OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 3 2024 monochromator crystals (mirrors), designs with fixed and non-fixed beam offset can be realized. Fig. 16 shows the positions of crystals corresponding to different grazing angles θ of incoming rays [36]. It follows from the figure that the beams exiting the monochromator are at the same height, i.e., its displacement is the same (h1 = h2 = h3). The beam offset depends on the way of attaching the optical elements. It is a question of whether the elements are rigidly connected or fixed independently of each other. A fixed beam offset is achieved by adjusting the gap g between the crystals. The constant beam displacement h is determined by the following relationship: 2 cos , h g = ⋅ θ (8) where g is the distance between the optical elements of the monochromator; θ is the beam grazing angle. For angles corresponding to the range 0 < θ < 45°, the second optical element should be extended (Fig. 16 a). If the grazing angles are larger than 45°, there is no need to extend the optical element (Fig. 16 b, c) [37]. Fig. 17 shows the case when the optical elements are rigidly connected to each other and, therefore, the gap g between them is the same [38]. When this scheme is realized, a change in the beam grazing angle (h1 ≠ h2 ≠ h3) also leads to a change in the displacement value h (h1 ≠ h2 ≠ h3). This is the case of non-fixed beam offset. In [37] a variant of the monochromator corresponding to such an arrangement of elements is described. a b c Fig. 17. Different magnitude of ray displacement (h1 ≠ h2 ≠ h3) when changing the angle of incidence θ (monochromator with non-fixed ray output) While developing a monochromator the choice of the crystal rotation axis is one of the main problems [39]. Three options of its location are possible (Fig. 18). According to one of it, the axis of system rotation of two crystals is located at the first optical element (at the point O1). It is also possible to locate the axis of rotation at the middle of the beam between the optical elements (at the point O2). According to the third option, the axis is located at the point where the beam falls on the second crystal of the monochromator (at point O3). The rotation point plays an important role in the geometry of the beam path in the monochromator. Fig. 18. Three possible arrangements of rotation axes of optical elements of monochromators
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