Shani G. Radiation Dosimetry: Instrumentation and Methods
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200
Radiation Dosimetry: Instrumentation and Methods
The microcomputer system consists of computer,
address latch, and program chip. The onboard clock oscil-
lator runs at 6 MHz and the program updates the meter
reading every one-half second. A block diagram of the
ASP-1 system is shown in Figure 3.125.
The RAM R-200 is a portable gamma meter designed
for measuring wide-range gamma radiation fields. It is
lightweight (approximately 500 g), in small dimensions
(80
35
130 mm), with emphasis on ease of operation
TABLE 3.40
PTW 23343 Ion Chamber Dimensions
Figure 3.118
Volume:
0.055 cm
3
Response:
1
10
9
C/Gy
Leakage:
2
10
15
A
Polarizing voltage: max. 300 V
Guard potential : max. 100 mV
Cable leakage: 3.5
10
12
C/(Gy
cm)
Wall material: CH
2
Polyethylene
Membrane thickness: 0.03 mm
Area thickness: 2.3 mg/cm
2
Electrode:
Acrylic, graphite-coated;
5.3 mm
Range of temperature:
15°C ….
40°C
Range of relative humidity: 20% … 75%
Ion collection time: 150V:0.20ms
300V:0.10ms
Ionization chamber type 23343 is a Markus chamber of volume
0.055 cm
3
.
The Markus chamber is the first chamber in the world specially
designed for electron dosimetry. Its small measuring volume
and watertight construction make this chamber ideal for mea-
surements in a water phantom, giving a good spatial resolution.
The diaphragm front allows measuring in the build-up region
of the electron field up to a depth of virtually zero. The mea-
suring volume is open to the surrounding air via the connecting
cable. This chamber can be delivered calibrated for absolute
electron dosimetry.
Source:
From Reference [102]. With permission.
TABLE 3.41
Farmer-Type Chamber Model A12 Dimensions
Figure 3.119
• Farmer chamber geometry • Axially symmetric
design • Homogeneous construction • Complete guarding
• Waterproof construction
• Collecting volume: 0.651 cm
3
• Collector diameter: 1.0 mm
• Wall thickness: 0.5 mm • Wall, collector, and guard material:
Model A12 - Shonka air-equivalent plastic C552 • Maximum
polarizing potential: greater than 1000 V • Gas flow capability
• Integrally flexible, low-noise cable • Separable minimum
scatter stem • Matching
60
Co cap • Handsome, rugged wooden
case • Inherent leakage currents: less than 10
15
amperes •
Cable: 50 Ohms, 29 pF/f, 2 m long • Signal connector: choice
of coaxial and triaxial BNC plugs and jacks and triaxial BNC
plug
Source:
From Reference [103]. With permission.
TABLE 3.42
Spokas Thimble Chamber Models A2, M2, P2,
T2 Dimensions
Figure 3.120
• Axially symmetric design • High collection efficiency •
Homogeneous construction • Complete guarding • Waterproof
construction • Gas flow capability • Integrally flexible, low-
noise cable • Separable minimum scatter stem • Matching
equilibrium caps • Handsome, rugged wooden case
• Collecting volume: 0.5 cm
3
• Collector diameter, 4.6 mm
• Wall thickness: 1.0 mm • Wall, collector and guard material:
Model A1–Shonka air-equivalent plastic C552; Model
M1–magnesium; Model P1–polystyrene equivalent; Model
T1–Shonka tissue-equivalent plastic A 150
• Maximum polarizing potential: greater than 1000 V • Inherent
leakage currents: less than 10
15
ampere • Cable: 50 Ohms,
29 pF/f, 2 m long • Signal connector: choice of coaxial and
triaxial BNC plugs and jacks and triaxial TNC plug • Nominal
calibration factor for
60
Co: 6.0 R/nC
Source:
From Reference [103]. With permission.
TABLE 3.43
Miniature Shonka Thimble Chamber Models
A1, M1, T1 Dimensions
Figure 3.121
• Axially symmetric design • Small gas volume • Homogeneous
construction • Complete guarding • Waterproof construction •
Gas flow capability • Integrally flexible, low-noise cable •
Separable minimum scatter stem • Matching equilibrium caps
• Handsome, rugged wooden case
• Collecting volume: 0.05 cm
3
• Collector diameter: 1.5 mm •
Wall thickness: 1.0 mm • Wall, collector, and guard material:
Model A1–Shonka air-equivalent plastic C552;
Model M1–magnesium;
Model T1–Shonka tissue-equivalent plastic A150
• Maximum polarizing potential: greater than 1000 V • Inherent
leakage currents: less than 10
15
amperes • Cable: 50 Ohms,
29 pP/f, 2 m long • Signal connector, choice of coaxial and
triaxial BNC plugs and jacks and triaxial TNC plug • Nominal
calibration factor for
60
Co: 6.0 R/nC
From Reference [103]. With permission.
Ch-03.fm(part 3) Page 200 Friday, November 10, 2000 12:00 PM
Ionization Chamber Dosimetry
201
and ergonomic structure. [106] The RAM R-200 meter
contains an internal detector for gamma field measurements
which covers a wide dynamic range of 0.1
Sv h
1
to 1
Sv h
1
. The meter includes two energy-compensated GM
tubes, high-voltage regulated power supply, signal pro-
cessing electronics, and embedded microprocessor cir-
cuitry with dedicated software for data processing and
display. A microprocessor-controlled auto ranging switch
determines the appropriate GM tube to be used according
to the dose rate. Additional functions include accumulated
dose measurement, malfunction detection and provision
of appropriate alarm, auto recognition of external probes,
and three serial RS-232 communications channels. The
RAM R-200 supports the logging of beta-gamma surface
contamination activity and dose-accumulation and dose-
rate measurements into its internal battery-backed-up
memory. The logged data includes location (provided
by an external GPS or a bar-code reader) and acquisition
time. The sophisticated software enables smooth analog
and digital display and fast response when abrupt changes
in the dose rate measurements occurs. The meter readout
is displayed on a large easy-to-read custom-designed
Liquid Crystal Display (LCD).
The RAM R-200 meter’s ability to concurrently mea-
sure and analyze radiation fields from both internal and
external probes enables dose-accumulated and dose-rate
alarms from the internal detector while the instrument
measures and displays readings from the external probe.
This feature improves the operator’s safety.
A dedicated integrated circuitry was designed and
implemented in order to achieve very low power consump-
tion, yielding prolonged system operability over 100 hours,
using a standard 9-V alkaline battery. The RAM R-200
probes can be operated using the RAM R-200 meter or
by direct connection to a PC via the serial port. Testability
and calibration of the meter and external probes are per-
formed directly from a PC. This concept for testability
significantly simplifies maintenance and operation.
The RAM R-200 system includes three external
probes: RG-40 for a high-range gamma field covering a
dose rate range of 1 mSv h
1
to 100 Sv h
1
, and RG-12
and RG-10 end-window GM probes for beta/gamma con-
tamination detection and measurement. Similar to the
meter, these probes are designed to withstand vibrations,
shocks, and extreme temperature conditions.
All RAM R-200 external probes include an internal
microcontroller circuitry, self-regulated power supply for
the internal electronics, high-voltage power supply, and
GM output signal processing electronics. The external
probe microcontroller performs data processing, enables
RS-232 serial communication, and executes a continual
built-in test for detection of malfunction conditions to
increase reliability and easy maintenance.
After connecting the external probes to the meter, the
latter performs automatic probe identification. No further
calibration or operator’s action is required.
Figure 3.126 shows the main board layout; Figure
3.127 shows a block diagram of the internal detector
board; Figure 3.128 shows a block diagram of the external
detector for high dose rate monitoring; Figure 3.129 shows
a block diagram of the CPU board; Figure 3.130 shows
the energy response of the internal detector; and Figure
3.131 shows the dose rate response of the R-200 detector.
The RAM R-200 meter and the RG-40 probe were
tested over a wide range of gamma fields. The intrinsic
error of the whole effective measurement range was less
than
10%. The results meet international standard
TABLE 3.44
Xradin Ionization Chambers, Spokas Planar
Chamber Models A11, P11, T11 Dimensions
Figure 3.122
• Planar geometry • Homogeneous construction • Complete
guarding • Waterproof construction • Gas flow capability • Low
mass collector • Integrally flexible, low-noise cable •
Separable minimum scatter stem • Handsome, rugged wooden
case
• Collecting volume: 0.62 cm
3
• Collector diameter: 20.0
mm • Window thickness: 1.0 mm • Body, collector, and
guard material: Model A11–Shonka air-equivalent plastic C552;
Model P11–Polystyrene equivalent plastic D400;
Model T11–Shonka tissue-equivalent plastic A 150
• Maximum polarizing potential: greater than 1000 V •
Inherent leakage currents: less than 10
15
amperes • Cable:
50 Ohms, 29 pF/f, 2 m long • Signal connector: choice of
coaxial and triaxial BNC plugs and jacks and triaxial TNC
plug • Nominal calibration factor for
60
Co :5.5 R/nC
Source:
From Reference [103]. With permission.
TABLE 3.45
Thin-Window Chamber Models AIITW, PIITW,
TIITW Dimensions
Figure 3.123
• Planar geometry • Homogeneous construction •
Complete guarding • Thin stretched conductive window •
Waterproof with cover over window • Low mass collector
• Gas flow capability • Integrally flexible, low-noise cable
• Separable minimum scatter stem • Matching equilibrium
caps • Handsome, rugged wooden case
• Collecting volume: 0.94 cm
3
• Collector diameter: 20.0
mm • Window: Conductive Kapton film, 3.86 mg/cm
3
•
Body, collector, and guard material: Model Al11TW–Shonka
air-equivalent plastic C552;
Model P11TW–Polystyrene-equivalent plastic D400;
Model T11TW–Shonka tissue-equivalent plastic A150
Source:
From Reference [103]. With permission.
Ch-03.fm(part 3) Page 201 Friday, November 10, 2000 12:00 PM
202
Radiation Dosimetry: Instrumentation and Methods
TABLE 3.46
Charecteristics of the NE-Technology Therapy Dosimetry Chambers
2571A 2577C 2581A 2532/3C 2536/3C
Sensitivity 4.0 rad/nC 40 Gy/
C 11.7 rad/nC 117 Gy/
C 5.2 rad/nC 52 Gy/
C 105 rad/nC 1050 Gy/
C 7.9 rad/nC 79 Gy/
C
Energy Range Photons
0.05–35MV
Electrons
5–35 MeV
Photons
0.05–35MV
Electrons
5–35 MeV
Photons
0.1–35MV
Electrons
5–35 MeV
Photons 7.5–100kV Photons 7.5–100kV
Max Dose Rate 3.5 krad/min 35 Gy/min 3.5 krad/min 35 Gy/min 44.0 krad/min 440 Gy/min 25 krad/sec 250 Gy/sec 4.8 krad/sec 48 Gy/sec
99% Eff. 250V 22 mrad/pulse 0.22 mGy/pulse 22 mrad/pulse 0.22 mGy/pulse 88 mrad/pulse 0.88 mGy/pulse 400 mrad/pulse 4 mGy/pulse 180 mrad/pulse 1.8 mGy/pulse
Length of table 10 m 10 m 10 m 1 m 1 m
Source:
From Reference [104]. With permission.
Ch-03.fm(part 3) Page 202 Friday, November 10, 2000 12:00 PM
Ionization Chamber Dosimetry
203
IEC1017-1 and ANSI N42 dose rate response require-
ments for gamma radiation rate meters. Energy and angu-
lar response are being tested.
Figure 3.131 shows the RAM R-200 meter’s internal
detector measurement accuracy from background (0.001
mSv/h) to 10 mSv/h. Adequate dose rate response from
background to 10 mSv/h was obtained with the low-range
GM tube ZP1201. The high-range GM tube ZP1313 test
shows good dose rate response from 500
Sv/h to 1Sv/h.
The meter’s software enables a wide range of gamma
radiation measurements from background to 1 Sv/h by
selecting the appropriate GM and without operator inter-
vention. To avoid continuous GM switching, a large over-
lapping is provided. Dead time and calibration correction
is performed continuously by the meter or external probe’s
microcontroller.
The RG-40 dose rate response was tested over a large
range of gamma fields, from 1 mSv/h to more than 100
Sv/h. This probe operates in a similar way to the RAM
R-200 meter by using two GM tubes. The lower-range
GM tube 4G60M measures from 1 mSv/h to 10 Sv/h. The
higher range GM tube 3G10 measures from 10 mSv/h to
a dose rate higher than 100 Sv/h. GM selection switching
is performed by the RG-40 microcontroller, according to
the internal probe software.
XIII. OTHER IONIZATION CHAMBERS
It has been recommended by several high-energy beam
dosimetry protocols [107] that parallel-plate chambers are
preferred for electron beam dosimetry, especially for low-
energy electron beams. Even in photon beams, parallel-plate
chambers have one advantage over cylindrical chambers in
that the replacement correction factor
P
repl
is unity.
P
repl
for
cylindrical chambers in proton beams is unity only when the
chambers are calibrated at
d
max
. The TG21 protocol recom-
mends the use of
P
repl
1 for parallel-plate chambers having
guarded field and internal heights and diameters on the order
of 2 and 20 mm, respectively.
FIGURE 3.111
PTW 23342 ion chamber.
Ch-03.fm(part 3) Page 203 Friday, November 10, 2000 12:00 PM
204
Radiation Dosimetry: Instrumentation and Methods
A Shonka model A2 parallel-plate chamber (Exradin
Instrumentation, Lisle, IL 60532) is made of conductive
plastic. It has a 1-mm-thick build-up, and the overall
chamber size is 4.5 cm diameter and 1.7 cm thick. Unlike
other commercial parallel-plate chambers, the Shonka
chamber takes advantage of using conductive plastic as
collecting electrodes, thus eliminating the need to intro-
duce foreign conducting materials to the chamber cavity.
One disadvantage of conducting plastic is that the chamber
requires a special provision to separate the collecting elec-
trode from the guard.
Solid water has a density of 1.03 gm/cm
3
, which is
similar to that of water. In addition, it is slightly conduc-
tive; therefore, it can be used as electrodes. A major advan-
tage of solid water is that its dosimetric characteristics
such as the electron density and the effective atomic num-
ber in high-energy photon and electron beams are similar
to those of water.
The HK chamber proposed by Kubo [107] produces
approximately eight times larger signals than the Attix
chamber. The Attix chamber has a 0.025-mm-thin
entrance window for build-up dose measurements,
whereas the HK chamber has a 4-mm-thick build-up mate-
rial, thick enough for
60
Co beam charge equilibrium, and
the guard width of the Attix chamber is 13.7 mm, whereas
the HK chamber has a 5-mm guard width.
Figure 3.132 shows a schematic cross-sectional dia-
gram of the HK chamber. The collector (A in Figure
3.132) is insulated from the guard (C) by a 0.5-mm-deep
and 25-mm–inner diameter polystyrene cup (B), into
which the collector is snugly fit. The thickness of the
insulator is 0.25-mm-throughout. The guard is separated
from the chamber body by a 0.25-mm-thick polystyrene
insert (E). The Attix chamber is schematically shown in
Figure 3.133. The front entrance window is a 0.025-mm-
thick Kapton impregnated with 5 mg/cm
2
-thick graphite.
The 12.7-mm diameter collector is made of 0.13-mm-
thick polyethylene coated with dag.
The cavity-gas calibration factors for the Attix and
HK chambers were determined against the PTW cylindri-
cal chamber using a 20-MeV electron beam, as recom-
mended by the TG21 protocol. The
N
gas
values from the
FIGURE 3.112
PTW 23344 ion chamber.
Ch-03.fm(part 3) Page 204 Friday, November 10, 2000 12:00 PM
Ionization Chamber Dosimetry
205
rest of the beams were normalized to that of the 20-MeV
electron beam. The
N
gas
values varied considerably,
depending on the type of beams used, i.e., from
1% to
almost
3%, contrary to the theory that
N
gas
is constant.
The HK chamber has a cavity volume which is
approximately eight times that of the Attix chamber.
Therefore, conceptually, the HK chamber is considered to
have a statistical advantage in the signal-to-noise ratios
over the Attix chamber. The Attix chamber has a three
times larger guard width than the HK chamber. Theoret-
ically, this should give a better definition of the collection
volume. If the guard widths are inadequate for low-energy
electrons, the assumption of
P
repl
1 will not hold, and
N
gas
at low- and high-electron energies should deviate
from each other. This was not observed. Therefore, both
chambers are assumed to have adequate guard widths.
[107]
Attix a
p
-
p
chamber [108] is available commercially
from Gammex/RMI, as the Model 449 plane-parallel ion-
ization chamber. A schematic diagram is shown in Figure
3.133, and Table 3.47 lists its specifications. Some notes
about this chamber include:
FIGURE 3.113
PTW 23323 ion chamber.
TABLE 3.47
Specifications of the Gammex/RMI Model 449
Chamber
—Dimensions: 6.0 cm O. D.
1.4-cm height, when removed from
its solid-water phantom slab.
—Materials:
Chamber body: Gammex/RMI solid water 457; Nylon screws.
Front wall: 0.025-mm-thick 4.8 mg/cm
2
conducting Kapton film,
operated at ground potential.
Collecting electrode: Thin graphite coating on a polyethylene
insulator 0.127 mm thick
1.27 cm diameter.
—Plate separation (air-gap width): 1 mm.
—Ion-collecting volume: Approx. 0.127 cm
3
, vented to atmosphere.
Guard ring: Outer diameter
4.0 cm, width
1.35 cm.
—Electrical leakage: Approx. 3
10
4
A.
—Cable: Low-noise triaxial; BNC male triaxial connector.
—Ion-collecting potential:
300 V or less, to be applied to collector
and guard electrodes through the electrometer’s low-impedance
terminal.
Source: From Reference 108. With permission.
Ch-03.fm(part 3) Page 205 Friday, November 10, 2000 12:00 PM
206 Radiation Dosimetry: Instrumentation and Methods
1. It is constructed almost completely of solid
water to minimize perturbation of the radiation
field when used in a solid-water phantom.
2. The 1-mm plate separation and 4-cm O.D.
guard ring allow it to approximate closely the
results of an extrapolation chamber without the
inconvenience.
3. It is completely guarded, not only against elec-
trical leakage but also against electron in-scat-
tering through the sides of the chamber, for the
photon build-up region and for electron beams.
4. The thin Kapton front wall allows measure-
ments to be made as close as 4.8 mg/cm
2
inside
the phantom surface and is easily replaceable
by the user if damaged.
5. The polarity effect is less than that of the other
p-p chambers that have a thin front wall.
Klevenhagen [109] developed a dosimetric method
based on an ionization chamber which has an uncalibrated
sensitive volume but behaves as a Bragg-Gray cavity in
high-energy radiation. The chamber has a variable volume
and is constructed from water-similar materials. It can be
used in a water phantom directly in a beam of a therapy
megavoltage machine under clinical conditions. The
chamber allows absorbed dose to be determined from first
principles, overcoming many of the problems encountered
with conventional dosimetry based on calibrated chambers.
The chamber is shown in Figure 3.134; it was designed
to work with small electrode gaps, typically between 0.5
and 5.5 mm, and it is reasonable to assume that the pres-
ence of such a small cavity does not disturb the electron
fluence in the irradiated medium. Under such circum-
stances, the ionization volume approximates a thin-air slice
surrounded by the medium with minimum perturbation of
the electron fluence.
The necessity to calibrate the chamber is overcome
by taking advantage of the fact that the mass of air
involved in the interactions with the electron beam can
be determined from the gradient of a relationship
between the measured ionization charge and the gap
width between the electrodes. The gradient is all that
FIGURE 3.114 PTW 31002 ion chamber.
Ch-03.fm(part 3) Page 206 Friday, November 10, 2000 12:00 PM
Ionization Chamber Dosimetry 207
needs to be known since the gradient defines the ioniza-
tion increment per increment of mass air, which follows
from
(3.209)
where is the increment of ionization charge per
increment of mass of air,
g
a
is the gap between electrodes,
A is the effective collector area, and p is the air density
corrected for ambient conditions.
The material used in construction is Perspex (Lucite),
which offers good mechanical stability and electron scat-
tering similar to water. Its plane-parallel geometry is well-
suited for electron dosimetry, and its elongated shape
ensures that any metal parts such as the stepping motor
and driving shaft are kept far enough from the beam to
avoid any significant perturbation of the electron fluence.
A cross section of the chamber is shown in Figure 3.134.
The collecting electrode (2) of about 22 mm diameter
and the surrounding guard ring (1) which has a width of
18 mm were formed on the front face of a mobile cylinder
(7) by depositing several layers of a graphite “dag” about
12 m thick. The fine surface finish was obtained by
machining off several micrometers from the surface. A
clean groove was then cut at about 11 mm radius to pro-
vide a uniform electric field and to stop leakage currents
from the HV electrode. Its width was sufficient to prevent
perturbation of the electron fluence from extending into
the sensitive volume. A 2-mm-thick high-purity graphite
cap was embedded under the collector at 8 mm depth and
was connected to the guard (1) in order to provide a perfect
screening for the collector and the signal lead (8), which
was guarded right up to the connection with the collector.
The entrance window (4) was made of a polyester film
12.5 m thick, coated with a thin layer of graphite “dag”
deposited on its inner surface to create a polarizing elec-
trode connected to the HV lead (5). The estimated thick-
ness of the coating was about 5 m. The polarizing voltage
for the chamber was provided from a variable voltage
source so that at each electrode spacing the same 150-V
mm
1
electric field strength was maintained. This was
found sufficient to achieve about 99.9% ion-collection
efficiency. The stability of the polarizing voltage supply
was better than 1%.
FIGURE 3.115 PTW 31003 ion chamber.
J
a
mg
a
m()1A()1
a
()
J
a
m
Ch-03.fm(part 3) Page 207 Friday, November 10, 2000 12:00 PM
208 Radiation Dosimetry: Instrumentation and Methods
It has been demonstrated that electret ionization
chambers (EIC), in which one or both electrodes of an
ionization chamber are replaced by very stable electrets,
could serve as integrating radiation dosimeters. [110] An
external power supply is not necessary since the electret
generates the electric field within the chamber volume.
Charge carriers produced by photon interaction with the
air in the sensitive volume migrate in the field of the
electret to the electrodes of opposite sign and cause a
decay of the electret charge. If the electric field generated
by the electret places the sensitive volume in saturation,
the difference in electret surface charge before and after
its use is linearly related to the air kerma received. The
natural decay rate of the electret surface charge is pre-
dictable and is included in the determination of the air
kerma received.
Characteristics of a radiation-charged electret dosim-
eter was described by MacDonald et al. [111] The dosim-
eter is based on a parallel-plate ionization chamber with
the exception that the collecting electrode is covered by
a thin polymer, Teflon or Mylar. During the charging of
the dosimeter, ions produced in the sensitive volume by
an external radiation source drift in the externally applied
electric field and become trapped on the polymer surface,
forming an electret. Once the external supply is removed,
a field across the sensitive volume is produced by the
electret charge, such that during any subsequent irradi-
ation, ions opposite in sign to those on the electret sur-
face are attracted to the electret and deplete the charge
layer in an amount proportional to the air kerma. The
remaining charge on the electret is read by an electrom-
eter through further irradiation. This technique allows
the dosimeter to be simultaneously charged and cali-
brated, used in the field, simultaneously discharged and
read, and reused again in situ without dismantling the
dosimeter.
FIGURE 3.116 PTW 30001 ion chamber.
Ch-03.fm(part 3) Page 208 Friday, November 10, 2000 12:00 PM
Ionization Chamber Dosimetry 209
FIGURE 3.117 PTW 30002 ion chamber.
FIGURE 3.118 PTW 23343 ion chamber.
FIGURE 3.119 Extradin model A12 ion chamber.
Ch-03.fm(part 3) Page 209 Friday, November 10, 2000 12:00 PM