Quantitative evaluation of ultracold neutron production and storage in superfluid helium

The production of ultracold neutron (UCN) in superfluid 4He is evaluated by use of the scattering cross-section model of liquid 4He developed by the authors in a wide range of temperatures from 0.1 to 4.2 K and neutron energies from 0.1 μeV to 10 eV. Typical results on the total cross section and angular distribution of UCN production are presented. The UCN density attainable in liquid 4He is estimated and found to exhibit saturation behavior at lower temperatures. It is shown that 1 meV neutrons serve for the UCN production effectively and that the UCN density reaches the maximum of 4.2×104 cm−3 below about 0.5 K where no significant up-scattering of UCN occurs and a UCN disappears only by neutron β-decay.