Regional Basic Professional Training Course in Korea

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2. Radiation Protection
The detector proper can take many forms. The most common is that of a cylindrical
conducting chamber containing a central electrode on the axis of the chamber and
insulated from it (see Fig. (2.7)). The usual gas filling for an ionization‐chamber detector
is dry air at atmospheric pressure (101.3 kPa). Other gases are sometimes used for
particular purposes, but in radiation protection work, air is commonly used.
An ionization chamber can be used to some extent for detection of all types of particles.
However, in radiation protection work it is most commonly used for detection of both
photons and high‐speed electrons. In the mean‐level mode of operation, the ionization
chamber can be used in two ways: (1) as a current‐measuring device yielding a quantity
proportional to the rate (amount per unit time) of arrival of the ionizing radiation within
the detector, and (2) as a voltage‐measuring device yielding a quantity related to total
radiation incident on the chamber during the entire period of measurement.
Principles of Operation
An ionization chamber demands that only ionization produced directly by incoming
radiation be collected. Therefore, for quantitative measurements, the detector must
furnish an output having a definite relationship to this ionization. To accomplish this goal,
a known fraction of the charge produced within the chamber must be collected and no
other ionization can take place. Therefore, it is necessary to understand how the electron
and residual positive ions, once formed, behave.
When an outer‐shell or valence electron is released from an atom, the immediate
inclination of electron and positive ion is to reunite. To avoid this “recombination” a
certain minimum level of voltage must exert enough force on the electron to separate it
from the positive ion and both of them on their respective ways. Generally the walls of
the chamber are the cathode; i.e., they have a negative potential or an excess of electrons.
Thus the positive ions, traveling slowly because they are relatively massive, migrate to