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U physicist Michael DuVernois in the Antarctica in his hunt for the mysterious source of cosmic rays.
Catching the rays
U researchers work to solve the enigma of cosmic rays
By Deane Morrison
Published on December 23, 2004
Astronauts riding the space shuttle can tell when they're passing over the South Atlantic. Former astronaut George "Pinky" Nelson once said that if he was resting during a mission and saw fireworks with his eyes closed, he knew his optical system was being bombarded by cosmic rays, which are relatively abundant over that part of the world. Actually, cosmic rays really aren't "rays" at all, but tiny particles that sail through space from the far reaches of the Milky Way galaxy and beyond. Most are the nuclei of atoms, ranging from solitary protons (the nucleus of the hydrogen atom) up to the nuclei of heavier elements, which contain both protons and neutrons.
These particles move close to the speed of light, and some of them carry the same amount of energy--read "wallop"--as a baseball thrown at 100 mph.Most of us never experience these mysterious messengers from the sky. Because cosmic rays carry an electric charge, the vast majority that head toward Earth are caught by our planet's magnetic field. The magnetic field has a weak spot over the South Atlantic, and that's why Nelson and other astronauts notice more "hits" there. Cosmic rays vary in the amount of energy they have; the most energetic just zip straight through Earth's magnetic field. But even these don't do any appreciable damage because when they enter the atmosphere, they almost always collide with molecules of atmospheric gases. It's these rare speedsters that intrigue U of M physicist Michael DuVernois. An assistant professor since 2000, he is part of a tradition of cosmic ray physics at the U that includes the late Phyllis Freier and Edward Ney, along with C.J. "Jake" Waddington, who still works in the field. "The ultra-high-energy cosmic rays come from outside the Galaxy," DuVernois says. These particles move close to the speed of light, and some of them carry the same amount of energy--read "wallop"--as a baseball thrown at 100 mph. What excites DuVernois and other cosmic ray physicists is the prospect of discovering the identity of the unseen Nolan Ryan hurling these cosmic fastballs our way. Whatever it is, it must be a source of unimaginable power. One way to unmask the dynamo is to look in the part of the sky where cosmic rays originate and see what's there. DuVernois is involved in two major experiments to find the "home address" of these visitors. He has his work cut out for him. The "fastballs" are so rare, each square mile of Earth's atmosphere gets only about one per year, on average. To catch these rays, one needs a lot of detectors spread over a huge area. Enter the Pierre Auger Array, an experiment being assembled on the windswept pampas of Argentina. The array is a network of particle detectors that will eventually cover an area the size of Rhode Island. Each detector is a tub containing 3,000 gallons of ultrapure water, a solar panel for power, and antennas for data transmission. They are placed about a mile apart. When a cosmic ray hits a molecule of gas in the atmosphere, it creates a shower of subatomic particles, just as a projectile hitting a glass vase produces a shower of glass. These particles can be detected if they go shooting through the pure water of a detector tank. The fallout from a big shower will hit several tanks, but the hits won't be simultaneous. From analyzing the sequence in which tanks register hits, the physicists can get an idea of the direction the particles came from. The array also contains sensitive light detectors that can see the particle showers. The showers cause atoms of nitrogen-the most abundant atmospheric gas-to glow like ultraviolet light bulbs, one after another along the downward path of the particles. DuVernois, whose main role is assembling the central data acquisition system for the project, has made many trips to Argentina to help place the detectors. The biggest problems? Frequent flats because of poor tire quality and burrowing animals called vizcachas, which like to get under the collector tubs. "The array is about one-third fully operational and is being put out into the field at a still increasing rate," says DuVernois. "About 14 or 15 more months of deployment, and we should be complete. Scientifically, the array is the largest in the world already." The international team of scientists operating the array, which is managed by the Department of Energy's Fermi National Accelerator Laboratory, will present its first scientific results at an international conference in India next summer. In the second experiment, physicists are flying a high-altitude balloon over Antarctica to pick up clues to where cosmic rays come from. The clues arrive as tiny, but very energetic, particles called neutrinos. Neutrinos are produced deep in space by many different processes, some of which may also generate cosmic rays. Neutrinos that approach Earth from the north will slip straight through the planet, emerging through the antarctic icecap. Occasionally, a neutrino will interact with matter in the ice, giving off a faint radio signal that can be detected by instruments aboard the balloon. Because neutrinos travel in straight lines, their paths point back to their origins in space.