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

Top layer hgs with two excitation energies at 27 kev

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top-layer (HgS) with two excitation energies, at 27 keV ( solid line ) and at 17.4 keV ( dashed line ), respectively
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472 B. Kanngießer and M. Haschke than the ones for the Hg L α line because of the mass attenuation coefficient for the S K α line, which is twice as high in the case of the cinnabar layer. Also the absolute countrates are two orders of magnitude smaller for the S K α line in comparison to the Hg L α line, even though the elements have almost the same density. The main reason lies in the photoelectric cross section for the S K-shell, which is already two orders of magnitude smaller than the one for the Hg LIII-shell for the same excitation energy. Additionally, the fluores- cence yield of the S K-shell is one order of magnitude smaller than the one for the Hg LIII-shell. On the other hand, the S K α line is enhanced by more fluorescence lines than the Hg L α line. In the case of the S K α line all L-Lines from the Hg as well as from the Pb of the bottom layer produce secondary enhancement. In the case of the Hg L α line only the Pb fluorescence lines from the LII- and LI-series (except the L β 2 line) have energies which can excite the Hg L α line. Thus, secondary enhancement is included in the calculations, as can be seen in Fig. 7.23. Pb is the only detectable fluorescence element in air from the bottom- layer (2PbCO 3 · Pb(OH) 2 ). Both curves of the respective excitation energy show the characteristic increasing attenuation with increasing thickness of the top-layer until the attenuation is so strong that the Pb fluorescence line is not detectable anymore. The crossing of both curves at a top-layer thickness of about 8 µ m shows the trade-off between the higher penetration depth of the 27 keV excitation radiation and the higher photoelectric cross section for the 17.4 keV excitation radiation. At top-layer thickness below 8 µ m the higher photoelectric cross section surpasses the higher penetration depth and vice versa for layer thickness over 8 µ m. This example gives the range of depth profiling for a heavy matrix. Infor- mation from the bottom layer can be gained only if the top layer thickness is in the order of several ten micrometers. In the case of a light matrix, for example, biological samples, the information depth can reach the mm range. In conclusion it can be said that with the new method elemental distribu- tions in 3D objects can now be measured in a non-destructive manner with a 3D resolution. At synchrotrons a resolution of a few micrometers is currently achievable by carrying out depth profiling on layered structures. Herewith major and minor elements can be distinguished in different layers, even if the same element is present in successive layers. The latter is the most severe restriction on conventional 2D micro-XRF. The same holds for the tabletop set-up if the sensitivity of the arrangement is taken one to two orders of mag- nitude lower. Further on, with a first evaluation and quantification approach
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