The most widely used portable radiation instrument is the geiger counter.
To use the idealised chamber as a geiger detector it has to be operated at a
higher voltage than that used for the proportional counter
. Looking back at
Fig. 6.4
shows that further increase of the voltage between the
electrodes of the chamber beyond the proportional range
causes increasing gas amplification until a point F is
reached at which the number of electrons
collected remains fairly constant as the voltage goes up. This plateau, FG,
is called the Geiger-Müller or GM region .
In this limiting region the tube
is delivering the maximum number of secondary
electrons it can produce. The
resulting pulse size is constant regardless of the initial number of primary
electrons . In other words, any ionizing event will
trigger the tube and give
the same sized pulse. These pulses are relatively large and can be easily
handled with simple electronics. Because of this, instruments operating in the
geiger region are fairly simple and are sensitive to low levels of
ionisation .
Further increase of the tube voltage above the point G will result in continuous arcing across the electrodes which may damage the detector.
It should be realized that in the above discussion it is the electric field rather than the voltage which is the relevant quantity. This means that the effects described above due to increasing voltage can be achieved equally or in combination with a reduction in the diameter of the central collection electrode .