VUV-SiPMs applied to BaF2 cross-luminescence detection for high-rate ultrafast timing applications

Inorganic scintillators are widely used for fast timing applications in high energy physics (HEP) experiments, time-of-flight positron emission tomography (TOF-PET), and time tagging of soft and hard X-ray photons at advanced light sources. As the best coincidence time resolution (CTR) achievable is proportional to the square root of the scintillation decay time it is worth to study fast cross-luminescence e.g. in BaF$_2$, with an intrinsic yield of about 1400 photons/MeV. However, emission bands in BaF$_2$ are located in the deep-UV at 195~nm and 220~nm, which sets severe constraints on the photodetector selection. Recent developments in dark matter and neutrinoless double beta decay searches have led to silicon photomultipliers (SiPMs) with photon detection efficiencies (PDEs) of 20-25~\% at wavelengths of 200~nm. We tested state-of-the-art devices from Fondazione Bruno Kessler (FBK) and measured a best CTR of 51$\pm$5~ps FWHM, when coupling 2$\times$2$\times$3~mm$^3$ BaF$_2$ crystals excited by 511~keV electron-positron annihilation gammas. Using these VUV-SiPMs we recorded the scintillation kinetics of samples from Epic-crystals under 511~keV excitation, confirming a fast decay time of 855~ps with 12.2~\% relative light yield and 805~ns with 84.0~\% abundance, together with a rise time of smaller 4~ps beyond the resolution of our setup. The total intrinsic light yield was determined to be 8500~ph/MeV. We also revealed a faster component with 136~ps decay time and 3.7~\% light yield contribution, extremely interesting for fastest timing applications. Timing characteristics and CTR results on BaF$_2$ samples from different producers and with different dopants (Yttrium, Cadmium and Lanthanum) will be given, which clearly show that the the slow 800~ns emission can be effectively suppressed. Such results ultimately are paving the way for high-rate ultrafast timing applications in medical diagnosis, range monitoring in proton or heavy ion therapy and high energy physics.