Physics and Engineering of Radiation Detection

640 Chapter 11. Dosimetry and Radiation Protection

Wall (Cathode)

Insulator

HV

Signal

Anode

Insulator

Insulator

Air

Air

Figure 11.4.3: A simple air filled ion chamber dosimeter.

the chamber inevitably has errors due to interaction of charges and radiation in the
wall material. We will later see how these errors are taken into account but first let
us see how the dose can be calculated from the chamber.
The radiation passing through the chamber produces electron-ion pairs that drift
in opposite directions under the influence of the externally applied electric potential.
The number of charge pairs created by the radiation and the total charge carried by
them can be calculated if one knows the energy deposited by the radiation and the
W-value of air through the following relations.

Nair =

Edep

Wair

(11.4.4)

⇒Qair = e

Edep

Wair

(11.4.5)

HereEdepis the total energy deposited by the radiation inside the active volume
of the detector. The chargeQairis a measurable quantity since it is proportional
to the current flowing through the chamber, which can be measured using suitable
electronics readout circuitry. IfQairis known, the deposited energyEdepcan be
calculated from the above relation. This energy can then be used to calculate the
absorbed dose according to

Dair =

Edep

Mair

=

WairQair

eMair

, (11.4.6)

whereMair is the mass of the air in the chamber. Up until now we have not
considered any sources of error. In other words, the dose calculated from the above
expression is good only for an ideal detector. A practical system, no matter how
perfectly it is built, introduces some uncertainties in the measurements. To take
these uncertainties into account we multiply the above expression by a correction
factorkt,thatis

Dair=

WairQair

eMair

kt, (11.4.7)

wherekt, to a large extent, depends on the design of the chamber as well as envi-
ronmental conditions. It can be factorized into individual correction factors exactly
as in equation 11.4.3.