Rarest radioactive event observed

The rarest radioactive event observed is the decay of xenon-124, an isotope with a half-life of 1.8 x 1022 years. The decay was observed in the XENON1T instrument, submerged in water 1,500 metres (4,921 feet) beneath the Gran Sasso mountains in Italy, and the results were published in Nature on 24 April 2019.

The experiment is run by the XENON Collaboration research team, with the intent of finding the most elusive substance in the universe: Dark Matter. The instrument is built to observe dark matter interacting with Xenon atoms, which are incredibly inert, but can also record any other interactions. The half-life of 1.8 sextillion years is a trillion times the age of the universe. The decay was the result of “double electron capture” which one of the co-authors of the study, Ethan Brown (USA), referred to as a "a rare thing multiplied by another rare thing, making it ultra-rare."