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

This paper validates the chemical and mechanical robustness of the COSINE-100U NaI(Tl) encapsulation for low-temperature operation in liquid scintillator by demonstrating stable performance over approximately 150 days at -33°C following initial compatibility checks.

The Gist

The Cosmic Detective's Deep Freeze: A Story of Ice, Light, and Sealed Secrets

Imagine you are a detective trying to catch a ghost. This isn't a spooky ghost, but a "dark matter" ghost—a mysterious particle that makes up most of the universe but refuses to interact with light or normal matter. To catch it, scientists use giant, ultra-sensitive cameras made of special crystals.

The COSINE-100 experiment is one such detective team. They use crystals made of Sodium Iodide (NaI), which glow (scintillate) when a dark matter particle bumps into them. But there's a catch: these crystals are like sponges. If they get even a tiny bit of moisture from the air, they get ruined. They also need to be surrounded by a liquid "shield" (liquid scintillator) to block out background noise.

The team wants to upgrade their experiment (COSINE-100U) to be even better. Their secret weapon? Freezing the crystals.

Why Freeze the Detective?

Think of a camera sensor. When it's cold, the "static" (noise) goes down, and the image gets clearer. Similarly, when these crystals are cooled to about -30°C (roughly -22°F), they become:

1. Brighter: They produce more light for every hit.
2. Sharper: They can distinguish between a real dark matter hit and a fake one much better.

But here is the big problem: How do you freeze a sponge that hates water, while it's swimming in a pool of liquid, without it cracking or leaking?

The "Winter Coat" Test

The scientists had to build a special "winter coat" (encapsulation) for the crystal. This coat had to:

• Keep the air out (so the crystal doesn't get wet).
• Keep the liquid shield out (so the liquid doesn't touch the crystal directly).
• Survive the stress of shrinking when it gets super cold.

To make sure this coat would work, they didn't just guess. They built a test prototype (a single crystal detector) and put it through a grueling "survival challenge."

The Challenge had three stages:

1. The Dry Run (Room Temperature):
   First, they put the crystal in a box and left it in normal air for 110 days.

   • The Metaphor: Imagine leaving a delicate, moisture-sensitive sandwich in a humid kitchen for months. If the wrapper has a tiny hole, the bread gets soggy.
   • The Result: The sandwich stayed perfectly dry. The seal held up against the air.

2. The Swim (Liquid Scintillator):
   Next, they dunked the sealed crystal into the liquid shield (which is like a giant tank of glowing oil) at room temperature for a week.

   • The Metaphor: Now, imagine dropping that sealed sandwich into a swimming pool. Does the water seep in? Does the wrapper dissolve?
   • The Result: The crystal stayed dry and happy. The liquid didn't leak in.

3. The Deep Freeze (The Real Test):
   Finally, they turned on the freezer. They didn't slowly cool it down; they slammed the temperature down to -33°C to stress-test the materials. They left it there for 150 days.

   • The Metaphor: Imagine putting that swimming sandwich into a deep freeze. The plastic wrapper shrinks, the bread shrinks, and the water around it freezes. Usually, things crack or separate under this stress.
   • The Result: Nothing broke. The crystal kept glowing just as brightly as before. The "seal" didn't leak, and the "glue" holding the sensors didn't pop off.

What Did They Learn?

The experiment was a huge success. Here are the key takeaways in plain English:

• The Crystal Got Superpowers: When frozen, the crystal actually got 5% brighter and its vision (energy resolution) got 9% sharper. It's like putting on high-definition glasses.
• The "Slow Motion" Effect: At freezing temperatures, the light from the crystal takes a little longer to fade away (like a slow-motion video). The scientists had to adjust their cameras to watch for a longer time to catch all the light, but this actually helps them filter out noise better.
• The Seal is Indestructible: The most important finding is that the special "winter coat" they designed is strong enough to handle the cold, the liquid, and the time. It didn't leak, crack, or let moisture in.

The Big Picture

This paper is basically a "safety report" before the main event. It proves that the scientists can safely freeze their dark matter detectors without ruining them.

Because this test worked, the COSINE-100U experiment is green-lit to start its real search for dark matter in March 2026. They will now run their full array of crystals at -30°C, hoping to finally catch that elusive dark matter ghost with their super-clear, super-bright, frozen eyes.

Technical Summary

Here is a detailed technical summary of the paper "Validation of the COSINE-100U NaI(Tl) Encapsulation for Low-Temperature Operation in Liquid Scintillator."

1. Problem Statement

The COSINE-100 experiment aims to test the annual modulation signal reported by DAMA/LIBRA using Sodium Iodide doped with Thallium (NaI(Tl)) crystals. The upcoming upgrade, COSINE-100U, seeks to enhance sensitivity to low-mass dark matter by operating the detector array immersed in a liquid scintillator (LS) veto at a cryogenic temperature of approximately −30°C.

Operating hygroscopic NaI(Tl) crystals in LS at cryogenic temperatures presents two critical engineering challenges:

1. Chemical Compatibility: Ensuring the encapsulation seals (specifically O-rings) remain chemically stable and do not degrade or leak when exposed to LS at low temperatures.
2. Thermo-Mechanical Stress: Verifying that the encapsulation can withstand the differential thermal contraction of materials (crystal, silicone pads, PMTs, and copper housing) during rapid cooling without compromising the optical coupling or the hermetic seal against humidity.

Prior to the full deployment of the COSINE-100U array, a dedicated validation of this encapsulation design was required to ensure long-term stability and optical performance.

2. Methodology

The authors constructed a prototype module using the NaI-037 crystal (70 mm diameter, 51 mm height, ~0.70 kg) and the specific encapsulation design intended for COSINE-100U.

• Encapsulation Design:

  • Direct Coupling: The crystal faces were coupled directly to two 3-inch high-quantum-efficiency PMTs (Hamamatsu R12669SEL) using 2 mm thick silicone pads (Eljen EJ-560) to eliminate quartz windows and maximize light yield.
  • Reflector: The crystal barrel was wrapped in PTFE-based Tetratex as a diffuse reflector.
  • Housing: The assembly was secured in a copper housing sealed with Viton O-rings, selected for chemical compatibility with LS and mechanical resilience at −30°C.
  • Environment: Assembly was performed in a glovebox with <10 ppm humidity and N2 flushing to minimize radon and moisture.

• Experimental Phases:

  1. Room Temperature (RT) Air Test: The detector operated in ambient air for ~110 days to verify the seal's integrity against humidity ingress.
  2. RT Liquid Scintillator Test: The detector was immersed in Linear Alkylbenzene (LAB)-based LS for one week to check for immediate chemical interactions.
  3. Low-Temperature (LT) Test: The system was placed in a commercial chest refrigerator and cooled directly to −33°C (simulating a rapid cool-down scenario) without a gradual ramp. It operated in LS at this temperature for ~150 days.

• Data Acquisition:

  • A $^{241}$Am source provided a fixed 59.54 keV $\gamma$-ray calibration line.
  • Waveforms were digitized (500 MHz, 12-bit). The integration window was adjusted from 8 $\mu$s (RT) to 16 $\mu$s (LT) to account for longer decay times.
  • Key metrics monitored: Light yield (PEs/keV), energy resolution, scintillation decay time, and long-term stability of the peak position.

3. Key Contributions

• First Validation of Cryogenic LS Operation: This study provides the first long-term validation of the specific COSINE-100U encapsulation design (direct PMT coupling + Viton seals) under the combined stress of liquid scintillator immersion and cryogenic temperatures.
• Characterization of Scintillation Properties at −33°C: The paper details the change in scintillation decay dynamics at low temperatures, demonstrating the necessity of a three-component decay model (fast, intermediate, and slow) compared to the two-component model used at room temperature.
• Engineering Stress Test: By cooling the system directly to the minimum temperature without a ramp, the authors successfully stress-tested the mechanical robustness of the seals and optical coupling against thermal shock.

4. Results

• Scintillation Decay Time:

  • At RT, the decay is well-described by a two-component model (Fast $\tau_1 \approx 220$ ns, Slow $\tau_2 \approx 1030$ ns).
  • At −33°C, a three-component model was required. A new slow component ($\tau_3 \approx 5049$ ns) emerged.
  • The fast component fraction dropped from 88.8% (RT) to 29.4% (LT), while the intermediate and slow components increased significantly. This confirms non-radiative quenching suppression and trapping state delays at low temperatures.

• Light Yield and Resolution:

  • Light Yield: Increased from 21.9 ± 0.2 PEs/keV at RT to 23.2 ± 0.2 PEs/keV at −33°C (a ~5.6% improvement).
  • Energy Resolution: Improved from 4.02 ± 0.15% to 3.66 ± 0.13% at the 59.54 keV peak.

• Long-Term Stability:

  • Air Phase: No degradation in light yield over 110 days, confirming seal integrity against humidity.
  • LT Phase: Over 150 days of operation at −33°C in LS, the light yield remained constant at 23.11 ± 0.04 PEs/keV.
  • Degradation Rate: A linear fit yielded a slope of $p_1 = (1.5 \pm 0.9) \times 10^{-3}$ PEs/keV/day, statistically consistent with zero. This rules out failure modes such as LS infiltration into the PTFE reflector or crystal surface clouding.

5. Significance

The results conclusively demonstrate that the COSINE-100U encapsulation design is chemically and mechanically robust for operation in liquid scintillator at cryogenic temperatures.

• Validation for Physics Run: The successful validation clears the path for the full COSINE-100U physics run, scheduled to begin in March 2026 at Yemilab.
• Enhanced Sensitivity: The observed ~5.6% increase in light yield and improved energy resolution at low temperatures will significantly enhance the experiment's sensitivity to low-mass dark matter (specifically Spin-Dependent WIMP-proton interactions).
• Operational Confidence: The study confirms that rapid cool-downs and long-term immersion in LS do not compromise the hermetic seal or optical coupling, mitigating major risks associated with the upgrade.