OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 3 2024 Focusing monochromators When conducting research using synchrotron radiation, there are problems that require either a focused or a divergent beam. One of the approaches allowing to form focused beams is based on using bent crystals. The basic configurations of monochromators with bent crystals are described below. Fig. 10 b shows the Laue scheme based on the passage of a “white” beam of synchrotron radiation through a monochromator crystal. According to it, the polychromatic SR beam enters the crystal, where it experiences reflection at an angle to the atomic planes satisfying the Wolf – Bragg condition. Monochromator crystals realizing diffraction according to the Laue scheme have been widely applied in solving problems related to the necessity of focusing synchrotron radiation [35]. The background was the development of the Yvette Cauchois spectrograph in the early 1930s, which showed that curved crystals allowed focusing X-ray radiation [40]. The course of the rays corresponding to this work (Fig. 19 a) can be considered as a modified Laue scheme, which is called the Cauchois – Johann scheme [40]. Fig. 19 b shows the scheme which is called the Cauchois – Johansson scheme [40]. The continuations of the atomic planes of the curved crystal converge at point N, and the continuations of the incident rays converge at point F’. In the case of radiation falling from the convex side, as shown in Fig. 19 a, b X-ray radiation is focused on spot F. The points F’, N, as well as the set of points lying in the spot F, are located on the same circle with radius R, called the focal one. The scheme shown in Fig. 19 c is called the Johann scheme [41]. Fig. 19 d shows the Johanson scheme [42]. For cases in Fig. 19 c, d, the radiation source I and the focus spot should be located on the focal circle. a b c d Fig. 19. Schemes of ray path in focusing monochromators with curved crystals: a – Cauchois – Johann scheme (modified Laue scheme); b – Cauchois – Johansson scheme; c – Johann scheme (atomic planes and focal circle have different radii of curvature); d – Johansson scheme (atomic planes and focal circle have the same radii) By directing the radiation from the inside of the crystal, the beam can be broadened as shown in Fig. 20 [43]. Fig. 20. Laue monochromator with bent crystals expanding the beam In addition to the Cauchois scheme, which assumes that the radiation falls on the outer side of the crystal, the focusing effect can also be achieved using the Johann (Fig. 19 c) and Johansson (Fig. 19 d) schemes, in which the radiation falls on the crystal from its inner side. Both schemes are based on Bragg diffraction of radiation. The difference between them is related to the radius of curvature of the crystals.
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