Sum peak in gamma-energy spectra of NaI(T1) detectors

The factors affecting the sum peak of NaI(T1) detectors has been studied experimentally, namely the intrinsic photopeak efficiency, the geometry factor, the energy and the intensity of the incident photon beam. Several radioactive sources were used for the measurement of the intrinsic photopeak efficiency of the two NaI(T1) detectors used in this work (51 mm × 51 mm and 76 mm × 76 mm).⁸⁸Y,⁶⁰Co and²²Na, were used for sum peak determination. Different source to detector distances, to produce different values of geometry factor, and different source activities were used. The effect of geometry on the intrinsic photopeak efficiency is very pronounced at high geometry factor. The linear relation between the relative intensity of the sum peak and both the geometry factor and the intensity of the incident photons, facilitate the quantitative prediction of the sum effect on the energy response function of NaI(T1) detectors.