In theory, an ionisation chamber can also be operated as a proportional counter or as a Geiger counter . The voltage applied to the electrodes would determine the mode in which the chamber operates. In practice however, the design of the chamber will be different in each case.
For the following discussion we will assume that we have a cylindrical detector with a central and outer electrode which can operate in any of the three modes by simply changing the voltage.
Let us also assume a beta particle ionises the gas in
the detector and produces 100 ion pairs . If the
detector is operated as an ionisation chamber, 100 electrons will be
collected at the anode when the correct voltage is
applied. The effect of changing the electrode voltage
is illustrated in
Fig.6.4 
.
Figure 6.4:
Characteristics of gas-filled tubes.
In the region AB recombination of ions is occurring. When the region BC is reached all the 100 ions the beta particle produces are collected. This plateau is called the ionisation chamber region. As the voltage is increased beyond C, more ions than the beta particle produced are being collected. This is because the primary electrons, created by the beta particle, are accelerated towards the anode and gain sufficient energy to cause further ionisation in the gas, producing secondary electrons . These secondary electrons can also go on to give further ionisation if the voltage is high enough. This effect is called gas amplification.