Figure 26   Photomultiplier Tube Schematic Diagram

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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


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