Measurement of the kinetic energy in He-4 through the superfluid transition by very high-energy neutron scattering

Measurements of the kinetic energy of atoms in liquid He-4 have been made using neutron Compton scattering at wavevector transfers between 90 and 150 Angstrom(-1). The kinetic energy shows a marked decrease from 16.2 K at a temperature of 2.5 K, to 13.9 K at 1.3 K. Three different measurements were made using energy analysers with different resolutions. Good agreement was obtained between the measurements using different resolutions and with previous measurements and calculations of the kinetic energies. For the best-resolution data, a line shape analysis was made in order to derive the non-Gaussian components of the momentum distribution. Values of kinetic energies are consistent with a Bose condensate fraction of similar to 10%. The measurements show that eV neutron scattering provides an accurate method of measuring kinetic energies in quantum fluids.