Characterization of a bent Laue double‐crystal beam‐expanding monochromator. (19th October 2017)

• Record Type: Journal Article
• Authors: Martinson, Mercedes; Samadi, Nazanin; Shi, Xianbo; Liu, Zunping; Assoufid, Lahsen; Chapman, Dean
• Is Part Of: Journal of synchrotron radiation. Volume 24:Part 6(2017)
• Page Start: 1146
• Page End: 1151
• Publication Date: 2017-10-19
• Subjects: beam expander -- double bent Laue monochromator -- Lang topography -- Berg–Barrett topography -- finite‐element analysis; Synchrotron radiation -- Periodicals; Free electron lasers -- Periodicals; 539.73505
• DOI: 10.1107/S1600577517014059
• Languages: English
• ISSNs: 0909-0495

Abstract

Abstract : A previously reported bent Laue double‐crystal monochromator was found to have areas of missing intensity in the final X‐ray beam. Measurements of the shape of the bent crystal wafers have been made using mechanical and diffraction methods to evaluate the crystal system and provide insight into potential methods of mitigating the non‐uniformities in the beam. Abstract : A bent Laue double‐crystal monochromator system has been designed for vertically expanding the X‐ray beam at the Canadian Light Source's BioMedical Imaging and Therapy beamlines. Expansion by a factor of 12 has been achieved without deteriorating the transverse coherence of the beam, allowing phase‐based imaging techniques to be performed with high flux and a large field of view. However, preliminary studies revealed a lack of uniformity in the beam, presumed to be caused by imperfect bending of the silicon crystal wafers used in the system. Results from finite‐element analysis of the system predicted that the second crystal would be most severely affected and has been shown experimentally. It has been determined that the majority of the distortion occurs in the second crystal and is likely caused by an imperfection in the surface of the bending frame. Measurements were then taken to characterize the bending of the crystal using both mechanical and diffraction techniques. In particular, two techniques commonly used to map dislocations in crystal structures have been adapted to map local curvature of the bent crystals. One of these, a variation of Berg–Berrett topography, has been used to quantify the diffraction effects caused by the distortion of the crystal wafer. This technique produces a global mapping of the deviation of the diffraction angle relative to a perfect cylinder. This information is critical for improving bending and measuring tolerances of imperfections by correlating this mapping to areas of missing intensity in the beam.