The Big Events

It was as if they went out to catch butterflies, and caught the ISS - the International Space Station. It wasn't supposed to happen. Cosmic ray researchers were dumbfounded when their "Fly's Eye" detector in the high Utah desert in the western USA turned up an incoming particle from space with an energy six times higher than their theory allowed. Two years later, on the other side of the world, a Japanese detector recorded another of these "impossible" events. These two carefully documented cosmic rays, whose energy is so high it defies explanation, have spurred the effort to build a new detector big enough to capture and study many more of these high-energy particles, and to try to discover where they came from.

The Fly's Eye Event

The highest-energy cosmic ray ever detected was observed on October 15, 1991 by the Fly's Eye cosmic ray detector in Utah, USA. The detector is located in the desert in Dugway Proving Grounds 75 miles southwest of Salt Lake City. The Fly's Eye detects cosmic rays by observing the light that they cause when they strike the atmosphere. When an extremely high-energy cosmic ray enters the atmosphere, it collides with an atomic nucleus and starts a cascade of charged particles that produce light as they zip through the atmosphere. The charged particles of a cosmic ray air shower travel together at very nearly the speed of light, so the Utah detectors see a fluorescent spot move rapidly along a line through the atmosphere. By measuring how much light comes from each stage of the air shower, one can infer not only the energy of the cosmic ray but also whether it was more likely a simple proton or a heavier nucleus.

The Utah researchers measured the energy of the unusual cosmic ray event in 1991 to be 3.2x10²⁰ eV. They were stunned by their observation. They had previously believed that such energetic particles could not exist in the universe, because theory said the particles should rapidly lose their energy in collisions with the universal microwave radiation left over from the Big Bang. Thus, very high-energy particles now pose a cosmic mystery that has inspired a worldwide collaboration to begin planning the vast new detector called the Pierre Auger Cosmic Ray Observatory.

The AGASA Cosmic Ray Event

Akeno, Japan, a village about 120km west of Tokyo, was the home of the world's largest surface array for detecting very high-energy cosmic ray air showers, until overtaken by the Pierre Auger Observatory in 2004. The Akeno Giant Air Shower Array (AGASA) consists of 111 particle detectors spread about a kilometer apart over an area of 100 square kilometers. Each detector occupies a small hut 2.2 square meters in area. Construction of the array began in 1987; it has been measuring cosmic ray air showers ever since its completion in 1991.

On December 3, 1993, the AGASA array recorded a very large air shower. This very special event was particularly well measured because the air shower fell completely inside the detector array and arrived from a nearly vertical direction. This air shower was produced by a cosmic ray with an energy of about 2x10²⁰ eV. This is the highest-energy cosmic ray observed at AGASA; and, like the Fly's Eye event in Utah, it has an energy well above that expected from any known source.

Scales of Energy

Scientists measure the energies of fast-moving particles like those in cosmic rays and particle accelerators in units called electron volts, abbreviated eV. An electron volt is the amount of energy that one electron gains when it is accelerated by an electrical potential of one volt. (A flashlight battery has about 1.5 volts.) Electrons in a television set are accelerated by the picture tube to an energy of about 50,000 electron volts. When they strike the screen, they make it glow.

The most powerful man-made particle accelerator, Fermilab's Tevatron, can accelerate protons to nearly one trillion electron volts. The highest-energy cosmic ray particle ever observed had an energy 300 million times higher than the protons at the Tevatron. Scientific notation, shown below, saves writing out the many zeros required for such large numbers.