7
Gas-Filled Detectors
71Principles of Gas-Filled Detectors
The operation of a gas-filled detector is based on the ionization of gas mol-
ecules by radiation, followed by collection of the ion pairs as charge or
current with the application of a voltage between two electrodes. The mea-
sured charge or current is proportional to the applied voltage and the
amount and energy of radiation, and depends on the type and pressure of
the gas.
A schematic diagram of a gas-filled detector is shown in Figure 7.1. When
an ionizing radiation beam passes through the gas, it will cause ionization
of the gas molecules and ion pairs will be produced depending on the type
and pressure of the gas. When a voltage is applied between the two elec-
trodes, the negative electrons will move to the anode and the positive ions
to the cathode, thus producing a current that can be measured on a meter.
At very low voltages, the ion pairs do not receive enough acceleration to
reach the electrodes and therefore may combine together to form the orig-
inal molecule instead of being collected by the electrodes. This region is
called the region of recombination (Fig. 7.2). As the applied voltage is grad-
ually increased, a region of saturation is encountered, where the current
measured remains almost the same over the range of applied voltages. In
this region, only the primary ion pairs formed by the initial radiations are
collected. Individual events cannot be detected; only the total current
passing through the chamber is measured. Because specific ionization
differs for a-,b-, and g-radiations, the amount of current produced by these
radiations differs in this region. The voltage in this region is of the order of
50–300 V. Ionization chambers such as dose calibrators are operated in this
region.
When the applied voltage is further increased, the electrons and positive
ions gain such high velocities and energies during their acceleration toward
the electrodes that they cause secondary ionization. The latter will increase
the measured current. This process is referred to as the gas amplification.
This factor can be as high as 10^6 per individual primary event depending on