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

Figure IV.12a: Pulse height distribution
(S) Gas proportional counter.
Figure IV.12b: Pulse height distribution
(Fe) Scintillation counter.
If argon is used as the counting gas component in gas proportional counters, an
additional peak, the escape peak (Figure IV.13), appears when X-ray energies are irradiated
that are higher than the absorption edge of argon.
Figure IV.13: Pulse height distribution (Fe) with escape peak.
The escape peak arises as follows:
The incident X-ray quant passes its energy to the counting gas thereby displaying a K electron
from an argon atom. The Ar atom can now emit an Ar Kα1,2 X-ray quant with an energy of 3
keV. If this Ar-fluorescence escapes from the counter then only the incident energy minus 3
keV remains for the measured signal. A second peak, the escape peak that is always 3 keV
below the incident energy, appears in the pulse height distribution.