Performance evaluation of the developed in-situ ortho-to-parahydrogen fraction measurement system for the ESS cryogenic moderator system

Abstract At ESS, we developed an in-situ ortho-to-parahydrogen fraction measurement system (OPMS) using a Raman spectroscopy. This system requires an accuracy of 0.1% parahydrogen (p-H2) fraction at around 20 K to observe ortho-to-parahydrogen back-conversion phenomena induced by the neutron irradiation. Initially, we succeeded in detecting the p-H2 peak at 1245 cm-1 (𝐽 = 4) in normal hydrogen (n-H2) at 0.3 MPa. This is because the theoretical peak intensity ratio of the third p-H2 (𝐽 = 4) to the Xirst orthohydrogen (𝐽 = 1), 𝐼(4)/𝐼(1), is 0.49%, which is equivalent to 𝐼(1)/𝐼(0) at 22.7 K. In this study, we measured the Raman spectra of n-H2 at ambient temperature and various pressures to clarify the effect of hydrogen density, 𝜌, on the accuracy of the 𝐼(4)/𝐼(1) ratio, instead of 𝐼(1)/𝐼(0). Additionally, we measured the Raman spectrum of gaseous hydrogen collected from equilibrium condition at 21 K, at ambient temperature and 0.7 MPa, to evaluate the accuracy of 𝐼(1)/𝐼(0) compared with that of 𝐼(4)/𝐼(1). The accuracy of 𝐼(4)/𝐼(1) decreased following a curve characterized by 𝜌 −0.95 , and the accuracy of 𝐼(1)/𝐼(0) follows a similar trend. For 𝜌 >0.54 kg/m3, the accuracy decreased below 0.1%. Therefore, it is expected that 𝐼(1)/𝐼(0) of liquid hydrogen (𝜌 =74.7 kg/m3) would achieve an accuracy of approximately ±0.001%.