A double electronic capture with the emission of two neutrinos does not happen often. So rarely in reality that the process is reputed to have a half-life that even exceeds the age of our universe. Yet it is one of those extremely rare events that researchers have been able to observe directly thanks to the Xenon 1T detector.
Xenon 1T: this is the small name that physicists give to the best-performing dark matter particle detector that exists today. It has been operated by the National Laboratory of Gran Sasso (Italy) since the end of 2015. And it owes its performance to the infinite precautions taken to achieve an incredibly low background noise for a detector of this size.
The hope of the researchers is to record the tiny flashes of light produced by particles of dark matter interacting with the xenon contained in the detector tank. But what they had not necessarily imagined was that the finesse of Xenon 1T would capture other phenomena that previously escaped observations. And in particular, the radioactive decay of xenon-124 (124 Xe).
The theory predicted that the isotope 124 of xenon had to undergo double beta decay by what physicists call a double electron capture with emission of two neutrinos. But no laboratory had been able to show it experimentally. It must be said that the half-life of 124 Xe is estimated at several thousand billion times the age of the universe. “This is the longest and slowest process ever observed directly,” says Ethan Brown, a physicist.
At the heart of matter
Concretely, what happened? At the heart of a xenon nucleus, a proton has turned into a neutron. This happens when an electron is captured by the nucleus. But in the case of the 124 Xe, the nucleus must simultaneously capture two electrons. Physicists speak of double electronic capture. For this to happen, the electrons must move closer to the nucleus at the right moment. And it is extremely rare.
“What we’ve seen is the reorganization of the electrons around the nucleus,” says Ethan Brown. “For when two electrons leave the procession to join the nucleus, two other electrons necessarily take their place on the orbit they occupied.”
For the first time ever, researchers have observed directly the radioactive decay of 124 Xe. An observation made possible by the obstinacy of physicists to reach a level of purity out of the ordinary. “A fascinating observation that pushes the frontiers of knowledge away from the most fundamental characteristics of matter,” enthuses Curt Breneman, Dean of the School of Science at the Rensselaer Polytechnic Institute (USA).
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