Energy Calibration of Electron Spectrometers

The energy calibration of any spectrometer requires the use of sources of known energy and preferably of monoenergetic sources. Monoenergetic electron sources are provided by accelerators and by radioisotopes emitting internal-conversion (IC) electrons (see Chap. 3). 

The advantage of the accelerators is their ability to provide a monoenergetic beam with any desired energy from zero up to the upper limit of the machine. The disadvantages are their expensive operation and the fact that the spectrometer has to be moved to the accelerator beam. 

IC emitters are relatively inexpensive to obtain and very easy to handle. They have the disadvantage that they emit not only IC electrons but also gammas. Thus, when a spectrum is recorded, the result includes both IC electrons and Ckimpton electrons created by gammas that interact in the detector. One may eliminate the Compton electrons by utilizing the X-rays that are also given off by the IC source. The X-rays are emitted in coincidence with the IC electrons, while the gammas, and therefore the Compton electrons too, are not. Thus, if the IC electrons are counted in coincidence with the X-rays, the Compton electrons will not be recorded. 

Pure beta-emitting isotopes exist and may be used for calibration, but only after the energy spectrum is cast into a form called the Kurie plot. The beta spectrum is continuous and extends from zero energy up to a maximum end point kinetic energy (see Fig. 13.12). Because of the shape of the spectrum, it is impossible to accurately determine the end point energy. However, from the 

If the left-hand side of Eq. 13.12 is plotted against e, the result is a straight line that crosses the energy axis at = q. The Kurie plot is a straight line for allowed beta transitions. A forbidden beta decay will show an upward curvature at the end.  

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Prominent advantages of these methods include multi-element simultaneous analysis via commonly well-spaced spectral lines, and chemical-state information accessible via smaU, but characteristic and measurable shifts or shape changes in the lines. The stability of surface layers under photon or electron irradiation limits the possible duration of data acquisition time. Seah [20] has described a system for the intensity/energy calibration of electron spectrometers used in AES and XPS, necessary for quantitative analysis. Both AES and XPS may be made quantitative with reasonably good precision, although a great deal of care is necessary. Riviere [21] has compared AES and XPS to other methods of surface analysis (SIMS, ISS, EPMA, RAIRS). 

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