Catch a Falling Ray

The mention of Argentina conjures any number of exotic or dramatic images … Eva Peron … dancing the tango … gauchos riding the plains … falling high-energy cosmic rays.

Well, perhaps high-energy cosmic rays haven’t worked their way into the travel brochures or spawned a Broadway musical just yet. Nevertheless, these rays – tiny particles from space that regularly pelt the earth – are the subject of one of the largest-scale scientific studies of its kind ever conducted, and one of the first parts of this project is now up and running in Argentina.

On a massive area of open plains just east of the Andes Mountains – a region known as Pampa Amarilla – a group of LSU professors, post-doctoral researchers and graduate students have been working on this $100 million cosmic-ray study with a broad international coalition at the new Pierre Auger Observatory.

The international coalition consists of some 250 scientists from 14 countries. The Auger facility, when completed, will span some 1,200 square miles and include more than 1,600 water tank detectors and several other structures, including a specialized observatory with 24 optical telescopes. The facility is managed by scientists from the Department of Energy’s Fermi National Accelerator Laboratory in Chicago.

With the recent commissioning of its 100th water tank detector, the Auger Observatory became the largest cosmic-ray experiment ever conducted. It continues to expand, and completion of the entire Argentina facility is expected in 2005.

LSU Associate Professor of Physics James Matthews, Dept. of Physics and Astronomy Chair Roger McNeil, postdoctoral researchers Rishi Meyhandan and Troy Porter, and several graduate students are helping to build this new facility, analyze the data gathered there and develop equipment and computer software and programming to aid in the experiments.

The background work on the project began more than 10 years ago, but the Argentinian facility only recently began operating and collecting data. Indeed, the LSU group is now analyzing the first round of information gathered at the facility.

A Cosmic Mystery

The project is large in scope because high-energy cosmic rays have baffled scientists for years and solving the mystery behind them requires a massive effort, explains Matthews. The structure of lower-energy rays – protons, nuclei, etc. – has been understood for some time, but scientists „don’t even know what the highest energy rays are or where they come from,“ he says.

„The only thing we know for sure about the high-energy rays is that they exist,“ Matthews says. „So, the best way to understand them is to collect and measure them … let them tell us what they are and where they are from.“

According to Matthews, cosmic rays were discovered more than a century ago. They strike the Earth from all different directions, all of the time, and they come in a range of „energies“ that measure not only how fast they move, but how much „punch“ they pack. Lower-energy rays are common, but high-energy rays are uncommon and have energy levels so high that they cannot be produced on earth.

„The highest energy rays are more than a billion times more energetic than any particles that can be produced in terrestrial (particle) accelerators,“ Matthews says. „It’s difficult to even imagine how to get particles to such energies.“

Rare rays

Because the particles are so rare, collecting or studying them individually ? with a detector in space, for instance ? would take „something very huge,“ Matthews says.

„If Tiger Stadium were floating in space, it might catch one every 20 years or so,“ he says.

However, Matthews says, there are two dependable ways to detect them. Each makes use of the fact that, when a high-energy cosmic ray strikes the earth’s atmosphere, it blasts apart into a shower of particles that fall to the ground, primarily in the form of electrons.

First, it is possible to observe the shower that develops in the atmosphere when the rays hit. Matthews says that the shower produces a weak fluorescence – „a line of faintly glowing atmosphere“ – that can be observed with the special „Fly’s-Eye“ fluorescence telescopes that observe the sky in all directions. Second, it is possible to collect or detect the falling particles using water-tank detectors widely dispersed on the ground.

However, in order for these methods to work for their project, Matthews says, certain criteria had to be met. The only way for a telescope to see the very brief, faint light of an atmosphere shower is on very dark, clear nights, meaning that the location could be nowhere near a major city. In addition, in order to successfully collect the falling particles, it was necessary to find a location with miles of available space where the numerous collection instruments could be placed and spaced appropriately.

The location in Western Argentina fit the bill nicely, Matthews says. In addition, there are a number of South American scientists with experience in studying cosmic rays who are taking part in the project. Thus, the Auger Observatory was established.

Nevertheless, Matthews explains, in order to make sure the project can view the entire celestial sky, a second facility will be required in the Northern Hemisphere. Two possible sites in remote areas of Utah and Colorado have been identified, but the Northern observatory won’t be up-and-running for a few more years.

„These highest-energy cosmic rays are messengers from the extreme universe,“ says Nobel Prize-winner Jim Cronin of the University of Chicago, who helped conceive the Auger experiment. „They represent a great opportunity for discoveries.“

Funding for the Pierre Auger Observatory in Argentina has come from 14 member nations. The United States 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. Further information on the project is available at http://www.auger.org.

Rob Anderson
LSU Media Relations
225-578-3871