5.4 Nature of Gas to be Used in Ionization Chamber
59
1. It should require least energy of ionization, i.e., the binding energy of outermost
electron of the atom should be low. Argon requires about 15.7 eV for its ionization.
In fact, the magnitude of the energy of ionization limits the energy of radiation
which can be measured by ionization counter.
2. The gas must have large atomic size to offer greater chance of interaction with
ionizing radiation.
3. The gas should be inert so that it does not react with a material of the counter.
4. It should be easily available, economical, otherwise the counter will become
expensive.
Argon is preferred in ionization chamber because it meets almost all these requirements.
5.5 Regions Suitable for Counting Purposes
From these discussions, it is clear that for detection and measurement of radioactivity,
ionization counter operating in (i) ionization region (ii) proportional region, and (iii)
Geiger region would be useful. In the foregoing section, we shall now devote time
to these three regions only.
A word of caution is perhaps needed here. It will be seen later in the section
describing proportional and Geiger region that there exists a flat plateau in both,
whereas no such plateaus are shown in Fig. 5.2. The reason being that current–voltage
characteristic as shown in Fig. 5.2 is obtained with an ionization chamber over a very
wide range of potentials. If plateaus as shown in Figs. 5.9 and 5.17 were to be shown
in Fig. 5.2 also, then X -axis scale would need to be so large that the length of the
potential axis would become too long to be plotted.
In Fig. 5.2 two graphs are shown, the upper one is for α-particles and the lower
for β-particles. Graph of α-particle is shown above the β-particle, as the former
has higher specific ionization than the latter. In other words, for radioactive sample
containing α-particles and β-particles of similar activity and comparable energy, αparticle will produce larger number of ion-pairs per unit volume in the ionization
chamber than β-particles. Moreover, α-particles always have specific energy (Fig.
5.3AB) hence magnitude of current would be very specific. On the other hand, βparticles possess energy anything from zero to a maximum value E max . As a result, the
amplitude of the current produced in the chamber would also have values from zero to
a maximum. This is shown in Fig. 5.3CD. Therefore, with β-particles, one observes
a spectrum of pulses with varying heights (Fig. 5.3D, small triangular pulses).
It should be noted that the slope of the curve in the proportional region corresponds
to the rate at which the collected charge changes with increasing voltage. This value
is same for an α- and β-radiation. The study of current–voltage characteristics curve
(Fig. 5.2) suggests that ionization counters could be designed to operate at ionization
plateau (B), proportional region (C), Geiger region (E), and spark region (beyond
F). The last two regions cannot differentiate between the type of radiations (e.g.,
59
1. It should require least energy of ionization, i.e., the binding energy of outermost
electron of the atom should be low. Argon requires about 15.7 eV for its ionization.
In fact, the magnitude of the energy of ionization limits the energy of radiation
which can be measured by ionization counter.
2. The gas must have large atomic size to offer greater chance of interaction with
ionizing radiation.
3. The gas should be inert so that it does not react with a material of the counter.
4. It should be easily available, economical, otherwise the counter will become
expensive.
Argon is preferred in ionization chamber because it meets almost all these requirements.
5.5 Regions Suitable for Counting Purposes
From these discussions, it is clear that for detection and measurement of radioactivity,
ionization counter operating in (i) ionization region (ii) proportional region, and (iii)
Geiger region would be useful. In the foregoing section, we shall now devote time
to these three regions only.
A word of caution is perhaps needed here. It will be seen later in the section
describing proportional and Geiger region that there exists a flat plateau in both,
whereas no such plateaus are shown in Fig. 5.2. The reason being that current–voltage
characteristic as shown in Fig. 5.2 is obtained with an ionization chamber over a very
wide range of potentials. If plateaus as shown in Figs. 5.9 and 5.17 were to be shown
in Fig. 5.2 also, then X -axis scale would need to be so large that the length of the
potential axis would become too long to be plotted.
In Fig. 5.2 two graphs are shown, the upper one is for α-particles and the lower
for β-particles. Graph of α-particle is shown above the β-particle, as the former
has higher specific ionization than the latter. In other words, for radioactive sample
containing α-particles and β-particles of similar activity and comparable energy, αparticle will produce larger number of ion-pairs per unit volume in the ionization
chamber than β-particles. Moreover, α-particles always have specific energy (Fig.
5.3AB) hence magnitude of current would be very specific. On the other hand, βparticles possess energy anything from zero to a maximum value E max . As a result, the
amplitude of the current produced in the chamber would also have values from zero to
a maximum. This is shown in Fig. 5.3CD. Therefore, with β-particles, one observes
a spectrum of pulses with varying heights (Fig. 5.3D, small triangular pulses).
It should be noted that the slope of the curve in the proportional region corresponds
to the rate at which the collected charge changes with increasing voltage. This value
is same for an α- and β-radiation. The study of current–voltage characteristics curve
(Fig. 5.2) suggests that ionization counters could be designed to operate at ionization
plateau (B), proportional region (C), Geiger region (E), and spark region (beyond
F). The last two regions cannot differentiate between the type of radiations (e.g.,