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Detectors at the HAWC Gamma Ray Observatory are protected by custom domes made of military grade fabric.
We have learned a lot about the universe by looking out from our planet. Scientists are discovering much can be learned about outer space by looking at our immediate surroundings as well. One of the most effective ways is by studying invisible light in the form of gamma rays.
The High Altitude Water Cherenkov (HAWC) Gamma Ray Observatory on the slope of Pico De Orizaba in the Mexican state of Puebla is one of the newest facilities for studying this type of light. The HAWC is set high in the mountains at 4,100 meters and currently has 300 Cherenkov detectors, more than 100 of which are currently active.
The Cherenkov method
Studying high-energy gamma rays helps scientists understand some of the most violent events in space, such as such as supernova explosions and the evolution of supermassive black holes. The gamma ray observatory operates by detecting gamma rays indirectly using the water Cherenkov method. This method requires nearly 50,000 gallons of water to be held by a bladder in large metal tanks 24 feet wide and 16.5 feet high. Each bladder houses photomultiplier tubes that help scientists determine the direction gamma rays are traveling.
In order to create a suitable environment for gamma ray observation, each detector is topped with a custom fabric covering. “Fabric allows us to have a reliable secondary light seal,” explains Patrick Younk, a scientist at the Los Alamos National Laboratory (LANL) in New Mexico. “The fabric seals around the perimeter of the cylindrical tank. We also put a gasket seal on the top of each water tank. The fabric arches over the rim of the gasket to complete the seal. It would have been more difficult to get a tight seal with a metal roofing option.”
The fabric
Denver Tent Co. of Denver, Colo., was commissioned to design and install the fabric covers. Jeff Greene, sales manager, says LANL specified fabric for the roof covers at the beginning of the project. “The fabric was constructed to form a dome cover for the water tanks,” he says. “It was shipped directly to the project site in Mexico. LANL [and the various agencies involved] were responsible for the installation of the water tanks and dome covers.”
LANL requested TEMPER fabric, manufactured by BondCote Corp., Pulaski, Va., for the dome covers. TEMPER, an acronym for Tent, Extendable, Modular, Personnel, is a frame-supported, general-purpose fabric tent system designed to provide shelter and environmental protection for U.S. military personnel in the field including forward battle areas. TEMPER is designed with military specifications for extreme weather resistance, flexibility, and light blocking.
“This specification covers four classes and two grades of fire-, water- and weather- resistant cloth,” says Greene. The dome covers also needed to withstand the snow loads and UV light they were exposed to in the high altitude environment.
“The inside of the fabric is black. This is a military specification to prevent people from seeing inside through the fabric, especially at night,” says Younk. “They don’t want light or shadows to get out. We didn’t want [visible] light to get in the tank. We have a light seal, called a bladder, and we needed the fabric as a secondary light seal. The steel tanks work for the sides, but for the roofs we needed the fabric to form a tight covering.”
Custom covers
Before any gamma rays enter a detector, and before any camera can detect and gather data, each water detector is protected from above by TEMPER fabric. To meet the strict demands of the research facility, Denver Tent needed to design and manufacture custom covers to fit each of the 300 detectors.
“Our greatest challenge involved welding on a curve,” says Greene. “If you imagine gift wrapping a basketball, that was what it was like trying to cover the dome. Even though the Temper Tent fabric is fairly pliable, fine-tuning the cut panels and assembling them proved difficult and time-consuming.”
In addition, the domes had to create a lightproof seal. “The other main challenge was closing (or sealing off) the top of the dome,” Greene explains. “We had to glue a cap at the peak. We didn’t have access to any machines that were capable of sealing off the top mechanically.”
Installation and other considerations
Each dome cover spans 24 feet in diameter and 5 feet in height and is designed to be flexible enough to fit multiple surfaces.
Installed by LANL at the project site, the covers are held snug to the sides of each detector with cable. “A cable was sewn into a pocket that runs around the whole perimeter of the fabric,” says Younk. “There are probably about 50 places where the cable is exposed. Each of those areas is brought over hooks that are on the walls that hold the cover securely to the structure.”
Fabric was chosen for this project because of its cost-effectiveness and light-blocking qualities, as well as for environmental and timeline factors. Younk says the project had a five to 10-year timeline for completion, something that would have been difficult and costly to achieve with non-fabric materials.
“We determined that using steel roofs would provide superior longevity, but would take us beyond our timeline,” he says. “Fabric put us into our sweet spot of lifespan and price. We discovered other benefits [to using fabric], as well. The assembly process was almost certainly easier, making installation less labor intensive and expensive.”
The observatory is also visible from Pico de Orizaba, Mexico’s highest mountain, and the third-highest peak in North America. LANL wanted the observatory to blend in with the environmental surroundings as much as possible. “We deployed this project in one of Mexico’s national parks,” Younk says, “and we wanted the fabric to be tan to blend with the surroundings. These are roofs of water tanks, which we can’t turn completely invisible but [fabric] was a much better option than having bright, shiny steel roofs in the sunshine visible from the mountain and the park.”
The project has been successful thus far. Fabric made from materials found on Earth is playing a part in studying high-energy cosmic rays that are expanding our understanding of some of the universe’s most colossal events. The observatory is able to gather information about how stars collapse into black holes, how super massive black holes behave in the center of interstellar gas clouds, and how dark matter collides with regular matter.
Jake Kulju is a freelance writer and editor based in Minneapolis, Minn.