Development and Initial Performance of an Upgraded NaI(Tl) Crystal Encapsulation for COSINE-100U

Imagine the universe is a giant, dark ocean, and most of the water is made of something we can't see, touch, or smell. Scientists call this invisible stuff "dark matter." We know it's there because it pulls on stars and galaxies like a hidden gravity, but no one has ever caught a single particle of it. It's like trying to find a specific ghost in a haunted house just by listening for footsteps. One famous experiment, called DAMA/LIBRA, claimed to hear those footsteps: they saw a signal that wiggled up and down every year, exactly like the Earth moving through a cloud of dark matter. But other scientists were skeptical. They wanted to hear the same sound with the same kind of "ears" to be sure it wasn't just a creaky floorboard or a drafty window.

To settle the argument, a team built a new set of ears using special crystals that glow when hit by particles. These crystals, made of sodium iodide, are like tiny, super-sensitive flashlights that flash whenever a dark matter particle (or a regular background particle) bumps into them. The challenge is that these crystals are very picky; they need to be kept dry, and the light they produce has to be captured perfectly to be counted. If the light gets lost or if the crystal gets dirty, the signal is drowned out by noise. This paper tells the story of how a team took an old set of these crystal-ears, gave them a major makeover, and put them in a brand-new, super-quiet underground home to see if they could hear the dark matter whisper any better.

The Crystal Makeover: From Foggy Windows to Crystal Clear

The scientists behind the COSINE-100 experiment decided to upgrade their detector, creating a new version called COSINE-100U. Think of their old setup like a camera with a thick, foggy glass window in front of the lens. Every time a flash of light (a photon) tried to get from the crystal to the sensor, it had to pass through this window, losing some brightness along the way. In the old design, these "windows" were actually thick quartz blocks that sat between the crystal and the light sensors.

For the upgrade, the team decided to smash the foggy windows. They removed the quartz entirely and glued the light sensors directly to the crystal using a thin, clear silicone pad. It's like taking a camera and taping the lens directly to the film, skipping the glass entirely. To make sure no light escaped from the sides, they also shaved the edges of the crystals at a 45-degree angle, acting like a mirror that bounces stray light back toward the sensors. They wrapped the whole thing in a shiny, white material (PTFE) that acts like a perfect mirror, and sealed it inside a copper box to keep out any liquid that might ruin the crystal.

The Results: Louder Flashes and Quieter Rooms

After building this new system, the team tested it in a room-temperature lab at the Yemilab facility in Korea. They ran the detector for 2462 hours (about 102.6 days) and compared the results to the old detector's final 698 hours (29.1 days) of data.

The results were a huge success. The new design made the crystals much brighter. In the old setup, the crystals produced an average of about 15 photoelectrons (tiny flashes of light) for every kiloelectronvolt (keV) of energy. In the new setup, that number jumped. The crystals now produce between 15.8 and 27.7 p.e./keV. Six out of the eight crystals even broke the 20 p.e./keV mark. One crystal, in particular, went from a dim 15.5 to a brilliant 27.7.

Even better, the upgrade saved two crystals that had previously been too dim to use. In the old experiment, Crystals 5 and 8 were so foggy they had to be ignored. After the makeover, Crystal 5 went from a weak 7.3 to a strong 17.8, and Crystal 8 went from a very dim 3.5 to a usable 15.8. They were brought back from the dead!

Cleaning Up the Noise

Making the crystals brighter is only half the battle; you also need to make the room quieter. The crystals can be bothered by "background noise," which comes from tiny amounts of natural radioactivity inside the crystal or on its surface. The team measured two types of this noise:

1. Bulk Alpha: Noise coming from deep inside the crystal.
2. Surface Alpha: Noise coming from the very outside skin of the crystal.

The measurements showed that the "bulk" noise (the deep stuff) went down slightly, which is exactly what you'd expect as the natural radioactive material inside slowly fades away over time. However, the "surface" noise dropped dramatically. Because the team was so careful when they cleaned and polished the crystals before sealing them, the surface noise was cut down by a huge amount. For some crystals, the surface noise dropped by nearly ten times.

The Final Picture

When the team looked at the energy spectra (the chart showing how many events happened at different energy levels), the new detector showed fewer "false alarms" at low energies. This is crucial because dark matter signals are expected to be very faint and low-energy. By making the crystals brighter and the background noise quieter, the COSINE-100U detector is now much more sensitive.

The paper concludes that this new design works exactly as planned. The crystals are brighter, the noise is lower, and the detector is ready for its next phase: running at a chilly -30 °C. The scientists believe that cooling the crystals will make them even better at spotting the faint whispers of dark matter. While this paper doesn't claim to have found dark matter yet, it has successfully built a much sharper pair of ears to listen for it.