Air shower (physics)

Cosmic ray air shower created by a 1TeV proton hitting the atmosphere 20 km above the Earth. The shower was simulated using the AIRES package. Animated 3d models of this and other showers can be found on COSMUS.

An air shower is an extensive (many kilometres (miles) wide) cascade of ionized particles and electromagnetic radiation produced in the atmosphere when a primary cosmic ray (i.e. one of extraterrestrial origin) enters the atmosphere. The term cascade means that the incident particle, which could be a proton, a nucleus, an electron, a photon, or (rarely) a positron, strikes a molecule in the air so as to produce many energetic hadrons. The unstable hadrons decay in the air speedily into other particles and electromagnetic radiation, which are part of the shower components.

The air shower was discovered by Bruno Rossi in 1934. By observing the cosmic ray with the detectors placed apart from each other, Rossi recognized that many particles arrive simultaneously at the detectors.[1] This phenomenon is now called an air shower.

Air shower formationEdit

Air shower formation in the atmosphere. First proton collides with an air molecule creating pions, protons and neutrons.

After the primary cosmic particle has collided with the air molecule, the main part of the first interactions are pions. Also kaons and baryons may be created. Pions and kaons are not stable, thus they may decay into other particles.

The neutral pions \scriptstyle\pi^0 decay into photons \scriptstyle\gamma in a process \scriptstyle\pi^0 \rightarrow \gamma + \gamma . The photons produced form an electromagnetic cascade by creating more photons, electrons and positrons.[2]

The charged pions \scriptstyle\pi^\pm preferentially decay into muons and neutrinos in the processes \scriptstyle\pi^+ \rightarrow \mu^+ + \nu and \scriptstyle\pi^- \rightarrow \mu^- + \nu. This is how the muons and neutrinos are produced in the air shower.[2]

Also, kaon may be an origin of muons, which means the decay process is \scriptstyle K^{+/-} \rightarrow \mu^{+/-} + \nu. In the other hand kaons can produce also pions via the decay mode \scriptstyle K^{+/-} \rightarrow \pi^{+/-} + \pi^0.[2]


The original particle arrives with high energy and hence a velocity near the speed of light, so the products of the collisions tend also to move generally in the same direction as the primary, while to some extent spreading sidewise. In addition, the secondary particles produce a widespread flash of light in forward direction due to the Cherenkov effect, as well as fluorescence light that is emitted isotropically from the excitation of nitrogen molecules. The particle cascade and the light produced in the atmosphere can be detected with surface detector arrays and optical telescopes. Surface detectors typically use Cherenkov detectors or Scintillation counters to detect the charged secondary particles at ground level. The telescopes used to measure the fluorescence and Cherenkov light use large mirrors to focus the light on PMT clusters.

The longitudinal profile of the number of charged particles can be parameterized by the Gaisser-Hillas function.

See alsoEdit


  1. ^ Rao, M. (1998), Extensive Air Showers, World Scientific, p. 5, ISBN 9789810228880 
  2. ^ a b c Rao, M. (1998), Extensive Air Showers, World Scientific, p. 10, ISBN 9789810228880 

External linksEdit

Last modified on 16 April 2013, at 08:21