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Successfully lowering a Dark Matter experiment a mile underground

11 November 2019

Q: How do you get a 5,000-pound, 9-foot-tall particle detector, designed to hunt for dark matter, nearly a mile underground? A: Very carefully.

Crews at the 4,850-foot level of the Sanford Underground Research Laboratory lay down metal sheeting to prepare for the arrival of the LUX-ZEPLIN central detector. (Credit: Matthew Kapust/Sanford Underground Research Facility)

The largest direct-detection dark matter experiment in the U.S.A., and a project which involves numerous research and engineering teams from the UK, has reached its latest milestone when the crews at the Sanford Underground Research Facility (SURF) in South Dakota last week strapped the central component of LUX-ZEPLIN, (LZ) below an elevator and s-l-o-w-l-y lowered it 4,850 feet down a shaft formerly used in gold-mining operations.

Dr Pawel Majewski from the Particle Physics Department of the UK’s Science and Technology Facilities Council has led the design, fabrication, cleaning, and delivery of LZ’s inner cryostat vessel and said, “It is extremely gratifying to finally see the unit holding the heart of the LZ experiment at last resting in its final place in the Davis Campus, one mile underground. The cryostat itself is a feat of engineering and the UK team have had to meet some very stringent and challenging requirements in building it, most particularly in making it from ultra-radio-pure titanium because of the huge mass of the cryostat – 2,000kgs.”

LZ is designed to hunt for theorised dark matter particles called WIMPs, or weakly interacting massive particles. Dark matter makes up about 27 percent of the universe, though we don’t yet know what it’s made of and have only detected it through its gravitational effects on normal matter.

The LZ experiment is 100 times more sensitive than its predecessor experiment, called LUX, which operated in the same underground space. Placing LZ deep underground serves to shield it from much of the steady bombardment of particles that are present at the Earth’s surface.

Professor Cham Ghag (UCL Physics & Astronomy), University College London LZ collaboration scientist, said: “Understanding the nature of the elusive dark matter is recognised as one of the highest priorities in science and we are building the most sensitive machine yet to detect WIMPS, which are the leading theoretical candidate for a dark matter particle.

“If WIMPS exist, billions of particles pass through your hand every second but to directly hunt this mysterious particle, we have to bury our detector deep underground to shield it from all the other particles which steadily bombard Earth’s surface.”

Dr Theresa Fruth, a postdoctoral research fellow at UCL who works on LZ’s central detector, said that keeping LZ well-sealed from any contaminants during its journey was a high priority – even the slightest traces of dust and dirt could ultimately affect its measurements.

“From a science perspective, we wanted the detector to come down exactly as it was on the surface,” she said. “The structural integrity is incredibly important, but so is the cleanliness, because we've been building this detector for 10 months in a clean room. Before the move, the detector was bagged twice, then inserted in the transporter structure. Then, the transporter was wrapped with another layer of strong plastic. We also need to move all our equipment a mile below the surface so that we can do the rest of the installation work underground.”

The central detector, known as the LZ cryostat and time projection chamber, will ultimately be filled with 10 tons of liquid xenon that will be chilled to -148ºF. Scientists hope to see tell-tale signals of dark matter particles that are produced as they interact with the heavy xenon atoms in this cryostat.

(Credit: Sanford Underground Research Facility)

The liquid form of xenon, a very rare element, is so dense that a chunk of granite can float atop its surface. It is this density, owing to the heavy atomic weight of xenon, that makes it such a good candidate for capturing particle interactions.

The cryostat is a large tank, assembled from ultrapure titanium, about 5 1/2 feet in diameter. It contains systems with a total of 625 photomultiplier tubes that are positioned at its top and bottom (see a related article). These tubes are designed to capture flashes of light produced in particle interactions.

LZ’s cryostat will be surrounded by a tank filled with a liquid known as a scintillator that will also be outfitted with an array of photomultiplier tubes and is designed to help weed out false signals from unwanted particle “noise.” And the cryostat and scintillator tank will be embedded within a large water tank that provides a further buffer layer from unwanted particle signals.

While LUX’s main detector was small enough to fit in the SURF elevator, LZ’s cryostat only narrowly fit in the elevator shaft.

It was first moved outside of a clean room at the surface level, picked up with a fork lift, and carried into position below the elevator cage. It was then attached to the underside of the cage with slings and straps, where it was slowly moved down to the level of the Davis Cavern, its final resting place.

Once detached from the elevator cage, it was moved using air skates on a temporarily assembled surface – akin to how an air hockey puck moves across the table’s surface. Because of the cryostat’s size, crews had to first temporarily remove underground duct work to allow the move.

Next steps for the experiment include having the cryostat wrapped with multiple layers of insulation, and a few other exterior components will be installed. Then it will get lowered into the outer cryostat vessel. It will then be a matter of months to hook up and check out all of the cables and make everything vacuum-tight. Most of the LZ work is now concentrated underground with multiple work shifts scheduled to complete LZ assembly and installation.

There are plans to begin testing the process of liquefying xenon gas for LZ in November using a mock cryostat, and to fill the actual cryostat with xenon in spring 2020. Project completion could come as soon as July 2020.

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