Signal Characteristics of Individual Crystals in a High Resolution BGO Detector Design Using PMT-Quadrant Sharing

The PMT-quadrant sharing (PQS) detector design allows very high resolution detectors to be achieved with 70% fewer PMT and cost. A common concern for the design is that there is a big gap (photo-insensitive area) between 4 circular PMT, and the photoelectron signal (pulse height) may be much lower for the central crystals. The pulse heights and energy resolution for each crystal in 3 different types of PQS blocks for 19 mm PMT have been measured. For a regular square 7 × 7 block (2.66 × 2.66 × 18 mm BGO): the maximum photopeak signals occurred at the corner crystal of the block. The signals for the worst central-5 crystals (sitting on space with no PMT connection) was found to have pulse heights of 0.87 that of the corner crystals. The 12 crystals (outside the central-5) coupling only to the glass wall but not photocathode had a relative pulse height of 0.92. The 8 crystals with partial exposure to photocathodes had 0.94. The energy resolution for individual crystal was found to be 22-30% with an average of 26%. Asymmetric photopeaks, especially for the corner crystals, were observed, which was found to be from the depth-of-interaction effect. In the latest PQS design, extended blocks with asymmetric light distributions were used on the 4 edges and 4 corners of a large detector module so that the previously unused (wasted) half-row of peripheral PMT are now covered by crystals. An asymmetric blocks, single-extended (7 × 8 blocks) was also tested. The pulse-height ratio between the worst and best group of crystals in the single-extended block was found to be 0.72 and that of the double extended block was also found to be 0.72. In a more demanding, higher spatial resolution 8 × 8 array (2.3 × 2.3 × 10 mm BGO) for mouse PET with shallower crystals, the pulse-height ratio was 0.73 with an average energy resolution of 20%. This study demonstrated that pulse height uniformity for the PQS design using circular PMT was excellent, which is better than the typical 4/1 pulse-height ratio in conventional block detectors.