Escape peaks

Tab. 4.1. Certain elements whose Ka peaks interfere with escape peaks of other elements using solid state detector SSD or silicon drift detector SDD [4.16].

When a proportional counter is. used in conjunction with a pulse-height selector, the occurrence of an escape peak may vitiate the results. Assume that the counter filling contains argon, whose K edge is at 3.87 A, and suppose that the pulse-height selector is set to select an x-ray line 3f shorter wavelength the intensity of which is to be measured. This line will excite the K lines of argon. To the extent that these lines are... 

Fig. 4. Gamma-ray spectrum of an irradiated stony meteorite following a 13 sec irradiation with a flux of approximately 108 14 MeVneutrons cm-2sec-1. Gamma-ray photopeaks of 28A1 and 18N produced by activation of silicon and oxygen, respectively, are prominent features of the spectrum. The features at 5.62 and 5.11 MeV are the first and second escape peaks due to pair production events of the primary 6.13 MeV 16N gamma-rays in the 3 X 3 Nal(Tl) detector...

The spectrum contains the 6.13 MeV peak accompanied by two escape peaks (5.62 and 5.11 MeV). These three peaks are entirely separate from the Pu gamma ray spectrum. 

The situation shown in Fig. 7-22(a) is unusual, in that there are more pulses in the escape peak than in the normal peak. If the window of a pulse-height analyzer is set to pass only the normal Mo Ka pulses, then the observed counting rate (=observed x-ray intensity) would be less than half the value observed with no analyzer at all because without an analyzer all pulses would be counted. The number of pulses in the escape peak will be larger, relative to that in the normal peak, the greater the fluorescence yield (Sec. 1-5) of the counter material and the lower the absorption coefficient of the counter material for its own fluorescent radiation. 

Fig. 7-22 Pulse-height distribution curves showing escape peaks (ep) in proportional counters for (a) Mo Kol radiation incident on a krypton counter and (b) Cu K

Escape peaks can be troublesome in x-ray spectroscopy. When several wavelengths are incident on the counter, the escape peak for wavelength /, may fall on or near the normal peak for wavelength ki, causing uncertainty in the identification of A,. 

Cu Ka. radiation is incident on a xenon-filled proportional counter. Calculate the ratio of the average pulse size in the escape peak to that in the normal peak. Compare your result with that of Fig. 7-22(b). 

For thin detectors, or detectors made of high-Z material—e.g., CdTe or Hglj—some X-rays may escape, thus forming the so called escape peaks (see Sec. 12.8). 

Figure 12.7 presents the spectrum of Na. The single- and double-escape peaks due to the 2.754-MeV gamma are clearly shown. The single- and double-... 

Figure 12.7 A gamma spectrum showing single- and double-escape peaks (from Chap. 4.4.2 of Bertolini and Coche).

The double-escape peak efficiency is important if the energy of the gamma E is greater than about 1.5 MeV, in which case pair production becomes important. The energy of the double-escape peak, equal to P — 1.022 MeV, is used... 

Double-escape / total detector escape peak ... 

Single-escape / total detector I escape peak peak efficiency j efficiency j j total counts in ... 

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