Tracks of electrons and positrons. These tracks show part of a shower of particles, moving from top to bottom through a hydrogen-neon bubble chamber. The shower was triggered by a collision between a neutrino and a neon nucleus. The tracks spiralling inwards anticlockwise are produced by electrons. Anti-electrons (or positrons) are spiralling inwards clockwise. The paired tracks appearing from a point source are electron-positron pairs formed from high-energy photons in a process known as pair-creation. This photograph was taken in the 15-foot bubble chamber at Fermilab. The region shown here is about 1 metre wide.