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. BaF
2
) 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, Cs
2
ZnCl
4
and Cs
3
ZnCl
5
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 Cs
2
ZnCl
4
and 1.03ns for Cs
3
ZnCl
5
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 Cs
2
ZnCl
4
and 1460ph/MeV for Cs
3
ZnCl
5
, this relates to 122 (Cs
2
ZnCl
4
) and 73 (Cs
3
ZnCl
5
) ph/MeV energy deposit. CTR measurements with a high-frequency setup reached 60ps (FWHM) using Cs
2
ZnCl
4
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 Cs
2
ZnCl
4
, air-coupled to VUV SiPMs, at 78ps (FWHM) is comparable to BaF
2
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 Cs
2
ZnCl
4
and only slight hygroscopicity for Cs
3
ZnCl
5
. Further measurements and simulations are planned to explore the prompt emission and to investigate light transport and sensitivity.