The Pierre Auger Observatory is an international cosmic ray observatory in Argentina designed to detect ultra-high-energy cosmic rays: sub-atomic particles traveling nearly at the speed of light and each with energies beyond 1018 eV. In Earth's atmosphere such particles interact with air nuclei and produce various other particles. These effect particles (called an "air shower") can be detected and measured. But since these high energy particles have an estimated arrival rate of just 1 per km2 per century, the Auger Observatory has created a detection area of 3,000 km2 (1,200 sq mi)—the size of Rhode Island, or Luxembourg—in order to record a large number of these events. It is located in the western Mendoza Province, Argentina, near the Andes. Construction began in 2000, the observatory has been taking production-grade data since 2005 and was officially completed in 2008. The northern site was to be located in southeastern Colorado, United States and hosted by Lamar Community College. It also was to consist of water-Cherenkov detectors and fluorescence telescopes, covering the area of 10,370 km2—3.3 times larger than Auger South. The observatory was named after the French physicist Pierre Victor Auger. The project was proposed by Jim Cronin and Alan Watson in 1992. Today, more than 500 physicists from nearly 100 institutions around the world are collaborating to maintain and upgrade the site in Argentina and collect and analyse the measured data. The 15 participating countries shared the $50 million construction budget, each providing a small portion of the total cost.
Physical background From outer space, ultra-high-energy cosmic rays reach Earth. These consist of single sub-atomic particles (protons or atomic nuclei), each with energy levels beyond 1018 eV. When such a single particle reaches Earth atmosphere, it has its energy dissipated by creating billions of other particles: electrons, photons and muons, all near the speed of light. These particles spread longitudinally (perpendicular to the single particle incoming route), creating a forward moving plane of particles, with higher intensities near the axis. Such an incident is called an "air shower". Passing through the atmosphere, this plane of particles creates UV light, invisible to the human eye, called the fluorescing effect, more or less in the pattern of straight lightning traces. These traces can be photographed at high speed by specialised telescopes, called Fluorescence Detectors, overlooking an area at a slight elevation. Then, when the particles reach the Earth's surface, they can be detected when they arrive in a water tank, where they cause visible blue light due to the Cherenkov effect. A sensitive photoelectric tube can catch these impacts. Such a station is called a water-Cherenkov Detector or 'tank'. The Auger Observatory has both types of detectors covering the same area, which allows for very precise measurements. When an air shower hits multiple Cherenkov Detectors on the ground, the direction of the ray can be calculated using basic geometrics. The longitudinal axis point can be determined from the densities in each affected ground station. Depending on the time difference of impact places, the angle of the axis can be determined. Only when the axis would be vertical, all ground detectors register at the very same moment in time, and any tilting of the axis will cause a time difference between earliest and latest touchdown.
Earlier observatories
Cosmic rays were discovered in 1912 by Victor Hess. He measured a difference in ionisation at different heights (using the Eiffel tower and a Hess-manned hot air balloon), an indication of the atmospheric thinning (so spreading) of a single ray. Influence of the Sun was ruled out by measuring during an eclipse. Many scientists researched the phenomenon, sometimes independently, and in 1937 Pierre Auger could conclude in detail that it was a single ray that interacted with air nuclei, causing an electron and photon air shower. At the same time, the third particle muon was discovered (behaving like a very heavy electron).
Overview
Surface detector (SD)
In 1967 University of Leeds had developed a water-Cherenkov detector (or surface station; a small water basin, 1.2 m deep; also called tank) and created a 12 km2 detection area Haverah Park using 200 such tanks. They were arranged in groups of four in a triangular (Y) ground pattern, the triangles in different sizes. The observatory worked for 20 years, and produced the main design parameters for the ground detection system at Auger Observatory. It was Alan Watson who in the later years led the research team and subsequently co-initiated Auger Observatory Collaboration.
Fluorescence detector (FD)
Meanwhile, from the Volcano Ranch (New Mexico, 1959–1978), the Fly's Eye (Dugway, Utah) and its successor the High Resolution Fly's Eye Cosmic Ray Detector called "HiRes" or "Fly's Eye" (University of Utah), the technique of the fluorescence detector was developed. These are optical telescopes, adjusted to picture UV light rays when looking over a surface area. It uses faceted observation (hence the fly's eye reference), to produce pixeled pictures at high speed. In 1992, James Cronin led the research and co-initiated the Auger Observation Collaboration.
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