For modern X-ray analytics in the lab as well as at synchrotron sources, multilayer X-ray optics are used for X-ray beam conditioning. The optics should deflect, monochromatize, focus or collimate the X-ray beam. For X-rays the real part of the refractive index n is nearly 1, so that X-rays cannot be shaped by lenses. The refractive index n = 1-d+ib consists of the dispersion d and the absorption b. A possibility of controlled X-rays beam shaping exists by using Bragg's diffraction.According to Bragg's condition, radiation of a certain wavelength k which illuminates a periodic structure with lattice constant d is deflected under the angle h. Different orders m are deflected in different directions. The perfection of the lattice as well as the lattice materials determines the reflectivity R.Optimized periodic structures of high reflectivity with adjustable d-values can be obtained by multilayer deposition. These multilayers can consist of up to several hundred layer pairs of two materials with single layers of a few nanometers thickness. The first order (m=1) can show a very high reflectivity of up to 95 %, and the peak widths and energy bandwidths can be 1-2 orders of magnitude larger than for single crystals. The multilayers consist of a heavy reflector and a light spacer, usually with a high dispersion contrast at the interface. For a good X-ray multilayer, the absorption should be low to achieve high reflectivity and reasonably sharp Bragg reflections.Using multilayers with a lateral layer thickness profile that are deposited onto curved substrates, optical elements can created that deflect and shape X-rays. Such so-called Göbel Mirrors consist, for example, of a parabolically curved substrate, and a multilayer coating with d-values typically in the range 3-5 nm. Due to the parabolically curved shape in conjunction with the d-value gradient, the divergent beams emitted from an X-ray source are transformed into a parallel beam bundle using Bragg diffraction (see Fig. 1).
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