Photomultiplier gain variation of a gamma camera due to a high flux irradiation

This work reports the effects on the gains of the photomultiplier tubes (PMTs) of a CardiaL (Elscint) gamma camera due to a high flux irradiation (HFI). One detector of this system received a non-uniform flux of 1.9-0.6 Mcps over 250 cm/sup 2/ areas for 30 minutes. This HFI caused PMT gain changes and, consequently, variations on the energy response of the detector. To evaluate the severity and the duration of the perturbation effect on the energy response, the photopeak position was monitored for 1 month with a /sup 99m/Tc point source. The 140 keV photopeak shifted to 158 keV soon after irradiation, reached the correct position after 9 days and moved to a new stable value of 132 keV after 15 days. In this stable condition an energy re-calibration reset the correct energy response of the gamma camera (photopeak position at 140 keV). This experience suggest that particular care should be taken to avoid exposures to high radiation fluxes that induce persistent gain shifts on the PMTs.