The MINOS Experiment

Main Injector Neutrino Oscillation Search

The headframe raises and lowers the elevator that carries tourists, workers, materials and equipment down into the historic Soudan mine.

The MINOS experiment, using facilities built by the Neutrinos at the Main Injector (NuMI) Project at Fermi National Accelerator Laboratory, will seek evidence for an extremely small mass for the subatomic particles called neutrinos.

To detect such a small mass—if it exists at all—requires observing the neutrinos after they traverse a very long path. Such an experiment, which interposes a long distance between the source of the neutrinos and the neutrino detector, is called a long-baseline experiment. Physicists have long recognized the possibilities for new physics insights offered by long-baseline neutrino oscillation experiments. Finding a neutrino source of sufficient intensity proved difficult, however. Like the beams from flashlights, beams of neutrinos fade out as they travel long distances. However, the new Fermilab Main Injector accelerator, which began operating in 1999, has the capability to provide intense neutrino beams, even at the distance required for a long-baseline experiment.

The new Main Injector will serve as the neutrino source for the MINOS experiment. The experiment's long baseline will begin at Fermilab, 40 miles west of Chicago, and end deep in a former iron mine in Soudan, a village in northern Minnesota, where MINOS will place its neutrino detector—735 kilometers, or about 450 miles, from the source.

Detectors Near and Far

The Soudan 2 lab, located in the underground physics lab in the former iron mine.

When the neutrinos arrive from Fermilab (a trip that will take 0.0025 seconds) they will encounter a massive detector, 5400 metric tons of iron studded with plastic-strip particle detectors, 800 meters beneath the Minnesota woods. Because neutrinos interact so rarely with other particles, only huge masses of material offer the chance of creating—and observing—an occasional interaction as the beam of neutrinos sails through. The detector will measure interactions of the neutrinos from the Fermilab beam, detecting about 9,000 neutrino interactions each year, out of five trillion neutrinos that pass through. A sandwich of steel and plastic scintillator detectors will measure the energy produced in the particle interactions and record the momentum of outgoing muon particles by tracking their paths as they go through the magnetized steel plates.

MINOS experimenters will measure and compare the properties of neutrinos as they leave the Fermilab site, one kilometer from the source, and when they arrive at the detector in Soudan. A difference between characteristics of the neutrino interactions in these two detectors would provide evidence for oscillation between types of neutrinos, and hence of neutrino mass. MINOS experimenters will build the near detector as a small-scale replica of the far detector, to make the two as similar as possible.

Physicists will build the new MINOS far detector in the Soudan Mine underground physics laboratory. A cavern in the underground lab is already home to an existing particle detector built in the 1980s to search for proton decay. On July 20, 1999, collaborators broke ground for a second, larger cavern to house the MINOS detector. The total cost for the near and far MINOS detectors will be about $45 million, and MINOS collaborators hope to begin taking data in 2003.

The Soudan Historic Site

Existing 1,000-ton detector in the Soudan mine.

The Soudan Underground Physics Laboratory, future home of the MINOS detector, is located in the Soudan Underground Mine State Park, operated by the Minnesota Department of Natural Resources. As the first iron mine in Minnesota, the mine is a registered national historic site. Mining techniques at Soudan from the 1880s emulated those used in Cornish tin mines. Mine operators recruited immigrants from eastern Europe, including many Finns, to create a work force that peaked at about 5,000 in the 1890s. Miners continued to extract pure hematite from the mine until market forces brought operations to a close in 1962. Park staff now provide public tours underground for 30 to 40 thousand people annually. The tours will continue during MINOS construction operation.