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. In a paper published in the journal Nature, researchers announced that they have observed the radioactive decay of xenon-124, which has a half-life of 1.8 X 1022 years.
Ethan Brown, an assistant professor of physics at Rensselaer, and co-author of the study, said:
“We actually saw this decay happen. It’s the longest, slowest process that has ever been directly observed, and our dark matter detector was sensitive enough to measure it.
“It’s amazing to have witnessed this process, and it says that our detector can measure the rarest thing ever recorded.”
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 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 the 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 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.
The photodetectors of the inner detector of XENON1T were tested in liquid xenon in the UZH laboratory. (Image: via Xenon Collaboration)
In most elements subject to decay, that happens when one electron is pulled into the nucleus. But a proton in a xenon atom must absorb two electrons to convert into a neutron, an event called “double-electron capture.” Double-electron capture only happens when two of the electrons are right next to the nucleus at just the right time, Brown said, which is “a rare thing multiplied by another rare thing, making it ultra-rare.”
When the ultra-rare happened and a double-electron capture occurred inside the detector, instruments picked up the signal of electrons in the atom re-arranging to fill in for the two that were absorbed into the nucleus. Brown said:
“Electrons in double-capture are removed from the innermost shell around the nucleus, and that creates room in that shell.
“The remaining electrons collapse to the ground state, and we saw this collapse process in our detector.”
The achievement is the first time scientists have measured the half-life of this xenon isotope based on a direct observation of its radioactive decay. Curt Breneman, dean of the School of Science, said:
“This is a fascinating finding that advances the frontiers of knowledge about the most fundamental characteristics of matter.
“Dr. Brown’s work in calibrating the detector and ensuring that the xenon is scrubbed to the highest possible standard of purity was critical to making this important observation.”
The XENON Collaboration includes more than 160 scientists from Europe, the United States, and the Middle East, and, since 2002, has operated three successively more sensitive liquid xenon detectors in the Gran Sasso National Laboratory in Italy. XENON1T, the largest detector of its type ever built, acquired data from 2016 until December 2018, when it was switched off.
The electronics to select the photodetectors were developed and built at the UZH. (Image: via Xenon Collaboration)
Scientists are currently upgrading the experiment for the new XENONnT phase, which will feature an active detector mass three times larger than XENON1T. Together with a reduced background level, this will boost the detector’s sensitivity by an order of magnitude.
Provided by: Rensselaer Polytechnic Institute [Note: Materials may be edited for content and length.]
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