AN ULTRA-LOW-TEMPERATURE DIFFRACTOMETER BASED ON AN HE-3-HE-4 DILUTION REFRIGERATOR USED FOR SYNCHROTRON-RADIATION X-RAY-DIFFRACTOMETRY ANDTOPOGRAPHY

The direct observation of crystal structure at ultra-low temperatures helps better understanding of phase transformations and phase stabilit y. Two sets of He-3-He-4 dilution refrigerators with a top-loading fac ility were installed at BL-3C(2) and 6C(1) of the Photon Factory. The first is used exclusively for topography of quantum crystals of solid He-3 and He-4 for studies of lattice defects. Typical examples of topo graphs that indicate an annealing effect of solid helium, understood i n terms of a quantum tunneling, are presented. The disappearance of do wnward diffraction Laue spots (2 theta(B) = 90 degrees) is discussed o n the basis of the temperature and other factors in solid helium. The second refrigerator is used for X-ray diffraction studies of crystalli ne 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 ex plained as an example of diffractometry.