The Soudan Mine opened 123 years ago
to unearth iron ore. Today physicists
have converted it into a lab where
they prospect for exotic particles.
On a gray midwinter dawn, a dozen scientists and technicians in hard hats gather outside the entrance to the Soudan Mine. Wearing heavy coats and clutching lunch bags, they step expectantly into the open cage of an 80-year-old hoist. An ancient engine chuffs as it lowers a steel gondola down the mine’s 2,341-foot-long shaft. The trip takes three minutes, but the passengers do not talk much. Instead, they listen idly to the icy air swooshing upward as the cage plunges into the guts of northeastern Minnesota’s Iron Range, away from the snowdrift-filled landscape and into a world of darkness.
The scene echoes an ordinary day at the mine more than a century ago. The Soudan Mine opened in 1884 atop one of the richest iron deposits in the world—hard hematite, about 65 percent pure. The ore quality kept Soudan going until 1962, when U.S. Steel decided that the mine had become too deep and hence too expensive to operate. New techniques made it cheaper to process lower-quality ore somewhere else. But the great depth that condemned the mine as a commercial venture has more recently given it new life as a research outpost, the Soudan Underground Laboratory. The half-mile layer of overlying rock creates a shield of peace and quiet unattainable anywhere on the surface of the earth. Physicists now burrow down deep here, oddly enough, to mine the heavens.
The gondola reaches level 27—the very bottom—and opens onto a state-of-the-art particle physics lab carved from two high-ceilinged caves. This is the home of the Cryogenic Dark Matter Search, inevitably abbreviated as CDMS.
The goal of CDMS is to hunt down another physics acronym: WIMPs, or weakly interacting massive particles. These enigmatic, hypothetical particles are the leading suspects in the search for dark matter, the unseen bits of whatever that are thought to make up the bulk of the matter in the universe. Ordinary atomic matter may seem like everything. After all, we are talking about all the stars as well as planets, comets, moons, the Crab nebula, black holes, brown dwarfs, the Pacific Ocean, you, me, cans of soup, and the family dog—all of it. Seven decades of astronomical research says that we are missing the big picture, however. There is four times as much dark matter as there is so-called ordinary matter, and we know hardly anything about it.
Today there is little doubt that dark matter exists. “It was very difficult at the beginning to imply that there were things out there that you couldn’t see,” says CDMS project coleader Bernard Sadoulet, an astrophysicist at the University of California at Berkeley. “Modern measurement techniques have authenticated the observations, but the problem is to detect whatever it is.” Whoever finds some is a virtual lock for the Nobel Prize. Actually uncovering a sample is . . . well . . . another matter. That is what draws researchers into the bowels of an abandoned Minnesota iron mine, working long shifts in what is surely one of the most tedious, yet potentially rewarding, jobs in all of physics.
A thick metal door leads to the CDMS lab, which is built into two connected caves carved out of solid rock, each about 100 yards long, 45 feet wide, and 45 feet high. The overall effect is James Bond modern—a high-tech workshop in the middle of nowhere, complete with exotic machinery and electronics, the brilliant light of scores of fluorescent lamps, and cranes in the ceiling to ferry heavy components from one place to another. The walls have been coated with sprayed concrete, making them look something like papier-mâché. The walls don’t sweat, though, and the air is dry enough to encourage chapped lips. There is no cafeteria, which explains the lunch bags: Everyone down here is here to stay until 3:30 p.m., when the first of two hoist trips takes people back to the surface.
The space may be cavernous, but by big-science standards, CDMS is a decidedly modest project. It has the participation of about 46 scientists from 12 universities and the U.S. Department of Energy’s Fermi National Accelerator Laboratory, or Fermilab. Project manager Dan Bauer, of Fermilab, says it cost about $30 million to set up and operate the project, bare-bones money compared with, say, the $8 billion that the Europeans are spending on their new particle accelerator, the Large Hadron Collider. The current version of CDMS went online last October and will continue quietly collecting data for the next two years.
The CDMS experiment is, expressed simply, nothing more than a cylindrical icebox covered with shielding and placed in a clean room to keep naturally occurring radioactivity from contaminating it. Hidden inside are the detectors, designed to track the particles that are invisible to us but which are meandering through every inch of the universe. The detectors, too, are unexpectedly simple things, disks of germanium about the size of hockey pucks that are placed together in five stacks. The guts of the experiment are no bigger than a piano stool. Padded with insulation, the whole dark-matter detector is about 10 feet high and looks like an outsize water heater.
