Physics and Engineering of Radiation Detection

638 Chapter 11. Dosimetry and Radiation Protection

Measurement Volume

Air

Enclosure

Cathode

Anode

Photons

Compensating Volume

Collimator

Guard Electrodes Guard Electrodes

Figure 11.4.2: Simplified diagram of a free in air ionization chamber

designed for air Kerma or dose measurement.

Let us now see how we can measure air Kerma using this chamber. Earlier in the
chapter we saw that air Kerma can be computed from (see equation 11.4.2)

Kair=

Wair

e(1−g)

X.

The exposureXhere was defined earlier by

X =

dQ

dm

=

dQ

ρdV

,

wheredmis the mass of air having densityρin the volume elementdV.dQis the
total charge of either sign created by the radiation in the volume elementdV.For
the case of the ionization chamber shown in Fig.11.4.2 this volume element is the
darker shaded measurement area. Substituting this expression for exposure in the
above expression for air Kerma we get

Kair=

Wair

e(1−g)

dQ

ρdV

. (11.4.1)

Note that this equation can only be used for an ideal detector since it does not take
into account the errors introduced by different parts of the system. The easiest way
to do this is to multiply the above equation by acorrection factorkt,whichgives

Kair=

Wair

e(1−g)

dQ

ρdV

kt. (11.4.2)

This correction factorktis composed of a number of correction factors related to
individual sources of error, that is

kt=kdkhkrkskfkpkikako, (11.4.3)