The half-life of the isotope xenon-124 is a trillion times longer than the age of the universe. Observing such a slow decay would seem impossible, but scientists working on the XENON Collaboration, a dark matter detection effort, have done exactly that.

According to researchers, the decay of xenon-124 is the "rarest thing ever recorded."

"If you had ten xenon-124 atoms, it would take about a trillion lengths of the entire universe to see five of them decay," Ethan Brown, an assistant professor of physics at the Rensselaer Polytechnic Institute in New York, told UPI.

"The way we make this measurement is to gather an enormous number of xenon-124 atoms and watch them for a relatively small amount of time, about a year in this case," Brown said. "By counting the handful of decays that we observe out of the huge number of atoms we can determine this huge half-life."

Brown is one of several scientists working on the XENON Collaboration, which runs the XENON1T experiment. XENON1T is a dark matter detector. It features a giant vat of super-pure liquid xenon. The noble gas is shielded from cosmic rays in a cryostat submerged in water and buried deep inside Italy's Sasso mountains.

Brown and his fellow collaborators look for dark matter by studying tiny flashes of light produced when particles interact with xenon inside the detector.

"The goal of the XENON1T experiment was always to try to detect dark matter," Brown told UPI. "That said, we realized early on that there are many other things that can be measured with such a sensitive detector, one of which is this rare decay of xenon-124. Thus, we actively searched for this decay in our data."

The evidence of xenon-124's decay came in the form of a proton inside the nucleus of a xenon atom converted into a neutron. In most decaying elements, the phenomenon occurs when one electron is pulled into the nucleus. However, the proton in a xenon atom has to absorb two electrons to transform into a neutron. The process is called "double-electron capture."

Double-electron capture only occurs when a pair of electrons are at just the right spot at just the right time -- an extremely rare event. But the extremely rare event happened inside XENON1T, and the detector's instruments picked up the signature of electrons in the xenon atom re-organizing to fill in for the two electron absorbed into the nucleus.

Scientists described their chance observation Wednesday in the journal Nature.

"This study demonstrates that the low background and large target mass of xenon-based dark-matter detectors make them well suited for measuring rare processes and highlights the broad physics reach of larger next-generation experiments," researchers wrote.

The XENON1T is currently switched off while it is being upgraded. When it is turned back on it, it will be XENONnT, a larger and more powerful detector.

"Our main goal is once again to try to detect and identify dark matter, but there are many other things that we can measure at the same time," Brown told UPI.

But researchers will continue to keep their eyes peeled for evidence of other rare phenomena.

"There are rare neutrino interactions from the sun, and an even more rare radioactive decay called neutrinoless double beta decay of Xenon-136, which is another million times more rare than the decay of Xenon-124, and only possible if the neutrino is its own antiparticle," Brown said. "Basically, having such a sensitive detector allows us to do all kinds of cool physics measurements that aren't accessible any other way."

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