Projects: Pierre Auger Observatory
On the pampas of western Argentina, the Pierre Auger cosmic-ray observatory is studying extensive air showers over an area of 3,000 square kilometers. These showers are generated by particles with far higher energies than any man-made accelerator can reach, and they continue to challenge our understanding.
Above 1019 eV (10 EeV) the rate of particles falling on the Earth's atmosphere is about 1/km2 a year. Thus detectors must be deployed over vast areas to accumulate useful numbers of events. Remarkably, this approach is practical because the cosmic rays generate giant cascades, or air showers, with more than 1010 particles at shower maximum for a 10 EeV primary cosmic ray. Some of the shower particles reach ground-level where they are spread over about 20 km2. The particles also produce fluorescence light by the excitation of atmospheric nitrogen, which provides an alternative and powerful means of detecting the showers and useful complementary information.
The strategy behind the design of the Pierre Auger Observatory is to study showers through detecting not only the particles, with an array of 1600 water Cherenkov detectors, but also the fluorescence light, using four stations, each with six telescopes overlooking the particle detectors. The water tanks are used to measure the energy flow of electrons, photons and muons in the air showers, while the faint light emitted isotropically as the shower moves through the atmosphere can be detected with the fluorescence telescopes. The observatory is now more than 75% complete and has been taking data for more than two years.
