An Ultra-Low-Temperature Diffractometer Based on an ³He–⁴He Dilution Refrigerator used for Synchrotron-Radiation X-ray Diffractometry and Topography

The direct observation of crystal structure at ultra-low temperatures helps better understanding of phase transformations and phase stability. Two sets of ³He-⁴He dilution refrigerators with a top-loading facility were installed at BL-3C₂ and 6C₁ of the Photon Factory. The first is used exclusively for topography of quantum crystals of solid ³He and ⁴He for studies of lattice defects. Typical examples of topographs that indicate an annealing effect of solid helium, understood in terms of a quantum tunneling, are presented. The disappearance of downward diffraction Laue spots (2θ_B = 90°) is discussed on the basis of the temperature and other factors in solid helium. The second refrigerator is used for X-ray diffraction studies of crystalline solids. A splitting of the Laue spots from Cs₂NaHoCl₆ below 150 mK, giving evidence of a cooperative Jahn–Teller transition of the first kind at the lowest transition temperature ever observed, is briefly explained as an example of diffractometry.