Development status of the in-situ measurement system for ortho-to-parahydrogen fractions for the ESS cryogenic moderator system

The Cryogenic Moderator System (CMS) is equipped with a catalyst to convert hydrogen from the ortho state to the para state in order to keep desirably high parahydrogen fractions of 99.5%. An in-situ measurement system for the ortho and para fractions of liquid hydrogen (OPMS) by means of a Raman spectroscopy with an accuracy of 0.1% is being developed to detect any undesirable shift towards a high orthohydrogen fraction caused by neutron scattering driven para-to-ortho back conversion. We have demonstrated that our developed sapphire window, through which the laser and backscattered photon travel, has endured the required pressure and thermal cycles in a liquid hydrogen environment. Meanwhile, a Raman optics system has been developed using normal hydrogen. Eventually, we have succeeded in detecting the parahydrogen peak at J = 4, which is our primary goal, because the peak ratio of parahydrogen at 1245 cm −1 to ortho hydrogen at 587 cm −1 corresponds to 0.5%. It can be expected that it would achieve our requirement for liquid hydrogen with higher density.