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
