149Chapter 3
Gas Filled Detectors
Radiation passing through a gas can ionize the gas molecules, provided the energy
delivered by it is higher than the ionization potential of the gas. The charge pairs
thus produced can be made to move in opposite directions by the application of an
external electric field. The result is an electric pulse that can be measured by an
associated measuring device. This process has been used to construct the so called
gas filled detectors. A typical gas filled detector would consist of a gas enclosure
and positive and negative electrodes. The electrodes are raised to a high potential
difference that can range from less than 100 volts to a few thousand volts depending
on the design and mode of operation of the detector. The creation and movement of
charge pairs due to passage of radiation in the gas perturbs the externally applied
electric field producing a pulse at the electrodes. The resulting charge, current,
or voltage at one of the electrodes can then be measured, which together with
proper calibration gives information about the energy of the particle beam and/or
its intensity.
It is apparent that such a system would work efficiently if a large number of
charge pairs are not only created but are also readily collected at the electrodes
before they recombine to form neutral molecules. The choice of gas, the geometry of
the detector, and the applied potential give us controlling power over the production
of charge pairs and their kinematic behavior in the gas.
In this Chapter we will look at general design considerations of gas filled detectors
and discuss their behavior in different conditions. Some of the extensively used
special types of gaseous detectors will also be discussed in this Chapter.3.1 ProductionofElectron-IonPairs
Whenever radiation interacts with particles in a gas, it may excite the molecules,
ionize them, or do nothing at all. To complicate the matter further, there are
different mechanisms through which these interaction could take place. In chapter
2 we visited some of these interaction mechanisms and found out that their gross
outcomes can be fairly accurately predicted by using statistical quantities such as
cross section and stopping power. Another quantity that is extremely important, at
least for radiation detectors, is the average energy needed to create an electron-ion
pair in a gas. This energy is referred to as theW-value.Itwouldbenaturalto
think that if the underlying radiation interaction processes are so complicated and
dependent on energy and types of particles involved then the W-value would be
different at each energy, for each radiation type, and for each type of gas. This is