Investigation of ortho⇆para hydrogen conversion by collisions with neutrons

Liquid hydrogen (LH) is constituted by para hydrogen (pH) and ortho hydrogen (oH) molecules. We analyzed the neutron scattering cross sections for pHoH, pHpH, oHpH, and oHoH reactions in LH. Results show that incident neutron energy lower than 14.7meV is not sufficient to achieve pHoH conversion. As energy increases, the cross section of pHoH 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 oHpH 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 pHoH induced by neutron inelastic scattering is not negligible.