Shani G. Radiation Dosimetry: Instrumentation and Methods

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170 Radiation Dosimetry: Instrumentation and Methods

reproduces the data for a water-walled (i.e., wall-less)

chamber with a diameter of 6.4 mm to within 0.25% and

is within 1% of all the IAEA-based data.

Using the protocol of the American Association of

Physicists in Medicine (AAPM) for the determination of

absorbed dose from high energy, the calibration factor

for the PTW M23343 plane-parallel ionization

chamber has been determined by Murali et al. [86] in

and high-energy electron beams. Two cylindrical cham-

bers of different wall materials and known values of N

gas

have been used as reference chambers. Results show that

the N

gas

value for the plane-parallel chamber determined

in the

Co beam is about 2% higher than the value

obtained in the high-energy electron beams. The discrep-

ancy is attributed to not taking into account a wall-

correction factor for the plane-parallel chamber while cal-

ibrating in the

Co beam. is calculated using the

relation

(3.175)

where c represents a cylindrical chamber; p represents a

plane-parallel chamber; M is the meter reading; P

ion

the

correction factor for the ion recombination; is the

ﬂuence correction factor (which depends on the dimen-

sions of the chamber); and is the correction factor

for the non-air equivalence of the wall material. and

for the plane-parallel chamber are assumed to be

unity (AAPM 1983).

The results for the values obtained with

Co and

the electron beams are presented in Table 3.24. It is seen

that the value obtained with

Co is 2.16% higher

than that obtained with the electron beams. The increase

can be recognized in both sets of measurements.

A set of dosimetric measurements was made by Huq

et al. [87] with Farmer-type PTW and Capintec ionization

chambers in solid water, PMMA, and polystyrene phan-

toms and exposed to a 4-MV photon beam from a Varian

Clinac 4S at Yale, a 10-MV photon beam, 6-and 15-MeV

electron beams, and a 25-MV photon beam. The results

are as follow.

TABLE 3.24

Values

Set I with PTW

(Gy C

–1

)

Set II with NE

(Gy C

–1

)

Co 0.4817 0.4812

at d  5 cm (2.16) (2.06)

Electrons (15 MeV) 0.4707 0.4710

max

 2.2 cm (0.17) (0.11)

Electrons (20 MeV) 0.4721 0.4722

max

 2.2 cm (0.13) (0.15)

Note: Values in parentheses show the percentage difference of the

value from the mean value (0.4715 Gy C

1

) obtained with the

electron beams.

Source: From Reference [86]. With permission.

gas

ion

repl

wall

()

 MP

ion

repl

wall

()



repl

wall

repl

gas

FIGURE 3.79 Calculated factors for all 17 Farmer-like chambers discussed by Rogers. The IAEA data set and AAPM values

of were used in the calculations. Chambers of the same material are shown by the same type of curve. (From Reference [85].

With permission.)

repl

Ch-03.fm(part 2) Page 170 Friday, November 10, 2000 11:59 AM

Ionization Chamber Dosimetry 171

The ratio of the electrometer readings corrected for

ionization-collection efﬁciency is equal to the ratio of

percent depth doses at and , i.e.,

(3.176)

where , and M are electrometer readings corrected for

ionization collection efﬁciency at and d

, respec-

tively. d

is a reference depth, f is a displacement factor (f 

0.5 for electrons and 0.75 for protons), and r is the inner

radius of the chapter. The above ratios for both photons and

electrons were obtained from dosimetry data in clinical use.

The ion chambers used for dose intercomparisons

were all 0.6-cm

Farmer-type chambers having different

wall, central electrode, and build-up cap materials.

For electron beams, all measurements were taken with

a source-surface distance (SSD) of 100 cm and a ﬁeld size

of 10

 10 cm at the phantom surface.

The basic equations that are used by both protocols for

the determination of absorbed dose to water are as follows:

1. AAPM protocol

• photons,

(3.177)

• electrons,

(3.178)

2. IAEA protocol

• photons and electrons,

(3.179)

The results of the IAEA absorbed dose to solid water are

compared with those of AAPM in Table 3.25.

Results of absorbed doses to solid water for 6-and 15-

MeV electron beams are given in Table 3.26 for a PTW

chamber. Measurements were performed by using both

chambers in solid water, PMMA, and polystyrene phantoms.

was determined by Reft et al. [88] for six com-

mercially available parallel-plate ionization chambers

using in-phantom and an in-air

Co irradiation and an in-

phantom irradiation with a high-energy electron beam. The

chamber characteristics for the six parallel-plate chambers

and three cylindrical chambers are listed in Tables 3.27 and

3.28, respectively. All parallel-plate chambers used were

designed to produce minimal perturbation in the electron

ﬁeld when placed in a homogeneous medium.

A 22-MeV electron beam with a 1515-cm

(0.15 

0.15- ) ﬁeld at a source to surface distance of 1 m was

used for all the electron irradiations. The cylindrical cham-

ber was placed with the axis of symmetry at and

perpendicular to the beam, and the parallel-plate chamber

was positioned perpendicular to the beam axis, with the

center of the inner surface of the front wall at .

For the same exposure in air from the

Co beam for

a pair of plane-parallel and cylindrical chambers at the

same point of measurement, one can write

(3.180)

where the measured charge is corrected for temperature,

pressure, polarity effect, and ion-collection efﬁciency. In

the revised notation of the AAPM protocol by Rogers, and

the letter of clariﬁcation by Schulz et al. and are

related by

(3.181)

TABLE 3.25

Absorbed Dose Intercomparisons for 4, 10, and 25-MV Photon Beams in a Solid Water Phantom

Chamber

PTW 0.63 0.988(0.99)

0.995 1.000 0.996 0.996(0.99)

Capintec 0.63 0.987(0.99)

0.995 0.996 0.994 0.991(0.99)

PTW 0.73 0.992 0.989 1.001 0.998 0.993

Capintec 0.73 0.991 0.989 0.998 0.997 0.990

PTW 0.78 0.995(0.995)

0.989 1.001 1.000 0.995(0.99)

Capintec 0.78 0.992(0.99)

0.989 0.999 0.997 0.991(0.98)

Average 0.993  0.002(0.994)

Note: has a value of 1.005 for the PTW chamber and 1.006 for the Capintec chamber, and has a value of unity

for both chambers. The 4, 10, and 25-MV data are shown in rows having values equal to 0.63, 0.73, and 0.78, respectively. [87]

Values given in parentheses are from theoretical intercomparison in Reference [22] of Chapter 2.

Source: From Reference [87]. With permission.

TPR

------

w,air

()

L



()

air

water

----------------------

wall

---------

%DD

P eff

repl

---------------------

IAEA

d()

AAPM

d()

---------------------

N

gas

cel

TPR

fr d

-------

%DD d

()

%DD d

()

--------------------------------------



fr

water

() MN

gas

L



()

gas

plastic

ion

wall

repl









()

plastic

water

ESC

water

() MN

gas

L



()

gas

plastic

ion

wall

repl





()

plastic

water



plastic

water



() M

w,air

()

cel



gas

max

air,pp,Co

xpp,

air, cyl, Co

xcyl,



gas

We()N

gas

wall

comp

ion

L



()

air

wall







()

wall

air



Ch-03.fm(part 2) Page 171 Friday, November 10, 2000 11:59 AM

172 Radiation Dosimetry: Instrumentation and Methods

for any one chamber, where corrects for the lack of

charged particle equilibrium in the chamber walls,

corrects for ion-collection efﬁciency, corrects for

any compositional material differences between the cham-

ber wall and build-up cap, is the mean restricted

collision mass stopping power, is the mean mass

energy absorption coefﬁcient, and W/e is the mean energy

expended per unit charge in dry air. Rearranging Equation

(3.181) and using Equation (3.180),

(3.182)

If the product of terms for the parallel-plate

chambers is known, can be obtained from the ratio

of charge measurements and dosimetric quantities given

in TG-21.

The parallel-plate and cylindrical chamber of like

material were positioned in the phantom with their res-

pective points of measurement at a depth of 5.0 g/cm

(50 kg/m

) and exposed to a

Co beam under identical

irradiation conditions. A 10

 10-cm

ﬁeld deﬁned at a

source-to-detector distance of 1 m was used for the

exposures.

Results for the determination of using the three

calibration procedures are presented in Table 3.29.

TABLE 3.26

Absorbed Dose Intercomparisons in Electron Beams (PTW Chamber)

Energy

(MeV) Phantom

6 Solid water 0.996 0.998 0.990 1.000 0.997 (0.987)

15 Solid water 0.999 0.999 1.000 1.000 1.010 (1.005)

6 PMMA 0.993 0.998 0.994 1.000 0.999 (0.989)

15 PMMA 0.998 0.999 1.000 1.000 1.010 (1.007)

6 Poly 0.993 0.998 0.993 0.977 0.973 (0.965)

15 Poly 0.994 1.000 1.000 0.999 1.005 (1.007)

Average 0.999  0.014 (0.993  L0.017)

Note: For this chamber has a value of 1.008 and as has a value of 1.005.

Values given in parentheses are from the theoretical intercomparison.

Source: From Reference [87]. With permission.

TABLE 3.27

Physical Characteristics of the Parallel-Plate Chambers Used by Reft et al.

Guard

Sensitive air-

volume Entrance Window Buildup Cap

Chamber Width Radius Depth

Thickness

(kg/m

)

Radius

(10

–3

Thickness

(kg/m

)Type Model (10

–3

m) (10



m) Material Material

NACP … 3.0 5.0 2.0 Graphite 0.900 15 4.10 Graphite

Markus 30–329 0.2 2.7 2.0 Polyethylene 0.023 15 5.00 Acrylic

Holt 30–404 5.0 12.5 2.0 Polystyrene 4.200 … … …

(Carbon-coated)

Capintec PS-033 0.5 8.1 2.4 Mylar 0.005 5.00 Polystyrene

(Aluminized)

Exradin P-11 5.1 10.0 2.0 Polystyrene-equiv. 1.180 22.5 4.00 Polystyrene

(Conducting

plastic)

Attix 449 13.5 6.35 1.0 Conductive

Kapton

0.048 30.0 4.95 Solid Water

(RMI)

Source: From Reference [88]. With permission.

w,air

()

L 



()

air

med

S 



()

med

water



----------------------------------------------

repl

---------

------



med

water

-------------

IAEA

()

AAPM

()

-----------------------

cel

N

gas

wall

ion

comp

L









gas,pp

We()N

xcyl,

air,Co

[]

cyl

L



()

wall

air







()

air

wall

[]



wall

comp

ion

[]

wall

comp

gas,pp

Ch-03.fm(part 2) Page 172 Friday, November 10, 2000 11:59 AM

Ionization Chamber Dosimetry 173

The question whether results of measurements made

in different phantom materials using different ionization

chambers are consistent with each other was investigated

by Huq et al. [89] Two 0.6-cc Farmer-type chambers, a

PTW N23333, NA 30-351, NA 30-352, and a Capintec

PR 06G chamber were used. They consisted of very dif-

ferent walls, build-up caps, and central electrode materi-

als. The wall and build-up cap material of the PTW cham-

ber was made of PMMA, and the central electrode was

made of aluminum. By contrast, the Capintec chamber

had an air-equivalent wall, polystyrene build-up cap, and

an air-equivalent plastic central electrode.

Absorbed dose at a point in a water phantom was

determined from measurements in a nonwater (or water)

phantom using the following expression:

(3.183)

where is the dose in water at a depth ;

is the dose to the gas in the chamber per unit elec-

trometer reading corrected for temperature, pressure, and

ion collection efﬁciency; is the ion-chamber

reading corrected for polarity and ion recombination at

the effective depth in the medium; is the

ratio of mean restricted collision stopping powers of med

to air; is the ratio of average unrestricted colli-

sional stopping powers of wat to med; is the differ-

ence in electron ﬂuence between nonwater and water

phantoms at water equivalent depths; and is a

replacement correction that accounts for the change in

photon and electron-beam ﬂuence that occurs because of

the replacement of phantom material by the air cavity.

If measurements are made in a nonwater-equivalent

phantom, then the protocol recommends that the depth in

the nonwater-equivalent phantom be scaled to that of the

water-equivalent phantom by the following expression:

(3.184)

If cylindrical chambers are used for depth-ionization mea-

surements, then replacement corrections are needed

to convert depth-ionization data to depth-dose data.

TABLE 3.28

Physical Characteristics of the Cylindrical Chambers Used by Reft et al.

Inner

Axial length

Entrance Window

Thickness

Build-up cap

Thickness

Chamber

Inner

Radius N

gas

Type Model (10

–2

m) (10

–2

m) Material (kg/m

) Material (kg/m

) 10

Gy/C

Farmer 30–35

0.305 2.20 Acrylic 0.59 Acrylic 5.30 4.57

PTW

Farmer N30–351

0.304 2.20 Graphite 0.96 Graphite 4.89 4.62

PTW

Farmer 2571

0.315 2.25 Graphite 0.65 Graphite 4.89 3.99

NEL

a,b

Farmer-type chamber available from Nuclear Associates, Carle Place, NY 11514.

Farmer-type chamber available from Nuclear Enterprises America, Fairﬁeld, NJ 07006.

Source: From Reference [88]. With permission.

TABLE 3.29

Summary of Results for ( 10

Gy/C) Using the Three Different Calibration Methods

22E

Co (Phantom)

Co (Air)

Chamber N

gas,pp

wall,pp

gas,pp

wall

comp

gas,pp

NACP 2.9900 13.530 3.0176 1.027 13.380 3.0000 1.0250 13.49 13.5  0.1

Markus 10.280 46.700 10.3140 1.000 46.920 1.0340 0.9970 47.25 47.0  0.3

Holt 0.6157 2.797 0.6258 1.000 2.807 0.6199 0.9904 2.821 2.81  0.01

Capintec 1.2120 5.506 1.1719 0.960 5.475 1.1640 0.9579 5.476 5.49  0.02

Exradin 0.9326 4.236 0.9518 1.000 4.269 0.9400 0.9984 4.243 4.25  0.02

Attix 5.1680 23.480 5.2790 1.015 23.640 0.5223 1.0060 23.61 23.6  0.1

Source: From Reference [88]. With permission.

gas pp,

M[]

cyl

M[]

cyl

M[]

cyl

gas

〈〉

wat

() N

gas

corr

med

()L



()

air

med

S



()

med

wat



med

wat

repl



wat

() d

wat

gas

corr

med

()

med

L



()

air

med

S



()

med

wat



med

wat

repl

water

med



eff

med

wat

med

----------



repl

Ch-03.fm(part 2) Page 173 Friday, November 10, 2000 11:59 AM

174 Radiation Dosimetry: Instrumentation and Methods

Figure 3.80 shows the relative depth-dose data for 6-

and 15-MeV electron beams generated by using a PTW

chamber in solid water, PMMA, and clear polystyrene

phantoms, using TG-25 except for the bremsstrahlung tail

section of the curves. Similar data for the Capintec cham-

ber are given in Figure 3.81. The ﬁgures clearly show that

the depth-dose data measured in a solid water phantom

are essentially indistinguishable from those obtained from

measurements made in the PMMA and clear polystyrene

phantoms at both energies. Thus, if one measures depth

ionization in a solid water, PMMA, or clear polystyrene

phantom and follows the recommendations of the TG-25

protocol to convert relative depth ionization to relative

depth dose, then no signiﬁcant differences will be

observed between the relative depth doses obtained from

measurements made in the solid water, PMMA, and clear

polystyrene phantom.

The AAPM TG-39 protocol has proposed three dif-

ferent methods of calibrating plane-parallel ionization

chambers, i.e., in-phantom irradiation with a high-energy

electron beam and in-phantom and in-air

Co irradiation.

To verify the consistency of the three methods, Almond

et al. [90] have measured values using each of these

techniques for the ﬁve most commonly used plane-parallel

chambers considered by the protocol. Their results dem-

onstrate that the measured values for the three dif-

ferent methods for any of the chambers agree to within

0.6%.

The ﬁve plane-parallel chambers considered by the

AAPM TG-39 protocol were used and designated as

FIGURE 3.80 Central axis relative depth-dose data for 6- and 15-MeV electron beams measured by using a PTW chamber in solid

water, PMMA, and clear polystyrene phantoms; (a) 6-MeV and (b) 15-MeV electron beams. Solid circles are data for solid water;

open circles are for clear polystyrene; and crosses are for PMMA phantom. (From Reference [89]. With permission.)

gas

Ch-03.fm(part 2) Page 174 Friday, November 10, 2000 11:59 AM

Ionization Chamber Dosimetry 175

“Capintec,” “Exradin,” “Holt,” “NACP,” and “PTW-

Markus.” Their characteristics and individual parameters

are listed in Table 3.30.

The comparisons were made against a PTW graphite

cylindrical Farmer-type chamber, number N30001G, with

an ADCL exposure factor and a cav-

ity-gas calibration factor , which was

calculated by using The

experimental setup is schematically shown in Figure 3.82.

Intercomparison between the plane-parallel chamber

and the PTW Farmer cylindrical chamber was performed

in a

Co beam in air using a 10  10-cm

(0.10  0.10 m

)

ﬁeld deﬁned at 0.80 m. The experimental setup is shown

in Figure 3.83. Under these conditions, the exposure

calibration factor for the plane-parallel chamber is given

(3.185)

where and are, respectively, the meter readings

of the cylindrical and plane-parallel chambers, corrected

for temperature, pressure, and polarity; and ,

are, respectively, the exposure calibration factors for

radiation of the cylindrical and plane-parallel chambers.

was then calculated from the ratios of 

, given by the AAPM TG-39 protocol.

The intercomparison between the plane-parallel

chamber and the PTW Farmer chamber was performed in

FIGURE 3.81 Central axis relative depth-dose data for 6- and 15-MeV electron beams measured by using a Capintec chamber in

solid water, PMMA, and clear polystyrene phantoms; (a) 6-MeV and (b) 15-MeV electron beams. Solid circles are data for solid

water; open circles are for clear polystyrene; and crosses are for PMMA phantom. (From Reference [89]. With permission.)

5.455 RnC()

gas

cyl

4.653 RnC()

gas

 N

ion

()

cyl

8.53 10

3



cyl

-------------------



cyl

gas

ion

()

Ch-03.fm(part 2) Page 175 Friday, November 10, 2000 11:59 AM

176 Radiation Dosimetry: Instrumentation and Methods

Co beam at a depth of 5 g/cm

(50 kg/m

) in a phantom

of a material selected to match that of the plane-parallel

chamber as described in the AAPM TG-39 protocol. The

experimental setup is shown in Figure 3.84. Under these

conditions, is given by

(3.186)

where and are, respectively, the meter readings

of the cylindrical and plane-parallel chambers under the

above conditions, corrected for temperature, pressure, and

polarity; is the factor that corrects the ionization

recombination measured by the two-potential method; and

is the correction factor for the replacement of phan-

tom material by the cavity of the cylindrical ionization

chambers. is taken as unity in the AAPM TG-21

protocol for the plane-parallel chambers, so it does not

appear in the above equation. is the correction factor

for the chamber materials being different from that of the

phantom.

TABLE 3.30

Physical Characteristics of Plane-Parallel Ionization Chambers Used by Almond et al.

Chamber

Wall: Material

Thickness

(cm)/(mg/cm

)

Collector: Material/

Diameter

(cm)

Guard Ring

Radial Width

(cm)

Electrode

Spacing

(cm)

Nominal

Volume

(cm

)

Recommended

Build-Up/Thickness

(cm)/dia.(cm)

Aluminized Conducting air- Conducting air-

Capintec polyethylene equivalent equivalent 0.24 0.5 Polystyrene/

0.001/0.5 plastic 1.6 plastic 0.05 0.48/3.0

Conducting Conducting Conducting,

Exradin Polystyrene polystyrene 2.0 polystyrene 0.5 0.2 0.6 Polystyrene/

0.1/104 0.48/4.5

Polystyrene Graphite Graphite

Holt 0.4/418 polystyrene 2.5 polystyrene 0.5 0.2 1.0 No additional

Graphite and

NACP Mylar 0.5 and Graphite/1.0 Graphite/0.3 0.2 0.16 Graphite/0.3/3.0

0.008/93

Graphite Graphite PMMA Graphite

PTW- Polyethylene 0.54 PMMA 0.07 0.2 0.055 Acrylic/0.42/3.0

Markus 0.0025/2.3

Source: From Reference [90]. With permission.

FIGURE 3.82 Experimental setup for the electron-beam method. (From Reference [90]. With permission.)

gas

ion

repl

wall

()

cyl

ion

wall

()

-------------------------------------------------------



cyl

ion

repl

wall

Ch-03.fm(part 2) Page 176 Friday, November 10, 2000 11:59 AM

Ionization Chamber Dosimetry 177

The values obtained for each chamber by using

three different calibration methods are shown in Table

3.31a. [90]

Calibration of

192

Ir PDR brachytherapy source was per-

formed by Reynaert et al. [91] in a water phantom at short

distances (1.0, 2.5, and 5.0 cm) using an NE2571 Farmer-

type ion chamber. To convert the measured air-kerma rate

in water to dose rate to water, a conversion factor (CF) was

calculated by adapting the medium-energy x-ray dosimetry

protocol for a point-source geometry. The CF was calcu-

lated by Monte Carlo simulations, where the source-ion-

ization chamber geometry was modeled accurately. In a

second method, a combination of Monte Carlo simulations

and measurements of the air-kerma rate in water (at 1.0,

2.5, and 5.0-cm distance) and in air (1-m distance) was

used to determine the CF. The following values were

FIGURE 3.83 Experimental setup for the

Co in-air method. (From Reference [90]. With permission.)

FIGURE 3.84 Experimental setup for the

Co in-phantom method. (From Reference [90]. With permission.)

gas

Ch-03.fm(part 2) Page 177 Friday, November 10, 2000 11:59 AM

178 Radiation Dosimetry: Instrumentation and Methods

obtained (medium-energy x-ray protocol): CF (1 cm) 

1.458; CF (2.5 cm)  1.162; CF (5.0 cm)  1.112 (1





0.7% for the three distances of interest).

A conversion factor (CF) relates the dose rate to water

to the measured air-kerma rate in the phantom

(3.187)

where is the air-kerma rate measured by the ion

chamber (cavity plus wall), which can be derived from

the chamber reading. An in-water calibration has the

advantage that one is measuring geometry and scatter

conditions in the correct beam.

To convert the obtained result for to ,

the two following expressions are combined:

(3.188)

where the air-kerma calibration factor for

Co was used

in the ﬁrst expression and that for the

192

Ir radiation quality

(without build-up cap) in the second; is a correction

for the non-air equivalence of the chamber wall and the

build-up cap; and is a correction for attenuation in the

wall and the build-up cap. M is the chamber reading. Then

(3.189)

The conversion from to introduces an

extra uncertainty on the

CF obtained with this approach.

The conversion factor

CF is obtained directly by dividing

by .

For the calibration factor with build-up cap for the

192

Ir radiation quality obtained by the calibration proce-

dure, Reynaert et al. found  0.988

and at the three dis-

tances of interest in the water phantom.

To determine the scatter contribution to the total

kerma, the photon ﬂuence spectra obtained in the EGS4

simulations were split into primary and scattered photon

spectra and multiplied by to remove the depen-

dence. The spectra were normalized per emitted photon.

These results are plotted in Figure 3.85. At distances larger

than 6 cm, the contribution of scattered photons to the

kerma is larger than that of the primaries.

can be deﬁned by the following expression

(3.190)

where is the calibration factor for kerma in

Co and

the index H denotes the high-energy photon dosimetry

protocol. can be deﬁned as the conversion factor

obtained from the high-energy photon protocol, or

(3.191)

The behavior of a chamber during measurements, with

variation of response with beam quality, was measured by

National Physical Laboratory (NPL).[92] The results, nor-

malized to 1 mm Cu HVL, are shown in Figures 3.86 and

3.87. As can be seen from these ﬁgures, the air-kerma

TABLE 3.31

(a) Summary of Results for , Using Three Different Calibration Methods.

(b) Percentage Difference from the Average Values of

(a)

Electron Beam

Co in Air

Co in phant. Average

Chamber N

gas

(10



Gy/C) N

gas

(10



Gy/C) N

gas

(10



Gy/C) N

gas

(10



Gy/C)  s.d.

Capintec 5.973 5.947 5.906 5.942  0.6%

Exradin 4.614 4.591 4.649 4.618  0.6%

Holt 2.740 2.755 2.766 2.754  0.5%

NACP 14.990 14.820 14.920 14.910  0.6%

PTW-Markus 48.510 48.929 49.074 48.838  0.6%

Uncertainty 2.1% 2.6% 2.3%

(b)

Chamber Electron beam

Co in air

Co in phantom

Capintec 0.5 0.1 0.6

Exradin 0.1 0.6 0.7

Holt 0.5 0.0 0.4

NACP 0.5 0.6 0.1

PTW-Markus 0.7 0.2 0.5

Source: From Reference [90]. With permission.

gas

r() CF K

det

r()

det

det,phot

gas

att

1 g()M and K

det

kIr

192

M

att

det, phot

kIr

kCo

att

1 g()

------------------------------------------------



det,phot

w,phot

det,phot

Ir, air

192

()N

()

Ir, water

192

()N

()0.987



2

r() N

MCF



------



---





w,air

wall

cel



Ch-03.fm(part 2) Page 178 Friday, November 10, 2000 11:59 AM

Ionization Chamber Dosimetry 179

calibration factors for the NE2561 and NE2571 vary by

less than 1% between 0.15 mm Cu and 4 mm Cu. The

responses of the (short) NE2577 and PTW Grenz ray

chambers are slightly less ﬂat, varying by 2% and 4%,

respectively. If the NACP electron chamber is included,

the variation becomes much greater (25%).

For Farmer types of chamber, the variation in response

with HVL has been extracted from data measured during

routine calibration. This is shown in Figure 3.88, normal-

ized to 3.5 mm Cu.

As can be seen, the variation of response for the

NE2581 is greater than might be expected. Although this

graph is plotted over a greater range (down to 0.068 mm

Cu), there is still about a 15% difference between 1.0 and

0.15 mm Cu, with signiﬁcant variation between individual

chambers. Experience suggests that this may also be typ-

ical of chambers of the Farmer type which use either

conducting or graphite-coated plastic outer electrodes.

Experimental investigation of a simple design of a

plane-parallel electron chamber, which has very thin lay-

ers of copper (0.018 or 0.035 mm) as conducting material

was done by Ma et al.[93] The results show that the

factors (proportional to the product of water/air stopping

power ratio and perturbation factor) for converting the in-

phantom air-kerma-calibrated chamber reading to the

absorbed dose to water are nearly constant for incident

electron energies between 4 and 11 MeV for prototype

chambers with 0.018-mm-thick copper layers and between

4 and 15 MeV for chambers with 0.035-mm-thick copper

layers.

FIGURE 3.85 Calculated total kerma is split into separate contributions of scattered and primary photons. The kerma are multiplied

by . At distances larger than 6 cm, the scatter contribution is larger than that of the primaries. The results are normalized at

1 cm. This ﬁgure can be compared with the results obtained by Russell and Ahnesjö. [91a] (From Reference [91]. With permission.)

FIGURE 3.86 The variation of the chamber calibration factor with beam quality. (From Reference [92]. With permission.)



Ch-03.fm(part 2) Page 179 Friday, November 10, 2000 11:59 AM