X-Ray Fluorescence Analytical Techniques - CNSTN : Centre ...

IV.2 Compton Edge
Figure II.8: Escape effect.
At low energy of the spectrum lies the Compton shoulder. This rise in the background is
caused by high-energy photons incoherently scattered from the front side of the detector
crystal, leaving only a small fraction of their energy with the recoiled Compton electron in the
detector (Figure II.9). The energy at which the Compton edge occurs is given by the formular
beneath. Both the detector resolution and multiple scattering tend to smear out this sharp
edge.
IV.3 Resulting Spectral Background
Figure II.9: Compton edge.
Figure II.10 shows the spectrum obtained by monochromatic excitation of 17.5 keV and
a Si(Li) detector. The width of the coherent scatter peak reflects the detector resolution at 7.4
keV. The incoherent peak is much broader due to the range of scattering angles included
about the nominal 90° scattering angle. The low energy tail on the incoherent peak extending
down to about 10 keV is primarily due to multiple Compton scattering in the specimen.
The major background represented by the cross-hatched area, is due to incomplete
charge collection in the Si(Li) detector. This occurs when a portion of the positive and
negative charges produced in the detector by the 16.8 and 17.4 keV photons recombine before
they are collected. The result is a pulse of abnormally low amplitude recorded at a lower than
normal energy. The intensity of background due to incomplete charge collection is a function
of detector quality and X-ray energy.