Analysis of gamma spectra is very interesting since it has a structure, and workers must distinguish between true pulses to be analyzed and accompanying pulses from different radiation sources. We will show the structure of the gamma spectrum in the example of cobalt-60 measured by the NaI(Tl) scintillation detector and the HPGe detector. The HPGe detector separates many closely spaced gamma lines, which is very beneficial for measuring multi-gamma emitting radioactive sources.
Cobalt-60 is an artificial radioactive isotope of cobalt with a half-life of 5.2747 years. It is synthetically produced by neutron activation of cobalt-59 in nuclear reactors. Cobalt-60 is a common calibration source found in many laboratories. The gamma spectrum has two significant peaks, one at 1173.2 keV and another at 1332.5 keV. Good scintillation detectors should have adequate resolution to separate the two peaks. For HPGe detectors, these peaks are perfectly separated.
As can be seen from the figure, there are two gamma-ray photopeaks. Both detectors also show response at the lower energies, caused by Compton scattering, two smaller escape peaks at energies 0.511 and 1.022 MeV below the photopeak to create electron-positron pairs when one or both annihilation photons escape, and a backscatter peak. Higher energies can be measured when two or more photons strike the detector almost simultaneously, appearing as sum peaks with energies up to the value of two or more photopeaks added.
Compton Continuum
In the crystal, a gamma-ray undergoes many interactions, but for intermediate energies, Compton scattering dominates. In Compton scattering, the incident gamma-ray photon is deflected through an angle Θ for its original direction. The photon transfers a portion of its energy to the recoil electron. The energy transferred to the recoil electron can vary from zero to a large fraction (maximum E) of the incident gamma-ray energy because all scattering angles are possible. The size of the scintillation crystal changes the ratio between the photopeak and Compton continuum. For an infinitely large spherical detector centered around a source, no photons would be able to escape, and only a photopeak would be seen on the spectrum. For very small detectors, the chance for a photon to leave after Compton scattering is high, and the Compton continuum would be larger than the photopeak.
Compton Edge
The Compton edge is a feature of the spectrograph that results from the Compton scattering in the scintillator or detector. This feature is due to photons that undergo Compton scattering with a scattering angle of 180° and then escape the detector. When a gamma-ray scatters off the detector and escapes, only a fraction of its initial energy can be deposited in the sensitive layer of the detector. It depends on the scattering angle of the photon and how much energy will be deposited in the detector. This leads to a spectrum of energies. The Compton edge energy corresponds to the full backscattered photon. The counts between the Compton edge and the photopeaks are caused by multiple Compton scattering events, where scattered gamma photon exits the sensitive material.