182 Chapter 3. Gas Filled Detectors
affect their performance. This is true for at least the low resolution systems
working in moderate to high radiation fields.Even though the ionization chambers are perhaps the most widely used detectors,
still they have their own limitations, the most important of which are listed below.
Low current: The current flowing through the ionization chamber is usu-
ally very small for typical radiation environments. For low radiation fields the
current could not be measurable at all. This, of course, translates into low sen-
sitivity of the system and makes it unsuitable for low radiation environments.
The small ionization current also warrants the use of low noise electronics cir-
cuitry to obtain good signal to noise ratio.Vulnerability to atmospheric conditions:The response of ionization cham-
bers may change with change in atmospheric conditions, such as temperature
and pressure. However the effect is usually small and is only of concern for
high resolution systems.3.5 ProportionalCounters
We saw earlier that the maximum pulse amplitude that can be achieved in a parallel
plate ionization chamber is directly proportional to the number of charge pairs cre-
ated by the incident radiation (see equation 3.4.4). This implies that for situations
when the incident particle energy is not very large or the flux is small, the pulse
amplitude may not be large enough to achieve acceptable signal to noise ratio.
Any increase in pulse amplitude is therefore tied to increase in the number of
electron-ion pairs. The easiest way to achieve large number of charge pairs is to
allow the primary charges produced by the incident radiation to create additional
charges. We have seen this phenomenon in the section on Avalanche Multiplication.
There we discussed that the primary charges are capable of producing secondary
ionizations in the gas provided they achieve very high velocities between collisions.
The process leads eventually to avalanche multiplication and consequently a large
pulse at the output.
The basic requirement for the avalanche to occur is therefore application of very
high electric potential between the two electrodes. Parallel plate geometry is very
inefficient for this purpose because the electric lines of force near the anode and cath-
ode have the same density. Even if we manage to operate a parallel plate chamber
at the breakdown voltage, still it is not possible to attain acceptable proportional-
ity between the applied voltage and the output pulse amplitude. The reason is of
course the dependence of the pulse amplitude on the point of interaction of radiation.
Cylindrical geometry solves both of these problems. Typically a cylindrical propor-
tional counter is similar to a cylindrical ionization chamber, though with mechanics
that can withstand higher electric potentials.
A typical proportional counter is shown in Fig.3.5.1(a). The anode in this cham-
ber is in the form of a thin wire stretched across the center of the chamber while
the wall of the cylinder acts as the cathode. This geometry ensures a higher electric
field intensity near the anode wire as compared to the cathode. This non-uniformity
in the electric field ensures, among other things, better electron collection efficiency