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Real-time monitoring of the orthohydrogen fraction in a liquid hydrogen moderator

by Erik B Iverson, Brian D Degraff, John J Denison, Bernard W Riemer, Franz X Gallmeier
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Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
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We present the first direct measurements of the orthohydrogen fraction in an operating liquid hydrogen moderator loop at a high powered neutron source. We have developed a Raman immersion probe capable of characterizing the orthohydrogen and parahydrogen makeup of a liquid hydrogen loop approximately every ten seconds, which can see orthohydrogen fractions lower than 0.20 without difficulty. At the Spallation Neutron Source, where the time-averaged power deposition in the studied moderator system is around 0.02 W/cm, the orthohydrogen fraction increases with beam power and equilibrates at levels of 0.40, and responds to beam trips in a dynamic fashion. Additionally, we find that without deliberate catalyst systems in place, the relaxation due to the piping in the system has insignificant effect in catalyzing the relaxation of freshly condensed hydrogen toward thermodynamic equilibrium, and that the radiation levels associated with activation in a non-operating spallation neutron source target-moderator-reflector assembly likewise has insignificant effect on the liquid hydrogen kinetics.