For the first time in history, scientists directly observedAn exotic form of radioactive decay called a two-neutrino double electron capture. This decay, observed in xenon-124 atoms, occurs so rarely that in order for the xenon-124 sample to be halved, it will take 18 sextillion years (18 with 21 zeros), respectively, it is extremely difficult to detect. The long-awaited observation of a two-neutrino double electron capture, which was reported on April 25 in Nature, lays the foundation for scientists who are trying to see an unseen, even more rare version of such a decay: neutrinoless double electron capture.
The rarest decay of atoms
Observation of this process would confirm thatsubatomic neutrino particles have their own anti-material particle partners, and would help to unravel the mystery of why our Universe is almost entirely made up of matter, and not of antimatter.
“The community was quite noisy after the release of this result,” says Lindley Winslow, a physicist at the Massachusetts Institute of Technology.
Xenon-124 is an isotope, the form of the element with suchsame number of protons, but with a different number of neutrons (the number 124 means the total number of protons and neutrons in the nucleus). It is one of the few radioactive isotopes that decay as a result of two-neutrino double capture of electrons. But atoms are so rarely subjected to this decay, that scientists need to track a huge number of xenon atoms to get a chance to witness it.
Elementary particle physicist Christian Wittweg fromThe University of Munster in Germany and its colleagues were looking for the signature of a two-neutrino double electron capture using the XENON1T detector, a machine commonly used to hunt dark matter. This device, protected from background radiation in the underground Gran Sasso National Laboratory in Italy, contains about three metric tons of xenon.
With a double neutrino electron capturethe atomic nucleus captures two electrons from the surrounding electron shells, converting two protons in the nucleus into neutrons and spitting out two neutrinos. Although the neutrinos themselves do not allow themselves to be detected, the process of capturing electrons emits x-rays and throws out other electrons from the atom, which can be detected.
From February 2017 to February 2018 XENON1Tcollected double electron capture control signals with two neutrinos about 126 times. From this it became clear that the half-life of xenon-124 is about 18 sextillion years, which is the longest half-life ever measured directly from a radioactive substance - and about a trillion times longer than the age of the Universe.
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