198 Chapter 3. Gas Filled Detectors
A good example of an external quenching circuit is the one that rapidly drops
the anode voltage right after the beginning of a discharge. This is essentially equiv-
alent to delivering a negative pulse of high amplitude to the detector, thus rapidly
removing the space charge. The rise time of such a pulse is kept very small (on the
order of several tens of a nanosecond) to ensure high efficiency in decreasing the
dead time (8).
3.6.E Advantages and Disadvantages of GM Counters
The advantages of GM counters are:
Simplicity in design: GM counters are perhaps the easiest to build and
operate in terms of readout electronics.Invulnerability to environmental changes: Since the magnitude of the
output pulse in GM detectors is very high, therefore they work almost inde-
pendent of changes in temperature and pressure.Following are some of the disadvantages of GM counters.Energy/Particle discrimination:The pulse height of the GM counters is
not proportional to the energy deposited by the radiation and therefore they
can not be used to measure dose or discriminate between types of radiations
or their energies.Low dynamic range: The dead time losses in GM counters increase with
radiation strength. The effect can be reduced by decreasing the size of the
chamber, though at the expense of reduced sensitivity. Due to this the dynamic
range of GM counters is very limited.3.7 SourcesofErrorinGaseousDetectors
3.7.A RecombinationLosses
Ideally the measured ionization current in an ionization chamber should consist of
all the electron-ion pairs generated in the active volume. However, due to different
losses, the electrons and ions are not fully collected. For precision measurements,
these losses must be taken into account. The recombination of electrons and ions
is one of the major source of uncertainty in measurements especially at high fluxes
of incident photons. Intuitively one can think that the recombination rate should
depend directly on the concentration of charges. This suggests that the rate of
change in the number of positive and negative charges should be proportional to the
number of charges themselves, that is
dn+
dt= S−αn+n− (3.7.1)
dn−
dt= S−αn+n−. (3.7.2)