Field Notes - Get Neutrinos Here

Visitors to Soudan Underground Mine State Park now have a second tour available to them. In addition to seeing where iron miners toiled deep underground, visitors can also see where researchers are exploring the frontiers of science. The park offers regular tours of the high-energy physics lab that houses a recently completed neutrino detector operated by the University of Minnesota.

Physicists call neutrinos "the ghosts of the universe" because they have no electrical charge, seldom interact with other matter, and have little or no mass. But the subatomic particles are numerous. Trillions pass through our bodies on a sunny day.

"These particles are kind of a shadow universe," said university physics professor Marvin Marshak. "There are so many of them that, even if they have a very small mass, the total would be equal to all the visible matter we can see in the universe."

To better understand neutrinos, researchers designed MINOS-the Main Injector Neutrino Oscillation Search project. After construction at the Fermi National Accelerator Laboratory near Chicago is finished in 2005, the lab will begin shooting neutrinos 450 miles through the earth to the Soudan detector. Changes in the neutrinos between when they leave Fermi and when they arrive at Soudan will help researchers calculate the particles' mass. Because neutrinos are so numerous, even a small mass for each could have a large impact on helping scientists better understand how the universe works. More than 200 scientists from six countries are involved in the $181 million project, funded largely by the U.S. Department of Energy.

Work on the Soudan detector began 2,341 feet underground in the Soudan Underground Laboratory in 1999.

"We went deep underground so that the cosmic rays are filtered out, so we can see these slippery guys," said UM-Duluth assistant professor of physics Alec Habig.

The first step was to create a chamber 300 feet long, 50 feet wide, and 45 feet high in solid rock.

"There is a look of awe when people walk in just because of the size of the room," said assistant park manager Jim Essig. "They don't expect to see something that size underground."

It took a little longer than two years to excavate the chamber. It took another two years to assemble the 5,500-ton detector, made of 486 plates of inch-thick steel, each 27 feet high and wide. Between the plates are strips of light-sensitive plastic to detect the brief flashes of light created when neutrinos hit atoms in the steel.

The 100-foot-long detector was finished in August. Researchers will use it to study neutrinos found naturally in cosmic rays until the beam from Chicago is turned on. The detector will operate for at least three years past that.

"It wouldn't be unexpected if it kept going for longer," Habig said. "Something we weren't expecting will pop up, and we'll probably want to reconfigure it and run it longer to see what's really going on."

The detector is on the same level that the state park was already using for tours of the historic mine. The 75-minute tours of the physics lab, offered twice daily during the summer, have proven popular.

"We limit the tours to about 40 people a day, and we're filling those tours about three-quarters of the time," Essig said. "The lab's workers have done a really good job working with our staff to help them understand and be able to explain the science of high-energy physics to visitors."

Science isn't the lab's only attraction. Another is a 60-by-25-foot mural by Minnesota artist Joseph Giannetti. It has a burst of yellow symbolizing energy, and representations of atoms and a nebula, scientific equations, researchers, and the mine's entrance. The mural "puts a little different twist on the tour," Essig said. "It ties directly to the science and the history of the mine."

Soudan Underground Mine State Park offers morning and afternoon tours of the laboratory from Memorial Day weekend through Labor Day. For more information or to schedule a group tour, call 218-753-2245.

Steve Kuchera, freelance writer.