Technological breakthrough creates unprecedented conditions for detecting ultra-rare events

Scientists from the Laboratory for Instrumentation, Biomedical Engineering and Radiation Physics (LIBPhys), Faculty of Sciences of the University of Coimbra have reduced radioactive radon-222 (²²²Rn) in liquid xenon to just 430 atoms per tonne—five times lower than in similar experiments, setting new standards in dark matter detection.

This historic result, obtained within the framework of the international XENONnT experiment and published in the prestigious journal Physical Review X, demonstrates the system’s unrivalled sensitivity for direct dark matter detection. Located beneath 1,300 metres of rock at the Gran Sasso underground laboratory in Italy, XENONnT uses six tonnes of ultra-purified xenon to minimise cosmic radiation.

José Matias-Lopes, coordinator of the Portuguese team, explains: “Radiation passing through the target produces tiny light and charge signals. Most signals come from known sources, allowing precise calculation of expected events.”

Achieving these radon levels required an advanced cryogenic distillation column and meticulous material selection, down to the smallest screws, to minimise background radiation.

XENONnT now operates at the location on Earth with the lowest ever background radiation, opening a new era in dark matter research. It also enables the study of extremely rare phenomena, including solar axions, neutrinos with anomalous magnetic moments, and coherent elastic neutrino–nucleus scattering (CEvNS).

“Under these conditions, XENONnT confirms its role as a world-leading low-energy particle observatory. It can perform high-precision neutrino measurements and probe extremely rare events, including the double-beta decay of ¹²⁴Xe and ¹²⁶Xe isotopes, as well as a wide range of dark matter candidates, up to the neutrino fog limit,” concludes Matias-Lopes.

The scientific article, “Radon Removal in XENONnT down to the Solar Neutrino Level”, is available here.