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Cosmic Rays and Air Showers Explained
Cosmic rays are protons and atomic nuclei arriving from space, some with energies far beyond anything particle accelerators can reach. When one strikes the nucleus of an air molecule 15–30 km up, it sets off an air shower: a cascade that can contain billions of particles by the time it reaches the ground.
How the cascade grows
- The first collision makes mostly pions.
- Neutral pions decay at once into two photons, which start an electromagnetic cascade: photons turn into electron–positron pairs, and electrons radiate new photons, roughly doubling the particle number every radiation length (about 37 g/cm² of air).
- Charged pions either hit another nucleus or decay into muons, which are penetrating and mostly reach the ground.
Shower maximum
The cascade grows until particle energies drop to about 85 MeV, then it dies away. The depth where it peaks is called Xmax. A 1 PeV (10¹⁵ eV) proton makes about a million particles at its maximum.
What showers tell us
Iron nuclei start their showers higher (smaller Xmax) and make more muons than protons of the same energy. By measuring Xmax and muon numbers, observatories like the Pierre Auger Observatory in Argentina and LHAASO in China work out what cosmic rays are made of, and where the most energetic ones come from.
Frequently asked questions
What is a cosmic ray?
A high-energy particle from space, usually a proton or an atomic nucleus. The most energetic ones carry millions of times more energy than particles in the LHC.
What is an air shower?
The cascade of secondary particles produced when a cosmic ray collides with the atmosphere. It can contain billions of electrons, photons and muons.
Why do muons reach the ground?
They are penetrating, and they live long enough because of relativistic time dilation: at nearly the speed of light their internal clocks run slow.
What is Xmax?
The atmospheric depth at which an air shower has the most particles. It depends on the energy and type of the original cosmic ray.