Atomic Distribution in Liquid Helium by Neutron Diffraction

The angular distribution of 1. 04 A neutrons scattered by liquid helium has been measured for a series of temperatures between 1. 6^ and 5. 04^ over the angular range 3^ to 70^. The scattering patterns are characterized by one principal maximum at an angle of 19. 60. 5^ over a large range in liquid density. No marked change in the scattering pattern accompanies the transition. It is shown that for large-angle scattering the liquid behaves like a system of free helium atoms. Scattering intensities normalized to the differential cross section for a free helium atom have been transformed to give the radial distribution functions at 5. 04^, 4. 24^, and in the region of 2^. The first maximum in the density is at 3. 5₀ A for the lowest temperature and at 3. 6₈ A for the highest. The density change is not accounted for by the shift of the maximum, and a decrease in the number of neighbors is postulated. The mean radial positions assigned to the first and second neighbors using the analysis of this paper agrees with a close-packed configuration of symmetrical particles, but the average number of first neighbors appears to be less than a complete close-packed arrangement, in keeping with the random arrangement expected for a liquid.