Summer 2000 -- Vol. 17, No. 1

High-Energy Cosmic-Ray Effort Includes Penn State Contingent

For almost 100 years, high-energy cosmic rays have been a mystery for Earth-bound scientists.

Although their existence has been documented and some research efforts have discovered their enormous energy--in several cases energy levels that surprised the researchers themselves by going beyond the theoretical limits of their experiments--the source of the high-energy rays remains unknown.

Through the Pierre Auger Project (pronounced "o-ja"), a collaboration of about 300 scientists from 19 countries has developed a plan for large-scale observations to address this mystery. In terms of scientific collaboration and the sheer size of its two planned observation sites, the project represents a major commitment to high-energy cosmic rays--whose energy surpasses that of anything that can be generated by a particle accelerator used in classroom and research settings.

In the past, researchers utilized air balloons and smaller ground-based observatory arrays to study high-energy cosmic rays. Their approach to the problem was important and set the groundwork for further study. Still, those efforts pale in comparison to the Auger Project. While those earlier experiments were valuable, they simply were not on the scale of this attempt.

"It's exciting to be part of something so big and so important," said Jim Beatty, associate professor of physics and astronomy and astrophysics at Penn State. "There are so many people involved. Although we have challenges on many levels, from communication and organization to the science itself, the project has such a great potential impact that it's easy to remain focused."

As the project's task leader for surface detector electronics, Beatty must remain focused. Among the contingent of nearly 300 scientists, he's one of only a dozen designated as a task leader. He supervises work involving about a quarter of the project's $50 million budget.

Along with Beatty, Penn State's contingent for the project includes: Stéphane Coutu, assistant professor of physics; Mike DuVernois, a graduate student in physics; Patrick Allison, a graduate student in physics; Scott Posey and John Passeneau, electronics staff; and several undergraduate students who assist with the testing of photomultipliers for the project's cosmic-ray detectors.

"We have a fair number of undergraduates who, at one time or another, have helped out with various aspects of the project," Beatty said. "It's a situation where everyone involved really has contributed."

Most scientists believe low-energy cosmic rays originate in our galaxy, as particles from exploding stars pick up energy from moving magnetic fields when they wander around the galaxy. Scientists do not believe the magnetic fields in our galaxy have enough energy to accelerate particles to the speeds displayed by high-energy cosmic rays, though.

Through the Auger Project, which begins this year with 40 cosmic-ray detectors situated in Mendoza, Argentina, they plan to study the particles that result from the cosmic rays after they hit Earth's atmosphere and shower to the ground. By studying the particles, scientists believe they can trace their path backward to first determine how the high-energy cosmic rays approached Earth and then to pinpoint their origin.

Even though the particles disperse as they come through the atmosphere and hit the ground, that does not pose an overwhelming problem for project scientists.

"Those particles come in fast, and do not move far off their original course. It's like a shotgun," Beatty said. "You aim the shotgun and not every pellet of the shot hits one spot, but if you look at the center . . . that's where the guy was shooting. What we can determine from the particles are the cosmic ray's direction and energy (by the amount of particles that hit the ground). We can understand something about the kind of ray it was just by the types of particles present when the shower reaches the ground."

Organizing the observatory array into a hexagonal shape also assists in the project. Eventually each site, initially in Argentina, and later in Millard County, Utah, will feature 1,800-square-mile grids--an area about the size of Rhode Island--with each of the detectors inside the grid located about a mile from each of the others.

Such a large ground area was required for the observatory array because high-energy cosmic rays are so rare. In order to capture just a few such rays, scientists must cast a huge "net."

The scientists selected a hexagonal grid for their research because of its uniformity. With the potential for particles to enter the detector field from any direction, the hexagon shape allows the best possible target area for particles to land.

"It's the most symmetrical arrangement--if you pick one detector, every other one of its nearest neighbors is the same distance away," Beatty said. "If you look at it from an angle and walk around, it has the smoothest variation of its properties."

Integral parts of that "net" are the detectors themselves. Each of the self-contained, solar-powered units includes a 3,000-gallon water tank. When particles pass through the tanks, the water allows scientists to detect "Cherenkov radiation." That's possible because the particles are traveling at about 186,000 miles per second, but the water allows light to travel at only 140,000 miles per second.

When this happens, the electric field of the particle cannot keep up with its motion and an electromagnetic shockwave, known as "Cherenkov radiation," is generated. In a sense, the particles generate a wake, similar to what a fast-moving boat does on the surface of the water. At each detector station, instruments will measure the number of particles that arrive. When particles hit a station, a small computer in the detector will communicate with computers at a central data center to determine if the particles are part of a large shower. At the data center, measurements from each of the detector stations will be combined to determine the direction and energy of the cosmic ray.

The array will measure about 3,000 cosmic-ray events each year, with about 30 detected at the highest energy levels. Those most powerful, high-energy cosmic rays carry an energy of about 50 Joules--roughly the equivalent of a second serve in tennis, when a tennis ball travels more than 30 miles per hour.

A second detection system will utilize the faint florescent glow caused by collisions of shower particles with air molecules during cosmic-ray air showers. On dark, moonless nights, finely tuned light sensors can measure that fluorescence. The amount of light generated depends on the number of particles in the shower and the cosmic ray's initial energy. Using the direction and shape of the light can help determine the ray's initial direction.

Scientists hope the project's combination of its large scale and advanced technology will help them to better understand the high-energy cosmic rays that bombard Earth.

By working to determine the origin of the rays, the scientists also hope to gain a better understanding of our universe.

Back to Science Journal Summer 2000 Index

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