Momentum density of hcp and liquid helium-4 by inelastic neutron scattering

A measurement of the momentum density in hcp and liquid 4He by inelastic neutron scattering is reported. Using the Low Resolution Medium Energy Chopper Spectrometer at the Intense Pulsed Neutron Source at Argonne National Laboratory, momentum transfers in the range 12 to 22.5 A-1 were attained. At these momentum transfers, the momentum density of the sample is related to the dynamic structure factor by the impulse approximation. The measured momentum distribution is Gaussian and the kinetic energy is larger than proposed by existing theories. Data were taken on two solid samples, the first was a 19.45 cm3/mole hcp solid, the second was 18.20 cm3/ mole; both solid samples were maintained at 1.70 K during data collection. Data were also taken on a liquid sample with a molar volume of 18.20 cm3/mole at 4.00 K. At 1.70 K the two solid samples are essentially in their ground states so that the measurement is of the ground state momentum density. The liquid sample was included to see if the difference between the liquid and solid momentum density at the same molar volume was observable