If the energy of the incoming X-rays is greater than the absorption edge of the detector gas, it can produce characteristic X-rays from the gas and produce what is termed an escape peak. The name derives from the fact that some energy of the incoming X-ray is escaping as characteristic X-rays from the detector gas.
For example, Ec for Ar is 3.2 keV and any X-rays with higher energy can excite Ar-Ka X-rays (E = 2.95 keV). The production of characteristic X-rays from the gas decreases the apparent energy of the incident X-ray and yields a separate peak offset towards lower energy by 3.2 keV. The size of the escape peak is a function of fluorescent yield, w, and is generally smaller than the characteristic peak because relatively few X-rays excite the gas compared with the number detected; however, in some cases its size can be quite large (Figure 220.127.116.11).
|Figure 18.104.22.168. Escape peak: (A) In argon (wK = 0.12); (B) in krypton (wK = 0.65) (after Maurice et al. 1979).|
Escape peaks can be a problem if they interfere with peaks of elements of interest. For example, the Ca-Ka (Ar) escape peak partially overlaps the P-Ka peak, potentially causing problems in analysis of apatite, Ca5(PO4)3(F,Cl,OH).
Copyright 1997-2003, James H. Wittke
Last update: 01/18/2006 01:47 PM.