Pressure Effect in the Atomic Distribution in Liquid Helium by Neutron Diffraction

Abstract

The angular distribution of ${1.06}_4$ A neutrons scattered from liquid helium at temperatures and pressures in the range 1.2°K to 4.24°K and 0 to 51.3 atmospheres for densities up to 0.184 gram/cc has been measured at about 210 equally spaced points in the angular range 5° to 62°. With increasing liquid density, the principle maximum moves to larger angles and increases in height. The liquid structure factors are given for densities of 0.166 gram/cc and 0.184 gram/cc. The density distribution functions are deduced for each of the scattering patterns. A study of these gives 2.27±0.08 A as the nearest distance of approach of two atoms in the liquid. The number of neighbors under the first and second coordinate shells changes from about 6.5 to 8.5 atoms and from about 9 to 5.5 atoms, respectively, for density changes from 0.095 gram/cc to 0.184 gram/cc. The corresponding change in the ratio of their spacings is from ${1.47}_5$ to 1.38, which values are close to $\sqrt{2}$, the theoretical ratio for a close packed lattice. The analysis that the density changes in the liquid and during the solid-liquid transformation cannot be accounted for on the basis of a uniform dilation of a basic structure. The changes in the distribution function caused by pressure are different from those caused by temperature along the normal vapor pressure line.