Experimental time resolution limits of modern SiPMs and TOF-PET detectors exploring different scintillators and Cherenkov emission

Solid state photodetectors like silicon photomultipliers (SiPMs) are playing an important role in several fields of medical imaging, life sciences and high energy physics. They are able to sense optical photons with a single photon detection time precision below 100 ps, making them ideal candidates to read the photons generated by fast scintillators in time of flight positron emission tomography (TOF-PET). By implementing novel high-frequency readout electronics, it is possible to perform a completely new evaluation of the best timing performance achievable with state-of-the-art analog-SiPMs and scintillation materials. The intrinsic SiPM single photon time resolution (SPTR) was measured with Ketek, HPK, FBK, SensL and Broadcom devices. Also, the best achieved coincidence time resolution (CTR) for these devices was measured with LSO:Ce:Ca of 2 x 2 x 3 mm$^{3}$ and 2 x 2 x 20 mm$^{3}$ size crystals. The intrinsic SPTR for all devices ranges between 70 ps and 135 ps FWHM when illuminating the entire 3 x 3 mm$^{2}$ or 4 x 4 mm$^{2}$ area. The obtained CTR with LSO:Ce:Ca of 2 x 2 x 3 mm$^{3}$ size ranges between 58 ps and 76 ps FWHM for the SiPMs evaluated. Bismuth Germanate (BGO), read out with state of-the-art NUV-HD SiPMs from FBK, achieved a CTR of 158$\pm$3 ps and 277$\pm$3ps FWHM for 2 x 2 x 3 mm$^{3}$ and 2 x 2 x 20 mm$^{3}$ crystals, respectively. Other BGO geometries yielded 167$\pm$3ps FWHM for 3 x 3 x mm$^{3}$ and 235$\pm$5 ps FWHM for 3 x 3 x 15 mm$^{3}$ also coupled with Meltmount ($n$ = 1.582) and wrapped in Teflon. Additionally, the average number of Cherenkov photons produced by BGO in each 511 keV event was measured to be 17$\pm$3 photons. Based on this measurement, we predict the limits of BGO for ultrafast timing in TOF-PET with Monte Carlo simulations. Plastic scintillators (BC422, BC418), BaF$_{2}$, GAGG:Ce codoped with Mg and CsI:undoped were also tested for TOF performance. Indeed, BC422 can achieve a CTR of 35$\pm$2 ps FWHM using only Compton interactions in the detector with a maximum deposited energy of 340 keV. BaF$_{2}$ with its fast cross-luminescence enables a CTR of 51$\pm$5ps FWHM when coupled to VUV-HD SiPMs from FBK, with only ∼22% photon detection efficiency (PDE). We summarize the measured CTR of the various scintillators and discuss their intrinsic timing performance.