Characterization of a bent Laue double-crystal beam-expanding monochromator.

Mercedes Martinson,Dean Chapman,Lahsen Assoufid,Xianbo Shi,Zunping Liu,Nazanin Samadi

doi:10.1107/s1600577517014059

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.

Highlights

The BioMedical Imaging and Therapy (BMIT) beamlines at the Canadian Light Source would greatly benefit from an increase in the vertical size of the X-ray beam, which would enable dynamic imaging of animal samples that are larger than currently possible with the 7 mm and 11 mm vertical heights of the bending-magnet and insertion-device beamlines, respectively
The effect of this distortion on the diffraction angles was carefully measured by analysing diffraction images produced by bent crystals in a variation of Berg–Barrett topography
Several measurement techniques were used to characterize a bent Laue double-crystal beam-expanding monochromator used at the BMIT beamline at the Canadian Light Source

Summary

Introduction

The BioMedical Imaging and Therapy (BMIT) beamlines at the Canadian Light Source would greatly benefit from an increase in the vertical size of the X-ray beam, which would enable dynamic imaging of animal samples that are larger than currently possible with the 7 mm and 11 mm vertical heights of the bending-magnet and insertion-device beamlines, respectively. Using the design parameters for the bending frames (0.5 m and 5 m cylindrical slabs), the actual bend radii predicted for the crystals were 0.518 m and 5.06 m, respectively, with anticlastic bend radii of 51.4 m and 70.8 m This indicates that the second crystal may be more adversely affected by anticlastic bending due to its larger bend radius and lower tolerance for small irregularities. As the anticlastic bend radii are on the same order of magnitude and perpendicular to the diffraction plane, we do not expect serious effects on intensity, despite the comparatively larger ratio between principle and anticlastic bend radii in the second crystal This analysis did not predict the hole of missing intensity as this was likely caused by imperfections in the physical bending frame.

Mechanical measurements

Conclusion

Findings

Funding information