How do you observe a process that takes more than 1 trillion times longer than the age of the universe? The XENON Collaboration research team did it with an instrument built to find the most elusive particle in the universe -- dark matter. National Science Foundation-funded researchers are announcing that they have observed the radioactive decay of xenon-124, which has a half-life of 18 sextillion (that’s 18 followed by 21 zeros) years. The XENON Collaboration runs XENON1T, a 1,300-kilogram vat of super-pure liquid xenon shielded from cosmic rays in a cryostat submerged in water 1,500 meters deep beneath the Gran Sasso Mountains of Italy. The researchers search for dark matter (which is five times more abundant than ordinary matter, but seldom interacts with ordinary matter) by recording tiny flashes of light created when particles interact with xenon inside the detector. And while XENON1T was built to capture the interaction between a dark matter particle and the nucleus of a xenon atom, the detector actually picks up signals from any interactions with the xenon. 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."
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