LEAD - The purest copper in the world is being formed at the Sanford Underground Laboratory.
Scientists with the Majorana Demonstrator are using that copper to machine even the tiniest nuts and bolts of their equipment, as well as for a shield with the vacuum-sealed cryostat that will hold the enriched germanium detectors, they are taking great care to ensure the highest level of purity for an experiment that seeks to detect neutrinoless double beta decay, one of the rarest radioactive decays to ever be seen in a laboratory.
In fact, John Wilkerson, of the University of North Carolina, who is the principal investigator for the Majorana Demonstrator, said his team is taking an extra step and assigning laser-engraved serial numbers to each copper part that is machined in their ultra-clean room. The numbers tell scientists which copper plate the parts came from. That way, if they find a piece that is not pure enough, they know exactly where it came from and can dispose of the whole lot.
The process to electroform and machine the parts for the Majorana detector is extensive and meticulously accomplished in the cleanest environment imaginable. It starts with highly purified commercial copper from Finland. Then, students and scientists from the Pacific Northwest National Lab and the South Dakota School of Mines and Technology, working under the direction of South Dakota School of Mines and Technology Chemist Cabot-Ann Christofferson, dissolve the copper in sulfuric acid “baths.” The copper is poured onto stainless steel mandrels that measure 13 inches in diameter by 24-25 inches high. When an electrical current is run through the bath, the individual copper atoms adhere to the mandrel, while the uranium and thorium elements fall away from the steel.
Once the purified copper has stuck to the mandrels, it is transported to the Majorana machining room, where Rapid City machinist Randy Hughes, of Adams ISC, works his magic. He bakes the mandrels in a large oven at about 600 degrees. When the ensemble comes out, it is dropped into a vat of cold, highly purified water, where the copper separates from the mandrel.
“It makes a very dramatic sound when it separates,” said Sanford Lab Communications Director Bill Harlan. “It’s almost like a mini-explosion.”
From there, Hughes gets busy cutting the copper on lathes, and flattening it out into large sheets. Some of those copper sheets will be used as a shield for the Majorana detectors, while others are used to craft tiny nuts, bolts and other parts for the experiment.
“Plates are used for the inner shielding of the detector,” said Wilkerson. “Also, all the parts that we are using inside our cryostats, all the string parts, are made out of ultra-clean materials and they are largely made out of copper because that is the cleanest material.”
Scientists with the Majorana Demonstrator have been electroforming copper for the experiment since July 2011, and Wilkerson said they are a little more than halfway done. That means they have about another year’s worth of electroforming and machining to do.
“We’re really making good progress there,” he said.
All of that work is to maintain purity for a series of germanium detectors that are about the size of a soda can, crafted from strings of highly enriched germanium. There are about five germanium crystals on a string and about seven strings in a cryostat.
“Pure germanium crystals work as semiconductor electronics and it turns out they provide some of the best detection of radioactivity that you can do in terms of properties and efficiency and resolution,” Wilkerson said.
The cryostats, Wilkerson said, are vacuum sealed and cooled to negative 320 degrees Fahrenheit. Majorana scientists are building two detectors using enriched germanium, and one prototype detector that uses natural germanium. The prototype will help scientists test the components of the assembly, to ensure the Majorana Demonstrator will work correctly once it is assembled.
But, after several months of electroforming their own copper and machining it into parts, which they will use to build their detector that will ultimately break through scientific barriers, scientists won’t know whether the copper is pure enough until they actually test the detectors. There is no other way in the world to test copper that pure, scientists said.
“We are looking for the rarest decay that will have ever been observed in a laboratory,” Wilkerson said. “The radon in your house has a four-day half life. That means that after four days, half of it has gone away. That also relates back to the array. The numbers are mind boggling. We are looking for half lives on the order of 1025 to 1027. The age of the universe is something like 1011.”
Ultimately, Wilkerson said, the Majorana Demonstrator will be searching for a rare occurrence called neutrinoless double beta decay, which will prove that neutrinos are their own antiparticle. This, he said, will be a major discovery. The Majorana Demonstrator experiment will help scientists determine whether they can achieve the sensitivity levels necessary to justify building a much larger Majorana experiment in the future. The Majorana Demonstrator collaboration includes groups from 18 institutions from the U.S., Canada, Russia, Japan and China.