World's Largest Cosmic Ray Detector Slovenia and Pierre Auger Observatory

Pierre Auger Observatory

Cosmic rays can be simple protons, electrons, or even photons, or they may be heavy nuclei such as oxygen or iron. Since the discovery of cosmic rays more than 90 years ago much progress has been made in understanding the nature of those of relatively low energy. However, despite decades of effort the origin of the most energetic cosmic rays with energies tens of millions of times greater than any produced in terrestrial particle accelerators, remains a mystery. The problem is two fold: the flux at these energies is extraordinarily low -- 1/km²/century; and, the primary cosmic rays themselves are never directly observed. Instead, their nature is inferred from the properties of the enormous air showers which they initiate high in the atmosphere.

The Pierre Auger Observatory project is a broad based international effort to study the highest energy cosmic rays. Two giant detectors, each covering 3000 km², are to be constructed in the Northern and Southern hemispheres. Each will consist of a surface array of 1600 particle detectors operating in conjunction with an atmospheric fluorescence detector. Surface arrays measure the lateral distribution of particles in air showers when they strike the ground; fluorescence detectors view the longitudinal development of showers as they move downward through the atmosphere. These two complementary techniques form a uniquely powerful instrument to study the nature of extreme energy cosmic rays. The objectives of the Pierre Auger Observatory are to measure the arrival direction, energy, and mass composition of the cosmic ray events with the unprecedented statistics and accuracy. Indeed, several thousands events per year with energy above 10¹⁹ eV will be detected after the Southern Pierre Auger Observatory completion in few months. On the other hand, the hybrid detection technique implemented by Auger (when the same events are studied using both fluorescence and shower particle techniques) allows to perform important cross-calibrations, to reduce uncertainties on the reconstructed shower parameters and to better control the systematics.