EDS spectral artifacts
Several artifacts can be produced in ED X-ray spectra because of the way the X-rays are detected and processed. Three common artifacts are the internal fluorescence peak, escape peaks and sum peaks.
Internal fluorescence peak
X-rays coming into the detector can generate Characteristic X-rays from the Si crystal. These X-rays are processed in the usual way and result in a small Si Kα peak in the ED spectrum, even when there is no Si in the sample being analysed. Improvements in design and construction of semiconductor detectors has minimized these artifacts, but they may be present in older detectors.
Sum peaks
Sum peaks are produced when the pulse processor cannot distinguish between two X-rays that arrive almost simultaneously. Instead of recording two X-rays with either the same or different energies, one X-ray with energy equal to the sum of the energies of the two incoming X-rays is recorded and plotted in the spectrum.
Escape peaks
Escape peaks are produced when instead of all of the energy of an incoming X-ray being converted to electron-hole pairs, a Si Kα X-ray is generated from the silicon detector crystal. The energy measured for the incoming X-ray is reduced by the magnitude of the Si Kα X-ray, i.e. 1.74 keV, and an escape peak is detected in the ED spectrum. Less than 2% of incoming X-rays will fluoresce Si Kα X-rays, so escape peaks form minor peaks located at an energy 1.74 keV less than the energy of related major peaks in the spectrum.
Sum peaks and escape peaks become more significant at high X-ray count rates or if the dead time is >60%. Commercial EDS software packages generally mark the positions of escape peaks and sum peaks related to the major Characteristic X-ray lines so that they can be readily identified in ED spectra.
In quantitative ED microanalysis, corrections must be made for spectral artifacts. The X-rays attributed to escape peaks and sum peaks must be subtracted from the spectrum, and the counts returned to the peaks where they should have been recorded. This correction procedure is known as pulse pile-up correction.
