In the movie "Shrek," the title character tells Donkey, "Ogres are like onions! They have layers." Vince Guiseppe uses the same analogy to describe the Majorana Demonstrator shield.
Since 2011, Guiseppe, Assistant Professor of Physics at the University of South Carolina, has been overseeing the construction of Majorana's six-layered shield. In September, they began work on the final, and most pivotal layer: the electroformed copper shield, or as Guiseppe calls it, "The innermost part of the onion."
In most situations, the innermost part goes in first. But this is a special case. Almost every piece of the experiment's innermost portion is made of electroformed copper, which grows slower than the speed of hair (copper grows 33 millionths of a meter per day compared to .44 millimeters for hair).
"We've been building parts for the experiment for a long time," Guiseppe said. "We did this part last because there's not much you can do with the inner shield until you have all of the innermost shield assembled and ready to go in at the same time."
When completed, the 2-inch-thick inner shield will include two five-sided boxes (two of the sides are on the modules). Each side is made up of four-inch thick plates, for a total of 40 plates that weigh about a ton. "That's a fair fraction of the total amount of copper that's been grown for this experiment," Guiseppe said.
The shield was designed to keep out as much radiation as possible, with each layer getting progressively 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.
For several months Majorana has been collecting data using Module One. "As we've looked at the data, it's clear there is no electroformed copper shield," Guiseppe said. "With the installation of the inner shield, we could see backgrounds decrease by a factor of 10. That's really significant."
The team began assembling the first box of the inner shield in October. It was installed last week. Installation requires extreme care and precision. Every piece has to fit perfectly(test assemblies were done inside the machine shop) including the screws that hold the plates together. The box was built on the shield platform then moved into place using Teflon rails that fit grooves cut into the outer copper shield (copper on copper is sticky and would make it impossible to get the inner shield in place). Special equipment was used to move the box inside the shield.
"We made a big fanfare when the first box was installed," Guiseppe said. ?It's a testament to the design, engineering and machining."
The second box was assembled last week and installed today. When the inner copper shields are installed on the modules, Module 1 will be moved back into the shield and once again begin collecting data.
"The whole collaboration is excited," Guiseppe said. "Now we'll start honing in on some of the details that will allow us to get even better."