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What is an X-ray Crystal Optical Element?


It is an X-ray optical element that utilizes Bragg diffraction by crystal lattice planes and is used to monochromatize X-rays or to reflect single-wavelength X-rays in a specific direction.


Channel-cut Crystal Monochromator


As shown in the figure, an x-ray crystal optical element that uses the two inner walls of a groove formed in a single crystal block as reflective surfaces. By selecting the angle of incidence, X-rays of a specific wavelength can be cut out and emitted in the same direction as the incident direction. It is simpler, more compact, and more stable than using two crystals.


How to make a channel-cut crystal


Generally, a groove is formed on a single crystal block by grinding, residual strain is removed by wet etching, and the surfaces are finished by polishing. An issue is a residual strain caused by the polishing process because it is difficult to apply the high-precision polishing method shown in the figure to such inner surface of the groove.


Technology to remove residual strain


We use plasma etching using atmospheric pressure plasma generated around a wire electrode, which was developed at Osaka University. Residual strain can be removed by chemically removing minute amounts of the crystal surface layer.


Residual strain removal example


When there is residual strain due to polishing, scratch-like dark lines appear in the X-ray reflectance image even though the surface looks mirror-like, and the X-ray reflectance ratio is also reduced. Plasma etching can remove the residual strain layer, resulting in a uniform X-ray reflectance image with a reflectance ratio almost in line with theoretical values.


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