AMANDA, Light of my Ice...: Research

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.

The design of IceCube, to be built around the existing AMANDA neutrino telescope, which is shown as a yellow cylinder. (Image by Darwin Rianto, courtesy of Ice Cube Collaboration. This material is based upon work supported by the National Science Foundation under Grant Nos. OPP-9980474 [AMANDA] and OPP-0236449 [IceCube], University of Wisconsin-Madison.)

Direction of neutrino events detected by AMANDA-II, the second generation AMANDA instrument. 90 degrees indicates looking toward the North Pole, and the “equator” indicates the horizon. The red band below the horizon is the edge of the background from muons and atmospheric neutrinos produced above the South Pole. The data points in this map are attributed to neutrinos produced in the atmosphere above the northern hemisphere. (Image provided by the AMANDA-II collaboration)

Layout of IceCube. (Image by Darwin Rianto, courtesy of Ice Cube Collaboration. This material is based upon work supported by the National Science Foundation under Grant Nos. OPP-9980474 [AMANDA] and OPP-0236449 [IceCube], University of Wisconsin-Madison.)

AMANDA functioned as a testbed and demonstrated the capability to detect neutrinos. In addition, to operate as a telescope it also had to accurately determine the direction of a neutrino’s path in space. To test its “pointing ability,” AMANDA was operated simultaneously with a much smaller and highly accurate particle detector. When the results were compared, the investigators found that AMANDA could correctly specify the muon path to within less than three degrees, sufficient accuracy for a neutrino telescope. The image of the neutrino sky shows AMANDA data.

With proof of concept established, the investigators are substantial enlarging AMANDA into an instrument, aptly named IceCube, with an array of detectors filling an entire cubic kilometer of ice. IceCube will consist of 4,800 detectors hung from 80 strings, in a volume large enough to detect neutrinos from distant astronomical sources, as shown in the two diagrams.

These astronomical neutrinos must be separated from the background of “atmospheric” neutrinos produced in secondary cosmic ray interactions, that is, from the decay of the products of cosmic ray-induced nuclear disintegrations. To distinguish between atmospheric and astronomical neutrinos, IceCube will rely on its greater size and efficiency to increase substantially the counts of detected neutrinos. The atmospheric neutrinos will be spread out uniformly, as in the above sky map, but the astronomical neutrinos will be concentrated around the direction of the neutrino source. IceCube must detect enough neutrinos so that an astronomical event in the distant universe will show up when the averaged background count is subtracted out.

In the past, as astronomers have extended their observations to microwaves, radio waves, and also to cosmic rays, each new kind of observation revealed fundamental processes at work in the universe or made possible the discovery of whole new classes of astronomical objects. Neutrino astronomy promises just such exciting results.