99.8% parahydrogen at the J-PARC pulsed spallation neutron source

Liquid hydrogen moderators composed of 99.8% parahydrogen associated with light-water premoderator at the J-PARC pulsed spallation neutron source have demonstrated high performance in the extraction of high intensity cold and thermal neutron beams. In the design stage, simulations have shown not only high total neutron intensity in the coupled moderator but also a local neutron-brightness increase at its edges. The edge-effect-brightness increase is also exploited in the design of the European Spallation Source (ESS) moderators, which are also based on 99.8% parahydrogen, but are thin (thickness: 3 cm) to further increase brightness. In this study, the spatial distribution of the neutron brightness at the surface of the coupled moderator in the J-PARC pulsed spallation neutron source was directly measured to experimentally validate the calculated edge-brightness enhancement. The brightness distribution at the moderator surface predicted by Monte Carlo simulation was clearly observed. As a result, the validity of the simulation tools used in the design-optimization process of the J-PARC and ESS moderator is confirmed.