High-frequency SiPM readout advances measured coincidence time resolution limits in TOF-PET

Scintillator based radiation detectors readout by SiPMs successively break records in their reached time resolution. Nevertheless, new challenges in time of flight positron emission tomography (TOF-PET) and high energy physics are setting unmatched goals in the 10 ps range. Recently it was shown that high frequency (HF) readout of SiPMs significantly improves the measured single photon time resolution (SPTR), allowing to evaluate the intrinsic performance of large area devices; e.g. FBK NUV-HD SiPMs of mm$^{2}$ area and 40$\mu$m single photon avalanche diode (SPAD) size achieve 90 ps FWHM. In TOF-PET such readout allows to lower the leading edge detection threshold, so that the fastest photons produced in the crystal can be utilized. This is of utmost importance if a high SPTR and prompt Cherenkov light generated by the hot-recoil electron upon 511 keV photo-absorption should improve timing. This paper shows that high-frequency bipolar transistor readout of state-of-the-art SiPMs coupled to high-performance scintillators can substantially improve the best achievable coincidence time resolution (CTR) in TOF-PET. In this context a CTR of 158 $\pm$ 3 ps FWHM with 2 x 2 x 3 mm$^{3}$ BGO crystals coupled to FBK SiPMs is achieved. This faint Cherenkov signal is as well present in standard LSO scintillators, which together with low SPTR values (<90 ps FWHM) improves the CTR of 2 x 2 x 3 mm$^{3}$ LSO:Ce:Ca coupled to FBK NUV-HD 4 x 4 mm$^{2}$ with 25 $\mu$m SPAD size to 61 $\pm$ ps FWHM using HF-electronics, as compared to 73 $\pm$ 2 ps when readout by the NINO front-end ASIC. When coupling the LSO:Ce:Ca crystals to FBK NUV-HD SiPMs of 4 x 4 mm$^{2}$ and 40$\mu$m SPAD size, using HF-electronics, a CTR of even 58 $\pm$ 3 ps for 2 x 2 x 3 mm$^{3}$ and 98 $\pm$ 3 ps for 2 x 2 x 20 mm$^{3}$ is achieved. This new experimental data will allow to further discuss the timing limits in scintillator-based detectors.