Under ice

Because neutrinos pass through Earth unimpeded, the AMANDA project uses the planet to block out all other signals. In fact, the telescope points down into Earth, rather than up.

The AMANDA array consists of 677 basketball-sized optical modules spaced along strings dangling in a series of holes burned into the ice. A large array like this is needed because cosmic neutrinos are rarer than those generated nearby, and detecting them is a tall order.
 
 

Less than one in a million neutrinos, on its way through Earth, crashes into a proton, Halzen explained. When one does, a charged particle called a muon is released and sent streaking through the ice. The muon radiates blue light. (This light is known as Cherenkov radiation and has been called the optical equivalent of a sonic boom.)

If one of these collisions occurs near an optical module, AMANDA senses the glow, and from its shape researchers can pinpoint the direction from which it came.

"It's really a proof of concept that the instrument is working and that larger instruments can be built," said Morris L. Aizenman, senior science associate in the Mathematical and Physical Science Directorate of the National Science Foundation.

Aizenman, who was not involved in the research, said it is now just a matter of time and funding before a more sensitive instrument can be constructed that should be able to detect cosmic neutrinos. He added that because neutrinos can go where photons cannot, it's possible that in addition to revealing new information about known phenomena, they could reveal objects or events that we had not even thought of, because we've never been able to see them.

Up next: IceCube and Antares

Halzen and his colleagues have plans to build a much larger detector known as IceCube. It would consist of 4,800 optical modules -- seven times as many as the current detector -- spread through a cubic kilometer of ice.

A proposal for the ambitious project has been approved by the national science foundations in Sweden and Belgium, Halzen said, and awaits approval by the U.S. National Science Foundation, which along with other organizations funded AMANDA.

Other neutrino detectors have been built, most in abandoned mines, but none can be scaled up to reach the sensitivities necessary to detect cosmic neutrinos, researchers say. A group of European researchers plan an underwater neutrino detector called Antares (Astronomy Neutrino Telescope Abyss Research) for 2003. Antares would sit on the floor of the Mediterranean Sea off the coast of France.

Meanwhile, AMANDA has already been scaled up and may produce some cosmic findings of its own.

"We are approaching the size of detector that, according to the best theoretical estimates, is required to detect signals beyond the Sun," Halzen said.

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