Changes in the energy response of a dedicated gamma camera after exposure to a high-flux irradiation

This work reports the effects of the gain variation of the photomultiplier tubes (PMTs) observed on a cardiac dedicated gamma camera after accidental high-flux irradiation. One detector of this dual-headed 90 degrees-fixed ga mma camera was accidentally left uncollimated during a quality assurance pr ocedure on the other detector with a Co-57 flood source (259 MBq) and recei ved a non-uniform high flux of 1.9-0.6 Mcps over 25 000 mm(2) areas for abo ut 30 min. To evaluate the severity and the duration of the perturbation ef fect on the energy response of the detector, the photopeak position was mon itored for about 1 month with a Tc-99m point source. The 140 keV photopeak shifted to 158 keV soon after irradiation, reached the correct position aft er 9 days and moved to a stable value of 132 keV after 15 days. Afterwards, a new energy calibration reset the photopeak position at 140 keV and the c orrect energy response of the gamma camera. This experience suggests that p articular care should be taken to avoid exposures to high radiation fluxes that induce persistent gain shifts on the PMTs of this system.