Bent Laue X-ray Beam Expander

Abstract

Synchrotron imaging beamlines around the world all suffer from a similar limitation, namely a beam that is smaller in the vertical direction than the horizontal. This can produce a beam that is so small in the vertical direction that some imaging applications are limited or even impossible. At the BioMedical Imaging and Therapy (BMIT) beamline facility at the Canadian Light Source (CLS), the vertical beam sizes on the Bend Magnet (BM) and Insertion Device (ID) beamlines are 7 mm and 11 mm, respectively. This limited vertical beam size results in several limitations. Micro-computed-tomography experiments requiring multiple rotations to produce a full three-dimensional representation of the sample result in longer scan times and possible reconstruction errors due to misalignment between rotations. Similarly, projection images requiring vertical scans to cover the entire two dimensional field of view extend acquisition times and lead to potential stitching errors between exposures. Dynamic phase-based imaging (i.e. movies), which are being used for some of the most cutting edge biomedical imaging research taking place worldwide, is virtually impossible with samples larger than the vertical beam size. This problem has been solved at other synchrotrons by building very long beamlines and allowing the beam to naturally diverge to a larger field of view, however this was not possible for BMIT due to budgetary and geographical limitations. In order to vertically expand the beam, a bent Laue double crystal monochromator was used in a non-dispersive divergent geometry to ultimately produce a beam expansion of 12× the incident height. Improvements were made to the system to preserve the quality of transverse coherence in the beam, allowing phase-based imaging techniques to be performed with a larger field of view. This was achieved by carefully matching the geometric and single-ray focal points in the so-called “magic condition.” The quality of the expanded beam was compared to that produced by the beamline’s standard flat Bragg double crystal monochromator and was found to differ in divergence by less than 10% between the two monochromator systems. Further testing was done to evaluate the criticality of matching the two focal types, and to determine at least a minimum energy range over which the system could be used reliably. These tests showed that the system is much more flexible than previously believed, with energy ranges of at least ±5 keV producing images wherein the vertical and horizontal edge width differ by less than 1%, indicating that the expander does not adversely affect the beam in the diffraction plane. Despite the improvement to the diffraction and focus characteristics of the system, there was an ongoing issue with areas of missing intensity in the beam. The hypothesis that this was caused by imperfect bending of the second crystal has been confirmed using diffraction and mechanical measurement techniques.