Inelastic Scattering of Thermal Neutrons from Liquid Helium

TLDR

Neutron scattering from liquid helium has been investigated over a broad range of wave‑vector transfers, revealing behavior consistent with nearly free helium atoms at large Q. The study employed time‑of‑flight rotating‑crystal and triple‑axis spectrometers to measure temperature dependence at both small and large wave‑vector transfers. At 1.1 K the data display a sharp one‑phonon peak plus a broad diffuse background, with well‑defined excitations above the λ point that sharpen at large Q and evidence for macroscopic occupation of the zero‑momentum state, all of which are compared with theoretical predictions.

Abstract

The scattering of neutrons by liquid helium has been studied over a wide range of wave-vector transfers, 0.2 < Q < 9.0 Å −1 , and energy transfers [Formula: see text]. The measurements were performed with both a time-of-flight rotating crystal spectrometer and a triple-axis crystal spectrometer. The scattering at 1.1 °K showed a sharp one-phonon peak and a broad diffuse background. Measurements have been made of the energy and intensity of both components. At large wave-vector transfers the scattering is of the form expected from a system of almost free helium atoms. Measurements of the temperature dependence of the results at the smallest and largest wave vectors have been made. In the former case they show well-defined excitations above the λ point which are better defined for large wave vectors, while in the latter case they can be interpreted as giving evidence for the macroscopic occupation of the zero-momentum state. The results are compared with the available theoretical predictions.