The Influence of Depth of Interaction and External Cross-Talk on the Timing of BGO-Based Detectors with μSiPMs for TOF-PET

ToF-PET has significantly improved PET image quality over the past decades. Recently, BGO has re-emerged as an attractive scintillator due to its high density, low cost, and potential for fast timing via prompt Cherenkov emission. However, using the fast emission in BGO is challenging due to its low Cherenkov yield and mixture with scintillation light. The DIGILOG project addresses these challenges using finely segmented SiPMs (μ SiPMs) with a dedicated readout. We previously showed that this approach can reach coincidence time resolutions (CTRs) of roughly 30 ps for 3 mm long BGO crystals in an idealized simulation. However, PET systems require longer scintillators, whose CTR is strongly affected by the depth-of-interaction (DOI). Moreover, external SiPM cross-talk can further degrade timing performance, especially in densely packed detector geometries. In this study, we investigate the influence of both DOI and external cross-talk on the timing performance of BGO- μ SiPM detectors. We measure the delay of external cross-talk using a Broadcom NUV-MT SiPM with and without coupling a 15 mm BGO crystal and find that the average delay time of cross-talk photons corresponds to the travel time for twice the scintillator length. With a suitable combination of rise time and threshold, chosen below the average delay, we believe that the μSiPM's timing performance will not be influenced significantly by external cross-talk. For the DOI investigation, we employ simulations of a 20 mm BGO crystal coupled to a full SiPM and to a 10×10μSiPM array. The results show that the μSiPM's CTR is significantly impacted by DOI with a front irradiation CTR of 125 ps and the CTR for a fixed DOI of 15 mm at the end of the crystal going down to 47 ps. This makes an appropriate DOI calibration necessary for optimal timing.