THE RESOLUTION FUNCTION OF A TRIPLE-CRYSTAL DIFFRACTOMETER FOR HIGH-ENERGY SYNCHROTRON-RADIATION IN NONDISPERSIVE LAUE GEOMETRY

The k-space resolution function of a triple-crystal diffractometer is calculated for an arrangement of three perfect silicon single crystals Bragg diffracting in nondispersive Laue geometry. A comparison is mad e with the results of measurements using synchrotron radiation in the energy range from 80 to 150 keV. In this case, absorption is very weak and according to dynamical theory the width of the diffraction patter n of thick perfect single crystals is proportional to the wavelength l ambda, whereas its Lorentzian tails are proportional to lambda(2). Tog ether with the fact that the Bragg angles are only of the order of 2 d egrees, this leads to a concentration of the starlike k-space resoluti on function into a narrow band parallel to the reciprocal-lattice vect or G. For diffraction of 80 keV synchrotron radiation at the silicon 1 11 reflection, the full width at half-maximum (FWHM) of the intensity distribution in the scattering plane is 1.1 x 10(-5) Angstrom(-1) perp endicular to G and 2.2 x 10(-4) Angstrom(-1) parallel to G. The observ ed differences in the contributions from monochromator and analyzer cr ystal to the resolution function are explained by the finite width of the electronic window of the detector counting chain and the non-Bragg scattering contribution from the crystals. If annealed Czochralski-gr own silicon single crystals with a mosaicity of similar to 3'' are use d as monochromator and analyzer, the resolution is reduced by one orde r of magnitude, but for studies of imperfect samples or of diffuse sca ttering large gains in intensity can be accomplished this way.