MJD requires precision machining
In the clean room at the Ross Campus, 4,850 feet below the surface, the Majorana Demonstrator (MJD) project is growing the world's purest electroformed copper. The copper is then machined in the world's deepest clean machine shop into a variety of parts, some of which are thinner than a hair's width. It's all part of MJD's effort to discover neutrinoless double-beta decay, which just might tell us why matter defeated anti-matter. This story is the second in a two-part series that focuses on the engineering behind the science of MJD.
The hum of machines fills the clean machine shop where machinist Randy Hughes is concentrating on a tiny piece of plastic. The plastic, called Vespel, will become an electrical connector for the germanium detectors inside the cryostats. At the end of the piece of Vespel are eight barely visible holes—home for eight gold-plated brass pins, each thinner than a single hair strand. But the pins weren't fitting right.
"It's just one or two thousandths of an inch off and that's not close enough," said Matthew Busch, R&D Engineer at Duke University/Triangle Universities Nuclear Laboratories. "Randy is trying to come up with an optimal drill size to give the holes the ideal fit for the pin."
MJD's experiment is made up of thousands of parts and pieces—almost all copper—that vary in size, from the cryostats to the copper shields to the connectors. Each piece is machined underground to minimize exposure to cosmic rays and other radioactive materials. The shop is filled with different machine tools, all computer controlled. The arbor press pushes the pins into the Vespel. The lathe machines the outer layer of the copper while it's still on the mandrel, a slitting saw cuts the copper cylinders in half, and a 70-ton press smashes the copper pieces flat. A laser engraver traces serial numbers into each piece of copper, allowing scientists to follow the copper to its origins. Finally, the EDM, or electrical discharge machine, vaporizes copper as it cuts hundreds of tiny parts that are identical to within two ten-thousandths of an inch.
Additionally, an electron beam welder (e-beam) is used for two high-quality welds on each cryostat. The e-beam welder is a unique tool that can cost up to $10 million. Rather than buy one, MJD chose to have the welding done in Indianapolis. "It's an intrinsically clean process because it's done in a vacuum chamber," Busch said.
Still, because portions of the cryostats must be brought above the surface, the team takes extra precautions when moving them. "The copper has 30-90 days maximum it can be on the surface," Busch said. However, cosmic radiation increases as elevation increases. "So, every day at 1,000 feet of elevation is like two days at sea level. All of that has to be factored in."
The team drives the copper to Indianapolis and they are the only ones allowed to touch the copper?and then only while wearing two pairs of gloves. "We load the material into the welder. When it cools we pull it out, put it back in the box and drive it back to the lab."
Because the experiment needs such intricate pieces, things don't always fit right—like the Vespel connectors. That?s when Busch and Hughes have to really get creative. "We can't buy anymore tools because there is no more room," Busch said. "So we have to modify the tool or the design. We have to determine how we can make what we need with the tools we have. We're really fortunate to have a full-time machinist who gets excited about making things the way we need them."
Currently, MJD is collecting data with a prototype that is not made with the ultra pure copper. And that's a good thing because it allows the scientists, engineers and machinist to make the changes necessary to ensure the experiment begins its data collection under optimal conditions.
"The whole idea is to test everything we do before we begin the ultra-pure experiment," said John Wilkerson, Principle Investigator and the John R. and Louise S. Parker Distinguished Professor of Physics and Astronomy at the University of North Carolina. "We're not learning physics from what we're doing, yet. But we've learned a lot in terms of getting everything working the right way. This is how we do it."