Science Underground: Going to Great Depths

The Davis Campus of the Sanford Underground Research Facility -- the Halls of Ivy it’s not.

The word “campus” brings to mind neo-Gothic bell towers and green lawns, not tunnels and caverns almost a mile underground. But that’s what the Davis Campus at the Sanford Underground Research Facility (SURF) looks like, 4,850 feet down in the Homestake Mine in the Black Hills of South Dakota. Although SURF has weathered a series of funding ups and downs worthy of the rise and fall of the elevator cages that once took miners, and now scientists, to work in the mine, last year the U.S. Department of Energy successfully assumed sponsorship of SURF’s scientific program from the National Science Foundation. DOE will maintain the mine as a home for existing and proposed projects to investigate dark matter, the secrets of the elusive neutrino, and other research that can only be done deep down in the dark.

DOE’s Lawrence Berkeley National Laboratory (Berkeley Lab) is the Department’s lead institution for SURF, and Kevin Lesko of the Nuclear Science Division still wears the hard hat he put on many years ago when he first championed the Homestake Mine, then on the verge of closing down, as the ideal place to do neutrino science and other underground research. The official dedication of SURF’s Davis Campus will be held on May 30, 2012, when scores of dignitaries from the state and federal governments, along with visiting scientists and members of the press, will trudge the subterranean burrows to hear science lectures in echoing halls of rock.

The Campus is named for Raymond Davis Jr., who beginning in the 1960s did the seminal experiment that made neutrino science one of the most exciting scientific challenges of the 21st century. Davis was searching for neutrinos from the sun with an immense barrel filled with cleaning fluid. The Homestake Mine was still going strong when he started – it was the richest gold mine the country had ever seen, perhaps best known for making the Hearst family fortune – but after many years of checking and rechecking his results and his experimental design, Ray Davis had still found only about a third the number of solar neutrinos that theorists expected.

This eventually led to the idea that neutrinos must constantly oscillate among three “flavors” – electron neutrinos (the kind made in fusion reactions that fuel the sun), plus muon neutrinos and tau neutrinos, named for their association with the electron’s heavier cousins. Davis’s cleaning-fluid experiment was sensitive to only one, the electron neutrino. Presumably the other flavors were there too, the sun’s neutrinos having mixed and changed with them while on the eight-minute trip to Earth, but Davis’s experiment couldn’t see them. After 30 years the idea of neutrino oscillation was indeed confirmed, and in 2002 Davis was awarded the Nobel Prize in Physics.

Davis’s experiment was housed in a hall now refurbished for use by the LUX search for weakly interactive massive particles (WIMPs), leading contenders for still-mysterious dark matter. Right around the corner from LUX is the Majorana Demonstrator, an experiment a hundred times more sensitive to radioactivity and other background signals than any like it ever built, which is setting out to prove that the search for the never-seen phenomenon of neutrinoless double-beta decay – the tell-tale signal that neutrinos are their own antiparticles and that the textbooks will have to be rewritten – is at least possible.