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.