AMANDA, Light of my Ice...

An underwater telescope called AMANDA, frozen deep in Antarctic ice, peers down at ghostly neutrinos that pass through Earth from above the Northern Hemisphere. (See Neutrino Nomads) Since neutrinos hardly interact with matter at all, they are ideal carriers of astronomical information across the vast reaches of the universe, quite impervious to scattering by interstellar matter or bending by magnetic fields. The same properties, however, that let neutrinos travel here unobstructed—no charge and hardly any mass—also make them extraordinarily difficult to detect.

A neutrino interacts with the nucleus of an oxygen atom in the atmosphere, producing a muon. As the muon moves through Antarctic ice, it gives off Cherenkov radiation, shown in blue, which is detected by instruments in the spheres. As the muon moves through the array of these spheres, it creates a track that specifies its direction in space. (Image provided by the AMANDA-II Collaboration).

AMANDA researchers Darryn Schneider and Katherine Rawlins hold one of the light-detection modules

AMANDA researchers Darryn Schneider and Katherine Rawlins hold one of the light-detection modules (Photograph by Melanie Conner, courtesy of the National Science Foundation).

Two AMANDA investigators fasten a light detector to a cable, so the detector can be lowered into the hole melted in the Antarctic ice

Two AMANDA investigators fasten a light detector to a cable, so the detector can be lowered into the hole melted in the Antarctic ice (Photograph by Josh Landis, courtesy of the National Science Foundation).

Neutrinos do interact occasionally with atomic nuclei, setting off a reaction that produces elementary particles called muons. The muons are emitted in the same direction in space as the neutrino was moving, so a measurement of the muon’s direction specifies the direction of the neutrino source. The muons move at the speed of light in vacuum, faster than the speed of light in the ice, and consequently they give off “Cherenkov” radiation in passing through the ice, in somewhat the same way that a moving ship makes a wake. So to find neutrinos, AMANDA detects light.

The light detectors, each about the size of a bowling ball, hang like beads on a string on long cables lowered into cylindrical holes in the ice. The holes are melted by a stream of hot water that produces a liquid cylinder about 50 cm in diameter and 2.5 km deep. Over the day it takes the water to refreeze, a cable, studded with as many as 42 detectors, is lowered into place.

The detectors form a three-dimensional array, located at the South Pole. As the muons pass through the ice, successive detections of the Cherenkov radiation yield a series of points along a line through the array and thus determine the muon’s path. Moreover, precise timing of the detector signals reveals which way along this path the muon moved, so the direction in space of the neutrino source can be specified.

A major complication in this experiment is the presence of a huge background of muons produced above Antarctica. When cosmic rays, mostly protons, interact with nuclei of nitrogen and oxygen in the atmosphere, muons are produced in large numbers and indeed are detected by AMANDA. To reject this background, precise timing, as mentioned above, reveals the direction of the muon track and rejects signals of muons moving downwards. Having the detectors point down helps, too, as does burying the detectors deep in the ice. As for muons produced above the northern hemisphere, the roughly 10,000 km of intervening rock filters them out as they pass through Earth to the South Pole.

Why build a neutrino telescope in such an inhospitable place as Antarctica?

The solid ice provides a stable matrix for detectors.
The ice is dark, clear, and free of radioactivity.
Management of the experiment takes place on the solid surface—a much more stable platform than, say, a ship.

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