11.5. Microdosimetry 653
probability of this happening is small due to smaller volume (see Fig.11.5.1(a)). The
dose measured in this case will then be higher than the dose expected in the tissue.
Another effect that can add uncertainty to the measurements is the scattering of
electrons. An incident particle can scatter off another particle before entering the
active volume of the chamber. If the scattered particle deposits its energy withing the
chamber volume, it will lead to a larger pulse or an extra event, thus overestimating
the dose (see Fig.11.5.1(b)). Fig.11.5.1(c) shows an effect that is said to produce
V-tracks. A neutron interacting with a nucleon in the wall material may break
it up into two heavy charged particles. These particles traverse the gas volume in
different directions forming a V, hence the name V-tracks. A relative smaller volume
tissue will not see both the particles. This effect is significant only at high neutron
energies, of the order of 20MeVand above. Another class of events that can cause
error in measurements are related to delta rays. The delta rays produced outside the
chamber’s active volume can also lead to double events as shown in Fig.11.5.1(d).
A read tissue, on the other hand will miss delta rays at a higher probability.
The discussion in the preceding paragraph leads us to the conclusion that chamber
walls is not a good idea. A good approach would then be to build awall-less
chamber. The reader might recall that during discussion on conventional dosimetry
we introduced a wall-less ionization chamber, in which the volume was subtended
by the electric lines of force and the beam of incident particles. That is, there was
no physical wall defining the active volume of the chamber. The same approach
has been adopted in building TEPCs. However such dosimeters suffer from small
variations in the electric field intensity, which lead to uncertainties in the definition
of active volume.
Another approach to building a TEPC is to use gridded walls. Using a gridded
wall minimizes the wall material and hence reduces the uncertainties associated with
the wall effects. The main advantage of this approach is that the active volume of
the chamber remains well defined even with slight variations in the electric field
intensity. The disadvantage, of course, is that the counter is not really wall-less
and therefore suffers from associated uncertainties, though at a smaller scale than a
walled counter.
B.2 Solid State Nuclear Track Detector ((SSNTD)
Earlier in the chapter we discussed track etch dosimeters. Such solid state nuclear
track detectors or SSNTDs can also be used as microdosimeters. To remind the
reader, a particle passing through a track etch detector leaves behind a damaged
zone. This zone can be revealed through the process of chemical or electrochemical
etching. The reason these detectors can be used as microdosimeters is that the track
parameters are directly related to the linear energy transfer and the type of incident
particle. In modern systems these parameters are determined through an automated
process, which is less prone to human errors. The apparatus generally consists of an
optical image analyzer with a built-in microscope.
Determination of theLET is done through a parameter called etch rate ratio
given by
V=
Vt
Vb