Figure 26 Photomultiplier Tube Schematic Diagram

Radiation Detectors SCINTILLATION COUNTER A schematic cross-section of one Figure 26 Photomultiplier Tube Schematic Diagram type of photomultiplier tube is shown in Figure 26. The photomultiplier is a vacuum tube with a glass envelope containing a photocathode and a series of electrodes called dynodes. Light from a scintillation phosphor liberates electrons from the photocathode by the photoelectric effect. These electrons are not of sufficient number or energy to be detected reliably by conventional electronics. However, in the photomultiplier tube, they are attracted by a voltage drop of about 50 volts to the nearest dynode. The photoelectrons strike the first dynode with sufficient energy to liberate several new electrons for each photoelectron. The second-generation electrons are, in turn, attracted to the second dynode where a larger third-generation group of electrons is emitted. This amplification continues through 10 to 12 stages. At the last dynode, sufficient electrons are available to form a current pulse suitable for further amplification by transistor circuits. The voltage drops between dynodes are established by a single external bias, approximately 1000 volts dc, and a network of external resistors to equalize the voltage drops. The advantages of a scintillation counter are its efficiency and the high precision and counting rates that are possible. These latter attributes are a consequence of the extremely short duration of the light flashes, from about 10^-9 to 10^-6 seconds. The intensity of the light flash and the amplitude of the output voltage pulse are proportional to the energy of the particle responsible for the flash. Consequently, scintillation counters can be used to determine the energy, as well as the number, of the exciting particles (or gamma photons). The photomultiplier tube output is very useful in radiation spectrometry (determination of incident radiation energy levels). Rev. 0 Page 47 IC-06