Structure of Solid Helium by Neutron Diffraction

Neutron diffraction patterns for six samples of polycrystalline solid helium have been measured over the angular range 5° to 62° using ${1.06}_4$ A neutrons. Four of these specimens were held at ${1.1}_5$°K and 66 atmospheres while the other two were maintained at ${1.8}_8$°K and at 66 and 69 atmospheres, respectively. The diffuse scattering was small at low angles and increased with increasing angle up to 45° 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 ${1.1}_5$°K and 66 atmospheres. The positions of the lines are fitted by a hexagonal close-packed structure in which $a_0=3.53\mathrm{}\pm 0.03$ A and $c_0=5.76\mathrm{}\pm 0.05$ A. Within experimenal error no difference is observed between the patterns at ${1.8}_8$°K and ${1.1}_5$°K. The calculated density of solid helium at ${1.1}_5$°K and 66 atmospheres is 0.214±0.006 gram/cc. The mean isothermal compressibility of the solid at ${1.1}_5$°K between the solidus and 66 atmospheres is (3.1±0.8)×${10}^{-3\mathrm{}}$ ${\mathrm{atmosphere}}^{-1\mathrm{}}$.