Growing Copper Deep Underground: SD Mines Plays Integral Role in Successful MAJORANA DEMONSTRATOR Experiment

Much of the experiment’s copper is processed underground to remove both natural radioactivity (such as thorium and uranium) and radioactivity generated above ground when cosmic rays strike the copper. Electroforming relies on an electroplating process that over several years forms the world’s purest copper stock. Ultrapure copper is dissolved in acid and electrolytically forms a centimeter-thick plate around a cylindrical stainless-steel mandrel. Any radioactive impurities are left behind in the acid. Here collaborator Cabot-Ann Christofferson of the South Dakota School of Mines & Technology measures the thickness of copper pulled from an electroforming bath. Credit: Sanford Underground Research Facility; photographer Adam Gomez

The collaborators working on the MAJORANA DEMONSTRATOR have published a study in the journal Physical Review Letters showing the success of the experiment housed in the Sanford Underground Research Facility (SURF). The success of the MAJORANA DEMONSTRATOR opens the door for the next phase of the experiment and sets the stage for a breakthrough in the fundamental understanding of matter in the universe. 

The experiment, led by the Department of Energy’s Oak Ridge National Laboratory, involves 129 researchers from 27 institutions and six nations. The South Dakota School of Mines & Technology was an integral part in facilitating the underground laboratory space at SURF and helped lead the effort to build the ultra-pure components needed to construct a successful experiment. 

“The goal was to demonstrate the feasibility and capability to build a larger one-ton experiment,”  says Cabot-Ann Christofferson, the Liaison and a Task Leader within the  MAJORANA Collaboration at the Sanford Underground Lab and an instructor in the Department of Chemistry and Applied Biological Sciences at SD Mines. “You have to show you can even reach the low-level background needed to observe this rare decay process, and we were able to reach that goal.” 

The MAJORANA DEMONSTRATOR collaboration seeks to answer some fundamental questions about the makeup of the universe. If equal amounts of matter and antimatter had formed in the Big Bang more than 13 billion years ago, one would have annihilated the other upon meeting, and today’s universe would be full of energy but no matter to form stars, planets and life. Yet matter exists now. That fact suggests something is wrong with Standard Model equations describing symmetry between subatomic particles and their antiparticles.

“The excess of matter over antimatter is one of the most compelling mysteries in science,” said John Wilkerson of ORNL and the University of North Carolina, Chapel Hill. Wilkerson leads the MAJORANA DEMONSTRATOR. “Our experiment seeks to observe a phenomenon called ‘neutrinoless double-beta decay’ in atomic nuclei. The observation would demonstrate that neutrinos are their own antiparticles and have profound implications for our understanding of the universe. In addition, these measurements could provide a better understanding of neutrino mass.” One of their keys to success depends on avoiding background that could mimic the signal of neutrinoless double-beta decay.

That was the key accomplishment of the MAJORANA DEMONSTRATOR. Its implementation was completed in South Dakota in September 2016, nearly a mile underground at the Sanford Underground Research Facility. Siting the experiment under nearly a mile of rock was the first of many steps collaborators took to reduce interference from background. Other steps included a cryostat, a device used to keep part of the experiment very cold, made of the world’s purest copper and a complex six-layer shield to eliminate interference from cosmic rays, radon, dust, fingerprints and naturally occurring radioactive isotopes.

“We were building a ship in a bottle in a sense,” says Christofferson. SD Mines researchers helped oversee the effort to build some of the components of the experiment, including the ultra-pure copper needed for the cryostat, detector parts and shielding.

To manufacture the world's purest copper, Christofferson and others on the team first dissolved high-purity oxygen-free high-conductivity copper in sulfuric acid. They then used an atypical electric current to pull copper atoms out of solution and deposit them on stainless steel mandrels all while controlling crystal structure and purity. Through this process, the team was able to grow copper virtually free of any impurities. In fact, this copper’s impurity level can only be measured on the scale of parts per quadrillion or 10-15 the purest formed copper in the world to date. The copper was then machined, all underground in a clean room, into the various parts needed to build the MAJORANA DEMONSTRATOR.  

“There was over 100,000-man hours spent underground on this experiment, and SD Mines personnel comprise more than 20 percent of that the time spent underground,” Christofferson says.  She also notes that SD Mines is a small school where students can get hands-on experience in major research projects. “Undergraduate students across multiple disciplines at SD Mines were involved in this large-scale DOE funded experiment. They did everything from chemistry to simulations work. A graduate student also made significant advancements in alloy clean materials in this project and the research is on-going,” she says.  

The MAJORANA Collaboration’s results coincide with new results from a competing experiment in Italy called GERDA (for GERmanium Detector Array), which takes a complementary approach to studying the same phenomenon. “The MAJORANA DEMONSTRATOR and GERDA together have the lowest background of any neutrinoless double-beta decay experiment,” said ORNL’s David Radford, a lead scientist in the experiment. 

The DEMONSTRATOR was designed to lay the groundwork for a ton-scale experiment by demonstrating that backgrounds can be low enough to justify building a larger detector. Just as bigger telescopes collect more light and enable viewing of fainter objects, increasing the mass of germanium allows for a greater probability of observing the rare decay. With 30 times more germanium than the current experiment, the planned one-ton experiment would be able to spot the neutrinoless double-beta decay of just one germanium nucleus per year.

The MAJORANA DEMONSTRATOR is planned to continue to take data for another two or three years. Meanwhile, a merger with GERDA is in the works to develop a possible one-ton detector called LEGEND, planned to be built in stages at an as-yet-to-be-determined site.

LEGEND 200, the LEGEND demonstrator and a step towards a possible future ton-scale experiment, will be a combination of the germanium and copper used in GERDA, MAJORANA and additional new detectors. Scientists hope to start on the first stage of LEGEND 200 by 2021. 

SD Mines will continue to be involved in this next phase of research. Christofferson has been named to the LEGEND technical board and her team will use the infrastructure in SURF already set up to grow ultra-pure copper needed in the LEGEND 200 experiment. 

A ton-scale experiment, LEGEND 1000, would be the next stage if approved. “This merger leverages public investments in the MAJORANA DEMONSTRATOR and GERDA by combining the best technologies of each,” said LEGEND Collaboration co-spokesperson (and long-time MAJORANA spokesperson up until last year) Steve Elliott of Los Alamos National Laboratory.

The title of the Physical Review Letters paper is “Search for Neutrinoless Double Beta Decay in 76Ge with the MAJORANA DEMONSTRATOR.”

Funding came from the U.S. Department of Energy Office of Science and the U.S. National Science Foundation. The Russian Foundation for Basic Research and Laboratory Directed Research and Development Programs of DOE’s Los Alamos, Lawrence Berkeley and Pacific Northwest national laboratories provided support. The research used resources of the Oak Ridge Leadership Computing Facility and the National Energy Research Scientific Computing Center, DOE Office of Science User Facilities at Oak Ridge and Lawrence Berkeley national laboratories, respectively. Sanford Underground Research Facility hosted and collaborated on the experiment.

UT-Battelle manages ORNL for the Department of Energy's Office of Science. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit http://science.energy.gov/.

Adapted from a release by ORNL.

 

Last edited 6/28/2018 1:05:55 PM

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