The universe is almost 14 billion years old, which is an inconceivable length of time by human standards. Compared to some physical processes, however, it is but a moment. There are radioactive nuclei that decay on much longer time scales. An international team of scientists now has directly measured the rarest decay process – or the longest half-life – ever recorded in a detector. Using the XENON1T detector, which mainly searches for dark matter at the Gran Sasso National Laboratory, the researchers were able to observe the decay of Xenon-124 atomic nuclei for the first time. The determined half-life measured for Xenon-124, which is the time after which half of the radioactive nuclei present in a sample have decayed, is about one trillion times longer than the age of the universe.
This makes the observed radioactive decay, the so-called double electron capture of Xenon-124, the rarest process ever seen happening in a detector. "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," said Professor Christian Weinheimer from the University of Münster, whose group lead the study. In addition, the new result provides information for further investigations on neutrinos, the lightest of all elementary particles the nature of which is still not fully understood. XENON1T is a joint experimental project of about 160 scientists from Europe, the United States of America, and the Middle East. German partners in the project are the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg as well as the universities of Münster, Freiburg, and Mainz. The results have been published in Nature.
The XENON1T detector searching for dark matter
The Gran Sasso Laboratory of the National Institute for Nuclear Physics (INFN) in Italy, where scientists are currently searching for dark matter particles, is located about 1,500 meters beneath the Gran Sasso massif, well protected from cosmic rays which can produce false signals. Theoretical considerations predict that dark matter should very rarely "collide" with the atoms of the detector. This assumption is fundamental to the working principle of the XENON1T detector: its central part consists of a cylindrical tank of about one meter in length filled with 3,200 kilograms of liquid xenon at a temperature of minus 95 degrees Celsius. When a dark matter particle interacts with a xenon atom, it transfers energy to the atomic nucleus, which subsequently excites other xenon atoms. This leads to the emission of faint signals of ultraviolet light that sensitive light sensors located in the upper and lower parts of the cylinder detect. The same sensors also detect a minute amount of electrical charge released by the collision process.
The new study shows that the XENON1T detector is also able to measure other rare physical phenomena, such as double electron capture. To understand this process, one should know that an atomic nucleus normally consists of positively charged protons and neutral neutrons, which are surrounded by several atomic shells occupied by negatively charged electrons. The element xenon occurs in nature in different variants, which differ only in the number of neutrons in the nucleus. One of these so-called isotopes, Xenon-124, for example, has 54 protons and 70 neutrons. In double electron capture, two protons in the nucleus simultaneously "catch" two electrons from the innermost atomic shell, transform into two neutrons, and emit two neutrinos. The other atomic electrons reorganize themselves to fill in the two holes in the innermost shell. The energy released in this process is carried away by X-rays and so-called Auger electrons. However, these signals are very hard to detect as double electron capture is a very rare process hidden by signals from the omnipresent natural radioactivity. One of the tasks of the German groups is to develop new methods to reduce interference signals from radioactivity.
Measuring double electron capture
This is how the XENON Collaboration succeeded with this measurement: the X-rays from the double electron capture in the liquid xenon produced an initial light signal as well as free electrons. The electrons were moved towards the gas-filled upper part of the detector where they generated a second light signal. The time difference between the two signals corresponds to the time it takes the electrons to reach the top of the detector. Scientists used this interval and the information provided by the sensors measuring the signals to reconstruct the position of the double electron capture. The energy released in the decay was derived from the strength of the two signals. All signals from the detector were recorded over a period of more than one year, however, without looking at them at all as this was conducted as a blind experiment. This means that the scientists could not access the data in the energy region of interest until the analysis was finalized to ensure that personal expectations did not skew the outcome of the study. Thanks to the detailed understanding of all relevant sources of background signals it became clear that 126 observed events in the data were indeed caused by the double electron capture of Xenon-124.
Using this first-ever measurement, the physicists calculated the enormously long half-life of 1.8×1022 years for the process. This is the slowest process ever measured directly. It is known that Tellurium-128 decays with an even longer half-life, however, its decay has never been observed directly and its half-life was inferred indirectly from another process. The new results show how well the XENON1T detector can detect rare processes and reject background signals. While two neutrinos are emitted in the double electron capture process, scientists can now also search for the so-called neutrino-less double electron capture which could shed light on important questions regarding the nature of neutrinos.
"The analyses developed under the lead of the University of Münster are an extremely valuable contribution to our understanding of the forces in the atomic nucleus and to our search for new physics," said Professor Uwe Oberlack of Johannes Gutenberg University Mainz (JGU). Oberlack is one of the founding members of the XENON Dark Matter Project. Before joining JGU in 2010, he worked for ten years in this field and in high energy astrophysics in the US. "It is the close collaboration of all participating institutions that provides the basis for the excellent work of the detector. We are curious how our search for dark matter will proceed in the next phase of the project."
Status and outlook of the experiment
The XENON1T detector acquired data from summer 2016 until December 2018 and was then switched off. Currently, the XENON Collaboration scientists are upgrading the experiment for the new XENONnT phase, with an active detector mass three times larger than before. Together with further suppression of interference signals, this will boost the detector's sensitivity by an order of magnitude. The German groups will also have a leading role in this phase of the project.
In Germany, the XENON1T experiment received financial support from the Max Planck Society, the German Federal Ministry of Education and Research (BMBF), and the German Research Foundation (DFG). International funding came from the USA, Switzerland, Italy, Israel, Portugal, France, Sweden, the Netherlands, and the European Union.
XENON Collaboration, Observation of two-neutrino double electron capture in 124Xe with XENON1T,
Nature 568, 532-535, 25 April 2019,
The XENON experiment at Mainz University
XENON1T: Enlightening the Dark
press release "XENON1T proves to be the most sensitive detector on Earth searching for WIMP dark matter" (18 May 2017)