Tungsten anode

Figure 2 Typical spectrum from an X-ray tube with a tungsten anode operated at 150 kVp. Fluorescence peaks occur at 57.98, 59.32,67.24, and 69.08 keV.

Figure 26 Spectra from four explosive samples and four non-threat materials. The differing structures of these spectra form the basis for identifying the target material. Incoherent scattering contributes to the background continuum and produces two peaks associated with the X-ray fluorescence off the tungsten anode. This is seen most clearly in the spectra with water or motor oil as targets.

Figure 26 shows the spectra from four explosive samples and as well as nonthreat materials. These spectra exhibit strong peaks slightly below 59 and 69 keV, which are unrelated to coherent scatter. These peaks arise from incoherent (Compton) scatter of fluorescent radiation at 59 and 69keV, which is produced from tungsten anodes. Scatter off water shows no apparent CXRS peaks. The four explosive samples clearly show characteristics peaks, which differentiate them from the non-threat materials and from each other. HeuristicaUy, the location and relative amplitudes of these peaks provide an effective means for detecting explosives. 

J.M. Boone, T.R. FeweU and R.J. Jennings, Molybdenum, rhodium and tungsten anode spectral models using interpolated polynomials with application to mammography, Med. Phys. 24(12) (1997) 1863-1874. 

The electrical potential of tungsten in solutions of different acids, bases, and salts has been measured against certain standard electrodes at 25 C. The tungsten does not behave as an insoluble electrode, but sends ions into the solutions. Under certain specified conditions— for examine, with high-current densities (2 amperes per square decimetre) in aqueous alkalis, but with low-current densities in aqueous solutions of acids and salts—-the tungsten anode becomes passive. The passivity appears to be due to adherent films of hydrated oxides. The electrochemical equivalent of tungsten has been found to be 0-3178 mg. per coulomb," which is in close agreement with the theoretical value. 

The most useful radiation source for atomic absorption spectroscopy is the hollow-cathode lamp, shown schematically in Figure 28-17. It consists of a tungsten anode and a cylindrical cathode sealed in a glass tube containing an inert gas, such as argon, at a pressure of 1 to 5 torn The cathode either is fabricated from the analyte metal or serves as a support for a coating of that metal. 

FIGURE 7.5. Thoriated tungsten cathode (left) and thoriated tungsten anode (right) for xenon short-arc lamps used for wide-angle cinema projection. By courtesy of Plansee AG, Austria. 

The hollow cathode is the most frequently used atomic absorption line source. A cupped cathode made of the element to be quantitated and a tungsten anode are positioned in a glass tube which is filled with an inert gas at reduced pressure. The end of the tube is sealed with an optically transparent quartz window. When an electrical potential is struck between the electrodes, the inert gas at the anode is ionized and moves toward the cathode. The element in the cup is sputtered into the gas and excited by the discharge to higher electronic states. The lamp emits intense lines due to resonance radiation. The emission will also show lines characteristic of the electrode itself as an impurity. When feasible, the electrode may be made of the element to be analyzed, thereby avoiding this possible interference. Lamps are available for over 60 different elements and are readily obtainable,... 

Two basic CEDM techniques are under development temporal subtraction and dual-energy techniques. Both these techniques use intravenous injection of an iodine-based contrast agent. However, the low-energy exposures used in mammography are not optimal for the visualization of iodine. Indeed, the X-ray beam generated from a molybdenum/rhodium/tungsten anode and a molybdenum /rhodium/silver filter in conventional mammography was developed to maxi-... 

Figure 3.6 Sensitivity curves for an energy-dispersive system with a tungsten anode. Arrows mark the absorption-edge energies, dashed lines represent extrapolation. (Reprinted by courtesy of EG G ORTEC.)...

Table 3.4 compares detection limits with secondary fluorescers to the results with the RMF method and 15-kV broadband excitation [16,17]. Four different fluorescence analyzers were tested (units A, B, C, and D), and the results were corrected for differences in performance for the energy-dispersive spectrometers employed on each unit. Unit A used a chromium anode tube, and unit B used a tungsten anode tube. Unit A was a commercial, general-purpose instrument. Unit B was specifically designed for atmospheric aerosol analysis, where closer coupling between the tube, fluorescer, sample, and detector could be employed with some sacrifice of insensitivity to specimen-positioning errors. Table 3.5 lists the x-ray tube operating conditions required for Table 3.4. For medium- to high-atomic-number elements, the secondary fluorescer method provides detection limits equivalent to the RMF element, but requires much higher x-ray tube power. For light elements.

May, J.E. Haley, A.J. (1970) Electroplating with auxiliary platinum-coated tungsten anodes. US Patent 3,505,178 April 7, 1970. 

Fig. 5.1. Spectra of the Straton tube at 140 and 80 kV potential. The peaks represent the characteristic lines of the tungsten anode and the continuous spectrum is a result of Bremsstrahlung. The mean photon energies are 53 and 71 keV, respectively...

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