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

All these approximations are equivalent to the

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All these approximations are equivalent to the insertion of a transition layer to which the roughness is attributed. In practice such models are useful to evaluate the mean roughness of the layer and not a particular roughness at one layer of the stack. In some cases, the computations using the transition layer model can lead to an increase of the reflectance or transmittance value. It is completely in disagreement with the physical reality. In practice, both transmittance and reflectance values must be lower than those computed for a perfect multilayer because the light is scattered in all directions. 3.4.3 HOPG-based Optics A. Antonov, I. Grigorieva, B. Kanngießer, V. Arkadiev, B. Beckhoff The Concept of Monochromators Monochromators nowadays constitute an essential part of many modern set- ups for X-ray diffractometry and X-ray fluorescence analysis (XRF). They determine not only the energy and the bandwidth of the beam, but also to a large extent its spatial and angular characteristics. Monochromators can be used to modify both the primary and the secondary beams in XRF. In the first case, monochromators contribute to form a primary beam with the spectral parameters, which are optimal for a particular problem of material analy- sis or medical investigation. They eliminate undesirable parts of a continuous spectrum and disturbing characteristic lines, thus suppressing the background and increasing the sensitivity of the analytical method. In the second case, monochromators are applied for collecting and analysing the secondary (flu- orescence) radiation detected. They are mostly used as dispersive elements for identifying characteristic lines of the elements, which may be present in a sample. Crystal monochromators are based on the phenomenon of X-ray diffrac- tion. Spectral selective reflection decreases significantly the total intensity of the beam. First, large parts of a spectrum are drastically reduced. Usually these parts are considered to be useless or even disturbing for an applica- tion under consideration and constitute an undesirable background. Second, the intensity of the line or narrow energetic band to be monochromatized decreases because the reflection coefficient is always smaller than 100%. To compensate these losses of the useful radiation, one can utilize a focusing geometry, which enables obtaining large intensities of monochrochromatized beams within small focal spots.
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144 A. Antonov et al. Decisive demands for effective focusing are a high reflectivity and a large angular acceptance of the optics employed. Perfect crystals usually have large values of reflection coefficients but they reflect X-rays in a very nar- row spectral region and have small acceptance angles. High spectral resolution E/E 10 4 to 10 5 of these crystals leads to a relatively poor effectiveness for X-ray focusing. Besides, realizing a focusing geometry with strongly bent perfect crystals is a nontrivial task. For applications with moderate demands
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