DIGILOG: A digital-analog SiPM towards 10 ps prompt-photon tagging in TOF-PET

Functional imaging techniques like positron emission tomography (PET) are an essential tool in an aging society. Despite impressive advances in microelectronics, photodetectors and scintillation materials, PET is still awaiting a breakthrough in terms of reduced cost and increased performance. Large potential is seen in ultraprecise time-of-flight (TOF), aiming at coincidence time resolutions (CTRs) better than 30 ps. However, state-of-the-art TOF-PET systems are still far away from this goal, achieving typical CTRs of 214 ps (FWHM). Several proposals have been put forth, whereas the most promising is to use prompt photon emission, e.g. Cherenkov radiation in BGO crystals, which are cheap to produce, thus contributing to drastic cost cutting. However, Cherenkov detection is challenging due to its limited photon yield, which in turn requires a very high photon detection efficiency (PDE), low dark count rate (DCR) and extremely fast and innovative electronic readout schemes. Recent analog silicon photomultipliers (aSiPMs) meet the first two targets, but not the latter.In the Digilog project we envisage to unite the best of these two worlds, combining high PDE, low DCR and an exceptional SPTR. To reach this goal, we will segment state-of-the-art aSiPMs into smaller clusters, called µSiPMs. A balanced segmentation of the electronic readout will make it possible to efficiently detect the first scintillation and Cherenkov photons, with a manageable granularity at system level. The µSiPM signals will feature photon-density time walk correction and photon counting. We envision to create 3D-stacked sensors where the electronics will be housed in a CMOS bottom-tier and the µSiPMs in the top-tier chip. Preliminary measurements on first µSiPM test-structures already reached PDE and DCR close to their commercial counterparts, while an SPTR of 25 ps FWHM, close to our sub-20 ps goal, has been achieved.