HIGH ENERGY PHYSICS
Underwater neutrino detector?
Flooded Iron Range mine pit to test next big idea
Marshall Helmberger
submitted photo
Researchers with the University of Minnesota sink a prototype neutrino detector in a flooded mine pit at the former LTV site earlier this month as part of a pilot project.

REGIONAL—When it comes to neutrino detectors, size matters. And that’s the idea behind a pilot project that is using an Iron Range mine pit to test whether giant underwater neutrino detectors can be the key to building these enormous devices at much lower cost.

“The advantage of a big detector is more interactions,” said University of Minnesota physicist Marvin Marshak, who directs the university’s underground research program.

The idea behind such a detector is pretty simple. Neutrinos are subatomic particles that are so small that they can, and regularly do, pass right through seemingly solid objects, including the Earth. Neutrinos can do this because atoms are composed of a small central core, surrounded mostly by space— space through which extremely tiny particles, like neutrinos, can pass without incident.

But every once in a great while, a neutrino will hit a solid atomic core, comprised of protons and neutrons, and will interact in a way that scientists can detect and study. To increase the odds of being able to witness such an event, scientists direct a beam of trillions of neutrinos at a massive object that’s wired up with sensitive light detectors, and then wait for impacts.

Not surprisingly, the more massive the object, the shorter the wait, so scientists have been gradually increasing the size of their detectors. But really, really big detectors come with an equally hefty price tag. The recently-completed 14,000-ton NOvA detector near Ash River, for example, cost well over $200 million.

While the NOvA detector is nearly three times the size of the MINOS detector located in Soudan, it’s still not big enough to obtain the volume of data that researchers would like. So far, the largest neutrino detector in the world is a 50,000-ton device in Japan, but scientists working with the University of Minnesota are now hoping to create a detector twice that size.

But getting there will take time, and plenty of preliminary planning and testing to prove their idea can work. The idea, developed over the past year, involves essentially encasing millions of gallons of pure water inside an underwater structure that’s rigged with light detectors. In July, a team of researchers from the U of M and University College London sunk a prototype version of the underwater detector to the bottom of a mine pit on the former LTV site. According to project manager Jerry Meier, the pit, known as 2W, was selected because it was deep enough— at least 200 feet— and because it was within the path of the Fermilab neutrino beam currently directed at Soudan.

Marshak said the team originally looked at Lake Superior, but determined that limited access, big waves, and other factors made it impractical. While the mine pit solved most of those challenges, it take approval from a number of different entities, including Cliffs Natural Resources, Mesabi Nugget, PolyMet, and RGGS, mineral rights trust. “It’s a pretty good collaborative effort,” said Meier.

The prototype version, which the team built at their Soudan surface headquarters, is pretty small, holding only about 50 tons of water. But it’s main objective is only to demonstrate that the concept can work. “If it works out well, then we go to the Department of Energy and the British Science Council to fund either a 10 or 25-kiloton version, which would be yet another preliminary step to engineering at 100-kiloton version.”

The device uses water from the mine pit, which is quite pure, but recirculating pumps and filters further improve the water’s quality inside the device.

The device is hooked electronically to a small surface building through an umbilical-like cord that provides power and allows computers to record any interactions between neutrinos and any atoms within all the water. The team has also sunk a number of environmental monitors, including a video camera, so they can keep an eye on any developments under the water.

Major cost reduction possible

If the concept proves workable, it could reduce the cost of building large neutrino detectors by more than a factor of ten. Currently, said Marshak, construction of a large detector costs about $15 million for every 1,000 tons. “We’re looking to get this below $1 million per 1,000 tons,” said Marshak. If so, that’s less than seven percent of the current cost of constructing such a device.

Researchers achieve such savings mostly by eliminating the need to purchase and install the material that creates the mass of the detector. In the case of the MINOS detector in the U of M’s underground lab at Soudan, steel plates were carefully lowered into the mine and erected to form the bulk of the detector. For the NOvA detector, scientists shipped in millions of gallons of mineral oil to provide that detector’s mass. But if their latest idea, first developed by a British colleague, Jenny Thomas, pans out, the team won’t have to haul, or even pay for, the raw material of their detector. “Essentially, we’re just instrumenting the water that’s already there,” said Marshak. Just based on cost, says Marshak, “if you want to go to really large detectors, then you’re pretty much restricted to a water detector.”

Window of opportunity

The research team is hoping to take advantage of the fact that the United States has only one operating neutrino beam, and it’s aimed at northeastern Minnesota. While Fermilab and a number of groups are working to develop a second beam, aimed at the Homestake Mine in South Dakota, Marshak said that alternative likely won’t be available for a number of years. “That beam is a $400-500 million project, so it won’t happen anytime soon,” said Marshak. “And that’s even if all the money was available, which it isn’t. So, it’s safe to say that for the next ten years or so, the only beams in the world will be the one in Japan, or the one aimed at Soudan.”

In other words, if it’s going to happen any time soon, it likely will happen here. “We can learn a lot before anyone else could even get to it,” said Meier.

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