<h3>Extensive Air Showers</h3>
Over the last 70 years, physicists have studied cosmic rays with energies in excess of ~1014 eV by using the Earth's atmosphere itself as part of the detection equipment .This technique takes advantage of the fact that interaction between high-energy cosmic rays and the air produces a correlated cascade of secondary particles. The process begins with the collision of the primary cosmic ray with a nucleus near the top of the atmosphere. This first collision typically produces more than 50 secondary particles, a majority of which are pi-mesons* (usually referred to as pions).
Pions come in three different flavors: positively charged, negatively charged, and neutral. All pions are unstable, but the charged pions are relatively long-lived and will most probably collide with another nucleus before decaying. The subsequent collisions are similar in nature to the primary collision. This process then leads to a cascade of particles which is referred to as a "hadronic shower".
One third of the pions produced are neutral. The neutral pions are very short-lived and will almost all decay into a pair of photons (gamma rays) before interacting with nuclei in the atmosphere. The photons interact with the nuclei in the air to produce electron-positron pairs, which in turn will produce photons via the "bremsstrahlung" process. This cascading process leads to the formation of an "electromagnetic shower". The hadronic shower itself is continuously producing neutral pions and thus initiating secondary electromagnetic showers along its path.
While high-energy cosmic rays are believed to consist mostly of charged nuclei. Gamma rays have been observed with energies as high as ~1012 eV. In the case of a gamma-ray primary particle, the particle shower produced would be purely electromagnetic. Generically, both types of cascades are called "extensive air showers" (EAS).
Extensive air showers were discovered in the 1930's by French physicist Pierre Victor Auger. In addition to his contributions to the field of cosmic rays, Pierre Auger was most well known for his discovery in the 1920's of a spontaneous process by which an atom with a vacancy in the K-shell achieves a more stable state by the emission of an electron instead of an X-ray photon, commonly known as the Auger Effect. This process forms the basis for the technique of Auger Electron Spectroscopy developed in the 1960's for characterizing surface properties of materials. Pierre Auger held the position of the Head of the Natural Science Sector at UNESCO during the years 1948-1959. Between 1962-1967, he served as Director General for the European Space Research Organization, the forerunner of the European Space Agency (ESA).
As an EAS develops into the atmosphere, more and more particles are produced. A small fraction of the kinetic energy of the primary particle is converted into mass energy. The remaining kinetic energy is then distributed over the shower. The process of multiplication continues until the average energy of the shower particles is insufficient to produce more particles in subsequent collisions. This point of the EAS development is called the "shower maximum". Beyond the maximum, the shower particles are gradually absorbed with an attenuation length of ~200 g/cm2 (rigorously this is a measure of the depth of material penetrated by the shower, which will be explained below).
*Properties of elementary particles can also be found at the web-site of the Particle Data Group.