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

TL;DR

This study validates the COSINE-100U NaI(Tl) detector encapsulation's stability and robustness for low-temperature operation in liquid scintillator, supporting its use in dark matter searches.

Contribution

The paper provides experimental validation of the encapsulation's long-term stability and compatibility at -33°C in liquid scintillator, enabling future low-temperature dark matter experiments.

Findings

Stable optical performance over 150 days at -33°C

No degradation observed during long-term immersion in liquid scintillator

Encapsulation proved chemically and mechanically robust

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.