Achieving fast timing performance with multiplexed SiPMs

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 excellent time resolution required for high SNR gain using silicon photomultipliers (SiPM) requires many resource heavy high bandwidth readout channels. A method of multiplexing many SiPM signals into a single electronic channel would greatly simplify ToF PET systems. However, multiplexing SiPMs degrades time resolution because of added dark counts and signal shaping. In this work the relative contribution of dark counts and signal shaping to timing degradation is simulated and a baseline correction technique to mitigate the effect of multiplexing on the time resolution of analog SiPMs is simulated and experimentally verified. A charge sharing network for multiplexing is proposed and tested. Results show a full width at half maximum (FWHM) coincidence time resolution of 232 +/- 2 ps for a single 3 mm x 3 mm x 20 mm LYSO scintillation crystals coupled to an array of sixteen 3 mm x 3 mm SiPMs that are multiplexed to a single timing channel (in addition to 4 position channels). A 4x4 array of 3 mm x 3 mm x 20 mm LFS crystals showed an average FWHM coincidence time resolution of 278 +/- 7 ps using the same timing scheme. All experiments were performed at room temperature with no thermal regulation. These results show that excellent time resolution for ToF can be achieved with a highly multiplexed analog SiPM readout.