A bent Laue-Laue monochromator for a synchrotron-based monochromatic x-ray computed tomography system

A bent Laue-Laue monochromator for wide, fan-shaped synchrotron x-ray beams was designed and constructed for the program Multiple Energy Computed Tomography (MECT) at NSLS. MECT uses monochromatic x-ray beams from the NSLS's X17B superconducting wiggler for computed tomography (CT) with improved image quality. The new monochromator uses Czochralski-grown Si{dollar}langle111rangle{dollar} crystals, which are 0.7 and 1.4 mm thick and are cut with thick ribs on their upper and lower ends. The crystals are bent cylindrically, using 4-rod benders employing two fixed and two movable rods. The bent-crystal feature of the monochromator was needed to solve the difficulties we had with the flat Laue-Laue design previously used in MECT, which include: (a) inadequate beam intensity, (b) excessive beam intensity fluctuations, and (c) instability of the beam's horizontal profile shape. Compared with flat Laue-Laue monochromator, the bent Laue-Laue device tested at 42 keV and 108 keV showed about 10 fold larger beam flux, about 5 times better beam stability, 10 fold less beam harmonic contamination at the bending radius of 15 m, and a smaller energy bandwidth at certain bending radii. The present work also allowed (a) better understanding of the basis for the beam-smiling effect in bent-crystal monochromators, (b) development of a new technique of utilizing the phase-space diagram method as a tool to understand, characterize, and evaluate double-bent crystal systems, and (c) a refined theoretical method of estimating the beam-harmonic contamination in bent-crystal monochromators.