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The Majorana Demonstrator is looking for a rare form of decay called neutrinoless double-beta decay. If they find it, it could tell us why matter exists.

Two researcher monitor the Majorana Demonstrator experiment.

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.

Electroforming baths in the cleanroom

Growing copper

In its finished form, Majorana is made up of more than 6,600 pounds of copper and more than 5,000 parts and pieces—some as tiny as the head of a pen; others measuring 2 feet square—nearly all of which were made from ultra-pure copper grown on the 4850 Level of Sanford Lab. The first step to building this highly sensitive experiment? Electroforming the purest copper in the world. It’s a simple, but slow process.

Copper nuggets were dissolved in acid baths to remove trace impurities. Then, an electric current was added causing the copper atoms to adhere to a stainless steel cylinder called a mandrel, growing to a thickness of about 5/8 of an inch over a 14 month-period—approximately 33 millionth of a meter per day. Once electroformed the copper was taken to the world’s deepest clean machine shop a kilometer away in the Davis Campus.

"Majorana went to great lengths to ensure the materials used in the experiment would not contribute to backgrounds," said Cabot-Ann Christofferson, chemist for the Majorana and the South Dakota School of Mines & Technology. "The copper is such an integral part of low-background experiments, that it will be one of the technologies used going forward."

Machinist examines a copper plate

Precision machining

Every part of the Majorana experiment was machined underground to minimize exposure to cosmic radiation. And every part had to fit perfectly to ensure the experiment runs correctly.

“If it’s just one or two thousandths of an inch off, it’s not close enough,” said project engineer Matthew Busch of Duke University/Triangle University Nuclear Laboratories.

Inside the clean machine shop, machinist Randy Hughes used a lathe to machine the outer layer of the copper, a slitting saw to cut the copper cylinders in half, a 70-ton press to flatten the copper pieces and a wire EDM—electrical discharge machine— to vaporize copper as it cut hundreds of tiny identical parts. If things didn’t fit right, they had to get creative, Busch said.

“We couldn’t buy more tools because there was no more room. So, we modified the tool or the design if things didn't fit the way we needed them to," Busch said. 

The science

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.

Researchers assemble strings inside the glovebox

Assembling Majorana

Having the world’s cleanest copper isn’t enough if you can’t keep your experiment clean. That’s why the experiment was assembled deep underground in a nitrogen-filled glovebox housed in a class-1,000 cleanroom.

Before entering the cleanroom, scientists donned cleanroom garb—Tyvek suits, masks, hoods, special shoe coverings and two pairs of gloves. Once inside the cleanroom, they replaced the outer glove with a new one then headed to the glovebox where they placed their already gloved hands inside huge black rubber gloves covered with another pair of latex gloves. This was done to protect the experiment, not the researchers. Once fully garbed, they began assembling the strings of detectors that reside inside two cryostats. Each cryostat contains about seven strings of 4-5 germanium crystals.

It was challenging and delicate work, involving hundreds of custom-made parts for each string. And everything had to be assembled in a particular order. Each detector is encapsulated in copper then stacked in strings and tied together with cables—most of which are no thicker than a strand of hair—and attached to the cryostat. Many of the parts connect everything to a data collection system inside the cleanroom.

“It’s a detailed, highly specialized procedure that came from many revisions,” said Tom Gilliss, a graduate student at the University of North Carolina.

Majorana installs the inner copper shield

Shields are like onions

In the movie “Shrek,” the title character tells Donkey, “Ogres are like onions! … They have layers.” The same can be said of Majorana’s six-layered shield, said Guiseppe who oversaw the construction of the shield.

Designed to keep out as much radiation as possible, each layer is cleaner as it gets closer to the heart of the experiment. The outer layer is polyethylene, which slows neutrons. The second layer is scintillating plastic, which detects muons. The third layer is an aluminum radon enclosure that keeps out room air, while the fourth layer is made of lead bricks to block gamma rays. Finally, a rectangular box of ultrapure commercial copper surrounds the electroformed copper shield.

But the most critical layer—the one closest to the experiment and the last to be installed—is made of electroformed copper: two five-sided boxes made of 40, 1/2-inch thick plates that, together, weigh about a ton. Majorana began collecting data long before the shield was completed and released positive results as early as 2015.

“Just two months after installing the electroformed shield, we saw a huge difference,” said Guiseppe. “It was like night and day.”

Ultra-pure copper

5,500 electroformed copper parts were used in the experiment, all were machined underground.

    Total weight of the shield

    The breakdown:

    • 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.