“What really impressed me was the trip down,” said astrophysicist James Buckley, PhD, speaking of the vertical mile he traveled to get to the site of an underground dark-matter experiment. “You can see you’re moving at a pretty good clip, which, by the way, is three times slower than the cage used to drop when it was a mine. It took us 10 minutes to get down a mile. You just watch the earth flashing by and every once in a while you go past a boarded up tunnel.”
The mine is the Homestake Mine, a played-out gold mine in Lead, S.D., that has been converted into a warren of underground chambers housing physics experiments that need to be shielded from cosmic radiation. One of these experiments is the Lux detector, designed to detect WIMPs (weakly interacting massive particles). WIMPs are hypothetical subatomic particles thought to make up dark matter in much the same way that electrons and quarks make up ordinary matter. But compared to other particles, WIMPs are elusive and interact only rarely with ordinary matter, and so far Lux hasn’t found any.
Buckley, a professor of physics in Arts & Sciences at Washington University in St. Louis, is a co-investigator on a team building a supersized version of Lux, called Lux-Zeplin, that will be roughly 100 times more sensitive to dark matter than its predecessor. Lux-Zeplin and two other dark-matter experiments survived a stringent “downselection” of competing dark-matter experiments and received Department of Energy and the National Science Foundation funding this July.
An enormous vat of xenon
The problem with finding WIMPs is not that they are rare, but that they rarely interact with ordinary matter. “A billion dark matter particles go through you every second, but you’d have to wait a thousand years for one of them to interact with you,” Buckley said. The detector is essentially a huge vat of liquid xenon, a meter and a half tall and a meter and a half across.” The idea is that rare collisions between WIMPs and the nuclei of the xenon atoms will leave detectable signatures in the form of sequential light signals.
Xenon is present in Earth’s atmosphere as a trace gas, but only at a level of about one part per 11.5 million. Usable amounts of xenon are produced as a byproduct of the separation of oxygen and nitrogen in liquid air. The liquid oxygen contains small quantities of krypton and xenon, which can in turn be separated and concentrated by further processing. Because it is rare, xenon is pricey. The new detector calls for a million liters of it, which Buckley calculates is 20 percent of the world’s annual production of xenon. In 1999 a liter of xenon cost $15, so the cost of filling Lux-Zeplin back then would have been $15 million. But the price has since spiked because of the popularity of xenon halogen lighting. The physicists hope LEDs will make sufficient inroads in the lighting market to bring it down again before they have to place their order Moe here Hunting for dark matter in a gold mine.