92
5 Ionization Counters
The fifth column represents aluminum while the third and fourth columns represent mica windows of different thickness. It is obvious from Table 5.1 that for counting weak energetic β-emitters, mica window with a thickness about 0.9 mgcm
−2
should be preferred. The unit of thickness is given in mgcm
−2 because absorption of
radiation depends upon thickness (cm) and the density of the material, i.e., mg/cm
3
× cm = mg cm
−2 .
The liquid sample of weak β-emitter, however, cannot be counted by a liquid
G.M. counter, because its wall thickness is around 50 mgcm
−2 . It should be noted
that the values given in Table 5.1 give the probability of transmission of β-particles
through these windows, and it should not be confused with the efficiency of counting
because efficiency of counting depends upon geometry of counting, source thickness,
efficiency of ion-pair production inside the counting chamber, etc.
5.15 Limitations of Ionization Counters
Certain limitations of the ionization counters are enumerated here:
1. γ -rays being highly penetrating can easily enter the counter. At the same time, it
can also escape the active area of the counter before it gets a chance to interact with
the enclosed gas molecules to produce primary ion-pairs. Therefore, the efficiency
for γ -ray counting with ionization counters is about 1% or less, whereas with βparticles or α-particles (provided they can enter the counter region), the efficiency
can be as high as 80%.
2. Liquid samples of weak β-emitters or α-emitters cannot be counted by the liquid
G.M. counter with a high efficiency because the wall thickness of the glass window
is around 50 mgcm
−2 . Windowless proportional counter could be used for such
counting, provided a provision is made to load the liquid sample into the counter.
Such arrangements are difficult to achieve due to greater chances of contaminating
counting chamber during loading or unloading of sample. Therefore, there is a
need to develop a counter specially to measure the activity of solid/or liquid
samples of α-emitter, weak β-emitter and γ -ray emitters. This is discussed in the
next chapter.
Summary
In this chapter, we studied the impact of particulate and electromagnetic radiation on gas under various potentials applied to a counter consisting of gas
and two electrodes (anode and cathode) enclosed in a closed cylinder. How
this behavior differs with α-particulate radiation and β-particulate radiation
are also discussed. Based on these types of behavior, different types of ionizations counters have been developed (ionization counter, proportional counter,
and G.M. counter). The principles of each of these counters and characteristic
5 Ionization Counters
The fifth column represents aluminum while the third and fourth columns represent mica windows of different thickness. It is obvious from Table 5.1 that for counting weak energetic β-emitters, mica window with a thickness about 0.9 mgcm
−2
should be preferred. The unit of thickness is given in mgcm
−2 because absorption of
radiation depends upon thickness (cm) and the density of the material, i.e., mg/cm
3
× cm = mg cm
−2 .
The liquid sample of weak β-emitter, however, cannot be counted by a liquid
G.M. counter, because its wall thickness is around 50 mgcm
−2 . It should be noted
that the values given in Table 5.1 give the probability of transmission of β-particles
through these windows, and it should not be confused with the efficiency of counting
because efficiency of counting depends upon geometry of counting, source thickness,
efficiency of ion-pair production inside the counting chamber, etc.
5.15 Limitations of Ionization Counters
Certain limitations of the ionization counters are enumerated here:
1. γ -rays being highly penetrating can easily enter the counter. At the same time, it
can also escape the active area of the counter before it gets a chance to interact with
the enclosed gas molecules to produce primary ion-pairs. Therefore, the efficiency
for γ -ray counting with ionization counters is about 1% or less, whereas with βparticles or α-particles (provided they can enter the counter region), the efficiency
can be as high as 80%.
2. Liquid samples of weak β-emitters or α-emitters cannot be counted by the liquid
G.M. counter with a high efficiency because the wall thickness of the glass window
is around 50 mgcm
−2 . Windowless proportional counter could be used for such
counting, provided a provision is made to load the liquid sample into the counter.
Such arrangements are difficult to achieve due to greater chances of contaminating
counting chamber during loading or unloading of sample. Therefore, there is a
need to develop a counter specially to measure the activity of solid/or liquid
samples of α-emitter, weak β-emitter and γ -ray emitters. This is discussed in the
next chapter.
Summary
In this chapter, we studied the impact of particulate and electromagnetic radiation on gas under various potentials applied to a counter consisting of gas
and two electrodes (anode and cathode) enclosed in a closed cylinder. How
this behavior differs with α-particulate radiation and β-particulate radiation
are also discussed. Based on these types of behavior, different types of ionizations counters have been developed (ionization counter, proportional counter,
and G.M. counter). The principles of each of these counters and characteristic