Physics and Engineering of Radiation Detection

11.4. Active Dosimetry 637

A.1 FreeinAirIonChamberDosimetry

As the name suggests, this type of ionization chamber measures exposure or Kerma
in free air. Such chambers are commonly used to determine air Kerma for x-rays.
The main idea behind this method is to ensure secondary electron equilibrium or
SEE. The SEE is said to exist if the energy taken away by the secondary electrons
escaping from a small mass element is compensated by the energy brought into the
element by the secondary electrons produced outside the element. This concept is
graphically depicted in Fig.11.4.1. The primary electrons, shown by small open cir-
cles, loose all their energies within the measurement volume (large circle) but the
secondary electrons (solid circles), having larger range, deliver their energies outside
the volume element. This amounts to a loss of energy and consequent underesti-
mation of the dose. But if the irradiation is constant outside the volume element
as well then the secondary electrons generated there may deposit their energies in-
side the measurement volume. Now, if the volume element is very small then we
can assume that the secondary electrons escaping the element will be compensated
by the secondary electrons generated outside but absorbed inside the measurement
volume.

Primary Electron Secondary Electron

Photons

Figure 11.4.1: Concept of secondary electron equilibrium for free air

ion chamber dosimetry. See text for explanations.

The requirement of secondary electron equilibrium can, to a good approximation,
be realized in a parallel plate ionization chamber. A simplified diagram of such
a dosimeter is shown in Fig.11.4.2. The small shaded volume element subtended
by the narrow beam and the electric lines of force between anode and cathode
form the measurement volume. The energy absorbed in this volume is actually
measured and constitutes the absorbed dose or air Kerma. The secondary electrons
escaping from this volume are compensated by the secondary electrons produced in
thecompensatingvolume elements on either side. The guard electrodes are mainly
used tosmooth outthe electric field non-linearities at the edges of the anode and
also provide safety against high voltage on the anode.