Hunting for dark matter in old gold mineshafts

The Sanford Underground Research Facility in South Dakota used to be at the heart of gold mining in the United States. The mine has been repurposed as a world-class science facility in a bid to help researchers in a host of fields -including physics, biology, geology, engineering and education. Here’s Dr Jaret Heise, science director at the Sanford Underground Research Facility…

Jaret - I am sitting in Lead, South Dakota on the western edge of the state in the geographic centre of the United States, where starting in 1876, gold Mining was the name of the game. And for 125 years, that was the main industry in this town. In this area, over 40 million ounces of gold was extracted from this, the largest gold mine in the Western Hemisphere. And we have been fortunate to inherit this fabulous resource and are now dedicating it as a science laboratory for various science disciplines.

Chris - Is it a vertical shaft? What's the anatomy of the mine as it were?

Jaret - Yeah, the main access underground is via two main shafts. They're named for former mining superintendents. The Yates shaft and the Ross Shaft. Equally important is how we are able to ventilate the underground workings. And we have two principal ventilation shafts with large fans on the end to suck air down those two personnel shafts to bring, I don't know, half a million cubic feet per minute into the underground spaces.

Chris - And how deep is it?

Jaret - The facility extends all the way to about 2.4 kilometres or 8,000 feet. Right now, we are only able to access down to about 5,000 feet or a little over 1.5 kilometres with the Yates and raw shafts. To go to the deeper sections, you would need internal shafts, and some of the lower elevations of our facility continue to be underwater, and we don't have the shafts to the deeper areas refurbished to allow us to gain access to the 8,000 foot level.

Chris - Does it get hot down there?

Jaret - It does. On the 48, 50 foot level, which is our main level for physics and other disciplines, the rock temperature is around 30 degrees Celsius and all the way down at the bottom of the facility, 8,000 feet or 2.4 kilometres, the rock temperature is over 50 degrees Celsius.

Chris - So what attracted you there in the first place? Why would a bunch of physicists and geologists and biologists want to go down a massive deep hole?

Jaret - Lots of good reasons. I'm glad you asked. There are a number of unique attributes at our facility that attract researchers from a host of disciplines. For physics it's the fact that we are a deep site with a rock overburden that shields cosmic ray muons. On the surface of the Earth, if you hold up your hand, three cosmic ray muons are going through your hand every second. On the 48-50 foot level, or 1.4 kilometres below the surface, that flux that was three per second translates to one per month. And if you're a physics researcher exploring very rare processes, searching for dark matter, discovering properties of neutrinos, then that affords you a very quiet environment to do those studies.

Chris - Is that what you're looking for?

Jaret - Yeah. One of our premier physics experiments called the LUX-ZEPLIN Experiment is in fact searching for evidence of dark matter. They are using 10 tonnes of xenon looking for collisions with WIMP dark matter. Now a WIMP is a weakly interacting massive particle, a special category of candidates for dark matter in our universe. And the collaboration just earlier this week updated their results and demonstrated that they are once again the most sensitive experiment anywhere in the world looking for this type of dark matter.

Chris - What are they actually looking for there? What does the xenon do and how does it tell us where the dark matter's there?

Jaret - Sure. So in this quiet environment on a 48-50 foot level, they have essentially a thermos inside a large ultrapure water shielding tank to make a very quiet environment. They're looking for light signatures, energy deposition. As a WIMP particle interacts with the nucleus of the xenon atom producing light, and that signature of light can be distinguished from more natural and more well-known types of backgrounds like a beta decay or a gamma ray particle.

Chris - You mentioned that it's not just particle physics that's going on here, there's a lot of engineering type stuff happening as well, as well as geology and even biology. So what are those other projects that you've got going on down your mind then?

Jaret - Yeah, I'm glad you asked. We have in total about 30 different experiments taking advantage of our facility. Some involve large collaborations, mostly on the physics experiment side, but there are a number of smaller groups in the biology, geology, and engineering disciplines that are also taking advantage of SURF. One group was funded by NASA as part of their search for life on other planets. And so they were doing some research and development at our facility with instruments that were taken to Mars on the 2020 mission. So they were testing instruments that characterise the rock and could help analyse for signs of life on another planet, on Earth in our facility.

Chris - And Jaret, do any of your scientists ever slope off, and you notice them tapping on the wall with a pickaxe...

Jaret - Scientists certainly don't stray too far from the clean labs, because that's where we have the espresso and the wi-fi, which are crucial ingredients for science. But not only that, but it's well known that the areas that the science caverns are very far away from the mined areas where the gold was deposited. And today, those areas aren't very stable, because they didn't care to have those areas accessible for decades. They were just for long enough to get the gold out and move on to the next area.