Validation of the COSINE-100U NaI(Tl) Encapsulation for Low-Temperature Operation in Liquid Scintillator

TL;DR

The study validates the COSINE-100U encapsulation for cryogenic operation in LAB-based liquid scintillator by subjecting a NaI(Tl) detector module to ~110 days at room temperature and ~150 days at $-33^{\circ}C$ in LS. It demonstrates stable optical performance, a temperature-dependent shift in scintillation decay to extended multi-component behavior, and a modest LT light-yield gain with better energy resolution. The long-term stability test rules out chemical incompatibilities and mechanical failures, supporting the readiness of COSINE-100U for a physics run at $-30^{\circ}C$ to enhance sensitivity to low-mass dark matter. The results establish both the feasibility and practical benefits of operating hygroscopic NaI(Tl) crystals immersed in LS at cryogenic temperatures, with integration windows and pulse-shape characteristics adjusted accordingly.

Abstract

The COSINE-100U (upgrade) will enhance the sensitivity of the COSINE-100 dark matter search by operating the detector array immersed in liquid scintillator (LS) at $-30^oC$. To validate the detector design for these conditions, we constructed a module using the COSINE-100U encapsulation and performed a dedicated long-term stability study. The module was first monitored at room temperature for ~110 days in air, followed by a one-week immersion in LAB-based LS to verify initial compatibility. Upon confirming stable optical performance, the temperature was lowered to $-33^oC$. During approximately 150 days of continuous operation at low temperature, we observed no degradation in performance. These results demonstrate the chemical and mechanical robustness of the encapsulation, confirming its suitability for the COSINE-100U physics run.

Figures (4)

• Figure 1: Assembly sequence of the NaI-037 NaI(Tl) detector: (a) the polished crystal after surface treatment; (b) attachment of the silicone optical pad to the PMT window; (c) the crystal coupled to two 3-inch Hamamatsu R12669SEL PMTs; (d) wrapping of the crystal and PMT assembly with PTFE-based Tetratex reflector; (e) installation of the PTFE holder to secure the crystal and PMTs; and (f) insertion of the assembled detector into the upgraded COSINE-100U copper housing.
• Figure 2: Photographs of the NaI-037 experimental setup. (a) Room temperature configuration: the encapsulated detector placed inside the lead shielding (top) and the lead bricks surrounding the setup (bottom). (b) Low-temperature configuration: the DAQ rack and refrigerator used for measurements (top) and the NaI-037 detector immersed in LS inside the refrigerator at $-33^{\circ}\text{C}$ (bottom).
• Figure 3: Accumulated waveforms for 59.54 keV events from an $^{241}\text{Am}$ source at room temperature (RT) and $-33^{\circ}\text{C}$ (LT), with the corresponding best-fit multi-exponential models overlaid. The extended tail observed at $-33^{\circ}\text{C}$ reflects the increased contribution of the intermediate and slow decay components.
• Figure 4: Light yield monitoring of the NaI-037 detector. (a) Time evolution of the light yield at room temperature in ambient air conditions. (b) Time evolution of the light yield at $-33^{\circ}\text{C}$ while immersed in LS. The solid lines represent fits to a constant function. The consistent light yield observed throughout the measurement periods demonstrates that the COSINE-100U encapsulation is sufficiently stable to maintain optical performance during long-term operation in LS at $-30^{\circ}\text{C}$.