A sharp crack, a loud hiss, a cloud of steam and leaping droplets of instantly boiled water announced the birth of another batch of the world's purest copper Thursday afternoon.
The process is called a "quench," and it's happening several times a month now in the Majorana Demonstrator experiment?s clean-room machine shop nearly a mile underground.
"That's our plate stock," Majorana Project Engineer Matthew Busch of Duke University said after the quench. Busch designed the machine shop and the ultra-pure copper parts being produced there. This batch of copper, he explained, will become part of a 2-inch-thick shield that will protect Majorana from background radiation.
But first, the copper itself must be purified. Copper on the surface is polluted by cosmic radiation, so Majorana brings very pure copper nuggets 4,850 feet underground, to a clean room near the Ross Shaft, where it is dissolved in acid baths then electroformed onto cylindrical stainless steel molds, called "mandrels."
The copper-coated mandrels–each about 23 inches tall by 13 inches in diameter–are then transported just over half a mile from the electroforming lab to the Majorana machine shop. Machinists Randy Hughes and Russ Bauman of contractor Adams-ISC use a lathe to remove the rough copper surfaces. Then comes the "quench." To prepare for the event, the mandrels are baked in an oven at 600 degrees F. for six hours.
Late Thursday afternoon, Hughes cleared the area around the oven doors and delivered a safety briefing. Then, working with Baumann, he opened the oven and rolled in a small counterweighted crane. Using insulated mitts, Hughes attached a hook to the hot mandrel, then he and Bauman rolled it out of the oven. Next, they rolled a stainless steel tank of deionized water into position underneath the mandrel. Baumann used a hand lever to lower the hot mandrel into the room-temperature water.
Then, BANG! The first contact of hot copper and cold water produced a crack loud enough to startle observers. Hot metal instantly boiled the water, but the hissing gradually diminished as the mandrel was submerged. The copper wanted to contract, Busch explained, but it could not because the stainless steel mandrel had not yet cooled.
Bauman stopped lowering the mandrel when the copper was submerged. He allowed it to cool for a couple of minutes, then he submerged the mandrel until the stainless steel at the top hit the water. There was more steam, and within seconds, the stainless steel had shrunk enough to allow a hollow drum of ultra-pure copper to slide off the mandrel into the tank.
Voila! New plate stock. In a year or so, Busch said, this batch of copper will be used to make a box that will help shield the experiment from radiation.