Physics and Engineering of Radiation Detection

11.5. Microdosimetry 651

whereD(y) represents the normalized dose distribution as a function of lineal energy.
It can be evaluated from

D(y)=

1

y ̄t

yf(y). (11.5.9)

Substituting this into the expression for ̄ydreveals that the dose average lineal energy
is simply the second moment of the distributionf(y), that is

y ̄d=

1

y ̄t

∫

y^2 f(y)dy. (11.5.10)

The prime objective of microdosimetry is the determination of lineal energy dis-
tribution along particle’s track. Since the dimensions involved in such a study are
extremely small, typically ranging from a nanometer to a few micrometers, the
experimental techniques are also somewhat modified versions of the conventional
dosimetry.
It was mentioned earlier that statistical fluctuations constitute the most signifi-
cant source of errors in microdosimetry, provided the dosimeter has been carefully
designed. The quantities we just described for microdosimetry are all statistical
in nature and therefore suffer from uncertainties because of statistical fluctuations.
These fluctuations can be due to a number of factors, such as

delta rays, which are the electrons emitted as a result of radiation interaction

in the medium,


lineal energy variation with particle track,

energy straggling,

range straggling, and

chord length variations.

11.5.BExperimentalTechniques

Microdosimetry techniques are somewhat different from the conventional dosimetry.
The reason is that in conventional dosimetry one is only concerned with macroscopic
deposition of energy while this macroscopic picture is not much useful to derive
microdosimetric quantities. In this section we will survey some of the commonly
used micrdosimeters..

B.1 Tissue Equivalent Proportional Counter (TEPC)

We have already discussed the working principle of proportional counters at various
places in the book. In essence, these counters work on the principle of charge
multiplication in the presence of externally applied high electric field. The tissue
equivalent proportional counter is a modified version of the conventional proportional
counter. The basic difference lies in constructing the chamber such that it is a close
in physical properties to a tissue as possible. A dosimeter that is notequivalentto
the tissue can not be measure the true dose absorbed by the tissue.
The most common approach to building such a chamber is based on simulating
the tissue volume by a relatively larger filling gas volume. The requirement of