Davis Campus ready for science
The floors are steel grids or ultra-smooth surfaces of burnt-orange epoxy-resin over concrete. The ceilings are acoustic tile or overhead conduits packaged in shiny foil. The walls are ho-hum cinder block—except when they suddenly bulge into Gaudiesque undulating freeforms, the shotcreted natural rock face intruding into the architectural tedium. It's the Davis Campus on the 4,850-foot level of the Sanford Underground Research Facility in what was formerly the Homestake gold mine in Lead, S.D. The mine has been repurposed for science, and the Davis Campus was officially opened during a dedication ceremony on May 30, 2012.
Deep underground is the only place to do the kind of leading-edge physics experiments that will soon be under way in these laboratories. The Large Underground Xenon experiment, or LUX, will search for dark matter. The Majorana Demonstrator is the next step in an exhaustive search to show that subatomic particles called "neutrinos" are their own antiparticles. Majorana will do this by detecting something never before observed: neutrinoless double-beta decay. Both LUX and Majorana require the absolute minimum of background interference. The Davis Campus, nearly a mile underground, is more than a million times quieter than a surface facility.
“The 4,850 feet of rock screens out most cosmic rays, and the surrounding rock is lower in radioactivity by a factor of 10 or more than other underground locations, including even those deeper than the Davis Campus,” says Kevin Lesko of Lawrence Berkeley National Laboratory’s Nuclear Science Division. Lesko is Principal Investigator for the Sanford Lab, whose operations are funded by DOE through Berkeley Lab.
Majorana Demonstrator’s goal is to prove that background noise at the Davis Campus is indeed “quiet” enough to be worth the expense of searching here for neutrinoless double-beta decay, a process with an estimated half-life longer than a trillion times the age of the universe (if it happens at all). LUX too, a search for weakly interacting massive particles (WIMPs), is not only the most sensitive search yet, it’s a precursor to a bigger detector to be placed in the same spot, if it’s quiet enough.
Quiet was the reason Ray Davis came from Brookhaven Lab in the mid-1960s, while Homestake was still a working gold mine, to look for neutrinos from the sun. He found only a third the number he expected, and the result was modern neutrino science, including the revelation that neutrinos have mass and come in flavors that oscillate among themselves. Three decades later he was recognized by a Nobel Prize.
Davis died in 2006. His widow, Anna, was on hand with their son and daughter-in-law to help dedicate the Davis Campus, bringing along an official replica of his Nobel Prize medal. (The original is in a bank vault.) Sanford Lab officials presented her with a fragment of the Davis neutrino experiment, which was torn down to prepare the cavern for LUX. The 25-ton water tank that will shield LUX now stands in the same space excavated for Davis.
The official opening of the underground campus thus covered a wide spectrum of significance, with different meanings for different visitors. Notables included South Dakota Gov. Dennis Daugaard, former Gov. Mike Rounds, philanthropist T. Dennis Sanford of the eponymous Sanford Laboratory, and Ron Wheeler, Executive Director of the South Dakota Science and Technology Authority, the lab’s operator; the Department of Energy was represented by James Siegrist, Associate Director of Science for High Energy Physics in the Office of Science, and by James Symons, Berkeley Lab’s Associate Director of General Sciences, as well as Kevin Lesko and many other scientists from the national laboratories, including Fermilab’s Deputy Director Young-Kee Kim.
For the state, the dedication was a vindication that more than $40 million of public investment and $70 million in private support, not to mention the virtual gift of the Homestake mine itself, would not be in vain. The National Science Foundations in December 2010 decided to stop funding what was to have been the Deep Underground Science and Engineering Laboratory (DUSEL). DOE, until then a junior partner in DUSEL planning but the sponsor of major experiments to come, found itself facing an important decision—whether to continue the project. In July 2011, DOE did decide to fund the continued operation of the Sanford Lab and its existing experiments, and to pursue options for future experiments. One of them, the Long Baseline Neutrino Experiment (LBNE), would fire neutrinos from Fermilab and catch (some of) them at Homestake. The DOE is reviewing LBNE in 2012, and NSF will continue to support individual experiments as well. One proposed experiments, Dual Ion Accelerators for Nuclear Astrophysics (DIANA), would study the nuclear reactions that fuel stars.
Those proposals, however, are for the future. Today researchers are about to start two experiments that could, even in the near term, help answer some of the most challenging questions facing physics in the 21st century.
When asked what would happen if Majorana finds nothing at all, James Siegrist said, “If there’s no observation, it tells us that neutrinoless double-beta decay doesn’t occur within well-defined limits, so neutrinos are not Majorana particles, they’re Dirac particles. That’s a step forward.”
And if LUX finds no WIMPs? Tom Shutt, the LUX co-PI from Case Western Reserve University, gave a straightforward answer. “We know dark matter interacts gravitationally and doesn’t interact via electromagnetism or the strong nuclear force. So if doesn’t interact weakly either, we start looking for a fifth force in the universe.”
“There’s a result no matter what,” Siegrist said. “DOE is going to benefit greatly from this data, with a good look at the future of physics.”
Paul Preuss is a science writer at Berkeley Lab. Bill Harlan is Communications Director for the Sanford Lab.