Tiny neutrino merits big project

Peter Kauffner

University researchers are designing a $50 million facility in northern Minnesota to answer one of the most vexing questions in physics — whether neutrinos have mass.
The neutrino is sometimes called the “nothing particle” because it is believed to have no mass and no electric charge.
It is a subatomic particle that so rarely interacts with anything else that one could pass through the earth billions of times before colliding with any other particle, said Jeffrey Nelson, a physics research associate who is helping design the detector.
This project, which is called the Main Injector Neutrino Oscillation Search, is being designed by more than 200 physicists from institutions based in the United States, China, Russia and Britain. It is financed primarily by a grant from the U.S. Department of Energy.
The neutrino detector is being built half a mile below the surface of the Soudan Underground Mine State Park, which is located on the Iron Range between the towns of Soudan and Tower.
As the oldest iron mine in Minnesota, the site is a popular tourist attraction. About 30,000 people from the United States and other countries take the underground tour of the mine annually, said Herbert Lamppa, mayor of Tower.
“They do stop in town; they do spend money, (the mine) does have an impact,” he said.
The detector must be placed deep underground in order to shield it from cosmic rays. A neutrino detector on the earth’s surface would be overwhelmed with extraneous data.
The new detector will be placed in an offshoot of the manufactured cavern that currently houses Soudan-2, a proton decay detector that has been in operation since 1989.
“Right now (the proposed cavern) is a hole in the ground about two inches in diameter–a core sample. Next year, we will get bids from contractors for excavation,” Nelson said.
When the detector becomes operational around 2001, the Fermilab National Particle Accelerator near Chicago will begin shooting neutrinos through the ground in the direction of the Soudan Mine.
If it is determined that the neutrinos detected in Soudan have “oscillated,” or changed in composition, during their 450 mile trip, it will mean that they have mass.
“If neutrinos can oscillate, they definitely have mass,” Nelson said.
About 10,000 tons of steel, roughly equivalent to the weight of a U.S. Navy frigate, will be used in the project. The detector will measure 164 feet by 26 feet.
“Basically (the Soudan Detector Hall) is going to be a sandwich of steel planes and detectors. It could be something like a thousand planes of steel and a thousand planes of detectors,” Nelson said.
If a neutrino collides with the nucleus of an atom in the detector, a shower of subatomic particles would be produced and these particles would leave traces in the steel.
“Something like 10 billion neutrinos have to go through (the detector) for one of them to interact,” Nelson said. The detector is expected to record about 12,000 neutrino interactions a year.
If researchers discover that neutrinos do have mass, it might help resolve several scientific mysteries.
One enigma is the question of “dark matter.” Some scientists estimate that 90 percent of the universe’s mass is currently undetected, because the way the universe is expanding can’t be accounted for with observable matter alone.
If each of the neutrinos, because of their large number in the universe, had only a tiny mass, the missing matter could be accounted for.