How can one cosmic ray create billions of particles in an air shower?

A single high-energy cosmic ray creates billions of particles in an extensive air shower through a repeating cascade process as it interacts with the Earth's atmosphere.12

When a primary cosmic ray particle enters the atmosphere, it collides with atmospheric nuclei, triggering a chain reaction of secondary particles. This cascade is primarily driven by two repeating mechanisms:32

• Pair production: A high-energy photon interacts with the electric field of an atomic nucleus and is converted into an electron and a positron.2
• Bremsstrahlung: These high-energy charged particles (electrons and positrons) are decelerated by the electric fields of nuclei, causing them to emit high-energy photons.24

This cycle repeats continuously as long as the particles possess sufficient energy, with each stage multiplying the total number of particles. As the shower penetrates deeper into the atmosphere, it grows in size until it reaches a maximum; eventually, the particles lose energy through ionization and other interactions, causing the cascade to gradually decrease in size. This process results in a vast shower of secondaries, including photons, electrons, and muons, which can spread over many square kilometers by the time they reach ground level.21