One of the unsolved mysteries of physics is the existence of dark matter: in order to explain the movements of all visible objects in the universe, there must be five times this mass of invisible matter. However, the properties of this dark matter and its behaviour when interacting with visible matter are still unknown.
In the global search for the detection of dark matter, the XENONnT experiment at the INFN Laboratori Nazionali del Gran Sasso (LNGS) in Italy stands out as one of the world's most sensitive detectors in the field. It aims at measuring the interactions of these hypothetical dark matter particles with xenon atoms. Shielded deep underground from cosmic rays and maintained at -95 °C to keep xenon in its liquid state, the detector requires an environment with virtually no contamination, achieved by the unique design of the detector and the use of ultra-radiopure materials. However, even then trace amounts of natural radioactivity can create false signals, obscuring the rare events scientists are looking for.
The international XENONnT collaboration, comprising over 190 scientists from 30 institutions worldwide, has now achieved a major technological improvement by significantly reducing one of the most problematic contaminants- radon, a radioactive gas.
The core of this innovation is a cryogenic distillation system that purifies the liquid xenon inside the detector. The new system has reduced radon-induced radioactivity to an extraordinarily low level: a billion times lower than the natural radioactivity of the human body and a factor of four lower than the previous record level in XENONnT. The new distillation column continuously purifies the xenon, lowering the radon concentration down to just 430 radon atoms per tonne of liquid xenon, making their background contribution as low as that from solar neutrinos. The analysis of the corresponding data was carried out by Dr. Florian Jörg at the Max-Planck-Institut für Kernphysik. This measurement sets a new benchmark for purity in xenon-based detectors and dramatically enhances sensitivity to rare particle interactions, that could finally reveal the nature of dark matter.
"This achievement demonstrates the effectiveness of the international collaboration," says Prof. Christian Weinheimer from the University of Münster, whose team led the development of this technology. "This breakthrough allows us to push the limits of particle physics and get one step closer to solving the mystery of dark matter."
The Collaboration's full evaluation of this lowest radon background rate ever achieved is accepted for publication in Physical Review X this month.
Original publication
Aprile E. et al. (2025): Radon Removal in XENONnT down to the Solar Neutrino Level. Physical Review X 15, 031079, DOI: 10.1103/zc1w-88p6
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