NEUTRINOS: CLUES TO THE MOST ENERGETIC COSMIC RAYS
PhysOrg.com
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APril 20 2010
(PhysOrg.com) — ARIANNA, a proposed array of detectors for capturing
the most energetic cosmic rays, is being tested in Antarctica with
a prototype station built last December on the Ross Ice Shelf by a
Berkeley Lab team. By detecting neutrino-generated signals bounced
off the interface of water and ice beneath the shelf, scientists
hope to pinpoint the still unidentified sources of ultra-high-energy
cosmic rays.
We’re constantly being peppered by showers of debris from cosmic rays
colliding with atoms in the atmosphere. Cosmic rays aren’t actually
rays, of course, they’re particles; ninety percent are protons, the
nuclei of hydrogen atoms, and most of the rest are heavier nuclei like
iron. Some originate from our own sun but most come from farther off,
from the Milky Way or beyond.
"The most energetic cosmic rays are the rarest, and they pose the
biggest mystery," says Spencer Klein of Berkeley Lab’s Nuclear Science
Division. He compares the energy of an ultra-high-energy (UHE) cosmic
ray to a well-hit tennis ball or a boxer’s punch – all packed into
a single atomic nucleus.
"If they’re protons, they have about 40 million times the energy of
the protons accelerated at the Large Hadron Collider," Klein says.
"With present technology we’d need to build an accelerator around
the sun to produce protons that energetic. Not only do we not know
how these cosmic accelerators work, we don’t even know where they are."
Being electrically charged, even the most energetic cosmic rays
are forced to bend when they traverse interstellar magnetic fields,
so it’s not possible to extrapolate where they came from by looking
back along their paths when they arrive on Earth.
Yet they can’t come from too far away. Klein explains that because
cosmic rays lose energy by plowing into the photons of the cosmic
microwave background as they travel, "the ones that we observe must
come from the ‘local’ universe, within about 225 million light years
of Earth. This sounds like a long distance, but, on cosmic scales,
it isn’t very far."
In all that volume of "nearby" space, sources capable of producing
such high-energy nuclei have not been clearly identified. One clue to
the origin of the highest-energy cosmic rays is the neutrinos they
produce when they interact with the very cosmic microwave photons
that slow them down.
How to find a cosmic accelerator
"Neutrinos have important advantages as observational tools," says
Klein. "The only way they interact is through the weak interaction, so
they aren’t deflected by magnetic fields in flight, and they easily
slip through dense matter like stars that would stop the cosmic
rays themselves."
The flip side is that it’s quite a trick to catch neutrinos, especially
those produced by rare events. Locating neutrinos produced by UHE
cosmic rays needs a detector covering a huge area.
Which is how Klein came to find himself tent-camping on the Ross Ice
Shelf last December (the middle of summer in Antarctica), along with
his colleague Thorsten Stezelberger of the Lab’s Engineering Division
and camp manager Martha Story from the Berg Field Center, a support
service at McMurdo Station, the main U.S. base in Antarctica. Klein
and Stezelberger were setting up a prototype station for the proposed
ARIANNA array of neutrino detectors (ARIANNA stands for the Antarctic
Ross Ice Shelf Antenna Neutrino Array).
Unlike such neutrino detectors as SNO in Canada, Daya Bay in China,
Super-Kamiokande in Japan, or IceCube, the huge neutrino telescope
under construction deep in the ice at the South Pole, ARIANNA doesn’t
need miles of rock or the Earth itself to filter out background
events. That’s because ARIANNA will be looking for an unusual kind
of neutrino signal known as the Askaryan effect.
ARIANNA will observe the shower of electrons, positrons, and other
particles produced when a neutrino interacts in the ice below the
ARIANNA detectors. In 1962, Gurgen Askaryan, an Armenian physicist,
pointed out that these showers contain more electrons than positrons,
so have a net electric charge. When a shower develops in ice, this
moving charge is an electrical current which produces a powerful
pulse of radio waves, emitted in a cone around the neutrino direction.
The energy shed by particles moving faster than the speed of light
in a medium like glass or water (light moves through water at
only three-quarters of its speed in vacuum) is called Cherenkov
radiation, and is perhaps most familiar as the blue glow made by
fast-moving electrons in a pool surrounding a nuclear reactor. The
same visible-light-wavelength Cherenkov radiation is used to detect
charged-particle events created by neutrinos in detectors like IceCube.
Instead of optical wavelengths, ARIANNA observes Cherenkov radiation
at radio wavelengths; the strength of the radio signal is proportional
to the square of the energy of the neutrino that gave rise to it. To
capture these signals, ARIANNA will use radio antennas buried in the
snow on top of the ice.
An energetic neutrino striking the upper atmosphere creates a shower
of particles in which electrons predominate. When the shower enters
the ice, it sheds Cherenkov radiation in the form of radio waves,
which reflect from the interface of ice and water and are detected
by antennas buried in the snow.
The Ross Ice Shelf makes an ideal component of the ARIANNA detector –
not least because the interface where the ice, hundreds of meters
thick, meets the liquid water below is an excellent mirror for
reflecting radio waves. Signals from neutrino events overhead can
be detected by looking for radio waves that have been reflected from
this mirror. For neutrinos arriving horizontally, some of the radio
waves will be directly detected, and some will be detected after
being reflected.
As envisaged by its principal investigator, Steven Barwick of UC Irvine
– who visited the Ross Ice Shelf in 2008 – ARIANNA would eventually
be comprised of up to 10,000 stations covering a square expanse of
ice 30 kilometers on a side.
Neutrinos on ice
Ten thousand stations is the eventual goal, but the first step is to
see whether just one station can work. During the Antarctic summer,
solar panels will provide power for the radio antennas under the snow
and the internet tower that sends data back to McMurdo Station, via a
repeater tower on nearby Mt. Discovery. During the long, dark winter,
it’s hoped that the power will come from wind turbines or a generator.
When the temperature is mostly below freezing even summer camping is a
challenge, as Klein and Stezelberger found. With all supplies brought
in by helicopter, the team set up three tents for sleeping, a larger
(10 foot by 20 foot) tent as a kitchen, dining room, laboratory
and office, and a small tent for a toilet. Instead of tent pegs,
the tents are held down by guy ropes tied to "deadman anchors."
"For each rope, we dug a two-foot-deep hole and buried a long bamboo
stake with the rope tied to it," Stezelberger explains. "When it was
taut, we refilled the hole with snow – a fair bit of work."
On the second day the team unpacked and assembled the six-foot tall
station tower, made of metal pipes anchored to plywood feet under the
snow. The tower holds four solar panels, a wind turbine, and antennas
for receiving time signals from global positioning satellites, and
for communicating via Iridium communications satellites.
Klein, Stezelberger and Story spent the third day assembling,
testing, and burying the neutrino-detecting antennas in six-foot-deep
trenches in the snow. On the fourth day an internet tower – network
communications were invaluable for sending data north, and for
allowing people to work remotely on the station computer – was brought
in by helicopter and erected by a four-person crew, who stayed for
lunch. "Fortunately they brought their own," Klein remarks. "We were
wondering how we’d feed everyone with only four forks, four spoons,
and four knives."
After another week, which was mostly spent testing instruments,
including bouncing radio signals off the water-ice interface, plus
two days waiting for the weather to clear so that helicopters could
pick them up, the team finally struck camp. After packaging their
gear in slings to be picked up by subsequent flights, they climbed
aboard a chopper and returned to base, leaving behind a functioning
station intended to survive the oncoming winter.
Klein and Stezelberger made it back to Berkeley Lab by the last day
of December. Klein, aided by UC Irvine’s Barwick and graduate student
Jordan Hanson, neutrino physicist Ryan Nichol of University College
London, and Lisa Gerhardt of Berkeley Lab’s Nuclear Science Division
(herself recently returned from work on IceCube at the South Pole),
spent the next weeks analyzing the data from the ARIANNA prototype
station on the ice, as it continued to report via the internet. The
stream of information included housekeeping data and scientific data
in the form of antenna signals.
"Wind had generally been so calm during the week and a half we spent
on the ice, we were afraid the wind generator wasn’t going to be
sufficient for the station’s power needs during the winter," Klein
says. "But after we left, the wind picked up and the wind turbine
started functioning, which encouraged us."
The antenna data was also instructive, and there was a lot of it –
signals from natural background noise and from man-made sources. An
event every 60 seconds was the "heartbeat" pulse emitted by the
station itself, which the team had set up to check the detector.
"But there were other, unexpected periodic signals, pairs separated
by almost exactly six seconds, their rate varying over 24 hours,"
Gerhardt says. Periodic signals strongly hint at man-made sources. "We
think they’re probably from the switching of the power supplies for
the internet hardware."
Other events, aperiodic, were part of the irreducible background,
including thermal noise due to molecular motion in the equipment. This
set a natural limit to the detector’s performance but should be
improved with better equipment.
One thing the prototype station hasn’t seen is an energetic neutrino,
and Klein doesn’t expect it to catch one. If the prototype survives the
winter, the next step will be a group of five to seven such stations
with equipment custom-designed to do the job. The full array is far
in the future.
"One real event would be an accomplishment," says Klein, "and it
might take a hundred stations to achieve even that. UHE cosmic rays
are extremely rare. If we can track just one back to its origin,
we’ll have made a tremendous advance in neutrino astronomy."