Scientists have spotted the
"rarest event ever recorded", in a major breakthrough as part of
attempts to solve a dark matter mystery.
The process takes more than one trillion times the age of the universe.
And it has been spotted by scientists for the first time.
The breakthrough discovery
was made by scientists who worked with a special instrument, built to track
down dark matter, the most elusive particle in the universe. The all-important
event was the radioactive decay of xenon-124.
"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," said Ethan Brown, an assistant professor of physics at Rensselaer, and co-author of the study. "It's an amazing to have witnessed this process, and it says that our detector can measure the rarest thing ever recorded."
The research came from the
XENON Collaboration, which runs an instrument known as XENON1T. That is
a 1,300-kilogram vat fille dwith super-pure liquid xenon, which can be
shielded from cosmic rays by being buried in water, 1,500 meters beneath
the Gran Sasso Mountains in Italy. It is run by scientists
looking for dark matter, which is five times more abundant than ordinary
matter, but has never been directly observed. They hope to study it by watching
for the little flashes of light that are created when particles hit the xenon inside
the detector.
Though it is looking for the
interaction between dark matter particles and the nucleus inside the xenon
atoms, it can actually see any signals that happen inside the tub. It was one
of those interactions that scientists spotted, when they saw a proton inside
the nucleus of a xenon atom change into a neutron. That happens as a
consequence of a very rare event: when a proton absorbs two electrons, which is
called "double-electron capture".
That can only happen when
two of those electrons are right next to the nucleus, at exactly the right
time. That is "a rare thing multiplied by another rare thing, making it
ultra-rare", Brown said. Scientists spotted those electrons re-arranging
as they made way for the two electrons that had been absorbed into the nucleus.
"Electrons in double-capture are removed from the innermost shell around the nucleus, and that creates room in that shell," said Brown. "The remaining electrons collapse to the ground state, and we saw this collapse process in our detector."
The discovery allows
scientists to see the half-life of the xenon isotope directly, watching its
radioactive decay as it happens. Such an achievement has never been possible
before, the researchers said.
"This is a fascinating finding that advances the frontiers of knowledge about the most fundamental characteristics of matter," said Curt Breneman, dean of the School of Science. "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."
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