From Sanford Underground Research Facility: “Five years later, the hunt continues”

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Sanford Underground levels

From Sanford Underground Research Facility

October 29, 2018
Erin Broberg

Second-generation dark matter detector prepares to continue the search for WIMPs.

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The LZ cryostat undergoes leak tests in the Surface Lab cleanroom. Matthew Kapust

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.

Women in STEM – Vera Rubin
Fritz Zwicky discovered Dark Matter when observing the movement of the Coma Cluster

Coma cluster via NASA/ESA Hubble

But most of the real work was done by Vera Rubin

Fritz Zwicky from http:// palomarskies.blogspot.com


Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)


Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL)


Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970. https://home.dtm.ciw.edu

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.

U Washington Large Underground Xenon at SURF, Lead, SD, USA


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

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.

LBNL LZ project at SURF, Lead, SD, USA


LZ Dark Matter Experiment at SURF lab

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.

Reducing radon

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.”

Cryostat

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.

Acrylic tanks

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.

See the full article here .


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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.
LUX/Dark matter experiment at SURFLUX/Dark matter experiment 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.

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.

Fermilab LBNE
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.