Structure of Solid Helium by Neutron Diffraction

Abstract

Neutron diffraction patterns for six samples of polycrystalline solid helium have been measured over the angular range 5\ifmmode^\circ\else\textdegree\fi{} to 62\ifmmode^\circ\else\textdegree\fi{} using ${1.06}_{4}$ A neutrons. Four of these specimens were held at ${1.1}_{5}$\ifmmode^\circ\else\textdegree\fi{}K and 66 atmospheres while the other two were maintained at ${1.8}_{8}$\ifmmode^\circ\else\textdegree\fi{}K and at 66 and 69 atmospheres, respectively. The diffuse scattering was small at low angles and increased with increasing angle up to 45\ifmmode^\circ\else\textdegree\fi{} beyond which the intensity decreased in much the same way as observed in the scattering patterns from liquid helium. Six lines were observed in the patterns recorded at ${1.1}_{5}$\ifmmode^\circ\else\textdegree\fi{}K and 66 atmospheres. The positions of the lines are fitted by a hexagonal close-packed structure in which ${a}_{0}=3.53\ifmmode\pm\else\textpm\fi{}0.03$ A and ${c}_{0}=5.76\ifmmode\pm\else\textpm\fi{}0.05$ A. Within experimenal error no difference is observed between the patterns at ${1.8}_{8}$\ifmmode^\circ\else\textdegree\fi{}K and ${1.1}_{5}$\ifmmode^\circ\else\textdegree\fi{}K. The calculated density of solid helium at ${1.1}_{5}$\ifmmode^\circ\else\textdegree\fi{}K and 66 atmospheres is 0.214\ifmmode\pm\else\textpm\fi{}0.006 gram/cc. The mean isothermal compressibility of the solid at ${1.1}_{5}$\ifmmode^\circ\else\textdegree\fi{}K between the solidus and 66 atmospheres is (3.1\ifmmode\pm\else\textpm\fi{}0.8)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$ ${\mathrm{atmosphere}}^{\ensuremath{-}1}$.