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- Crystal Polishing laboratory
The Crystal Polishing laboratory provides advice, support, research and development in the field of crystal X-ray optics.
It also manufactures and characterises single-crystal X-ray optics for ESRF beamlines and external customers.
The Crystal Polishing laboratory manufactures perfect crystal monochromators for all ESRF beamlines. Most crystals are made from 100mm diameter, high-purity silicon float-zone ingots of up to 1m long that are manufactured industrially. For the production of monochromators these ingots must be transformed into objects of various size and shape depending on the final application.
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Monochromators come in a great variety, ranging from simple rectangular plates to complex Bragg-Laue crystals and nested channel-cut devices for high spectral and angular resolution. |
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Preparing the crystals: the raw ingots are oriented on an X-ray single crystal diffractometer. |
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Circular diamond saw |
Subsequent processing consists of fine grinding and lapping of the surfaces exposed to the X-ray beam, either using grinding wheels or by hand (e.g. inside channel-cut crystals), followed by chemical etching to remove the surface damage (fig. 4). According to specifications, the surface can be polished. This is necessary for applications that require minimal perturbation of the X-ray wavefront, e.g. imaging techniques where surface structure must be avoided. Chemo-mechanical polishing (CMP) is generally the final step following polishing of the etched surface using very fine diamond powder. The best surface finish presently obtained is about 1Å (rms) while the best figure (flatness) for a thick specimen is around 1 µrad (rms).
About one ton of silicon single crystal material has been processed and many different types of flat or bent, also directly cooled objects have been produced. X-ray crystal optics, like beamline concepts, constantly evolve following the general evolution of instrumentation for synchrotron X-rays to meet new scientific goals.
Critical issues are precise, strain-free mounting and bonding of the crystals on supports for cooling and bending. Here the traditional art and skill of handcrafting meet with the modern tools of mechanical and thermal engineering such as finite element analysis. A critical ingredient for the production of high performance crystal optics is the detailed understanding of the diffraction processes in perfect, curved or otherwise deformed crystals that allow us to predict their effect on X-ray beam propagation and on the final performance of optical schemes.