29 October 2019
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. The detector was moved through the passageway via air skates to smooth its journey.
(Credit: Matthew Kapust/Sanford Underground Research Facility)
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.
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 theorized 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 recognized 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.”
Jack Bargemann, Simon Fiorucci, Alvine Kamaha, Charles Maupin, Jake Davis, Jeff Cherwinka, STFC’s Pawel Majewski, and Doug Tiedt, welcome the arrival of the LUX-ZEPLIN central detector to the 4,850-foot level at the Sanford Underground Research Laboratory. The detector, which is lying on its side, will ultimately be surrounded by several other tanks.
(Credit: Matthew Kapust/Sanford Underground Research Facility)
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 minus 148 degrees Fahrenheit. 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, 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.
The LZ collaboration now has over 200 participating scientists and engineers who represent 38 institutions around the globe – with UK scientists, supported by the STFC, representing about a quarter of the collaboration. The cryostat is not the only contribution the UK is making to LZ – although it is the largest. In fact, STFC on behalf of the UK is providing £4.5million worth of funding to the project over 5 years, also including calibration delivery, photomultiplier tubes and internal monitoring sensors.
The UK contribution includes the universities of Bristol, Edinburgh, Liverpool, Oxford, Sheffield, Imperial College London, Royal Holloway, University College London and a team at STFC’s Rutherford Appleton Laboratory.
Between the size of the device, the confines of the space, and the multiple groups involved in the move of the cryostat, the entire moving process required rigorous attention to both the design and the scheduling. Prior to rigging the detector under the cage, testing took place with other cranes to see how it would react when suspended. Analysis and testing was also completed to ensure it would remain steady and straight in the shaft. The ride lowering the cryostat to the depth of 4850 feet was slow, approx. 100 feet per minute and took 45 minutes compared to the normal lift speed of 13-15 minutes.
More information about dark matter.
More information about the LZ and the LZ collaboration.
More information about SURF.
Founded in 1931 on the belief that the biggest scientific challenges are best addressed by teams, Lawrence Berkeley National Laboratory and its scientists have been recognized with 13 Nobel Prizes. Today, Berkeley Lab researchers develop sustainable energy and environmental solutions, create useful new materials, advance the frontiers of computing, and probe the mysteries of life, matter, and the universe. Scientists from around the world rely on the Lab’s facilities for their own discovery science. Berkeley Lab is a multiprogram national laboratory, managed by the University of California for the U.S. Department of Energy’s Office of Science.
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The Sanford Underground Research Facility’s mission is to enable compelling underground, interdisciplinary research in a safe work environment and to inspire our next generation through science, technology, engineering, and math education. For more information, please visit the Sanford Lab website.
Last updated: 05 November 2019