Prominent peculiarities of X-ray diffraction of solid helium

Abstract

Since3He and4He atoms are the lightest stable mono-atoms with atomic number 2, they cause a variety of macroscopic quantum effects, including large zero point vibrations in liquid and solid phases. Some quantities relevant to X-ray diffraction of solid helium, i.e., the atomic-scattering factor, mass-absorption coefficient, the reciprocal of which is the characteristic length defined by absorption and the extinction distance etc., show either larger or smaller values by approximately two orders of magnitude compared with 3d transition metals. An incidental disappearance of Laue spots due to the downward X-ray diffraction of solid helium (2θB = 90°) could be explained by two factors. One is that the fraction of recoil-free emission is weakened by the great ratio of the huge recoil energy to the phonon energy through the zero-point energy in the Debye-Waller parameter. The other is that the great difference in the linear scatterer density in downward diffraction from that in forward diffraction results from the irradiation of a line-focus X-ray beam to take section topographs weakens the downward X-ray diffraction. The dislocation line of solid helium could not be observed with sharp contrast. Considering the two-orders higher magnitude of the extinction distances and the high dislocation density of more than 2.6· 105cm−2, dynamical X-ray diffraction seems to lose validity.