The half-life of a process is the time after which half of the radioactive nuclei present in a sample have decayed away. Using the XENON1T dark-matter detector, a 1,300-kg vat of super-pure liquid xenon shielded from cosmic rays in a cryostat submerged in water deep 1.5 km beneath the Gran Sasso mountains of Italy, physicists from the XENON Collaboration were able to observe the decay of xenon-124 atomic nuclei for the first time. The half-life measured for xenon-124 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.

Not all atoms are stable. Depending on their makeup, some will stabilize themselves by releasing subatomic particles and turning into an atom of a different element — a process called radioactive decay.

We’re much more familiar with radioactive elements like uranium and plutonium — these are the wild teenagers of radioactive elements, constantly hurling off particles. Radon-222, for example, has a half-life of just four days.

Some elements, however, decay very slowly. Xenon-124 is one such elder statesman, though physicists have estimated its half-life at 160 trillion years as it decays into tellurium-124. The Universe is presumed to be merely 13 to 14 billion years old. The new finding puts the half-life of xenon-124 closer to 18 sextillion years.

“Half-life doesn’t mean it takes that long for each atom to decay. The number simply indicates how long, on average, it will take for the bulk of a radioactive material to reduce itself by half,” said Dr. Christopher Tunnell, a physicist at Rice University and a member of the XENON Collaboration.

“Still, the chance of seeing such an incident for xenon-124 is vanishingly small — unless one gathers enough xenon atoms and puts them in the ‘most radio-pure place on Earth.’ A key point here is that we have so many atoms, so if any decays, we’ll see it. We have a (literal) ton of material.”

“We actually saw this decay happen. It’s the longest, slowest process that has ever been directly observed, and our dark matter detector was sensitive enough to measure it,” said Dr. Ethan Brown, a physicist at Rensselaer Polytechnic Institute and a member of the XENON Collaboration.

“It’s an amazing to have witnessed this process, and it says that our detector can measure the rarest thing ever recorded.”

The evidence for xenon decay was produced as a proton inside the nucleus of a xenon atom converted into a neutron. In most elements subject to decay, that happens when one electron is pulled into the nucleus. But two protons in a xenon atom must simultaneously absorb two electrons to convert into two neutrons, an event called double-electron capture.

“Double-electron capture only happens when two of the electrons are right next to the nucleus at just the right time, which is a rare thing multiplied by another rare thing, making it ultra-rare,” Dr. Brown said.

When the ultra-rare happened, and a double-electron capture occurred inside the detector, instruments picked up the signal of electrons in the atom re-arranging to fill in for the two that were absorbed into the nucleus.

“Electrons in double-capture are removed from the innermost shell around the nucleus, and that creates room in that shell. The remaining electrons collapse to the ground state, and we saw this collapse process in our detector,” Dr. Brown said.

“The new results show how well the XENON1T detector can detect very rare processes and reject background signals,” said Professor Laura Baudis, an astroparticle physicist at the University of Zurich and a member of the XENON Collaboration.

“While two neutrinos are emitted in the double-electron capture process, we 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 results were published in the April 25 issue of the journal Nature.

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E. Aprile et al (XENON Collaboration). 2019. Observation of two-neutrino double electron capture in 124Xe with XENON1T. Nature 568: 532-535; doi: 10.1038/s41586-019-1124-4