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

On monochromatization and high requirements on the

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on monochromatization and high requirements on the intensity of the reflected radiation mosaic crystals reveal themselves as the most suitable candidate. Among the known mosaic crystals, highly oriented pyrolytic graphite (HOPG) crystals, both bent and flat ones, belong to the category most widely used. Monochromators in XRF X-ray fluorescence spectroscopy is based on the measurements of fluorescent radiation emitted by the element, which is excited by X-rays having a pho- ton energy higher than the corresponding absorption edge of the respective element. Although qualitative interpretation of the spectra obtained is rather simple, the reliable quantitative analysis of the elemental concentrations is often a nonlinear optimization problem due to various mutual absorption and secondary enhancement effects modifying the fluorescence intensities. In addition, XRF detection limits (DL) are affected by the shape of spectral background partially induced by scattered excitation radiation and its inten- sity depending on the major matrix constituents of the sample. Monochromatic excitation can drastically decrease the spectral back- ground below fluorescence lines associated with photon energies consider- ably lower than that of the excitation radiation. Furthermore, a fundamental parameter based XRF quantification is, in general, more reliable in the case of a monochromatic than polychromatic excitation radiation. This is due to the fact that the shape of the spectral distribution of the excitation radiation is often not known well enough when X-ray tubes, in particular in conjunction with nonmonochromatizing optics, are employed. For a given count rate capa- bility of the energy-dispersive detection system, the sensitivity of XRF can be improved in the case of a monochromatic excitation if enough primary inten- sity is available to achieve the highest count rate allowed by the given detector. In energy-dispersive X-ray fluorescence analysis (EDXRF), the conven- tional techniques aimed at either a quasimonochromatic excitation or a reduction of scattered excitation radiation involve the use of secondary tar- gets, selective filtration of the source primary radiation or the production of polarized primary radiation by means of Barkla scattering. However, all these approaches substantially decrease the intensity of primary X-ray tube radia- tion by several orders of magnitude, thus considerably reducing the sensitivity of EDXRF analysis. An alternate way to modify the spectrum of the primary radiation is offered by diffraction-based X-ray optics that selectively reflects a narrow band of the beam spectrum. Especially important for XRF is the fact that
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X-Ray Optics 145 Bragg-reflection monochromators are meanwhile available as doubly curved crystals with an application-adapted geometry and permit to compensate the reflection-caused losses of the useful radiation by collecting the primary radi- ation within a large solid angle of acceptance. Besides, such monochromators
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