Study of TaSi 2 -Si Crystals with High Energy Synchrotron Radiation as Tunable Wide-bandpass Monochromator and Analyzer
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Study of TaSi2 -Si Crystals with High Energy Synchrotron Radiation as Tunable Wide-bandpass Monochromator and Analyzer Optics U. Rütt1 , S.R. Stock2 , Z.U. Rek3 1 Materials Science Division, Argonne National Lab, Argonne, IL, USA 2 School of Materials Sci & Eng, Georgia Inst of Technology, Atlanta, GA 30332-0245, USA 3 Stanford Synchrotron Radiation Lab, Stanford, CA, USA
ABSTRACT Single crystals of the in situ composite TaSi2 -Si are candidates for wide bandpass monochromators and analyzers for x-ray experiments. Two large crystals (32 x 39 x 7 mm3 and 32 x 39 x 30 mm3 ) were studied in detail using 115 keV synchrotron radiation at the BESSRC beamline at the APS. For transmission geometry, the Si(111) reflection and 39 mm crystal thickness, rocking curve widths of 95 arc sec with reflectivities of 50% were measured over the entire crystal. Turning the crystal to an angle of 45° to the incident beam and translating it through the beam allowed investigation of the influence of crystal thickness on diffracted intensity. For sample thicknesses ranging between 5 mm and 12.5 mm, the full width at half maximum (FWHM) of the rocking curve increased linearly from 29 arc sec to 53 arc sec. The greatest gain in integrated intensity was obtained for 8-9 mm thickness where FWHM = 40 arc sec. If this crystal were used as a monochromator for synchrotron radiation, it would provide 40 times more intensity than a perfect silicon crystal and an energy bandwidth of 1 keV at 115 keV photon energy. With the increase of the rocking curve FWHM, a plateau across the peak maximum developed; its width increased from 5 arc sec to 25 arc sec when the crystal thickness increased from 5 mm to 12.5 mm, respectively. This plateau allows these crystals to be used as analyzer crystals in diffraction experiments, i.e., as very narrow slits to suppress background. A plateau width of about 5-20 arc sec for the analyzer crystal is needed to insure that the entire intensity scattered from a sample is delivered to the detector. The simple expedient of changing the crystal thickness alters the plateau width of the analyzer crystal and tunes its acceptance to the needs of the sample under study. INTRODUCTION Directional solidification of eutectic mixtures often produces aligned arrays of a second phase in a continuous matrix; for semiconductor-metal mixtures closely-spaced, cylindrical metallic rods in a single crystal matrix often result. Originally developed for microelectronic applications, the Si-TaSi2 composites were also found to be useful wide bandpass optical elements for synchrotron radiation, particularly for hard x-radiation [1-4]. Characterization of the crystals’ x-ray optical performance included x-ray diffraction topography and double- and triple-axis diffractometry including reciprocal space mapping, and this included studies at 8 keV [1,2], at 120 keV [3] and at various energies spanning 6 keV and 80 keV [4]. The earlier work was done on crystals grown prior to 1990 at GTE Laboratories, and growth of Si-TaSi2 crystals has recently resumed
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