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Soudan: Anatomy of an underground laboratory

By Bill Harlan, Journal staff

SOUDAN, Minn. - International Falls often wins the title of coldest place in the United States, but the coldest place in the entire universe is a hundred miles east, in the tiny north woods community of Soudan.

The ultra-cold spot - the size of a breadbox - is actually 2,341 feet underground, in the University of Minnesota's Soudan Underground Laboratory. Cryogenically super-cooled crystals in this experiment could make the first observation of a mysterious substance called "dark matter."

"This would be a very, very big deal," University of Minnesota physicist Long Duong said recently, during a break from searching for the cause of a temporary glitch in the sensitive detector. "Headlines all over the world."

Underground laboratories shield sensitive experiments from background cosmic radiation. Super-sensitive detectors like the dark-matter experiment at Soudan are unlocking some of the best-kept secrets of the universe.

The Soudan Underground Laboratory is one of four sites proposed for a larger, deeper national underground laboratory.

As the only working lab among the four candidates, Soudan offers a window into what the future might look like at Lead, S.D., if the National Science Foundation picks the Homestake gold mine for the nation's deep lab.

So let's descend to the coldest spot in the universe - but first, the lay of the land.
The north woods

On the surface, Soudan and Lead share some characteristics. Each town is surrounded by national forest. Each has a closed mine that was formerly the economic mainstay. Each community now relies mostly on tourism.

The big difference between Lead and Soudan is water. Even a wet year in the Black Hills is dry compared to the soggy, dense boreal forests of northeastern Minnesota.
The Soudan mine is 230 miles north of Minneapolis, at the southern edge of the Boundary Waters Canoe Area Wilderness - a million-acre tract whose 10,000 lakes represent nearly half a million acres of surface water. The wilderness area lies within the 3 million acres of the Superior National Forest - more than double the size of Black Hills National Forest.

Soudan also is in the heart of Minnesota's Iron Range, named for the iron ore that fueled the region's economy, but the Soudan Mine closed in 1962.
Today, maybe 30,000 to 40,000 people live within a 50-mile radius of Soudan, which is much smaller than Lead. (The only store in Soudan is called "The Only Store in Soudan.")
Into the iron mine

The Soudan Mine now is a state park, and the underground tours to the rough-hewn tunnels of the iron mine and the high-tech caverns of the lab attract about 5,000 visitors a year, hosted by rangers from the Minnesota Department of Natural Resources and local science teachers.

During the off season, the first noisy descent into the mine, in a metal cage, is at 7:30 a.m. Absent tourists, it's a dark ride. (Park rangers install a light bulb in the cage for summer visitors, and park rangers don't pack the paying visitors into the cage so tightly. Still, a few times a week, a claustrophobic tourist demands a quick return to the surface.)
The cage station at 2,341 feet looks and feels like a station in the upper levels of the Homestake mine. It is 50 degrees year round at this level. Bare light bulbs provide a minimum of light, and if you step off the cage and turn left, you'll find yourself in a pitch-dark, 10-foot by 10-foot tunnel heading into the old mine.

In the summer, park rangers load tourists on "man cars" hauled behind a small, yellow electric locomotive, then haul them three quarters of a mile into the mine to dimly illuminated stopes - mined-out caverns similar to Homestake's stopes.

Mannequins dressed as miners are positioned behind "jumbo" drills - again, similar to equipment at Homestake.

But one feature at Soudan is absent at Homestake. The black shapes that whoosh by tourists' heads in the Soudan stopes are not their imagination. About 5,000 bats winter in the mine - somehow finding their way to its deepest recesses, where the air remains mysteriously fresh without exhaust fans.

Into the lab
"We don't even know how the air gets in here," Bill Miller said.

Miller, 52, is manager of the science side of the old iron mine - Soudan Underground Laboratory.

The lab helped him realize a dream shared by many former Homestake miners - a high-tech job in science that allows him to live in an area he loves. "I always knew I was going to live in Ely and be a canoe guide," he said, but after he fell in love and started a family, he needed a real job.

Miller was working construction when he signed on as a laborer at Soudan 21 years ago to help build the lab. He had an aptitude for the work. Today, he not only manages Soudan, he's helping to plan another experiment, with a budget of a quarter of a billion dollars, at an even more remote site farther north.

Miller also serves as a spokesman for the lab, talking to civic groups and leading the occasional reporter on a tour.

Take a sharp right from the cage to a wide tunnel behind the shaft and you'll come to a giant set of metal doors that open onto Bill Miller's world.
The main cavern at the Soudan lab is 45 feet tall by 40 feet wide and 271 feet long.

The walls are lined with gray Shotcrete - a blow-on concrete that provides a uniform, popcorn texture to the walls.

Those little black splotches hanging on the wall are bats, but don't worry. They're dead, probably of dehydration. Despite the soggy terrain above, Soudan is a relatively dry mine and the lab is bone dry. Bats that venture into the lab don't last long. (By contrast, Homestake, in a dry climate, is a much wetter mine.)

The dominant image in the big main chamber is not science but art: a reddish, orange-ish Joseph Giannetti mural 25 feet tall and 60 feet wide that depicts great moments and people in particle physics - including the late Ray Davis, who won a Nobel Prize for his 30-year experiment at Homestake.

The neutrino target
As the eye travels left, down the length of the chamber, the huge octagonal end of the MINOS detector looms into view.

The Main Injector Neutrino Oscillation Search detector is the biggest current experiment at Soudan. This 6 kiloton behemoth features 486-inch-thick steel plates - the octagons - that sandwich plastic "scintillators" hooked to 186,000 separate fiber-optic data channels.

MINOS is the target for a beam of neutrinos fired from a 4,000-foot gun barrel that's 450 miles away, at the Fermi National Laboratory, just outside Chicago.

Neutrinos are chargeless subatomic particles created in stars like our own sun, which are so tiny they can zip through an entire planet without hitting anything.

The steel plates in MINOS increase the odds a neutrino passing through the detector will hit something and the effects of the collision will register on the scintillators.

Precise measurements of those aftereffects are adding weight to a discovery first detected by Davis at Homestake - that neutrinos, which come in three types, or "flavors," can change flavor in flight.

Flavor changing means neutrinos have mass, but physics' standard model of how the universe works originally called for massless neutrinos. MINOS and experiments in Japan and Canada are all confirming that Davis was right and the model was wrong.

WIMPs in the cold
Neutrinos, however, are no longer the most elusive particle in the universe, which brings us, at last, to the coldest spot in the universe.

That honor might go to "weakly interacting massive particles" or WIMPs - that is, if they exist at all. So far, they've never been detected.

WIMPs, Duong says, may constitute much of a mysterious substance called "dark matter." He's at a loss to describe the reaction to detecting WIMPs - except to predict headlines - but a Discover magazine story this month called such a discovery "a lock for a Nobel Prize."

Scientists posit dark matter because the way galaxies move suggests there's more gravity than can be accounted for in the observable universe.

Neutrinos move at near light speed, so their collisions, though rare, are dramatic. WIMPs, however, move relatively slowly, at just a thousandth of the speed of light, which means they need a detector in a place that's very quiet and very still.

That's where the CDMS 2 Detector comes in.

The Cryogenic Dark Matter Search is in a second large cavern at Soudan, originally excavated for a larger proton-decay experiment, now concluded.

CDMS 2 is much smaller - about the size of a decent commercial refrigerator. Visitors to the cramped "clean room" that houses the CDMS 2 wear the same white, paper outfits Duong wears. "We don't want any radon," said Jim Beaty, an engineer and a native "Iron Ranger" who provides technical assistance to Duong and other physicists.

Even the salt in perspiration contains trace radon, Beaty explains, so on go the gloves, hat and booties.

Underground science

Underground physics labs - whether 2,341 feet deep at Soudan or 7,400 feet deep at the proposed Homestake site - all share a common goal: quiet. Deep labs reduce the noise of the universe so researchers can detect the slightest of interactions.

Thus, another lab under construction at Soudan, the "low background counting facility," will have even more stringent clean-room protocols than CDMS 2. Scientists will use that lab to construct, monitor and calibrate ultra-sensitive detectors.

Even a lab like Soudan, with a small local footprint, is expensive - more than $100 million so far. A DUSEL likely would cost $300 million or more to construct, not counting experiments that would be added over the years.
Why spend that money?

Scientists theorize that WIMPs, neutrinos and other phenomena are related to the violent moments following the creation of the universe - the Big Bang. Underground science already is helping to reveal the inner workings of the universe.

Still, it's all very theoretical. Duong on his personal Web site offers a more philosophical basis for underground science. Duong quotes explorer Fridjtof Nansen, whose book, "Farthest North," described an 1893 attempt to reach the North Pole. Nansen wrote: "The history of the human race is a continual struggle from darkness toward light. It is therefore to no purpose to discuss the use of knowledge. Man wants to know, and when he ceases to do so, he is no longer man."

For a comparison of all four sites being considered, go to www.rapidcityjournal.com/features/snews/mines/.
For continuous updates and reports from reporter Bill Harlan about the selection process and his travels to the four sites, visit The Final Four of Physics blog at http://www.rapidcityjournal.com/blogs/mines/
Contact Bill Harlan at 394-8424 or bill.harlan@rapidcityjournal.com