After visiting the point of origin of the MINOS neutrinos on our Fermilab tour at the beginning of the trip, it seemed a fitting conclusion to stop by their destination as my own road neared its end. So with Lizzie in Mexico, I made the Summer’s last science-related stop at the Soudan Mine with my friend Sam on our way back across the country.
As discussed in our previous post, the MINOS experiment uses a beam of neutrinos called NuMI (Neutrinos at the Main Injector) produced by decaying protons from Fermilab’s Main Injector. These neutrinos travel 450 miles through the earth to the 2341-foot deep Soudan Mine in northeast Minnesota (and beyond, of course), where physicists can isolate the Far Detector from just about any interference. Despite the fact that the detector is shaped like an enormous stop sign, only a tiny number of neutrinos obey the symbolic request: of all the trillions of neutrinos produced by NuMI, the Far Detector sees only about one a day.
A rich iron mine between 1884 and 1962, Soudan remains the oldest and deepest mine in the Minnesota. Now it’s a state park where visitors can experience the underground world of a miner and see the physics laboratory that has sprung up since the mine’s closing. The primary difficulty with visiting is that the mine is remote even by mid-American standards: only around 30,000 people live within 50 miles of the site, and even the physicists mostly run the detector remotely. Our route did take us through both of Bob Dylan’s hometowns (Duluth and Hibbing), though, so we got to drive down Highway 61 listening to Highway 61 Revisited.
In a sense, there isn’t anything more to the MINOS Far Detector than what Lizzie and I saw of the Near Detector at Fermilab: it’s an enormous horizontal pile of hundreds of octagonal metal plates. That said, there are nearly twice as many of them — 486 vs. 282 — and each one is considerably larger, so the Far Detector has a more pronounced way of looming over you.
Above all, the fact that the detector is housed in a mine causes complications. Everything required to build the 6,000-ton detector, including the enormous metal plates themselves, had to fit inside the existing elevator shaft: digging a huge 350-foot hole for a science experiment is one thing; digging a huge 2341-foot hole (and keeping it stable) is something else entirely. The Soudan detector’s plates are therefore made up of sections, each of which was lowered separately before being combined in the detector hall itself and moved into position by one of the physics world’s ubiquitous ceiling cranes. Amazingly, the job came in ahead of schedule and under budget, in part because local layoffs provided an eager and expert workforce.
The mine’s isolation from cosmic rays and temperature variations also makes it ideal for the Cryogenic Dark Matter Search II (CDMS II), the second experiment currently operating out of Soudan. We couldn’t get in to see the detectors so pictures are scant, but the lab focuses on the search for Weakly Interacting Massive Particles (WIMPs), which are even more difficult to detect than neutrinos and may play a significant role in Dark Matter. Since its detectors reach their highest sensitivity just a fraction of a degree above absolute zero, CDMS II is also home to the coldest place in the known universe.
Which brings us to the bats. They were everywhere, hanging upside down from walls and pieces of equipment, flying around our heads from the moment we stepped out of the elevator. I’d hazard a guess that there aren’t any other major experiments that let bats fly through their beamline, but that’s part of what makes mine-based neutrino work unique. If it turns out that bats have neutrino-damping properties no one is yet aware of, MINOS is taking a major hit. (Somehow, I don’t get the sense that anyone is worried.)
The bats also bring to mind an extracurricular visit of Lizzie’s and mine to the Museum of Jurassic Technology in Los Angeles. It is impossible to succinctly describe the museum to someone who has not been there, though founder and curator David Wilson (of no known relation to Fermilab’s Robert Wilson) does as good a job as anyone: “We’re a small natural history museum with an emphasis on curiosities and technological innovation… We’re definitely interested in presenting phenomena that other natural history museums are unwilling to present.” (quote taken from Mr. Wilson’s Cabinet of Wonder, Lawrence Weschler’s wonderful book about the museum). Which is to say, half of it, improbable or not, is absolutely real; the other half of it is utter nonsense, at least as far as the rest of reality is concerned. But it’s all presented with a deadly seriousness that both mocks and pays a deep homage to the objective, authoritarian tone of most museums.
My favorite exhibit was about a fictional species of bat that emits x-rays instead of sound waves, granting it the ability to fly through solid objects. As with the rest of the museum, there was an inordinately detailed account of its extraordinary discovery and scientific meaning. Supposedly frustrated by all attempts to capture the creature — how, after all, does one study something that can fly through any container? — a team of scientists led by the estimable but entirely fictional Donald R. Griffith (notably not the real bat zoologist Donald R. Griffin) constructed a detector in the South American rainforest out of five 20-foot-tall spokes of eight-inch-thick lead, each 200 feet long. After more than two months of waiting they finally managed to catch one bat, which they identified using a specially designed portable X-Ray viewer. A large slab of lead sits behind glass at the Museum — but naturally without an x-ray viewer with which one could actually see the bat that it claims to encase.
The notion is ludicrous, of course. Weschler describes how one skeptic calculated that “Dr. Griffith” would require on the order of 190,000 assistants to transport 9,154,000 pounds of lead into the rainforest to build the bat detector.
Obviously impossible. And yet as I stared at a battleship’s worth of steel, which had been carefully lodged 2,341 feet below the surface of northern Minnesota in pieces and assembled into 486 horizontally stacked sheets, all to detect one or two of the relevant particles a day, with bats fluttering all around it…
Real science has a way of being even more ludicrous than the wildest of imaginings. Which, presumably, is exactly David Wilson’s paradoxical point.