Review of the Gamma Spectrum Structure
I. Principle Features
A. Photopeak: Principle gamma photon energy; due primarily to photoelectric absorption, but can contain energy from both primary and secondary interactions; possibility of multiple photopeaks in nuclides with multiple gamma photons (see Coincidence Peak).
B. Compton Plateau: Single Compton scattering events with scattered gamma photon exiting crystal; energy deposited depends on angle of photon-electron interaction; maximum energy at 180° interaction angle (see Compton Edge).
C. Compton Edge: Maximum energy of single Compton scattering event; Compton edge energy given by E2/(E + 0.255), where E = initial photon energy (note: this energy represents the energy of the Compton recoil electron, which is absorbed is the crystal).
D. Compton Valley: Multiple Compton scattering events with scattered gamma photon exiting crystal.
II. Secondary Features
1. Interactions in Crystal
A. Iodine Characteristic X-Ray Escape Peak: Photoelectric absorption by K-shell electron in iodine of NaI(Tl), resulting in a K-shell vacancy. The K --+ L transition for iodine = 28 keV. If this characteristic x-ray exits the crystal 28 keV is removed from the photopeak, resulting in a secondary peak 28 keV below the photopeak. However, for gamma energies above -200 keV, the x-ray escape peak is lost in the spread of the photopeak.
B. Annihilation Escape Peak: Pair production in crystal; only seen if principle photon energy is greater than 1.02 MeV. If both 0.51 MeV annihilation photons escape the crystal, 1.02 MeV is removed from the photopeak, resulting in a secondary peak 1.02 MeV below the photopeak. If one annihilation photon is absorbed and the second exits the crystal, then 0.51 MeV is removed from the photopeak, resulting in a secondary peak 0.51 MeV below the photopeak. The probability is greater that one annihilation photon will be absorbed.
C. Coincidence Peak: Simultaneous absorption of two gamma photons in crystal, resulting in a peak higher in energy than the photopeak. The gamma photons may result from serial isomeric transitions or simultaneous decays in separate radioatoms.
2. Interactions in Source-Detector Shield
A. Lead (Pb) Characteristic X-Ray Peak: Photoelectric absorption by K-shell electron in lead of shielding, resulting in a K-shell vacancy. The K --> L transition for lead = 72 keV. If this characteristic x-ray is absorbed in the crystal then a secondary peak at 72 keV is observed. This peak may be reduced by increasing the distance between the detector and shield or using a lower Z shielding material.
B. Backscatter Peak: Gamma photon interacts by Compton mechanism in shield and is backscattered (180° interaction) out of shield into crystal. Maximum photon energy in this interaction is ~200 keV regardless of initial photon energy. This results in a secondary peak at -200 keV.
C. Annihilation Peak: Pair production in shield by high energy (>1.02 MeV) gamma photon. This results ; in two 0.51 MeV annihilation photons. Due to geometry, only one of these photons can be absorbed in the crystal. If this occurs a secondary peak at 0.51 MeV is observed.
3. Miscellaneous
A. Bremsstrahlung: High-energy beta particles or secondary electrons may interact via bremsstrahlung in source, shield, or crystal, resulting in a continuum of energies, especially at the lower end of the spectrum.
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