In football, a coach has to be pretty desperate to put in the second string. The Majorana Demonstrator does things a little differently. Its second string (or cryostat) likely is the best.
"Our experiment design includes a prototype, followed by two cryostats," said Adam Bradley, a postdoc with Lawrence Berkeley National Laboratory who works on the Majorana Demonstrator project. "With each cryostat, we've learned what not to do or what to do better or how to be more efficient."
Last year, the Majorana team decommissioned its prototype and installed the first cryostat into the shield. Recently, they finished assembling the last string for cryostat two and are preparing to deploy the vessel into the shield. Each cryostat contains seven strings, providing a total of approximately 44 kilograms of enriched and natural germanium.
Building strings for the Demonstrator is challenging and delicate work, involving hundreds of custom-made parts for each string. And each part must be assembled in a specific order, said Tom Gilliss, a graduate student at the University of North Carolina. "It's a detailed, highly specialized procedure that came from many revisions."
Why such rigid procedures? "We have specific low-background needs," said Brandon White, a postdoc at Los Alamos National Laboratory (LANL). Low backgrounds are so important to the experiment, he added, that the collaboration tracks every part?some of which are as small as the head of a pen or as thin as a strand of hair. "We know the history of every part that goes into every detector unit."
The Majorana experiment is looking for a rare form of radioactive decay called neutrinoless double-beta decay. The process is so rare, said Ralph Massarczyk, also a postdoc at LANL, that a single event could be lost if there is a lot of background.
"The demonstrator will let us show that we can build a good experiment that is well-shielded in a good place," Massarczyk said. "Then we'll see how things go."