A mile underground, a historic gold mine turns to cutting-edge science

Why is the world around us made of solid matter when prevailing theories of physics predict an equal amount of antimatter? What can neutrinos, the mysterious particles that pass through our bodies in the trillions every second, tell us about the history and future of the universe? These questions lie at the heart of the planned Deep Underground Neutrino Experiment (DUNE), a major international collaboration hosted by Illinois physics research center Fermilab.

For the second post in a series exploring the project and the complex being built to house it (find the first here), we spoke with Elaine McCluskey, the Long-Baseline Neutrino Facility’s (LBNF) project manager.

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Can you give a quick overview of LBNF?

LBNF will eventually be part of the Sanford Underground Research Facility in Lead, South Dakota. One set of neutrino detectors will be located there, and another set will be here at Fermilab in Illinois, which will also host the beamline facility that makes the neutrinos. That will be the world’s largest, highest-intensity beamline, and it will send neutrinos 1,300 km [808 miles] through the earth.

What do you find most interesting and innovative about the project?

One of the project’s main challenges is to make sure that that beam of neutrinos is aimed at the right place. The alignment has to be very precise. We’ve successfully done this for another neutrino beam to Minnesota. We got it down to within half an inch, so we have high confidence that we can do this again.

Minnesota neutrino detector

Minnesota neutrino detector

Another important consideration is maximizing the amount of neutrinos available — the number of neutrinos is an important parameter. We’re building a very large facility here at Fermilab to make these neutrinos from protons, building off of the existing Fermilab accelerator complex. That process is something we understand very well, but we’re trying to find ways to make it more efficient in order to make more neutrinos.

Fermilab particle accelerator in Batavia, Illinois

Fermilab particle accelerator in Batavia, Illinois

And then building the facilities we need to support these very large far detectors in South Dakota is another challenge. Basically, we’re creating large caverns a mile underground to shield the experiment from cosmic rays.

Shielding experiments from cosmic rays

The detectors will be housed inside large refrigerated boxes of argon, which is kept at 87 kelvin [−303 degrees Fahrenheit]. These boxes are called cyrostats; they’re required to purify and recirculate the argon, which helps to make sure that we don’t get a lot of background noise in the experiment.

Neutrino event in liquid argon

Neutrino event in liquid argon

Our partner in Europe, CERN, has already committed to providing the first of these four very large cryostats that we need to house the detectors in South Dakota.

They’re working with a company that’s adapting a technology used to transport liquefied natural gas for scientific use. We’re testing the technology here at Fermilab, and at CERN they’re building two next-step larger detectors that will be used for a prototyping program. From there we’ll scale them up to these very large vessels that we need to build — each is 18 by 18 meters [59 feet] in cross section and about 65 meters [213 feet] long. The scaling up of that technology is one of the most innovative things we’re doing on this project.

The Sanford lab site was an old gold mine, so we are taking advantage of existing infrastructure. We will be rehabbing some of that infrastructure because it dates from the historic mining days.

LBNF caverns

LBNF caverns

Is that why this location was chosen? I’m assuming there aren’t that many existing sites a mile underground.

Well, it’s a bit of serendipity. When we first started to think about this experiment some years ago, we knew we needed a certain distance between the point where the neutrinos are created and the point where they’re detected. This is because when neutrinos travel, they change from one type of neutrino to another. Physicists can model how they’ll change as they move over particular distances, and for the type of science we want to do, a distance of 1,300 km was determined to be the sweet spot.

So because Fermilab is the only place in the United States where neutrinos are made, we looked for deep underground sites in a 1,000 to 1,500 km (621 to 932 miles) circumference around the Chicago area. There weren’t a lot of options.

Luckily, the Sanford lab was started around this time. The gold mine there was abandoned in the early 2000s, and the National Science Foundation saw it as a potential deep underground research lab. The state of South Dakota has been extremely supportive of the project. So we were able to build on work that had already been done — some physics experiments are already taking place at this mile-underground level today.

LBNF caverns

LBNF caverns

Can you tell me about some of the decisions that you could draw on precedent for and others that required you to start from scratch?

The fact that a federal laboratory is partnering with a state laboratory is unusual, whereas two federal laboratories collaborating would be very straightforward. We’ve had to create some new legal mechanisms for working together. The Sanford Underground Research Facility is owned by the State of South Dakota and run by an agency called the South Dakota Science and Technology Authority. Fermilab is partnering with that organization, and there will be a lease for that property in order to allow the Department of Energy to use federal dollars to build this facility there.

From an engineering perspective, the caverns we’re creating are not extraordinary in comparison to underground caverns like underground power stations or very large subway stations. The logistics challenges, however, are very real. The Sanford lab is in the Black Hills of South Dakota, where there is not a lot of flat land. The site is literally on top of two hills with a valley in between. There are two access shafts, one on top of each hill. There’s not a lot of flat space available to stage construction. So creating areas to manage the construction activities is one of the first logistical challenges we have to deal with. Everything is going in and out of one shaft for the construction work — it’s like having a single door on the side of the building.

Scheduling is also extremely complicated. The construction will overlap schedule-wise with the installation of these giant cryostats, the cryogenic systems, and the detectors. So we’re figuring out how to have all these activities go on at the same time while minimizing the disruption to the existing science experiments in the Sanford lab. We’re going to be doing blasting for probably at least three years, and we will have to work with our neighbors in order to plan that in cooperation with their experimental program.

 

Keep an eye on the site in the coming weeks for an interview with Arup’s project manager for the effort, Josh Yacknowitz.

 

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