A delta ray is a secondary electron with enough energy to escape a significant distance away from the primary radiation beam and produce further ionization.
Collectively, these electrons are defined as delta radiation when they have sufficient energy to ionize further atoms through subsequent interactions on their own.
Delta rays appear as branches in the main track of a cloud chamber (See Figs.
These branches will appear nearer the start of the track of a heavy charged particle, where more energy is imparted to the ionized electrons.
In a bubble chamber, electrons will lose their energy more quickly than other particles through Bremsstrahlung and will create a spiral track due to their small mass and the magnetic field.