Effect of microcell size on timing performance of silicon photomultipliers for ToF-PET imaging

The timing resolution of a Silicon Photomultiplier (SiPM)-based scintillator detector depends on its ability to detect optical photons in the first ns after a scintillation event and is thus directly proportional to its photon detection efficiency (PDE). As a result, an SiPM with a larger microcell size, and thus larger fill factor and PDE, is expected to demonstrate better timing resolution. However, coincidence timing resolution measurements we performed with Hamamatsu 3×3 mm² SiPMs with 25, 50 and 100 um microcells showed that the timing resolution of larger microcell devices are adversely impacted by larger dark count rates. A simple high pass filter can be used to reduce the impact of the dark counts on the baseline of an SiPM output and thus improve the timing resolution to reflect what is expected from a device with larger fill factor and PDE.