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By Peter Rejcek
ARIANNA proposes to use radio waves to capture the
elusive particles
The Chinese philosopher Lao Tzu once wrote that a journey
of a thousand miles begins with a single step.
For a team of physicists hoping to learn more about the
high-energy universe, the journey toward building an array
of 10,000 instruments for just that purpose began this
past season with a single prototype deployed on a 600-meter-thick
ice shelf.
“We’re trying to find the sources of ultra-high-energy
cosmic rays in the universe,” explained Spencer Klein
, leader of the three-person field team that set up the
detector in an area called Moore’s Bay, more than
100 kilometers from McMurdo Station . Those galactic cosmic
rays — rays being something of a misnomer for the
highly charged particles — pack the energy of a well-hit
tennis ball in just one particle as it hits the Earth’s
upper atmosphere and bursts into a trillion smaller bits.
Nothing on Earth — not even the Large Hadron Collider
— is capable of producing a particle of comparable
energy, according to Klein. To put it in perspective:
One would need to build an accelerator around the sun
to produce these particles, Klein said.
“That tells us that somewhere there are these extremely
high-energy accelerators in the universe,” said Klein,
a physicist in the Nuclear Science Division at Lawrence
Berkeley National Laboratory (LBNL) . “We would very
much like to know where these cosmic accelerators are,
and how they work.”
Unfortunately, cosmic rays themselves are lousy tracers
for getting back to the source of these high-powered galactic
accelerators. Interstellar magnetic fields bend the electrically
charged particles as they hurl through space, meaning
there’s not a straight line back to the spot that
first spit them out.
Physicists need a different type of probe — a neutrino.
Yes, the same subatomic particle with no charge and little
mass that the IceCube Neutrino Observatory at the South
Pole is being built to detect.
IceCube uses strings of sensors buried deep in the icecap
to detect not the neutrino itself but the light created
by neutrino collisions in the ice. The collision creates
a negatively charged particle called a muon, which continues
in the same direction as the neutrino. And as it travels
through the ice, it produces a cone of blue light (called
Cherenkov radiation) that IceCube detects.
But Klein and his colleagues, including the project’s
principal investigator Steve Barwick with the University
of California-Irvine , want to detect even higher-energy
neutrino events than IceCube will with its strings of
ice-encased photomultipliers. At ultra-high energies,
IceCube is too small to detect the tiny flux, Klein said.
They want to use radio waves.
The grand scheme is called ARIANNA, for Antarctic Ross
Ice shelf Antenna Neutrino Array. It would consist of
up to 10,000 stations, each consisting of four to eight
TV-like antennae embedded just below the surface of the
ice, spread over more than 100 square kilometers. The
antennae are connected to an electronics box for processing
the radio pulses. Solar panels would charge the detector
in the summer.
For the winter, Klein said the team is testing a wind
generator on the prototype instrument, which would place
limits on power consumption during the dark Antarctic
months.
The ARIANNA detectors search for radio waves produced
by neutrino interactions in the ice. The radio pulses
come from the particle showers produced when the neutrino
converts its energy into matter. Radio waves in ice are
particularly attractive, Klein said, because you get radio
emissions from the entire particle shower. Also, radio
waves can travel up to a kilometer in the ice.
“That means if you’re building stations you
can put them on a roughly 1-kilometer grid, and you need
fewer of them,” Klein said.
The physicists also believe the water below the ice shelf
is an additional advantage. The ice-water interface reflects
radio waves, bouncing back any emissions from downward-traveling
neutrinos.
Popular theories for the source of high-energy particles
include something called active galactic nuclei, which
are galaxies with a super massive black hole at the center.
Colliding black holes or collapsing giant stars are also
possible sources.
“Those are just theories, and it could just as well
be something else,” Klein said.
Klein, LBNL engineer Thorsten Stezelberger, and Martha
Story from McMurdo’s Berg Field Center, spent 11
days in a field camp setting up the prototype radio detector,
which will stay out for a year. In addition, work included
characterizing properties of the ice, such as measuring
the radio wave reflections from where the ice meets the
water.
Klein, also involved with the IceCube experiment, has
been to the South Pole, but the ARIANNA fieldwork was
new for the physicist. “Going out with three people
and a tent was a very different experience. It was a small
group, so everybody was doing everything,” he said.
If all goes as planned — the prototype station survives
the winter, the site demonstrates the necessary properties
— the ARIANNA team hopes to move forward a proposal
to deploy a five- to seven-station array with new hardware
specifically designed for the project.
“That would be the next step. You don’t go
from one station to thousands,” Klein said.
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