Section 7.1.4.5. Escape peaks

The pulse-amplitude distribution may have two or more peaks, even when monochromatic X-rays are used (Parrish, 1966). Absorption of the incident X-rays by the counter-tube gas or scintillation crystal may cause X-ray fluorescence. If this is re-absorbed in the active volume of the counter only one pulse is produced of average amplitude A₁ proportional to the incident X-ray quantum energy (k = constant) However, the gas or crystal has a low absorption coefficient for its own fluorescent radiation, hence, some quanta of the latter of energy e₂ may escape from the active volume of the counter, the amount depending on the geometry of the tube, gas, windows, etc. The average amplitude of the escape pulses is Thus,

The pulse-height analyser discriminates against pulses only on the basis of their amplitudes. When it is set to detect X-rays of energy e₀, it is also sensitive to X-rays of energy . For example, when using an NaI scintillation counter for Cu Kα, e₀ = 8 keV, and for the escape X-rays I Kα, e₂ = 28.5 keV. A pulse-height analyser set to detect X-rays of energy 8 keV is also sensitive to X-rays of energy 36.5 keV, because, from equations (7.1.4.3) and (7.1.4.4), In Figs. 2.3.5.3(c), (d) and 7.1.4.1(d), the escape peak E.P. shows clearly at 0.35 Å, the wavelength of 36.5 keV X-rays. There may be a number of weak escape peaks arising from the stronger powder reflections. In practice, the escape peak should not be confused with a small-angle reflection. It can be tested by reducing the X-ray tube voltage to below the absorption-edge energy of the element in the detector from which it arises.