Effects of SiPM multiplexing on timing performance

Using time of flight (ToF) measurements for positron emission tomography (PET) is an attractive avenue for increasing the signal to noise (SNR) ratio of PET images. However, achieving fast timing resolution required for high SNR gain using silicon photomultipliers (SiPM) requires many resource heavy high speed readout channels. A method of multiplexing many SiPM signals into a single electronic channel would greatly simplify ToF PET readout systems. In this work the effect of multiplexing on the timing resolution of analog SiPMs is modeled, simulated and tested. The simulations and experiments show that baseline fluctuations from cumulative uncorrelated dark noise are the most important cause of timing degradation, but their effects can be mitigated with baseline correction. A charge sharing network for position sensitive multiplexing is proposed and tested. Results show a full width at half maximum (FWHM) coincidence timing resolution of 232 +/-2ps for single 3mm × 3mm × 20mm LYSO scintillation crystals with 16 SiPMs multiplexed to a single timing channel (in addition to 4 position channels). Measurements with a 4×4 array of 3mm × 3mm × 20mm LFS crystals show excellent crystal separation with a minimum ratio of distance between crystal peaks to standard deviation of crystal peaks of 11.9. The mean FWHM coincidence timing resolution of the 4×4 LFS array was 278 +/-7ps. All experiments were performed at room temperature with no thermal regulation. These results show that fast timing resolution for ToF can be achieved with highly multiplexed analog readout.