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.