Some materials, when they absorb ionising radiation, produce seconday electrons by the photoelectric, Compton scattering and pair production proceses that generate photons of light. These materials are called scintillators. The light is emitted in a short pulse or flash and the amplitude absorbed so that the level of ionising radiation absorbed. If a meterial of a high atomic number os used, the sensitivity is increased. The most common material used as a scintillator is sodium iodide which is activated by the addition of a small amount of thallium.
Sodium iodide produces a very large amount of light compared to other scintillators.
It takes about 10 nanoseconds for the light to reach it's maximum after the ionising radiation is absorbed. This is it's dead-time . The scintillator is contained in a thin aluminium can, to prevent light entering the outside. The can is sealed to keep moisture out because iodide is hygroscopic, that is it absorbs water from the atmosphere. The inside of the can is coated with powdered titanium dioxide, which acts as a reflector to direct all the light from the scintillator through a quartz or plexiglass window in the rear of the can.