Neutronic analysis of a liquid- 4 He source for ultra-cold neutron production and storage

Ultra-cold neutron (UCN) production and storage in a liquid-4He source are evaluated by means of multi-group neutron-transport analysis. Using the cross-section model developed by the authors, a set of group constants for liquid 4He at many different temperatures down to 0.1 K is generated. Major results thus obtained are as follows. As the temperature is decreased below about 0.5 K, the UCN density shows a saturation behavior because of no significant up-scattering and UCN loss by neutron β-decay only. Single down-scattering of a 1-meV neutron and also multiple scattering of a higher-energy neutron contribute largely to UCN production. A high UCN density of 1×105 cm-3 is possible with a high reflectivity of a storage vessel, say over 0.9999.