7 - Scintillation counters

Summary

One of the most commonly used particle detectors is the scintillation counter. A fraction of the energy lost by a charged particle can excite atoms in the scintillating medium. A small percentage of the energy released in the subsequent deexcitation can produce visible light. The technique has been used since the earliest investigations of radioactivity, when, for instance, Rutherford used scintillating ZnS crystals in his alpha particle scattering experiments.

In modern detectors light produced in the scintillator is propagated through light guides and directed onto the face of a photomultiplier tube. Photoelectrons emitted from the cathode of the tube are amplified to give a fast electronic pulse, which can be used for triggering or timing applications.

The scintillation process

We define a scintillator to be any material that produces a pulse of light shortly after the passage of a particle. The phenomenon is closely related to fluorescence, which is usually defined to be the production of a light pulse shortly following the absorption of a light quantum. “Shortly” here refers to time intervals on the order of 10 ns or less. Phosphorescence is a third phenomenon involving light emission, but in this case the molecules are left in a meta-stable state, and the emission may occur much later than the initiating event.

Both inorganic and organic scintillators have been discovered. The scintillation process is different for the two groups.