Single-particle dynamics of the solid heliums from deep inelastic neutron scattering

Abstract

Previous neutron-scattering research on solid heliums has been restricted to small momentum transfers, Q, both by large Debye–Waller factors and by scientific interest being restricted to collective modes and their interactions. It has also been limited by insufficient sources of neutron beams of about an electronvolt in energy, which are required to apply the method of deep inelastic scattering to solids. Making use of a spallation neutron source and a suitable chopper spectrometer, one can now collect data in which the dynamic structure factor S(Q,E) directly reflects the atomic-momentum distribution n(p). For 4He, the major determining factor on this single-particle property is the density of the sample, not its microscopic structure. At a strictly constant number density, samples of hexagonal close-packed (hcp), body-centered cubic (bcc), and normal liquid show identical S(Q,E)'s for Q's near 200 nm−1, at current levels of precision. The case of bcc 4He is used to illustrate the method, and the derived kinetic energy is compared with theoretical predictions. The applicability of these results in other areas of solid-helium physics is indicated.