Nuclear Physics

Rarest nuclear radioactive decay

Print edition : May 24, 2019

The XENON Experiment underground. (Left) The water tank with a poster showing what is inside. (Right) The three-storey service building. Photo:

AN international team of scientists working with a detector built for dark matter has, as a bonus, been able to directly measure the rarest decay process ever recorded.

The universe is almost 14 billion years old. This duration is, however, tiny compared with the time taken for physical processes such as the radioactive decay of some nuclei. Using the dark matter detector XENON1T at the Gran Sasso National Laboratory of the National Institute for Nuclear Physics, Italy, the team observed for the first time the decay of xenon-124 nuclei. The half-life measured for the xenon-124 decay process, called double electron capture, is about one trillion times longer than the age of the universe. Half-life is the time over which half of the radioactive nuclei present in a sample decay. The observed radioactive decay is the rarest process ever measured in a detector. It provides information for further investigations on neutrinos, the lightest of all elementary particles whose nature is still not understood fully.

XENON1T is a joint experimental project of about 160 scientists from Europe, the United States and West Asia. “The fact that we managed to observe this process directly demonstrates how powerful our detection method actually is—also for signals which are not from dark matter,” pointed out Christian Weinheimer of the University of Munster, whose group led the study. Using this first ever measurement, the half-life for the process has been estimated to be 1.8×1022 years, making it the slowest process ever measured directly. The atom tellurium-128 is known to have an even longer half-life as estimated indirectly from another process.

The Gran Sasso Laboratory is about 1,400 metres beneath the Gran Sasso massif in order to protect the detector from cosmic rays that can produce false signals. The XENON1T detector consists of a central part that has a metre-long cylindrical tank filled with 3,200 kg of liquid xenon at a temperature of –95° C. Its working is based on the principle that according to theory dark matter should collide very rarely with the atoms of the detector. The new results demonstrate the efficiency of the detector to detect rare processes and reject background signals. While two neutrinos are emitted in the double electron capture process, scientists can now also look for the so-called neutrino-less double electron capture, which could throw new light on the nature of neutrinos.

The scientists are currently upgrading the experiment for the new “XENONnT” phase, which will feature a three-times-larger active detector mass. Together with a reduced background level, this will boost the detector’s sensitivity by an order of magnitude.