Fast scintillators are desired in many radiation detection applications, especially where excellent timing resolution is required or in high count rate environments. Examples include time-of-flight positron emission tomography (TOF-PET), TOF computed tomography (TOF-CT), SiPM-based photon counting CT (PCCT), GHz hard X-ray imaging, and various high energy physics (HEP) experiments^[1]. To push the limits of timing performance and approach the proposed target of 10 picoseconds coincidence time resolution (CTR)^[2], ultrafast scintillators with decay times on the order of ~1 nanosecond (ns) may be necessary. However, commercially available inorganic scintillators meeting this criterion are scarce, and although some plastic scintillators have decay times on this scale, the low density and Zeff limit their use.

A promising avenue towards achieving ultrafast emission in inorganic crystals is the mechanism known as core valence luminescence (CVL), or cross luminescence. Compared to other fast emission processes, CVL is particularly interesting due to the overall well-balanced set of properties that can be obtained with such materials – sub-ns decay times, moderate densities, good chemical stability, and relatively bright emission. CVL can be observed in many wide-bandgap halides containing “CVL-active” ions (K+, Rb+, Cs+, and Ba2+)^[3], with the most notable example being BaF2, which is currently employed or being considered for use in various fast timing applications. While other CVL crystals have been explored over the years, none have outperformed BaF2 sufficiently to transition from the research stage into commercial viability.

Cs2ZnCl4 and Cs3ZnCl5 are promising new ultrafast CVL scintillators with 1-2 ns single-component decay times that are currently being investigated at the Scintillation Materials Research Center (SMRC)^[4]. These materials offer several distinct advantages over the state-of-the-art BaF2 – namely longer wavelength emission (peaking near 300 nm) that better aligns with the spectral sensitivity of common photosensors, as well as the absence of a long decay components (several hundred ns) that can lead to severe pulse pileup when count rates are high (Fig. 1).

Located at the University of Tennessee, Knoxville, the SMRC stands as an academic research facility at the forefront of scintillator materials research. With in-house capabilities spanning materials synthesis, crystal growth, and the characterization of physical, optical, and scintillation properties, it provides a unique setting for the exploration and development of novel scintillators. A key focus of the ongoing work is on scaling up crystal growth of Cs2ZnCl4 and Cs3ZnCl5 to sizes conducive to commercial production using the vertical Bridgman method. The goal is to grow crack-free optically clear crystals upwards of 25 mm to 50 mm (1″ to 2″) in diameter. Encouragingly, successful growth of a 38 mm Cs2ZnCl4 crystal has been achieved (Fig. 2), demonstrating its potential scalability.

Additional developments at the SMRC have included improved light yields for both Cs2ZnCl4 (1,980 ph/MeV at 662 keV) and Cs3ZnCl5 (1,430 ph/MeV at 662 keV) owing to the high optical transparency and low defect concentration achieved with recently grown crystals^[4]. This relatively bright CVL emission makes these materials more practical to use and should ultimately lead to benefits in timing performance given that CTR is inversely proportional to the square root of the light yield. Their favorable scintillation properties and ability to be fabricated in large sizes now position Cs2ZnCl4 and Cs3ZnCl5 as potential contenders for radiation detection applications where BaF2 has conventionally been considered.

References

^[1]C. Dujardin et al., “Needs, Trends, and Advances in Inorganic Scintillators,” IEEE Trans. Nucl. Sci., vol. 65, no. 8, pp. 1977-1997, (2018), doi: 10.1109/TNS.2018.2840160

^[2]P. Lecoq, “Pushing the Limits in Time-of-Flight PET Imaging,” IEEE Trans. Radiat. Plas. Med. Sci., vol. 1, no. 6, pp. 473-485, (2017), doi: 10.1109/TRPMS.2017.2756674

^[3]P.A. Rodnyi, Core–valence luminescence in scintillators, Radiat. Meas., vol. 38, no. 4, pp. 343-352, (2004), https://doi.org/10.1016/j.radmeas.2003.11.003.

^[4]D. Rutstrom et al., “Improved light yield and growth of large-volume ultrafast single crystal scintillators Cs2ZnCl4 and Cs3ZnCl5,” Opt. Mat., vol. 133, (2022), https://doi.org/10.1016/j.optmat.2022.112912