The universe is almost 14 billion years old. An inconceivable length of time by human standards - yet compared to some physical processes, it is but a moment. There are radioactive nuclei that decay on much longer time scales. An international team of scientists has now directly measured the rarest decay process ever recorded in a detector. Using the XENON1T detector which mainly searches for dark matter at the INFN Gran Sasso National Laboratory, the researchers were able to observe the decay of Xenon-124 atomic nuclei for the first time. The half-life of a process is the time after which half of the radioactive nuclei present in a sample have decayed away. “The half-life measured for Xenon-124 is about one trillion times longer than the age of the universe,” says Dr. Alexander Fieguth, who carried out a major part of the experimental analyses as part of his PhD thesis at the University of Münster - for which he received a University Dissertation Prize.
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,” says Prof. 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 whose nature is still not fully understood. XENON1T is a joint experimental project of about 160 scientists from Europe, the US and the Middle East.
The story in detail:
A sensitive detector for the search of 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 1,400 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 -95° C. 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 which are detected by means of sensitive light sensors located in the upper and lower parts of the cylinder. The same sensors also detect a minute amount of electrical charge which is 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. 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 which is hidden by signals from the omnipresent natural radioactivity. “One of the tasks we have in the XENON experiment is to develop new methods which can be used to reduce interference signals from radioactivity as far as possible,” explains Christian Weinheimer.
The measurement of the 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. “For over a year we recorded all the signals which occurred in our detector, but without looking at them immediately,” says Christian Wittweg, who is doing his PhD at Münster University.
The reason for this is that experiment was conducted in a “blind” fashion. 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 the atom Tellurium-128 decays with an even longer half-life, however, its decay was never observed directly and the 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. The double electron capture observed involves one decay channel emitting two neutrinos. This channel also provides initial important findings for subsequent measurements of the so-called neutrino-less double electron capture. “By discovering this - which has not yet been achieved - answers could be found to important questions relating to the nature of neutrinos,” Alexander Fieguth confidently asserts. The former physics PhD student at Münster University is now working at Stanford University.
Status and outlook of the experiment:
XENON1T acquired data from 2016 until December 2018 when it was switched off. 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.
E. Aprile et al. (2019): Observation of two-neutrino double electron capture in 124Xe with XENON1T. Nature