Radioactive waste from nuclear power plants can take a long time to decay. For plutonium-239 the half-life - that is the time until half of the atoms of a sample have decayed - is no less than 24,000 years. But this is nothing compared to the half-life of the noble gas xenon-124, as an international research team with collaborators from the University of Zurich has now shown.
In physics things sometimes go very fast, sometimes very slowly. The movement of light is very fast, for example: The electromagnetic waves travel 300,000 km per second and take just over one second to reach the Moon from Earth. Other processes take more time, sometimes even much more time. Physicists involved with the XENON experiment in Italy have recently come across a disintegration process of unimaginable slowness: it takes, believe it or not, as much as 1.8 x 1022 years until 50 of 100 atoms of the noble gas xenon-124 have decayed. The half-life of the unstable xenon isotope is thus a trillion times longer than the age of the universe. It is the slowest process of its kind ever measured in a detector. Scientists recently reported on the discovery in the journal Nature.
Protons Catch Electrons
Radioactive decay is when atomic nuclei transform into other, more stable nuclei with the release of energy. Decays can occur in different ways. One form is called electron capture: Here, (positively charged) protons from the atomic nucleus bond with (negatively charged) electrons and thereby form neutrons. This process can also be observed with xenon-124, an atom with a nucleus containing 54 protons and 70 neutrons. If this atom decays, two protons simultaneously capture two electrons from the innermost shells and bind with them to form neutrons, emitting two neutrinos. Because there are now two electrons missing, the electrons move from the outer shells to the two free places on the innermost, low-energy shells. As a consequence of this process energy is emitted in the form of X-rays, and at the same time so-called Auger electrons are released .
By measuring the X-ray radiation and the Auger electrons, physicists of the XENON experiment were able to experimentally observe the decay of xenon-124 atoms and determine the inconceivably long half-life of the noble gas. The scientists used the data recorded by the XENON1T detector during one year. From the data, they proved exactly 126 decays .
Tank with 3.2 Tons of Liquid Xenon
The XENON experiment is one of about a dozen experiments currently underway at the Gran Sasso Laboratory, 120 km northeast of Rome. The laboratory, which belongs to the National Institute of Nuclear Physics (INFN), is located in the Gran Sasso Mountains. A 1400 meter rock layer protects the three rock caverns where the experiments are carried out against the disturbing influence of cosmic radiation. The XENON1T detector is the third in a family of XENON projects, after XENON10 and XENON100. One hundred sixty physicists from Europe, the USA and the Middle East are participating in the detector experiments. This includes the research group of Laura Baudis, Professor of Particle Physics at the University of Zurich.
"The XENON1T detector is a cylindrical tank with 3.2 tons of liquid xenon cooled to -95 ° C. This makes it the largest time projection chamber of this construction type to date," says Laura Baudis. Switzerland has been heavily involved in the detector project since the beginning, as Baudis points out: "We were responsible for testing the new photosensors sensitive to vacuum ultraviolet radiation, which we performed in our laboratory in Zurich using liquid xenon. Also, we were responsible for the development and construction of the readout electronics of the photosensors, and we built and operated the calibration system of the photosensors. We also played a leading role in the design and construction of the inner detector (the time projection chamber) and conducted cooling tests. We also tested many detector materials for their radioactivity with our Ge detector (Gator) at the Gran Sasso lab. This was crucial for the low background - ie the low impact of disturbing influences - of the XENON1T detector. "
Search for Dark Matter
The primary objective of the XENON experiment in the heart of the Gran Sasso Mountains is to detect dark matter, a form of matter that most physicists are convinced exists, but its existence has not yet been proven. The researchers from eleven countries who are participating in the XENON experiment hope to be able to prove the existence of dark matter particles in collision with xenon atoms. Theory predicts that in such collisions, a weak ultraviolet light beam is emitted, which can be detected with the sensors of the detector. At the same time, the sensors should detect the low electrical charge released by the collision process.
The big goal - the detection of dark matter - has not yet been achieved with the XENON experiment. However - almost incidentally - the half-life of xenon-124 was determined. The University of Zurich was instrumental in the data analysis. "For me, these measurements show the incredible sensitivity of the XENON1T detector to register extremely rare events," says Laura Baudis on the importance of the recent measurement. "After measuring the weak interaction process of double-electron capture in xenon-124 for the first time, we can use it to test core models that predict half-lives. These are also important for the predictions of neutrino-less xenon-124 decay, which according to theoretical predictions also occurs. "
Further Upgrades Planned
In order to make the XENON detector even more sensitive than it is already, it will be upgraded to the XENONnT, which will contain 8.4 instead of the current 3.2 t Xenon. Later, under the project name DARWIN, particle physicists want to increase the amount of Xenon in the detector even more to 50 t . The hunt for dark matter and the deepened understanding of neutrinos has just begun.
Author: Benedikt Vogel
 The long half-life of double electron capture makes it extremely rare, and the process has escaped detection for decades. In 2νECEC, two protons in a nucleus are simultaneously converted into neutrons by the absorption of two electrons from one of the atomic shells and the emission of two electron neutrinos (νe). After the capture of the two atomic electrons, mostly from the K shell, the filling of vacancies results in a detectable cascade of X-rays and Auger electrons. The nuclear binding energy Q released in the process (on the order of 1 MeV) is carried away mostly by the two neutrinos, which are not detected within the detector. Thus, the experimental signature appears in the kiloelectronvolt, rather than the megaelectronvolt, range. The process is illustrated in Fig. 1: Schematic of two-neutrino double electron capture. Link to the .