Characterization of pure-NaI crystal with SiPM readout at low temperatures

NaI crystals are used as target materials for several experiments related to dark matter direct searching and coherent elastic neutrino-nuclear scattering detection. The signal event rate rises exponentially with decreasing detector thresholds. Increasing the detector’s light yield is the primary means of lowering the threshold for scintillation detectors. This study investigated the scintillation characteristics of an undoped NaI crystal at low temperatures.

Using a 2.5 \(\times \) $\times$ 2.5 \(\times \) $\times$ 2.5 cm \(^3\) $_{}^3$ crystal and two Hamamatsu S14161-6050HS SiPM arrays, the light yields of pure NaI crystal under various temperatures were measured with a \(^{241}\) $_{}^{241}$ Am \(\gamma \) $\gamma$ source. In addition, the change in the scintillation efficiency of \(\alpha \) $\alpha$ particles was also investigated with this system.

The scintillation efficiency gradually increased as the temperature decreased. At 77 K, the directly measured light yield was 13.50 ± 0.03 photoelectrons per keV when SiPMs were operated at an overvoltage of 3.8 V, 16 times larger than at 233 K. After eliminating the effect of SiPM crosstalks and after-pulses, a light yield of 9.48 ± 0.21 photoelectrons per keV primarily generated by the crystal scintillation was achieved. The scintillation efficiency of \(\alpha \) $\alpha$ particles exhibited a 3.15 times increase in efficiency at 77 K compared to 233 K.

This light yield enhancement provides low-temperature pure NaI crystals with a competitive edge in detecting nuclear recoil signals. The absolute light yield of nuclear recoil signals in low-temperature NaI crystals should be further investigated.