DEEP SCIENCE
Soudan laboratory hosts open house
Keith Vandervort
K. Vandervort
A group of visitors to the underground physics lab learn more about the research going on at the bottom of the Soudan Mine.

SOUDAN – Former miners, families, students, senior citizens and anyone looking for an adventure spent part of their day last Saturday in a physics laboratory more than 2,000 feet below the surface of the earth learning about the basic building blocks of the universe.

More than 200 people from around the area took part in an open house, which hasn’t been offered at the facility since 2010.

The Soudan Underground Laboratory is operated by the University of Minnesota in partnership with the Fermi National Accelerator Laboratory, the Minnesota Department of Natural Resources, and the CDMS and MINOS Collaborations.

The lab is located almost a half-mile underground and is designed to explore fundamental questions about the structure of the universe.

Tourists travel in darkness about 10 miles an hour in small cars or cages, which hold no more than six people, about 2,000 feet down at an angle which follows the rock formation to the lab entrance.

Steve Wilson, who has lived in Tower for more than 15 years, toured the lab for the first time. “It was weirdly fascinating on many levels,” he said. “Just think, the miners who went down there so long ago thought they were doing something worthwhile for our country. How could they have known what they dug out down there would someday be used to study the universe.”

As a student who admittedly struggled though high school physics a long time ago, Wilson said he “couldn’t imaging the complexity of the science that is done right here.”

Parts of the tour included several stations with scientists trying to explain in everyday terms what happens in the lab.

Chanda Welch of Tower took her six-year-old son, Hunter and his friend Kellin Pratt, 4, to see the lab.

“I felt in awe of what they do there,” she said “It was so amazing to have the opportunity to see that right here in Tower. To think they do that right here.”

She said her son was very impressed with the elevator ride down the mine shaft. “He also liked the (liquid nitrogen) experiment where he got to crush the frozen flowers,” she said. “And they got a souvenir for show-and-tell at school.”

The Soudan Underground Laboratory is a general-purpose science facility which provides the deep underground environment required for a variety of sensitive experiments.

The lab currently hosts two large projects: MINOS, which investigates elusive and poorly-understood particles called neutrinos; and CDMS, a “dark-matter” experiment which may help explain how galaxies are formed. Both labs were built for basic research—exploring how the universe works—but similar efforts have spawned practical (if unforeseen) byproducts, including advanced medical imaging techniques and even the worldwide web.

Most particle physics experiments are looking for extremely small particles that both exist naturally and are produced in high energy physics labs, for experimental purposes, around the globe. The sensitive detectors of the Soudan Underground Physics Lab study these particles.

Naturally-produced particles from around the galaxy constantly bombard the Earth’s surface. These particles, sometimes referred to as cosmic rays, would overload a detector if placed on the surface.

By the time these particles reach the Earth’s surface they have become mostly muons, electrons, positrons, and gamma rays. Most cosmic rays cannot travel very far through rock. If the detectors are located underground, most of the cosmic rays are blocked by the rock.

On the surface, a detector the size of the human hand would detect about two muons per second. Down in the lab, one-half mile underground, that same detector would detect only two muons per week.

In 1980, Marvin Marshak, a physicist from the University Of Minnesota, was looking for a location to set up an apparatus to detect the decay of protons, an event which no one had ever observed before (nor has anyone yet). The apparatus was designed to detect the particles that would be produced when a proton decays. These same particles are produced in other radioactive decay, including reactions occurring inside the sun.

These particles constantly shower the Earth’s surface, as a result the detector needed a place that was protected from these background particles.

Marshak took the Soudan Mine Historic tour and thought that the Soudan Mine would be the perfect place. It was deep, therefore, few background particles could reach down to the lab. It was safe because there was no blasting from an active iron mine. And yet, it still had a functional elevator and pump system because it was a historic site. 

In the 11 years the experiment ran about 500 of the atmospheric neutrinos that traveled the ½ mile through the Earth’s surface were observed in the Soudan Two detector. The neutrino data that were collected prompted further investigation of neutrino oscillation and helped pave the way for the MINOS experiment currently running in a neighboring cavern.

The MINOS cavern is 270 feet long, 50 feet wide, and 40 feet high. The CDMS II cavern is similar in shape but only 230 feet long. The surrounding rock

formation is not iron ore but Ely Greenstone, an especially strong rock that is about 2.7 billion years old.

Nearly 100,000 tons were excavated to build the lab—all hoisted to the surface, six tons at a time. Some was used beneath the parking area west of the Engine House, while the rest was piled on site.

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