The hunt for dark matter has been fruitless so far, but scientists searching for the elusive particle have discovered another rarity: the radioactive decay of xenon-124.
Xenon is a pretty unremarkable element. It's an invisible, colorless, odorless, tasteless noble gas that doesn't react much with other chemicals.
One of its isotopes, xenon-124, has a half-life of a whopping 1.8 x 10 22 years. In other words, on average it would take about a trillion times the current age of the universe – 13.7 billion years – for half of the atoms in a xenon-124 sample to decay.
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Luckily, researchers working at the XENON1T detector, a 3,200 kilogram tank containing pure liquid xenon used to find dark matter*, didn't have to wait that long to observe the spectacle. "We actually saw this decay happen," said Ethan Brown, an assistant professor of physics from Rensselaer Polytechnic Institute, this week.
The XENON1T detector has a total mass of 3,500kg, 3,200kg of which is liquid xenon at -95 degrees Celsius, and is buried 1,500 metres beneath the Gran Sasso mountains in Italy.
Brown noted: "It's the longest, slowest process that has ever been directly observed, and our dark matter detector was sensitive enough to measure it." The results have been published in Nature.
The researcher called it "the rarest thing ever recorded". An atom of xenon-124 has to undergo a process known as "double-electron capture" to decay into an atom of tellurium-124. Two protons have to capture two electrons to convert into a neutron. It requires not one but two electrons that have to be in the right location – by the nucleus – at the right time.
'Vanishingly small number'
"The chances of observing a single atom decay are vanishingly small, about 1 in 1022 over a year of watching," Brown told The Register. "We get around this by having an enormous number of atoms, then that vanishingly small number gets multiplied by a huge number to let us see a handful of events over about a year of looking."
Brown estimates there are over a septillion xenon-124 atoms – that's 1.6 x 1024 for you nerds – spread out in the tank. (Don't forget: 124Xe composes about 0.1 per cent of naturally occurring xenon.) The detector looks out for characteristic flashes of light released from the telltale signs of a dark matter particle interacting with a xenon atom.
These flares are also produced when xenon-124 decays. As a xenon-124 atom decays into tellurium-124, the two electrons are removed from its innermost shell creating a gap. As the rest of the electrons reshuffle around the tellurium-124 nucleus, the atom emits x-rays and electrons that can be detected.
It's the first time that the decay has been identified and observed, previous experiments did not have the same level of sensitivity to measure such interactions. Although the researchers haven't found what they're looking for, the xenon-124 decay is still useful, Brown insists.
"This is an immensely important step in rare event physics like dark matter detection. This shows just how well we are able to operate this largest liquid xenon experiment ever run, and demonstrates how confident we are in the data coming out of our measurements. This is a huge step in observing rare phenomena, and serves as validation of all of the measurements from the XENON collaboration in the search for dark matter." ®