Scintillating properties of Cs${_2}$ZrCl${_6}$ crystals in the temperature range of 5-300 K

TL;DR

This study probes the temperature dependence of scintillation in Cs$_2$ZrCl$_6$ CZC crystals under γ and α irradiation across 5–300 K, revealing a LY maximum around 135–165 K (γ ≈ $56{,}900$ photons/MeV, α ≈ $19{,}700$ photons/MeV) with RT values near $53{,}300$ photons/MeV (γ) and $16{,}000$ photons/MeV (α). The α quenching factor rises from $QF=0.30$ at RT to $QF=0.36$ at 135 K, while decay-time constants for α and γ pulses shift substantially with temperature, and pulse-shape discrimination remains robust down to ~180 K but declines at 135 K. The authors attribute the nontrivial LY behavior to self-trapped excitons in [ZrCl$_6$]$^{2-}$ clusters and discuss the potential of Ce$^{3+}$ doping to enhance speed and discrimination, identifying the 135–165 K window as the practical operating range. An approximate absolute LY of $LY_{abs} \approx 75{,}200$ photons/MeV is discussed under ideal transport conditions, underscoring the CZC crystal’s promise for rare-decay searches while highlighting areas for spectral and transport optimization.

Abstract

A new comprehensive study on the Cs${_2}$ZrCl${_6}$ (CZC) crystal scintillating properties under different types of irradiation was performed over a wide temperature range from 5 to 300 K. The light yield (LY) at room temperature (RT), measured under irradiation by 662 keV $γ$ quanta of $^{137}$Cs, was evaluated to be 53,300 $\pm$ 4,700 photons/MeV corresponding to approximately 71% of its estimated absolute value. The maximum light emission was observed in the temperature interval 135-165 K, where the LY reached 56,900 photons/MeV and 19,700 photons/MeV for $γ$ quanta and $α$ particles, respectively. The quenching factor (QF) for $α$ particles increases smoothly from QF = 0.30 at RT to QF = 0.36 at 135 K. The shape of scintillation pulses induced by $α$ particles is characterized by three time-constants (0.3, 2.5 and 11.8 $μ$s at RT), whereas the average pulse of $γ$ induced events is characterized by two time-constants (1.3 and 11.5 $μ$s at RT). At the same time, scintillating properties and pulse-shape discrimination capability of the CZC exhibit an acute deterioration at temperatures below 135 K. The optimal operating conditions to maximize the scintillating performance of undoped CZC crystals are discussed.

Figures (9)

• Figure 1: Energy spectrum collected at RT with the CZC sample irradiated by 662 keV $\gamma$ quanta from a $^{137}$Cs source. Analyzing the spectrum from low to high energy, the narrow peak near zero corresponds to noise events with no corresponding scintillation light. The peak around 7,000 ADU$\mu$s is due to Ba unresolved X-rays (31.8 and 32.2 keV). The back-scattering peak and Compton edge at 40,000 ADU$\mu$s and 100,000 ADU$\mu$s, respectively, are present and well separated. The full absorption peak at 136,970 ADU$\mu$s is well defined with the energy resolution FWHM = 4.7%.
• Figure 2: (Left) The Cs${_2}$ZrCl${_6}$ crystal sample (20 $\times$ 14 $\times$ 6 mm) mounted in the crystal holder during installation at the cryostat. The thermocouple is mounted on a cold finger near the sample holder to ensure careful temperature monitoring. The $^{241}$Am $\alpha$ source, a small silver-colored tablet, can be seen at the bottom of the sample holder. (Right) The schematic view of the measurement setup with the Cs${_2}$ZrCl${_6}$ crystal installed in the optical cryostat used for the light collection efficiency (LCE) coefficient simulation in the Geant4 software package. Where: “A” is the crystal sample; “B” is the Teflon reflecting tape (1 mm thick); “C” is the gold-plated cold finger with an attached crystal holder; “D1” and “D2” are the fused quartz internal windows of the cryostat ($\diameter$40 $\times$ 1 mm); “E” is the fused quartz external window of the cryostat ($\diameter$40 $\times$ 2 mm); “F” is the fused quartz entrance window of the PMT ($\diameter$28 $\times$ 1 mm); “G” is the photocathode layer of the PMT; “H” is the PMT internal volume; “I” is the light-tight black-colored PMT holder that covers rest of the entrance window “E”. Area to the right of the window “E” represents internal volume of the optical cryostat, while area to the left of the window “E” stays at ambient pressure and temperature. Distances: “A-D1” = 1.5 mm, “D1-D2” = 3.2 mm, “D2-E” = 4.2 mm, “E-F” = 1.0 mm.
• Figure 3: Energy spectrum collected at RT with the Cs${_2}$ZrCl${_6}$ crystal mounted in the optical cryostat. The red curve represents the best fit of the 662 keV full absorption peak from $^{137}$Cs with a sum of Gaussian and exponential functions. The black curve exhibits the best fit of the 4.7 MeV $\alpha$ peak from $^{241}$Am with the CrystalBall function. Integral is expressed here in ADU$\mu$s.
• Figure 4: Temperature dependence of the Light Yield (LY) and Quenching Factor (QF, purple curve) of the Cs${_2}$ZrCl${_6}$ crystal under excitation by $\gamma$ quanta (red curve) and $\alpha$ particles (black curve). The LY is expressed in photons/MeV.
• Figure 5: Best fit curves of average $\alpha$ and $\gamma$ events recorded at RT are overlaid on the actual average pulses. The average pulses of each type were obtained through the addition of 1324 and 415 individual pulses induced by $\alpha$ (black curve) and $\gamma$ (red curve) events, respectively. These events were selected near the mean value of the corresponding peaks, and normalizing them to the same area.