High voltage photodetector calibration for improved timing resolution with scintillation detectors for TOF-PET imaging

The development of scintillators, photodetectors and electronics with excellent timing properties has made possible bench-top time-of-flight (TOF) measurements with sub 200 ps coincident timing resolution. In a light sharing configuration, losses in light collection result in degraded detector timing resolution. Photodetector uniformity in a light sharing configuration between many detectors is a significant limiting factor in the achievement of similar whole detector timing resolution. Furthermore, it is noticed that low gain photomultiplier tubes (PMT) have poorer timing resolution, but timing resolution improves as gain is increased. We implement a method of individually programming the photocathode and dynode bias voltages of a detector of Photonis XP20D0 PMTs. High voltage calibration of photodetectors will achieve better signal-to-noise-ratio (SNR) than variable-gain-amplifiers (VGA) or delay circuits in the signal path that introduce new sources of noise. Independent control of both the photocathode and dynode voltages allows for nearly orthogonal changes to the gain and transit time. By adjusting the lowest energy PMTs to higher gains, we also improve the intrinsic timing performance of those tubes thereby further improving system timing resolution.