Digging deep for tiny particles

Than Tibbetts

Half a mile underground in northeast Minnesota, a team of scientists and engineers is working to unravel one of the universe’s tiniest mysteries.

The Main Injector Neutrino Oscillation Search team is looking at a beam of neutrinos – tiny particles similar to electrons but without electric charges – being fired from the Fermi National Accelerator Laboratory in Illinois, 40 miles west of Chicago.

To do that, the team built a 6,000-ton detector 2,341 feet below the surface of an underground iron mine in Soudan, Minn. The team detected its first neutrinos March 19.

Physics professor Marvin Marshak, one of six University faculty members working on the project, said the goal is to determine the mass of neutrinos.

“If they have mass, then they have a component of the dark matter in the universe,” he said. “We are very interested in dark matter in terms of the formation and expansion of the universe.”

Scientists still don’t know what dark matter is, but they know some type of unseen matter is exerting gravitational force.

Neutrinos rarely interact with any type of matter, and most of the particles shot from the national laboratory’s accelerator will fly through 450 miles of solid rock, past the Soudan detector and into space.

Neutrino sources vary, Marshak said. Some were created in the big bang, some are produced by stars and some from interactions between cosmic rays and the Earth’s atmosphere.

Much to the scientists’ excitement, Soudan lab manager Bill Miller said, they have detected three neutrinos in the last month.

Miller said that the project hopes to “catch” one neutrino every four hours when the national laboratory’s accelerator is running at full power.

The massive Soudan detector, made of layers of steel and special plastic 26 feet wide, had to be brought down from the surface piece by piece in a narrow mine shaft elevator.

Similarly, the 40-foot-high cavern that houses the detector required 100,000 tons of rock be excavated and lifted to the surface, 6 tons per trip.

The half mile of solid rock above the detector shields it from cosmic rays, according to the University’s high energy physics group Web site.

The $174 million project will collect data for a year or two before scientists can draw any conclusions, Miller said.

Spokesman Kurt Riesselmann said the national laboratory has a smaller version of the detector at the head of the beam.

This detector is used to compare the results against the Soudan detector, he said.

Marshak said that, unfortunately for the project, the national laboratory has had to shut down the neutrino beam because of problems with one of those targets.

“That’s one of the problems doing research,” he said. “Neutrino targets are – well, you don’t buy them at Wal-Mart. It takes time to figure out what’s wrong.”

Riesselmann said the team has also proposed a second neutrino detector in northern Minnesota. The detector would be located along the Ash River Trail, just south of Voyager’s National Park.