Neutron reflection from a liquid helium surface

Abstract

The reflection of neutrons from a helium surface has been observed for the first time. The 4 He surface is smoother in the superfluid state at 1.54 K than in the case of the normal liquid at 2.3 K. In the superfluid state we also observe a surface layer~200 Å thick which has a subtly different neutron scattering cross-section, which may be explained by an enhanced Bose-Einstein condensate fraction close to the helium surface. The application of neutron reflectometry described in this paper creates new and exciting opportunities for the surface and interfacial study of quantum fluids.

Influence of the liquid helium meniscus on neutron reflectometry data

“…The SLD profile is also good replication of that shown in Ref. 6 as in this paper we have refitted the original data set with the new fitting package to check the validity of the initial model. For the scientific implications of the SLD profile 4 He mixture.…”

“…5 Much later the reflection of neutrons was observed for the first time from the free surface of liquid 4 He. 6 These experiments showed that the 4 He surface is smoother in the superfluid state at 1.54 K than in the case of the normal liquid at 2.3 K. In the superfluid state a $200 Å thick surface layer was found that has a subtly different neutron scattering cross-section, which may be explained by an enhanced Bose-Einstein condensate fraction close to the helium surface.…”

“…In our initial experiments on neutron reflection from liquid helium surfaces 6,8,9 at the ISIS neutron scattering facility, we used the general-purpose time-of-flight (TOF) polarized reflectometer CRISP; 17 in later experiments we used the nextgeneration reflectometer POLREF. 18 The experimental set-up used for the liquid helium experiments is described in Ref.…”

“…6. It should be noted at this point that the density of liquid 4 He changes with temperature and has to be taken into account in the initial calculation of the He isotope SLD.…”

A dynamically adjustable wavelength-sensitive neutron filter

“…Gas detectors filled with 3 He are commonly used for the detection of thermal neutrons, due to their high efficiency and low gamma sensitivity [1]. Standard 3 He detectors on advanced neutron scattering instruments allow us to cover a broad range of scattering angles and momentum transfer vectors Q. For neutron spectroscopy, however, the need to measure the neutron energy transfers can make the instrument design much more complicated.…”