By Karl Leif Bates
An international team of neutrino hunters, including Duke physicist Phil Barbeau, is publishing an update this week on the first two years of an ambitious experiment called EXO-200. (The cool name stands for 200-kilogram Enriched Xenon Observatory.)
What the team is after is “neutrinoless double-beta decay,” a particular reaction that’s expected once in 10 to the 25 years in a given molecule of xenon. At about a billion times longer than the age of the universe, that’s a little long to make the funding agencies wait for results, “so we use Avogadro’s number to fight that,” said Barbeau, who is an assistant professor of physics and member of the Triangle Universities Nuclear Lab.
EXO-200 is a 40-centimeter cylinder of very thin, almost balloon-like copper containing 200 kg of pure Xenon 136. That capsule is in turn surrounded by a vacuum, another thin copper shell, a foot-thick layer of lead, chilled to -100C and buried 2,150 feet underground in a New Mexico salt dome. They really don’t want any intrinsic radiation confusing things.
Unfortunately, the scientific team has been locked out of their underground lair for months by safety concerns following a radiation leak at the federal facility, so the experiment is on hold until things get cleaned up.
EXO-200 is so sensitive that even the super-tiny amount of radiation contained in a human fingerprint would throw it off, so all of this has to be super-cleanroom-clean too.
If the device can see a neutrinoless double-beta decay, it would strongly suggest the existence of a theoretical Dirac neutrino of unknown mass, AND that this particle exists simultaneously as a particle and its own anti-particle. And that, in turn, would poke another hole in the Standard Model of physics.
If this is all starting to sound a little crazy, just know that these little fellas are candidates for both dark matter and dark energy, the huge missing variable in the mass and energy of the entire universe. So finding them would be helpful. (Here’s a really good article (PDF) in Physics World by Stanford physicists.)
Xenon 136 isotopes occur naturally at about 20 percent of a population of xenon, so the scientists sent a $3 million ton of xenon to Russia to be spun in the sorts of industrial centrifuges that were formerly used to purify fission materials for nuclear bombs. Swords become plowshares, the Russians keep the lighter Xenon and re-sell it, and the scientists get their enriched 136 for the experiment.
EXO-200 is one of several competing experimental designs that are being used as a proof of concept for the Department of Energy. As physicists are wont to do, they’re drawing up plans to build a detector that is five to 25 times bigger, but first these teams have to figure out which approach works best.
Barbeau said this week’s update on EXO-200 is essentially that they haven’t seen anything so far, other than establishing that the detector is working really well and probably capable of catching the neutrinoless double-beta when it happens. “One thing that’s a constant with the neutrino is that it’s always surprising us,” Barbeau said.
The update appears as an advance online paper in Nature June 4.
EXO-200’s international team of physicists is supported by the Department of Energy and National Science Foundation in the United States, the NSERC in Canada, SNF in Switzerland, NRF in Korea, RFBR in Russia and DFG Cluster of Excellence “Universe” in Germany.