Experimental verification of dynamical diffraction focusing by a bent crystal wedge in Laue geometry

Abstract

Recent X‐ray optical experiments have validated previous theoretical work on focusing by bent crystals in Laue geometry. It was shown that a suitable choice of the crystal thickness significantly improves the focusing performance of the crystal by combining the dynamical focusing of the Borrmann triangle with the geometrical focusing obtained by the crystal bending. The best focus size achieved during the experiments was less than 5 µm and is in practice limited by the source demagnification for the considered setup.

Bent perfect crystals as X-ray focusing polychromators in symmetric Laue geometry

“…Mocella et al (2004Mocella et al ( , 2008 The polychromatic focusing condition (1) can be expressed as R cos B ¼ 2pq=ðp À qÞ, which implies p e ¼ q e ¼ 2pq=ðp þ qÞ. Mocella et al (2004Mocella et al ( , 2008 The polychromatic focusing condition (1) can be expressed as R cos B ¼ 2pq=ðp À qÞ, which implies p e ¼ q e ¼ 2pq=ðp þ qÞ.…”

“…In the approach of Mocella et al (2004Mocella et al ( , 2008, the illumination on the crystal entrance surface is considered to be incoherent, as a consequence of polychromaticity. There would therefore be no influence of the source-to-crystal distance on the dynamical focusing behaviour.…”

“…In the design of an energy-dispersive X-ray absorption spectroscopy beamline, one uses a positionsensitive detector downstream of a sample placed in the focal position. According to Mocella et al (2004Mocella et al ( , 2008, the focal size should be minimized when the dynamical and polychromatic foci are made to coincide by the choice of the crystal thickness (see also Sanchez del Rio et al, 1994). The optical performance of such a crystal (polychromator), in particular the size of the focal spot, depends on the crystal thickness via the 'dynamical focusing' effect, which was described decades ago by Afanasev & Kohn (1977) and Aristov et al (1978Aristov et al ( , 1980 in the flat crystal case, and by Kushnir & Suvorov (1982) in the bent crystal case; see also the book by Pinsker (1978).…”

Dynamical focusing by bent, asymmetrically cut perfect crystals in Laue geometry

“…It is found that the focusing performance can indeed be strongly improved by weak asymmetry, as compared to the case of symmetric geometry. We intend to continue the present analysis by considering the important case of a polychromatic source instead of the monochromatic source considered in the present work; for this purpose, it will be necessary to combine the present analysis with the condition for polychromatic focusing which is cos 2 =q À cos 1 =q ¼ 2=R (Chukhovskii & Krisch, 1992;Mocella et al, 2004Mocella et al, , 2008Guigay et al, 2013) and to discuss the influence of the dependence on the X-ray energy of the lateral displacement x c pointed out in the present investigation. In the case of a symmetric reflection, according to equation (34) with s ¼ 0, x c ' Àðt sin B Þ=2, if q and R cos B are much smaller than p; the lateral displacement is consequently approximately proportional to the X-ray energy.…”

Spectrometer for hard X-ray free-electron laser based on diffraction focusing

“…It was experimentally confirmed by Aristov et al (1978Aristov et al ( , 1980Aristov et al ( , 1982, and by Koz'mik & Mikhailyuk (1978). Recently the effect of focusing in bent-crystal systems was considered theoretically by Mocella et al (2004Mocella et al ( , 2008, and by Nesterets & Wilkins (2008) as a way for improving polychromatic focusing.…”