The Historic Cherokee Caverns near Knoxville, Tenn., tell a fascinating story about the geological history of the Tennessee Valley. The Caverns are a tourist attraction that often hosts special events?a music festival, Christmas in the Cave, and a ?not-too-scary? Halloween party. But the Caverns are more than a tourist stop, they also play an important role in the Majorana Demonstrator Project.
Majorana uses enriched germanium detectors in its search for a rare form of radioactive decay called neutrinoless double-beta decay. The detectors are manufactured in Oakridge, Tenn., then stored at the back of the caverns in special safes to protect them from cosmic rays.
?The cave is privately owned, so we had to arrange extra security measures,? said Brandon White, a Postdoctoral Researcher with Oak Ridge National Laboratory (ORNL). Those measures included a reinforced ?bat-friendly? door. ?Yes, there are bats living in the caverns and we had to make sure our safety precautions wouldn?t harm them,? White said. So far, bats and scientists seem to be getting along just fine.
When Majorana needs more detectors, researchers call on White, who has made all 11 trips from Tennessee to Lead since February 2013. Each detector is mounted securely inside a cryostat then packed into wooden crates lined with thick foam padding. White has 36 hours to reach Lead. ?We would need more shielding if the detectors were going to be on the surface any longer.?
?Cosmic rays can change the germanium material into other radioactive isotopes. Spending less time at higher elevations or increasing the amount of shielding around the detectors could reduce the rate of production of these isotopes,? said Alan Poon, Group Leader in the Nuclear Science Division at Lawrence Berkeley National Laboratory.
Once packed, White and another Majorana collaborator begin the trip west. They pack the crates at 4:30 a.m. then hit the road: I-40 across half of Tennessee, I-70 across Missouri, I-29 from Kansas City to Sioux Falls, I-90 across South Dakota. It?s a 1,464-mile trek, with a single stop in Omaha, Neb. ?We stay in a very tall hotel in a room on the lower level. The other rooms and the concrete serve as shielding.?
The detectors are monitored throughout the journey to keep an eye on cosmic ray hits. ?Brandon has this down to a science,? Poon said. ?The cosmic ray count rate is lowest when the detectors are in the hotel room while Brandon is sleeping.?
As White drives across the country, he hits ever higher elevations, increasing exposure to cosmic rays. ?We try to time the trips so we arrive at Sanford Lab in time to take the 3:45 p.m. cage to the 4850 Level,? White said. ?If we arrive early, we wait in Rapid City.?
Once underground, the detectors are cooled to liquid nitrogen temperatures, then run through a series of tests and calibrations to ensure they meet experiment standards. White made his final trip from Oak Ridge in July 2015 when he picked up the last seven of 35 detectors. All seven have been tested and will be used in Module 2, which is expected to be operational next year.
?In every collaboration, there are always a few go-to people who make sure everything runs smoothly,? said Poon. ?Brandon has made many contributions. He took the responsibility to ensure the safe delivery and the testing of each detector. He has literally blessed every single enriched detector in this experiment!?