Sagitally focusing scanning monochromator produces 0.4-mm focus (abstract)

Abstract

A sagitally focusing stage has been constructed for the double-crystal scanning monochromator on beam line X9-A at the NSLS which can be exchanged with the normally used flat crystal stage. The bending device located at 12.0 m from the source is designed to accept a beam up to 200-mm wide. The monochromator with bending stage can be scanned over Bragg angles from 10° to 71° corresponding to photon energies from 11.4 to 2.1 keV with Si-111 crystals. In conjunction with the vertically focusing mirror a point focus of 0.38 mm×0.18 mm [horizontal×vertical, full width at half maximum (FWHM) each] has been achieved at a focal length of 3.8 m (center of a four-circle diffractometer). Focused at the back of the experimental hutch (focal length=5.4 m) the focal size was 0.55 mm×0.30 mm. The measured horizontal width of the focus equals the calculated size of the demagnified source and is independent of the horizontal convergence used. The horizontal focus produced by the crystal is very clean: the full width at 1% of maximum is 1.5 mm and at 0.01% it is about 4 mm. The flux into the focus is 5×1011 photons/s at a photon energy of 7.4 keV and beam current of 200 mA. The average flux density in the center of the focus (FWHM area) is 5×1011 photons/s/mm2. The preliminary crystal presently used has been made from a thin Si plate of 0.4-mm thickness, 80-mm width, and 75-mm length (in the direction of the beam). Steel ribs of 0.6-mm thickness have been glued at 3-mm pitch to the back of the crystal in order to stiffen the plate and reduce anticlastic bending.1 Plates have been glued to this crystal plate to extend the width to size the bending stage. The crystal was cut in 111 orientation with a 4° angle between the lattice planes and the crystal surface. The asymmetric cut was used to increase the angle of incidence and thus decrease the effect of the remaining anticlastic bending. The bending stage has a bending couple at both ends of the crystal in order to produce the desired curvature of the crystal. The bending couples can be adjusted to produce a conical shape as needed for the particular choice of focal length.2 Small piezotranslators push on the bending couples to remove small amounts of twist in the crystal introduced by alignment errors of the bending couples. All of the adjustment drives are outside of the monochromator tank and transmitted to the bending stage via linear feedthroughs and cables. A full description of this device and the crystal with glued ribs will be submitted shortly to this journal. This work was supported by Grant RR-01633, National Institutes of Health, Division of Research Resources.