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Bismuth germanate (BGO) shows good properties for positron emission tomography (PET) applications, but was substituted by the development of faster crystals like lutetium oxyorthosilicate (LSO) for time-of-flight PET (TOF-PET). Recent improvements in silicon photomultipliers (SiPMs) and fast readout electronics make it possible to access the Cherenkov photon signal produced upon 511 keV interaction, which makes BGO a cost-effective candidate for TOF-PET. Tails in the time-delay distribution, however, remain a challenge. These are mainly caused by the high statistical fluctuation on the Cherenkov photons detected. To select fast events with a high detected Cherenkov photon number, the signal rise time of the SiPM was used for discrimination. The charge, time delay and signal rise time was measured for two different lengths of BGO crystals coupled to FBK NUV-HD SiPMs and high frequency readout in a coincidence time resolution setup. The recorded events were divided into 5 × 5 categories based on the signal rise time, and time resolutions of 200 ± 3 ps for 2 × 2 × 20 mm3 and 117 ± 3 ps for 2 × 2 × 3 mm3 were measured for the fastest 20% of the events (4% in coincidence). These good timing events can provide additional information for the image reconstruction in order to increase the SNR significantly, without spoiling the detector sensitivity. Putting all photopeak events together and correcting for the time bias introduced by different numbers of Cherenkov photons detected, time resolutions of 259 ± 3 ps for 20 mm long and 151 ± 3 ps for 3 mm long crystals were measured. For a small fraction of events sub-100 ps coincidence time resolution with BGO was reached for a 3 mm short pixel. [ABSTRACT FROM AUTHOR]

Future colliders will set stringent requirements on the performance of detector materials in terms of timing and radiation hardness. Scintillating garnet crystals proved to satisfy the latter, while the former can be improved through technological developments. In this work, optical and scintillation properties of Cerium-doped Gd 3 Al 2 Ga 3 O 12 (GAGG:Ce) single crystals were studied upon gamma radiation excitation. Several 2x2x3 mm 3 and 2x2x10 mm 3 samples from various producers were characterized in terms of light output, transmission, scintillation kinetics and coincidence time resolution (CTR). Light output was measured using a 137Cs radioactive source, ranging between 27900 and 49500 photons per MeV. Scintillation emission time profiles were measured with 511 keV gamma excitation, and the fastest samples displayed components below 70 ps rise time and 50 ns decay time. CTR was measured employing silicon photomultipliers (SiPMs) obtaining a best value of 87 ± 2 ps full width at half maximum, significantly improving on past state-of-the-art GAGG. [ABSTRACT FROM AUTHOR]

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 mm3 and mm3 size crystals. The intrinsic SPTR for all devices ranges between 70 ps and 135 ps FWHM when illuminating the entire mm2 or mm2 area. The obtained CTR with LSO:Ce:Ca of mm3 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 ps and 277 ps FWHM for mm3 and mm3 crystals, respectively. Other BGO geometries yielded 167 3 ps FWHM for mm3 and 235 5 ps FWHM for mm3 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 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), BaF2, GAGG:Ce codoped with Mg and CsI:undoped were also tested for TOF performance. Indeed, BC422 can achieve a CTR of 35 2 ps FWHM using only Compton interactions in the detector with a maximum deposited energy of 340 keV. BaF2 with its fast cross-luminescence enables a CTR of 51 5 ps 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. [ABSTRACT FROM AUTHOR]