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

With a combination of eg si k α si l α or si k α w

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With a combination of, e.g., Si K α /Si L α or Si K α /W N α , it is possible to determine both the thickness and the composition of the top layer. From the Si K α intensity the thickness can be determined. After that, the Si con- centration can be computed from the Si L α or W N α intensity. While this is described here as two sequential steps, the discussed back-calculation schemes for FPs allow this to be done simultaneously.
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5 Quantitative Analysis 379 It is also possible to use all four lines simultaneously. This has the advan- tage that the accuracy of the analysis can be estimated, because there are four degrees of freedom but only two unknown parameters. 5.6 Complex Excitation Effects and Light Elements N. Kawahara 5.6.1 Indirect Excitation Processes in the Low Energy Region Led by the advances of the techniques in modern instruments such as syn- thetic multilayer monochromators or thin detector windows, XRF applications spread out to lower energy regions and lighter element analysis. There are par- ticularly an increasing number of analytical applications in the field of thin and layered samples which require measurements of L or M (and even N-) lines at low energies. This section discusses some unusual indirect excitation processes in the regime of low energies, which are mostly negligible at higher energies where direct excitation is efficient and the dominating factor. At lower energies, however, excitation by conventional X-ray tubes is inefficient because the low energy limit of the available tube photons is often much higher than the binding energies of the shells of interest, and indirect excitation effects gain (relative) importance. They may even by far outweigh direct excitation. 5.6.2 Secondary Excitation by Electrons When a primary X-ray photon excites an atom, a photo-electron and an Auger-electron are generated as well as a fluorescent photon. For light ele- ments such as C, B, and Be, the wide gap between their absorption edge energies and the low energy limit of the spectral distribution of tube pho- tons results in a very low fluorescent photon emission probability and the emission of photo-electrons with relatively high energy. Considering the fact that a single electron can originate a long sequence of (inner shell) ionizations while photoelectric excitation is basically a single-event process, the excitation by photo-electrons can cause a rather large contribution compared to direct excitation. When light element analytes are contained in a heavy element ma- trix, photo-electrons may be abundantly emitted by the matrix elements as well and often Auger-electrons with relatively high energies cause further enhancement. Mathematical expressions for the characteristic X-ray excitation by elec- trons have been reported mainly in the field of electron probe microanalysis (EPMA). The number of photons, n i ( E 0 ), emitted from the i -shell of ana- lyte atoms by interaction with a single electron of initial kinetic energy E 0 is expressed in the following equation, which was originally reported by Green and Cosslett [22].
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