Upgrade of NaI(Tl) crystal encapsulation for the NEON experiment

The Ghostly Dance of Neutrinos

Imagine the universe is filled with a ghostly swarm of particles called neutrinos. These particles are the ultimate introverts; they have no electric charge and almost no mass, meaning they can zip through entire planets, stars, and even your own body without ever bumping into a single atom. For decades, scientists knew these ghosts existed, but catching them was like trying to photograph a shadow in a hurricane. In 2017, a team finally caught a glimpse of them interacting with matter in a specific way called "coherent elastic neutrino-nucleus scattering" (CEνNS). Think of it like a ping-pong ball hitting a bowling ball; the bowling ball barely moves, but it does wobble. Detecting that tiny wobble is incredibly hard, but if we can, it helps us understand the fundamental rules of the universe, how stars explode, and even how to keep an eye on nuclear power plants from a distance.

To catch these wobbles, scientists need detectors that are super-sensitive and super-quiet. They use crystals that glow when hit by energy, surrounded by a liquid that acts like a shield against background noise. However, just like a high-tech camera, these detectors are only as good as their housing. If the housing leaks or gets dirty, the signal gets lost in the static. This is the story of the NEON experiment, a team of scientists trying to build the perfect "ghost catcher" using a nuclear reactor as their flashlight.

The Leaky Suit and the New Armor

The NEON experiment, located at the Hanbit nuclear power plant in South Korea, set out to detect those tiny wobbles using a special type of crystal called NaI(Tl). In late 2020, they installed 13.3 kilograms of these crystals, which were dipped in a bath of liquid scintillator (a glowing liquid) to help block out unwanted noise. The plan was simple: let the reactor's anti-neutrinos hit the crystals, watch them glow, and count the light.

But things didn't go according to plan. During the first year of testing, the team noticed two major problems. First, the crystals started getting dimmer over time. Second, the detectors were picking up weird, noisy signals that looked like static on a radio. After some detective work, they realized the culprit was the "suit" the crystals were wearing. The original design had a small flaw: the bottom of the light-sensing tubes (photomultipliers) was exposed to the liquid scintillator. Because the glass necks of these tubes weren't perfectly shaped and the seal wasn't tight enough, the liquid slowly leaked inside.

Imagine wearing a raincoat that has a tiny hole in the zipper. At first, you might not notice, but after a few hours of rain, the inside gets wet, your clothes get soggy, and you can't move properly. That's what happened to the crystals. The liquid scintillator seeped in, got on the crystal's surface, and even created tiny white spots due to humidity. This "wetness" blocked the light, causing the crystals to lose their brightness (light yield) and creating that annoying static noise. The team realized that if they didn't fix the suit, they would never catch the ghosts.

The Upgrade: A Copper Fortress

In April 2022, the NEON team decided to upgrade their gear. They took the detectors apart in a super-clean room (a glovebox) where the air was so dry it was almost empty of water. They cleaned the crystals with alcohol, polished them until they were shiny, and wrapped them in special Teflon sheets to bounce light back efficiently.

Then, they built a new "armor." Instead of just sealing the tubes loosely, they placed the crystal and its light-sensors inside a solid copper case. This case was designed like a heavy-duty pressure cooker. They used special rubber O-rings and tightened stainless steel bolts with a torque wrench to ensure the seal was perfect. Most importantly, they used special waterproof cable glands for the wires coming out of the case, ensuring that neither the liquid scintillator nor the outside air could ever sneak in again.

They also swapped out two of the smaller crystals for larger ones, bringing the total weight of the detector up to 16.7 kg. By the time they turned the system back on in April 2022, the new design was ready for its debut.

The Results: Bright, Quiet, and Stable

The results of the upgrade were immediate and impressive. For over a year, the new detectors ran without a single hiccup. The weird static noise that had plagued the old system vanished completely. More importantly, the crystals stayed bright. In the first year of operation, the light output remained steady, proving that the new copper fortress successfully kept the liquid out.

The team measured how much light the crystals produced for every unit of energy they received. They found that the crystals were incredibly efficient, producing between 21.8 and 25.6 "photoelectrons" (tiny flashes of light) for every 1 keV of energy. This is a huge improvement and exactly what they needed to see the faint wobble of a neutrino. The data showed that the light yield didn't drop over time, confirming that the new encapsulation design was stable and effective.

Why This Matters

This paper doesn't just say "we fixed a leak." It proves that with a better design, we can build detectors that are stable enough to catch the most elusive particles in the universe. The NEON team showed that by sealing the crystals in a copper case and keeping the liquid scintillator out, they could maintain high performance for over a year. This success suggests that the NEON experiment is now ready to start its main mission: hunting for those elusive neutrino wobbles. Furthermore, this new "copper armor" technique is so promising that other experiments, like COSINE-100, are already planning to use similar upgrades to improve their own searches for dark matter and neutrinos. The ghost hunters have finally found a suit that keeps them dry.