INSTRUMENTATION FOR TXRF AND GI-XRF - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 198

element the sensor is composed of and does not produce its own chain of
electron-hole pairs. On the one hand, this photon is lost for detection and the
detector efficiency is subject to the discontinuities already shown in Figures
3.24 and 3.25. On the other hand, the residual energy shows up as an individual
photon of the actual energy E

E K β . Such“packages”of energy or
“photons”appear as a separate peak in the spectrum. Figure 3.32 shows a few
such escape peaks due to some strong mother peaks. As is shown in the figure,
their appearance is quite different for a Si(Li) or SDD detector on the one hand
and for an HPGe detector on the other hand.
An effect similar to the escape-peak phenomenon will occur if secondary
electrons instead of X-ray photons escape from the sensor volume, mainly from
a near-surface layer. This effect is called incompletechargecollection . It leads to
a reduction and tailing of the mother peak. The spectral background on the
low-energy side of the mother peak is thereby lifted but only slightly ( < 0.1% of
the peak height in a distance > 500 eV).
The position of escape peaks is dependent on the position of the mother
peak. Their peak height is mainly dependent on the fluorescence yield and the
mass-absorption coefficient of the sensor material. This is demonstrated in
Figure 3.33 for a Si(Li) detector and an HPGe detector. Escape peaks will only
arise if the mother peak lies“above”the energy of the respective absorption

E K α or E

Figure3.32. Energy-dispersive spectrum of FeBr 2 excited by a Mo X-ray tube and recorded by a
Si(Li) detector (semilogarithmic). The Si escape peaks of iron and bromine are filled with dots. If an
HPGe detector is used instead of the Si(Li) detector, four escape peaks of bromine and even one of
molybdenum will distinctly appear whereas the two Si escape peaks will vanish. The Ge escape
peaks are marked in black. Figure from Ref. [2], reproduced with permission. Copyright1996,
John Wiley and Sons.