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Abstract
The BioMedical Imaging and Therapy beamlines at the Canadian Light Source are used by many researchers to capture phase-based imaging data. These experiments have so far been limited by the small vertical beam size, requiring vertical scanning of biological samples in order to image their full vertical extent. Previous work has been carried out to develop a bent Laue beam-expanding monochromator for use at these beamlines. However, the first attempts exhibited significant distortion in the diffraction plane, increasing the beam divergence and eliminating the usefulness of the monochromator for phase-related imaging techniques. Recent work has been carried out to more carefully match the polychromatic and geometric focal lengths in a so-called `magic condition' that preserves the divergence of the beam and enables full-field phase-based imaging techniques. The new experimental parameters, namely asymmetry and Bragg angles, were evaluated by analysing knife-edge and in-line phase images to determine the effect on beam divergence in both vertical and horizontal directions, using the flat Bragg double-crystal monochromator at the beamline as a baseline. The results show that by using the magic condition, the difference between the two monochromator types is less than 10% in the diffraction plane. Phase fringes visible in test images of a biological sample demonstrate that this difference is small enough to enable in-line phase imaging, despite operating at a sub-optimal energy for the wafer and asymmetry angle that was used.
Highlights
At the Canadian Light Source (CLS) in Canada, the BioMedical Imaging and Therapy (BMIT) bend magnet (BMIT-BM) beamlines and insertion device (BMIT-ID) (Wysokinski et al, 2007, 2013) have been very successful in their mission to image biological tissue and conduct live animal imaging studies (Pratt et al, 2014)
We discovered that the beam expander destroyed the phase characteristics of the beam in the vertical direction and caused blurring of horizontal knife-edges placed at longer sample-to-detector distances
Using = 3.33, = 0.22, fs = 22 m and bend radius R = À0.5 m, we find that the magic condition is Mercedes Martinson et al Phase-preserving beam expander 803
Summary
At the Canadian Light Source (CLS) in Canada, the BioMedical Imaging and Therapy (BMIT) bend magnet (BMIT-BM) beamlines and insertion device (BMIT-ID) (Wysokinski et al, 2007, 2013) have been very successful in their mission to image biological tissue and conduct live animal imaging studies (Pratt et al, 2014). A compromise is to use readily available off-cut crystals with asymmetry angles close to ideal, and to allow some variance in the Bragg angle Both geometric and single-ray focal lengths are a function Figure 2 of the Bragg angle B, the asymmetry angle and the crystal Upper-sign and lower-sign geometries. It provides only the magnitude of the change in Bragg angle, not the sign This is important because a pencil beam will create either a virtual or real focus depending on the upper- or lower-sign geometry as well as the orientation of the crystal concavity relative to the source. Using the small-angle approximation, l2=fp 1⁄4 2Ár0ot, and solving for the polychromatic focus, we have fp 1⁄4 This agrees with a previous result (Sutter et al, 2008) that was derived for a bent Laue crystal in the lower-sign orientation.
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