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  • Stone-Cold Telescope Will Spot Cosmic Neutrinos

Photonics Spectra
Jun 1999
Hank Hogan

MADISON, Wis. — Antarctica is stone cold, covered by a miles-deep sheet of ice. But that ice makes the South Pole ideal for a cosmic neutrino telescope.

That's why nearly 100 researchers representing 14 universities and institutes from across the US and Europe have been working on the Antarctic Muon and Neutrino Detector Array. With extensive support from the National Science Foundation, the telescope is under construction with completion expected next winter.

Bob Morse, a senior scientist in the physics department at the University of Wisconsin, is overseeing the job. He explained why the instrument had to go south.

This drawing shows the Antarctic Muon and Neutrino Detector Array as of July 1998. The latest version of the telescope, when complete, will contain 754 photomultiplier tubes. Courtesy of Douglas Lowder, University of California at Berkeley.

"We need to be deep to get away from cosmic ray muons," he said. "We basically needed a block of ice about two miles thick that was clear, and that had a scientific infrastructure that would support a modern-day, complicated experiment. If you plug all those conditions in, there is only one place that comes up, and that is the South Pole."

Neutrinos are born deep in the heart of supernovas, quasars and neutron stars, carrying otherwise invisible information. As neutrinos travel, they usually breeze by like ghosts. But, occasionally, they interact with the rest of the universe. The resulting faint flash of blue light is like a bow wave, indicating where the neutrino came from and its energy.

Like other neutrino telescopes, the Antarctic Muon and Neutrino Detector Array spots these flashes using a string of photomultiplier tubes. Seven 2.4-km-long strings -- each holding 48 photomultiplier tubes -- will be added to those already sunk into the ice.

When complete in February next year, this version will contain 754 8-in. Hamamatsu photomultiplier tubes. The strings will be frozen in place, resulting in a stable and dark environment, suitable for neutrino hunting. Investigators expect to spot one a day.

This approach is not without problems, however. For one, the site is cold and remote. Also, the telescope will not be able to look at different parts of the sky because it will be sitting on the Earth's axis of rotation.

But the initial results are promising. The cosmic neutrino detector eventually may be scaled up to a cubic kilometer across, with as many as 6000 photomultiplier tubes spotting 30 or so neutrinos a day.

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