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

Due to the anisotropic emission characteristics of

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Due to the anisotropic emission characteristics of the scattered radiation based on the classical dipole radiation it is advantageous to place a detector in such a position that only the isotropic emission of the fluorescence signal is detected. So the combination of TXRF with polarized radiation leads to a lower background. Moreover, the use of monochromatic primary radiation improves the background conditions because only photons of one energy can be scattered. An increase in sensitivity can be attained by using a tunable intense excitation source, enabling the exciting energy to be adjusted to just above the absorption edge of the element of interest. Synchrotron radiation with its outstanding properties offers new possi- bilities for improving the performance of TXRF. The intense beam with a
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Methodological Developments and Applications 523 Detector Plane of polarization E-vector Atomic dipoles from sample Anisotropic dipole emission characteristic Fig. 7.55. Schematic view of the effect of using polarized radiation continuous spectral distribution from photon energies in the infrared to high energy X-ray photons as well as the linear polarization in the orbit plane and its natural collimation are features best suited for excitation in total re- flection geometry. Figure 7.56 shows the spectral brightness of Beamline L at HASYLAB allowing excitation by photons between a few eV and 100 keV. For optimal excitation conditions the spectral distribution can be modi- fied by elements like cut-off mirrors, monochromators, and filters. Details can be found in [190]. Multilayer monochromators are best suited for the com- bination with synchrotron radiation excited XRF [191, 192]. In comparison to crystal monochromators they offer a larger bandwidth (∆ E/E 0 . 01), which leads to a much larger photon flux on the sample. Another advantage is the possibility of selecting the excitation energy just below a matrix element with high concentration and just above the absorption edge of the element of interest (“selective excitation”), with the drawback, however, of an increased background due to Raman scattering. The combination of TXRF with synchrotron radiation allows various geo- metrical arrangements for reflector and detector. Figure 7.57 shows three possi- bilities. For geometry A the polarization effect is fully utilized by positioning
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524 C. Streli et al. 10 8 20 0 40 60 Energy (keV) Brightness (ph/s/mrad 2 /mm 2 /0.1%bw) 80 100 10 9 10 10 10 11 Fig. 7.56. Spectral distribution of Beamline L at HASYLAB the detector axis in the plane of the orbit. Scattered radiation is not emitted in that direction. The sample is excited efficiently, and full homogenous illu- mination of the sample by the width of the beam in the horizontal plane is given. There are hardly any losses due to the collimators because the beam is naturally collimated in the vertical plane with 0.1 mrad to 0.2 mrad diver- gence depending on the energy. The detection of the fluorescence signal is not optimal because the detector must be sidelooking to take advantage of the polarization effect. The fluorescent radiation has a long path in the sample to
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