Trial measurement for para-to-orthohydrogen back conversion under the Fe(OH)₃ catalyst condition at J-PARC cryogenic moderator system

Abstract The J-PARC cryogenic hydrogen moderator system (CMS) was developed to provide supercritical hydrogen with a parahydrogen(pH 2 ) fraction exceeding 99% at a pressure of 1.6 MPa and a temperature of 18 K to three moderators. Parahydrogen-to-orthohydrogen (oH 2 ) conversion, referred to as back conversion, occurs in high neutron irradiation field. An oH 2 -to-pH 2 catalyst was installed to ensure the required pH 2 fraction. This study aims to clarify the effect of the occurrence of the back conversion under catalyst condition in the J-PARC CMS. We developed a new method to intentionally increase oH 2 fraction by around 1% to simulate the back conversion phenomenon under off-beam conditions. Normal hydrogen (nH 2 ) at ambient temperature and a flow rate of 1.33 g/s was continuously injected into the CMS, where supercritical hydrogen (34.1 K, 1.6 MPa and 97.7 g/s flow rate) was circulated. The increase in oH 2 fraction was estimated to be 0.9%. The pH 2 fractions in hydrogen captured at downstream of the catalyst were measured using ESS Raman spectroscopy. This demonstrated that the catalyst could maintain the pH 2 fraction in the CMS at a theoretical value in thermal equilibrium conditions.