Slotted surface treatment of position-sensitive NaI(Tl) detectors to improve detector performance

We have investigated cutting slots in the entrance surface of 1-in-thick NaI(Tl) crystals, which are used in large position-sensitive detectors, to narrow the detector light response function (LRF). The LRF describes the spread of light from the crystal to the photomultiplier tube (PMT) array. A narrow LRF will, therefore, reduce pileup effects in these detectors at high count rates, and so improve the count-rate performance of NaI(Tl)-based positron emission tomography (PET) scanners. The slot pitch was kept fixed at 5 mm to take into consideration the time required to cut the slots and the slight reduction in sensitivity for the slotted detector. Slots will narrow the LRF and simultaneously make the detector response to position changes discrete in nature. Our investigation focuses on optimizing the slot depth and PMT readout array to achieve a narrow LRF with a detector spatial resolution close to that of the 5-mm slot pitch. Our measurements and simulations show the narrowing of the LRF for large two-dimensional (2-D) detectors with increasing slot depth. However, using 50-mm PMTs the variation of the point spread function between adjacent slots, together with the discrete 5-mm sampling, results in a worsening of the detector spatial resolution for a slotted detector compared to a similar unslotted detector. Simulations show that using smaller 39-mm PMTs will result in better position sampling and a reduction in the variation of the point spread function (PSF) in between adjacent slots. The 2-D detector with 15-mm-deep slots and 39-mm. PMTs results in a detector spatial resolution which is similar to that of an unslotted detector. However, the LRF of this slotted detector is significantly narrower (about 35%), thus reduced event pileup would be expected at high count rates