Neutronic study of UCN production and storage in a liquid 4He source

The production and storage of ultracold neutrons(UCNs) in liquid 4He are evaluated by means of a multigroup neutron transport analysis. Several sets of group constants for eight different temperatures between 0.1 and 2.5 K in a neutron energy region of 0.1 μeV to 10 eV are generated. A model UCN source of pure liquid 4He is analyzed, in which UCNs are produced by down-scattering of cold neutrons, stored optically by total reflection at the surface, and destroyed by the neutron β-decay only. The following results are obtained: (a) As lowering temperature below about 0.5 K, UCN density exhibits saturation behavior because of no significant up-scattering of UCNs. (b) In addition to single down-scattering of a 1 meV neutron, multiple scattering of a higher-energy neutron contributes largely to UCN production, nearly doubling UCN density. (c) A high reflection coefficient of, say, greater than 0.9999, is necessary to yield a high UCN density of 1.21×105 cm−3 at 0.1 K.