Tracking Down Source Of High-Energy Cosmic Rays -- Argentina Site Preparations Begin

These giant detectors will be part of the new Pierre Auger Observatory, a $50 million international project led by University of Chicago Nobel laureate James Cronin and Alan Watson of the University of Leeds. Cronin and Watson took part in a groundbreaking ceremony on March 17 in Argentina's Mendoza Providence, the Auger Observatory's Southern Hemisphere site. Construction will begin soon on a large-scale prototype detector array that may go into operation by the end of the year.

The Auger Observatory will practice a new form of astronomy rooted in particle physics, Cronin said. "Nature is trying to tell us something by showing us something extraordinary," he said. "These are the most energetic particles in the universe. There's nothing more energetic."

The groundbreaking ceremony puts the observatory one step closer to realization following long and arduous efforts to build a coalition of 250 scientists in 19 nations to support the Auger Project.

"I have worked on the highest-energy cosmic rays for 35 years and on the Auger project for seven and a half," Watson said. "I passionately want to find out where the highest-energy cosmic rays come from, and I think this project is the most effective way to do it."

Cronin and Watson named the Auger Observatory for the scientist who discovered showers of cosmic ray particles in 1938. Auger conducted research at Chicago in 1942, even launching hot-air balloon experiments from the University's Stagg Field to study cosmic rays. His daughter, Mariette Auger Berl, who attended the groundbreaking ceremony in Argentina, studied arts and literature at Chicago before joining the Free French Forces during World War II.

The origin of high-energy cosmic rays has remained one of the great mysteries of science since Auger's discovery. Low-energy cosmic rays continually strike every square yard of the Earth from all directions. The Auger Observatory will seek cosmic rays that have more than a hundred million times more energy than the particles produced in the world's most powerful particle accelerator at Fermi National Accelerator Laboratory. Only one of these particles strikes any square kilometer (six-tenths of a mile) area in a century.

Scientists suspect that the high-energy particles originate outside the galaxy at a distance of approximately 150 million light years--relatively close by astronomical standards.

"They cannot come from very far away because they interact with the cosmic microwave background radiation," Cronin said. When the particles collide with this low-energy microwave radiation, the afterglow of the big bang, the cosmic rays lose much of their energy.

Cosmic ray particles can only be directly observed above Earth's atmosphere. The Auger Observatory will detect them indirectly. When cosmic ray particles strike the atmosphere, they collide with air molecules, producing a shower containing many millions of particles that fall to Earth over a wide area.

When complete, the Auger Observatory will consist of the Argentina array and another array in Utah to be built later. Each site will consist of 1,600 particle detector stations covering 1,860 square miles. Each station will sit approximately one mile apart, equipped with a 3,000-gallon water tank rigged with instruments that will measure the number of particles passing through. Central computers will combine measurements of the number of particles and their arrival times at each station to determine the direction and energy of the original cosmic rays that set off the shower.

The second detection system will use finely tuned light sensors that will be able to detect the faint glow caused by the collisions between shower particles and air molecules on dark, moonless nights.

Auger Project scientists plan to officially begin their cosmic ray experiments in 2001. Construction of the southern observatory will continue in stages until completion in 2003.

A few high-energy cosmic ray showers already have been detected with existing arrays. The Fly's Eye cosmic ray detector in Utah detected the most powerful one ever recorded on Oct. 15, 1991. The shower packed 300 million times more energy than the Fermilab particle accelerator can generate.

Even the runner-up event, detected at Japan's Akeno Giant Air Shower Array on Dec. 3, 1993, released an energy far beyond what even the most violent exploding stars could achieve.

Scientists can only speculate about the causes of these high-energy particle bursts. One theory suggests that they result from topological defects--stresses and strains in space comparable to faulting and folding within the Earth's crust. Although ideas abound in scientific literature, Cronin said, "we are nowhere near understanding these things."

The 19 nations represented in the Auger Project are Argentina, Armenia, Australia, Bolivia, Brazil, China, Czech Republic, France, Germany, Greece, Italy, Japan, Mexico, Poland, Russia, Slovenia, the United Kingdom, the United States and Vietnam.