First demonstration of long-distance electron drift in liquid xenon with the Xenoscope time projection chamber
The Xenoscope experiment at the University of Zurich successfully demonstrated long-distance electron drift over 2.6 meters in liquid xenon using cosmic-muon-induced signals, providing precise velocity measurements and a critical benchmark for the development of next-generation large-scale liquid-xenon time projection chambers.
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Deep beneath the surface of the earth, in the quiet dark where light cannot reach, physicists search for the invisible stuff that makes up most of the universe. They call this dark matter, and to find it, they need detectors that are both incredibly large and exquisitely sensitive. One of the most promising tools for this job is a tank filled with liquid xenon, a heavy, noble gas that has been cooled until it turns into a liquid. When a particle from space or the dark matter itself bumps into an atom in this liquid, it creates a tiny flash of light and knocks loose some electrons. These electrons are the key. If scientists can catch them and measure how long they take to travel through the liquid, they can figure out exactly where the collision happened. But for the next generation of these massive detectors, which will hold tens of tons of liquid, the electrons must travel much farther than ever before. The big question is whether they can survive a journey of several meters without getting lost or absorbed by impurities in the liquid.
At the University of Zurich, a team of researchers built a special device called Xenoscope to answer this question. It is a tall, vertical cylinder filled with liquid xenon, standing 2.6 meters high. This height is significant because it matches the scale of the future, giant detectors planned for the next decade. The researchers wanted to see if electrons could drift all the way from the bottom of this tank to the top without vanishing. To do this, they did not use a machine to create the particles; instead, they used nature itself. They waited for cosmic rays, specifically muons, which are high-energy particles created when particles from space hit the atmosphere. These muons rain down on Earth constantly, passing right through the building and the tank. When a muon slices through the liquid xenon, it leaves a trail of ionization, knocking electrons loose along its path.
The team set up a system to catch these muons as they entered the tank. They placed two plastic panels on opposite sides of the outer container, tuned to detect muons passing through at a specific, shallow angle. When both panels saw a muon at the same time, they sent a signal to start a timer. This gave the researchers a precise starting point. They then watched the top of the tank, where a grid of light sensors waited to catch the electrons that had drifted up from the muon's path. The electrons, pushed by an electric field, travel upward through the liquid. When they reach the top, they are pulled into a gas layer, where they create a second, delayed flash of light. By measuring the time between the muon's arrival and this second flash, the team could calculate how fast the electrons were moving and whether they made it the full distance.
The experiment worked. The researchers successfully detected electrons that had traveled the entire 2.6-meter length of the tank. They found that the electrons did not disappear; they survived the long journey. By analyzing the timing of the signals, they measured the speed of the electrons under two different conditions. At one setting, the electrons moved at a speed of 1.320 millimeters per microsecond. At a slightly stronger electric push, they sped up to 1.436 millimeters per microsecond. These measurements are precise, with very small margins of error, and they confirm that the liquid xenon in the tank was pure enough to allow charge to travel over these meter-scale distances.
This result is a crucial step forward for the field. It proves that the technology needed to build the next generation of dark matter detectors is viable. If the electrons could not survive a drift of this length, the massive tanks planned for the future would not work as intended. The researchers also noted that while they successfully tracked the electrons, the initial flash of light from the muon was too faint to be seen clearly in their setup, a limitation caused by the lack of reflective material inside the tank. However, the main goal was achieved: they demonstrated that charge transport over such long distances is possible and measurable.
The team also compared their findings with previous measurements taken in smaller tanks or at different temperatures. Their new data fits well with what is already known, but it extends the range of certainty to a much larger scale. This gives confidence to the designers of the upcoming observatories, which aim to hold up to 80 tons of liquid xenon. The success of Xenoscope means that the physics community can move forward with the design of these giant instruments, knowing that the electrons will behave as expected even when they have to travel more than two meters. The experiment did not discover dark matter itself, but it cleared a major hurdle, showing that the path to finding it is open.
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