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 sources of radiation. We will show the structure of the gamma spectrum on the example of cobalt-60 measured by NaI(Tl) scintillation detector and by the HPGe detector. The HPGe detector allows the separation of 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 for the creation of 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.
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 (e.g. 2 keV plus 1332.5 keV in case of cobalt-60) or simultaneous decays in separate radionuclides (e.g. 2 x 1173.2 keV in case of cobalt-60).