廣延空氣簇射是高能宇宙射線進入地球大氣層的現象。當宇宙射線與大氣層的空氣粒子相互作用，產生若干粒子；而新生粒子又再產生更多粒子。此複合過程稱為粒子級聯。級聯粒子群有如一塊擴展中的薄餅，以近乎光速行進。
Extensive Air Shower (EAS) happens when a cosmic ray enter Earth's atmosphere. When a cosmic ray particle hits air particles, several particles are created, then particles cascades are created susequently. The expanding sheet of particle cascade flies in nearly speed of light.
級聯結構示意圖 Schematic diagram of cascade
直到粒子的平均能量低於約 8x107 電子伏特，級聯便會由吸收過程取代。「薄餅」的粒子數目會隨之而下降；但範圍則繼續擴展。截至到達地面前，「薄餅」厚約數米，闊愈百米。
When particle energy is lowered to about 8x107 eV, the cascade process stop and the particles start to be absorbed. The number density of particle decrease while the size is still expanding. It is as thin as a few meters and as wide as a few hundred meters when it touches the ground..
粒子產生和數量示意圖 Schematic diagram of particle creation
由光子引發的空氣簇射包含電子（e-）、反電子（e+）和 g 射線；而由核子引發的則包含 m 子、中微子( n )和重子（即質子（p）、中子（n）和 p 介子）。除 m 子和中微子外，大部份簇射粒子都會被吸收而無法到達地面。
A cascade triggered by a photon causes electrons(e-), positrons(e+) and g rays; that triggered by a nucleus evolves into muons( m ), neutrinos( n ) and baryons (protons(p), neutrons(n) and pi-mesons( p ) ). Apart from muons and neutrinos, most cascade particles cannot reach the ground.
到達地面的簇射粒子數目會隨不同因素而改變，包括原初宇宙射線的能量、觀測高度等。因此，透過比較不同觀測站的數據，可以推算原初宇宙射線的能量、入射方向等資料。
The number of cascade particles changes according to the incident cosmic ray energy, observation height, etc. Hence the properties, such as energy and incident direction, of the incident cosmic ray can be calculated according to data from stations.