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 1.06₄ A neutrons. Four of these specimens were held at 1.1₅°K and 66 atmospheres while the other two were maintained at 1.8₈°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 1.1₅°K and 66 atmospheres. The positions of the lines are fitted by a hexagonal close-packed structure in which a₀=3.53+/-0.03 A and c₀=5.76+/-0.05 A. Within experimenal error no difference is observed between the patterns at 1.8₈°K and 1.1₅°K. The calculated density of solid helium at 1.1₅°K and 66 atmospheres is 0.214+/-0.006 gram/cc. The mean isothermal compressibility of the solid at 1.1₅°K between the solidus and 66 atmospheres is (3.1+/-0.8)×10^-3 atmosphere^-1.