From Sanford Underground Research Facility: “Success of experiment requires testing”

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Sanford Underground levels

From Sanford Underground Research Facility

November 13, 2018
Erin Broberg

Tomasz Biesiadzinski, project scientist for SLAC National Accelerator Laboratory (SLAC), works on the mock PMT [photomultiplier tubes] array. Erin Broberg

“The LZ detector is kind of like a spacecraft,” said Tomasz Biesiadzinski, project scientist for SLAC National Accelerator Laboratory (SLAC). “Repairing it after it’s installed would be very difficult, so we do everything we can to make sure it works correctly the first time.”

LZ Dark Matter Experiment at SURF lab

LBNL LZ project at SURF, Lead, SD, USA

Biesiadzinski himself is responsible for planning and carrying out tests during the assembly of time projection chamber (TPC), the main detector for LUX-ZEPLIN experiment (LZ). Currently being constructed on the 4850 Level at Sanford Underground Research Facility (Sanford Lab), this main detector consists of a large tank that will hold 7 tonnes of ultra-pure, cryogenic liquid xenon maintained at -100o C. All the pieces of this detector are designed to function with precision; it’s Biesiadzinski job to verify that each part continues to work correctly as they are integrated. That includes hundreds of photomultiplier tubes (PMT).

Test run

The most recent test was piecing together an intricate mock array for the PMTs, which will detect light signals created by the collision of a dark matter particle and a xenon atom, inside the main detector. In a soft-wall cleanroom in the Surface Laboratory at Sanford Lab, Biesiadzinski and his team carefully practiced placing instruments like thermometers, sensors and reflective covering. They practiced installing routing cabling, including PMT high voltage power cables, PMT signal cables and thermometer cables.

“Essentially, we wanted to gain experience so we could be faster during the actual assembly. The faster we work, the more we limit dust exposure and therefore potential backgrounds,” said Biesiadzinski. “It was also an opportunity to test fit real components. We did find that there were some very tight places that motivated us to slightly redesign some small parts to make assembly easier.”

These tests will make the installment of the actual LZ arrays much smoother.

“LZ’s main detector will have two PMT arrays, one on the top of the tank and one on the bottom,” Biesiadzinski explained. “The bottom array will hold 241 PMTs pointing up into the liquid Xenon volume of the main detector. The top array will hold PMTs 253 pointing down on the liquid Xenon and the gas layer above it in the main detector.”

In total, there will be 494 PMTs lining the main detector. If a WIMP streaks through the tank and strikes a xenon nucleus, two things will happen. First, the xenon will emit a flash of light. Then, it will release electrons, which drift in an electric field to the top of the tank, where they will produce a second flash of light. Hundreds of PMTs will be waiting to detect a characteristic combination of flashes from inside the tank—a WIMPs’ telltale signature.

“Both arrays—top and bottom—record the light from particle interactions inside the detector, including, hopefully, dark matter,” said Biesiadzinski. “This data allows us to estimate both the energy created and 3D location of the interaction.”

Catching light

The PMTs used for LZ are extremely sensitive. Not only can they distinguish individual photons of light arriving just a few tens of nanoseconds apart, they can also see the UV light produced by xenon that is far outside the human vision range. The X-Y location of events in the detector can be measured using the top PMT array to within a few millimeters for sufficiently energetic events.

To insure every bit of light makes its way to a PMT, the inside surfaces of the arrays are covered with Polytetrafluoroethylene (PTFE or teflon), a material highly reflective to xenon scintillation light, in between the PMT faces.

“This way, photons that don’t enter the PMTs right away—and are therefore not recorded—are reflected and will get a second, third, and so on, chance of being detected as they bounce around the detector,” said Biesiadzinski.

Researchers will also cover the outside of the bottom array, including all of the cables, with PTFE to maximize light collection there. Light recorded there by additional PMTs that are not part of the array, allow us to measure radioactive backgrounds that can contaminate the main detector.

Keeping it “clean”

In addition to being very specific, these PMTs are also ultra-clean.

“By clean, we mean radio-pure,” said Briana Mount, director of the BHUC, where 338 of LZ’s PMTs have already been tested for radio-purity.

The tiniest amounts of radioactive elements in the very materials used to construct LZ can also overwhelm the rare-event signal. Radioactive elements can be found in rocks, titanium—even human sweat. As these elements decay, they emit signals that quickly light up ultra-sensitive detectors. To lessen these misleading signatures, researchers assay, or test, their materials for radio-purity using low-background counters (LBCs).

“Our PMTs are special made to have very low radioactivity so as to not overwhelm a very sensitive detector like LZ with background signal,” said Biesiadzinski.

Testing the PMTs at the BHUC allows researchers to understand exactly how much of a remaining background they can expect to see from these materials during the experiment. Mount explained that most of the samples currently being assayed at the BHUC are LZ samples, including cable ties, wires, nuts and bolts.

“We have assayed every component that will make up LZ,” said Kevin Lesko, senior physicist at Lawrence Berkeley National Lab (Berkeley Lab) and a spokesperson for LZ. “At this point we have performed over 1300 assays with another 800 assays planned. These have kept BHUC and the UK’s Boulby LBCs fully occupied for approximately 4 years. These assays permit us ensure no component contributes a major background to the detector and also allows us to assemble a model of the backgrounds for the entire detector before we turn on a single PMT.”

For a visual description and breakdown of LZ’s design, watch this video created by SLAC.

Fritz Zwicky discovered Dark Matter when observing the movement of the Coma Cluster.

Fritz Zwicky from http://

Coma cluster via NASA/ESA Hubble

But most of the real work was done by Vera Rubin a Woman in STEM

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.

See the full article here .

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Stem Education Coalition

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.

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.

Fermilab 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.

LBNL LZ project at SURF, Lead, SD, USA