Five years ago, lead scientists for the Large Underground Xenon (LUX) experiment presented the first scientific results to come from the 4850 Level of Sanford Lab since Ray Davis’ Nobel-winning research in the 1960s. And the results were big.
After a run of just over three months operating a mile underground, LUX had proven itself the most sensitive dark matter detector in the world.
“LUX is blazing the path to illuminate the nature of dark matter,” said Brown University physicist Rick Gaitskell, co-spokesperson for LUX with physicist Dan McKinsey of Yale University, at the time.
Dark matter, so far observed only by its gravitational effects on galaxies and clusters of galaxies, is the predominant form of matter in the universe—making up more than 80 percent of all matter. Weakly interacting massive particles, or WIMPs—so-called because they rarely interact with ordinary matter except through gravity—are the leading theoretical candidates for dark matter. The mass of WIMPs is unknown, but theories and results from other experiments suggest a number of possibilities.
LUX’s mission was to scour the universe for WIMPs, vetoing all other signatures. It would continue to do just that for another three years before it was decommissioned in 2016.
In the midst of the excitement over first results, the LUX collaboration was already casting its gaze forward. Planning for a next-generation dark matter experiment at Sanford Lab was already under way. Named LUX-ZEPLIN (LZ), the next-generation experiment would increase the sensitivity of LUX 100 times. SLAC physicist Tom Shutt, a previous co-spokesperson for LUX, said one goal of the experiment was to figure out how to build an even larger detector.
“LZ will be a thousand times more sensitive than the LUX detector,” Shutt said. “It will just begin to see an irreducible background of neutrinos that may ultimately set the limit to our ability to measure dark matter."
This month, we celebrate five years of LUX, and look into the steps being taken toward the much larger and far more sensitive experiment. The following are just a few of the steps being taken by the LZ collaboration to make an experiment 30 times bigger and 100 times more sensitive—all in the pursuit of WIMPs.
Renovating the Davis Cavern
To make room for this scaled-up experiment, renovations had to occur inside the Davis Cavern.
“Planning for this renovation started several years ago—even before LUX was built,” said John Keefner, underground operations engineer. “We had to refit the cavern and existing infrastructure to allow for the installation of LZ.”
The Davis Cavern renovation project included removing an existing cleanroom, tearing down a wall between two former low-background counting rooms, installing a new hoist system, building a work deck and preparing the water tank itself to accommodate the larger cryostat.
In addition to hosting the experiment nearly a mile underground to escape cosmic radiation, additional protections had to be put in place, including a radon-reduction system that was installed to further ensure the experiment remains free of backgrounds that could interfere with the results.
Radon, a naturally occurring radioactive gas, significantly increases background noise in sensitive physics projects. The radon reduction system pressurizes, dehumidifies and cools air to minus 60 degrees Celsius before sending it through two columns, each filled with 1600 kg of activated charcoal, which remove the radon. The pressure is released, warmed and humidified before flowing into the cleanroom.
“Our detectors need very low levels of radon,” said Dr. Richard Schnee, who is head of the physics department at SD Mines and a collaborator with LZ. Schnee heads up the SD Mines team that designed a radon reduction system that will be used underground. While the radon levels at the 4850 Level are safe for humans, they are too high for sensitive experiments like LZ, which go deep underground to escape cosmic radiation, Schnee explained. “We will take regular air from the facility and the systems will reduce the levels by 1,000 times or more.”
The arrival of the LZ cryostats at Sanford Lab in May 2018 marked a significant milestone in the LZ project, as the cryostat was several years in the making and is a key component in the experiment.
The cryostat works in a similar way to a big thermos flask and keeps the detector at freezing temperatures. This is crucial because the detector uses xenon, which at room temperature is a gas. For the experiment to work, the xenon must be kept in a liquid state, which is only achievable at about minus 148 degrees Fahrenheit.
After being delivered to the surface facility at Sanford Lab, the outer cryostat vessel of the cryostat chamber spent five weeks being fully assembled and leak-checked in the Assembly Lab clean room. It has now been disassembled and packaged for transportation from the surface to the underground location on the 4850 Level. The inner cryostat vessel also passed its leak test.
Water tank passivation
To ensure unwanted particles are not misread as dark matter signals, LZ's liquid xenon chamber will be surrounded by another liquid-filled tank and a separate array of photomultiplier tubes that can measure other particles and largely veto false signals.
“The LUX water tank needed a number of ports added or modified to support the LZ infrastructure. We also added the capability to install small hoisting equipment on the ceiling of the tank,” said Simon Fiorucci, a physicist with Lawrence Berkeley National Laboratory, who oversaw LUX operations at Sanford Lab and will serve in a similar role for LZ.
Once these steps were completed, the entire inside of the tank had to be re-passivated to prevent rusting during its many years of service ahead. Finally, the tank was filled to the brim and monitored for a week to ensure there were no leaks.
Additionally, LZ will include a component not present in LUX—nine acrylic tanks, filled with a liquid scintillator, will form a veto system around the experiment, allowing researchers to better recognize a WIMP if they see one.
The acrylic tanks, or more precisely the liquid scintillator inside the tanks, are crucial in bringing the experiment to a new level of sensitivity—100 times greater than LUX—by identifying neutrons, which can mimic dark matter signals.
“Recent dark matter searches have found that neutrons can be a pernicious background,” said Carter Hall, former LZ spokesperson and professor of physics at the University of Maryland. “The acrylic tanks and their liquid scintillator payload will provide a powerful neutron rejection signal so LZ is not fooled.”
These are just a few of the many steps being taken to ensure that LZ once again scours the universe with pristine accuracy.
“We want to do again what we did five years ago—create the most sensitive dark matter detector in the world,” said Dr. Markus Horn, research scientist at Sanford Lab and a member of the LZ collaboration.