Absolute 60Co characterization based on gamma–gamma coincident detection by two NaI(Tl) detectors

A gamma–gamma coincidence measurement method was used for the characterization of 60Co. A two-dimensional analysis of coincident events allows the careful calculation of the number of counts, when each of the two γ-rays emitted by 60Co is in the photopeak area in each of the NaI(Tl) detectors. The standard formulas that were applied for absolute calibration of 125I (Eldridge–Crowther formulas) were modified for the case of 60Co decay. These modified Eldridge–Crowther formulas were applied to the absolute calibration of 60Co samples measured by the two NaI(Tl) detectors both in coincidence and normal modes. Measurements were performed for three sources of different activities and for two positions of each source between the two NaI(Tl) detectors. Results are in good agreement with the measurements of the same samples done by the 4πβ-γ method.