Physics and Engineering of Radiation Detection

162 Chapter 3. Gas Filled Detectors

E 1

E 2

E 2 E 1

Geiger−

Pulse Height

Bias Voltage

>

region

limited proportionality

Mueller

continuous discharge

region

proportional

recombination

region

ion chamber region

Figure 3.3.1: Variation of pulse height produced by different types of de-

tectors with respect to applied voltage. The two curves correspond to two

different energies of incident radiation.

is the onset of the so called ion chamber region. In this region further increasing
the high voltage does not affect the measured current since all the charges being
produced get collected efficiently by the electrodes. The current measured by the
associated electronics in this region is called saturation current and is proportional
to the energy deposited by the incident radiation. The detectors designed to work
in this region are called ionization chambers.
It is almost impossible to completely eliminate the possibility of charge pair
recombination in the ion-chamber region. However, with proper design, ionization
chambers having plateaus of negligibly small slopes can be built.

3.3.C ProportionalRegion

In the previous chapter we studied the process of production of electron-ion pairs
by the passage of radiation. This type of ionization is referred to as the primary
ionization. If the charges produced during primary ionization have enough energy
they themselves can produce additional electron-ion pairs, a process called secondary
ionization. Further ionization from these charges is also possible provided they have
enough energy. Obviously this process can occur only if a high enough electric
potential exists between the electrodes so that the charges could attain very high
velocities. Although the energy gained by the ions also increases as the bias voltage
is increased, the electrons, owing to their very small mass, are the ones that cause
most of the subsequent ionizations.
Thismultiplicationof charges at high fields is exploited in the proportional detec-
tors to increase the height of the output signal. In such a detector the multiplication