Transport properties of Ca2NaMg2V3O12 garnet

The results from dc conductivity and thermopower measurements on Ca2NaMg2V3012 single crystals are reported. The charge transport is shown to occur via tnermally activated hopping of electrons situated at V4+ tetrahedral sites. The importance of correlation effects at higher concentrations is emphasized

Replaceable Be and Al windows for x-ray beam lines

Construction details and results of on-going performance tests are presented for two ultra-high vacuum compatible, free-convection cooled window assemblies in which the window disk is easily replaceable. In the Be window assembly, an 0.25 mm thick Be disk, 38 mm in diameter is sealed into a stainless steel adapter flange using 0.5 mm diameter pure Al wire. The assembly is leak tight to better than 1 x 10/sup -10/ atm cm/sup 3//sec before and after multiple bakeouts to 250/sup 0/C. However, an unexplained temperature-dependent He leak, with a maximum magnitude in the low 10/sup -10/ atm cm/sup 3//sec range at 250/sup 0/C, appears during bakeout. Possible causes for this leak are discussed. In the Al window assembly, a 25 ..mu..m thick Al disc, 60 mm in diameter acts as the sealing medium as well as the window material over a 10 mm by 50 mm opening. This assembly remains leak tight to better than 1 x 10/sup -10/ atm cm/sup 3//sec throughout multiple bakeouts to 150/sup 0/C and has been successfully tested in white and monochromatic beam lines at the NSLS x-ray ring. 25 refs

Liquid gallium cooling of silicon crystals in high intensity photon beam

The high-brilliance, insertion-device-based, photon beams of the next generation of synchrotron sources will deliver large thermal loads (1 kW to 10 kW) to the first optical elements. Considering the problems that present synchrotron users are experiencing with beams from recently installed insertion devices, new and improved methods of cooling these first optical elements, particularly when they are diffraction crystals, are clearly needed. A series of finite element calculations were performed to test the efficiency of new cooling geometries and new cooling fluids. The best results were obtained with liquid Ga metal flowing in channels just below the surface of the crystal. Ga was selected because of its good thermal conductivity and thermal capacity, low melting point, high boiling point, low kinetic viscosity, and very low vapor pressure. Its very low vapor pressure, even at elevated temperatures, makes it especially attractive in uhv conditions. A series of experiments were conducted at CHESS in February of 1988 that compared liquid gallium cooled silicon diffraction crystals with water cooled crystals. 2 refs., 16 figs., 1 tab

CT imaging of small animals using monochromatized synchrotron x rays

Rats and chicken embryos were imaged in vivo with a prototype Multiple Energy Computed Tomography (MECT) system using monochromatized x rays from the X17 superconducting wiggler at the National Synchrotron Light Source. The CT configuration coated of a horizontal low-divergence, fan-shaped beam, 70 mm wide and 0.5 mm high, and a subject rotating about a vertical aids. A linear-array high-purity Ge detector with 140 elements, each 0.5 mm wide and 6 mm thick, was used with a data acquisition system that provides a linear response over almost six orders of magnitude of detector current. The dual photon absorptiometry (DPA) algorithm was applied to images of the rat head acquired at 20 and 45 keV to obtain two new images, one representing the low-Z, and the other the intermediate-Z clement group. The results indicate that the contrast resolution and the quantification accuracy of the images improve stepwise; first, with the monochromatic beam and, second, the DPA method. The system is a prototype for a brain scanner