On the Prospects of BaF₂ as a Fast Scintillator for Time-of-Flight Positron Emission Tomography Systems

Future time-of-flight positron emission tomography (TOF-PET) will be in need of ultrafast scintillation materials, with potential seen in cross-luminescence (CL). BaF2, for example, shows a sub-100-ps decay time with 300 photons produced per MeV. However, it poses challenges, such as medium radiation length, low photofraction, moderate light yield, and vacuum ultraviolet (VUV) emission around 200 nm. Recent developments in UV-sensitive solid-state photodetectors (SiPMs) have the potential to establish CL for ultrafast TOF in PET. In this work, we aim to study BaF2, read out by these new VUV SiPMs, as a viable alternative to cerium-doped lutetium-yttrium oxyorthosilicate (LYSO:Ce). In order to investigate the prospects of BaF(2 )in TOF-PET, we compare the CTR of 2 x 2 x 20 mm(3) undoped and yttrium-doped BaF2 crystals, readout by Hamamatsu S133703050CN SiPMs with a maximum photon detection efficiency (PDE) of 14% at 200 nm and air coupling, to LYSO:Ce:Ca crystals of the same size. With high-frequency readout electronics, we reached 233 ps with undoped BaF2 and 213 ps with BaF2:Y, while the performance of LYSO:Ce:Ca was 125 ps (measured with HPK S14160-3050HS, PDE similar to 60% and glue coupling). Conducting measurements at different depth of interaction positions shows a pronounced impact on the CTR. Furthermore, we investigated the performance for systems of BaF2 with TOFPET2c ASIC measurements and Geant4 simulations for effective sensitivity comparisons to LYSO:Ce:Ca.