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

Cannot ignore this effect when both the anode and the

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cannot ignore this effect when both the anode and the window are grounded, which is the usual case for side-window tubes. The angle between the target surface and the normal to the window (anode angle) can amount to 15–20 , so that the total aperture of the exit X-ray beam can reach 30–40 . Dual-anode geometry can be realized in side-window tubes. For that, a thin layer of a light element covers a target made of a heavy element (e.g., Rh X-ray window Evacuated glass envelope W-emitter Wehnelt cathode Tungsten target Cu-anode Fig. 2.4. Design of a side-window X-ray tube for dental applications
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40 V. Arkadiev et al. on W). Depending on the value of the high voltage, two types of spectra can be obtained with the same X-ray tube. When the accelerating voltage is low, the penetration depth of electrons is also small and x-radiation is excited mainly within the upper layer consisting of the light element. At large values of the accelerating voltage electrons penetrate deep into the second target, exciting the spectrum of the heavy element, which dominates in this case. Another possible arrangement of an exit window is shown in Fig. 2.5. Here, the cathode has a ring shape, the takeoff angle amounts to 90 and the exit window is normal to the tube axis. Generally, such end-window geometry allows realizing smaller anode-window (respectively anode-sample) distances. Besides, the cathode and the window are usually grounded while the anode has a high positive potential. As a result, the bombardment of the window by the scattered on the anode electrons is strongly suppressed. However, the anode–window distance remains restricted, mainly because of the danger of a disruptive discharge between the anode and the window. Self-absorption of the excited X-radiation in the target is minimized due to the large takeoff angle. This fact makes end-window tubes especially attractive for low-energy applications. Besides, the tubes are considered to give more isotropic emission spectra. On the other hand, small anode-cathode distances typical for the design of the end-window tubes limit the value of the high voltage, which usually does not exceed 60 kV. In transmission-anode X-ray tubes the target is a thin layer deposited di- rectly on an internal side of an exit window made of a beryllium film. The elec- tron beam strikes the target at right angle and the excited photons go through the target and the beryllium window further in the same direction. Usually, a continuous spectrum of transmission-anode X-ray tubes is suppressed to a certain extent due to the absorption of bremsstrahlung in the target foil, especially at low energies and above the absorption edge of the excited line, so that one can regard these tubes as possessing an intrinsic filtration. Low contribution of the continuous spectrum leads to the reduction of the back- 5 6 7 9 8 4 1 2 3 1. X-ray beam, 2 Be-window, 3 Electron beam, 4 Ring cathode, 5 Anode, 6 Tube housing assembly, 7 Insulating tube for cooling, 8 Cooling connections, 9 High-voltage connections Fig. 2.5. Design of an end-window X-ray tube for fluorescence analysis
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