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

The measured characteristics of the microlens are

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The measured characteristics of the “microlens” are shown in Table 3.3. Due to the smaller focal distance F 2 , the spot size on a sample is also smaller. The effective distance D eff is ca. 2.7 mm for the energy range 16.7–23.2 keV. Table 3.2. Measured characteristics of the “minilens” E/keV 3–5 5–7.5 7.5–10 10–15 15–20 20–25 25–30 Spot size/ µ m 38 40 41 41 35 31 33 Intensity gain 2,892 7,078 7,929 8,070 8,502 5,631 1,550
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X-Ray Optics 105 0.001 0.01 0.1 1 10 100 1000 10000 0 10 20 30 40 50 E (keV) Intensity (cts) a b MoK FeK Fig. 3.8. Spectra obtained by scattering on PMMA. (a) direct beam; (b) beam at the exit of the optics. Measurement condition: 50 kV, 50 µ A, Si(Li) detector Table 3.3. Measured characteristics of the “microlens” E keV 2.5–6.5 6.5–10.2 10.2–16.7 16.7–23.2 23.2–30.0 Spot size µ m 25 25 22 20 20 Intensity gain 1,380 1,460 1,590 1,160 345 Figure 3.9 shows the energy dependence of the focal spot size measured and calculated according to (3.4), respectively. As discussed above, the focal spot size is expected to increase when radiation energy decreases. However, the measured values depend weakly on the energy and they decrease even below 4 keV. Only in the region of 25 keV does the measured value approach that of the calculated one. The result can be explained by the fact that X-rays with relatively large incidence angles become quickly extinct while propagating through the channels due to the stronger absorption. This effect leads to a certain “self-collimation” of the beam and is especially significant in the low- energy region, where absorption becomes stronger. In spite of large values of the critical angle at low energies, only photons with small incidence angles can transmit through the lens channels. Some increase of the focal spot size in the energy interval 25–30 keV as compared to 20–25 keV in Fig. 3.9 is obviously connected with the edge effects at the pinhole. The effective size of the pinhole increases due to smaller absorp- tion of high-energy photons in the pinhole material. To test this assumption, a spot scan with a 10- µ m wire containing 94% Cu and 6% Sn was carried out (see Fig. 3.10). This experiment was important to show if the so-called “halo effect” may be responsible for the increase of the focal spot at high energies. In many
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106 V. Arkadiev and A. Bjeoumikhov 0 50 100 150 200 250 300 0 5 10 15 20 25 30 E (keV) Spot size (mm) measured calculated Fig. 3.9. Calculated and measured energy dependences of the focal spot size for the “minilens” 0 2000 4000 6000 8000 10000 12000 14000 - 0.06 - 0.04 - 0.02 0 0.02 0.04 0.06 X (mm) Intensity (cts/10s) CuKa SnKa Fig. 3.10. Spatial distributions of Cu K α and Sn K α -yields while scanning the lens focal spot with a 10- µ m wire (Ua = 50 kV) publications the existence of such a transmission halo around the main sharp focus of polycapillary lenses is mentioned. This effect occurs in the high-energy region and deteriorates the spatial resolution of the analysis of heavy elements [80]. The measured Sn K α -distribution in Fig. 3.10 does not give any evidence of the “halo effect” for the lens under consideration: the focal spot does not become blurred for radiation energies above the Sn K-edge of absorption ( > 29 . 2 keV). This result confirms our assumption that in our pinhole scan
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