Abstract
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.
Highlights
Time of flight positron emission tomography (TOF-PET) substantially benefits from continually improving the coincidence time resolution (CTR), with the goal of reaching a CTR of 10 ps full width at half maximum (FWHM) (Lecoq 2017)
BGO and prompt photon emission In figure 10 we show the measured scintillation characteristics of BGO recorded with a time correlated single photon counting (TCSPC) setup described in Gundacker et al (2016b)
The weighted photon detection efficiency (PDE) of the HPK silicon photomultipliers (SiPMs) for BGO Cherenkov emission in the range of 310–850 nm is 44.9% and, slightly better than that of the FBK SiPM (39.2%). This is due to the higher PDE of the HPK SiPM at larger wavelengths (∼500 nm), and to some extent explains why the measured CTRs with HPK-SiPMs are slightly better than the predicted MC values; bearing in mind that the MC simulations are adapted to FBK-SiPMs
Summary
Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Stefan Gundacker1,2,5 , Rosana Martinez Turtos2 , Nicolaus Kratochwil2,3, Rosalinde Hendrika Pots2,4, Marco Paganoni1,2, Paul Lecoq2 and Etiennette Auffray2 Keywords: TOF PET, silicon photomultiplier (SiPM), single photon time resolution (SPTR), coincidence time resolution (CTR), Cherenkov emission in BGO for fast timing, 10 ps challenge, scintillation kinetics
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