Effect of photomultiplier gain-drift and radiation exposure on 2D-map decoding of detector arrays used in positron emission tomography

Arrays of scintillation crystals optically coupled to four single-anode photomultipliers (PMT) are commonly used in PET cameras. The accuracy of the crystal-position decoding relies on the relative gain stability of all 4 PMT over time. The drift in the gain of one PMT relative to the other three distorts the two dimensional (2-D) crystal-decoding map and causes wrong crystal assignment of a gamma hit. We have studied gain stability and its effect on the 2-D decoding map using 4 Philips XP1911 PMT (19 mm diameter) coupled in the quadrant-sharing mode (PQS) to a position-sensitive BGO array (7×7) over 100-day period. The effect on PMT gain due to radiation exposure of the block was also studied. Using 662 keV gamma rays from a Cs-137 source and the collimated tight of a blue LED coupled to the block we measured the individual PMT gain drift, total composite-energy pulse heights and 2-D decoding maps. Some of the drifts were temperature dependent. The measurements were also corrected for temperature variations of the technique. It was found that throughout the duration of the experiment all four PMT presented similar and continuous gain shifts averaging an 11% drop after 100 days