Biomedical Engineering Reference
In-Depth Information
10
Methods of Radiation Detection
This chapter describes ways in which ionizing radiation can be detected and mea-
sured. Section 10.7 covers special methods applied to neutrons. In Chapter 12 we
shall see how these techniques are applied in radiation dosimetry.
10.1
Ionization in Gases
Ionization Current
Figure 10.1(a) illustrates a uniform, parallel beam of monoenergetic charged parti-
cles that steadily enter a gas chamber across an area
A
with energy
E
and come to
rest in the chamber. A potential difference
V
applied across the parallel chamber
plates
P
1
and
P
2
gives rise to a uniform electric field between them. As the par-
ticles slow down in the chamber, they ionize gas atoms by ejecting electrons and
leaving positive ions behind. The ejected electrons can immediately produce addi-
tional ion pairs. If the electric field strength, which is proportional to
V
, is relatively
weak, then only a few of the total ion pairs will drift apart under its influence, and a
small current
I
will flow in the circuit. Most of the other ion pairs will recombine to
form neutral gas atoms. As shown in Fig. 10.1(b), the current
I
can be increased by
increasing
V
up to a value
V
0
, at which the field becomes strong enough to collect
all of the ion pairs produced by the incident radiation and its secondary electrons.
Thereafter, the current remains on a plateau at its saturation value
I
0
when
V
>
V
0
.
Since it is readily measurable, it is important to see what information the satu-
ration current gives about the radiation. If the fluence rate is
˙
cm
-2
s
-1
, then the
˙
˙
=
˙
intensity
E
.
If
W
denotes the average energy needed to produce an ion pair when a particle of
initial energy
E
stops in the chamber, then the average number
N
of ion pairs pro-
duced by an incident particle and its secondary electrons is
N
of the radiation (Section 8.8) entering the chamber is given by
E
/
W
. The average
charge (either
+
or
-
) produced per particle is
Ne
, where
e
is the magnitude of the
electronic charge. The saturation current
I
0
in the circuit is equal to the product
of
Ne
and
=
˙
A
, the total number of particles that enter the chamber per unit time.
Therefore, we have
