2.5.3.7. Satellite Peaks

At high overvoltages more than one electron may be ejected from an atom. In this case, the overall structure of the electron shells is changed and X-rays are produced with slightly lower energies than those produced by single electron ionization. These X-rays appear as small satellite peaks near the characteristic radiation peaks. Another important source of satellite peak production is the Auger process.

Generally, satellite peaks are not important, except when there is a large overvoltage. For example, using E_c of 15 keV to produce Fe-Ka is about optimum (15 / 7.112 = 2.1) but produces an eight-fold overvoltage for Si-Ka (15 / 1.838 = 8.2). This results in a larger intensity of Si satellite peak and decreased intensity of the Si-Ka line. In addition, satellite peaks lower the peak-to-background ratios because more X-rays fall outside PHA window (see far below). In routine analysis, the production of satellite peaks is only a potential problem for elements of Z < 20. However, this is not usually a significant problem. The intensity of the Si-Ka peak at E_o 15 keV is satisfactory, because more Si-Ka X-rays are produced and detected than Fe-Ka X-rays, owing to higher abundances of silicon in geological materials and better spectrometer and crystal efficiencies for Si-Ka.