64
5 Ionization Counters
counter operating in this region does not need any amplifier. This counter cannot
be used to differentiate the pulses of α-particles from β-particles.
It may be appearing as a confusing statement that in ionization chamber the pulses
are of microvolt range and in the proportional region they are of millivolt and yet we
say that pulses produced by α- and β-particles can be distinguished in this region. On
the other hand, pulses produced in Geiger region being of 1 volt cannot distinguish
the nature of radiation. Fact is that pulses produced in ionization or proportional
regions need to be amplified to 1-volt pulse height before they can be recorded by
the instrument. What we mean by such statement is that the average height of pulses
produced in the ionization chambers are of microvolt range, but within this range
there are variations in their pulse height according to the energy of interacting radiation. When these pulses are amplified to 1-volt range, the variation in the pulse
height is not lost. Same is the case with the proportional region. In the Geiger region,
however, all pulses are produced by the interacting radiation to almost same pulse
height and hence cannot be differentiated at all.
Now, we are in a position to discuss the three counters, as mentioned earlier, in
somewhat greater depth.
5.7 Ionization Counters
Ionization counter operates in the region B as shown in Fig. 5.2. This counter can
be made of various designs. One typical form of an ionization counter is shown in
Fig. 5.6a. It consists of a metallic cylindrical chamber containing a central conducting
electrode located at the axis of the chamber but insulated from the main body.
A proper voltage is maintained between the wall of the chamber (as a cathode,
normally is earthed) and the central electrode (as an anode). The chamber is often
filled with dry air at atmospheric pressure but other gases like argon may also be
chosen for this purpose.
Since the ionization counter operates in the region B (Fig. 5.2), ion-pairs formed
by the primary ionization would only be responsible to produce current. We have seen
earlier, that pulses produced by these ion-pairs are of low magnitudes (of the order
of microvolts) and hence a sophisticated amplifier is needed. Moreover, since the
pulses produced by primary ionization process depends upon nature of the radiation
(i.e., α-particles or β-particles, or γ -rays), they can be differentiated by the counter.
For this purpose, a pulse height analyzer (or a discriminator bias, which allows the
pulses to be recorded of height greater than the height set by the unit) is used, which
distinguishes pulses of different energies. The basic problem in this counter is that
pulses produced by radiations like α-particles or β-particles or γ -rays would be
almost of same height as that produced by cosmic radiations (commonly known as
the background radiation) present in the environment. A sophisticated pulse height
analyzer must, therefore, be used to differentiate cosmic radiations from radiations
5 Ionization Counters
counter operating in this region does not need any amplifier. This counter cannot
be used to differentiate the pulses of α-particles from β-particles.
It may be appearing as a confusing statement that in ionization chamber the pulses
are of microvolt range and in the proportional region they are of millivolt and yet we
say that pulses produced by α- and β-particles can be distinguished in this region. On
the other hand, pulses produced in Geiger region being of 1 volt cannot distinguish
the nature of radiation. Fact is that pulses produced in ionization or proportional
regions need to be amplified to 1-volt pulse height before they can be recorded by
the instrument. What we mean by such statement is that the average height of pulses
produced in the ionization chambers are of microvolt range, but within this range
there are variations in their pulse height according to the energy of interacting radiation. When these pulses are amplified to 1-volt range, the variation in the pulse
height is not lost. Same is the case with the proportional region. In the Geiger region,
however, all pulses are produced by the interacting radiation to almost same pulse
height and hence cannot be differentiated at all.
Now, we are in a position to discuss the three counters, as mentioned earlier, in
somewhat greater depth.
5.7 Ionization Counters
Ionization counter operates in the region B as shown in Fig. 5.2. This counter can
be made of various designs. One typical form of an ionization counter is shown in
Fig. 5.6a. It consists of a metallic cylindrical chamber containing a central conducting
electrode located at the axis of the chamber but insulated from the main body.
A proper voltage is maintained between the wall of the chamber (as a cathode,
normally is earthed) and the central electrode (as an anode). The chamber is often
filled with dry air at atmospheric pressure but other gases like argon may also be
chosen for this purpose.
Since the ionization counter operates in the region B (Fig. 5.2), ion-pairs formed
by the primary ionization would only be responsible to produce current. We have seen
earlier, that pulses produced by these ion-pairs are of low magnitudes (of the order
of microvolts) and hence a sophisticated amplifier is needed. Moreover, since the
pulses produced by primary ionization process depends upon nature of the radiation
(i.e., α-particles or β-particles, or γ -rays), they can be differentiated by the counter.
For this purpose, a pulse height analyzer (or a discriminator bias, which allows the
pulses to be recorded of height greater than the height set by the unit) is used, which
distinguishes pulses of different energies. The basic problem in this counter is that
pulses produced by radiations like α-particles or β-particles or γ -rays would be
almost of same height as that produced by cosmic radiations (commonly known as
the background radiation) present in the environment. A sophisticated pulse height
analyzer must, therefore, be used to differentiate cosmic radiations from radiations