High-efficiency coherence-preserving harmonic rejection with crystal optics

Abstract

This work reports a harmonic-rejection scheme based on the combination of Si(111) monochromator and Si(220) harmonic-rejection crystal optics. This approach is of importance to a wide range of X-ray applications in all three major branches of modern X-ray science (scattering, spectroscopy, imaging) based at major facilities, and especially relevant to the capabilities offered by the new diffraction-limited storage rings. It was demonstrated both theoretically and experimentally that, when used with a synchrotron undulator source over a broad range of X-ray energies of interest, the harmonic-rejection crystals transmit the incident harmonic X-rays on the order of 10−6. Considering the flux ratio of fundamental and harmonic X-rays in the incident beam, this scheme achieves a total flux ratio of harmonic radiation to fundamental radiation on the order of 10−10. The spatial coherence of the undulator beam is preserved in the transmitted fundamental radiation while the harmonic radiation is suppressed, making this scheme suitable not only for current third-generation synchrotron sources but also for the new diffraction-limited storage rings where coherence preservation is an even higher priority. Compared with conventional harmonic-rejection mirrors, where coherence is poorly preserved and harmonic rejection is less effective, this scheme has the added advantage of lower cost and footprint. This approach has been successfully utilized at the ultra-small-angle X-ray scattering instrument at the Advanced Photon Source for scattering, imaging and coherent X-ray photon correlation spectroscopy experiments. With minor modification, the harmonic rejection can be improved by a further five orders of magnitude, enabling even more performance capabilities.

Selected advances in small-angle scattering and applications they serve in manufacturing, energy and climate change

“…The much greater stability afforded by vertical crystal rotation axes enables use of higherorder crystal reflections with narrower rocking curves and hence lower minimum attainable q values. Note that, owing to dynamical scattering considerations, use of Si 220 or 440 (or many other hkl) X-ray optics while employing an Si(111) upstream monochromator results in the effective elimination of X-ray harmonics without the need for harmonic rejection mirrors (Zhang et al, 2018a). The new USAXS instrument stages operate significantly faster than the previous instrumentation, including the core USAXS flyscan operation.…”

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