OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 3 2024 In the Johansson scheme, the radius of the focal circle and the radius of curvature of the crystal coincide. In the Johann scheme, the radius of the focal circle is smaller. Crystals installed in focusing monochromators can be deformed in various ways. A distinction is made between sagittal [44] and meridional [45] bending (Fig. 21). a b Fig. 21. Curved optical elements. Sagittal bending of a crystal with radius Rs (a) and meridional bending with radius Rm (b) Depending on the type of crystal bending, the focal length of the monochromator is determined according to the formulas: 2 sin 2 sin ; s pq R f p q = θ ⋅ = θ + (9) 2 2 , sin sin m pq f R p q = ⋅ = θ + θ (10) where p is the distance from the source to the crystal; q is the distance from the crystal to the sample; f is the focal length. To adjust the focal length of the monochromator, it is necessary to provide a change in the radius of curvature of the crystal. Monochromators are equipped with mechanical devices that make it possible to adjust the necessary curvature of the crystal. Fig. 22 shows the device of the monochromator described in [46]. The device is equipped with two crystal mounting nodes using a scheme that provides a fixed beam output. This is realized by moving the Fig. 22. Curved optical elements of the fixed beam offset monochromator
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