Timing alignment study of PMT-Quadrant-Sharing (PQS) detectors for time-of-flight PET

We developed a digital signal processing method to measure the coincidence timing resolution of a 13×13 (4×4×20 mm³) PQS LYSO block coupled with Hamamatsu R9779 PMT and investigated the time alignment effect. A single LYSO (4×4×20 mm³) coupled with R9779 was used as a timing measurement reference which has a resolution of 214 ps (FHWM). Signals from PMTs were captured by oscilloscope and analyzed in Labview. Linear fitting and extrapolation to the baseline of the pulse rising edge was used to get the pulse arrival time. By adjusting the output cable length of each PMT, the timing resolution of each crystal in the block was measured with both aligned and unaligned PMT timing. The relationship between the timing resolution of each crystal and the delay time of one of the four PMT was investigated. The result shows that a timing misalignment of 500 ps only worsens the timing resolution by 20 ps. Further more, the regional timing variance on the photocathode of R9779 was also measured by coupling a single LYSO to different locations on its glass window. The coincidence timing resolutions of the 13×13 block with two set-ups of different PMT rotational orientations were compared. The result shows that the timing response difference on the photocathode of R9779 which was measured to be 110 ps has no obvious effect to the timing resolution.