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Thanks to the XENON1T dark matter detector housed in the Gran Sasso mountains in Italy, scientists recorded one of the rarest events ever detected: a special type of radioactive decay in xenon-124.
It's an amazing feat, because the decay of this isotope is extremely slow. In fact, xenon-124 has a half-life of 1.8 x 10 for a power of 22 years – about a trillion times longer than the age of the Universe.
In half-life, radioactive decay refers to the time it takes for half of the atomic nuclei in a given sample to change spontaneously in one of many types of radioactive decay, which often involves spitting or protons, neutrons, and electrons in various combinations.
In this case, a team of researchers managed to observe a special event called double electron capture, where two protons in a xenon atom simultaneously absorbed two electrons, producing two neutrons – described by the team as "a rare thing multiplied by another rare thing, making it ultra-rare ".
This exciting observation has taken place thanks to the incredibly accurate calibration of XENON1T – the instrument is designed to detect hypothetical dark matter particle interactions with atoms in 1,300 kilograms (2,866 pounds) of 39 xenon isotope packaged in the tank of the device.
But in this case, sensors designed to observe such interactions captured the disintegration of the isotope itself, leading to a rare observation of a different type.
"We have actually seen this degradation happen," says one of the researchers, Ethan Brown, of the Rensselaer Polytechnic Institute (RPI) in New York. "It's the longest and slowest process ever directly observed, and our dark matter detector was sensitive enough to measure it."
"It's amazing to have witnessed this process and it is said that our detector can measure the rarest thing ever recorded."
Scientists had never previously directly observed the radioactive decay of this xenon isotope, although its half-life was theorized from 1955. It represents direct evidence of something we have been seeking for decades.
What is really happening is that XENON1T detects the signals emitted by the electrons in the atom by rearranging itself to replace the two that were captured in the nucleus. As Gizmodo points out, it is not entirely possible to be considered a discovery, but it is still an incredible observation.
"The double-capture electrons are removed from the innermost shell around the nucleus, which creates room in this shell," says Brown. "The remaining electrons have collapsed in the ground state, and we have seen this process of collapse in our detector."
Although XENON1T was designed for dark matter research, it also shows how these instruments can lead to other important discoveries. This last observation could tell us more about neutrinos, abundant but difficult to detect particles that scientists have been looking for for decades.
In this case, the researchers found a dual electron capture with two neutrinos – the result of the electron rearrangement meant that two neutrinos were emitted by the atomic nucleus. The next challenge they want to take up is to detect a double electron capture without neutrinol – an event even rarer than this one.
This, in turn, could help reveal some of the deepest secrets of particle physics.
"This is a fascinating discovery that advances the frontiers of knowledge on the most basic characteristics of matter," says Curt Breneman of RPI, who did not participate directly in the 39; study.
"Dr. Brown's work to calibrate the detector and ensure that xenon was washed to the highest degree of purity was essential for making this important observation."
The search was published in Nature.
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