“We’ve already seen this decay happen. It’s the longest and slowest process ever directly observed, and a dark matter detector was sensitive enough to measure it.” Ethan Brown, assistant professor of physics at Rensselaer Polytechnic Institute on a process that takes more than a trillion times the age of the universe. “It’s amazing to witness this process, and she says our detector can measure the rarest thing ever recorded.”
Detected in the search for dark matter
The XENON Cooperation The research team did this with an instrument designed to find the most elusive particle in the universe – dark matter. In a research paper published in Nature, researchers announce that they have observed the radioactive decay of xenon-124, which has a half-life of 1.8 x 10^22 years, or a half-life of 18 sextillion years, which is one. A trillion times longer than the age of the universe.
XENON Collaboration powers the XENON1T, a 1,300kg vessel of ultra-pure liquid cosmic ray-protected xenon in a deep-water submersible cooler located 1,500m under the Gran Sasso Mountains in Italy. Researchers search for dark matter by recording tiny flashes of light that arise when particles interact with xenon inside the detector. And while XENON1T is designed 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 xenon.
Many elements undergo a type of decay called electron capture, in which an electron combines with a proton in the nucleus, producing a neutron. But for a xenon atom, two protons each have to absorb an electron almost simultaneously to turn into two neutrons, an event called “double electron capture”.
The double electron capture only occurs when two electrons are next to the nucleus at just the right time, Brown said, “a rare thing that doubles into something else rare, making it very rare.”
When an extremely rare, double electron capture occurred inside the detector, the devices picked up the signal of electrons in the atom to rearrange them to fill the two that were absorbed into the nucleus.
Electrons in double capture
“The electrons in the double capture are removed from the inner shell surrounding the nucleus, and that creates space in that shell,” Brown said. “The remaining electrons collapse to the ground state, and we saw this breakdown process in our detector.”
When a near-impossible becomes possible
This achievement is the first time that scientists have measured the half-life of this isotope of xenon based on direct observation of its radioactive decay. With 6 x 10^27 xenon atoms in the container, each decaying with the same half-life of 1.8 x 10^22 years, a nearly impossible discovery becomes a possibility.
“This is a remarkable discovery that advances the frontiers of knowledge about the fundamental properties of matter,” he said. Kurt Brennemann Dean of the College of Science. “Dr. Brown’s work in calibrating the reagent and ensuring that the xenon was purified to the highest possible level of purity was critical to making this important observation.”
The XENON Collaboration group includes more than 160 scientists from Europe, the United States and the Middle East, and since 2002, has operated three of the most sensitive liquid xenon detectors in succession at the Gran Sasso National Laboratory in Italy. The XENON1T, the largest detector of its kind ever, obtained data from 2016 through December 2018, when it was shut down. Scientists are currently upgrading the experiment to the new XENONnT stage, which will feature an active detector mass three times greater than XENON1T. Combined with the low background level, this will enhance the sensitivity of the detector by an order of magnitude.
Image source: The core of the Abell 3827 galaxy cluster is visible at the top of the page, as dark matter lenses the background galaxies into arcs and reflective, distorted structures. (NASA/ESA/Richard Massey)
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Maxwell Mo, astrophysicist, NASA Einstein Fellow, University of Arizona. Max can be found two nights a week investigating the mysteries of the universe at Kate Peak National Observatory. Max received his Ph.D. in astronomy from Harvard University in 2015.