by Sharon ButlerIt's sort of like fitting a square peg into a round hole--only worse.
Physicists working on the MINOS (for Main Injector Neutrino Oscillation Search) experiment need to cart a giant 5,400-ton detector down into the bowels of the earth, into a mine dug a half mile below the surface in Soudan, Minnesota. The detector is the instrument they'll use to test whether any of the muon neutrinos in a beam from Fermilab have oscillated into tau or electron neutrinos. If the particles have morphed from one kind into another, then, drawing on the theory of quantum mechanics, physicists can safely deduce that neutrinos have mass.
The problem is: The only vehicle the physicists have for transporting the monster detector underground is an antique elevator shaft with two double-decker cages barely able to hold 10 people on each level. Little changed since the days when Soudan was a working iron ore mine decades ago, the aging elevators rattle and shake as they descend a pitch-black hole into the mine, which they share with a large bat population.
The detector can be broken into modular components, but even then the task of loading and unloading is a topological problem that defies mental picture.
That's why the MINOS scientists decided they needed a mockup of the shaft and the elevator cages, and a full-size prototype of the detector. The pieces are now nearly completed out in the New Muon Lab at Fermilab.
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Rob Plunkett, a Fermilab physicist and MINOS scientist, says the lowering of the detector into the cavern is complicated by a number of factors.
For one, no radical reconfigurations are allowed to accommodate the detector components because the Soudan mine is a National Historic Monument, overseen by the Minnesota Department of Natural Resources and managed by the University of Minnesota.
Luckily, the two elevator cages, which are raised and lowered by a pulley system, are not the original design anyway, says Bill Miller, who oversees the Soudan 2 laboratory currently in the mine. They no longer have "skips," the buckets with wheels used to lift the iron ore. And they now have two decks instead of one. Consequently, minor changes have been permitted. In particular, each cage will have one extra deck, giving it an extra 11 feet in length. When the floors of the decks are removed, the extra length will accommodate the 24-foot-long sheets of steel that make up one of the stop-sign-shaped plates of the detector. One six-ton plate is loaded into the cage at a time, disassembled into long rectangular sheets that look like bookcase shelving.
The MINOS collaboration has estimated that it will take about 1,000 trips to get the layers of steel down into the mine, plus another 200 for the scintillator, the fiber-optic cables that look like fluorescent fish line.
Those trips will take an estimated 22 months, in part because--a second complication--the mine is open to tourists from Memorial Day through Labor Day. The collaboration has agreed to use the elevators only at night during the summer tourist season.
Limited space down in the MINOS cavern will further complicate the operation. As the pieces of the detector are being lowered into the mine, workers will have to assemble the machine because there is little space for storage.
Assembly itself is difficult. The four slabs of steel that make up one plate have to be laid down on a frame known as a strongback. A second plate is welded to the first, and scintillator is attached. The entire layer--steel plus scintillator, like the noodles and tomato sauce of a lasagna dish--is then stood on end, with the strongback connected, and moved into place on rails. The strongback is required because the steel, only an inch thick, is not rigid. "It's like standing sheets of tissue paper on their edges," said Plunkett.
As the layers are assembled, they will each have to be tested because, once built, the detector cannot be taken apart.
Plunkett said that all these interlocking operations have required intricate choreography, detailing how many people and what equipment need to be where and when. Those procedures are still being refined to ensure that as little time as possible is lost. "If things get mixed up, the line stops," Plunkett said.
To assist in the choreography, the scientists are videotaping the loading, unloading, and assembly of the detector components. That will give them a better understanding of the time required for each step in the process.
Enactments have already helped. The loading of a piece of plywood in the shape of one of the steel slabs pointed out the need for another piece of equipment--a powered winch.
When the construction of the detector begins in earnest in the Soudan mine, The Minnesota Department of Natural Resources plans on offering a daily tour of the MINOS cavern, along with the usual tour of the old iron ore operations. Thousands of people will be able to watch, as, piece by piece, the MINOS physicists assemble their multiton detector for an era of new physics.
|last modified 9/17/1999 email Fermilab|