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°K and 5.04°K over the angular range 3° to 70°. The scattering patterns are characterized by one principal maximum at an angle of 19.6±0.5° over a large range in liquid density. No marked change in the scattering pattern accompanies the $\lambda$ 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°K, 4.24°K, and in the region of 2°K. The first maximum in the density is at ${3.5}_0$ A for the lowest temperature and at ${3.6}_8$ 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.