Experiment Seeks Neutrinoless Double-beta Decay

Collaborators of the Majorana Demonstrator, an experiment led by Oak Ridge National Laboratory, have shown they can shield a sensitive, scalable 44-kilogram germanium detector array from background radioactivity. The accomplishment, according to the team, is critical to both developing and proposing a much larger future experiment—with approximately a ton of detectors—to study the nature of neutrinos. These electrically neutral particles interact only weakly with matter, making detection difficult.

“The excess of matter over antimatter is one of the most compelling mysteries in science,” says John Wilkerson, head of the project at ORNL and professor of physics at the University of North Carolina, Chapel Hill. 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, a fact that suggests something is wrong with Standard Model equations describing symmetry between subatomic particles and their antiparticles.

The Majorana team seeks to observe a phenomenon called “neutrinoless double-beta decay” in atomic nuclei, an observation of the nucleus falling apart that could demonstrate that neutrinos are their own antiparticles and profoundly adjust scientific understanding of the universe, according to Wilkerson.

In a common mode of decay, a neutron inside the nucleus emits an electron (called a “beta”) and an antineutrino to become a proton, while in two-neutrino double-beta decay, two neutrons decay simultaneously to produce two protons, two electrons and two antineutrinos. Both have been observed. The Majorana team, however, is looking for evidence for a similar decay process that has never been observed, where no neutrinos are emitted.

Researchers from nearly a dozen experiments have sought neutrinoless double-beta decay, and many future experiments have been proposed. In 2015, a report from the US Nuclear Science Advisory Committee to the Department of Energy and the National Science Foundation deemed a US-led ton-scale experiment to detect neutrinoless double-beta decay a top priority for the nuclear physics community.

Researchers work on the delicate wiring of a cryostat. Image: Sanford Underground Research Facility, M. Kapust

Researchers work on the delicate wiring of a cryostat. Image: Sanford Underground Research Facility, M. Kapust

A key accomplishment for the Majorana researchers was finding a way to avoid background that could mimic the signal of the decay. To achieve this, the team not only staged the experiment nearly a mile underground at the Stanford Underground Research Facility—future home of the DUNE liquid argon neutrino detectors—but also built a custom copper cryostat and a complex six-layer shield to eliminate interference from cosmic rays, radon, dust, fingerprints and naturally occurring radioactive isotopes.

“If you’re going to search for neutrinoless double-beta decay, it’s critical to know that radioactive background is not going to overwhelm the signal you seek,” says ORNL’s David Radford, a lead scientist in the experiment.

The Majorana Demonstrator uses germanium crystals as both the source of double-beta decay and the means to detect it. Germanium-76 (Ge-76) decays to become selenium-76, which has a smaller mass. When germanium decays, mass gets converted to energy that is carried away by the electrons and the antineutrinos. “If all that energy goes to the electrons, then none is left for neutrinos,” Radford says. “That’s a clear identifier that we found the event we’re looking for.”

The chances of spotting a neutrinoless double-beta decay in Ge-76 are rare, however, no more than one for every 100,000 two-neutrino double-beta decays. Using detectors containing large amounts of germanium atoms increases the probability of spotting the rare decays, but none have been detected since June 2015.

Researchers plan to continue taking data for another two or three years while also coordinating a merger with GERDA, a complementary study in Italy, to develop a possible one-ton detector called LEGEND, planned to be built in stages at an as-yet-to-be-determined site. http://www.ornl.gov