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A 100-Telescope Net for Gamma Rays

An artist's conception of a future ground-based gamma-ray system. (Image courtesy of J. Buckley, University of Washington. Click for larger image.)

One hundred telescopes, stretching one after another in every direction for half a kilometer, point toward the heavens every night to observe the most energetic processes in the universe—everything from supermassive black holes to exploding stars. For now, this tableau is just a dream. But a team of SLACers, in collaboration with researchers around the world, are working to make it a reality.

"This is an exciting project, one that will reveal the universe's most energetic gamma rays: the ones above one tera-electron volt," said astrophysicist Stefan Funk of the Kavli Institute for Particle Astrophysics and Cosmology. "But the project is still in the very initial stages, and there's much to be done."

Funk is a member of the Advanced Gamma-ray Imaging System, or AGIS, collaboration, which is designing the next big gamma-ray telescope. With ten times more sensitivity to gamma rays than previous experiments—including the Fermi Gamma-ray Space Telescope—the array will detect sources that are too distant and too weak for current telescopes to pick up, adding missing detail to scientists' gamma-ray map of the universe. The AGIS telescope will also be substantially better at pinpointing the direction from which each gamma ray came, helping researchers to better understand the origin of the rays and how these ultra-high energy particles are produced. The project is still in the early phases of research and development, and has not yet secured funding outside the collaborators' home institutions. If all goes according to plan, the collaboration will start building the array around 2014.

Gamma rays, the universe's highest energy form of light, are thought to emanate from jets of plasma streaming from enormous black holes, supernovae and other astronomical sources. These fast-moving light rays stream through space and continually strike the Earth's atmosphere. Each strike produces an electron–positron pair that races along for a mere moment before it, too, interacts with atmospheric gas and creates another gamma ray of slightly lower energy. This process repeats again and again, creating a "shower" of particles—each with slightly less energy than its precursors—until the last gamma-ray does not have enough energy to create a new electron-positron pair and the shower ends, typically about 10 kilometers above the Earth's surface.

"And that's a good thing, because otherwise there wouldn't be life on Earth," Funk said.

Before it ends, however, the particle shower performs a neat trick. The high-energy particles travel faster than light through the atmosphere, creating a burst of visible light called Cherenkov light. Following in the footsteps of previous ground-based experiments, the AGIS telescope will use the Earth's atmosphere as an extension of its detector, tracing the gamma rays by observing this light.

In order to view enough of the Cherenkov light to be an order of magnitude more sensitive than previous experiments, the AGIS collaboration will build between 50 and 100 telescopes, all spread over a square kilometer of land. Each telescope will use a fleet of small mirrors acting as one single large mirror to focus the Cherenkov light onto an array of photomultiplier tubes—essentially, a very sensitive digital camera—at its center. Existing ground-based gamma-ray detectors use a very similar design, but on a much smaller scale; the largest such arrays currently use only four such telescopes.

"These [preexisting] telescopes are pieces of art," Funk said. "They are more or less each handmade," something that won't be economically feasible if the AGIS collaboration needs to mass produce as many as 100 telescopes. "So now we're doing R&D to determine how best to build them," Funk continued. "We need to determine the optimal size of the telescopes, the ideal altitude and spacing between them. At the same time, we're also working to improve the technology."

One technological hurtle is the shutter speed of each telescope's camera. Although the Cherenkov light is so bright that it drowns out the background light of the night sky, it does so only for a tiny fraction of a second. The longer the photo exposure, the more background light enters the camera, washing away the bright but fleeting Cherenkov light. As part of the AGIS R&D effort, collaborators at Argonne National Lab and U.C. Santa Cruz are working on new photon detectors that could replace the photomultiplier tubes. The goal is a shutter speed of 10 nanoseconds—about one million times faster than an average handheld point-and-shoot camera.

KIPAC researchers are exploring the next step in the process: the readout electronics. With such a quick shutter speed, an AGIS camera will have the ability to take half a billion photos a second, if it doesn't require any dead time between photos. But that's a big "if." The camera will take extremely high-resolution photos and will need to export that electronic information before taking the next photo. The lengthier the dead time, the more likely that the camera will miss a fleeting particle shower.

"Here at KIPAC, we're essentially looking at how we can take the output, change it into a more usable form, and digitize it," Funk said. "The hard part is doing that very quickly, very cheaply and very reliably. Fortunately, a lot of physics experiments have these same requirements, and there is quite a bit of expertise at SLAC."

—Kelen Tuttle
SLAC Today, March 10, 2009