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U Washington Majorana Demonstrator Experiment at SURF


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.


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.”


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.


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 out of many revisions of the experiment,” said Tom Gillis, a graduate student at the University of South Carolina.


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.”

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

144,500 pounds
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. Abstract on Legend.

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.

MPG GERmanium Detector Array (GERDA) at Gran Sasso, Italy

Gran Sasso LABORATORI NAZIONALI del GRAN SASSO, located in the Abruzzo region of central Italy

“This merger leverages public investments by combining the best technologies of each,” said LEGEND Collaboration co-spokesperson Steve Elliott of Los Alamos National Laboratory.

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About us.
The Sanford Underground Research Facility in Lead, South Dakota, advances our understanding of the universe by providing laboratory space deep underground, where sensitive physics experiments can be shielded from cosmic radiation. Researchers at the Sanford Lab explore some of the most challenging questions facing 21st century physics, such as the origin of matter, the nature of dark matter and the properties of neutrinos. The facility also hosts experiments in other disciplines—including geology, biology and engineering.

The Sanford Lab is located at the former Homestake gold mine, which was a physics landmark long before being converted into a dedicated science facility. Nuclear chemist Ray Davis earned a share of the Nobel Prize for Physics in 2002 for a solar neutrino experiment he installed 4,850 feet underground in the mine.

Homestake closed in 2003, but the company donated the property to South Dakota in 2006 for use as an underground laboratory. That same year, philanthropist T. Denny Sanford donated $70 million to the project. The South Dakota Legislature also created the South Dakota Science and Technology Authority to operate the lab. The state Legislature has committed more than $40 million in state funds to the project, and South Dakota also obtained a $10 million Community Development Block Grant to help rehabilitate the facility.

In 2007, after the National Science Foundation named Homestake as the preferred site for a proposed national Deep Underground Science and Engineering Laboratory (DUSEL), the South Dakota Science and Technology Authority (SDSTA) began reopening the former gold mine.

In December 2010, the National Science Board decided not to fund further design of DUSEL. However, in 2011 the Department of Energy, through the Lawrence Berkeley National Laboratory, agreed to support ongoing science operations at Sanford Lab, while investigating how to use the underground research facility for other longer-term experiments. The SDSTA, which owns Sanford Lab, continues to operate the facility under that agreement with Berkeley Lab.

The first two major physics experiments at the Sanford Lab are 4,850 feet underground in an area called the Davis Campus, named for the late Ray Davis. The Large Underground Xenon (LUX) experiment is housed in the same cavern excavated for Ray Davis’s experiment in the 1960s.
LUX/Dark matter experiment at SURFLUX/Dark matter experiment at SURF

In October 2013, after an initial run of 80 days, LUX was determined to be the most sensitive detector yet to search for dark matter—a mysterious, yet-to-be-detected substance thought to be the most prevalent matter in the universe. The Majorana Demonstrator experiment, also on the 4850 Level, is searching for a rare phenomenon called “neutrinoless double-beta decay” that could reveal whether subatomic particles called neutrinos can be their own antiparticle. Detection of neutrinoless double-beta decay could help determine why matter prevailed over antimatter. The Majorana Demonstrator experiment is adjacent to the original Davis cavern.

Another major experiment, the Long Baseline Neutrino Experiment (LBNE)—a collaboration with Fermi National Accelerator Laboratory (Fermilab) and Sanford Lab, is in the preliminary design stages. The project got a major boost last year when Congress approved and the president signed an Omnibus Appropriations bill that will fund LBNE operations through FY 2014. Called the “next frontier of particle physics,” LBNE will follow neutrinos as they travel 800 miles through the earth, from FermiLab in Batavia, Ill., to Sanford Lab.

Fermilab LBNE