Engineering Majorana's shield

Many experiments require equipment or methods that don't yet exist. In the case of the Majorana Demonstrator (MJD) project, the experiment includes creating electroformed copper parts and designing an intricate multi-layered shield that will protect the experiment from cosmic and terrestrial radiation. This story is the first in a two-part series that focuses on the engineering behind the science of MJD.

In its search for a rare form of radioactive decay, the Majorana Demonstrator experiment requires almost zero background noise. That's why researchers are building the experiment on the 4850 level inside a clean laboratory. But even before the experiment can begin collecting data, scientists must design, build and test all of the equipment. 

"We have to do a lot of engineering before we get to the actual data collection," said Vince Guiseppe, an Assistant Professor of Physics at the University of South Carolina. One element of the project is the 'layered' shield that is being built around the experiment itself. 

"The shield is like an onion, with layer after layer of different materials that progressively get cleaner as they get closer to the experiment," said Guiseppe, who is leading the construction of the shield. 

When completed, the shield will have six layers, all designed to minimize cosmic and terrestrial radiation coming into the experiment. The outer most layer, a 12-inch thick panel of polyethylene, slows neutrons. The second layer, scintillating plastic 'veto panels,' will detect muons, the most penetrating cosmic rays. Next, an aluminum radon enclosure will keep out room air, while the fourth layer, made of lead bricks set in a particular pattern, will block gamma rays. 

Finally, a rectangular box of ultrapure copper will surround a 2-inch thick layer of electroformed copper. These two layers are designed to protect the experiment from the radioactivity of the shield. "Ideally, we would want the shield to be made entirely of electroformed copper," Guiseppe said. "But it would take decades to grow enough."

The lead brick shield is perhaps the most complicated layer. It has three sections: a castle made of 3,400 brick pieces, and two portable monoliths, each of which will hold 570 bricks inside a copper frame. The monoliths will contain the germanium detector arrays and hardware, what Guiseppe calls "the heart of the experiment." They are portable so researchers can move the detectors in and out of the shield as needed. 

Each section requires an intricate interweaving of bricks to ensure every crack is covered. Guiseppe and a mechanical designer created the patterns using computer models. ?When we stack each layer of bricks we have specific instructions that have to be followed precisely,? Guiseppe said. 

Although the shield isn't complete and won't be when MJD begins collecting data this summer, Guiseppe said, "We're at a point now where it's actually a useful shield that will allow us to learn a lot."