On October 21, crews at the Sanford Underground Research Facility (SURF) in South Dakota strapped the cryostat containing the central component of LUX-ZEPLIN (LZ), the largest direct-detection dark matter experiment in the US, below an elevator and slowly lowered it 4,850 feet down a shaft formerly used in gold-mining operations. This final journey of LZ’s central detector to its resting place in a custom-built research cavern required extensive planning and involved two test moves of a “dummy” detector to ensure its safe delivery.
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. These tubes are designed to capture flashes of light produced in particle interactions.
“This was the most challenging move of a detector system that I have ever done in decades of working on experiments,” said Jeff Cherwinka, the LZ chief engineer from the University of Wisconsin, who led the planning effort for the move along with SURF engineers and other support.
Jake Davis, a SURF mechanical engineer who worked on the cryostat move, said, “Between the size of the device, the confines of the space and the multiple groups involved in the move, the entire process required rigorous attention to both the design and the scheduling. Prior to rigging the detector under the cage, we did testing with other cranes to see how it would react when suspended. We also completed analysis and testing to ensure it would remain nice and straight in the shaft.”
He added, “The ride was slow, right around 100 feet per minute. The ride to the 4,850-foot level typically takes 13-15 minutes. Today, it took close to 45 minutes. I rode in the cage, watching it through an inspection port in the floor. There was a huge sigh of relief after the move, but there’s still a lot of work ahead to finish LZ.”
Theresa Fruth, a postdoctoral research fellow at University College London 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 cleanliness; we’ve been building this detector for 10 months in a clean room. Before the move, the detector was bagged twice and inserted in the transporter structure. Then, the transporter was wrapped with another layer of strong plastic. We also need to move all our equipment underground so we can do the rest of the installation work.”
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 -148 ˚F. Scientists hope to see telltale signals of dark matter particles that are produced as they interact with the heavy xenon atoms in this cryostat.
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, which makes it a good candidate for capturing particle interactions.
Pawel Majewski of the Rutherford Appleton Laboratory in the UK, who led the design, fabrication, cleaning and delivery of LZ’s inner cryostat vessel for the UK Science and Technology Facilities Council, said, “It is extremely gratifying to see it… holding the heart of the experiment and resting in its final place in the Davis campus, one mile underground.”
Dark matter makes up about 27% 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. LZ is designed to hunt for theorized dark space matter particles called WIMPs, or weakly interacting massive particles. It 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.