Sky and Telescope
 AMANDA,
at the South Pole, records light flashes from muons created by
neutrinos from the northern sky. By looking through the Earth,
this technique suppresses contamination by cosmic-ray induced
muons that are ever-present. Sky & Telescope diagram.
"The
observation of neutrinos by a neutrino telescope deep in the Antarctic
ice cap, a goal that was once thought difficult if not impossible,
represents an important step toward establishing the field of
high-energy neutrino astronomy first envisioned 40 years ago."
So concludes E. Andrés (University of Wisconsin, Madison)
and a host of coinvestigators in the March 22nd issue of Nature.
They describe
the robust detection of muons — elementary particles with
more than 200 times the mass of an electron that are spawned when
a high-energy neutrino strikes matter, such as Antarctic ice.
The "telescope" employed for these observations was
AMANDA, the Antarctic Muon and Neutrino Detector Array sited at
the South Pole.
A neutrino
is electrically neutral, so its path through space isn't bent
by cosmic magnetic fields and points directly back to its source.
Unfortunately, these nearly massless particles almost never interact
with matter, so a big collecting area is needed, and that's where
AMANDA gains an advantage over other neutrino detectors. AMANDA
actually looks downward — through the Earth — for arriving
muons. When it sees one coming from the Northern Hemisphere, chances
are good that was neutrino-born and not created by cosmic rays
from overhead.
These upward-propagating
muons are detected by means of bluish Cherenkov radiation emitted
while they move through the highly transparent ice at relativistic
speeds. AMANDA employs 10 strings containing a total of 302 photomultiplier
tubes (PMTs) lowered up to 2 kilometers into the ice. By measuring
the precise times at which PMTs light up on different strings,
the scientists determine the direction from which the muon, and
hence the neutrino, came.
In the study,
the scientists report a diffuse, high-energy neutrino background
rate of one event every 19 hours, on average. Interestingly, no
neutrino point sources were found, such as would be expected from
supernova remnants or active galaxies.
The success
of this experiment bodes well for the construction of IceCube,
a much larger array having an effective area of 1 square kilometer
and consisting of 4,800 PMTs on 80 strings.
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