March 13, 2017
Vince Guiseppe stands next to an extra lead brick monolith, which keeps the shield sealed if a working module needs to be removed for service. Credit: Constance Walter
For nearly seven years, the Majorana Demonstrator Project’s “shield team” has been building the six-layered shield that surrounds the experiment on the 4850 Level. In early March, they placed the last piece of polyethylene on the outermost layer of the shield.
“I’m proud of what the team has produced,” said Vince Guiseppe, assistant professor of physics at the University of South Carolina. “This was a complicated project. Every layer was added at the right time and fit perfectly.”
The Majorana collaboration uses germanium crystals to look for a rare form of radioactive decay called neutrinoless double-beta decay. The discovery could determine whether the neutrino is its own antiparticle. Its detection could help explain why matter exists. The shield is critical to the success of the experiment.
Each layer of the shield was designed to target certain forms of radiation. “The closer the layer is to the experiment, the greater its impact,” Guiseppe said.
The most important layer is the electroformed copper that sits closest to the experiment. Comprised of 40, half-inch thick copper plates, it was grown and machined underground. “This is clearly the hallmark of our shield system in terms of purity and cleanliness protocols,” Guiseppe said. Surrounding that portion of the shield, is a 2-inch thick layer of ultrapure commercial copper.
Next is a “castle” built with 3,400 lead bricks. Two portable monoliths, each holding 570 bricks, support the cryostats filled with strings of germanium detectors and cryogenic hardware, what Guiseppe calls “the heart of the experiment.”
An aluminum box encapsulating the lead castle protects the experiment from naturally occurring radon. Every minute, the team injects eight liters of nitrogen gas to purge the air within the enclosure. “We don’t want any lab air getting in.”
Attached to the aluminum box are scintillating plastic “veto panels” designed to detect muons, the most penetrating of all cosmic rays.
Finally, there’s the 12 inches of polyethylene enclosing the entire experiment, including the cryogenics (chilled water heat exchangers moderate the temperature). The poly slows down neutrons that could cause very rare backgrounds. Why worry about such rare events? High-energy neutrons can bounce through just about anything, including the 22 inches of lead and copper shielding. If a neutron hits a copper atom, it could create a gamma ray right next to the experiment.
“The poly is the final defense against backgrounds in an experiment that requires extreme quiet,” Guiseppe said.
The entire shield, weighing 145,000 pounds, rests on an over floor made of steel with channels for the poly.
Jared Thompson, a research assistant, began his work with Majorana in 2010, etching lead bricks for the shield. In fact, in March 2014, he placed the last brick on the castle. And he was part of the group that recently placed the last piece of poly.
“It’s really exciting,” Thompson said. “A complete shield could mean a whole new data set down the road.”
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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 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.