Physics and Engineering of Radiation Detection

170 Chapter 3. Gas Filled Detectors

3.4.A CurrentVoltageCharacteristics.................

Fig.3.4.1 shows the current-voltage characteristics of an ionization chamber at differ-
ent incident radiation intensities. Normally the chambers are operated in the middle
of the plateau region to avoid any large variation of current with small variations
in the power supply voltage. This ensures stability and softens the requirement of
using very stable power supplies, which are normally very expensive.
As we stated earlier, the plateau of any chamber always has some slope but
normally it is so small that it does not affect the signal to noise ratio in any significant
way.
As shown in Fig.3.4.1, the form of the current-voltage characteristic curve of an
ion chamber does not depend on the intensity of the incident radiation. Quantita-
tively, two differences arise: one is the onset of the plateau region and the second is
the output current amplitude. The underlying reason for both of these differences is
the availability of larger number of electron-ion pairs at higher intensities. If the rate
of production of charge pair increases, then a higher electric field intensity will be
needed to eliminate (or more realistically tominimize) their recombination. Hence
the plateau in such a case will start at a higher voltage. Also, as we will see later, the
output current is proportional to the number of charge pairs in the plateau region
and therefore at higher intensities the plateau current is higher.

φ 1

φ 2

φ 3

φ 3 >>φ 2 φ 1

2

3

1

Bias Voltage

Output Current

Figure 3.4.1: Current-voltage

characteristic curves of an ion-

ization chamber at different in-

cident radiation intensities. The

output signal as well as the on-

set of the plateau (indicated on

the plot with 1,2, and 3) in-

crease with increasing intensity

or flux of radiation.

3.4.B MechanicalDesign

The mechanical design of an ion chamber consists of essentially three components:
an anode, a cathode, and a gas enclosure. The particular geometry of these parts
are application dependent. Fig.3.4.2 shows the two most common ion chamber
geometries and the electric field inside their active volumes. Both of these designs
have their own pros and cons, which we can only understand if we look at the
production and behavior of electron-ion pairs.

B.1 ParallelPlateGeometry....................

This simple design consists of two parallel plates maintained at opposite electrical
potentials (see Fig.3.4.3). Although, as we saw earlier, the curvature in the electric