From Sanford Underground Research Facility: “Life in the (low) background”

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From Sanford Underground Research Facility

March 15, 2019
Erin Broberg

Alan Smith set the gold standard for low background counting—a standard that guides the BLBF at Sanford Lab today.

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Alan Smith (right) explains data recorded at the Oroville facility, where the first-generation dark matter and neutrinoless double-beta decay experiment was housed. Seated next to Smith is Dave Martinez, key account manager for AMETEK ORTEC. (Circa 1994.) Photo courtesy of Lawrence Berkeley National Laboratory.

In 1953, Alan “Al” Smith arrived for his first day of work at Lawrence Berkeley National Laboratory (Berkeley Lab). Over the next seven decades, Smith advanced the science of low-background counting, working to push the capacity of detectors to see ever more minute signatures. He officially retired in 1994, but continued to come to the lab until 2018. By age of 92, Smith had created a “gold standard” of gamma-ray assay counting at Berkeley Lab and established a legacy within particle and nuclear physics research communities.

“Al’s career has always been in the background of nuclear science—literally,” said Keenan Thomas, Nuclear Counting Facility Manager at Lawrence Livermore National Laboratory, who was previously mentored by Smith at Berkeley Lab as part of the Berkeley Low Background Facility (BLBF) team.

Quieting the search

Rare-event searches, such as the Majorana Demonstrator’s search for neutrinoless double-beta decay or LUX-ZEPLIN’s (LZ) dark matter hunt, don’t just need to be shielded from cosmic rays—they also require some of the world’s cleanest materials. By “clean,” researchers mean radio-pure; they are looking for materials with the lowest concentrations of radioactive elements.

As radioactive elements such as uranium, thorium and potassium decay, they emit signals that quickly light up ultra-sensitive detectors and overwhelm rare-event signals. To lessen these misleading signatures, researchers use low-background counters (LBCs), which can detect even the tiniest amounts of radioactivity and assay all materials and components.

“Low-background counting is a tedious, yet critical, quality-control step toward success of rare-event experiments,” said Thomas. “Low-level assays are one of our last lines of defense to ensure that an experiment will be successful in achieving the sensitivity to the very weak signals it was designed for—and not be masked by spurious background signals generated from many different sources, including the materials in the detector itself.”

Going deeper for science

Smith’s career both paralleled and propelled advances in low-background counting. When Smith began his work in the 1950s, nuclear science was relatively new and booming, with many measurements yet to be made. The Bevatron particle accelerator came online at Berkeley Lab in 1954, and Smith was initially tasked with assessing the stray neutrons it produced.

LBNL Bevatron

To do this, he placed large aluminum discs called “activation foils” around the perimeter of the Bevatron to “capture” neutrons that scattered from the accelerator. By assaying these foils later with sodium iodide detectors, Smith determined the amount of neutrons produced using weak signals from the trace amounts of radioactivity generated in the foils.

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Smith places an aluminum disk sample exposed to radiation at the Bevatron into a lead and concrete box containing a detector at the low-level radiation counting facility, circa 1950s.
Photo courtesy of Lawrence Berkeley National Laboratory

Smith realized that the Bevatron itself created a dynamic background, which clouded the measurements. To reduce this noise, a low background “cave” was constructed using a single pour of concrete, with walls ranging from 4-6 feet thick. Smith and a graduate student, Harold Wollenberg, carefully counted dozens of samples of concrete source materials to find a supply with the lowest levels of naturally occurring radioactive elements. The dense walls reduced external backgrounds from the Bevatron, cosmic rays and natural radioactivity in the environment—all with the benefit of not emitting a significant amount of background themselves. This cave became the surface location for the Berkeley Low Background Facility (BLBF) and is still in use today.

In the 1980s, Smith became involved with the University of California-Santa Barbara and Berkeley Lab double-beta decay experiment, which used high-purity germanium detectors. Originally installed in the low-background cave at Berkeley Lab, researchers soon realized the backgrounds from cosmic rays were still too high. Smith turned to his professional connections at the underground Hyatt Power Plant at Oroville Dam, to secure a space for the experiment to relocate a few hundred feet below ground. The underground space supplied sufficient overburden for the experiment, and transitioned into a remote location that housed low-background counters for BLBF for decades.

In 2014, the BLBF at Oroville was relocated to the Black Hills State University Underground Campus (BHUC) on the 4850 Level of Sanford Lab. Before the move, Smith assayed hundreds of samples from the 4850 Level, including natural rocks, concrete, shotcrete, paint and other materials to determine what kinds of backgrounds existed. His attention to these samples from the underground construction of the Davis Campus created ample material-radiopurity data that researchers now use in simulations for Sanford Lab experiments. Once the transition to BHUC was completed in 2015, Smith and his team received and analyzed data online at Berkeley Lab.

Today, the BLBF performs low background counting in its two unique facilities—the surface site at Berkeley Lab and the underground site at Sanford Lab. Researchers assaying their materials from a distance can monitor results in real-time, while relying on daily support from BHSU faculty and students and Sanford Lab staff. Support includes changing samples in the detectors, monitoring the liquid nitrogen systems that purge radon from inside the detectors and assistance in the installation of detectors underground.

“The campus at Sanford Lab is an ideal location for these counters,” said Kevin Lesko, senior scientist at Berkeley Lab. “Not only does its depth create a shield for the detectors, but it’s in the thick of major physics experiments—it’s where the action is.”

Both facility locations owe much of their design and creation to Smith’s meticulous measurements.

Creating a “gold standard”

Throughout his career, Smith pushed the capabilities of low-background counting, striving to make more and more sensitive measurements. As technology advanced, his methods of detection graduated from sodium iodine (NaI), to germanium lithium-drifted (GeLi) and eventually to high-purity germanium detectors (HPGe). With each advancement, his methods became more sensitive and intrinsic backgrounds needed to be reduced even further. Reducing backgrounds necessitated knowledge in the natural radioactivity of common materials used in the shielding and detectors themselves.

As he measured countless materials for various projects, Smith developed a deep expertise that became crucial to research into neutrinos, neutrinoless double-beta decay and dark matter—experiments that required extreme radiopurity in detector materials to detect extremely rare, weakly-interacting signals.

His vast knowledge of the backgrounds in common materials, including metals, composites and ceramics, brought many researchers at Berkeley Lab to him first when designing experiments. Researchers would ask what they should use to create a component of their experiment, and Smith would quickly point them toward the materials with the lowest levels of natural radioactivity.

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Smith displaying the Michael Nitschke Award for Technical Excellence he received in 2001. Photo courtesy of Lawrence Berkeley National Laboratory.

“We have immense trust in Al’s judgement,” said Alan Poon, deputy director of the Berkeley Lab Nuclear Science Division, who worked with Smith on multiple projects, including the design and construction of the Majorana Demonstrator detector. “Looking at the counting data from a sample, he could sometimes give us an entire history of the material and the impact it would make on our experiment.”

When Smith left Berkeley Lab in 2018, he had contributed to dozens of high-profile physics experiments—including Nobel Prize-winning SNO, KATRIN, the Daya Bay Reactor Neutrino Experiment, LUX [see below], LZ [see below], Majorana [see below], CUORE, DM-Ice, LBNE (the precursor to LBNF/DUNE [see below]), Double Chooz, KamLAND, and more—all while living in the background.

SNOLAB, a Canadian underground physics laboratory at a depth of 2 km in Vale’s Creighton nickel mine in Sudbury, Ontario

KIT Katrin experiment

Daya Bay, approximately 52 kilometers northeast of Hong Kong and 45 kilometers east of Shenzhen, China

CUORE experiment,at the Italian National Institute for Nuclear Physics’ (INFN’s) Gran Sasso National Laboratories (LNGS) in Italy,a search for neutrinoless double beta decay

Double-Chooz – Two identical detectors are to be installed near the Chooz nuclear power plant, in the French Ardennes, at different distances from the reactors

KamLAND-Zen detector, an electron antineutrino detector at the Kamioka Observatory, an underground neutrino detection facility near Toyama, Japan

“I don’t think we would have achieved what we did without Al’s work,” said Poon. “He set the gold standard for low-background counting.”

Looking forward

Sanford Lab benefits tremendously from the impacts of Smith and his team. His scrupulous records of underground backgrounds inform researcher’s simulations, while the materials he screened are part of the the Majorana Demonstrator and LZ dark matter detectors.

Most notably, his influence is seen at the BLBF on the 4850 Level, where six low-background counters quietly collect data using high-purity germanium detectors.

BLBF LBNL& SURF

Here, the same detectors once used for measuring materials for LUX and the Majorana Demonstrator will continue counting for next-generation experiments, including LZ, LEGEND and the Deep Underground Neutrino Experiment.

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.

LBNL LZ project at SURF, Lead, SD, USA

LBNL LZ project 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.

LBNL LZ project at SURF, Lead, SD, USA

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.”