Scattering of Cold Neutrons on Gel Samples Formed by Impurity Clusters in Superfluid He-II

Abstract

The approach we have proposed earlier for a large increase in ultracold neutron density was based on equilibrium thermalization of cold neutrons during their diffusive motion in a large sample of an impurity-helium gel which consists of weakly bounded deuterium or heavy water nanoclusters in He-II cooled below a few mK. It is clear that the nanoparticles should provide a sufficiently large cross-section of coherent scattering of cold neutrons, but the scattering of neutrons on impurity gel samples in He-II has been never investigated earlier. In this paper we present results of the first observations of the scattering of a beam of cold neutrons with mean velocities from 30 to 160 m/s on heavy water and deuterium gel samples in He-II at temperatures T∼1.6 K. It has been observed that the beam transmission through the heavy water gel sample has been changed from ∼0% to ∼50% upon increasing the neutron velocity from 30 to 160 m/s, and that the neutron transmission trough the deuterium gel sample is less than 5% at all velocities. The angular distribution of the neutron scattering changes with variations in the neutron velocity: for both the gels at large neutron energies the scattering is directed in a front hemisphere mainly, and at low energies the angular distribution of scattered neutrons is close to uniform.