Reference

NBS

Publi - cations

NATL INST. OF STAND & TECH

A11107 211Q3M

NBSIR 77-1203

Central-Axis CO Ionization

Measurements in Graphite As

A Function of Phantom

Diameter, Depth, and Field

Size

Steve R. Domen

Center for Radiation Research

National Measurement Laboratory

National Bureau of Standards

Washington, D C. 20234

Issued September 1 978

QC

100

U56

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

77-1203

APR I? 1379

NBSIR 77-1203

CENTRAL-AXIS

6

°CO IONIZATION

MEASUREMENTS IN GRAPHITE AS

A FUNCTION OF PHANTOM

DIAMETER, DEPTH, AND FIELD

SIZE

Steve R. Domen

Center for Radiation Research

National Measurement Laboratory

National Bureau of Standards

Washington, D.C. 20234

Issued September 1978

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman. Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Central-Axis Co Ionization Measurements in Graphite as a

Function of Phantom Diameter, Depth, and Field Size

Steve R. Domen

Center for Radiation Research

National Measurement Laboratory

Abstract

Ionization measurements along the central axis were made

in a graphite phantom irradiated with cobalt-60 gamma rays.

The measurements were made under the following conditions:

phantom diameters of 15, 20, and 30 cm; 15 depths from 1 2

to 39 g/cm ; and square field sizes of 8.3, 10.5, 12.4, and

17.4 cm at a fixed detector position of 1 m from the

source. Empirical fits to the data aid in correcting

calorimeter comparisons to a common geometry.

Key Words: Absorbed dose; cobalt-60; graphite; calorimeter comparisons; phantom size.

I. INTRODUCTION

In view of the importance of achieving accurate and uniform measurements in radiation therapy and radiation protection, and because

those goals are linked to comparisons of calorimetric primary standards,

this investigation was initiated to aid in the comparison of calorimeters constructed of graphite and irradiated with cobalt-60 gamma rays.

Because comparisons of those instruments will be meaningful only if they

refer to a common geometry, an investigation was made to study the variation of absorbed dose in graphite along the central axis as a function

of phantom diameter, depth of measurement, field size, and backscattering thickness. This investigation required ionization measurements using 185 experimental setups: phantom diameters of 15, 20, and

2

30 cm; 15 depths from 1 to 39 g/cm ; and square field sizes of 8.3,

1
10.5, 12.4, and 17.4 cm. The data permit interpolation between those

values including approximate extrapolations to an infinite diameter

phantom. The smallest phantom diameter of 15 cm was that of the

National Bureau of Standards (NBS) graphite calorimeter [Ij. When

calorimeters of different diameters are compared, correction factors

must be specially determined, such as in the recent BNM-LMftl/NBS

comparison [2] where a correction was applied to measurements made

with a 3-cm diameter calorimeter for comparison with measurements made

with the NBS 15- cm diameter calorimeter.

A program for the international comparison of absorbed-dose

calorimeters has been established at the Bureau International des

Poids et Mesures (BIPM). All calorimetric measurements must be compared

with the BIPM graphite ionization chamber, enclosed by a 30- cm diameter

phantom, which is the intermediary instrument for the calorimetric

comparisons. This investigation will aid in correcting measurements

to a specified, or to an infinite, diameter phantom.

II. EXPERIMENTAL SETUP

Figure 1 shows the experimental setup. A spherical graphite

ionization chamber (similar to the one described in reference [3]),

1.27 cm inner diameter, was enclosed near the front surface of a

graphite phantom, 15.2 cm diameter by 10.0 cm thick, which are the

outside dimensions of the NBS calorimeter. The distance between the

graphite ion collecting electrode and the cobalt-60 source was kept

fixed at 1 m. Measurements were made at different depths by laying

2
graphite discs (1.7 g/cm }, 15.2 cm in diameter and of various thickness,

on the phantom. Measurements were also made by enclosing those bodies

with graphite sleeves, one 20.3 cm outside diameter by 15.3 cm inner

diameter, and another of 30.5 cm outside diameter by 15.3 cm inner

diameter. Height adjustment screws permitted the sleeves to be raised

or lowered so that their top surfaces were at the same height as the

top surface of the 15.2 cm diameter disc nearest to the source. Three

2.5 cm thick graphite rings (r-| , r^, ana r-J were raised, one at a

time, and placed against the rear of the graphite block simulating the

NBS portable calorimeter to determine if the graphite behind the ion

chamber provided an essentially infinite backscattering thickness.

The placement and removal of all the massive pieces of graphite caused

small deflections in the wooden supports. However, the distance

between the source and collecting electrode was always fixed within

0.025 mm by means of an adjusting nut (not shown) until the dial

indicator showed that the collecting electrode returned to its initial

position.

Beam profile measurements were made of the four square field sizes

by exposing film in air at 1 m from the source, and then measuring the

film density along lines bisecting opposite sides. The distances

(in centimeters) across the 90% and the 50% points of central density

were, respectively, 5.3 and 8.3, 7.4 and 10.5, 9.5 and 12.4, and 14.3

and 17.4. The beam sizes at the 50% points are used in this report.

3
III. RESULTS

Measurements showed that the backscattering ring (r-j) increased the

response by (0.07 _+ 0.03)%, and that there was no further significant

increase in response with ring r

2

also in position. All three rings

(r-j, r

2

, and r^) were in position during the following measurements.

A complicated factor that affects the detector response is geometry,

such as the square field sizes that range from being larger in area to

being much smaller than the circular phantom cross sections. Another is

the unknown effects of coll ima tor- scattered and leakage radiation that

can vary with different source heads over the phantom front surface.

These considerations , the small increase in detector response with

phantom diameter, plus the fact that only three measurements made were

at a given depth, make it clear that the infinite diameter response can

be estimated only roughly. Fortunately, the further increase in response

beyond the largest (30 cm) phantom size is quite small, and this is not

an important region because phantoms of larger diameter are not in use

in international comparisons. Nevertheless, it is still of some interest

to estimate the correction to infinite phantom diameter, however rough

that estimate may be. Empirical fits to the data points seemed adequate.

The relative current is plotted against the reciprocal of the square of

the phantom diameter in Figure 2. The measurements were made at a depth

2 of 4.7 g/cm and with the 10.5 cm field, close to standard conditions

for international comparison of calorimeters. The increase in current

is only 0.5% when the phantom diameter, D, is increased from 15 to

30 cm. A useful equation for the range of 15 <_ D<_ 30 cm is of the type:
Y = a„ + a-r X + a 0 X , where X = D a o I c

In this range, the curve has no point of inflexion which is a reasonable

physical expectation. The numerical coefficients of this equation are

tabulated in Tables 1 to 4. This equation with three constant coefficients passes, of course, through the three data points, but the change

in detector response as a function of D (at a particular depth) is not

determined from any such single curve. It is determined from results

derived from the curves (Y.) plotted as a function of depth. An ilCt lustrative example is given below.

Tables 1 to 4 summarize the results of the measurements, corrected

for cobalt-60 decay. Column 1 lists the depth of measurement , and

column 2 lists the diameters of the phantom as D(l), D(2), and D{3),

where

D(l) = 15.24 cm, D(2) = 20.32 cm, and D(3) = 30.48 cm.

Column 3 lists the relative measured ionization chamber current. Column

4 shows the percent current increase relative to the 15.2 cm diameter

phantom. Columns 5 to 7 list the coefficients of the equation

2 -2

Y = a„ + a,X + a 0 X calculated from the three values of D and Y shown

a 0 l c in columns 2 and 3. Table 5 shows the percent increase in response

calculated from equations Y and Y„ when the phantom is increased from

Ot p 30.5 cm to infinite diameter.

5
Other empirical fits to the data can be made. An equation of the

type:

Y

B

- b

0 + b

2

X

2 + b/

also passes through the data points, but most of the curves resulted in

points of inflexion in the range of 15 <_ 0 <_ 20 cm. Mainly for this

reason, the coefficients of Y 0 were not included in Tables 1 to 4.

p However, the curves, Ya and Y^, were used to compare calculated response

increases when a 30.5 cm diameter phantom is increased to infinite

diameter.

The central axis measurements for each field and phantom size as a

function of depth were least-squares fitted to the polynomial

2 3 4

y = cQ + c-jX + C2X + c^x + c^x ,

where y is the detector response and x is depth in mass per unit area,

determined from micrometer and mass measurements of the 15.2 cm diameter

plates. The coefficients are listed in Table 6 for the different field

sizes and phantom diameters shown in columns 1 and 2, respectively.

Figure 3 is an illustrative example that shows the calculated

difference in response of an 18 cm diameter calorimeter compared to one

of 15 cm diameter for the 10.5 x 10.5 cm field. The percent increase in

response of the former is plotted as a function of the measurement depth,

x. The points were calculated from the equation for Y , the coefficients

of which are listed in Table 2. The points are represented by the linear

equation y = 0.021x + 0.13.

6
The cobalt-60 source was In an Eldorado Super G head manufactured

by Atomic Energy of Canada Limited. It is important to note that the

results might have been somewhat different, if the experiment had been

done with a different source head but with the same field dimensions,

because of differences in scattering and energy spectral distributions

incident on the phantom. Thus the results reported here are, at best,

an approximation for correcting two different diameter calorimeters if

they are compared in another cobalt-60 source apparatus.

IV. SUMMARY

Comparison of calorimeters requires corrections to identical

irradiation conditions. This investigation aias in determining those

corrections from ionization measurements made under the following

conditions: phantom diameters of 15, 20, and 30 cm; 15 depths from 1 to

39 g/cra

2

; square field sizes of 8.3, 10.5, 12.4, and 17.4 cm at the 50X

points; a fixed source-detector distance of 1 m; and saturated backscattering thickness.

V. ACKNOWLEDGEMENT

This work was supported in part by the National Cancer Institute,

National Institute of Health, Bethesda, Maryland.

7
REFERENCES

Domen, S. R. and Lamperti , P. J., A heat-loss-compensated calorimeter: theory, design, and performance, J. Res. Nat. Bur. Stand.

(U.S.), 78A (Phys. and Chem.) No. 5, 595610, (Sept. -Oct. 1974).

Guiho, J. -P., Simoen, J. -P., and Domen, S. R., Comparison of

BNM-LMRI and NBS absorbed-dose standards for ^°Co gamma rays,

Metrologia, No. 2, 63-68, (1978).

Loftus, T. P., and Weaver, J. T., Standardization of

60

Co and 137Cs

Gamma-ray Beam in Terms of Exposure, J. Res. Nat. Bur. Stand.

8
Table 1. Resents with an 8.3 x 8.3 cm Co field

6raphite

Depth

Phantom

Diameter

Relative Current

Y* Increase from

a 15. 2 cm dia.

Phantom

Coefficients of . \ - V a

l X + a 2

X<

(cm) (X) a

o

a-,

1 a2

1.4 DO ) « 15.24 28.710 J 2 4 o.oc

D(2) » 20.32 28.748 1 s

c 0.13 28.768 -17.2 -2700

0C3) * 30.48 28.763 09 4 0.18

2.0 0(1

)

28.255 08 4 0.00

n(2) 28.289 10 3 0.12 28.301 2 .

-2900

0(3) 28.300 12 6 0.16

2.9 DO) 27.631 19 4 0.00

0(2) 27.670 08 3 0.14 27.702 -9.6 -1600

0(3) 27.690 05 4 0.21 4.7 DO ) 26.079 19 4 0.00

D(2) 26.126 06 3 0.18 26.146 1 .2 -3 SO0

D{3) 26! 143 15 3 0.24

6.4 DO) 24.559 02 2 0.00

D(2) 24.596 01 2 0.15 24.650 -21 .4 o Dili 24.629 08 3 0^28

8.8 DO

)

22.485 08 0.00

D(2) 22.539 12 3 0.24 22 . 536 -15.6 -1930

0(3) 22.569 00 3 0.37

11.3 DO) 20.162 28 i 0.00

0(2) 20.241 07 3 0.39 20.300 -14.9 -4000

• 0(3) 20.280 12 3 0.58

1 -. . 0 DO) 18.066 14 3 0.00

D(2) 18.123 18 3 0.32 18.181 -20.1 -1560

0(3) 18.158 03 3 0.51 17.5 DO) 15.237 04 3 0.00

D(2) 15.303 20 4 0.44 15.368 -21 .9 -1990

0(3) 15.342 02 3 0.69

21.4 D(l) 12.614 22 3 0.00

0(2) 12.673 02 3 0.47 12.750 -31 .8 0 D(3) 12.716 08 3 0.81 24.7 DO ) 10.642 06 3 0.00

D(2) 10.704 20 3 0.59 10.763 -19.6 -2000

0(3) 10.740 09 3 0^92

28.1 DO) 8.937 21 4 0.00

0(2) 8.996 14. 3 0.66 9.067 -23.4 - 430

0(3) 9.036 26 3 i.n

31.9 DO) 7.264 17 3 0.00

D(2) 7.314 26 3 0.70 7.381 -27.5 0 D(3) 7.352 10 3 1.22

" 35.7 0(1) 5.978 10 2 0.00

6.021 03 2 0.72 6.081 -24.1 c 811 6.056 11 3 1.30

39.1 DO) 4.950 09 3 0.00

D(2) 4.990 18 3 0.81 5.044 -21.9 0 D(3) 5.021 11 5 1.43

* The first entry is 28.710 with a mean error of 0.0064 for the mean of 4 measurements.
Taole 2. Results with a 10.5 x 10.5 cm Co Field

Graphite Phantom Relative Current

ueptn Diameter Y

(g/cm

2

) (cm)

1.4 0(1) - 15.24 29. 3-aZ ! c 0(2) - 20.32 <: y . jso 1

c

•J D(3) « 30.48 3

2.0 0(1) 28.893 22 4 0(2) 28.951 10 3

?fi 991 14 4

2.9 0(1) 28.305 AOUo J 0(2) 28.374 12 4 0(3) 28.398 22 3

4.7 DO) £0. O 1

i i t y'X< 26.899 04 3 0(3) 13 3

6.4 DO) 1 b

A1 0(2) 25.443 27 3 25.491 07 3 8.8 DO) i. J . JUU a C 3 0(2) 23.469 05 3 0(3) 23. 518 25 4 11.3 0(1) Zl . !<£/ ACUD J 0(2) 21 .216 25 4

21 . 286 19 3 14.0 0': i 9.U4Z u/ A

0(2' 19.141 25 3 obi 19 202 02 3

17.5 DO) 16.202 Go aJ >r 16 309 09 3 3(3

}

u/ 21.4 0(1) 13.527 22 3 0(2) &4n 17 3 0(3) U . / u3 U7 -?

24.7 DO) 11 .495 ** T 3

:

"i! 7G . KmUU

07 3 10 3

28.1 0(1) 9.707 1

3 0(2) 9.806 08 3 0(3) 9.864 07 3 31.9 0(1) 7.940 09 3 0(2) 3.024 21 3 3<3) 8.091 22 3

35.7 dc: £.55? 10 3

3(2! 6.642 V. 3 0(3) 6.697 15 3

39.1 : : 11 3 0(2) 5.527 11 3 0(3) 5.579 29 4

Increase from

a 15.2 cm d1a. Phantom

(%)

0.00

0.21 0.30

0.00

0.20

0.34

0.00

0.25

0.33

0.00

0.31

G.-.7 0.00

0.34

0.53

0.00

0.44

0.65

0.00

0.42

0.75

0.00

0.52

0.84

0.00

0.56

1.05

0.00

0.84

1.31 0.00

0.92

1.47

0.00

1.01 1.62

Coefficients of

2

Y a + a,X + a,X

a 0 I t

a 2

00

06

90 0.00

1.14

1.97

o

,

1.26

2.21

29.435 -8.7 -3500

29.02'i -27.7 -460

28.400 3.9 -6050

26.963 -16.4 -4140

25.521 -24.2 -3290

23.543 -17.2 -5570

21.337 -49.0

19.245 -37.2 -2320

16.415 -36.8 -2960

13.746 -34.4 -3830

11.706 -36.2 -3000

S.904 -34.3 -2670

8.139 -46.6

6.740 -40.1

5.618 -37.3 0
Table 3. Results with a 12.4 x 12.4 cm Co Field

Graphite

Depth

(g/cm

2

)

Phantom

Diameter

(cm)

Relative Current

Y Increase from

a 15.2 cm dia.

Phantom

w

Coefficients of , Y = a + a,X + apX*

a O I t

j.

' D(2

D(3

D{1

D(2

D(3

D(l

D(2

D(3

D(l

D(2]

D(3

D(l

D(2

D(3

D(l L'2 D(3

0(1

0}2

D(3

D(l

0(2

D(3

DO

0(2

D(3

0(1

D(2

D(3

DO

D(2

D(3

DO

D(2

D(3

0(1

0(3

0(1

;.'2 D(3

DO

0(2

D(3

= 15.24

= 20.32

= 30.48

29.676 1 3

29.768 06 7 29.805 13 7 29.246

29.349

29.384

28.672

28.772

28.818

27.218

27.333

27.393

25.788

25.919

25.970

23.821

23.970

24.031

21.614

21.765

21.835

19.538

19.706

19.790

16.705

16.867

16.949

14.024

14.180

14.262

11.955

12.107

12.175

10.134

10.271

10.346

8.318

8.444

8.516

6.908

014 082

762

655

13 9 07 7 09 8 10

36

0606n

08

13 7 13 8 05 4 1717

03

1410

06

1205

08

11 4

23" 5 14 4 0503

2405

0918

06

07

1406 4 07 4 09 4 12 4 08 4 14 4

5.914

10

1102

0.00

0.31 0.44

0.00

0.35

0.47

0.00

0.35

0.51 0.00

0.42

0.64

0.00

0.51 0.70

0.00

0.63

0.88

0.00

0.70

1.03

0.00

0.86

1.29

0.00

0.97

1.46

0.00

1.12

1 .70 0.00

1.27

1.84

0.00

1.36

2.09

0.00

1.52

2.37

0.00

'

. >; 2.51 0.00

1.64

2.65

29.817 -3.8 -6790

29.389 4.5 -8740

23.838 -12.2 -6100

27.428 -27.4 -4950

25.986 -4.2 -9660

24.053 -8.8 -10500

21.869 -21.6 -8740

19.834 -32.0 -8540

16.995

14.310

34.5 -7620

-37.4 -6790

12.203 -15.5 -9750

10.390 -35.8 -5520

3.559 -35.7 -4740

7.112 -44.1 -1770

5.955 -35.4 -2220
Table 4. Results with a 17.4 x 17.4 cm Co Field

Graphite

Depth

(g/oa2

)

Phantom

Diameter

(cm)

Relative Current

Y Increase from

a 15.2 cm dia

Phantom

(*)

Coefficients of

2

Y

= a + a-iX »• a,X

a 0 I £ a

l a

2

1.4

2.0

2.9

4.7

6.4

o.3

11.3

M.O

17.5

21.4

24.7

28.1

31.9

35.7

39.1

DO) - 0(2)

D(3)

::\)

ill)

0(1)

D(2)

0(3)

0(1)

0(2)

DO)

0(2)

D{3;

DO)

m

0(2)

0(3)

0(1)

0(2)

0(3)

o(i)

0(2)

0:0,

D(2)

0(3)

DO)

0(2)

0(3)

DO)

0(2)

0(3)

D(l)

0(2)

do:

0(2)

0(1)

0(2)

0(3)

15.24

20.32

30.48

30.661 09 3 30.944 12 4 31.081 04 3 30.234 21 3 30.565 09 3 30.690 18 4 29.674 17 4 30.025 09 3 30.175 01 3 28.239 11 4 28.654 07 3 28.835 06 3 26.822 08 3 27.283 20 5 27.486 09 3 24.887 07 3 25.416 11 3 25.628 13 3 22.689 11 3 23.302 10 3 23.533 09 3 20.653 12 3

21 .295 21 4 21.529 09 3 17.823 07 3 18.509 00 3 18.761 04 3 15.107 20 3 15.793 04 3 16.033 23 3 12.996 04 3 13.656 19 4 13.895 25 4 11.116 08 3 11.728 07 3 11.956 33 3 9.222 08 3 9.778 04 3 9.985 11 3 7.721 06 3 8.207 19 3 8.396 08 3 6.493 04 3 6.921 16 4 7.081 12 4

0 . 00 0.92

1.37

0.00

1.09

1.51 0.00

1.18

1.59

0.00

1.47

2.11 0.00

1.72

2.48

0.00

2.13

2.98

0.00

2.70

3.72

0.00

3.11 4.24

COO

5.26

0.00

4.54

6.13

0.00

5.08

6.92

0.00

5.51 7.56

0.00

6.03

8.27

0.00

6.29

8.74

0.00

6.59

9.06

31 .151 -49.2 -15000

30.722 -2.9 -25700

30.235 -30.8 -23100

28.912 -42.7 -26400

27.571 -47.7 -29300

25.699 -24.7 -38100

23.593 -4.3 -47800

21.582 6.6 -51600

18.820 4.1 -54800

16.076 21.9 -57360

13.947 9.3 -53500

12.012 0.1 -48320

10.035 0.4 -44000

8.452 -13.6 -36300

7.122 -1.5 -33600
Table 5. Percent increase In detector response when a 30.5 cm diameier

phantom is increased to infinite diameter*

Side of square field (cm)

8.3 10.5 12.4 17.4

Depth

2,

(g/cm )

Ya

(%)

Y

3

{%)

Ya

(%)

Y

3

{%)

Ya

(%) (%)

Ya

[%)

h

{%)

1 4 0.08 0.01 0.05 0.03 0.04 0 . 03 0.23 0.13

2.0 0.00 0.01 0.11 0.04 0.02 0.03 0.11 0.11

2.9 0.04 0.02 0.01 0.02 0.07 0.04 0.20 0.14

4.7 0.01 0.02 0.08 0.04 0.13 0.06 0.27 0.17

6.4 0.09 0.04 0.12 0.05 0.06 0.05 0.31 0.20

8.8 0.08 0.04 0.11 0.06 0.09 0 07 G.28 0 22

11.3 0.10 0.05 0.25 0.10 0.15 0.09 0.26 0.26

l a n 1 H . U U. i

U. Ub n oo U. CC u. uy n oo U . CL U. 1 1 n oc U . co (\ OQ U . CO

17.5 0.17 0.07 0.26 0.11 0.27 0.13 0.32 0.35

21.4 0.27 0.11 0.30 0.13 0.34 0.16 0.28 0.39

24.7 0.22 0.10 0.36 0.16 0.23 0.14 0.37 0.45

28.1 0.34 0.14 0.41 0.18 0.44 0.20 0.47 0.50

31.9 0.40 0.15 0.62 0.25 0.52 0.23 0.51 0.54

35.7 0.43 0.17 0.65 0.25 0.70 0.27 0.68 0.60

39.1 0.47 0.18 0.72 0.29 0.69 0.27 0.57 0.59

* Calculated from Ya a^X +

2 h^k and Y

3 = bo

4 + b

4x\

where X = D" .
Table 6. Summary of central axis detector response

Field Size Phantom Equation Coefficients 1

(cm)

Diameter

(cm) c o 10 c-j 10

2

c

2 10

4

c, 10

6

c,

8.3

ii

ii

DO)

0(2)

D(3)

= 15.24

= 20.32

= 30.48

29.855

29.883

29.889

-7.481

-7.446

-7.399

-1 .775

-1 .777

-1 .802

9.125

9.080

9.141

-9.878

-9.800

-9.861

10.5

ii DO)

DC2)

D(3)

30.389

30.441

30.460

-6.946

-6.881

-6.820

-1 .912

-1 .933

-1 .962

8.843

8.852

8.914

-9.163

-9.153

-9.206

12.4

ii

ii

DO)

0(2)

D(3)

30.702

30.780

30.807

-6.736

-6. 635

-6.574

-1 .947

-1 .976

-1 .997

8.650

8.629

8.646

-8.794

-8.716

-8.708

17.4

ii

ii

DO)

D(2)

D(3)

31 . 677

31 .916

32.017

-5.677

-6.288

-6.084

-1 .848

-1.871

-1 .971

7 . 999

7 [561

7.769

-7.976

-7.278

-7.475

2 3

y = c

Q + c-jX + c

2 x + c^x where x is the depth in g/cm'
FIGURE LlGlNDS

Fig. 1 Experimental setup for determining the iondmetric response as a

function of phantom diameter, depth, field size, and backscattering thickness. The ion chamber was fixed at 1 m from

the source and centered on the beam axis.

Fig. 2 Relative current as a function of phantom diameter at a depth of

2 4.7 g/cm , with a field size of 10.5 x 10.5 cm in air at 1 m.

Fig. 3 Illustrative example snowing the percent increase in central

axis absoroec-dose rate in an 13 cm diameter calorimeter

compared to one of 15 cm diameter, for a 10.5 x 10.5 cm field

in air at 1 m, in a cobalt- 60 y-ray beam.
BEAM DIRECTION

GRAPHITE SLEEVES GRAPHITE BUILDUP

PLATES

HEIGHT ADJUSTMENT

FOR GRAPHITE SLEEVES

ION COLLECTING

ELECTRODE

GRAPHITE BLOCK

SIMULATING THE

NBS PORTABLE

CALORIMETER

DIAL INDICATOR
0 12 3 4 x \0 6

RECIPROCAL OF PHANTOM DIAMETER SQUARED s crrf 2

NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

| 1. PUBLICATION OR REPORT NO.

77-1203

2. Gov t Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTiTLE

Central -Axis 6 ^C0 Ionization Measurements in Graphite as a Function of Phantom Diameter, Depth, and Field Size

5. Publication Date September 1973

6. Performing Organization Code

7. AUTHOR(S)

Steve R. Domen

o. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract /Grarit No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

National Bureau of Standards

Department of Commerce

Washington, DC 20234

^3. Type of Report & Period

Covered

NBSIR Report

14« Sponsoring Agency Code*

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less {actual summary of most signi ficant information. If document includes a significant

bibliography or literature survey, mention it here.)

Ionization measurements along the central axis were made in a graphite phantom

irradiated with cobalt-60 gamma rays. The measurements were made under the

following conditions: phantom diameters of 15, 20, and 30 cm; 15 depths from 1 to 39 g/cm2

; and square field sizes of 8.3, 10.5, 12.4, and 17.4 cm at a fixed

detector position of 1 m from the source. Empirical fits to the data aid in

correcting calorimeter comparisons to a common geometry.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Absorbed dose; cobalt-60; graphite; calorimeter comparisons; phantom size.

18. AVAILABILITY QT Unlimited 19. SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES

I 1 For Official Distribution. Do Not Release to NT1S 20 UNCLASSIFIED

Order From Sup. of Doc, U.S. Government Printing Office Washington. D.C. 20402. SD Cat. No. CH

20. SECURITY CLASS

(THIS PAGE)

22. Price

1 ! Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151 UNCLASSIFIED

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USCCMM.OC 29042-P74

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