[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

Simultaneously to focus the beam and to enhance

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simultaneously to focus the beam and to enhance spectral flux at the sam- ple by several orders of magnitude. All these optical elements are united by the basic principle of Bragg, Bragg–Laue, or Bragg–Fresnel diffraction on
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88 A. Erko artificially made volume structure and differ from the other elements by the unique combination of useful properties. The first successful demonstrations of the Bragg–Fresnel principle were re- ported in 1985–1986 using multilayer [20, 22] and crystalline [21] substrates. The general principles of Bragg–Fresnel diffraction were first formulated in the work of Aristov [24]. The use of Fresnel focusing in combination with total external reflection was demonstrated in 1994 [25]. Since that time Bragg– Fresnel lenses have been used at several synchrotron radiation facilities to construct microprobes [26, 27] imaging beam monitors [28] and time-resolved systems [29]. The theory of Bragg–Fresnel optics has already been published in several works [7, 30]. Unfortunately, the main advantage of the Bragg–Fresnel optics, namely the combination of a monochromator and a focusing element in the same device, restricts the field of its potential applications. The neces- sity to design an optical element for a particular geometry and a fixed energy conflicts with the desired flexibility of the experimental arrangement and lim- its the number of possible experimental methods mainly to microfluorescence analysis and microdiffraction. The situation is additionally aggravated by the complexity of the Bragg–Fresnel optics technology and the very high cost of its production. Modern technology for Bragg–Fresnel and Fresnel reflection optics is based on evaporating or sputtering metals onto surfaces of crystals and multilay- ers [31]. The fabrication and successful tests of a synthesized X-ray hologram made with Ni phase-shift layer on a surface of Si [111] crystal was reported [32]. A linear Bragg–Fresnel lens placed onto the second crystal of a double-crystal monochromator was tested and will serve as a basic sagittal focusing ele- ment for the small angle scattering facility at the BESSY microfocus beamline project [33]. Capillary X-ray optics has been successfully applied with conventional X-ray sources. Straight glass monocapillaries can effectively transport X-radiation from the source to the sample thus increasing radiation inten- sity on the sample [34]. Special polycapillary arrays with curved channels can be used for transforming divergent radiation from a point source into a qua- siparallel beam or for focusing a divergent beam onto a small spot [35, 36]. Straight polycapillary arrays have been used for X-ray imaging and for beam splitting and filtering [37]. Capillary optics in synchrotron beamlines is mainly applied for focusing radiation onto a spot of µ m and sub- µ m size by means of tapered monocapillaries. The first experimental results on the beam focus- ing and filtering properties of mono- and polycapillaries using synchrotron radiation were obtained at BESSY (Berlin) [38] and LURE (Orsay).
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