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In their search for dark matter, scientists from the XENON Collaboration are using one of the world's most sensitive dark matter detectors, XENONnT at the Gran Sasso Laboratory of the National Institute of Nuclear ÌÇÐÄÊÓƵics INFN in Italy, to detect extremely rare particle interactions. These could provide clues about the nature of dark matter. The problem, however, is that tiny amounts of natural radioactivity generate background events that can mask these weak signals.

The XENONnT experiment has made a breakthrough by significantly reducing one of the most problematic contaminants—radon, a radioactive gas. For the first time, the research team has succeeded in reducing the detector's radon-induced radioactivity to a level a billion times lower than the very low natural radioactivity of the human body.

The underlying technology, which the XENONnT consortium in the current issue of the ÌÇÐÄÊÓƵical Review X, was developed by a team led by particle physicist Prof Christian Weinheimer from the University of Münster.

The XENONnT experiment measures the interactions of hypothetically predicted dark matter particles with atoms of liquid xenon, a noble gas. The 8.5-ton detector operates at around minus 95 degrees Celsius deep below Earth's surface in order to eliminate as many background events as possible.

The detector's exceptional sensitivity is due to the extraordinary purity of the liquid xenon achieved thanks to the unique design of the detector and the use of ultra-radiopure materials. However, even traces of dissolved radon and its radioactive decay products can produce flashes of light that resemble the signals being sought. Since radon, a product of long-lived isotopes from the formation of our solar system, is present in virtually all materials, it accounts for a significant portion of the natural radiation exposure in humans.

To reduce radon events even further, the XENONnT team developed a cryogenic distillation system for the continuous purification of xenon. This process specifically removes radon and reduces its concentration in xenon by a factor of 4 to just 430 radon atoms per ton of liquid xenon, as determined by the XENONnT group from Max Planck Institute for Nuclear ÌÇÐÄÊÓƵics at Heidelberg.

The background events caused by radon are thus about as rare as the extremely rare background caused by neutrinos, which originate from nuclear fusion inside the sun and cannot be shielded. Thanks to radon removal, measurements can be carried out with practically no radioactive background.

"The technology paves the way for larger, even more sensitive detectors such as the planned XLZD liquid xenon observatory, which will be ten times larger," explains Weinheimer. "XENONnT brings us one step closer to solving the mystery of dark matter."