Dual-anode tube

A wide variety of x-ray tube anode materials are available. The more commonly used anodes are chromium, rhodium, tungsten, and molybdenum. In addition, aluminum, copper, silver, gold, and platinum have all been employed as single anodes. Dual-anode tubes, combining two different anode materials, are also used to provide more flexibility in the choice of excitation condition. Tungsten with chromium and molybdenum with tungsten are two of the more common combinations. Other pairs have also been used to optimize excitation for a particular application (e.g., rhodium combined with tungsten). 

Radiation Source. The radiation source used in ESCA is a standard X-ray anode tube, as described in Chapter 8. Soft X-rays are used, with A1 and Mg being the most common anodes. Many commercial systems offer a dual anode X-ray tube so that the analyst can switch between excitation wavelengths. It is very important that the X-ray source be monochromatic, with a linewidth extending over as narrow an energy range as possible. A1 and Mg have narrow K emission lines. The Mg K line has an energy of... 

In dual Mg/Al X-ray tube, the anode is bombarded by electrons emitted from two independent filaments that are held at ground potential. The acceleration of electrons is caused by the positively biased anode at 15kV. The accelerated electrons hit only the nearest anode face. These electrons produce core holes in the anode target materials by electron impact ionization. The vacancies can relax by emission of characteristic X-rays that illuminate the sample. By switching the filament, one can get MgKa E = 1253.6eV ) or AlKa ( x = 1486.6 eV) lines with inherent resolution (AF fwhm) of 0.7eV... 

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