Investigation of ortho↔para hydrogen conversion by collisions with neutrons

• Published in: Journal of nuclear science and technology Vol. 55; no. 5; pp. 478 - 483
• Main Authors: Mei, Longwei, Liu, Cong, Shen, Fei, Wang, Songlin, Hu, Zhiliang, Zhou, Bin, Liang, Tianjiao
• Format: Journal Article
• Language: English
• Published: Tokyo Taylor & Francis 04.05.2018; Taylor & Francis Ltd
• Subjects: Conversion; cross section; cryogenic moderator; Hydrogen; Inelastic scattering; Liquid hydrogen; MCNP; Neutron flux; Neutron scattering; neutron source; Neutrons; Ortho hydrogen; Para hydrogen; para-ortho hydrogen conversion; Scattering cross sections; thermal neutron scattering
• ISSN: 0022-3131; 1881-1248
• DOI: 10.1080/00223131.2017.1412363

Liquid hydrogen (LH) is constituted by para hydrogen (pH) and ortho hydrogen (oH) molecules. We analyzed the neutron scattering cross sections for pH→oH, pH→pH, oH→pH, and oH→oH reactions in LH. Results show that incident neutron energy lower than 14.7 meV is not sufficient to achieve pH→oH conversion. As energy increases, the cross section of pH→oH reaction increases sharply. If incident neutron energy is larger than 30 meV, branching fraction of pH molecules being converted into oH molecules upon inelastic scattering with the neutrons approaches 100%. As for the opposite process, branching fraction of oH→pH conversion is 10%-20% when incident neutron energy is lower than 30 meV. This conversion fraction oscillates with increasing incident neutron energy, eventually stabilizing at 30%. Based on the cross sections of the four reaction channels, we calculated the corresponding conversion rates based on a reactor cold neutron source model. There is 0.12% pH molecules of LH moderator would be converted to oH for one month operation . If neutron flux increased by 2 orders of magnitude, more than 10% pH would be transformed into oH, which means relative conversion rate of pH→oH induced by neutron inelastic scattering is not negligible.