Scintillator Paddle Primer

A scintillator "paddle" is simply a paddle shaped device used to detect the passage of a charged particle through it. It has 2 basic parts. The first part, through which the charged particle passes, is a piece of transparent scintillator, typically plastic, but other materials are also used. (Our paddles are cast from polystyrene plastic.) The scintillator is doped with a fluor that glows feebly when some of the kinetic energy of the charged particle is deposited in the scintillator and transferred to the fluor molecules. The de-excitation of the fluor molecules produces light that travels through the scintillator and enters the front face of a photomultiplier tube (pmt) attached to one end of the paddle. The sides of the scintillator are well-polished so that the scintillation photons have a high probability of being totally internally reflected from the sides of the paddle. The scintillator is always covered with an opaque material so that the only light reaching the pmt is scintillation light.

The pmt is the second piece of the paddle and is a device that converts light into electric charge. It is a cylindrical shaped object that contains a set of carefully positioned electrodes ("dynodes"). A high voltage (typically, 1000 Volts) is applied to the pmt so that an electric field is produced between the dynodes. Additionally, the inside surface of the pmt is coated with a thin material ("photocathode") that light can easily knock electrons from. A photon from the scintillator enters the pmt, strikes the photocathode and ejects an electron. The strong electric field inside the pmt accelerates the electron so that when it strikes the next electrode, it liberates a few more electrons, each of which is accelerated until it strikes the next dynode, after which the entire process is repeated. The overall effect is that about a million electrons are produced at the end of the dynode chain for each photon that strikes the pmt.

The final electron avalanche produced at the last electrode ("dynode") leaves the pmt and is detected by electronics attached to the back of the pmt.