First results of the NEXT-100 detector using $^{83m}$Kr decays

Deep within the subatomic world, there is a rare and mysterious event that physicists have been hunting for decades: the neutrinoless double beta decay. In the standard rules of nature, certain atoms can transform by emitting two electrons and two invisible particles called neutrinos. However, if a specific type of neutrino exists—one that is its own antiparticle, known as a Majorana particle—this process could happen without releasing those neutrinos at all. Finding this decay would rewrite our understanding of the universe, proving that matter and antimatter are more deeply linked than we thought. To catch this fleeting event, scientists need detectors of immense sensitivity and precision, capable of distinguishing a single, rare signal from a sea of background noise. The challenge lies not just in building a large enough container, but in ensuring that every tiny bit of energy released inside is measured with perfect clarity.

A team of researchers known as the NEXT collaboration has been working on this challenge using a specialized detector filled with high-pressure xenon gas. Their latest effort, the NEXT-100 experiment, recently began its first phase of operation deep underground at the Laboratorio Subterráneo de Canfranc in Spain. This facility is designed to be a giant, three-dimensional camera that can track the paths of electrons with incredible detail. The goal is to eventually fill the detector with a heavy, enriched version of xenon and wait for the rare decay to occur. Before they can begin that long search, however, they must prove that the machine works exactly as intended. In their first major report, the team describes how they tested the detector's vision using a safe, artificial source of energy to map out its performance and ensure it is stable enough for the years of data collection ahead.

To test the machine, the researchers introduced a tiny amount of a radioactive gas called krypton-83m into the xenon-filled chamber. This gas acts as a perfect, known source of energy, decaying in a way that releases a precise, tiny burst of energy equivalent to 41.5 kiloelectronvolts. Because this energy is so low and consistent, it serves as a reliable ruler against which the detector can be measured. As the krypton atoms decay throughout the volume of the detector, they create flashes of light that are picked up by two different sets of sensors. One set of sensors, located at one end of the cylinder, measures the total amount of light to determine the energy of the event. The other set, a grid of tiny light sensors on the opposite side, captures the shape and position of the light to reconstruct exactly where the event happened in three dimensions. By analyzing millions of these events, the team could create a detailed map of how the detector responds to energy at every single point inside the chamber.

The results of this initial calibration were highly encouraging. The researchers found that the detector's response was not perfectly uniform; the amount of light collected varied slightly depending on where the event occurred, influenced by the geometry of the chamber and the behavior of the electrons as they drifted through the gas. However, by using the data from the krypton decays to create a correction map, they were able to adjust the readings and make the detector's vision uniform across the entire volume. After applying these corrections, the team measured the detector's ability to distinguish between different energy levels, a quality known as energy resolution. In the full volume of the detector, they achieved a resolution of 4.37 percent. When they focused on a central, well-behaved region of the detector that mimics the conditions of their future high-pressure operation, the resolution improved to 4.16 percent.

These numbers are significant because they indicate that the detector is performing better than its design targets. The team calculated that if they were to look for the much higher energy of the neutrinoless double beta decay, which is about 2.458 megaelectronvolts, the detector's performance would scale up to an energy resolution of approximately 0.5 percent. This is well below the one percent threshold required to effectively separate the rare signal from background noise. Furthermore, the study showed that the detector is remarkably stable over time. Over a period of three months, the sensors and the gas inside remained consistent, with the light output varying by less than two percent and the purity of the gas remaining high enough to allow electrons to travel freely without being lost.

The success of this first run with the NEXT-100 detector marks a critical step forward in the search for neutrinoless double beta decay. By proving that the machine can map its own internal landscape with such precision and maintain that performance over time, the collaboration has validated the technology needed for the next phase of the experiment. The team plans to increase the pressure of the xenon gas to its full operating level in early 2026, at which point the detector will be ready to begin its long-term search for the rare decay. For now, the data confirms that the instrument is not only built to the right specifications but is also functioning with the stability and clarity required to potentially uncover one of the most profound secrets of the universe.