Performance of nearly fixed offset asymmetric channel-cut crystals for X-ray monochromators.

Ronald Frahm,Benjamin Bornmann,Tang Li,Vadim Murzin,Zhen Hong,Dirk Lützenkirchen-Hecht,Qianshun Diao

doi:10.1107/s1600577519011123

Abstract

X-ray double-crystal monochromators face a shift of the exit beam when the Bragg angle and thus the transmitted photon energy changes. This can be compensated for by moving one or both crystals accordingly. In the case of monolithic channel-cut crystals, which exhibit utmost stability, the shift of the monochromated beam is inevitable. Here we report performance tests of novel, asymmetrically cut, channel-cut crystals which reduce the beam movements by more than a factor of 20 relative to the symmetric case over the typical energy range of an EXAFS spectrum at the Cu K-edge. In addition, the presented formulas for the beam offset including the asymmetry angle directly indicate the importance of this value, which has been commonly neglected so far in the operation of double-crystal monochromators.

Highlights

A double-crystal monochromator (DCM) is typically equipped with sets of Si(111), Si(220) or Si(311) crystals to obtain monochromatic X-rays especially from synchrotron radiation sources. Both crystals are mounted together on one goniometer, which rotates both crystals to the desired Bragg angle which defines the wavelength and photon energy E
If we assume a horizontal axis for the Bragg rotation and a separation between both crystals D, the height offset of the exit beam with respect to the incoming white radiation is given by h 1⁄4 2D cosðÞ: ð1Þ
The QEXAFS monochromator is equipped with a liquid-nitrogen-cooled symmetric Si(111) channel-cut crystal

Summary

Introduction

A double-crystal monochromator (DCM) is typically equipped with sets of Si(111), Si(220) or Si(311) crystals to obtain monochromatic X-rays especially from synchrotron radiation sources Both crystals are mounted together on one goniometer, which rotates both crystals to the desired Bragg angle which defines the wavelength and photon energy E. All mechanical movements introduce vibrations, and angular inaccuracies occur Considering that both crystals have to remain stable in angle to a small fraction of the Darwin width, and with the large distances between monochromators and experiments at highenergy synchrotron radiation sources, such inaccuracies become very difficult to handle. The accuracy of the realized crystal orientation with respect to the desired lattice planes specified by different manufacturers increased from about Æ 0.1 to nowadays Æ 0.05 or less This asymmetry uncertainty is not considered to be a major issue. The reduction of vertical walk by asymmetric crystals was discussed, for example, by Hrdy (1989), Smither & Fernandez (1994) and Smither et al (2012), but all without specific experimental verification

Theory of asymmetric channel-cut crystals

Asymmetric channel-cut crystals for nearly fixed offset exit beam

Design of the channel-cut test crystal

Characterization of the test crystal

Energy scans for calibration of the Bragg angle