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Radioactive radon settles on detector components and creates interference that mimics signals from dark matter particles.
Abstract
Dual-phase xenon time projection chambers catch rare events, like dark matter particles bumping into nuclei. By listening only to ionization signals, you can pick out single-electron events — as faint as a whisper in a quiet room. But there's a pervasive background buzz, which, it turns out, comes from isotopes in the radon-222 decay chain settling on the electrodes. A new first-principles model of this background matches data from the LZ and LUX experiments, and also points to ways to reduce it in future detectors.
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A dark matter detector is like a room where you're trying to hear a whisper while static electricity crackles nearby. This crackle is produced by radon atoms—a radioactive gas that seeps out from materials and air. Settling on metal grids inside the liquid xenon chamber, they knock out electrons when decaying, mimicking the signal from a dark matter particle.
Physicists built a model of this noise and compared it with data from the LUX and LZ experiments. The calculations matched, confirming the culprit. Now it's clear: to hear the whisper, you need to get rid of the crackle—either by cleaning the components more thoroughly or by changing the design, since the sought-after events carry energy of just a few electrons.
A curious twist: radon is born from uranium, which is present in trace amounts in the materials of the detector itself—the setup creates its own interference.
The interference distorts the energy spectrum—the distribution of electrons where physicists look for anomalies pointing to new physics beyond the standard model. Eliminating this background will bring us closer to solving the mystery of the Universe's hidden mass.
🎯 Radon, which hinders the search for dark matter, is the same gas that accumulates in basements and is the second leading cause of lung cancer.
