NIST Measurement Services: NEW NIST PUBLICATION

January 1991

Absorbed-Dose Calibration of

Ionization Chambers in a

®°Co Gamma-Ray Beam

John S. Pruitt

U.S. Department of Commerce

National Institute of Standards and Technology

NIST MEASUREMENT SERVICES:

Absorbed-Dose Calibration of Ionization Chambers

in a ^°Co Gamma-Ray Beam

John S. Pruitt

Center for Radiation Research

National Measurement Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899

October 1990

U.S. DEPARTMENT OF COMMERCE, Robert A. Mosbacher, Secretary

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

John W. Lyons, Director
National Institute of Standards and Technology Special Publication 250-40

Natl. Inst. Stand. Technol. Spec. Publ. 250-40, 54 pages (Oct. 1990)

CQDEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1990

For sale by the Superintendent of Documents, U.S. Government Printing Office, Wasfiington, DC 20402-9325
Abstract

Measurement Service C.l from National Institute of Standards and

Technology (NIST) publication SP 250 provides calibrations for customerowned ionization chambers so that they may be used to determine absorbed

dose to water in ^^Co gamma-ray beams. The calibrations are based on calorimetric measurement of absorbed dose to graphite in a graphite

phantom. Transformation of the calibrations to a water phantom is made

with a specially-designed graphite ionization chamber, and requires

knowledge of photon mass attenuation coefficients and the perturbation

of the graphite chamber in the water medium. The determination of these

quantities is described in detail, along with the operational techniques

normally used to transfer the calibration to customer-owned chambers.

Appendix A lists experimental data used to test the photon-f 1 uence

scaling theorem. Appendices B, C, and D describe international

comparisons of the chamber calibrations, and Appendix E shows a sample

calibration report.

i i i
TABLE OF CONTENTS

page

1. The Calibration Service 1 2. The Genera! Approach 1

3. The Graphite Calorimeter 2

4. The Graphite Transfer Ionization Chamber 6

5. The Chamber Calibrations in a Graphite Phantom 11

6. Calibration Transfer to Water 19

6.1 Theory 19

6.2 Evaluation of the Coefficients 21

6.3 The Chamber Currents in Water 24

6.4 The Absorbed Dose Rate in Water 24

7. Operational Procedures 28

7.1 Exposure Techniques for Various Chambers 28

7.2 Current Measurements and Corrections , 29

7.3 Quality Assurance Measurements , 31

7.4 Chamber MPPK-281 31

8. Calibration Uncertainties 32

Appendix A Test of the Photon Fluence Scaling Theorem 35

Appendix B U.S. - U.S.S.R. Comparison 41

Appendix C U.S. - Canada Comparison 43

Appendix D U.S. - Sweden Comparison 44

Appendix E Sample Calibration Report 45

References 49

i V
1. The Calibration Service

NIST publication SP 250 lists Measurement Service C.l (X-Ray and GammaRay Measuring Instruments), which includes the calibration of suitable ionization chambers so that they can be used to determine absorbed dose to water in

a phantom irradiated with '^^Co gamma rays. The calibrations are based on measurements of absorbed dose with a graphite calorimeter. These measurements

have been used to calibrate the vertical 10-kCi ^^Co source in room B036 in

NIST building 245 so that the absorbed dose rate to water is known as a function of water depth and field size at several positions in the beam.

A report entitled "The Graphite Calorimeter as a Standard for Absorbed

Dose for Cobalt-60 Gamma Radiation," has been published It contains

much of the information presented in this report.

2. The General Approach

The 10-kCi Theratron F ^oCo source in room B036 of NIST building 245 has

been mounted on the ceiling of that room to produce a vertical gamma-ray beam,

as shown schematically in figure 1. The Theratron head contains adjustable

jaws for collimation of the beam. In the work discussed here, the beam cross section was always square, of size s x s at the collimator, or f x f at the

detector.

The absorbed dose rate in a graphite

phantom was measured with a graphite

calorimeter at a source distance Zg, and

a graphite depth x with a field size f . The absorbed dose ^ate in a water phant8m

at a source distance z^ and a water depth

with a field size f^ was obtained from

this measurement by a transfer technique

utilizing current measurements at the

calibration points in both phantoms, with

a graphite transfer ionization chamber,

published photon mass attenuation coefficients, and a measured chamber perturbation correction. The transformation from

the graphite dimensions {z„,x^,f^.) to the

water dimensions (z^,x^,f^J was determined

by the requirement that the photon spectra

at the two positions should be similar.

As shown in section 6.4 this was accomplished with a simple scaling technique

where the scaling factor depended only on the relative densities of atomic electrons

in the two phantoms.

By this means the Theratron F source

was calibrated so that at a fixed distance

z^^ the absorbed dose rate to water was

SOURCE

Figure 1. Schematic diagram of

vertical ^^Co source, showing

source distance (z), collimator

size (s), field size (f), and

phantom depth (x)

.

^Numbers in brackets indicate the literature references at the end of the

paper.

1
determined as a function of water depth for several field sizes f^, in the

absence of the chamber. If then another ionization chamber is placed at

(z^,x^,f^), it can be calibrated in terms of the absorbed dose rate to water

per unit current, and can be used in other ^oco beams with similar yeometry to

determine absorbed dose rates.

3. The Graphite Calorimeter

The graphite calorimeter has been described in the NBS Journal of Research

in an article entitled "A Heat-Loss-Compensated Calorimeter: Theory, Design,

and Performance" [2]. The figures discussed below come from that article.

Figure 2 is a schematic cross section of the calorimeter showing the

various elements. The core is a single graphite disc, 20 mm in diameter and

2.75-mm thick. The surrounding jacket consists of a graphite base plate with

holes for air evacuation, as shown, and a graphite cap. The jacket dimensions

are such that core and jacket have the same heat capacities. The core is

mounted on the jacket base plate with polystyrene supports. The jacket is

similarly mounted on the much more massive graphite shield and the shield in

turn in the even larger graphite medium. All four of these elements, core,

jacket, shield, and medium were machined from a single graphite block and have

the same density, 1.70 g/cm^. Thermal isolation of these four elements is

improved by evacuation of the gaps separating them, and by coating the

surfaces of the shield and medium with aluminized polyethylene terepthal ate,

as shown in figures 3 and 4. The shield cap and jacket cap have been removed

in figures 3 and 4 to show the core.

Temperature-sensing thermistors are embedded in each of the four

components, and the core and shield also each contain a higher resistance

thermistor used for heating purposes. The medium is heated by wire coils

embedded in its rear surface. Its temperature is maintained at 303 K by the

thermoregul ator shown in figure 2.

Temperature changes in the various components are measured with the

Wheatstone bridge circuit of figure 5, which can be used to follow changes in

any of the four temperature-sensing thermistors, C(core), J( jacket),

S(shield), and M(medium). Initially, with the calorimeter components at their

equilibrium temperatures, the variable resistances are adjusted so that the

bridge is balanced in any of the five modes listed in figure 5. Rq (or Rj)

equals the resistance of thermistor C (or J), and the currents through

thermistors C and J do not change when they are switched out of the bridge

circuit. Then the Mode Switch can be moved to any of the five positions

without disturbing equilibrium.

The bridge output voltage, between 0 and 0' in figure 5, is amplified and

displayed on a chart recorder as a function of time. Figure 6 is a schematic

drawing of a typical run, divided into three parts, an initial drift, a heating segment, and a cooling segment. The sawtooth changes during heating

are caused by changes in R^ large enough to keep the recorder pen on the chart

paper. The temperature rise observed here is proportional to the fractional

change in R^ during the run:

= R"^(0)

"

2
CENTRAL

PLANE

Figure 2. Cross section diagram of graphite calorimeter, showing components

core, jacket, shield, and medium.

3
Figure 3. Calorimeter core-jacket-shield assembly with the shield-cap and

jacket-cap removed.

where the

are R^{0)

hR^ > 0,

ohms

i niti al

and

and a 1 s

and

the chart

f i nal

aR,

values of

. with

cal i brati on in ohms per scale division, d^ 1s the

chart distance between initial and final

drifts, each extrapolated to the mid-run,

The total energy is proportional to the

observed temperature rise, so:

E = k f m

(2)

where the constant k is determined from a calibration run.

Chart records for radiation runs and

calibration runs look superficially

similar, but differ principally because

of the size of the correction factors, F,

which come from the extrapolations.

Returning to figure 6, eq (1) can be

rewritten as:

(aR^ + ad)

where d is also shown in figure 6, and;

F = (aR^ + ad)

(4)

Figure 4. Enlargement of

calorimeter core-jacket-shield

assembly with the caps removed,

4
R^(0) «4*-R,(0)--^ ARjj — R^(0;-AR,

Figure 6. Schematic diagram of a calorimeter chart record, showing the

methods used to seek a null and to correct for heat losses.

5
Correction factors for calibration runs in modes C and C + J, and

for radiation runs in mode C are shown as a function of run length in

figure 7 as Fq, Fq^j, and Fq, respectively. The reason for this behavior

is the differences in the heat loss

to their surroundings by the

calorimeter components contributing

to the signal. During radiation

runs, core, jacket, and shield are heated uniformly. In mode C only the

core contributes to the signal, heat

loss to the jacket is negligible

except for long runs, and the correction factors are small. During

calibration runs the core is heated

directly but the jacket and shield

are not. The jacket temperature

rises slowly because of radiation

from the core, but is always much

smaller than that of the core. Heat

losses from the jacket are much

smaller than those from the core, and

the correction factors are considerably reduced when the jacket is added

to the components contributing to the

signal. Thus calibration runs are normally made in the C + J mode and

radiation runs in the C mode.

For the calorimeter measurements

described in this report, the

calorimeter was placed in a ringshaped graphite phantom, with an inside diameter of 15 cm, an outside

diameter of 30 cm, and a depth of 15 cm. This effectively increased the

outer diameter of the calorimeter from 15 cm to 30 cm and increased the

calorimeter response by as much as 1%, depending on the field size and

phantom depth.

4. The Graphite Transfer Ionization Chamber

Figure 8 shows two photographs of a type PLl ionization chamber, showing

the entire chamber and an enlargement of the front end. This chamber was designed for the transfer of calorimetric measurements of absorbed dose from a graphite phantom to a water phantom. 2 The chamber and its properties were

first described in 1976 in a report entitled "Ionization Chamber for AbsorbedDose Calibration" [3]. Schematic cross sections of the front end and

mid-section of the chamber appear in figure 9, showing how the components fit

2The chamber shown in figure 8 is PLl-13, made of tissue-equivalent plastic.

The other PLl chambers are graphite, which has less photographic contrast.

500

TIME, s

1000

Figure 7. Calculated calorimeter heat

loss corrections as a function of

time. Subscripts refer to mode, C

for core alone, and C+J for core

plus jacket. Superscripts refer to

type of operation, c for calibration

and r for irradiation.

6
I 1

Figure 8. Photographs of PLl-type ionization chamber, showing square-ended

high-voltage electrode, high-voltage lead, and air hole.

Dimensions are in Millimeters

83

Figure 9. Cross section diagrams of PLl front end (with air cavity) and

middle (with cable connection).
together. They are held together with either clear epoxy or conducting epoxy,

depending on whether or not the juncture is meant to transmit current. More

detailed dimensions are shown in databook 761, page 19 (DB 761/19). The

assembly procedure that proved most useful is described in DB 779/99.

The chamber air gap is thimble shaped, with a thickness of 1.25 mm and an effective volume of 0.3 cm^. The collecting and guard electrodes, on the

inside surface of the thimble, are both graphite, and are separated by an 0.08-mm thick polyethylene terephthal ate washer. The collecting electrode is

connected to the coaxial cable central conductor by means of a 6-cm long

aluminum rod. Similarly, the guard electrode is connected to the coaxial

cable braid by means of an 8-cm long aluminum tube. The polarizing or highvoltage electrode, on the outside surface of the air thimble, is graphite, and

is separated from the guard electrode by an acrylic insulator. The highvoltage connection is made via the separate wire shown in figure 8.

The PLl chamber has a square end because it was designed to simulate a graphite rod of infinite length when in a phantom. It is placed in a cylindrical hole crossing the phantom and aligned with the center of the air gap,

taken to be a point 6.6 mm from the square end, on the beam axis. A passive

graphite rod fills the rest of the hole. Since both rod and chamber have

square ends, there are no air gaps

outside the chamber. The graphite,

acrylic plastic, and polystyrene

phantoms used for this report are each

cylindrical in shape, with the axis of

symmetry coinciding with the beam

axis. The cylindrical hole in each case runs along a diameter, about

10 mm below the top surface. The

water phantom is a plastic box,

30 X 30 cm in cross section, and open

at the top. The cylindrical hole in

this case is the inside of a plastic

tube (12.7-mm I.D., 15.9-mm O.D.)

running between the centers of two

opposite sides.

There are at present 11 PLl-type

chambers available for use, with serial

numbers ranging from PLl-10 to PLl-20.

The first three, PLl-10 to PLl-12,

were constructed in 1974 October. The

remaining eight were assembled about

3 years later. Of these, PLl-13 is

constructed of A-150 tissue-equivalent

plastic, and the other seven are of

graphite.

The recombination characteristics Figure 10. Ion recombination characof the PLl chamber are shown in teristics of PLl-type chamber,

figure 10, which is reprinted from showing dependence of ion current

reference [3]. For polarizing poten- on absorbed-dose rate for

tials less than 60 V, the chamber potentials less than 60 V.

8
current is a function of the dose rate. For potentials larger than 60 V, ion

multiplication appears. At 60 V, the chamber is well behaved and reproducible.

Consequently, the PLl chambers are always used with a polarizing potential of

± 60 V.

The sensitivities of the chambers relative to PLl-11 have been measured on several occasions, in both graphite and water phantoms. These comparisons are summarized in table 1, which comes from DB 827/152. The relative sensitivities

range from 1.05 for PLl-10 and PLl-12 to 1.18 for PLl-13. Only PLl-13 shows a large difference in relative sensitivity between water and graphite phantoms

(0.75%). Presumably this is because tissue-equivalent plastic does not respond

to spectral changes in the same way that graphite does. The original set,

PLl-10 to PLl-12 have lower sensitivities because the external sleeves between

front and mid-sections (see fig. 9) are made of acrylic plastic rather than

graphite.

Table 1. Chamber sensitivities relative to chamber PLl-11. G and W signify graphite

and water phantoms, respectively. The coefficient of variation, V, Is also

shown.

Date and Phantom

Chamber 75 Apr

G

77 Sep

W 77 Dec

G

78 Jan

G

78 Apr

W 79 Mar

G 80 Jul

G 80 Jul

W 81 JulW Average V, %

PLl-10 1.0545 1.0574 1.0540 1.0535 1.0556 1.0547 1.0550 0.13

12 1.0548 1.0557 1.0540 1.0543 1.0548 1.0542 1.0546 0.06

13 1.1848 1.1760 1.1804 0.53

14 1.1219 1.1207 1.1213 0.08

15 1.1011 1.0989 1.1000 0.14

16 1.1244 1.1241 1.1243 0.02

17 1.0918 1.0909 1.0928 1.0911 1.0917 0.08

18 1.0764 1.0756 1.0760 0.05

19 1.1021 1.0995 1.1003 1.0999 1.1005 0.10

20 1.1152 1.1120 1.1133 1.1126 1.1133 0.12

The resistance between collecting and guard electrodes has been measured

and recorded for each chamber at several times between 1977 October and

1988 June. The measurement circuit is shown in figure 11, where the source of

power was a 1.35 V mercury cell for all except the most recent set of measurements. For the 1988 June set, the source of power was a 1.5 V alkaline cell.

As shown in figure 12, the resistances are all between 10^ ft and IQi^ ^, The

original set, PLl-10 to PLl-12 have consistently higher resistances than the more recent models, but they seem to vary by as much as a decade during those

10.7 years.

9
To Chamber

3 - Way Reversing Toggle

Battery Box

-Braid

Central Conductor

.Plug To

|=» Electrometer Ground

To

Electrometer

Figure 11. Circuit used to measure input resistances of PLl-type ionization

Chambers.

1977 1978 1978 1980 1981 1988 Oct. April Aug. Aug. Aug. June

Figure 12. Resistance history of ionization chambers PLl-10 to PLl-20,

covering more than 10 years.

10
All reported PLl currents in this document are averages of currents

measured with positive and negative high voltage polarities. Each measured

current is the sum of two components, an ion current generated in the chamber

air gap by interaction between air molecules and secondary electrons, and a parasitic current generated in the body of the collecting electrode by

secondaries coming to rest. These two components can be separated by making

measurements with both positive and negative high-voltage polarity. The ion

current component will change sign when the polarity changes, while the

parasitic component will not.

If I"*" and I" are currents measured with positive and negative polarity,

respectively, the ion current is:

I = 1+ - I- (5)

where I" is normally a negative number. The parasitic current is (I"*" + I")/2

and may be either positive or negative. It has no immediate application, but

is usually recorded because abrupt changes may indicate measurement problems.

The currents reported for customer ionization chambers, on the other

hand, are usually measured with a single high voltage polarity, as specified

by the customer.

5. The Chamber Calibrations in a Graphite Phantom

The NIST 10-kCi Theratron F ^^Co gamma-ray source was calibrated with a graphite phantom at source distances of Zg = 0.654, 0.80, 0.90, 1.00, 1.10,

1.20, and 1.25 m. The beam sizes were determined by square metal jigs

temporarily fitted into the collimator opening, with sides of s = 24.0, 28.0,

33.4, 40.5, and 50.8 mm. Not all collimator sizes were used at each distance.

The combinations actually used are shown in table 2, which lists the field

sizes f„ at the detector. These were calculated from measurements of the beam

Table 2. Field sizes in graphite, fg, mm

Source

di stance

z, m

Collimator size

s , mm

24.0 28.0 33.4 40.5 50.8

0.654 52 62 75 95

0.800 76

0.900 86

1.000 80 95 115 145

1.100 105

1.200 114

1.250 86 100 119 144 181

11
profile at z„ = 1.00 m when

s = 33.4 mm. The profile measurements are shown in figure 13.

They were made in air with chamber

PLl-11, and its accompanying

graphite backup rod, mounted in

the water phantom box. What is

plotted in figure 13 is chamber

current as the box is moved in

either a North-South direction or

an East-West direction. The box was always positioned so that the

direction of motion was perpendicular to the direction of the

chamber axis of symmetry. The

beam profile does not drop to zero

at large distances because of

scatter from the bottom of the

box, but if this background

component is subtracted from the

measured currents, and if the

field size is defined as the full

width at half maximum, the average

field size in the two perpendicular directions is 95.2 mm. Since

the field size varies as the product of collimator size and source distance, the general equation is:

6 -4 -2 0 2 4 6 8 10

Distance from Beam Cenler.cm

Figure 13. ^^Co beam profile measured

with PLl-type ionization chamber

mounted in empty water-phantom box.

fg = 2.85 s Zg (6)

where fg and s are in millimeters and Zg is in meters.

The chamber calibrations in graphite were made with the PLl-11 ion

chamber in a phantom 30 cm in diameter and 17-cm thick. The NIST portable

graphite calorimeter (15 cm in diameter and 10-cm thick) was encased in a graphite sleeve with an inner diameter of 15 cm and an outer diameter of

30 cm. Extra graphite plates were added to the bottom of the calorimeter so that the thickness of graphite behind the calorimeter core was 28 g/cm2. in

order to protect the calorimeter, it was used with about 2.1 cm of the sleeve

phantom projecting above the surface of the calorimeter. It was necessary to

correct for scatter from this projecting ring, as discussed below, correcting

to a situation where the calorimeter phantom has a flat surface like that of

the ion chamber phantom.

The two phantoms were mounted on a sliding table so that they could be

alternately inserted into the ^^Co beam. When in the beam, the center of the

calorimeter core and the center of the ion chamber were equidistant from the

source. Measurements were made as a function of depth in the phantoms by

adding 15-cm diameter graphite discs, surrounded by 30-cm diameter graphite

rings (where needed) with a 15-cm internal diameter. The top surface of the

discs was seldom in the same plane as the top surface of the rings, but the

plane separation was never as large as 3 mm. It was shown in a subsidiary

experiment that the error introduced by this mismatch was never as large as 0.1%.

12
The depths in the two phantoms were also never identical but they

differed by only 0.03 g/cm2. The minimum depth in the ion chamber phantom was

1.62 g/cm2, while that in the calorimeter phantom was 0.864 g/cm2 for the

calorimeter itself plus 0.123 and 0.660 g/cm2 for two discs which were always

in place, giving a total of 1 .647 ~ 1.65 g/cm2.

The chamber calibration data are listed in table 3. They were obtained

in a series of three experiments performed between August 1978 and April 1979.

The earliest experiment was performed at a source distance of Zg = 1.00 m (see

fig. 1). An initial test of variation with source distance was included,

using a single phantom depth (5.08 g/cm2), a single collimator opening

(33.4 mm), and source distances of = 0.8, 0.9, 1.0, 1.1, and 1.2 m.3 The

second experiment was performed at a source distance of Zg = 0.654 m and was immediately followed by measurements at a source distance of z^ = 1.25 m. The

0.654 m distance was chosen because as shown in the following Section, photon

spectra at that distance in graphite are similar to photon spectra at the

reference distance of 1.00 m in water. The 1.25-m distance was chosen to

provide calibrations suitable for the Soviet i ntercompari sons described in

Appendix B.

The calorimeter data from these experiments are recorded in a file folder

labeled "^oco measurements in B036" in the possession of Steve R. Domen.

Summaries are also included, and copies of these summaries have been inserted

in DB 843/44-46. The calorimeter dose rates listed in table 3 are averages of

5 individual runs at z = 0.654 m and of 10 individual runs at z^ = 1.00 m and

Zg = 1.25 m. The listed dose rates have been corrected for:

(a) excess scatter from the projecting lip of the ring phantom,

mentioned above. These corrections were determined in a subsidiary

ion chamber experiment by adding rings to the ion chamber phantom.

The results are listed in table 4, which shows that the correction

is less than 0.1% if t^he projecting rings are not directly irradiated, that is, if f /2 < 150 mm, where f is one side of the square

field at the chamber (fig. 1).

(b) source decay from a reference date (t = 0 on 1979 Jan 1, where t is

time). The correction factor used was exp(0 .0003600 t), with t measured in days. The coefficient 0.0003600 day-i was determined

from the published ^oCo half life of 5.2714 years [4].

(c) shutter timer errors. These were measured with the ion chamber,

comparing currents measured with the run length determined by the

shutter timer with currents measured with the run length determined

by an electronic gate. The shutter timer was found to be in error

by 0.078 s, which was added to the original exposure time for each

calorimeter run. The runs themselves varied in length from 118 to

365 s, with most about 250 s long.

3The depth 5.08 g/cm2 was chosen because it could be reached with both the

calorimeter (0.864 + 4.218 g/cm2) and the ion chamber (1.62 + 0.123 + 0.152 + 0.660 + 0.992 + 1.534 g/cm2)

.

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CO CD 1* r-~. 1—I O CD oo oo 00 CO CM 1-1 O CT> 1—1 I—I I—I 1—I o o o o o o

en CD oo lo

1—I 1—I T—I ! I I I o o o o o

oo ^ CD ^ cr> LO CO CD LO

CM CM 1—1O CTl

CO LO 00 r-l

O CO LO

CM CM 1—1O CTl

1-1 1-1 1-1 1-1 o

CDO CD CD

O

oo CO CO LO CM CO 1—I I—I I—I o o o

CM 1- o o o

CD CM CO CM oo

O 1—I

1—I oo 1—1 CM CM «—I 1—1 O CTl

CO CO oo

«:^-

O 1—I

CM CM 1—1

CO CM

O CJ^ 1—I o CD O CTl

1—1 1—1 o O CD O O O O

oo CD CO • • • 1—1 1—1 I—I o o o o o

CO CM CM

oo CTl CTl

O CO

CTl r~-

<Ti oo

«^ CO 00

r-H r-^ CM 1-1 O

CO COCO

cri oo O CD • • o o

LO CO r-^ r~-- CM CTl CD 1—I oo 00 CO «^ LO

1—I CO LO LO oo 1—t 1—I

oo

CD Looocooo^c^r^cn COi-lOOC»OOCOLO

1—ICO«d-LOLOlOCOi—I

ooo

LO CO oo r-^ oo

CO 1—1 CO CO CO «^

1—1 CO LO CO CO I—I o

{_o

C/1 oc

+J cn E

t/5 ISl Q

LO

CO o o O O 00 CTl

CD

CDo o o O CD

T-1 CM oLOCM

14
Table 4. Measured ring factors, the ratio (PLl-11 current with rings

present)/(PLl-ll current with rings removed). The listed o are standard deviations of the mean of 12 or more measurements. The

rings project above the central face of the graphite phantom by

(3.05 - 0.59 x) cm.

Source Aperture Phantom Ring

di stance si ze depth factor cr z, m s, mm X, g/cm2

DU .o 1i .UUUo nnm n nnnn U .UUUU

It; J . ID i .UUU^ U .UUUi

/in c 1 CO L .be i .UUUi U .UUUI

Q 1 C 6 .10 i .UUUi U .UUUI

1.00 50.8 1.62 1.0023 0.0003

3.15 1.0016 0.0002

4U .D i .0^1 1 nm n i .UUiU U .UUUc

It; J . i D i .UUU^i U .UUUD

o J .4 1 fi9 i .UUU4 U .UUUi

J . i 0 i .UUUd U .UUU

1

CO .u 1 .

1i . nnn? uuu

/

u .uuuo

1 t; 0.13 i . uuu

J

u .uuu i

1.25 50.8 1.62 1.0062 0.0003

3.15 1.0030 0.0001

40.5 1.62 1.0028 0.0003

3.15 1.0015 0.0002

33.4 1.62 1.0015 0.0001

3.15 1.0007 0.0001

28.0 1.62 1.0005 0.0001

3.15 1.0003 0.0001

(d) impurities in the calorimeter core, using a correction factor of

1.0002 [5].

(e) The presence of an acrylic plastic ring in the graphite phantom.

This ring constitutes the outer wall of the calorimeter, as shown in

figure 2, and has an inner diameter of 14 cm and an outer diameter

of 15 cm. The correction factor used was 1.0002 [5].

15
(f) the presence of an air yap in the calorimeter phantom during the

first experiment, where = 1.00 m. This gap was produced by

plastic tape around the edge of the top surface of the calorimeter,

inserted to protect the calorimeter core from excess pressure when

extra plates are added. The correction was studied with ion chamber

measurements, and the correction factor varied from 1.0001 at

px = 3.18 g/cm2 to 1.0005 at px = 11.59 g/cm2, where x is depth

in the graphite phantom in cm, and p is the graphite density in

g/cm3.

(g) radiation escaping from the sides of the phantom. This was studied

with ion chamber measurements in a graphite phantom 17.5-cm thick

with a diameter which could be changed from 15 cm to 20 cm to 30 cm. The chamber depth could be changed by adding graphite discs on top

of the phantom, which at all times had a flat top surface, with no projecting rings. Measurements were made with four different square

fields, from 83 x 83 mm to 174 x 174 mm, at a variety of mass depths

between 1.4 g/cm2 and 39.1 g/cm2, using phantom diameters of 15, 20,

and 30 cm. The results are listed in internal report NBSIR 77-1203

(September 1978). At each depth for each field size, the measured

currents were fitted to a polynomial of the form (b + b^D"^ + b^0p8), where Dp is phantom diameter. From each polynomial could be

calculated the percent increase in detector response when Dp is

increased from 30 cm to infinity. This is also the percent loss of

scattered radiation at that mass depth for that field size. The

numbers obtained form a relatively regular array listed as Yg(%) in

table 5 of the internal report. This array was in turn fitted to

the equation Yg(%) = A exp(af(j) where f is field size in millimeters, and A 5nd a are functions of mass depth, pXg, in g/cm2:

A = -0.0007 + 0.00113 (pXg)

a= 0.0277 - 0.00166 (pXg) + 0.000064 (pXg)2 .

(These approximations are valid only for pX < 14.0 g/cm2.) The

correction factor used for radiation escaping from the photon sides

is 1 + 0.01 A exp(afg), which never exceeded 1.003.

(h) the presence of vacuum gaps surrounding the calorimeter core and

jacket. These gaps perturb the distribution of scattered radiation

incident on the core. Monte Carlo calculations of this effect have

recently been reported [6] for the NIST calorimeter geometry. These

indicate that at mass depths of 5 g/cm2 and 17 g/cm2, measurements

of absorbed dose should be multiplied by correction factors of

1.0020 ± 0.0005 and 1.0050 ± 0.0005, respectively. The dose rates

listed in table 3 have all been multiplied by the factor

(1.00075 + 0.00025 (px^)), which reduces to the given factors at 5

- and 17 g/cm2.

The values of 0 listed in table 3 differ slightly from those in the

calorimeter summary (DB 843/44-46) because corrections (g) and (h) had not

been applied, and because a decay rate of 0.0003625 day-i had been used rather

than the present rate of 0.0003600 day-^. The values of C listed in the ion

chamber summaries (DB 810/21-11 and DB 812/32-35) also lack corrections (g)

and (h) and in addition had been multiplied by z2.

16
The ion chamber data from these experiments are recorded in

DB 801/116-200, OB 810/1-14, 137-200, and DB 812/1-30. The results are summarized in DB 812/32-35 for source distances z„ = 0.654 m and 1.25 m. The

ion chamber currents listed in table 3 have been corrected for:

(aa) air density changes inside the chamber, using the correction factor

[101.325(T + 273.15)]/[(Tp + 273.15)P], where T is air temperature

in degrees Celsius, Tp is a reference temperature (22 °C) and P is

air pressure in kilopascals (101.325 kPa = 1013.25 mbar = 760 mmHg = one standard atmosphere)

.

(bb) source decay froin a reference date (t = 0 on 1979 Jan 1, where t is

time in days). The correction factor used was exp(0 .0003600 t).

(cc) tel ethermometer errors, using a correction factor of 1.0004

(OB 801/148-149).

(dd) instrumental errors, using a correction factor of 1.0015 (DB 801/7).

(ee) radiation escaping from the sides of the phantom, using the

calorimeter correction (g) , above.

(ff) the difference in mass depth between ion chamber and calorimeter

measurements using the same discs. Linear corrections were used,

fitted to the two nearest mass depths. For example, for

Zq = 0.654 m and s = 50.8 mm, the partially corrected ((aa) through

(ee) ) currents at mass depths of 5.84, 8.34, and 11.56 g/cm2 in the

ion chamber phantom are 0.4369, 0.3983, and 0.3526 nA, respectively.

The interpolated currents at the calorimeter mass depths of 5.87,

8.37, and 11.59 g/cm2 are 0.4364, 0.3979, and 0.3522 nA, respectively, as shown in table 3. The only mass depth for which this

correction was not needed was 5.08 g/cm2, which could be attained

with different sets of discs for the ion chamber and for the

calorimeter, as mentioned earlier.

The currents listed in table 3 do not agree with those in the databook

summaries (DB 810/21-22 and DB 812/32-35) because correction (ee) had not been

applied to the databook currents, which had been multiplied by z2, and had

used a reference temperature T^ = 20 °C in correction (aa) rather than the

22 °C used here.

The graphite chamber calibration factors listed under Ngp^p^ in table 3 are the simple ratios of dose rate to current. It can be seen that these

factors vary with both field size and phantom depth. As shown in reference

[1], the graphite calibrations with the 10-kCi Theratron F ^^Co source can be

fitted to the equation:

Vaph = ^graphtl + k Jl-exp[-,^( px^- px^^)] ) . k^(l-exp[-,f ( f^-f ^1 )] (7)

where Ng^^p^ is the value of Ng^^p^ at the reference mass depth pXg and the

reference field size fg. The exponent coefficients were chosen to be

= 0.25 cm2/g and yf= 0.025 mm"!. With these choices, the best-fit values

17
of N^pjp^, k^, and are listed in table 5, along with pxj, f^, and the

coefficient of variation of each fit.

Table 5. Parameters and coefficients for eq (7), the calibration of chamber

PLl-U in a graphite phantom at 22 °C and 1 atm. The coefficient of

variation, V, was calculated for 3 degrees of freedom.

Source Pre- sel ected Least Squares

di stance

z

g ^f

r PXg f^

g graph k X ^f V m cm2/g mm" 1 g/cm2 mm Gy/yC %

0.654 0.25 0.025 5.56 65.4 101.07 -0.00333 0.00595 0.09

1.00 0.25 0.025 5.00 100.0 101.59 -0.00227 0.00465 0.14

1.25 0.25 0.025 5.00 100.0 101.52 -0.00217 0.00411 0.22

The values of px^ = 5.56 g/cm^

and fg = 65.4 mm for the source distance Zg = 0.654 m were chosen

to transform into the water phantom

values px^ = 5.00 g/cm2 and

ff, = 100 mm at a source distance of

z^ = 1.00 m, as discussed in

section 6.4. The transformation

of f and z requires multiplication

by 1.53, but the transformation of

px is more complicated:

5.56 g/cm2 = 3.27 cm in graphite,

which when multiplied by 1.53

yields 5.0 cm e 5.0 g/cm2 in water.

Figure 14 illustrates the

variation with mass depth, field

size, and source distance,

predicted by eq (7) using the

reference values Zg = 1.00 m.

pxfjvg = 5.0 ^ g/cm2, 3, -M, , f[^ . = The g 100 mm.

two curves show change in Ng^^p^

when either pXg or fg is varied,

with the other parameters held

constant. The three points show

that variation with distance alone

is no larger than 0.15% over the

range of distances covered here.

a

z

<

(/) r)o

X

Icc

UJa

w

H

ir<

Q.

DCO

HO

<

<

QCm

<

o

UJo

z

<

X

o

DEPTH IN GRAPHITE px, g/cm2

3 5 7 9 11 13

80 90 100 110 120 130 140 150

SIDE OF SQUARE FIELD f, mm

Figure 14. Dependence of PLl-type

calibrations on depth in graphite

phantom (top scale), and field size

(bottom scale). The plotted points

show dependence on source distance

(mi ddl e seal e)

.

18
o

o

z

oco

o

<

z

g

<

OCm

-I<

o

101.6

101.4

101.2

Fi nal 1y , figure 15

compares the measured calibration variation with source distance at a mass depth of

pXg = 5.08 g/cm2 and a collimator size s = 33.4 mm

with the predictions of eq (7),

using eq (6) to relate field

size and collimator size. The

agreement is excellent.

6 . Calibration Transfer to

Water

6 . 1 Theory

The following treatment is

based on the general formalism

described by Loevinger in 1981

[7]. Consider an ionization

chamber immersed in an absorbing and scattering medium

irradiated by ^^Co gamma rays.

It is a thick-walled chamber so that all electrons reaching the

cavity arise in the chamber

walls rather than in the

medium, which may be either

graphite or water. The ratio

of the mean absorbed dose rate

in the chamber cavity air when the medium is graphite to the mean absorbed

dose rate in the cavity air when the medium is water, is:

101.0

O Single Measurement

• From Equation

SOURCE DETECTOR DISTANCE z, m

Figure 15. Comparison of measured and

predicted (eq (7)) PLl-U graphite phantom

calibrations at several source distances

and a fixed phantom depth. The line is

drawn arbitrarily to fit the points.

j^graph .graph

air ^ ai r i«water .water ^

' air air

where I represents chamber current. In this notation the superscripts refer

to the material of the media and the subscripts to the local chamber

materi al s

.

In each medium the cavity air absorbed dose rate can be related to the

dose rate in the chamber wall material (if the cavity is filled with wall

material) by using the Bragg-Gray equation [8]:

^graph

_waiL_

^graph

ai r

S/ Pwall

graph

S/p

ai r

5'water

wal 1

0'water

ai r

S/p,

S/p

.water

wal 1 (9)

ai r

19
where S/p represents an electron mass stopping power averaged over the spectrum of electrons reaching the cavity. The positions of the chamber in the

two media are always chosen so that source distances and depths are inversely

proportional to the electron densities in the media. Under these conditions

the relative spectra of photons and electrons at the cavity are the same [9]

(also see Appendix A) and the stopping power average ratios are the same in

graphite and in water.

Then from (9):

and substituting in (8)

.graph

'^wall

^graph

ai r

D#water

wal 1

[5 water

ai r

(10)

^graph

'wall

[5 water

wal 1

.graph

^air

.water

^air

(11)

Next, consider the ionization chamber (with its cavity filled with wall

material) in the water medium. The ratio of the absorbed dose rate to water

at the center of the chamber position with the chamber removed to the absorbed

dose rate to the wall material at the center of the chamber position with the

chamber present is:

15 water

water

Dwater

wal 1 (3

^en^P^water

'^en/P^wall

water

^wall

(12)

where B is the ratio of absorbed dose to the collision part of kerma, and

ygp/p is a mean mass energy-absorption coefficient for the photon spectrum at

the chamber center position. The factor 4^ ater'^^ all

ratio of photon

fluence at the center position without/wi t)n the cliamber present and corrects

for the increased attenuation of the chamber compared to water.

Eliminating from (11) and (12) and rearranging terms:

^water .water ^water ^ ^en'^'^^ water ^water / '^wall | Wr=^air ^

, , M T^Ti^ ) ^^3)

^wall (^en/p^wall ^wall \^ai r /

20
ivhich is the water calibration equation. It describes how the absorbed dose

rate in water with no chamber present C^^^^-gp) can be calculated from a chamber current measured in water (I^^p )» a measured chamber calibration in

graphite (I^^g^i^'^/Iai'p^'^ ] > absorbed-dose-to-kerma ratios (B^^^gp/ -| )

, a ratio

of average mass energy-absorption coefficients [( Mgn^ ^^water^^ ^^en^ P^wal 1 1' a measured fluence ratio

(^'y^a-^gp/^^^al ] ) • particular chamber used here

the wall material is graphite, out this equation can be applied to other wall

materi al s

.

6.2 Evaluation of the Coefficients

The ratio of absorbed dose to kerma does not differ by more than a few

tenths of one percent for different materials [7], so that the ratio

(Bwater/ Bwal 1 ) ^^^^ '^^ taken to be unity, with an uncertainty of 0.1 percent.

The mass energy-absorption coefficients for graphite and for water were taken from Hubbell [10] as a function of photon energy. They were averaged

over photon energy using photon spectra predicted by Bruce and Johns [11] for

60Co radiation incident on a water phantom, and photon spectra predicted by

Seltzer et a2_. [12] for soqo radiation incident on a carbon phantom. If

P(k,x) r^preTents a spectral distribution, where k is photon energy and x is

depth in the phantom, the ratio of average mass energy-absorption coefficients

is:

tWp)water / ^^en/^Wer * ^ ^(^'^)^^

R = = — (14)

y

°°

0

where the ( Ug^/ p) come from Hubbell. The same spectrum is used in both

numerator and denominator of this ratio [9]. The Bruce-Johns predictions

cover a range of water depths and three field sizes, as listed in Table 6A.

The Seltzer predictions are for a fixed geometry, and are listed in Table 6B

as a function of water depth and field size. (Seltzer assumed a graphite

density of 1.80 g/cm2, which transforms into an electron density of 1.62

relative to water, so that spectra at a depth of Xg in graphite are similar to

spectra at a depth of 1.62 Xg in water.) It can be shown by interpolation of

the Bruce-Johns data that their predictions differ by no, more than a few

tenths of one percent from those of Seltzer. The ratios used come from the

matrix equation:

2 ^ i i R =

y y f^ M. . xJ (15)

^ UQ jio ^

w

where f^ is field size in water in mm and x^^ is water depth in mm. The matrix

components, M^.:, are listed in table 6C. Equation (15) generates the values

of R listed in table 6B to within 0.02% and those listed in table 6A to

withVn 0.25%. The variation with both x^^ and f^^ is shown to be almost

negligible in figure 16.

21
Table 6. Ratio of average mass energy-absorption coefficients,

(uen/p)water/(^en/p)wa1l ' ^ function of water depth and field

size f^

6A Calculated using Bruce-Johns spectra

Xy^, mm f^^= 50 mm 100 mm 200 mm

0 1.11078 1.11095 1.11118

20 1.11088 1.11109 1.11152

50 1.11097 1.11128 1.11195

100 1.11114 1.11157 1.11251

200 1.11107 1.11166 1.11300

Calculated using the spectra of Seltzer et al

x^, mm f^, mm R

11.3 115.6 1.11171

27.5 116.6 1.11198

50.2 . 118.1 1.11243

76.1 119.8 1.11294

102.1 121.5 1.11349

124.7 123.0 1.11366

6C Matrix components

= 1.11104 = 1.50 E-5 = -2.53 E-8

= 5.10 E-6 M^^ = 3.21 E-9 M^^ = 1 -^^ ^"10

M^Q = -1.12 E-8 M^^ = 3.50 E-10 = -1.61 E-12

22
The fluence ratio ^Water/'lV^al 1 (called here a "replacement factor")

was determined from replacement

measurements made with three nesting

graphite sleeves. These were 10-cm

long and fitted on the outside of the

acrylic tube spanning the water

phantom described in section 4. With

the phantom filled with water and a PLl graphite chamber inside the tube,

measurements were made of the decrease

in current when the water surrounding

the acrylic tube was replaced by

graphite sleeves with total wall

thicknesses of 2.5, 5.1, and 7.6 mm. Any graphite sleeves not being used were pushed out of the beam but were

still underwater, so that the water

depth remained constant.

These measurements are plotted

as a function of wall thickness in

figure 17 (which also shows similar

measurements made with acrylic and

polystyrene nesting sleeves).

Replacement of water by graphite

reduces the chamber current by 0.156%

per mm of wall thickness, or 0.078%

per mm increase in diameter. By

extrapolation, it follows that

replacement of water by a graphite

rod 12.3 mm in diameter (the PLl

chamber) would decrease the central

fluence by 0.96%, so that

^water/'J^wall = 1-0096.

The replacement measurements

for graphite and acrylic are sum- marized in DB 810/132. Those for

polystyrene are in DB 827/164-168.

It should also be mentioned here

that a footnote in reference [1] is

misleading. This footnote states

that the published replacement

correction factor, 0.068% per mm,

has been corrected for the difference

between the average graphite phantom

density and the graphite sleeve

density. It can be seen from eq (12)

above that the replacement factor does

not involve the graphite phantom, and

since the sleeves and chambers were

made from the same graphite rod, no density change correction was needed.

1.114

.113

.112

FIELD SIZE , mm

150 X I50>

0 50 100

DEPTH IN WATER X, mm

Figure 16. Ratio of mean mass energyabsorption coefficients of water

and of graphite as a function of

field size and depth in a water

phantom.

•E 1.00

<

c

2>3

oi—

a> s>E

ca JZO

0.99

0.98

1

1 1 1 Slope, %/mm

0.0002

0

+ —

om^ —

0.156\^

0 Polystyrene

+ Acrylic

X Graphite

1 1 1 2 4 6 8 Tube Wall Thickness, mm

Figure 17. Measurements of replacement factors obtained by sliding

tubes of polystyrene, acrylic

plastic, and graphite over PLl-U

in the water phantom.

23
6.3 The Chamber Currents in Water

The water currents were measured with chamber PLl-11 as a function of

depth, x^, and collimator size, s, in a series of five experiments performed

between 1978 October 27 and 1982 February 3. The currents listed in table 7A

have all been corrected for:

(a) air density changes inside the chamber, using the correction factor

[101.325(T + 273.15)7(295.15 P)], where T is air temperature in

degrees Celsius and P is air pressure in ki 1 opascal s,

(b) source decay from t = 0 on 1979 Jan 1, using the correction factor

exp(0 .0003600 t) , where t is time in days,

(c) tel ethermometer errors, using a correction factor ranging from

1.0004 (DB 801/148-149) to 1.0006 (DB 827/63).

(d) digital voltmeter errors ranging from 0.9990 (DB 827/60) to 0.9992

(DB 827/173) to 1.0009 (DB 810/7).

(e) extra attenuation of the acrylic tube, using the correction factor

1.0006, which comes from the replacement measurements of figure 17.

The original current measurements can be found in DB 810/52-54 for

experiment #1, in DB 812/127-129 for experiment #2, in DB 812/188-191 for

experiment #3, in DB 827/76 for experiment #4, and in DB 827/170-171 for

experiment #5. Of the original data only the capacitance values, the measured

voltage changes, and the measured values of correction (a) were retained.

6.4 The Absorbed Dose Rate in Water

The absorbed dose rate in a water phantom comes from eq (13), which may

be rewritten:

^wlXw-y = 1-0096 R^C^.fJ Kx^.s) f

(Xg.fg) (16)

where C (x ,f ) = S^T" , Kx ,s) = -? (x ,f ) = [59^?P^I9!'^P^ and R

w^ w' w^ water' ^ w' ' air I g g wall air y comes from eq (15) and table 6C. What remains to be done is to rewrite

(3/1 (x ,f ) as a function of x and f . This can be done by scaling source ^ g g' WW J :y distances, field sizes, and phantom depths inversely with electron densities.

It is shown in reference [9] that if this is done, photon spectra will have

the same relative shapes in different materials, so that eq (10) is valid, and

consequently, eqs (13) and (16) are also valid.

24
Table 7. Current from chamber PLl-11 and absorbed dose rate to water at 1 m as

a function of depth in water, x^, and collimator size, s. Currents

are corrected to 22°C and 1 atm. Currents and dose rates are both

corrected to 1979 Jan 1.

7A PLl-11 current 7B Dose rate to water

Water I(x^,s)

,

nA D(x^,s), mGy/s

U t: |J L 1

1

w s=28.0 33.4 40.5 50.8 s=28.0 33.4 40.5 50.8

mm mm mm mm mm mm mm mm mm

Q ^ n 1 U . i z) C.0 nU

9nci 1

. ^lUO i 91 Q A 9"? R

n

^0 . ou

1 n Q n 1 Q99 n 1 QA9 U . Lyoc n 9nn9 n 90 A7 9 1 QO c i .oy 9 9 9 9 QQ c .oy 00 /I c; ilo . 40

U . iyo4 00 9Q ^1(1 . (ly n 1 QCQ U . iODO n 1 Q 1 1 u . i y 0 4

91 99 91 7fi 9 9 9Q 99 Q 7

n 1 Q/i 9 U . ioM- (1 90 QA

oD .U n 1 777 U . i / / / U . i o<i 3 nu .

1i oQ7n/ u u . i y (1

J

90 1 c; 90 7

1

9 1 90 c i . ^y 91 Q ^1 L . y O 00 . o U . i / DO n 1 Q 1 /I U . iODi ij . 1 y i 4 9n ni 90 fii 91 1 Q ^ i . iy 9 1 Q "3 ^1 i .OO

Q nu • 1± 7Rn / OU 11 Q7 . 85OJ

/17 A nU 1 fiR"^ . 1 Oo J 1 Q Dfi 91 ni

0 i .o U . 100'+ n 1 7nQ n 1 7fin nU . 1817 io i

/

18 73 1 Q 38 90 nn 9(1 fiQ

3 O • U . 1 DO J 1 Q 1 fi J- -7 . J. U

U . i sJ 3 J n 1 fidfi n 1 701 0 1 761 18 0? 1 8 66 1 Q C u . u o

U J. • t U . i DoD X O • J J

o / • o 0U 1 '^'^fi . i 3 OU n 1 fi/ift n 1711 1 7 ?7 18 7? 1 Q d7

7 ^ Q U . i 0.50 1 7 4*^

81 R VJ . 1 *-r C.O n 1 dR8 nU 1 . i J tJ n 1 fii ? 16.14 16.85 17X / . 53o o 18 33

85.5 0.1399 0.1456 0.1515 0.1583 15.81 16.49 17.20 18.00

94.2 0.1389 15.72

96.1 0.1322 0.1384 0.1442 0.1511 14.94 15.67 16.36 17.17

96.5 0.1314 0.1504 14.86 17.10

104.6 0.1261 0.1318 0.1379 0.1450 14.25 14.92 15.65 16.48

114.7 0.1245 14.10

25
The number of electrons per cm^ in graphite with a mass density of

1.70 g/cm3 is 1.70 x 6 N^/12.011 = 0.8492 N^, where is Avogadro's number.

The electrons/cm^ in water with a mass density of 1 g/cm^ is (2x1 + 1x8)

N;^/(2xl.0080 + 1x16.0) = 0.5551 N;^. The ratio of these is graphite/water = 1.53, so that the conditions will be satisfied if:

source distance = 1.53 Zg

field size f^ = 1.53 fg

phantom depth = 1.53 Xg (17)

where the subscripts w and g refer to water and graphite, respectively.

As for the transformation of currents, this comes from eq (6), which in

light of (17) can be rewritten:

f f

^ = = 2.85 s . (18)

g w

This shows that although field size and source distance transform according to

eq (17), collimator size, s, is invariant. That is, photon spectra at points

(z^,x^,f^) in a water phantom are similar to photon spectra at points

(Zg,Xg,fg) in a graphite phantom if the same collimator is used in each case.

The ratio l5/I(x^,f^), which is the same as N^^,^. (x^ ,f ^) , can, in light

of (7) and (17), be ?ew?itten as: ^'^P^ ^

^

Ngrlphll ^ ^(l-exp[-Cx(x^-50)]) + (l-exp[-^^(f^-100)] }] (19)

with both X and f in mm. This equation is to be used at a source distance

w w

of z^ = 1 m, so the parameters Ngp^p^> k^, k^, and e,^ must all be

transformed from the corresponding graphite parameters for a source distance

of Zg = 1/1.53 = 0.654 m. Then, from table 5, N^^^p^ = 101.07 Gy/yC,

k^ = -0.00333, k^ = 0.00595, = 0.0278 mm"!, and ^ = 0.0164 mm-i.

(Equation (19) for water at z = 1.00 m will track eq (7) for graphite at w

z = 0.654 m only if k and k^ remain unchanged and if 5.56 y = 50 £, and

65.4 = 100

Then, putting (19) into (16), the dose rate to water at a source distance

of 1 m is:

0 (x ,f ) = 1.0096 R (x ,f ) I(x ,f ) n""^^ , [1+k ri-exp[-p (x -50)]]

w^ w' vi' w' w' ^ w' w' graph L x^ ^x^ w

'

(20)

+k^(l-exp[-Cf(f^-100)])] mGy/s .

26
For a particular collimator size, s, the field size comes from (18). The

currents I(x^,f^^) are measured at 1 m with chamber PLl-11 and are corrected to

22 °C and 1 standard atmosphere pressure.

The calculated dose rates for the currents listed in table 7A are shown

in table 7B. They refer to 1979 Jan 1. For each collimator size, the water

absorbed dose rates at 1 m have been fitted to an equation of the type:

^w(^''^ = ^i^^) \) exp(-0.01 xj mGy/s (21)

The coefficients C^(s) are listed as a function of s in table 8 along with

coefficients of variation, V. As can be seen, the coefficients of variation

are all smaller than 0.2%.

Table 8. Coefficients C^(s) in water absorbed-dose rate eq (21), on 1979 Jan 1. The coefficient of variation, V, was calculated for

4 degrees of freedom.

Col 1 imator size, mm

Coefficients 28.0 33.4 40.5 50.8

22.45 22.99 23.46 23.99

^1 0.1819 0.1828 0.1911 0.1975

^2 -2.49E-4 -0.16E-4 0.37E-4 1.62E-4

1.57E-6 0.45E-6 0.62E-6 0.42E-6

V, % 0.12 0.18 0.09 0.08

Finally, the calibration of PLl-11 in a water phantom at 1 m is the ratio

l5 (x ,f )/I(x ,f ), or

W^ W' Vl' ^ W' W^'

Nwater

2 2.

(PLl-11) = 102.04 il I f] M xJ) [1

i=0 j=0 ^ 'J "

- 0.00333(l-exp[-0.0278 (x^-50)])

+ 0.00595(l-exp[-0.0164 (f^-100)])] Gy/yC (22)

at 22 °C and 1 standard atmosphere, where the matrix elements M^j come from

table 6C. Both x,,WWand f,, are in millimeters, and z,„ = 1 meter.

w

27
7 . Operational Procedures

7 . 1 Expo s ure Techniques for Various Chambers

We have calibrated several types of ionization chambers for determination

of absorbed-dose rate in a ^^Co gamma-ray beam. In general, these can be

divided into two types, thimble chambers and flat chambers. The thimble

chambers, which include our own PLl chambers, have been inserted into the

plastic tube of the water phantom. At the time of writing (April 1989) four

calibration reports have been issued for customer-owned thimble chambers. Two

of the chambers were Exradin model A-1 chambers (SN 151 and 153), one was a Farmer probe, model 2505/3 (SN 1400), and one was a l-cm^ E.G.^G. model

(SN 43-A) . The first three were placed in the 12.7 mm I.D. tube and compared

with chamber PLl-U at water depths of about 50 mm, and a source distance of

1 m. In each case, the chamber was fitted with an acrylic adapter so that the

tube was filled with plastic rather than air. For the larger diameter

E.G.&G. chamber, the 12.7-mm I.D. plastic tube was replaced by a 19.1-mm I.D.

tube and again a special adapter was used to fill the tube with acrylic. In all cases, the calibration consisted of measuring the ratio of PLl-11 current

to current from the customer-owned chamber, for the same measurement

conditions, and multiplying by the appropriate PLl-11 calibration factor from

eq (22):

N,,,,,(customer) = jI^'^^'^ x N^^^^^(PLl-ll) (23)

where I is current.

When the customer-owned chamber fits in tne 12.7-mm I.D. tube, with its

adapters, the exposure of PLl-11 in the same tube is performed using a passive

graphite rod to fill the rest of the tube. When the 19.1-mm I.D. tube must be

used, PLl-11 and its passive rod are surrounded by an acrylic sleeve to

eliminate air pockets from the tube. Acrylic sleeves attenuate the photons

more than water does and this can be compensated for by multiplying the

measured current by (1 + 0.00042 t^g-]-]), where t^-,-]-] is the wall thickness of

the acrylic sleeve in millimeters. The coefficient comes from figure 17.

Most of the flat chambers calibrated here are model MPPK, manufactured by

Memorial Hospital in New York City.^ These are plane-parallel chambers 4.5 cm

in diameter and 1-cm thick, consisting of two conducting plastic discs, each

4-mm thick, separated by a 2-mm air gap. The chamber is centered in a 25-cm X 25-cm x 1-cm thick polystyrene plate which can form part of a 25-cm

square polystyrene phantom. The phantom used in this laboratory consists of

two 25-cm square polystyrene plates, each 6-mm thick, and a 30-cm diameter

polystyrene block, 10-cm thick. During calibration, the flat chamber plate

is sandwiched between the two 6-mm plates with the one labeled PST on top and

the one labeled PSB on the bottom. The entire sandwich sits on top of the

'^We have also calibrated a Capintec model PS-033, which differs in detail, but

for which the calibration techniques are similar.

28
30-cm diameter block, which acts as a backstop. The reference plane in the

chamber coincides with the inside surface of the top electrode, 4 mm below the

top of the chamber and its surrounding polystyrene plate. The chamber is

calibrated at two polystyrene depths, 4 mm, with the plate PST removed, and

10 mm, with the plate PST in place.

Comparison with PLl-11 is made by removing the flat chamber sandwich and

placing PLl-U with its passive graphite rod in a 12.7-mm diameter hole across

the 30-cm diameter block. The central axis of this hole is 9.5 mm below the

top surface of the block. (During calibration of the flat chamber, this hole

is either filled with a plastic rod or effectively removed by turning over the

block.) The source distances are adjusted so that both the central axis of

PLl-U during its use and the reference plane defined above for the flat

chamber are at a source distance of 0.984 meter [9]. This distance was chosen

so that photon and electron spectra would be the same in polystyrene as at a source distance of 1.00 m in water, transforming inversely with electron

density as in section 6.4. The number of electrons per cm^ in polystyrene

with a mass density of 1.049 g/cm3 is 1.049 x 7 N/^/13.019 = 0.5640 N/^, which

is 1/0.984 = 1.016 times the electron density in water, 0.5551 N^, where is

Avogadro's number.

The calibration equation for flat chambers is more complicated than

eq (23) because the PLl-11 current and the flat chamber current are measured

at slightly different depths in polystyrene. Equation (23) can be replaced by

the more general equation:

N„,,, J customer) = fi(<~erl ^ JCEkUnL , N^^.^ (PLl-U) (24) ""^'^ O(PLl-ll) I(customer) "^"'^

where the added D terms are absorbed dose rates to water at the chamber

positions with the chambers removed. When the water phantom is used with

thimble chambers, both PLl-11 and the customer chamber are placed in the same position, and (24) reduces to (23). In the flat chamber case, the ratio

• • D(customer)/D(PLl-ll) can be evaluated from eq (21) and table 8, where it must

be remembered that x^ i s a distance in water, which is 1.016 Xp , if Xp is in

polystyrene. For the present case, using a collimator size of s = 33.4 mm,

the predicted dose rates to water on 1979 Jan 1 at depths of 1.016 x 4 mm,

1.016 X 9.5 mm, and 1.016 x 10 mm are 22.79, 22.48, and 22.45 mGy/s,

respectively. Then the ratio D(customer)/l5(PLl-ll) is 22.79/22.48 = 1 .014

with plate PST absent (polystyrene depth = 4 mm) and 22.45/22.48 = 0.999 with

PST present (polystyrene depth = 10 mm).

7.2 Current Measurements and Corrections

The collecting electrode of the ionization chamber being studied is

connected via low-noise coaxial cable to the input of a Keithley 610C electrometer. This instrument has a dial readout that is usually turned off. In

its place, a Digitec 266 DC voltmeter (DVM) provides a digital readout of the

electrometer feedback voltage. The XI Output of the 610C is connected to the

Hi terminal of the DVM, and the Guard terminal of the 610C is connected to the

Lo terminal of the DVM. The 610C is used with the Feedback switch set to

Fast, and the mode switch set to 10"^, 10"^, or 10" i'^ coulombs full scale.

29
The electrometer is used with a locally made DVM timer, which controls

the Digitec DVM display. The timer output is connected to terminals D6 on the

DVM card-edge connector, so that the DVM displays the instantaneous electrometer feedback voltage only when it gets a positive pulse from the timer.

The front panel of the timer contains an ERC counter, an ITC comparator, and a Start button. When the Start button is pushed, the timer generates a pulse,

the DVM displays an initial voltage, and the counter starts counting elapsed

time. When the counter reaches the pre-set comparator number, it generates a second pulse, the DVM displays a final voltage, and the counter stops

counting. For the timer usually used, the time unit for both counter and

comparator is centi seconds . (For some of the local DVM timers, the time unit

is milliseconds.)

High voltage for the chamber is provided by a Keithley model 240A power

supply, capable of generating from 0 to 1200 V of either polarity. Measured

ionization currents are corrected to 22 °C and 1 atm (101.325 kPa = 1013.25 mbar = 760 mmHg) . Pressure measurements are made with a Wallace and

Tiernan FA139 aneroid barometer and temperature measurements with a Digitec

5810 thermistor thermometer.

The chamber current measured in amperes during a run is:

I = " (25)

where: C is the electrometer capacitance, in farads:

Vj and Vp are the initial and final DVM measurements, in volts;

At is the run time, in seconds, determined by the DVM timer.

The value of the capacitance C must be determined for each electrometer

scale by independent measurements. For the particular electrometer usually

used, NBS #184539, the capacitances measured in December 1987 were:

Full-scale (C) Capacitance (pF)

10-8 10,003.

10-9 1,012.5

10-10 104.02

(These numbers have changed by less than 0.1% from earlier measurements in

1982.)

A fully corrected current in a polystyrene phantom at a source distance

of 0.984 m can be written as:

= (I-I^_) X [101 . 325(273. 15+T)/(295. 15 P)] x (0.984)2 x Cp x Cpj (26)

where I is current measured during irradiation, is leakage current measured

with no radiation incident, T is temperature in degrees Celsius, P is pressure

in kPa, Cq is a correction for systematic errors in the DVM, and Cpy is a correction for systematic errors in the temperature and pressure measurements.

30
The term in square brackets is the correction to a reference air density, and

the (0.984)2 term corrects the current to a source distance of 1 m in a water

phantom.

Ordinarily, if 1^ is negligible, PLl-U )/!(-( customer) is equal to

I(PLl-ll)/I(customer) , since most of the factors cancel out, but the measured

currents should be fully corrected anyway, for the purposes of Quality

Assurance, as described below.

7.3 Quality Assurance Measurements

These consist of following the measurements of Ij-(PLl-ll) in the 30-cm

diameter polystyrene block and fitting them to the equation:

= A

6'°^^

(27)

where t is the number of days since 1979 Jan 1 and a = 0.0003600 day-i [4].

These measurements yield values of A in (27). The block consists of four

30-cm diameter polystyrene plates taped together (labeled PBl, PA4, PA5, and

PA6 in DB 827/65) with a total thickness of 9.3 cm. A 12.7-mm hole was drilled along a diameter 9.5 mm below the top surface of the block. During QA

measurements, the axis of symmetry of the block coincides with the beam axis

and the axis of the hole is 0.984 m from the source. PLl-U and its passive

graphite backstop fill the hole, with the beam axis passing through the

chamber at a point 6.6 mm from the square end. No extra absorbers are added

atop the block. Currents are measured and corrected as described in

section 7.2.

To date (October 1989) there have been five reliable measurements of

covering a span of more than 6 years. The data are listed in table 9A. The

values of C^and Cpj used to obtain I^-in each case are shown in columns 4 and

5, along with the databook references in parentheses. Table 9B shows the

calculated zero-time amplitudes A = exp(at), plus deviations from the

average. These deviations are all less than 0.2% in absolute magnitude,

providing assurance that the calibration quality has remained high.

7.4 Chamber MPPK-281

The NBS purchased a flat Memorial Hospital chamber (MPPK-281) in 1985.

In each flat chamber calibration since then, comparison with MPPK-281 has been

included with comparison of PLl-U in the 30-cm polystyrene block and the

customer's flat chamber. The average ratio of MPPK-281 current to PLl-11

current during the three most recent calibrations is 3.595 ± 0.17%

(DB 843/41). It is hoped that continued use of MPPK-281 with PLl-11 will

result in a reliable calibration of MPPK-281, which will enable PLl-U to be

eliminated from flat chamber calibrations.

At the same time, the comparison of PLl-U and other thimble chambers in

the polystyrene block is much more convenient than their comparison in the

water phantom. It is proposed to use this block, with additional polystyrene

plates added on top, to calibrate all thimble chambers that will fit in the

12.7 mm diameter hole in that block, using the methods described above.

31
Table 9. Corrected PLl-11 current in polystyrene block at source distance

0.984 m, phantom depth 9.5 mm, collimator size 33.4 mm, at 22 °C and

1 atm

9A As a function of time, t, measured from 1979 Jan 1

Number Date

81 Apr 2 82 Oct 21

84 Mar 8

87 Mar 24

87 Dec 4

Reference

Current Time

Cpj(DB) nA t,days

DB 827/69

843/1

843/17

843/37

843/40

0.9990(827/60)

0.9990(827/60)

0.9985(843/8)

0.9990(843/36)

0.9990(843/36)

1.0006(827/63)

1.0006(827/63)

1.0006(827/63)

1.0006(827/63)

1.0006(827/63)

0.14597

0.11876

0.09908

0.06640

0.06054

823

1390

1894

3005

3260

9B Calculation of zero-time amplitude A.

Number A, nA ^"^mean^"^^

1 0.19631 + 0.18

2 0.19588 - 0.04

3 0.19593 - 0.01

4 0.19588 - 0.04

5 0.19576 - 0.10

Mean 0.19595 ± 0.1%(m.e.)

8 . Calibration Uncertainties

The absorbed dose calibration of chamber PLl-11 can be transferred to

another (secondary) chamber by substitution in a water phantom in the ^^Co

gamma-ray beam. The component uncertainties that enter into the absorbed dose

rate to water (eq (16)) and into the calibration of the secondary chamber are given in table 10. Conventional statistical estimates of random uncertainties

are given as standard deviations of the mean, designated "type A," which can

be considered to be objective estimates. All other uncertainty estimates,

designated "type B," are subjective estimates, based on the experience of the

calibration staff. The type B uncertainties are estimated as realistic upper

bounds of the possible errors, and correspond very roughly to 99% confidence

1 imits.

The overall uncertainty IJ comes from combining the type A uncertainties

(s^) and the type B uncertainties (w-: ) by means of the equation:

U = 3/ ^ s? + ^ (w./3)2 (28)

where the terms (w^/3)2 are treated as variances [13]. This overall

uncertainty then, corresponds roughly to a 99% confidence limit when the

statistical uncertainties are included.

32
For the first three parts of table 10, the uncertainties are separated

into those involved in the direct measurements of calorimeter power and PLl-U

chamber current, and those involved in the determination of the corrections.

The correction factor uncertainties are listed separate ly in table 11. The

numbers listed for correction factors in table 10 are J Y w?" where the w. come

from table 11. In table 11, five component uncertainties have been taken to

be equal to zero, because the time interval between l.(b) and 2.(bb) was negligible, the geometry is such that l.(g) and 2.(ee) are equal, and the

l.(f) correction was not needed for the water calibration, for which only

z„ = 0.654 m was used.

Table 10. Uncertainty Analysis

Estimated uncertainty (%)

Type A Type B Component s^ w-j

1. Calorimeter

Power measurements 0.04 0.03

Correction factors 0.12

2. PLl-11 chamber in graphite

Current measurements 0.03 0.05

Correction factors 0.11

3. PLl-U chamber in water

Current measurements 0.03 0.05

Correction factors 0.11

4. Dose rate conversion, graphite to water

Absorption coefficient ratios 0.5

Replacement factor 0.2

B ratio 0.1

Positioning of chamber and calorimeter 0.1

Failure of PLl-11 wall buildup 0.15

Failure of scaling theorem 0.2

5. Chamber being calibrated

Current measurements 0.1 0.1

Positioning of chambers 0.2

Overall uncertainties, U Absorbed dose rate to water (1 through 4) 0.7%

Secondary chamber calibration (1 through 5) 0.8%

33
As shown in table 10 the overall uncertainty in determining the absorbeddose-rate to water is ± 0.7%, while that for calibrating any secondary chamber

is ± 0.8%. In both cases, the main contribution comes from the ± 0.5%

uncertainty assigned to Hubbell's absorption coefficient ratios. His estimate

of ± 0.3% [10] applies to monoenergeti c photons in the ^oco energy range. As

described in reference [1], this was arbitrarily increased to ± 0.5% because

of the presence of low-energy photons, for which the ratio is much less

accurately known.

The uncertainty for calibration of secondary chambers in calibration

reports can be conveniently rounded to ± 1%.

Table 11. Correction factor uncertainties

Estimated Uncertainty %

Component Type B ^i

1. Calorimeter corrections (section 5)

(a) Excess ring scatter 0.02

(b) Source decay 0.00

(c) Shutter timer 0.01

(d) Core impurities 0.1

(e) Acrylic ring 0.02

(f) Protection air gap (only for 2^=1.0 m) 0.00

(g) Escaping radiation 0.00

(h) Core air gaps 0.05

2. PLl-U graphite current corrections (section 5)

(aa) Air density changes 0.1

(bb) Source decay 0.00

(cc) Telethermometer errors 0.02

(dd) Instrumental errors 0.04

(ee) Escaping radiation 0.00

(ff) Mass depth difference 0.01

3. PLl-U water current corrections (section 6.3)

(a) Air density changes 0.1

(b) Source decay 0.02

(c) Telethermometer errors 0.02

(d) Digital voltmeter errors 0.04

(e) Acrylic tube attenuation 0.02

34
APPENDIX A - Test of the Photon Fluence Scaling Theorem

The purpose of this appendix is to present the data on which the conclusions of reference [9] are based. Currents from ion chamber PLl-11 were measured in the ^^Co beam from the 10-kCi Theratron F source using phantoms of

water, polystyrene, acrylic plastic, and graphite. These currents were measured as a function of depth in the phantoms for several source distances.

These distances were chosen to test the theorem that for photons that interact

with low-Z materials by means of the Compton effect, distances in these

materials scale inversely with the electron densities, and photon fluences

scale directly with the square of the electron densities.

5

The measurement conditions are shown in table 12 which lists the mean mass densities and the electron densities relative to water for the four media

used. Source-detector distances used in three separate experiments (A, B, and

C) are also shown. In each of the three experiments, the product of source distance and relative electron density is the same for each medium. At each

position, PLl-11 current was measured as a function of phantom depth, for the

four field sizes determined by the 50.8-mm, 40.5-mm, 33.4-mm, and 28.0-mm

collimator jigs (as summarized in DB 843/59-63).

Table 12. Test of scaling theorem; conditions and results

Water

Materi al

Polystyrene Acrylic Graphite

Mean mass density, g/cm^ 1 1.049 1.182 1.70

Relative electron density 1 1.016 1.149 1.53

Source-detector distance, m A 1.000

B 1.265

C 1.530

0.984

1.245

1.506

0.870

1.101

1.332

0.654

0.827

1.000

Total normalization factor A 1.008

B 1.022

C 1.026

1.012

1.024

1.032

1.004

1.016

1.024

0.980

0.995

1.004

PLl-11 replacement factor 1.010 1.010 1.007 1.001

The current measurements were all corrected to a temperature of 22 °C, a pressure of 101.325 kPa, and a time zero of 1979 Jan 1. In addition, each

current was multiplied by the square of the source distance, to correct it to

a distance of 1 m, to a first approximation. When all of the corrected

measurements for a given experiment and a given collimator jig were plotted as

a function of scaled phantom depth (true phantom depth x electron density of

^Electron density as used here means electrons per unit volume.

35
phantom/electron density of water), it was found that the curves for water,

polystyrene, acrylic, and graphite were always very similar in shape but

displaced in magnitude by as much as a few percent.

It was found by trial and error that if the corrected currents were

multiplied by the total normalization factors listed in table 12 as a function

of experiment and material, this displacement in magnitude could be reduced to

a few tenths of one percent. The normalized currents are listed in table 13,

where the units were nanoamperes before normalization. Also shown in table 13 are the least-squares polynomial coefficients in the equation:

I (x) =

{ I b. x'] exp(-0.01 x) (29)

^ i=0

where x is scaled phantom depth in mm. The coefficients of variation of the

fits, V, are also listed in percent in table 13. The curve-fitting residuals,

(I - Ip) are shown in percent in figure 18 for these 12 curves. The most

striking systematic variation is for the graphite residuals, which all have

the same general shape and the largest deviations. In general, the normalized

depth-current curves agree to within a few tenths of one percent, and the

scaling theorem appears to work within that limit.

The normalization factors of table 12 are not unity for several reasons.

First, with the ion chamber at different distances from the source, there will

be different amounts of air attenuation. The measurements can be corrected

for air attenuation by multiplying by the factor (1 + 0.0032 z) , where z is source distance in meters, and 0.0032 m" i comes from the mass energyabsorption coefficient for air at a photon energy of 1.17 MeV [10] and the

density of air at 22 °C.

Second, the measurements can also be corrected for replacement of the

graphite chamber by phantom material, using the replacement factors listed in

table 12. These were calculated from the data of figure 17, using the

following reasoning:

As in the evaluation of >l>watpr/^wal 1 section 6.2, it can be deduced

from figure 17 that insertion of a 12.3-mm diameter rod (the same size as the

chamber) in a water phantom reduces the central axis fluence by a factor of

1/1.0000 if the rod is polystyrene, by a factor of 1/1.0026 if the rod is

acrylic, or by a factor of 1/1.0096 if the rod is graphite. Replacing a graphite rod by an acrylic (or polystyrene) rod may be done in two steps.

Replacing a graphite rod by a water rod increases the fluence by the factor

1.0096 (hence the 1.010 replacement factor for water). Replacement of the

water rod by an acrylic (or polystyrene) rod decreases the fluence by a factor

of 1/1.0026 (or 1/1.0000). Thus, replacement of a graphite rod by an acrylic

(or polystyrene) rod increases the fluence by a net factor of 1.0096/1.0026 = 1.0070 (or 1.0096/1.0000 = 1.0096), giving the table 12 replacement factor of

1.007 (or 1.010).

36
Table 13. Normalized PLl-11 currents in 4 phantoms during 3 experiments, plus polynomial coefficients for eq (29) in text. The coefficient of variation. V, was calculated

for 4 degrees of freedom.

Experiment A Experiment B Experiment C

Collimator Size, mm

Scaled phantom 50.8 40.5 33.4 28.0 50.8 40.5 33.4 28.0 50.8 40.5 33.4 28.0

depth. mm Water Phantom

8.8 0.2094 0.2047 0.2004 0.1961 21.4 0.2013 0.1966 0.1922 0.1879

22.2 0.1912

23.5 0.2001

28.1 0.1930

28.3 0.2032

30.6 0. 1992 0. 1939 0. 1891 0.1843

36.3 0.1925 0.1873 0.1825 0.1775

49.1 0.1802

51.8 0.1828 0.1771 0.1719 0.1664

53.0 0.1860 0.1801 0.1747 0.1693

53.2 0.1826

56.2 0. 1692 59.0 0.1851

0 / . J 0.1721 0.1658 C.15C2 0.1C45

74.5 0.1719 0.1653 0.1595 0.1538

76.2 0.1620

81.5 0.1621 0.1554 0.1497 0.1436

90.5 0.1453

93.2 0.1561

93.6 0.1630

96.1 0.1519 0.1450 0.1392 0.1330

96.2 0.1574 0.1502 0.1441 0.1380

117.8 0.1428 0.1356 0.1291 0.1231

Polystyrene Phantom

9.6 0.2068 0.2025 0.1987 0.1948 0.2092 0.2048 0.2006 0.1964 0.2110 0.2066 0.2021 0.1976

28.7 0.1973 0.1924 0.1880 0.1837 0.2004 0.1955 0.1906 0.1860 0.2026 0.1977 0.1932 0.1879

47.9 0.1853 0.1797 0.1747 0.1698 0.1891 0.1835 0.1781 0.1731 0.1917 0.1862 0.1809 0.1755

72.8 0.1681 0.1617 0.1560 0.1506 0.1729 0.1664 0.1605 0.1549 0.1760 0.1700 0.1640 0.1582

97.5 0.1507 0.1437 0.1376 0.1320 0.1563 0.1491 0.1428 0.1370 0.1598 0.1532 0.1470 0.1410

122.4 0.1336 0.1263 0.1201 0.1146 0.1397 0.1323 0.1257 0.1199 0.1437 0.1369 0.1302 0.1242

Acrylic Plastic Phantom

9.7 0.2068 0.2023 0.1985 0.1947 0.2090 0.2046 0.2005 0.1964 0.2107 0.2062 0.2020 0.1974

29.1 0.1972 0.1921 0.1877 0.1834 0.2002 0.1951 0.1906 0.1861 0.2024 0.1973 0.1927 0.1877

47.9 0.1854 0.1796 0.1746 0.1698 0.1890 0.1834 0.1782 0.1732 0.1917 0.1861 0.1810 0.1756

74.8 0.1668 0.1602 0.1545 0.1490 0.1714 0.1650 0.1592 0.1535 0.1747 0.1685 0.1627 0.1569

102.5 0.1472 0.1400 0.1339 0.1282 0.1526 0.1455 0.1394 0.1334 0.1565 0.1497 0.1435 0.1374

128.6 0.1296 0.1221 0.1160 0.1105 0.1355 0.1282 0.1218 0.1159 0.1399 0.1327 0.1263 0.1202

Graphite Phantom

14.6 0.2049 0.1997 0.1956 0.1918 0.2070 0.2021 0.1979 0.1939 0.2086 0.2037 0.1993 0.1947

28.4 0.1980 0.1925 0.1878 0.1836 0.2008 0.1954 0.1908 0.1864 0.2029 0.1974 0.1927 0.1878

52.2 0.1715 0.1664 0.1868 0.1806 0.1753 0.1703

52.6 0.1827 0.1764 0.1891 0.1831 0.1779 0.1722

75.0 0.1666 0.1594 0.1535 0.1480 0.1710 0.1645 0.1586 0.1532 0.1746 0.1677 0.1621 0.1561

102.4 0.1284 0.1530 0.1460 0.1396 0.1338

103.9 0.1475 0.1396 0.1332 0.1567 0.1495 0.1432 0.1371

Polynomial Coefficients

lO^b,

lO^b,

V, %

0.2110

1.757

1.717 1.935

0.21

0.2062

1.726

0.264 4.565

0.15

0.2025

1.670

-0.590 5.904

0.19

0.1989

I. 623 -1.8)8

II. 018

0.18

0.2123

1.848

1.697 5.943

0.12

0.2079

1.789 0.758

6.491

0.11

0.2038

1.735 -0.007

6.581

0.12

0.1998

1.700

-1.141 9.526

0.13

0.2142

1.855

2.727 4.134

0.20

0.2098

1.793

1.804 4.881

0.18

0.2053

1.770

0.655 6.511

0.17

0.2012

1.684 0.188 6.860

0.21

37
0.4

0.0

0.4

0.4

0.0

-0.4

0.4

0.0

-0.4

(A)

20 40 60 80 100 120

0.4

0.0

-0.4

t

0.4

0.0

-0.4

0.4

0.0

0.4

KEY

X Water + Polystyrene o Acrylic Graphite

1 \ s = 40.5 mm

(B)

(B)

20 40 60 80 100 120

20 40 60 80 100 120

Scaled Phantom Depth, mm

0 20 40 60 80 100 120 Scaled Phantom Depth, mm

igure 18. Residual currents in tests of fluence scaling theorem, shown as a function of collimator size and scaled phantom depth for four materials.

38
The only complication in this argument is that the replacement of

graphite by acrylic (or polystyrene) must be done in an acrylic (or polystyrene) phantom, rather than in a water phantom. It would be expected that

these changes would modify the spectra incident on the rods, but not by enough

to change the numerical factors.

The replacement factor for graphite is not unity because the PL chamber

graphite density (1.79 g/cm^ - reference [3]) is larger than the graphite

phantom density (1.70 g/cm^ - table 12). The factor 1.001 was chosen by

assuming that y, the departure of the replacement factors from unity (0.010

for both water and polystyrene and 0.007 for acrylic) was proportional to x, the difference between the graphite chamber mass density and the mass density

of the phantom material (0,79 for water, 0.741 for polystyrene, and 0.608 for

acrylic). For the graphite phantom, with a mass density difference of

0.09 g/cm3, the least squares predicted departure is 0 .09( ^xy/ ^x2) = O.OOU.

The quantities plotted

in figure 19 as a function

of source distance are the

total normalization factors

of table 12 divided by both

the PLl-U replacement

factors of table 12 and the

air attenuation correction

factors. These ratios have

been labeled Residual

Normalization Factors F(z).

If the prediction that

photon fluence scales

directly with the square of

electron density were accurate, F(z) would be a constant. The reasons why

the Residual Normalization

Factor is not constant

probably include the fact

that the source is not a point source. Its effective

size is increased by scatter

inside the head and at the

col 1 imator.

M

re

re

C/3

1.02

1.01

1.00

E 0.99

0.98

0.97

1 1 1 1

X

a/

KEY

X Water + Polystyrene

o Acrylic • Graphite

I 1

1

1 0.6 0.8 1.0 1.2 1.4 Source Distance z, meters

1.6

Figure 19. Residual Normalization Factors from

test of fluence scaling theorem, shown as a function of source distance.

As a graphical example of the depth-current curves, figure 20 shows the

three normalized curves from table 13 for collimator size 28.0 mm, along with

the experimental points (or averages of the experimental points where they are too close to be separately graphed). The differences in shape are presumably

caused by photon spectral changes as the source distance increases from set A to set C. These differences are not large but they are measurable. At a scaled depth of 8 mm, the ordinate for curve C is 1.4% larger than that of curve A. At a scaled depth of 128 mm, curve C is 8.9% larger than curve A.

Thus spectral differences presumably account for a current difference of 7.5%

over that range. For larger apertures, the difference is somewhat smaller.

For s = 50.8 mm, the difference is only 6%.

39
0.20

0.18

0.16

E

0.14

0.12 -

Collimator Size

s - 28.0 mm

1

20 40 60 80 100

Scaled Phantom Depth, mm

120

Figure 20. Normalized depth-current curves for experiments A, B, and C for

collimator size 28.0 mm and several scaled phantom depths. These indicate

some photon spectral change as source distances increase from A to C.

40
Appendix B - U.S. - U.S.S.R. Comparison

A total of six graphite transfer ionization chambers were first calibrated in a graphite phantom at NIST. Their calibrations are summarized in

table 5 (for chamber PLl-11) and table 1, which shows the sensitivities

relative to PLl-U of the chambers used (PLl-14, 16, 17 ,18,19,20) . The chambers

were shipped to VNIIFTRI in Moscow in two sets on two different occasions,

where they were also calibrated cal orimetri cal ly in a ^^Co beam. The first

set (PLl-14, 16, 18) was calibrated in 1978. The second set ( PLl-17 ,19 ,20) was calibrated in 1981, using a new Soviet phantom-calorimeter assembly that

includes a pair of differentially connected calorimeters. The results of

these comparisons are shown in table 14, using the originally reported Soviet

calibrations and the more recent U.S. calibrations described in this manual.

The original i ntercompari son was made with preliminary calorimetric calibrations of PLl-14, 16, 18 using the 0.5-kCi ^oCo source in room B034 of NIST

building 245, rather than the 10-kCi ^^Co source in room B036 that was used

for all the calibrations described in the main body of this report. The

results of that original i ntercompari son were described in reference [14],

which listed the average Absorbed-dose (NIST)/Absorbed-dose (VNIIFTRI) = 1.022 ± 0.005. For these B034 calibrations, the source distance was 1.00 m,

the phantom depth was 5.0 g/cm2, and the field size was 120 x 120 mm. These

numbers differed sufficiently from the VNIIFTRI numbers (1.24-m, 3.4-g/cm2,

and 112-mm diameter field size) that the second i ntercompari son was undertaken. As shown in table 14, the average ratio Absorbed-dose (NIST)/Absorbeddose (VNIIFTRI) = 1.0002 for the 1981 i ntercompari son.

In all cases, the VNIIFTRI beam was circular and the NIST beam was square

in cross section, so that it was impossible to duplicate the measurement

conditions from one laboratory to the other. It was decided arbitrarily that

the most sensible comparison would be one in which the beam areas were identical, which accounts for the choice of NIST field sizes of 99.3 x 99.3 mm

for i ntercompari son #1 and 101.9 x 101.9 mm for i ntercompari son #2.

41
Table 14. U.S. — U.S.S.R. i ntercompari sons in graphite phantoms at 20 °C and

1 atm

Intercompari son #1

A. Measurement Conditions:

Parameter VNIIFTRI NIST

Source distance, m Phantom depth, g/cm2

Field size, mm

1.24

3.4

diameter 112

1.25

3.4

99.3 X 99.3

B. Calibrations:

Chamber VNIIFTRI

(1978)

NIST

(1989)

Gy(NIST)

Gy(VNIIFTRI)

PLl-14

PLl-16

PLl-18

11.30 nC/Gy

11.35

10.85

90.04 Gy/iiC 1.017

89.80 1.019

93.83 1.018

Average 1.018

Intercompari son #2

A. Measurement Conditions:

Parameter VNIIFTRI NIST

Source distance, m Phantom depth, g/cm2

Field size, mm

1.00

4.8

diameter 115

1.00

4.8

101.9 X 101.9

B. Calibrations:

Chamber VNIIFTRI

(1978)

NIST Gv(NIST)

(1989) Gy(VNIIFTRI)

PLl-17

PLl-19

PLl-20

10.81 nC/Gy

10.92

11.03

92.44 Gy/yC 0.9993

91.70 1.0014

90.65 0.9999

Average 1.0002

42
Appendix C — U.S, - Canada Comparison

Two graphite transfer ionization chambers were first calibrated in a water phantom at NIST. Their calibrations are summarized in eq (21), for

chamber PLl-11, and table 1, which shows the sensitivities relative to PLl-11

of the two chambers (PLl-17,19). The chambers were shipped to the National

Research Council (NRC) in Ottawa, where they were also calibrated

cal orimetri cal ly in a ^oco beam. The results of this i ntercompari son is shown

in table 15, using the originally reported Canadian calibrations and the more

recent U.S. calibrations described in this manual. On the average, the ratio

Absorbed-dose (NIST)/Absorbed-dose (NRC) = 1.0045. Based on the original U.S.

calibrations, this ratio was reported in reference [14] as 1.003 ± 0.002.

Table 15. U.S. —Canada (National Research Council) i ntercompari sons in water

phantoms at 0 °C and 1 atm (101.325 kPa)

A. Measurement Conditions:

Parameter NRC NIST

Source distance, m 1.0 1.0

Phantom depth, mm 50 50

Field size, mm 100 X 100 100 X 100

B. Calibrations

Chamber

NRC

(1978)

NIST

(1989)

Gy(NIST)

Gy(NRC)

PLl-17

PLl-19

96.0 Gy/viC

94.8

96.21 Gy/yC

95.44

1.0022

1.0068

Average 1.0045

43
Appendix D — U.S. - Sweden Comparison

Two liquid ionization chambers were transported from Sweden, where they

had been calibrated in terms of absorbed-dose to water in a ^^Co beam by three

methods: ai r-ioni zation-chamber dosimetry, water calorimetry, and ferroussulphate dosimetry. The internal agreement between these three methods was of

the order of 0.4%. These two chambers were then calibrated in the NIST ^^Co

beam at several depths in the water phantom. The NIST calibration data are shown in table 16A, and the NIST and Swedish calibrations are compared in

table 16B, where the Swedish numbers came from ferrous-sul f ate dosimetry.

The NIST calibrations in table 16 differ from the numbers recently

published in reference [15], for several reasons, most of which are mentioned

in sections 5, 6.2, and 6.3. The agreement between the NIST and Swedish

calibrations is not nearly as good as the published values (0.02% for

chamber #1 and 0.19% for chamber #2), but is still within the overall

uncertainty of the NIST calibration alone, without considering uncertainties

in the Swedish calibrations.

Table 16. Calibration of Swedish liquid ionization chambers in a water

phantom

A NIST measurements as a function of depth in water,

using the 50.8 mm collimator jig.

Chamber Source Water Absorbed Measured Chamber Average

number di stance depth dose-rate current cal i brati on m mm mGy/s nA Gy/yC Gy/yC

1 1.0000 33.8 12.247 0.19;'52 63.61 63.68

51.9 11.499 0.18056 63.69

52.4 11.478 0.18037 63.64

53.2 11.443 0.17984 63.63

73.1 10.566 0.16598 63.66

93.6 9.653 0.15150 63.72

113.1 8.804 0.13819 63.71

131.4 8.040 0.12614 63.74

2 1.0005 51.3 11.513 0.17960 64.11 64.14

73.4 10.542 0.16432 64.16

B U.S. — Swedish intercomparison

Chamber Sweden NIST Gy(NIST)

number (1982) (1989) Gy( Sweden)

Gy/yC Gy/yC

1 63.23 63.68 1.0071

2 63.82 64.14 1.0050

44
Appendix E — Sample Calibration Report

DG 8787/87 Page 1 o£ 4

DB 843/42

TFN 240907

1987 Dec 23

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

GAITHERSBURG, MD 20899

REPORT OF TEST

Parallel plate ionization chamber, mounted in a polystyrene plate

Model MPPK, Serial Number 278

Manufactured by Memorial Hospital

New York, NY 10021

Submitted by

Received at NBS 1987 Oct 21

Calibration date 1987 Dec 4

The calibration factors given in this report are quotients of the absorbeddose rate to water, measured in a polystyrene phantom irradiated with

cobalt- 60 gamma rays, divided by the ionization chamber current generated

by that radiation. The average ion current used to compute the calibration

factor is based on currents measured with the outer electrode of the triax

connector at the stated polarity and potential. The middle electrode was grounded and the central pin was connected to the electrometer input. The

currents were normalized to one standard atmosphere and 22 degrees Celsius.

Use of the chamber at other pressures and temperatures requires correction

of the ion currents to these reference conditions. The correction factor F is computed from the following expression:

F = (273.15 + T)/(295.15 H)

where T is the temperature in degrees Celsius, and

H is the pressure expressed as a fraction of a standard atmosphere.

(1 standard atmosphere = 101.325 kilopascals = 1013.25 millibars = 760

mmHg)

.

The absorbed-dose rate to water at the calibration position in polystyrene

was determined as described on page 4. The calibration factors given on page 3 apply only for the conditions given in the table, but the factors

vary slowly with depth in the phantom and with field width, and are nearly

independent of source-chamber distance.

45
DG 8787/87

1987 Dec 23

Page 2 of 4

The chamber was received with no inforraaClon about the chamber inner

dimensions. These were assumed to be the same as the dimensions of the

Memorial Hospital chamber described in J. G. Holt, et ai. , "Absorbed dose

measurements using parallel plate polystyrene phantoms," Int. J. Rad.

Oncol. Biol. Phys. 5, 2031-2038 (1979).

Calibration factors are given at two depths. At the smaller depth, no other plate was placed on top of the chamber block. At the larger depth, an additional 25-cm square, 6-mra-thick polystyrene plate was added. In

both cases, the chamber mount was backed by a 30-cm-diameter , 12-cm-thick

polystyrene block.

The uncertainty of the absorbed dose measurements is under investigation.

It is expected to be not greater than 2% at the 95% confidence level; the

accuracy of the current measurements is believed to be within a few tenths

of a percent.

Information on technical aspects of this report may be obtained from

J. S. Pruitt, Radiation Physics C214, National Bureau of Standards,

Gaithersburg, MD 20899, (301) 975-5587.

Calibration performed by J . S. Pruitt

Report approved by R. Loevinger

For the Director

by

Randall S. Caswell, Chief

Ionizing Radiation Division

Center for Radiation Research

National Measurement Laboratory

46
DG 8787/87

1987 Dec 23

Page 3 of '4

NATIONAL BUREAU OF STANDARDS REPORT OF CALIBRATION

Parallel Plate Chamber

Model MPPK Serial Number 278

Collection potential: -300 V

Not tested for atmospheric connection

1 2 3 4 5 6 Beam Calibration Factor Polystyrene Source Beam Dose

Code 22 deg C and 1 atm Depth Distance Width Rate

(Cy/nC) (mm) (m) (mm) (mGy/s)

60Co 0.0334 4 0.984 94 7 0.0333 10

During calibration, the cavity was positioned in the center of the beam

with the polystyrene plate perpendicular to the beam direction. The

chamber surface nearest the source of radiation had a small air vent hole.

A reference plane was defined, which is believed to coincide with the

inside surface of the electrode nearest the source of radiation. This

reference plane is parallel to the surface of the polystyrene plate, at a depth of 4 mm.

2E-15 A was the average leakage current measured during the calibration.

Checked by

EXPLANATION OF COLUMNS IN THE CHAMBER CALIBRATION TABLE

1 . The beam code identifies the radiation source used for the

calibration.

2. The calibration factor defined in the body of the report, in SI units

(grays per nanocoulomb) . Use of the calibration factor to

determine absorbed dose to water is discussed on page 4.

3. The depth from the phantom surface to the reference plane.

4. The distance between the radiation source and the reference plane.

5. The beam width in the reference plane measured in air between the

50-percent intensity lines. The cross-section of the beam was square

.

6. The absorbed-dose rate at which the calibration was performed. If the

chamber is used to measure an absorbed-dose rate that is signif tcancLy

different from chat used for the calibration it may be necessary to correct for ion recombination. The stated calibration factor has not

been corrected for ion recombination.

47
DG 8787/87

1987 Dec 23

Page 4 of 4

The NBS cobalC-60 beam had previously been calibrated In absorbed dose Co

water at a source distance of 1 meter as described In: J. S. Pruitt, S. R. Domen, and R. Loevlnger, "The graphite calorimeter as a standard of

absorbed dose for cobalt-60 gamma radiation," J. Res. Nat. Bur. Stand. 8^,

495-502 (1981). Figure 3 of this article Illustrates the dependence of an Ionization chamber calibration on phantom depth, field size, and source

distance. The calibration factors listed in column 2 of the table were obtained from this water calibration using current measurements in a polystyrene phantom, with the chamber reference plane at a source distance

of 0.984 meters, to insure a spectral distribution the same as at 1 meter

in water, as described in: J. S. Pruitt and R. Loevinger, "The photonfluence scaling theorem for Compton-scattered radiation," Med. Phys . 9,

176-179 (1982). The dose rate in a water phantom at source distance d in a cobalt-60 beam may be obtained from measurements of current In a polystyrene phantom at source distance 0.984 d, and Is:

D««t.r = (0.984)2 X i(0.984d) x N Gy/s

where I(0.984d) is the normalized current (in nA) in the polystyrene

phantom at 0.984 d, and N is the calibration factor (in Gy/nC)

from column 2. The water depths to which these calibrations apply are

the polystyrene depths listed in column 3 of the table, divided by 0.984.

48
REFERENCES

Pruitt, J. S., Domen, S. R., and Loevinger, R., "The Graphite Calorimeter

as a Standard of Absorbed Dose for CobaTt-60 Gamma Radiation," J. Res.

Natl. Bur. Stand. (U.S.) 86, 495 (1981).

Domen, S. R. and Lamperti, P. J.. "A Heat-Loss-Compensated Calorimeter:

Theory, Design, and Performance, J. Res. Natl. Bur. Stand. (U.S.) 78A , 595 (1974).

Pruitt, J. S. and Loevinger, R., "Ionization Chamber for Absorbed-Dose

Calibration," Proceedings of Symposium on Measurements for the Safe Use

of Radiation; 1976 March 1-4; Gai thersburg, MD, National Bureau of

Standards (U.S.) Spec. Publ . 456: 37-39, 1976 November.

NCRP Report No. 58, "A Handbook of Radioactivity Measurements and

Procedures (Second Edition)," National Council on Radiation Protection and

Measurements, Bethesda, MD, 20814 (1985), table 9.

Domen, S. R., private communication.

Quinn, T. J., "Report of the Director, Bureau Internationale des Poids et

Mesures (BIPM)," 1987 October-1988 September, Sevres, France.

Loevinger, R., "A Formalism for Calculation of Absorbed-Dose to a Medium

from Photon and Electron Beams," Med. Phys. 8-, 1 (1981).

Burlin, T. E., "Cavity-Chamber Theory," Chapter 8, Radiation Dosimetry,

Vol. 1, Fundamentals; Second Edition: edited by F. H, Attix, W. C. Roesch,

and E. Tochilin. Academic Press (1968).

Pruitt, J. S. and Loevinger, R., "The Photon-Fl uence Scaling Theorem for

Compton Scattered Radiation," Med. Phys. 9^, 176 (1982).

Hubbell, J. H., "Photon Mass Attenuation and Mass Energy-Absorption

Coefficients for H, C, N, 0, Ar and Seven Mixtures from 0.1 keV to

20 MeV," Radiat. Res. 70, 58 (1977).

Bruce, W. R. and Johns, H. E., "The Spectra of X-Rays Scattered in Low

Atomic Number Materials," Brit. J. Radiol. Supplement No. 9, British

Institute of Radiology, London (1960).

Seltzer, S. M., Hubbell, J. H., and Berger, M. J., "Some Theoretical

Aspects of Electron and Photon Dosimetry," Proceedings of IAEA International Symposium on National and International Standardization of

Radiation Dosimetry, 5-9 December 1977, Atlanta, GA; IAEA-SM-222/05;

International Atomic Energy Agency, Vienna (1978).

Colle, R. Private communication concerning a meeting of a Working Group

on the Statement of Uncertainties, Bureau Internationale des Poids et

Mesures, Sevres, France, October 1980.

Proceedings of the 5th Meeting of Section I of the Comite Consultatif

pour les Etalons de Mesure des Rayonnements lonisants (CCEMRI), 28-30 May

1979, at the Bureau International des Poids et Mesures, Sevres, France.

Mattsson, 0., Svensson, H., Wickman, G., Domen, S. R., Pruitt, J. S., and

Loevinger, R., "Absorbed Dose to Water: Comparison of Several Methods

Using a Liquid Ionization Chamber," submitted for publication in Acta

Radiologica Oncology.

49

NBS-n4A (REV. 2-BC) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NIST/SP-250/40

2. Performing Organ. Report No. 3. Publication Date

October 1990

4. TITLE AND SUBTITLE

ABSORBED-DOSE CALIBRATION OF IONIZATION CHAMBERS

IN A 6 0Co GAMMA-RAY BEAM

5. AUTHOR(S)

John S. Pruitt

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

(formerly NATIONAL BUREAU OF STANDARDS)

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

Same as item #6

10. SUPPLEMENTARY NOTES

QJ] Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200'Word or less factual summary of most significant information. If document includes a si gnificant

bibliography or literature survey, mention it here)

Measurement Service C.l from National Institute of Standards and

Technology (NIST) publication SP 250 provides calibrations for customerowned ionization chambers so that they may be used to determine absorbed

dose to water in ^^Co gamma-ray beams. The calibrations are based on calorimetric measurement of absorbed dose to graphite in a graphite

phantom. Transformation of the calibrations to a water phantom is made

with a specially-designed graphite ionization chamber, and requires

knowledge of photon mass attenuation coefficients and the perturbation

of the graphite chamber in the water medium. The determination of these

quantities is described in detail, along with the operational techniques

normally used to transfer the calibration to customer-owned chambers.

Appendix A lists experimental data used to test the photon-f 1 uence

scaling theorem. Appendices B, C, and D describe international

comparisons of the chamber calibrations, and Appendix E shows a sample

calibration report.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

absorbed dose; calibration; calorimeter; cobalt-60; ionization chamber; photon fl uence

13. AVAILABILITY

^Unlimited

I I For Official Distribution. Do Not Release to NTIS

r~Xl Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

54

15. Price

USCOMM-DC 6043-P80

THE SP 250 SERIES ON NIST MEASUREMENT SERVICES*

SP 250-1 Spectral Radiance Calibrations PB87 179883

SP 250-2 Far Ultraviolet Detector Standards PB87227609

SP 250-3 Radiometric Standards in the Vacuum

Ultraviolet PB87227625

SP 250-4 Fricke Dosimetry in High-Energy Electron Beams

PB881 10374

SP 250-5 Alpha-Partide Calibrations PB881 68620

SP 250-6 Regular Spectral Transmittance PB88 108550

SP 250-7 Radiance Temperature Calibrations PB88123674

SP 250-8 Spectral Reflectance PB88 109905

SP 250-9 Calibration of Beta-Partide-Emitting

Ophthalmic Applicators

PB88 108535

SP 250-10 Radioactivity Calibrations with the Gamma Ionization Chamber and Other

Radioactivity Calibration Capabilities

PB88 123708

SP 250-1 1 Dosimetry for High Dose Applications

PB88201587

SP 250-12 Neutron Personnel Dosimetry

PB87227617

SP 250-13 Activation Foil Irradiation with Californium

Fission Sources PB88217443

SP 250-14 Activation Foil Irradiation by Reactor Cavity

Fission Sources PB88217435

SP 250-15 Photometric Calibrations PB881 53747

SP 250-16 Calibration of X-Ray and Gamma-Ray

Measuring Instruments PB88211826

SP 250-17 The NBS Photodetector Spectral Response

Calibration Transfer Program

PB88201595

SP 250-18 Neutron Source Strength Calibrations PB88211818

SP 250-19 Calibration of Gamma-Ray-Emitting

Brachytherapy Sources PB89 193858

SP 250-20 Spectral Irradiance Calibrations PB88 123781 SP 250-21 Calibration of Beta-Partide

Radiation Instrumentation PB88201579

SP 250-22 Platinum Resistance Thermometer

Calibrations PB88 138367

SP 250-23 Liquid-in-Glass Thermometer

Calibration Sen/ice

PB89 128888

SP 250-24 Standard Cell Calibrations PB88 123690

SP 250-25 Calibration Sen/ice for Inductive Voltage Dividers SP 250-26 NBS Phase Angle Calibration Services PB88225636

SP 250-27 AC-DC Difference Calibrations PB892222616

SP 250-28 Solid-state DC Voltage Standard Calibrations PB881 68703

SP 250-29 Traceable Frequency Calibrations PB881 68364

SP 250-30 GOES Satellite Time Code Dissemination:

Description and Operation

PB88 168760

SP 250-31 Mass Calibrations PB89 153894

SP 250-32 A Calibration Sen/ice for 30 MHz

Attenuation and Phase Shift PB88238324

SP 250-33 A Calibration Sen/ice for Voltage Transformers and High-Voltage Capacitors

PB882252903

SP 250-34 High Vacuum Standard and Its Use

PB891 93841 SP 250-35 The Calibration of Thennocouples

^ and Thermocouple Materials PB89209340

SP 250-40 Absorbed-Dose Calibration of

Ionization Chambers in a

^"00 Gamma-Ray Beam

* Entries Entries containing a stock number {SN003-003-) and price can be purchased from the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402. GPO will accept checks, money orders, VISA, and MasterCard. For more infomation, or to place an order, call (202) 783-3238. Be sure to dte the stock number on all orders.

Entries containing PB numbers can be purchased from the National Technical Information Sen/ice, Springfield, VA 22161. NTIS will accept American Express in addition to the payment methods listed for GPO. For more information call (703) 487-4650; to place an order

call (800) 336-4700. Be sure to dte the PB number on all orders. ^ ^

Entries without stock or PB numbers are In preparation.
U.S. Department of Commerce

National Institute of Standards and Technology

(formerly National Bureau of Standards)

Gaithersburg, MD 20899

Official Business

Penalty for Private Use $300