Shh — don’t tell anyone that I’m online! I am writing from a secure, undisclosed location in northern Minnesota, where I am taking a vacation between the end of the ICHEP rush and the start of the fall semester. But longtime readers will remember my penchant for physics tourism, and it turns out that I am within an hour’s drive of the Soudan Underground Mine State Park. So I had to go visit!
The Soudan mine is the oldest iron mine in Minnesota, with amazingly pure ore deposits. But by the 1960’s, it wasn’t cost-effective to operate, and it was turned into an historic park. In the late 1970’s, particle physicists at the University of Minnesota, led by Marvin Marshak, realized that it would be a great place for physics experiment. With a half mile of rock and iron overhead, the mine would have a very low flux of cosmic rays, and thus there would be low backgrounds for searches for very rare processes. The hot thing to look for at that time was proton decay, which was predicted by some simple extensions to the standard model to be observable at reasonable rates. The iron in the mine was full of protons, so “all” that had to be done was to place a detector in the mine and watch and wait. The first Soudan experiment set a lower limit on the proton lifetime of more than 10^30 years.
Since then, other experiments have operated in the mine, and there are two there right now. The MINOS experiment is searching for neutrino oscillations. Fermilab produces a beam that is mostly muon neutrinos, which is directed 500 miles northwest towards Soudan. The MINOS far detector in the mine looks for neutrino interactions and sees how often the neutrinos observed are muon neutrinos, or some other flavor. Meanwhile, the CDMS experiment is looking for dark matter. If we live within a cloud of dark matter, then a dark-matter particle might interact with the CDMS detector, a germanium crystal that is kept at temperatures very near absolute zero. Such an interaction will excite phonon vibrations in the crystal, which can be detected.
Tours of the scientific facilities at Soudan are available twice a day during the summer, and it was encouraging to see how many people turned out for a physics lesson on the summer morning that I went. We all squeezed into a mine elevator (after it had been inspected for bats, who live in the mine), and headed downwards at a 78 degree angle to the lowest level of the mine; the trip takes about two and a half minutes. (I was expecting an open elevator car, but it was in fact totally enclosed, and thus less disconcerting than I had feared.) The mine is generally at a temperature of 50 F during all seasons, but the MINOS detector throws off enough heat from its electromagnet to make the experimental hall quite comfortable. Our tour guide was a high-school biology teacher from the area (I think Hibbing) who does this as a summer job; he said up front that he had to learn a lot to learn a lot to be able to give the tours. He did a fine job of explaining the physics behind the experiments. (I only caught one mistake, which was in a Fermilab-produced video that said that the Tevatron was the world’s highest-energy particle accelerator. True until last December.) CDMS requires a super-clean environment and thus it was off-limits to visitors, but we were able to get a good look at the MINOS detector, a long stack of iron plates instrumented with scintillating fibers.
I didn’t see any scientists on duty, but Fermilab is in the midst of a maintenance shutdown right now, and I also imagine that the the detector only operates with a skeleton crew anyway, as the site is quite remote, more than a four-hour drive from the Twin Cities. Any MINOS collaborators reading? How many of you have been up to Soudan for a visit? (And how many of you have a photo of yourself like the one of me below?)
So, dear readers, the next time you find yourself “up north” and want a break from the loons, be sure to stop by the mine to get a dose of particle physics!
