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

On the other hand the exit focal distance f 2 has to

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spectrum. On the other hand, the exit focal distance F 2 has to be selected for the more important part of the spectrum. There are numerous experimental results of the test measurements of lenses (see e.g., [79]). As an example, we present here the characteristics of two dif- ferent monolithic lenses. The main geometrical parameters of both lenses are collected in Table 3.1, while the meaning of these parameters is displayed explicitly in Fig. 3.7. Two values are specified for the lens diameters: the external diameter and the diameter of the internal polycapillary core. The terms “minilens” and “microlens” are stipulated to denote the difference in Table 3.1. Main geometrical parameters of two lenses “minilens” “microlens” L mm 101.5 36.8 F 1 mm 47.5 39.8 F 2 mm 15.3 16.8 D in (structure/shell) mm 4.6/5.3 1.7/3.2 D max (structure/shell) mm 6.7/7.5 2.3/4.2 D out (structure/shell) mm 2.5/2.7 1.5/2.8 Φ rad 6.7/7.5 0.043 R = F 1 + L + F 2 mm 164.3 93.4
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104 V. Arkadiev and A. Bjeoumikhov Source Focus Lens D in D max D out F 1 F 2 L F Fig. 3.7. Main geometrical parameters of an X-ray lens the length and focal distances which leads in its turn to a different spot size on a sample. “Microlenses” are more compact and have smaller focal distances (less than 10 mm) which allows to obtain spot sizes below 20 µ m. Typical spot sizes for “minilenses” lie in the range of 20–100 µ m. Both lenses under consideration are asymmetric, the entrance focal distance being three times larger than the exit one. Therefore, a focal spot size on a sample is expected to be smaller than the source size. Table 3.2 contains the measured physical parameters of the “minilens”: the focal spot size and the gain factor. A microfocus X-ray tube with a molybdenum anode (spot size 50 µ m) was used as source and a SDD X-Flash detector (manufacturer Bruker AXS Mi- croanalysis GmbH, Germany) was used for detection of X-rays. The intensity distribution in the focal spot and the spot size were measured by scanning with a 5- µ m pinhole at different energies. The intensity gain factor was calculated as the ratio between the intensities on a sample with and without the lens. The values of the intensity gain are very large and correspond to the effective distance D eff of less than 1.8 mm for the energy range 15–20 keV. Such intensities are hardly achievable without optics, because it is practically impossible to realize the anode–sample distance of 1.8 mm. Figure 3.8 shows the exit spectrum of the “minilens” measured with a scat- tering target made of PMMA (plexiglas). The spectrum of the direct beam through a 50- µ m pinhole at the same distance from the X-ray source is pre- sented for comparison. Both spectra give a good idea of the intensity gain achieved. Besides, they demonstrate clearly how the lens modifies the pri- mary spectrum. In this case a Si(Li) detector with an active area of 10 mm 2 was used for the measurement.
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