Sensitivity studies of the ortho/para H2 concentration on the neutronic performance of the Lujan center liquid H2 moderator using state-of-the-art computational tools

A liquid H2 moderator has been operational at the Manual Lujan Jr. Neutron Scattering Center (Lujan Center) since 1985. Detailed Monte Carlo calculations have been made to estimate the neutronic performance of the liquid H2 moderator cold source, using the state-of-the-art Los Alamos LAHET Code System and liquid H2 scattering kernels. The absolute neutron spectrum leaking from the moderator has been measured. In thermal equilibrium at 20 K, the moderator would convert to 100% para-hydrogen on some time frame. The authors show that the performance of the Lujan Center cold neutron source is sensitive to small fractions of ortho-hydrogen, and display these results as a function of the ortho/para H2 concentration. Integral data from the measured neuron spectrum have been combined with calculated predictions to estimate the ortho/para H2 concentration in the moderator. From these analyses, they estimate the ortho-hydrogen concentration in the liquid H2 moderator to be 6--9% at the time the neutron spectrum was measured