In 1937, Italian physicist Ettore Majorana hypothesized the Majorana fermion—a particle that could be its own antiparticle. If the theory proves true, it could unlock one of the greatest mysteries of the universe: why there is more matter than anti-matter—and why we exist at all.
The Majorana Demonstrator Project, located deep underground at Sanford Lab, uses 44 kilograms of natural and enriched germanium crystals placed inside two cryostats in the hopes of finding this particle, a rare form of decay called neutrinoless double-beta decay. The experiment is called a demonstrator because the collaboration needed to prove it could create a quiet enough environment to find what it is looking for. A unique shield and 4,850 feet of rock help block cosmic and terrestrial radiation from this highly sensitive experiment.
Now, after years of planning, designing and building the experiment, the collaboration has something to celebrate. In a study published in March 2018, the Majorana Collaboration showed it can shield a sensitive, scalable, 44-kilogram germanium detector from background radioactivity, which is critical to developing a proposed ton-scale experiment.
“We know that we created an environment that is incredibly clean and quiet,” said Vincent Guiseppe, a co-spokesperson for the Majorana Demonstrator and an assistant professor of physics and astronomy at the University of South Carolina. “These results give us a much better understanding of the always-elusive neutrino and how it shaped the universe.”
Guiseppe credits the results to the design of the experiment and the stringent cleanliness protocols put in place.
The Majorana Demonstrator collaboration believes germanium is the best material to detect neutrinoless double-beta decay. During the decay process, two electrons are ejected in the germanium. The electrons ionize the germanium, creating a very specific amount of electric charge that can be measured with special equipment. If they discover it, it could tell us why matter—planets, stars, humans and everything else in the universe—exists.
The process is so rare, the slightest interference could render the experiment useless. That’s why it was built deep underground, using electroformed copper that never saw daylight. Still, that wasn’t enough. To achieve the quietest background possible, they built the experiment inside a glovebox in a class-1,000 cleanroom, then surrounded it with a six-layered shield designed to protect it from any stray cosmic or terrestrial radiation.
5,500 electroformed copper parts were used in the experiment, all were machined underground.
Total weight of the shield
- Lead: 108,000 pounds
- Poly shield: 31,000 pounds
- Copper shielding: 5,500 pounds
The Majorana 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.
“When we started this project, there were many risks and no guarantee that we could achieve our goals, as we were pushing into unexplored territory,” said John Wilkerson, principal investigator of the experiment and the John R. and Louise S. Parker Distinguished Professor in the Department of Physics and Astronomy at the University of North Carolina.
“It’s very exciting to see these world-leading results. We’ve achieved the best energy resolution of any double-beta decay experiment and are among the lowest backgrounds ever seen.”
With 30 times more germanium than the current experiment, the ton-scale, called LEGEND (Large Enriched Germanium Experiment for Neutrinoless Double-Beta Decay), could more easily see the rare decay it seeks. The plan is to partner with GERDA (GERmanium Detector Array), a sister experiment located at Gran Sasso in Italy, and other researchers in the field.
"This merger leverages public investments by combining the best technologies of each," said LEGEND Collaboration co-spokesperson Steve Elliott of Los Alamos National Laboratory.