For example the information depth for y in the glass

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of the excitation and on the concentration of the element investigated. For example, the information depth for Y in the glass standard investigated with the synchrotron based set-up ranges from 250 to 300 µ m. The margin for the information depth indicates the margin for the matrix of the glass standard. In general, the synchrotron based 3D micro-XRF set-up gains a 2–20 times higher information depth. In order to gain more insight into the depth dependence of the fluorescence intensity produced we have calculated absolute countrates for fluorescence line intensities of a layer system. The layer system chosen is one investigated with the 3D confocal set-up (see Fig. 7.19) and is composed of two pigment layers, a cinnabar (HgS) layer on top of a lead white (2PbCO 3 · Pb(OH) 2 ) layer. In two series of calculations the thickness of both layers is varied from 2 to 100 µ m, respectively, whereas the other layer thickness is kept constant at 10 µ m. The incidence flux is taken to be 2 × 10 8 photons/s which is about the flux delivered by the BAMline. The incident angle Φ is 25 and the emergence angle Ψ is 65 . Both beams are supposed to be parallel. The solid angle of acceptance of the poly CCC (0.2 sr) was taken into account for the restricted field of view of a detector. The countrates falling onto the detector were calculated for two different excitation energies, namely 17.4 and 27 keV. The fundamental
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Methodological Developments and Applications 471 parameters needed for the calculations are taken from the actual compilations of Elam [37], Henke [38], and McMaster [39]. Figure 7.23 shows the thickness variation of the top-layer, the Cinnabar layer. The thickness of the bottom-layer, the lead white layer, is kept constant at 10 µ m. The fluorescence line intensities of the top-layer elements Hg and S increase with increasing layer thickness until a thickness is reached for which the self- absorption becomes so strong that the additional thickness does not contribute to an additional fluorescence signal. For the Hg L α line this critical thickness is reached at about 25 µ m for the 17.4 keV excitation energy and at about 40 µ m for the 27 keV excitation energy. The difference in the critical thickness for the two excitation energies can be explained by the lower attenuation for the 27 keV excitation energy. For all thickness the absolute countrates for the 17.4 keV excitation energy are higher which is due to the higher photoelectric cross section for this energy in comparison to the one for 27 keV. A similar behaviour shows the S K α fluorescence line for both excitation energies. Here, the critical thickness is reached at about 15 µ m for the 17.4 keV excitation en- ergy, whereas the 27 keV excitation energy is associated by a critical thickness of about 30 µ m. The critical thickness is thinner for both excitation energies Hg L a (9.9keV) Hg L a (9.9keV) Pb L a (10.5keV) Pb L a (10.5keV) S K a (2.3keV) S K a (2.3keV) 10 2 10 3 10 4 10 5 10 6 5 0 10 Absolute count rates (s - 1) 15 20 25 30 35 40 Layer thickness ( m m) 45 50 55 60 Fig. 7.23. Calculated fluorescence line intensities for a thickness variation of the
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  • Spring '14
  • MichaelDudley

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