IMBSIR 76-1117

Exposure Spectra from NBS

Vertical-Beam 60Co Gamma-Ray

Source

Margarete Ehrlich

Christopher G. Soares

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

December 1976

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 76-1117

EXPOSURE SPECTRA FROM NBS

VERTICAL-BEAM 60 Co GAMMA-RAY

SOURCE

Margarete Ehrlich

Christopher G. Soares

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

December 1 976

U S. DEPARTMENT OF COMMERCE. Elliot L. Richardson. Secretary

Edward O. Vetter. Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Acting Director

Contents

Page

1. Introduction 1

2. Method for deriving the exposure spectra for different

collimator apertures 2

2.1 Deriving exposure spectra from number-of-photon

spectra 2

2.2 Deriving exposure spectra for the field sizes of

of interest 3

3. Results 3

Table 1. Exposure in consecutive energy intervals for

different field sizes. The values are expressed

in percent of total exposure at a distance

of 1 . 0 m 5 Table 2. Exposure in consecutive energy intervals below

0.80 MeV, from scatter due to the housing and

collimator, and due to the source proper. The

values are expressed in percent of total

exposure at a distance of 1.0 m 6

4. References 7

i

I

Exposure Spectra from the NBS Vertical -Beam ^Co Gamma-Ray Source

Margarete Ehrlich and Christopher G. Soares

ABSTRACT

Exposure spectra at a distance of 1 meter from the NBS verticalfin beam Co gamma-ray source are presented in pabular forjn for field sizes

of practical interest. Included are total exposure spectra as well as a breakdown of the scatter contribution according to its origin in the source proper and in the source housing and collimator.

1. Introduction

As a part of a long-range program of providing adequate

characterization of the x- and gamma-ray beams employed in the instrumentcalibration program of the National Bureau of Standards (NBS), measurements were made of the spectrum in one of the ^Co gamma-ray beams used

in this program. Procedures for arriving at spectral distributions from

the experimental pulse-height distributions obtained at two distances

from the source, and for isolating the spectra due to scatter from the source proper and from the housing and collimator were reported elsewhere [1]. In practical applications to instrument calibration, one usually is interested not in the number of scattered photons in a particular photon-energy interval but in the contribution of these photons to

the total exposure -- information that can be derived readily from the

photon spectra. It is the purpose of this report to present, in tabular

form, such exposure spectra for the scattered radiation below 0.80 MeV, at

a distance of 1 meter from the NBS vertical-beam ^Co gamma-ray source,

for collimator apertures of practical interest.

1
2. Method for deriving the exposure spectra for different

collimator apertures

2.1 Deriving exposure spectra from number-of-photon spectra

Following the notation in the original paper [1], let s. be the

J number of scattered photons in the jth photon-energy interval of width

0.05 MeV. Expressed in percent of the total number of photons, N,

reaching the detector, the number of photons in the jth interval then

is 100 s./N. Similarly, at the location of the detector, let x- be

1 J the exposure due to scattered photons in the jth energy interval.

Expressed in percent of the total exposure, X, at the detector, the

exposure due to scattered photons in the jth interval then is

100 Xj/X E 100 s

j

E

j

|J ^

r

(E

j

) / X.

Q

“I V* where E. is the average photon energy, in the jth interval, y (E.) is

J ®** J the corresponding mass energy-absorption coefficient of air [2], and the

total exposure, X, is given by

n n

x s Ss jEj C<Ej>4 < N-S s

j

)[E 1.17

|J en

r(1

- 17 MeV > + E

l ,33<n ^ ' 33

J I J ’

In this expression, the sums are extended in equal 0.05-MeV intervals

from 0.10 to 1.00 MeV while, above 1.0 MeV, where the data for the

spectra of the scattered photons were rather poor, only two intervals

are considered, namely, 1.00 to 1.17 MeV and 1.17 to 1.33 MeV. The

data below 0.10 MeV, which may have been spurious, are excluded, since,

according to results obtained with Monte Carlo methods, the number of

scattered photons below 0.10 MeV to be expected in this source geometry

is negligibly small [3]. Converting number of photons to exposure makes

a possible contribution below 0.10 MeV even smaller, since it takes

about five photons of an energy between 0.03 and 0.10 MeV to make the same contribution to exposure as one unscattered ^Co photon.

Because of experimental evidence that the scatter contribution -- beyond a certain lower distance limit -- changes only slowly with distance

from the source [1], it is justified to take the experimental scatter

spectra obtained at a distance of 1.08 m to be identical, for all practical purposes, with scatter spectra at a distance of 1.00 m.

2
To obtain the effect of a variation in field size on the contribution of scattered photons to exposure, the following expression was

used

100 f,0) 7m (x

j x< 2))

(2)\ /l00x( 2)

' mhTim

where 100 (xh^ - x|

2 b/X ^ is the difference between the contribution

of scattered photons to exposure for the larger field size (superscript 1) and the correspondi ng contribution for the smaller field

size (superscript 2) expressed in percent of total exposure for the

larger field size; 100 x^Vx^ and 00 xj^/X^ are the percent

J contributions to exposure in the jth energy interval obtained from

the measured number spectra for the two field sizes; and and are the values for the corresponding total exposure for the two fields

obtained from ionization measurements.

2.2 Deriving exposure spectra for the field sizes of interest.

The exposure spectra were computed from the measured number

spectra [1] as outlined, and smooth curves were drawn of exposure

contribution in each energy interval as a function of field size used

in the measurements. The exposure contribution for the field sizes

and the distance of interest were determined by interpolation of these

curves. No interpolation was attempted for the rectangular field.

3. Results

The results are presented in tables 1 and 2. The scatter

contribution to exposure above 0.80 MeV, which, depending on field

size, was between 1 and 5 percent, was not tabulated separately, but was added to the exposure contribution due to unscattered photons in

the 1.17- and 1.33-MeV lines. This was done because the original data were poor for energies in the vicinity of the unscattered photons, and

did not lend themselves to interpolation. This shortcoming should not

influence the practical usefulness of the presented data, since, in

this energy region, the response of most instruments submitted for

calibration does not appreciably depend on photon energy.

3
In table 1, contribution to exposure is shown per energy interval,

in percent of the total exposure at a distance of 1 m, for the different

field sizes. Also listed at the bottom for each field size is the expo- sure contribution below 0.80 MeV, in percent of the total exposure.

This contribution is entirely due to scattered photons. The uncertainty

attached to its value represents one standard deviation, compounded from

the statistical error in the count rate, the unfolding error, and the

goodness of fit between the unfolded spectra and the measured pulseheight distributions [4]. Not included are the comparatively small

systematic errors, e.g., those due to limitations in the knowledge of

the absorption coefficients. For simplicity's sake, no uncertainties

are shown for the scatter contributions in the individual energy intervals. These uncertainties vary from interval to interval, being worst

at the very low and the very high energies. For the uncertainties in

each interval, the reader is referred to the photon spectra shown in

figs. 6 through 8 of the original paper [1], in which error bars are plotted for each point.

While table 1 shows exposure spectra including scatter from the source proper, the housing, and the collimator, table 2 shows the scatter

contribution from the housing and collimator alone, (for different field

sizes), and from the source alone. Also shown, for representati ve field sizes, are the increases in the scatter contributions due to

increases in field sizes. These latter contributions are expressed in

percent of the values of exposure obtained with the larger of the two

listed field sizes.

4
Table

1.

Exposure

in

consecutive

energy

intervals

for

different

field

sizes.

The

values

are

expressed

in

percent

of

total

exposure

ata

distance

of

1.0

m. 5
Table 2. Exposure in consecutive energy intervals below 0.80 MeV, from scatter

due to the housing and collimator, and due to the source proper. The

values are expressed in percent of total exposure at a distance of 1.0 m.

Percent Exposure in Energy Interval AE

AE (MeV

)

Due to Scatter by Housing and

for Field Sizes:

Col 1 imator.

6.25 cm x 6.25 cm 12.5 cm x 12.5 cm 31 . 25 cm x 31 . 25 cm 0.10 - 0,15 0.01 % 0.01 % 0.02 %

0.15 - 0.20 0.01 0.02 0.03

0.20 - 0.25 0.00 0.01 0.04

0.25 - 0.30 0.01 0.06 0.08

0.30 - 0.35 0.04 0.13 0.15

0.35 - 0.40 0.12 0.29 0.31

0.40 - 0.45 0.23 0.45 0.47

0.45 - 0.50 0.27 0.49 0.52

0.50 - 0.55 0.23 0.43 0.46

0.55 - 0.60 0.17 0.32 0.37

0.60 - 0.65 0.12 0.25 0.30

0.65 - 0.70 0.10 0.24 0.30

0.70 - 0.75 0.09 0.23 0.31

0.75 - 0.80 0.07 0.20 0.30

0.80

£

0.10

1.5 ± 0.7 % 3.1 ± 0.6 % 3.7 ± 0.5 %

AE (MeV

)

Percent Exposure in Energy Interval AE

Increase with Increasing Field Size*, Comparing

Due to Scatter

in Source

Proper

25 cm x 25 cm

and 5 cm x 5 cm

10 cm x 10 cm and 5 cm x 5 cm

25 cm x 25 cm

and 10 cm x 10 cm 0.10 - 0.15 0.02 % 0.01 % 0.01 % 0.03 %

0.15 - 0.20 0.03 0.02 0.01 0.08

0.20 - 0.25 0.05 0.04 0.02 0.25

0.25 - 0.30 0.12 0.09 0.04 0.41

0.30 - 0.35 0.17 0.14 0.04 0.46

0.35 - 0.40 0.25 0.19 0.07 0.41

0.40 - 0.45 0.34 0.25 0.10 0.33

0.45 - 0.50 0.35 0.26 0.11 0.26

0.50 - 0.55 0.31 0.23 0.10 0.20

0.55 - 0.60 0.27 0.19 0.09 0.17

0.60 - 0.65 0.24 0.15 0.09 0.16

0.65 - 0.70 0.24 0.14 0.10 0.15

0.70 - 0.75 0.24 0.14 0.10 0.15

0.75 - 0.80 0.25 0.14 0.12 0.15

0.80

E 2.8 ± 0.6 % 2.0 ± 0.7 % 1 .0 ± 0.8 % 3.2 ± 0.6 %

0.10

Percentage referred to the exposure measured for the larger of the two field sizes.

6
4. References.

[1] Ehrlich, M., Seltzer, S. M., Bielefeld, M., Trombka, J. I.,

Spectrometry of a ^Co gamma-ray beam used for instrument calibration, Metrologia 12, 169 (1976).

[2] Hubbell, J. H., Photon cross sections, attenuation coefficients,

and energy-absorption coefficients from 10 keV to 100 GeV,

NSRDS-NBS 29, Superintendent of Documents, U.S. Government Printing

Office, Washington, D.C. 20402 (1969).

[3] See, e.g., Appendix B, ICRU Report 18, Specification of High

Activity Gamma-Ray Sources; International Commission on Radiation

Units and Measurements, Washington, D.C. (1969).

[4] See, e.g., Trombka, J. I., Schmadebeck, R. L., Nuclear Instr.

Methods 62, 253 (1968).

7
N Hi- 1 I 4 A '(• fc / II u

U.S. DEPT OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. pi BI.K AT ION OR REPORT NO.

NBSIR 76-1117

2. Gov’t Accession

No.

3. Recipient's Accession No.

4. TIT 1.1 AND SI BT IT LI

. , 60

Exposure Spectra from NBS Vertical Beam Co Gamma-Ray Sources

5. Publication Date November 1976

6. Performing Organization Code

7. AUTHOR(S)

Margarete Ehrlich and Christopher Soares

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2407120

11. Contract/Grant No.

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

Same as No . 9

13. Type of Report & Period Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

Exposure spectra at a distance of 1 meter from the NBS vertical60 beam Co gamma-ray source are presented in tabular form for field sizes

of practical interest. Also tabulated are contributions to exposure

arising from source housing and collimator, as well as from the source

proper.

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

)

60

Co gamma-ray source; collimator; exposure spectra; gamma-rays; scattered radiation;

source housing; spectrometry.

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