SCIENTISTS SEARCH FOR SOURCE OF COSMIC RAYS
HOUGHTON, MI--Something out there is hurling incredibly energetic particles around the universe. But no one knows what that something is. Now scientists from 50 institutions representing 19 countries have joined forces in an attempt to solve the mystery and find the source of high-energy cosmic rays that have bombarded the earth since the beginning of time.
Named the Pierre Auger Project after the French scientist who discovered extensive showers of secondary subatomic particles caused by the collision of primary high-energy particles with air molecules, the unified effort is perhaps the largest truly international scientific collaboration in history.
The project will build two huge observatories--one in the southern hemisphere at a site in Argentina, and the other in the northern hemisphere at a location in Utah. Each observatory will extend over an area 10 times larger than the city of Paris, France and will combine air fluorescence detectors and an array of detectors located on the ground to measure extensive air showers produced when high-energy cosmic rays strike the earth's atmosphere. The ground array will operate continuously, while the fluorescence detectors will provide additional information on dark moonless nights.
The international cost of the project is $50 million for each of the two observatories. Funding for the U.S. participation in the construction and operation of the first observatory has been approved by the U.S. Department of Energy and the National Science Foundation. Ground breaking for the southern hemisphere observatory is scheduled for March 17 of this year. Both facilities are expected to be completed by 2004.
But what are cosmic rays anyway, and why are scientists so interested in them?
"Cosmic rays are fast-moving particles from space that constantly bombard the earth from all directions," explains Northern Hemisphere Project Spokesperson Dr. David Nitz of Michigan Technological University's Physics Department. "Most of the particles, at least those at energies low enough to make identification practical, are either the nuclei of atoms, or electrons. Single protons--the nuclei of hydrogen atoms--make up most of the nuclei, but a few are much heavier, ranging up to the nuclei of lead atoms." Nitz says cosmic ray particles travel at nearly the speed of light, which means they have very high energy. In fact, some of them are the most energetic of any particles ever observed in nature. The highest-energy cosmic rays have a hundred million times more energy than the particles produced in the world's most powerful man-made particle accelerator.
But the source of such super rays remains a mystery, and scientists love a mystery, because solving a mystery in nature provides the opportunity to learn something new about the universe. Investigating the mystery of high-energy cosmic rays is just such an opportunity.
"No one knows the source of the highest-energy cosmic ray particles,"says Nitz. "Most lower-energy cosmic ray particles that strike the earth come from within our own Milky Way Galaxy. Many probably come from the exploding stars we call supernovae."
Over time, some cosmic ray particles pick up energy from moving magnetic fields they encounter as they wander through space. This process of acceleration is well understood for low-energy cosmic rays accelerated by magnetic fields produced by the sun. In our own galaxy, scientists believe that the strong moving magnetic fields produced by supernovae explosions provide the energy for acceleration. Scientists believe that the very highest-energy cosmic ray particles come from sources beyond the Milky Way--but where?
"Wherever they come from, these highest-energy particles could hold secrets to the beginning of the universe," says Nitz. "We know of no source in the cosmos that could produce such energies, not even the power released by the most violent exploding stars. More powerful natural accelerators, therefore, must be responsible for these extraordinary rays, and these accelerators must lie outside our galaxy."
To discover the source of cosmic ray particles, scientists measure their energy and their direction as they arrive from space. To measure cosmic rays directly requires sending detectors to heights above most of the earth's atmosphere--using high-flying balloons and satellites. These techniques become impractical for the highest energy cosmic rays, which are extremely rare. However, cosmic rays can also be detected indirectly on the surface of the earth by observing the showers of particles they produce in the air--which is just what the Pierre Auger observatories are designed to do.
Nitz and his colleagues at Michigan Tech have received a 3-year $620,000 operating grant from the DoE to fund their participation in the project. An additional $600,000 in DoE construction funding will be granted to Michigan Tech to provide instrumentation for the southern hemisphere observatory. The Michigan Tech team will develop and supply custom application specific integrated circuits for the surface detector stations which identify and record in real time the passage of extremely high-energy cosmic rays. They will also oversee the design and implementation of the microwave radio system that provides the link for all information transfer to and from the four fluorescence "eyes" and the 1,600 surface detector stations at each observatory.
Overall Pierre Auger project leaders are Dr. James Cronin, professor of physics at the University of Chicago; and Dr. Alan Watson, professor of physics and pro-vice-chancellor at the University of Leeds, United Kingdom.
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For more information, contact David Nitz at 906-487-2274 or by email: dfnitz@mtu.edu
02/26/99-028