Distortion/Asymmetry of X-ray Peak from Gaussian Shape in EDS
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At high energies, the X-ray peak generated by the detector is reasonably well described by a Gaussian function (see page1755). However, most of measured X-ray peaks are broadened and are not perfectly Gaussian but have some asymmetrical distortions due to the way the electron-hole pairs are counted. In fact, the X-ray peak distortion is induced by non-uniform regions near the surfaces and sides of the detector. In this case, recombination of electron-hole pairs at the traps and recombination sites distorts the output.

According to the Hyperment function, the systematic, energy-dependent asymmetry of the X-ray peaks measured from the photon-detector interactions can be theoretically obtained by adding two analytical expressions S(E) and D(E) to describe the spectroscopic features. [1] Therefore, the measured X-ray line shape P(E) as a function of the analyzed photon energy E, can be given by,
         P(E) = S(E) + D(E) + G(E) ------------------- [1755]
         S(E) -- The Compton scattering of the photons within the detector.
         D(E) -- The incomplete charge collection in the dead layer of the detector.
         G(E) -- The major Gaussian peak.

In EDS measurements, there are mainly two typical artifacts that induce distortion (or deviation) from the Gaussian shape of an X-ray peak and its calibration:
        i) Deviations from the ideal Gaussian shape occur on the low energy side of the peak:
           Incomplete charge collection acts to change the peak shape and to shift the peak position to lower energies (e.g. at energies less than 3 keV, and especially at < 1 keV).
        ii) The effect of incomplete charge collection usually extends over all energies below the obviously affected peak, resulting in the so-called "background shelf".

The input count rate of EDS detector is normally selected so that the system deadtime is generally less than 30%-50% in order to minimize the effects of pulse pileup and peak distortion.

For the identification of the elements that have low-energy X-rays only, a single peak will eventually be available. In this case, the analyst must taken advantage of every feature available, such as the asymmetry of the L and M peaks due to the relative heights and separations of the Lα–Lβ and Mα–Mβ peak pairs.







[1] J. L. Campbell, A. Perujo and B. M. Millman, Analytic description of Si(Li) spectral lineshapes due to monoenergetic photons, X-Ray Spectrometry, 16(5), 195–201, 1987.