5.3 General Design of an Ionization Chamber
57
5.3.1.3 Proportional Region
When the voltage is further increased, primary ions are accelerated due to large
electrostatic field developed across the two electrodes. These accelerated primary
ions on interaction with gas molecules produce further ionization of gas molecules.
These new ion-pairs are called secondary ion-pairs, and process is known as secondary ionization of gas molecules. It must be remembered here that number of
ion-pairs formed by radiation has not altered. Only the number of ion-pairs was
formed initially by radiation, due to increase in the electrostatic field that gets amplified. The process of multiplication of ion-pairs to produce secondary ion-pairs is
also called gas amplification. As a result, the number of ions which reach the collecting electrode becomes more than initially produced by the interacting radiation.
Consequently, current starts to increase with increase in the potential (C). If voltage
is further increased, more and more secondary ion-pairs are produced. Current due
to ions collected at the anode rises rapidly with increasing voltage. This increase in
current collected at the electrode is proportional to the voltage. This voltage range
is referred to as the proportional region. Like the ionization region, in this region
also, ion-pairs produced per unit volume of gas is higher for α-particles than for βparticles. This is because the probability of interaction of α-particles with argon gas
is more than β-particles due to heavier mass of the former particles. Because of this,
for the same number of α-particles and β-particles, the former will produce more
number of ion-pairs. Hence, number of ion-pairs formed per unit volume would be
more with α-particles than with β-particles. Thus, in this region, one can differentiate
ionization produced due to α-particles from β-particles. This region has one more
added advantage over the ionization region; the magnitude of current is much higher
in the proportional region as compared to ionization region. Hence, one does not
need a sophisticated instrument to measure current in the proportional region.
5.3.1.4 Region of Limited Proportionality
Beyond the proportionality region, there is no strict proportionality between voltage
and amount of charge collected. This region is called the limited proportionality
region, and is shown in the segment (D). In this region, the slope of curves for
the two types of radiation (i.e., α-particles and β-particles) are not the same. This
is because the amount of charge that can be collected is limited by the particular
characteristic of the chamber in use.
5.3.1.5 Geiger Region
If the voltage is further increased, two curves coincide. It is observed that the charge
collected is not at all dependent on either the type of radiation or number of primary
ions initially formed. In fact, it depends only on voltage applied to the electrode. In this
region, field intensity around the center of the electrode is so high that any ion formed,
57
5.3.1.3 Proportional Region
When the voltage is further increased, primary ions are accelerated due to large
electrostatic field developed across the two electrodes. These accelerated primary
ions on interaction with gas molecules produce further ionization of gas molecules.
These new ion-pairs are called secondary ion-pairs, and process is known as secondary ionization of gas molecules. It must be remembered here that number of
ion-pairs formed by radiation has not altered. Only the number of ion-pairs was
formed initially by radiation, due to increase in the electrostatic field that gets amplified. The process of multiplication of ion-pairs to produce secondary ion-pairs is
also called gas amplification. As a result, the number of ions which reach the collecting electrode becomes more than initially produced by the interacting radiation.
Consequently, current starts to increase with increase in the potential (C). If voltage
is further increased, more and more secondary ion-pairs are produced. Current due
to ions collected at the anode rises rapidly with increasing voltage. This increase in
current collected at the electrode is proportional to the voltage. This voltage range
is referred to as the proportional region. Like the ionization region, in this region
also, ion-pairs produced per unit volume of gas is higher for α-particles than for βparticles. This is because the probability of interaction of α-particles with argon gas
is more than β-particles due to heavier mass of the former particles. Because of this,
for the same number of α-particles and β-particles, the former will produce more
number of ion-pairs. Hence, number of ion-pairs formed per unit volume would be
more with α-particles than with β-particles. Thus, in this region, one can differentiate
ionization produced due to α-particles from β-particles. This region has one more
added advantage over the ionization region; the magnitude of current is much higher
in the proportional region as compared to ionization region. Hence, one does not
need a sophisticated instrument to measure current in the proportional region.
5.3.1.4 Region of Limited Proportionality
Beyond the proportionality region, there is no strict proportionality between voltage
and amount of charge collected. This region is called the limited proportionality
region, and is shown in the segment (D). In this region, the slope of curves for
the two types of radiation (i.e., α-particles and β-particles) are not the same. This
is because the amount of charge that can be collected is limited by the particular
characteristic of the chamber in use.
5.3.1.5 Geiger Region
If the voltage is further increased, two curves coincide. It is observed that the charge
collected is not at all dependent on either the type of radiation or number of primary
ions initially formed. In fact, it depends only on voltage applied to the electrode. In this
region, field intensity around the center of the electrode is so high that any ion formed,