Cold neutron production in liquid para- and normal-H2 moderators

A neutron transport analysis is performed for liquid H2 moderators with 100% para and normal (ortho:para=0.75:0.25) fractions. Four sets of energy-averaged cross-sections (group constants) for liquid ortho- and para-H2 at melting and boiling points are generated and neutron energy range between 0.1 μeV and 10 eV is broken into 80 groups. Basic moderating characteristics are studied of a model cold-neutron source in a one-dimensional bare-slab geometry. It is shown that liquid para-H2 is superior in cold neutron production to liquid normal H2 on account of a para-to-ortho transition (molecular rotational excitation) and a good transmission property with a mean free path of about 10 cm. In the case of neutron extraction from the inside of the source, high intensity of cold neutrons is possible with liquid normal H2 at higher temperatures up to the boiling point.