Shani G. Radiation Dosimetry: Instrumentation and Methods
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210 Radiation Dosimetry: Instrumentation and Methods
The electret dosimeter resembles a parallel-plate
ionization chamber: the polarizing electrode is a thin
aluminum foil and the measuring electrode is a metal-
lized polymer foil with the polymer facing the polarizing
electrode over an air gap. A schematic diagram of a
chamber as connected to charging and discharging mea-
surement apparatus is shown in Figure 3.135. Chambers
were constructed with 1.5-mm-thick fiberglass backing,
various collector radii c (0.5 to 1.9 cm), guard-ring
widths
g (1.0 to 19.0 mm), and air gaps a (1 to 9 mm).
Small pin connectors were added to allow simple con-
nection to the current measurement equipment and
removal from the equipment when the dosimeter was to
be used for monitoring. Most of the measurements were
performed using a Hewlett Packard Faxitron x-ray sys-
tem that can operate up to 90 kV
p
. Some measurements
were also performed on a highly reproducible diagnostic
energy unit.
FIGURE 3.120 Extradin model A2, M2, P2, T2 ion chamber.
FIGURE 3.121 Extradin model A1, M1, T1 ion chamber.
FIGURE 3.122 Extradin model A11, P11, T11 ion chamber.
FIGURE 3.123 Extradin AIITW, PIITW, TIITW for chamber.
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Ionization Chamber Dosimetry 211
FIGURE 3.124 Values of k
Q
at 10 cm depth in accelerator photon beams as a function of %dd(10)
x
for cylindrical ion chambers
commonly used for clinical reference dosimetry. (From TG51.)
FIGURE 3.125 ASP-1 system block diagram. (From Reference 105. With permission.)
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212 Radiation Dosimetry: Instrumentation and Methods
FIGURE 3.126 RAM R-200. Main board layout.
FIGURE 3.127 Block diagram of the internal detector board.
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Ionization Chamber Dosimetry 213
FIGURE 3.128 Block diagram of the external detector board.
FIGURE 3.129 Block diagram of the CPU board.
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214 Radiation Dosimetry: Instrumentation and Methods
The surface charge density was measured as a function
of air kerma using the radiation discharge method via the
electrometer. Typical measurements of charge density
removed during discharge over a very large range of air
kerma and for various external applied voltages, V
0
, are
shown in Figure 3.136. These curves first rise linearly with
air kerma (or with time for constant air kerma rate) and
then saturate as the electret becomes neutral. The maxi-
mum level of surface-charge density removed corresponds
to the maximum charge density deposited during the
charging process.
The dependence of radiation-induced conductivity (RIC)
on radiation rate in 254-
m-thick Teflon polyfluoroethylene
propylene (FEP) film used in a electret ionization
chamber (EIC) was discussed and measured by Mac-
Donald and Fallone [112] with regard to radiation
dosimetry. RIC permits trapped surface charge to
migrate through the polymer and recombine at the col-
lecting electrode, which results in a reduced EIC charg-
ing efficiency and an overestimation of air-kerma in radi-
ation dosimetry.
The EIC does not require an external potential when
it is used as a radiation dosimeter because the trapped
charges on the electret provide the electric field for the
sensitive volume between the collecting and polarizing
electrodes. Exposure to radiation produces ions in the
air gap which drift towards the electret to neutralize the
charge on the electret surface. Within a certain range,
the air kerma is related to the difference between the
original electret charge and that remaining after the irra-
diation.
Although RIC affects the charging of electrets only
when ionizing radiation is used, when any EIC is used as
a dosimeter, irradiation of the chamber allows charge to
FIGURE 3.130 Energy response of the internal detector.
FIGURE 3.131a RAM R-200 meter–low range GM dose rate linearity.
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Ionization Chamber Dosimetry 215
be depleted from the electret surface because of ion
motion through the sensitive volume of the air gap and
through the bulk of the electret. The rate-of-charge loss
through the electret is time-dependent and nonlinearly
dependent on the rate at which the EIC receives air kerma.
To understand the limits imposed by RIC on the use of
the EIC as a personnel dosimeter, we have studied its
dependence on radiation rate both theoretically and exper-
imentally.
The assignment of dose at a point in a medium
from a measurement of exposure using an air-equiva-
lent ionization chamber involves a well-known conver-
sion factor (the f-factor) that includes the ratio of the
average mass energy absorption coefficients (
en
/
) for
the medium and for air. The f-factor is computed as
follows: [113]
(3.210)
where is the mean energy expended in air per
ion formed. In the integrals of Equation (3.210),
en
/
is the mass energy absorption coefficient and
(E) is
the differential fluence spectrum, as a function of the
photon energy E, at the point of interest. Because spec-
tra of x-ray beams used in a particular application are
often not known, investigators reduce consideration of
FIGURE 3.131b RAM R-200 meter-high range GM dose rate linearity.
FIGURE 3.132 A cross-sectional view of the HK solid-water parallel-plate chamber. The pancake-shaped cavity size is 2 mm high
and 35 mm in diameter. The front-wall thickness is 4 mm. Alphabetical symbols are (A)
25-mm-diameter collector, (B)
0.25-mm-thick polystyrene cup to separate the collector from the guard, (C) 5-mm-wide guard, (D) 1-mm-thick high-voltage
protective polystyrene insulator, (E)
0.25-mm polystyrene insulator to separate the ground from the chamber body, and (F) air-
communication hole. (From Reference [107]. With permission.)
f
med
W
e
-----
en
E()
[]
med
E
E()Ed
en
E()
[]
air
E
E()Ed
--------------------------------------------------------------
We
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216 Radiation Dosimetry: Instrumentation and Methods
FIGURE 3.133 Schematic cross sectional diagram of the Gammex/RMI model 449 chamber showing its major components. (From
Reference 102. With permission.)
FIGURE 3.134 Schematic diagram of the chamber (not to scale). 1, guard ring; 2, collecting electrode; 3, sensitive volume;
4, entrance window (12-
m-thick polyester film); 5, polarizing voltage lead; 6, external chamber wall (Perspex); 7, mobile cylinder
(Perspex) carrying the collector and guard ring; 8, signal lead; 9, screw driving the piston; 10, stepping motor for moving the collector;
and 11, connector box. (From Reference [109]. With permission.)
FIGURE 3.135 Schematic of radiation-charged electret ionization chamber and measurement apparatus. The external potential V
0
is applied during the charging mode but removed during the discharging mode. The leads to the external supply and electrometer
are removed when the EIC is used as a dosimeter. (From Reference [112]. With permission.)
Ch-03.fm(part 3) Page 216 Friday, November 10, 2000 12:00 PM
Ionization Chamber Dosimetry 217
the complete photon spectrum to that of a single photon
energy through the use of an equivalent photon energy
derived from half-value layer (HVL) measurements
according to
(3.211)
where Ê is the equivalent photon energy and HVL is the
thickness of the first half-value layer. This approach
reduces Equation (3.210) to
(3.212)
Mono-energetic f-factors in the energy range 1 keV to 1.25
MeV for several tissues, tissue substitutes, and radiation
dosimeters are reported in Table 3.48. These conversion
factors, in units of Gy R
1
10
2
, can be used in con-
junction with a measurement of exposure to estimate
absorbed dose at a point in a medium. (The data in this
note can also be used, with minor conversions, when the
measured quantity is air kerma.)
FIGURE 3.136 General dosimeter response for various geometries and V
0
, q
n
/A (where A is collector area), vs. air kerma for
(
a) constant air gap a and various initial charging voltages V
0
and (b) constant V
0
and various a. The dosimeter is formed of a
110-
m-thick Mylar electret exposed to 0.964 mGy s
1
(26-keV effective energy x-rays). (c) q
n
vs. air kerma for very low air
kerma rates of 0.049 mGys
1
. (From Reference [112]. With permission.)
E
ˆ
()
ln 2
HVL
------------
f
med
E
ˆ
()
W
e
-----
en
E()
[]
med
en
E()
[]
air
---------------------------------
Ch-03.fm(part 3) Page 217 Friday, November 10, 2000 12:00 PM
218 Radiation Dosimetry: Instrumentation and Methods
TABLE 3.48
Mono-Energetic f-factors for Tissues, Tissue Substitutes, and Radiation Dosimeters (Gy R
1
10
2
)
Tissues Tissue Substitutes Dosimeters
Adipose Blood ICRU10b ICRP23 ICRU44 Lung Soft
E (MeV) Tissue (Whole) Bone Bone Bone Tissue Tissue A-150 B-100 B-110 Water Alanine CaF
2
LiF
1.00E-03 0.638 0.924 0.836 0.908 0.918 0.923 0.901 0.549 0.780 0.930 0.989 0.744 1.27 1.02
1:50E-03 0.634 0.942 0.858 0.939 0.952 0.944 0.920 0.536 0.796 0.964 1.01 0.743 1.35 1.08
2.00E-03 0.630 0.952 0.871 0.960 0.973 0.953 0.928 0.528 0.809 0.989 1.02 0.742 1.41 1.11
3.00E-03 0.639 1.00 1.28 1.57 1.57 1.02 0.987 0.520 0.830 1.03 1.04 0.741 1.50 1.15
4.00E-03 0.572 0.925 1.20 1.49 1.50 0.939 0.921 0.461 0.760 0.948 0.940 0.663 1.40 1.05
5.00E-03 0.564 0.924 2.78 3.75 3.92 0.938 0.921 0.646 2.63 3.73 0.933 0.654 6.84 1.06
6.00E-03 0.558 0.923 2.96 4.01 4.19 0.938 0.922 0.659 2.82 4.02 0.928 0.647 7.41 1.06
8.00E-03 0.549 0.939 3.24 4.42 4.63 0.937 0.921 0.680 3.12 4.47 0.919 0.636 8.31 1.06
1.00E-02 0.542 0.941 3.45 4.73 4.95 0.936 0.921 0.697 3.35 4.82 0.913 0.628 8.99 1.06
1.50E-02 0.532 0.946 3.82 5.26 5.51 0.934 0.921 0.726 3.74 5.41 0.902 0.615 10.2 1.06
2.00E-02 0.528 0.948 4.04 5.58 5.85 0.933 0.921 0.749 3.99 5.78 0.895 0.608 10.9 1.06
3.00E-02 0.541 0.951 4.20 5.81 6.10 0.932 0.921 0.785 4.18 6.05 0.887 0.613 11.4 1.04
4.00E-02 0.587 0.954 4.01 5.51 5.78 0.934 0.925 0.822 4.01 5.76 0.891 0.645 10.8 1.01
5.00E-02 0.659 0.957 3.51 4.77 4.99 0.939 0.932 0.859 3.52 4.99 0.903 0.700 9.19 0.971
6.00E-02 0.739 0.960 2.91 3.87 4.03 0.945 0.940 0.892 2.92 4.03 0.919 0.761 7.24 0.928
8.00E-02 0.858 0.963 1.95 2.43 2.51 0.955 0.952 0.932 1.96 2.51 0.945 0.853 4.14 0.868
1.00E-01 0.917 0.964 1.45 1.69 1.73 0.961 0.959 0.949 1.46 1.73 0.959 0.899 2.54 0.840
1.50E-01 0.961 0.965 1.07 1.12 1.12 0.964 0.963 0.961 1.06 1.11 0.970 0.934 1.29 0.818
2.00E-01 0.970 0.965 0.982 0.991 0.985 0.966 0.965 0.963 0.978 0.980 0.973 0.941 1.02 0.814
3.00E-01 0.974 0.965 0.944 0.935 0.925 0.966 0.965 0.964 0.940 0.919 0.974 0.944 0.898 0.812
4.00E-01 0.975 0.965 0.936 0.923 0.912 0.966 0.965 0.964 0.931 0.906 0.974 0.945 0.872 0.812
5.00E-01 0.976 0.965 0.933 0.919 0.908 0.966 0.965 0.964 0.929 0.902 0.974 0.946 0.862 0.812
6.00E-01 0.976 0.965 0.932 0.917 0.905 0.966 0.965 0.964 0.927 0.899 0.974 0.946 0.858 0.812
8.00E-01 0.976 0.965 0.930 0.915 0.904 0.966 0.965 0.964 0.926 0.898 0.974 0.946 0.854 0.812
1.00E+00 0.976 0.966 0.930 0.915 0.903 0.966 0.966 0.964 0.926 0.897 0.975 0.946 0.853 0.812
1.25E+00 0.976 0.965 0.930 0.914 0.902 0.966 0.965 0.964 0.925 0.896 0.974 0.946 0.851 0.812
Source: From Reference 113. With permission.
Ch-03.fm(part 3) Page 218 Friday, November 10, 2000 12:00 PM
Ionization Chamber Dosimetry 219
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