The light emitted by the scintillator molecules is transferred as electric impulses by a photomultiplier. The main parts of the photomultiplier are the photocathode and the multiplying system (dynodes). In the scintillation detector, the scintillation crys­tal and the photomultiplier are coupled together (Figure 14.6). Very good optical connection is required between the scintillator and the photocathode, which is pro­vided by silicon oil with high viscosity.

The operation of the photomultiplier is as follows. The light emitted by the scintil­lator produces electrons (photo electrons) in the photocathode. The photocathode is usually located onto the inner wall of the input window. It is semipermeable for the input light. Most frequently, antimony or its compounds are used as photocathodes. The antimony is evaporated and deposited onto the inner wall. Then it is treated with alkali metals or little oxygen. The photocathode SbCsO is an example, with the high­est sensitivity at a 440 nm wavelength. This is close to the wavelength of the light emitted by most scintillators (about 400 nm). As mentioned previously, the addition of a secondary scintilator can modify the wavelength of the light if required.

The electrons emitted by the photocathode are transmitted to the dynodes of the multiplying system. The dynodes have an increasingly higher voltage, and the dif­ference between the adjacent dynodes is about 80—150 V; thus, the quantity of the electrons is multiplied from one dinode to another. At the end of the dynode sys­tem, the quantity of the electrons (i. e., the current) is high enough to be detected directly by the usual electronic devices. The sensitivity of the photomultipliers (namely, the signal-to-noise ratio) is very good.

The quantity of the electrons can be multiplied even by a factor of 108. The mul­tiplication factor of the electrons depends on the number, the geometry, and the voltage of the dynodes. Usually, 8—15 dynodes are applied. The multiplication factor is limited by the dark current of the photocathode, which is caused by the spontaneous electron emission of the photocathode. Cooling the photocathode decreases the dark current.

The electric impulse outputs from the photomultiplier are attenuated, discrimi­nated, and registered.