From Sanford Underground Research Facility: “LZ Time Projection Chamber assembly completed”

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From Sanford Underground Research Facility

August 2, 2019
Erin Broberg

Collaboration puts together the ‘heart’ of LUX-ZEPLIN dark matter detector.

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The recently assembled LUX-ZEPLIN xenon detector in the Surface Assembly Lab cleanroom at Sanford Underground Research Facility on July 26, 2019. Photo by Matthew Kapust Sanford Underground Research Facility.

On July 26, researchers working in the Surface Assembly Lab (SAL) at the Sanford Underground Research Facility (Sanford Lab) had quite an audience. Nearly a dozen onlookers, including researchers, technicians and one very curious writer, peered through two windows into the cleanroom. From this vantage point, they watched researchers carefully peel back a protective layer of foil to reveal—for perhaps the last time in half a decade—the innermost piece of the LUX-ZEPLIN (LZ) dark matter experiment.

What they revealed was LZ’s xenon detector, called a Time Projection Chamber, or TPC. Researchers recently completed the assembly of this impressive structure, a gleaming white column standing nearly nine-feet tall, that houses key components needed for LZ’s dark matter search.

“This xenon detector will be at the heart of the LZ dark matter experiment,” said Henrique Araújo, Imperial College London, who leads the LZ collaboration efforts in the UK and co-led the development of the TPC with Tom Shutt from SLAC National Accelerator Laboratory (SLAC). The U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) is leading the LZ project.

“The TPC is a complex system and it’s a major achievement to have it fully assembled,” Shutt said. “It takes us one important step closer to being able to look for dark matter. It is also gratifying because it involved assembling a large number of sub-systems designed and built by groups across the US and the UK over a number of years. So, it’s a coming together of sorts for the collaboration.”

While it was unwrapped, researchers in full-body cleanroom suits took final measurements and ran tests on the instrument, which will soon be sealed inside a cryostat vessel and transported to the 4850 Level of Sanford Lab. Once installed underground, the TPC will be hidden within layers of protective shielding until the experiment has finished taking data.

“We have some things in common with a space program,” said Araújo. “Before you launch, you do all of your work on the ground for years, perfecting the engineering so your instrument will work no matter what. LZ is a bit like a space experiment, just headed the opposite direction. We cannot expose it to underground air—that would compromise its performance. Once you deploy it underground, that’s it. It has to work.”

Piecing together the detector

The assembly of the TPC began in December 2018, when components first began arriving at Sanford Lab. Dozens of institutions across the globe had been fabricating components since 2015 or participating in the assembly.

“In creating these components, we paid a lot of attention to selecting and screening materials with low radioactive contamination and low radon emission to lessen any potential background interference within the detector,” said Tomasz Biesiadzinski, a project scientist with SLAC who has led the assembly effort at Sanford Lab. In all, tens of thousands of specially designed components were integrated into the detector.”

Since December 2018, the assembly team tallied 13,500 working hours at the SAL and drew from a broad reserve of expertise to properly address the mechanical, optical, electrical, cleanliness and background requirements of each component. With 250 members from 37 institutions around the globe and support from Sanford Lab’s support scientists and engineers, expertise covering all these areas was readily available.

“This type of experiment is still done the old-fashioned way—where the principal investigators, students, postdocs, engineers and technicians all work together to build it,” said Araújo. “The expertise that you need in order to assemble the experiment is so vast that you have to have a diverse group onsite. And working alongside people from these different backgrounds adds great joy to our time here.”

Cleanliness campaign

One researcher who contributed a substantial number of those hours was Nicolas Angelides, LZ collaboration member and graduate student at University College London, who presided over much of the cleanliness program for the TPC assembly.

“Dust particles can disrupt the detector signals,” said Angelides. “Dust also contains trace amounts of radioactivity, creating a background we need to control ahead of time.”

To protect against stray dust particles and radon—an atmospheric gas that could contaminate the detector—the entire assembly process took place within the Surface Assembly Lab, a laboratory space with a radon-reduction system and a class-100 clean room outfitted specifically for the TPC assembly. Within the clean space, strict cleanliness protocols are followed.

“All walls and floors are vacuumed and wiped down at least every week. Anything that can’t be wiped is put in an ultra-sonic bath, where sound waves are sent through a solvent to dislodge all small particles from every nook and cranny,” said Angelides.

High-efficiency air filters remove dust particles, some smaller than a single organic cell. If the air-particle concentration inside the room gets too high, an alarm will sound, alerting researchers to cover the detector. Because static electricity attracts dust, the assembly area is surrounded by neutralizing fans that quickly dissipate static charge. A total of twenty-six of these fans were pointed at the TPC alone.

Workers themselves pose a contamination risk to the experiment, as humans are a major source of dust. “We wear full-coverage cleanroom suits and follow a two-stage gowning procedure,” said Angelides. “Every step closer to entering the cleanroom is held to higher cleanliness standards and requires additional levels of gear. It takes a good quarter of an hour just to get to work!”

“What LZ has done more than any other project in the field is control the cleanliness of the materials and the assembly process,” said Araújo. “At the end of the day, nothing goes into the cleanroom or touches the detector that is not extremely clean.”

Generations of design

The design of LZ’s detector has been developed over decades of experimentation, including multiple iterations of the ZEPLIN program and the Large Underground Xenon (LUX) detector, from which LZ derives its name.

U Washington Lux Dark Matter 2 at SURF, Lead, SD, USA

The ZEPLIN program was the first to develop the liquid xenon TPC concept employed by LZ. In 2013, LUX had been declared the most sensitive dark matter detector in the world and retained that status until 2017—one year after it had been decommissioned.

“LZ sits on the shoulders of a number of smaller experiments,” said Araújo. “Each experiment solved their own issues at their own scale. By getting larger one step at a time, we have been able to search for new physics with ever larger experiments, and we are confident that LZ will work as it is designed to.”

Once underground, the detector will be cooled down and filled with ten tons of liquid xenon. This very dense liquid is an ideal medium for dark matter detection.

Researchers believe that if a dark matter particle interacts with a xenon atom, it will produce two flashes of light. The first flash occurs when the particle collides with the xenon atom; from this interaction some electrons are shaken off the xenon too. Then, guided by an imposed electric field, the electrons drift toward the top of the detector and are accelerated through a layer of gaseous xenon above the liquid, producing a second flash of light.

Although these flashes would be imperceptible to the human eye, the detector is lined with hundreds of photomultiplier tubes. These ultrasensitive sensors are capable of amplifying a signal from even a single photon of light.

“This TPC concept in which a single interaction produces two signals—the primary and secondary scintillations—is a powerful way to detect radiation,” Araújo said. “This is the technology that has been leading these dark matter searches because it allows us to say, with the precision of a few millimeters, where each interaction happens, and whether it is signal-like or background-like, which we can tell by the relative sizes of the two flashes of light.”

Direct detection of dark matter

Rigorous cleanliness standards, meticulous engineering and decades of experience all push LZ closer to its goal: detecting dark matter.

“A leading candidate for dark matter is the weakly interacting massive particle,” said Araújo. Different experiments world-wide are looking for this particle, endearingly nicknamed the WIMP (weakly interacting massive particle), within different regions of mass. LZ is designed to search for a particle within a mass region of a few protons to a few tens of thousands of protons.

“If there are particles in that mass range, we should have the world-leading sensitivity to spot them first,” said Araújo.

Major support for LZ comes from the DOE Office of Science, the South Dakota Science and Technology Authority, the UK’s Science & Technology Facilities Council and by collaboration members in South Korea and Portugal.

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Researchers peel back a protective layer of aluminum foil, revealing the recently assembled LUX-ZEPLIN xenon detector in the Surface Assembly Lab cleanroom at Sanford Underground Research Facility on July 26, 2019. Photo by Matthew Kapust, Sanford Underground Research Facility.

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Researchers examine the foil-wrapped LUX-ZEPLIN xenon detector that was recently assembled in the Surface Assembly Lab cleanroom at Sanford Underground Research Facility on July 26, 2019. Photo by Matthew Kapust, Sanford Underground Research Facility.

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The recently assembled LUX-ZEPLIN xenon detector stands nearly 9 feet tall in the Surface Assembly Lab cleanroom at Sanford Underground Research Facility on July 26, 2019. Photo by Nick Hubbard, Sanford Underground Research Facility.

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A researcher takes measurements of the recently assembled LUX-ZEPLIN xenon detector in the Surface Assembly Lab cleanroom at Sanford Underground Research Facility on July 26, 2019. Photo by Nick Hubbard, Sanford Underground Research Facility.

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A researcher snaps a photograph of the recently assembled LUX-ZEPLIN xenon detector in the Surface Assembly Lab cleanroom at Sanford Underground Research Facility on July 26, 2019. Photo by Nick Hubbard, Sanford Underground Research Facility.

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The recently assembled LUX-ZEPLIN xenon detector in the Surface Assembly Lab cleanroom at Sanford Underground Research Facility on July 26, 2019. Photo by Matthew Kapust, Sanford Underground Research Facility.

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A close-up of the top of the recently assembled LUX-ZEPLIN xenon detector in the Surface Assembly Lab cleanroom at Sanford Underground Research Facility on July 26, 2019. White PTFE reflective paneling lines much of the detector. From the outside, a viewer can see the stainless-steel outer rings of the electric grids, the back of the PMT array and some of the PMT cabling. Photo by Nick Hubbard, Sanford Underground Research Facility.

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Under ultraviolet light, research check for dust on the detector. Photo By: Nicolas Angelides, LZ Collaboration.

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About us.
The Sanford Underground Research Facility in Lead, South Dakota, advances our understanding of the universe by providing laboratory space deep underground, where sensitive physics experiments can be shielded from cosmic radiation. Researchers at the Sanford Lab explore some of the most challenging questions facing 21st century physics, such as the origin of matter, the nature of dark matter and the properties of neutrinos. The facility also hosts experiments in other disciplines—including geology, biology and engineering.

The Sanford Lab is located at the former Homestake gold mine, which was a physics landmark long before being converted into a dedicated science facility. Nuclear chemist Ray Davis earned a share of the Nobel Prize for Physics in 2002 for a solar neutrino experiment he installed 4,850 feet underground in the mine.

Homestake closed in 2003, but the company donated the property to South Dakota in 2006 for use as an underground laboratory. That same year, philanthropist T. Denny Sanford donated $70 million to the project. The South Dakota Legislature also created the South Dakota Science and Technology Authority to operate the lab. The state Legislature has committed more than $40 million in state funds to the project, and South Dakota also obtained a $10 million Community Development Block Grant to help rehabilitate the facility.

In 2007, after the National Science Foundation named Homestake as the preferred site for a proposed national Deep Underground Science and Engineering Laboratory (DUSEL), the South Dakota Science and Technology Authority (SDSTA) began reopening the former gold mine.

In December 2010, the National Science Board decided not to fund further design of DUSEL. However, in 2011 the Department of Energy, through the Lawrence Berkeley National Laboratory, agreed to support ongoing science operations at Sanford Lab, while investigating how to use the underground research facility for other longer-term experiments. The SDSTA, which owns Sanford Lab, continues to operate the facility under that agreement with Berkeley Lab.

The first two major physics experiments at the Sanford Lab are 4,850 feet underground in an area called the Davis Campus, named for the late Ray Davis. The Large Underground Xenon (LUX) experiment is housed in the same cavern excavated for Ray Davis’s experiment in the 1960s.

LBNL LZ project at SURF, Lead, SD, USA, will replace LUX at SURF

In October 2013, after an initial run of 80 days, LUX was determined to be the most sensitive detector yet to search for dark matter—a mysterious, yet-to-be-detected substance thought to be the most prevalent matter in the universe. The Majorana Demonstrator experiment, also on the 4850 Level, is searching for a rare phenomenon called “neutrinoless double-beta decay” that could reveal whether subatomic particles called neutrinos can be their own antiparticle. Detection of neutrinoless double-beta decay could help determine why matter prevailed over antimatter. The Majorana Demonstrator experiment is adjacent to the original Davis cavern.

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.”
We celebrate five years of LUX, and look into the steps being taken toward the much larger and far more sensitive experiment.

Another major experiment, the Long Baseline Neutrino Experiment (LBNE)—a collaboration with Fermi National Accelerator Laboratory (Fermilab) and Sanford Lab, is in the preliminary design stages. The project got a major boost last year when Congress approved and the president signed an Omnibus Appropriations bill that will fund LBNE operations through FY 2014. Called the “next frontier of particle physics,” LBNE will follow neutrinos as they travel 800 miles through the earth, from FermiLab in Batavia, Ill., to Sanford Lab.

FNAL LBNE/DUNE from FNAL to SURF, Lead, South Dakota, USA


LBNE

U Washington Majorana Demonstrator Experiment at SURF

The MAJORANA DEMONSTRATOR will contain 40 kg of germanium; up to 30 kg will be enriched to 86% in 76Ge. The DEMONSTRATOR will be deployed deep underground in an ultra-low-background shielded environment in the Sanford Underground Research Facility (SURF) in Lead, SD. The goal of the DEMONSTRATOR is to determine whether a future 1-tonne experiment can achieve a background goal of one count per tonne-year in a 4-keV region of interest around the 76Ge 0νββ Q-value at 2039 keV. MAJORANA plans to collaborate with GERDA for a future tonne-scale 76Ge 0νββ search.

CASPAR at SURF


CASPAR is a low-energy particle accelerator that allows researchers to study processes that take place inside collapsing stars.

The scientists are using space in the Sanford Underground Research Facility (SURF) in Lead, South Dakota, to work on a project called the Compact Accelerator System for Performing Astrophysical Research (CASPAR). CASPAR uses a low-energy particle accelerator that will allow researchers to mimic nuclear fusion reactions in stars. If successful, their findings could help complete our picture of how the elements in our universe are built. “Nuclear astrophysics is about what goes on inside the star, not outside of it,” said Dan Robertson, a Notre Dame assistant research professor of astrophysics working on CASPAR. “It is not observational, but experimental. The idea is to reproduce the stellar environment, to reproduce the reactions within a star.”