Thanks to the XENON1T Darkener Detector located under Italy's Gran Sasso Mountains, scientists have recorded one of the rare events that will ever be discovered: a particular type of radioactive decay in xenon-124.
It's a great experience because the degradation of this isotope is extremely extremely slow. In fact, xenon-124 has a half-life of 1.8 x 10 to the power of 22 years – about one billion times longer than the age of the universe.
In radioactive decay, the half-life refers to the time it would take for half the atomic nuclei in a given sample to spontaneously change through one of the many types of radioactive decay, which often involves spitting or capturing protons, neutrons. and electrons in different combinations.
In this case, a team of scientists managed to observe a special event called a double-electron capture, where two protons within a xenon atom simultaneously absorbed two electrons, resulting in two neutrons ̵
This exciting observation took place thanks to XENON1T's incredibly accurate calibration – the instrument is designed to detect interactions between hypothetical dark matter particles with 1,300 kg atoms (2,866 pounds) of xenon isotope packed into the device tank .
But in this case, the sensors designed to observe such interactions caught the decay of the isotope itself, leading to a rare observation of another kind.
"We actually saw this decay," says one of the researchers, Ethan Brown of the Rensselaer Polytechnic Institute (RPI) in New York. "It is the longest, slowest process ever observed directly, and our dark stock detector was sensitive enough to measure it."
"It's great to have seen this process and it says our detector can measure the rarest ever recorded."
Scientists have never before directly observed the radioactive decay of this xenon isotope, even though its half-life has been Theorized around 1955. It represents direct evidence of something we have been seeking for decades.  What actually happens is XENON1T detects the signals emitted by electrons in the atom which rearranges to fill the two trapped in the nucleus. As Gizmodo reports, it doesn't statistically hit the threshold to count as a discovery, but it's still an incredible sight.
"Double capture electrons are removed from the inner shell around the core, and it creates space in the shell," Brown says. "The remaining electrons collapse to the ground, and we saw this breakdown process in our detector."
Although XENON1T was built to look for dark material, it shows how these instruments can also lead to other important findings. This latest observation could teach us more about neutrines, abundant but difficult to detect particles, scientists have been hunting for decades.
In this case, the researchers saw a two-neutrino double electron uptake – the result of the rearrangement of electrons meant two neutrins were emitted by the atomic nucleus. The next challenge they want to take is to discover a neutrino-free double-electron capture – an event that is even rarer than this.
It can again help unlock some of the deepest secrets of particle physics.
"This is a fascinating find that promotes the boundary of knowledge of matter's most fundamental properties," says Curt Breneman of RPI who was not directly involved in the study.
"Dr. Brown's work on calibrating the detector and ensuring that the xenon is scrubbed to the highest possible purity standard was crucial to making this important observation."
The research has been published in Nature .