Cloud chamber photo of a shower" of electrons & positrons produced by a cosmic ray. The primary cosmic rays are atomic nuclei which collide with atoms in the upper atmosphere. A common product of these collisions is the neutral pion, which rapidly decays into pairs of gamma rays. In this picture, one of these gamma-rays initiates the cascade of electron-positron pairs at the top of the chamber. The electrons & positrons curve in different directions in the chamber's magnetic field. They also produce more gamma rays, through radiation & annihilation; when these interact with two horizontal sheets of lead (black) the process repeats itself, regenerating the shower each time."