sition of electrons from the upper shells, mainly the Lshell, to the Kshell.
The difference in binding energy between the K-shell electron (~88 keV)
and the L-shell electron (~16 keV) appears as lead Kx-ray of ~72 keV.
These characteristic x-ray photons may be directed toward the detector and
absorbed in it and may appear as a peak in the g-ray spectrum (see Fig. 8.3).
These photons can be reduced by increasing the distance between the
detector and the shielding material.
Backscatter Peak
When g-ray photons, before striking the detector, are scattered at 180° by
Compton scattering in lead shielding and housing, and the scattered
photons are absorbed in the detector, then a peak, called the backscatter
peak, appears in the g-ray spectrum (see Fig. 8.3). For high-energy photons,
the backscattered peak appears at 256 keV [see Eq. (6.3)]. This peak can be
mostly eliminated by increasing the distance between the shield and the
detector.
Iodine Escape Peak
Photoelectric interaction of g-ray photons with iodine atoms of the NaI(Tl)
detector usually results in the emission of characteristic Kx-rays. These x-
ray photons may escape the detector, resulting in a peak equivalent to
photon energy minus 28 keV (binding energy of the K-shell electron of
iodine). This is called the iodine escape peak, which appears about 28 keV
below the photopeak (Fig. 8.4). This peak becomes prominent when the
energy of the photon is less than about 200 keV, because, at energies above
Gamma-Ray Spectrometry 91Fig. 8.4. A spectrum of 81-keV g-ray of^133 Xe showing an iodine escape peak.