Spectrum contain small peaks which orginate from regions of the specimen other then analysed points, from the specimen holder, or from parts of the instruments. These are excited mainly by stray electrons (for example, those backscattered from the sample) and are more likely to be seen by EDS detector than by EDS detector. They can be minimized by good colllimation, but are difficult to eliminate completely. Figure 1 show solid angle of X-ray collection and figure 2 possible sources of remote X-ray excitation. One of the way to detect this stray radiation is use of the Faraday cup (figure 3). Figure 4 and 5 shows these artifacts.


Figure 1: Solid angle of X-ray collection from the point of view
the specimen (dark shading) and the detector (light shading).


Figure 2: Possible sources of remote X-ray excitation in an
electron beam/EDS system. Solid lines - electron paths,
dashed lines - X-ray paths.


Figure 3: Shematic illustration of a cross-section through a
Faraday cup suitable for detecting remote sources of X-ray
excitation in a electron beam instrument.


Figure 4: Spectra obtained in EPMA from the component of a
Faraday cup: a.) Ti block, directly excited, b.) Pt aperture, c.)
"in-hole" spectrum. The spectra indicate a source od stray
radiation at long range from the the incident beam since only
Ti X-rays are observed in the "in-hole" spectrum. The stray
radiation represents about 0.6 % of that excited by the direct
beam.


Figure 5: EDS spectrum of As excited by a 40 keV electrons
showing an anomalous background below 25 keV due to direct
entry of electrons scattered from the specimen into detector.
The X-ray continuum was fitted from the high energy and
of the spectrum.