THE entire Antarctic ice cap could be turned into a vast detector to look for
neutrinos. Researchers behind the project say that the continent-sized detector
could help solve one of astronomy鈥檚 biggest mysteries: where the highest-energy
cosmic rays come from.
鈥淚t鈥檚 a very bold experiment,鈥� says Pierre Sokolsky, an expert on high-energy
cosmic rays at the University of Utah in Salt Lake City. 鈥淭hese guys are the
best hope for a new technique that may be applicable not just for neutrinos but
for charged cosmic rays as well.鈥�
Cosmic rays are particles such as protons that bombard the Earth from space.
The standard model of particle physics predicts that these particles should not
have an energy of more than about 1020 electronvolts when they reach Earth, a
limit called the GZK cut-off. But Sokolsky and others have detected a few
particles with even higher energies.
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Astronomers believe that distant objects such as quasars or gigantic black
holes could be creating these particles, but they don鈥檛 know how. However, they
say that interactions between these particles and the cosmic background
radiation should create ultra-high-energy 鈥淕ZK neutrinos鈥�, which should also be
reaching Earth. Being able to study these neutrinos, could help astronomers
understand how the cosmic rays were created.
The trouble is that ultra-high-energy neutrinos are currently undetectable.
Now Peter Gorham of NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California,
and his colleagues think they have a way to spot them. If a GZK neutrino smashed
into the nucleus of an atom in a non-conducting material, it would generate a
powerful burst of radio waves. Gorham鈥檚 team recently confirmed this at the
Stanford Linear Accelerator Center in Menlo Park, California, using high-energy
photons to simulate a neutrino collision in a block of silica.
These collisions are so rare in nature that spotting them requires a huge
mass of a material transparent to radio waves. 鈥淭hat got us thinking about the
Antarctic ice cap,鈥� says Gorham. 鈥淏elow 鈥�20 掳C ice is almost
completely lossless to radio waves.鈥�
The researchers calculated that a balloon flying 40 kilometres above the
South Pole could monitor nearly a million cubic kilometres of ice at a time for
any telltale radio pulses. They have designed a receiver for the balloon,
equipped with an array of 36 radio antennas to monitor the ice cap for pulses at
frequencies between 300 and 1500 megahertz.
If NASA approves, the balloon-based instrument could be launched from McMurdo
Station in Antarctica in December 2004. Gorham believes that if GZK neutrinos
are reaching Earth, they should detect them within 30 days of flight.
鈥淚t is the first realistic detector concept that can probe for GZK neutrinos
at the expected levels,鈥� says project member Steven Barwick, an astrophysicist
from the University of California at Irvine. 鈥淣ow people have confidence that if
something is out there, we will find it.鈥�
If the team detects GZK neutrinos, it will be a sure sign that sources of
high-energy cosmic rays exist throughout the Universe. 鈥淭hat would be a very
big deal,鈥� says Sokolsky.