Particle shower
An electromagnetic shower begins when a high-energy electron, positron or photon enters a material.
At high energies (above a few MeV), in which the photoelectric effect and Compton scattering are insignificant, photons interact with matter primarily via pair production — that is, they convert into an electron-positron pair, interacting with an atomic nucleus or electron in order to conserve momentum.
High-energy electrons and positrons primarily emit photons, a process called bremsstrahlung.
These two processes (pair production and bremsstrahlung) continue, leading to a cascade of particles of decreasing energy until photons fall below the pair production threshold, and energy losses of electrons other than bremsstrahlung start to dominate.
The characteristic amount of matter traversed for these related interactions is called the radiation length
Beyond that point electrons are increasingly affected by multiple scattering, and the lateral size scales with the Molière radius
The propagation of the photons in the shower causes deviations from Molière radius scaling.
The mean longitudinal profile of the energy deposition in electromagnetic cascades is reasonably well described by a gamma distribution: where
About half of the incident hadron energy is passed on to additional secondaries.
The remainder is consumed in multiparticle production of slow pions and in other processes.
Another important characteristic of the hadronic shower is that it takes longer to develop than the electromagnetic one.
This can be seen by comparing the number of particles present versus depth for pion and electron initiated showers.
[citation needed] A simple model for the cascade theory of electronic showers can be formulated as a set of integro-partial differential equations.
It was from these air showers that the first muons and pions were detected experimentally, and they are used today by a number of experiments as a means of observing ultra-high-energy cosmic rays.
Some experiments, like Fly's Eye, have observed the visible atmospheric fluorescence produced at the peak intensity of the shower; others, like Haverah Park experiment, have detected the remains of a shower by sampling the energy deposited over a large area on the ground.
