World’s Largest Air Shower Array Now on Track of Super-high-energy Cosmic Rays
Pierre Auger Observatory seeks source of highest-energy extraterrestrial
particles
Batavia, Ill.- With the completion of its hundredth surface detector, the
Pierre Auger Observatory, under construction in Argentina, this week became
the largest cosmic-ray air shower array in the world. Managed by scientists
at the Department of Energy’s Fermi National Accelerator Laboratory, the
Pierre Auger project so far encompasses a 70-square-mile array of detectors
that are tracking the most violent-and perhaps most puzzling- processes in
the entire universe.
Cosmic rays are extraterrestrial particles-usually protons or heavier
ions-that hit the Earth’s atmosphere and create cascades of secondary
particles. While cosmic rays approach the earth at a range of energies,
scientists long believed that their energy could not exceed 1020 electron
volts, some 100 million times the proton energy achievable in Fermilab’s
Tevatron, the most powerful particle accelerator in the world. But recent
experiments in Japan and Utah have detected a few such ultrahigh energy
cosmic rays, raising questions about what extraordinary events in the
universe could have produced them.
“How does nature create the conditions to accelerate a tiny particle to such
an energy?” asked Alan Watson, physics professor at the University of Leeds,
UK, and spokesperson for the Pierre Auger collaboration of 250 scientists
from 14 countries. “Tracking these ultrahigh-energy particles back to their
sources will answer that question.”
Scientific theory can account for the sources of low- and medium-energy
cosmic rays, but the origin of these rare high-energy cosmic rays remains a
mystery. To identify the cosmic mechanisms that produce microscopic
particles at macroscopic energy, the Pierre Auger collaboration is
installing an array that will ultimately comprise 1,600 surface detectors in
an area of the Argentine Pampa Amarilla the size of Rhode Island, near the
town of MalargUe, about 600 miles west of Buenos Aires. The first 100
detectors are already surveying the southern sky.
“These highest-energy cosmic rays are messengers from the extreme universe,”
said Nobel Prize winner Jim Cronin, of the University of Chicago, who
conceived the Auger experiment together with Watson. “They represent a great
opportunity for discoveries.”
The highest-energy cosmic rays are extremely rare, hitting the Earth’s
atmosphere about once per year per square mile. When complete in 2005, the
Pierre Auger observatory will cover approximately 1,200 square miles (3,000
square kilometers), allowing scientists to catch many of these events.
“Our experiment will pick up where the AGASA experiment has left off,” said
project manager Paul Mantsch, Fermilab, referring to the Akeno Giant Air
Shower Array (AGASA) experiment in Japan. “At highest energies, the
astonishing results from the two largest cosmic-ray experiments appear to be
in conflict. AGASA sees more events than the HiRes experiment in Utah, but
the statistics of both experiments are limited.”
The Pierre Auger project, named after the pioneering French physicist who
first observed extended air showers in 1938, combines the detection methods
used in the Japanese and Utah experiments. Surface detectors are spaced one
mile apart. Each surface unit consists of a 4-foot-high cylindrical tank
filled with 3,000 gallons of pure water, a solar panel, and an antenna for
wireless transmission of data. Sensors register the invisible particle
avalanches, triggered at an altitude of six to twelve miles just
microseconds earlier, as they reach the ground. The particle showers strike
several tanks almost simultaneously.
In addition to the tanks, the new observatory will feature 24 HiRes-type
fluorescence telescopes that can pick up the faint ultraviolet glow emitted
by air showers in mid-air. The fluorescence telescopes, which can only be
operated during dark, moonless nights, are sensitive enough to pick up the
light emitted by a 4-watt lamp traveling six miles away at almost the speed
of light.
“It is a really beautiful thing that we have a hybrid system,” said Watson.
“We can look at air showers in two modes. We can measure their energy in two
independent ways.”
The Pierre Auger collaboration is in the process of preparing a proposal for
a second site of its observatory, to be located in the United States.
Featuring the same design as the Argentinean site, the second detector array
would scan the northern sky for the sources of the most powerful cosmic
rays.
Funding for the $55 million Pierre Auger Observatory in Argentina has come
from 14 member nations. The U.S. contributes 20 percent of the total cost,
with support provided by the Office of Science of the Department of Energy
and by the National Science Foundation. A list of all participating
institutions is available at http://auger.cnrs.fr/collaboration.html
Fermilab is a national laboratory funded by the Office of Science of the
U.S. Department of Energy, operated by Universities Research Association,
Inc.
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Photos are available at
http://www.fnal.gov/pub/presspass/press_releases/auger_photos/index.html
More information on the experiment is at http://www.auger.org/
