New materials for high-energy-resolution x-ray optics
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roduction X-ray techniques such as resonant inelastic-x-ray scattering (RIXS)1,2 and nuclear resonant scattering (NRS)3 are powerful tools for examining the subtle aspects of material structure and dynamics. These techniques find applications in many fields of science ranging from fundamental physics to biophysics, chemistry, materials science, and other interdisciplinary fields, including geophysics and environmental sciences. For these versatile and increasingly popular x-ray techniques, the clarity of the results can be enhanced by improving the energy resolution, which is determined by the x-ray optics. In the hard x-ray region (photon energies >∼5 keV), most high-energy-resolution optics use Bragg reflections in perfect crystals. For these energies, silicon has been the crystal of choice as it is readily available in large perfect crystals, has good mechanical and thermal properties, and its processing technology is well known. However, crystals other than silicon have the potential to provide high-energy resolutions (∼25 keV illustrated in Figure 1a, where the Curved Analyzer >10 × 10 >∼10,000 Figured RIXS minimum possible angular deviation ∼10 × 10 ∼100 Flat RIXS from exact backscattering (θB = 90°) is PSC Analyzer plotted as a function of x-ray energy for Area used is an indication for the size requirements of the perfect portion on the crystal material. silicon (cubic) and quartz (trigonal). As Note: PSC, post-sample collimation geometry; NRS, nuclear resonant scattering; RIXS, resonant can be seen, for high-symmetry silicon, inelastic x-ray scattering. • VOLUME • www.mrs.org/bulletin MRS 42 •ofJUNE Downloaded from https:/www.cambridge.org/core. Columbia University Libraries, on 12 Jun 2017 at 00:53:25, subject to theBULLETIN Cambridge Core terms use,2017 available at https:/www.cambridge.org/core/terms. https://doi.org/10.1557/mrs.2017.94
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NEW MATERIALS FOR HIGH-ENERGY-RESOLUTION X-RAY OPTICS
Figure 2. Schematics of different experimental geometries as indicated. Often, the detector will be an array detector, with, for example, the application of dispersion compensation10 to improve resolution. Also, in many operating spectrometers, the conventional resonant inelastic x-ray scattering (RIXS) setup has an array of several tens of analyzer crystals. Note: PSC, post-sample collimation geometry; NRS, nuclear resonant scattering.15
beam from the sample and use a flat crystal analyzer in a post-sample collimation (PSC) geometry; see discussion in Reference 14. Figured analyzers require relatively large crystals to be made5,15,16 (segmented, bent, or bonded) in order to achieve an appropriate curvature. These analyzers are also easily tiled, allowing large solid angles to be collected with potentially large arrays. On the other hand, the PSC scheme requires a collimating optic near the sample17–19 that, at present, usually cannot accept a large solid angle, and may be expensive to generate. However, only a relatively small analyzer crystal without any figuring is needed, which simplifies the analyzer fabrication
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