Timing limits of ultrafast cross-luminescence emission in CsZnCl-based crystals for TOF-PET, HEP and TOF-CT

Cross-luminescence (CL) has been shown to be one of the promising scintillation mechanisms to overcome timing limitations in TOF-PET, TOF-CT or high energy physics. However, well-known CL-emitters (e.g. BaF2) have so far not been established in practice due to their disadvantages, mainly the emission in the deep-UV, calling for the need to develop new CL-emitting scintillators. One promising candidate is CsZnCl-based materials with a decay time ~1-2ns and light emission ~300nm. In this work, Cs2ZnCl4 and Cs3ZnCl5 are examined regarding their scintillation emission time profile and coincidence time resolution (CTR) read out with commercially available and research SiPMs. With 2x2x3mm³ crystals and a time-correlated single-photon counting (TCSPC) setup decay times of 1.79ns for Cs2ZnCl4 and 1.03ns for Cs3ZnCl5 were measured with no long tails in the scintillation emission time profiles, validating previously published measurements. Additionally, a prompt emission with significant abundance of 5% and 6% was discovered. Assuming a light yield of 1980ph/MeV for Cs2ZnCl4 and 1460ph/MeV for Cs3ZnCl5, this relates to 122 (Cs2ZnCl4) and 73 (Cs3ZnCl5) ph/MeV energy deposit. CTR measurements with a high-frequency setup reached 60ps (FWHM) using Cs2ZnCl4 coupled to FBK VUV SiPMs with silicon oil (Dow Corning 200), which is on par with state-of-the-art LYSO:Ce,Ca glue-coupled to NUV-MT SiPMs at 58ps. We find that the CTR of Cs2ZnCl4, air-coupled to VUV SiPMs, at 78ps (FWHM) is comparable to BaF2 at 75ps (FWHM). With their fast decay time and high CTR, these crystals seem to be perfectly suitable for high-rate time-of-flight applications further supported by the non-hygroscopicity for Cs2ZnCl4 and only slight hygroscopicity for Cs3ZnCl5. Further measurements and simulations are planned to explore the prompt emission and to investigate light transport and sensitivity.