A bent Laue–Laue monochromator for a synchrotron-based computed tomography system

We designed and tested a two-crystal bent Laue–Laue monochromator for wide, fan-shaped synchrotron X-ray beams for the program multiple energy computed tomography (MECT) at the National Synchrotron Light Source (NSLS). MECT employs monochromatic X-ray beams from the NSLSʹs X17B superconducting wiggler beamline for computed tomography (CT) with an improved image quality. MECT uses a fixed horizontal fan-shaped beam with the subjectʹs apparatus rotating around a vertical axis. The new monochromator uses two Czochralski-grown Si〈1 1 1〉 crystals, 0.7 and 1.4 mm thick, respectively, and with thick ribs on their upper and lower ends. The crystals are bent cylindrically, with the axis of the cylinder parallel to the fan beam, using 4-rod benders with two fixed rods and two movable ones. The bent-crystal feature of the monochromator resolved the difficulties we had had with the flat Laue–Laue design previously used in MECT, which included (a) inadequate beam intensity, (b) excessive fluctuations in beam intensity, and (c) instability of the shape of the beamʹs horizontal profile. Compared with that earlier monochromator, the bent Laue–Laue device tested at 42 and 108 keV showed about a 10-fold larger beam flux, about 5 times better beam stability, 10-fold less harmonic contamination, and a smaller energy bandwidth at certain bending radii. The present work gave us better understanding of the basis for the beam-smiling effect in bent-crystal monochromators, and allowed us to refine the theoretical method of estimating the beam-harmonic contamination in bent-crystal monochromators.