Once a plastic scintillator is exposed to ionized radiation, electrons within the plastic scintillator become excited and move from their locked position within the valence band to the conduction band where they are free to move around, leaving an associated hole behind it.
An electron-hole pair, called an exciton, will continue moving throughout the lattice until it is trapped by a defect within the plastic scintillator or by a deliberately-introduced dopant, e.g. thallium or cerium. These defects are called activators, they create special areas within the plastic scintillator called activator sites and luminescent centers.
In the final stage, electrons trapped within these sites decay, emitting a photon causing – yep, you guessed it – luminescence.