In the second, taller building, the TALE telescopes target higher skies than their TA counterparts. Although the cosmic rays TALE will study still fall in the high-energy realm, they are less energetic than those identified by TA. The decreased energy means the showers end higher in the atmosphere, so TALE's telescopes peer above the horizon, looking for those faint ultraviolet flashes that occur when the cosmic rays collide with particles in the atmosphere.
The pair of buildings at Middle Drum tower over the desert, with exterior automatic doors stretching about 20 feet high, with only a few feet to the roof. TALE’s telescopes point higher into the atmosphere than TA’s, requiring greater height for the doorways through which they peer.
The two massive buildings are sealed tight. We pass through an office area where someone sits to monitor the fluorescence telescopes. Unlike the scintillation detectors, which aren't affected by light, the fluorescence telescopes are sealed off from sunlight during the daylight hours, because sunlight can permanently damage the mirrors. A sign on the door reminds us of the danger direct sunlight has for the instruments, and includes the image of a person having their face melted in the movie "Indiana Jones and the Raiders of the Lost Ark." This light sensitivity is so extreme, a sign on the road to the site requests that headlights be turned off and that flashlights be used with a red filter.
Each telescope consists of four round optical mirrors combined in a cloverleaf pattern. When the garage doors open on clear nights, the 3-inch mirrors collect light and focus it on a collection of 256 photomultiplier tubes. The channeled light should reveal any flashes on the horizon from cosmic rays. Like a powerful magnifying glass, it results in a very focused beam of light.
"We could melt glass with this thing," said Robert Cady, an assistant research professor at the University of Utah, who is working on the experiment.
Whenever the telescopes are operating, two people must be on site in case there's a problem. (With the telescope sitting in the middle of nowhere, safety concerns mean that no one is permitted to be at the site alone.) Most of the time, the work for those two employees is boring, Cady said, but their presence is necessary to protect the instruments.
"When something goes wrong, it goes really badly wrong," he said.
Among other things, the folks at the site must check the enormous garage doors to make certain they shut completely at the end of a run. If a mechanical issue keeps them from closing, each mirror must be covered. The tool of choice is king-size fitted sheets, which, Cady said, work perfectly.
Once or twice a year, the mirrors are washed to remove any accumulated dust, but the work must be done carefully to avoid scraping off the aluminum cover, Cady said.
Each cloverleaf sits on a metal frame, with its computer controls in a locker behind it. Everything in the giant warehouse is raised off the floor, thanks to lessons learned from a previous project, which suffered a rodent problem.
"Rats love to chew cables," Cady said. "We keep everything rat-proof, off the ground."
Hard to replace
Even though they're out in the middle of a desert, a few of the scintillation detectors have had to be replaced. A wet winter several years ago resulted in flooding, and several of the detectors were immersed. Cady and a colleague went out in a kayak to check on the instruments, and some had only their antenna sticking above the water. Those were trashed, and now sit in the corral. Another one was damaged in an auto accident when a motorist unaffiliated with the project crashed into it.
Simple exterior repairs can be made to the scintillation detectors in the field, but major repairs require them being taken back to Delta. There, the team can repair or replace major components in controlled conditions, without having to haul everything out to the middle of the desert.
While the detectors are inexpensive to replicate, the mirrors are another story. According to Cady, the equipment and space used to fabricate them no longer exists. So, while the mirrors themselves cost only a few thousand dollars, he estimated it would take more than $100,000 to get set up to build more. Fortunately, the project has 30 to 40 more mirrors in storage in Salt Lake City.
According to Cady, the biggest emergency event came at the beginning of the project, when three members of the team flipped their pickup truck in the desert. The helicopter that was placing the detectors carried the three into town, where an ambulance transported them to the hospital. All three survived.
Other problems include occasional wind damage to the detectors. Far more likely is that one of the team members will wind up stuck in the desert due to car trouble.
"We have the local mechanic on speed dial," Cady said.
Callahan often interacts with the people in the county, making sure they have an idea of what the giant array is doing. She sets up a booth at the state fair every year and welcomes the opportunity to share details with anyone who is interested in the Cosmic Ray Center.
The hunt for cosmic rays requires a location with a thin, dry atmosphere where secondary particles can travel easily to the detectors, which usually means deserts at high altitudes. Similar sites have been set up in Mexico, South America and Antarctica. There have also been cosmic-ray detectors on the International Space Station, which collect the actual cosmic rays and not secondary particle showers. Among these, the Telescope Array has perhaps the best location, only 3 hours south of Salt Lake City's airport, Callahan said. In addition, she's grateful for the support from Delta's community.
"There are only a few places in the world where you can do this kind of work," she said.
Originally published on Space.com.
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