NBS Measurement Services:

Calibration of

Beta-Particle Radiation

Instrumentation and Sources

John S. Pruitt

Christopher G. Soares

Margarete Ehrlich

NBS

Special

Publication

250-21

U.S. Department of Commerce

National Bureau of Standards

NBS MEASUREMENT SERVICES:

CALIBRATION OF BETA-PARTICLE

RADIATION INSTRUMENTATION

AND SOURCES

John S. Pruitt

Christopher G. Soares

Margarete Ehrlich

Center for Radiation Researcli

National Measurement Laboratory

National Bureau of Standards

Gaithersburg, MD 20899

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued April 1 988
Library of Congress Catalog Card Number: 88-600514

National Bureau of Standards Special Publication 250-21

Natl. Bur. Stand. (U.S.), Spec. Publ. 250-21, 75 pages (Apr. 1988)

CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1988

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

The calibration and related measurement services of the National Bureau

of Standards are intended to assist the makers and users of precision

measuring instruments in achieving the highest possible levels of

accuracy, quality, and productivity. NBS offers over 300 different

calibration, special test, and measurement assurance services. These

services al.low customers to directly link their measurement systems to

measurement systems and standards maintained by NBS. These services

are offered to the public and private organizations alike. They are described in NBS Special Publication (SP) 250, NBS Calibration Services

Users Guide .

The Users Guide is being supplemented by a number of special

publications (designated as the "SP 250 Series") that provide a detailed description of the important features of specific NBS

calibration services. These documents provide a description of the:

(1) specifications for the service; (2) design philosophy and theory;

(3) NBS measurement system; (4) NBS operational procedures; (5)

assessment of measurement uncertainty including random and systematic

errors and an error budget; and (6) internal quality control procedures

used by NBS. These documents will present more detail than can be

given in an NBS calibration report, or than is generally allowed in

articles in scientific journals. In the past NBS has published such

information in a variety of ways. This series will help make this type

of information more readily available to the user.

This document (SP 250-21), NBS Measurements Services: Calibration of

Beta-Particle Radiation Instrumentation and Sources, by J. S. Pruitt,

C. G. Soares, and M. Ehrlich, is the twenty first to be published in

this new series of special publications. It describes the NBS

calibration facility for beta-particle instruments and sources used in

radiation-protection dosimetry for the measurement of absorbed dose to

water. Inquiries concerning the technical content of this document or the specifications for these services should be directed to the authors

or one of the technical contacts cited in SP 250.

The Center for Radiation Research (CRR) is in the process of publishing

21 documents in this SP 250 series, covering all of the calibration

services offered by CRR. A complete listing of these documents can be

found inside the back cover.

NBS would welcome suggestions on how publications such as these might

be made more useful. Suggestions are also welcome concerning the need

for new calibration services, special tests, and measurement assurance

programs

Joe D. Simmons

Acting Chief

Measurement Services

Chris E. Kuyatt

Director

Center for Radiation Research
ABSTRACT

In a project funded jointly by the National Bureau of Standards (NBS) and

the Nuclear Regulatory Commission (NRC), NBS has developed a calibration

facility for beta-particle instruments and sources used in radiationprotection dosimetry. Central to this facility are beta-particle and nearly

monoenergetic electron beams characterized in terms of absorbed-dose rates to

water and in terms of pulse-height distributions of beta-particle and electron

spectra.

This documentation utilizes material previously published or otherwise

disseminated by members of the staff of the Ionizing Radiation Division of the

Center for Radiation Research (CRR) of the National Bureau of Standards (NBS).

After a description of the facility and the procedures, results are given of

(1) studies that led to the determination of absorbed-dose rates to water for

the NBS beta-particle and nearly monoenergetic electron beams, and to the

uncertainties entering into these determinations; (2) studies of the influence

of source configuration on beta-particle and nearly monoenergetic electron

spectra; and (3) an estimate of the overall uncertainties associated with the

proposed calibration service. Included also are the results of some calibrations similar to those that will be performed under the calibration

service employing the beta-particle sources, and of samples of studies

demonstrating the benefits of using the nearly monoenergetic electron beams in

special instrument studies and in a future extension of the calibration

service.

i V
TABLE OF CONTENTS

Page

Preface 111

Abstract 1v

List of Tables v1

List of Figures viii

1. History and Description of Service 1 1.1 Quantity for Reporting Calibration Results 1 1.2 Sources and Instruments Accepted for Calibration 1 1.3 Procedures for Calibration and Internal Quality Control 1

2. The Facility for Beta-Part icle Source and Instrument Calibration 2 2.1 Rationale for Facility Choice and for Preliminary Studies 2 2.2 Facility Design 2 2.3 The Beta-Part icle Sources 3

2.4 The Extrapolation Ionization Chamber 3

2.4.1 Setup 3

2.4.2 Chamber Construction 7 2.4.3 Principle of Use 7 2.4.3.1 Measurement of Ionization Current as a Function

of Electrode-Plate Separation 7 2.4.3.2 Correcting the Measured Ionization Current 9

2.4.3.3 Computation of Absorbed-Dose Rate to Water 14

2.4.3.4 Comparison of Magnitude of Corrections for

the Three Types of NBS Sources 15

2.5 The Parallel-Plate Transfer Ionization Chamber 15

2.5.1 Description 15

2.5.2 Principle of Use 17

3. The Nearly Monoenergetic Electron Facility 17

3.1 Introduction 17

3.2 Facility Design.. 17

3.3 The Electron Accelerators 18

3.3.1 Beam Handling 18

3.3.2 Beam-Exit Window 18

3.3.3 Frequent Remote Beam Viewing 18

3.4 The Beam Monitors 18

3.5 Use of Extrapolation Chamber to Measure Absorbed Dose to Water., 20

3.5.1 Requirements of Limitation in Beam Intensity 20

3.5.2 Dose (Rate) Measurements and Study of RadiationProtection Instruments 20

4. Characterization of NBS Beta-Part icle Beams 20

4.1 Determination of Absorbed-Dose Rates to Water; International

Comparisons 20

4.2 Influence of Measurement Geometry on Source Calibration 21

4.2.1 Dependence of Dose Rate on Off-Axis Shift of Detector.... 22

4.2.2 Dependence of Dose Rate on Precision of Source-toDetector Distance Setting 22

4.3 Spectral Characterization by Means of Measurements as a Function

of Depth in Plastic 24

V
TABLE OF CONTENTS (Continued)

Page

4.4 Spectral Characterization by Means of Pulse-Height Distributions. 24

4.5 Uncertainty in the Calibration of Beta-Part icle Sources and

Transfer Ionization Chambers Submitted to NBS 31

5. Characterization of Nearly Monoenergetic Electron Beams 33

5.1 Beam Cross Section 33

5.2 Determination of Range of Absorbed-Dose Rates to Water Available

for Instrument Calibration 33

5.3 Determination of Spectral Composition of the Electron Beam 37

6. Examples of Practical Studies Performed with the Beta-Particle Beams.. 41

6.1 Compatibility of Different ^Osr+^Oy Sources with Existing

Standards « 41

6.2 Comparison of 20hji and ^Osr+^Oy Source Calibrations at NBS

and at a DOE Laboratory 47

7. Examples of Studies Performed with the Nearly Monoenergetic Electron

Beams 47

7.1 Predicting Beta-Particle Response of Instruments from

Response to Nearly Monoenergetic Electrons 47

7.2 A Study of Rate Dependence of the Response of RadiationProtection Survey Meters 47

7.3 Role of Nearly Monoenergetic Electron Sources in the Development

of a New Generation of Survey Meters 48

8. Safety Considerations 48

8.1 Radiation Hazards 48

8.2 Electrical Hazards 49

8.3 The Nearly Monoenergetic Electron Facility 49

9. References 50

10. Appendices..... 52

Appendix 1. Sample of Calibration Report Form 52

Appendix 2. Report of a Trip to a DOE Laboratory by J. S. Pruitt 54

Appendix 3. Measurement Assurance Study by Means of PTW Transfer

Ionization Chamber, Model 2047 58

Appendix 4. Predicting Beta-Particle Response of Instruments from

their Response to Nearly Monoenergetic Electrons 60

Appendix 5. Role of Nearly Monoenergetic Electron Sources in the

Development of a New Generation of Survey Meters 63

11. List of Pertinent Tables and Reports on File in the Office of the

Dosimetry Group 66

11.1 Pertaining to the Corrections Leading from Ionization Measurements with The Extrapolation Ionization Chamber to Absorbed

Dose Rate to Water 66

11.2 Pertaining to Radiation Protection 66

vi
LIST OF TABLES

Page

Table 1. Characteristics of Amersham-Buchler Beta-Part icle Sources 4

Table 2. Typical Examples for Correction Factors to Measured

Ionization Currents 11

Table 3. Values of Parameters a and B for Air Density Correction Factor 14

Table 4. Comparison of Absorbed-Dose Rates to Water, at the Surface,

Obtained by NBS and PTB; and Absorbed-Dose Rates to Water at

Depth of Interest in Radiation Protection for Three of the

NBS Sources 21

Table 5. Uncertainties in the Determination of Absorbed-Dose Rate to

Water from Measurements with the Extrapolation Ionization

Chamber 34

Table 6. Uncertainty in the Calibration of Beta-Part icle Sources and

Transfer Ionization Chambers Submitted to NBS 35

Table 7. Examples of Pertinent Electron Beam Parameters Obtained Prior

to Improvement in the Monitor System 37

Table 8. Comparison of Different Types of ^Osp+soy Sources 42

vii

1
LIST OF FIGURES

Page

Figure 1. Theoretical Beta-Part icle Spectra for the Radionuclides

Employed 5

Figure 2. Beta-Particle Calibration Facility 6

Figure 3. Extrapolation Ionization Chamber: Cross Section through the

Front Portion; Schematic Representation 8

Figure 4. Current Measurements in the Extrapolation Ionization Chantoer

as a Function of Nominal Electrode-Plate Separation 10

Figure 5. Design of PTW Paral lei -Plate Transfer Ionization Chamber 16

Figure 6. Monoenerget ic Electron Beam-Handling Arrangement 19

Figure 7. Degree of Non-Uniformity of Dose Rate along a Horizontal line

through the Reference Point, Perpendicular to the Beam Axis 23

Figure 8. Dependence of Dose Rate on Source-to-Detector Distance 23

Figure 9. Ionization Current as a Function of Depth in PTP 25

Figure 10. Attenuation Curve in Polystyrene for the 2-mCi ^^Sr+^^^Y Source.. 26

Figure 11. top: Spectrum of ^'^''Pm Source, Used with Flattening Filter

bottom: Spectrum of ^o^t] Source, Used with Flattening Filter,

Showing Influence of Lead Collimator with Circular

Aperture, 3.2 mm in Diameter 27

Figure 12. Spectrum of ^OSr+^^Y Sources, Showing Influence of Flattening

Filter and of Detector Collimator.... 28

Figure 13. Spectrum of ^OSr+^^Y Sources Used without Flattening Filter,

Showing Influence of Source-to-Detector Distance 29

Figure 14. Energy Calibration Spectrum of 20/5-1 Obtained with the 5-mm

Silicon Surface-Barrier Detector 30

Figure 15. Effect of Added Polystyrene Absorbers on ^^Sr+^^Y Spectrum 32

Figure 16. Degree of Uniformity of Cross Section of Scanned Electron

Beam over the Range of Energies and Distances of Interest 36

Figure 17. Degradation of Nominal 200-keV, 300-keV, and 400-keV Electrons

in Different Thicknesses of Polyimide Added to the 0.025-mm

Polyimide Exit Window 38

Figure 18. Degradation of Nominal 1.5-MeV and 2.5-MeV Electrons in

Different Thicknesses of Aluminum Added to the 0.09-mm

Aluminum Exit Window 39

Figure 19. Degradation in Air of Nominal 200-keV to 2.5-MeV Electrons in

Different Thicknesses of Intervening Air Layers 40

Figure 20. loni zat ion-versus-Depth Curves for Three Different Types of

90Sr+90Y Sources 43

Figure 21. Comparison of Spectrum of the Buchler-Amersham and the NVLAP

Testing Laboratory's ^^Sr+^^Y Sources, on Semi -Loga rithmic

Scale 44

Figure 22. Comparison of Spectrum of the Amersham Experimental and the

NVLAP Testing Laboratory's ^^Sr+^^Y Sources, on SemiLogarithmic Scale 45

Figure 23. Comparison of Spectrum of Amersham Experimental ^Osr+soy

Source, Bare and Shielded 46

vi i i
1 . History and Description of Service

Between 1982 and 1985, the National Bureau of Standards (NBS), with

partial support from the U.S. Nuclear Regulatory Commission, developed a facility suited to establish a service for calibrating certain user-owned

encapsulated beta-particle sources and certain laboratory-standard transfer

instruments for application in radiation protection. This work was undertaken

in response to the need of the radiation-protection community for better

calibrations of beta-particle survey instruments and dosimeters.

1.1 Quantity for Reporting Calibration Results

The physical quantity of interest in radiation protection is the absorbed

dose (rate)^ to tissue, from which the effective dose equivalent (rate) and

the pertinent practical dose-equivalent (rate) quantity (or quantities) may be

derived. For the application in radiation protection, absorbed dose (rate) to

tissue may be set equal to absorbed dose (rate) to water. Therefore, source- calibration results will be reported by NBS in terms of absorbed-dose rate to

water at a fixed source-to-detector distance, and instrument-calibration

results will be reported as calibration factors, the quotients of absorbed

dose (rate) to water and the corresponding instrument reading ("scale

indication").

1.2 Sources and Instruments Accepted for Calibration

The following beta-particle sources and instruments are accepted:

(1) Encapsulated sources of ^Osr+soy, 20471^ i'+7pm, with activities

leading to absorbed-dose rates to water ranging from a few tenths of one yGy/s to a few mGy/s, for calibration in terms of absorbed-dose

rate to water, using the NBS extrapolation ionization chamber; and

(2) thin-window paral lei -plate ionization chambers suited for use as transfer instruments for the calibration of beta-particle sources in

terms of absorbed-dose rate to water. (See also sec. 2.5.)

Acceptance testing, handling, packing, and shipping of all materials and

reporting of results are in accordance with NBS policy. For details of the

NBS calibration service for beta-particle instrumentation and sources, see Chapter VI, Ionizing Radiation Measurements, test numbers 47035C - 47036C, of

NBS Special Publication 250, The NBS Calibration Service Users Guide

1989-1991 , which is in preparation at the time of writing (February 1988).

1.3 Procedures for Calibration and Internal Quality Control

Calibrations are performed at fixed source-to-detector distances. If a beam-flattening filter is to be used with the submitted source (see sec. 2.3),

it is to be supplied with the source. The calibration uncertainty for

^Throughout this Document, when a statement is made that can refer either to a particular radiation quantity or to its rate, the name of the quantity is

followed by the word "rate," in parentheses.

1
905r+90Y and 204y] sources is reported as 3 percent, that for ^'^^Pm sources as

5 percent, and that for thin-window paral lei -plate ionization chambers as

3 percent when calibrated with the ^°Sr + ^^Y and 204-|-] sources and 5 percent

when calibrated with the ^'^'^Pm source (see sec. 4.5). Consistency of

calibration results is ensured through internal quality-control procedures

comprising (a) the calibration in terms of absorbed-dose rate to water of an

NBS beta-particle source of known activity along with the submitted unknown

source incorporating the same radionuclide, or (b) the calibration of the NBS

thin-window paral lei -plate ionization chamber along with the chamber

submitted, for calibration in at least one of the calibration beams. Calibration data on the submitted source or instrument are considered acceptable if

the data obtained on the same day for the NBS source or instrument agree to

within the statistical uncertainty (type A uncertainty) stated in table 5 of

section 4.5. A sample report form is shown in appendix 1.

2. The Facility for Beta-Particle Source and Instrument Calibration

2.1. Rationale for Facility Choice and for Preliminary Studies

One of the reasons why protection measurements for beta particles are considerably more difficult than for photons is the beta particles' vastly

greater interaction in the media intervening between the source and the point

of measurement (the "reference point"), and the resulting greater change in

beta-particle spectrum with a change in the measurement configuration — and

consequently in most instances also in detector-response characteristics. As

a consequence, calibrations of radiation-measurement instruments in betaparticle beams can be considered strictly valid only for the particular

configuration in which they were performed, and are applicable to field

measurements in similar configurations only. Conversely, knowledge of the

degree to which changes in measurement configuration may produce changes in

beam characteristics and therefore in calibration results is of considerable

importance. Therefore, prior to establishing a calibration service, the

influence on spectral characteristics of beam-flattening filters and other

absorbing materials in the beta-particle beams, and the influence of changes

in source-to-detector distance, had to be investigated. Changes in absorbeddose rates with elevation above sea level were given special attention.

Because of the strong influence of spectral characteristics on instrument

response, NBS also did preliminary measurements of the response functions of

several measuring instruments in nearly monoenerget ic electron beams over a wide range of energies. The purpose of these measurements was to determine

how well the response in any beta-particle field may be deduced from an instrument's response function obtained with monoenerget ic electrons if the

beta-particle spectrum is known at the reference point.

2.2 Facility Design

The NBS facility for carrying out beta-particle measurements for

radiation-protection purposes consists of:

(1) a set of wel 1 -characterized beta-particle sources covering a wide

energy range;

2

.i
(2) an extrapolation ionization chamber suited for the determination of

absorbed-dose rates to water at distances from the beta-particle

sources appropriate for the study of radiation-protection

i nst ruments;

(3) a rugged (although thin-walled) pa ral 1 el -pi ate ionization chamber

calibrated in the beta-particle beams, and suited for use as a transfer instrument, i.e., for establishing traceability to NBS of

the results of beta-particle measurements performed by a secondarystandards laboratory; and

(4) an electrical interlock system causing the source shutter to close

automatically when the safety perimeter (established by infrared

beams) is penetrated.

The rest of section 2 deals with a more detailed description of these systems

and with thei r use.

2.3 The Beta-Particle Sources

NBS purchased the complete Amersham-Buchler beta-particle calibration

setup, initially standardized by the Physi kal i sch-Techni sche Bundesanstalt

(PTB). The setup consists of four encapsulated beta-particle sources, a source holder on a stand equipped with a shutter, separate beam-flattening

filters for each type of source, and associated electrical circuitry for

shutter operation and timing. The beam-flattening filters are plastic discs,

thicker in the center than on the periphery for ^^sr+^Oy and ^o^+j]

^ with a central circular opening for ^^^Pm. When installed perpendicular to the beams

and concentric with the beam axis at a pre-set distance from the sources, they

filter the beams selectively so as to achieve beam cross sections of relatively constant absorbed-dose rates over an area sufficient for instrument

calibration. (See also sec. 2.4.1 and 4.2.1.) Information on the properties

of the beta-emitting radionuclides employed and on source structure and

nominal activity is given in table 1. Figure 1 shows the approximate

theoretical beta-particle spectra of the three radionuclides [1,2]2, covering

a range of average energies from 0.06 to 0.8 MeV.

2.4 The Extrapolation Ionization Chamber

2.4.1 Setup

Figure 2 is a picture of the arrangement for producing the beta-particle

fields and for determining absorbed-dose rates to water in these fields by means of measurements with an extrapolation-ionization chamber. The betaparticle sources fit into the shuttered source holder. Different beamflattening filters are employed with the different radionuclide sources. The

extrapolation ionization chamber on the left was designed by the PTB and built

for NBS by Pychlau Technical Works (PTW). The body of the chamber is made of

polymethyl methacrylate (PMMA). Its diameter is 140 mm — which makes it large

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

Document

3
Table 1. Characteristics of Amersham-Buchler Beta-Particle Sources

(a) Source Structure and Activity

Radionuclide Half Life St ructure Source Nominal Activity

rlDt) ^ \\\Kj 1 j , Ud L c

90sr+90Y 28.5 y ^"^Sr carbonate 50 mg/cm^ Ag 1850(50)

pressed*^ into

Ag foil

+0.1 mm (77 mg/cm^)

steel

May 24, 1982

90sr+90Y 28.5 y ^•^Sr carbonate 50 mg/cm^ Ag 74(2);

pressed*^ into

Ag f oi

+0.1 mm (77 mg/cm^)

steel

Dec 16, 1982

3.78 y 20'+Tl pressed^

into Ag foil

20 mg/cm^ Ag 18.5(0.5);

Dec 16, 1982

2.62 y

I'+'^Pm pressed*^

into Ag foil

5 mg/cm^ Ag 518(14);

Dec 16, 1982

(b) Beta-Particle Energies and Ranges

Radionucl ide Avg. and Max.

Beta-Pa rt icle

Energies (MeV)^

Range in

air^ polystyrene

cm mg/cm2 cm mg/cm^

90Sr^

E = 0.196 41 49

Emax = 0-546 187 225 0.19 204

SOy E = 0.935 375 452

Emax = 2.284 1037 1249 1.10 1167

2oni E = 0.244 58 70

Emax = 0-763 291 351 0.30 319

E = 0.062 5.9 7.2

Emax = 0-225 51 61 0.052 55

^From PTB Certificates. In the Buchler manual, encapsulation of the 2 mCi

90sr+90Y source is listed as 50 mg/cm^ Ag plus 1 mg/cm^ Au.

^During the rolling stage.

^Average and maximum energies are for the unmodified spectra.

^Air at reference conditions (22°C and 101.325 kPa).

^Practically none of the beta particles from ^°Sr penetrate the combined

filtration of (1) the source encapsulation, (2) the air between source and

detector, and (3) the beam-flattening filter.

4
Beta-Particle Energy, MeV

Figure 1. Theoretical Beta-Particle Spectra for the Radionuclides Employed

[2], Plotted is the number of beta particles per MeV for one transition

from the original state, against beta-particle energy in MeV,

5
Figure 2. Beta-Particle Calibration Facility. The source-support stand is

shown at the right, the extrapolation ionization chamber at the left, and a beam flattening filter supported by wires in-between. The pictured flattening

filter is for the ^'+^Pm source, and is designed for use at the pictured

source-to chamber distance of 20 cm. The source shutter is shown in the open

position. The chamber is mounted on a stand that can be moved from side to

side across a table, which in turn is mounted on a cart, with wheels running

on the tracks shown at the bottom.

6
enough to simulate a phantom of infinite size. The plate separation of the

ionization chamber is variable and can be accurately determined by means of a micrometer measurement (see screw on end of chamber at left). The thin

entrance window for the beta particles is constructed of polyethylene

terephthalate (PTP). The source shutter is electrically interlocked, with

infrared beams and photocells defining a safety perimeter as long as the

shutter is open.

The chamber is mounted on a stand that can be moved from side to side across the table, for investigating beam uniformity in the calibration plane

(which is the plane perpendicular to the beam axis at the reference distance).

The reference distances (distances from source to reference points) have been

chosen to be 20 cm for ^'*'^Pm, and 30 cm for both ^Osr+aoy and 204j], Variations in dose rate with reference distance can be studied by moving the

chamber cart along the tracks.

2.4.2 Chamber Construction

Figure 3 shows a schematic CROSS-secti on through the front end of the

extrapolation chamber, which was'' .ori gi nal ly designed by Boehm [3]. The highvoltage electrode through which incident beta rays enter the chamber is a 2.6 mg/cm^ graphite-coated PTP foil. The collecting electrode and the guard

electrode are both graphite coatings on the acrylic piston. The plate separation, d, is changed by moving the piston with the aid of a micrometer screw

(not shown),

lonization-chamber volume is taken to be the product of the area of the

collecting electrode, A = (Tr/4) (effective diameter)^, and the plate separation

(air gap), d, of the collecting electrodes. The effective diameter and

consequently the area A can be measured accurately with a traveling

microscope. Accurate determination of d is usually more difficult but can be

readily accomplished by extrapolation measurements.

2.4.3 Principle of Use

The following is a guide to the use of the extrapolation chamber for

determining absorbed-dose rate to water at the points of interest in the betaparticle beams. Covered are the steps leading from the initial ionization

measurements to the fully corrected absorbed-dose rates, including a detailed

discussion of the required corrections. While background references are given

only in special cases (e.g., for the recombination and diffusion corrections,

for scatter corrections and for the correction for measurements at high

altitudes), the various tables and reports giving the relationships used in

the derivation of the other corrections and the values of the pertinent

constants entering in these relationships are on file in the office of the

Dosimetry Group. (See sec. 11.1 for a list of documents.)

2.4.3.1 Measurement of Ionization Current as a Function of Electrode-Plate

Separation

The ionization current is obtained from a measurement with an electrometer of the charge accumulated on a calibrated capacitor after irradiation

of the extrapolation ionization chamber over a measured period of time.

7
PMMA

Brass

Aluminum

Graphite-Coated

PTP Foi

Electrodes:

Collecting

High Voltage

Guard

Fig. 3. Extrapolation Ionization Chamber: Cross Section through the Front

Portion; Schematic Representation.

The collecting and guard electrodes (graphite coating on PMMA) are

separated by a 0.2-mm wide circular groove (shown of exaggerated size). The

distance between the midpoints of the two grooves (distance between the

indicated dashed lines) defines the effective diameter of the collecting

volume. In this particular instrument, the diameter is 30 mm. The entrance

window is a 2.6 mg/cm^ PTP foil. For the measurements, the plate

separation, d, (also referred to as air gap or chamber depth) is usually

varied between about 0.5 and 2.5 mm. During any experiment the voltage

gradient across the chamber is kept constant, at a value between 10 and

40 V/mm.

8
Figure 4 shows the results of current measurements as a function of plate

separation with the three types of sources, after application of certain

corrections. These corrections are introduced in eq. (1) below, and are subsequently discussed in detail. The corrected currents vary linearly with

plate separation to a good approximation. Linearity is important since the

slopes of the curves are used in the computation of absorbed-dose rate, as seen in eq. (2), section 2.4.3.3. The average of the absolute values of the

deviations from the lines drawn, as determined from least-squares computations, are 0.03 fA for the I'+^Pm points and 0.02 fA for the 204ji and ^Osp+soy

points, and are too small to be shown in the figure.

2.4.3.2 Correcti.ng the Measured Ionization Current

The corrected ionization current, I^, is given by

K = I nc. Ilk- , (1)

where I is the measured ionization current, the quantities c^- are correction

factors relating to the use of the particular type of ionization chamber with

a particular beta-particle spectrum, and the correction factors kj relate to

environmental conditions and source properties, and are independent of the

type of ionization chamber employed. Sample values of the correction factors

for the three types of sources are given in table 2. Following is a discussion of these correction factors:

(a) CfQ-j] is required if the measurement is not carried out with a foil

of the desired thickness. For radiation-protection applications, one is

interested in absorbed-dose rates behind 7 mg/cm^ of tissue-equivalent

material — a thickness considerably larger than that of the entrance window —

which one obtains by taping additional PTP foil to the front face for the

measurements. A foil attenuation correction is required only for comparisons

with results of others who extrapolate to zero wall thickness (see, e.g.,

sec. 4.1), It is large for the relatively low-energy beta- particle spectrum

of ^'+'^Pm because the calibration distance is close to the range of a large

fraction of these beta particles. For ^°Sr+^°Y, the correction is much

smaller in magnitude and of opposite sign, indicating buildup rather than

attenuation. The correction factors were calculated from measurements of

current as a function of foil thickness as CfQ^-j = Iq/Ix* where Iq is the

ionization current in the absence of a foil (extrapolated) and l^ the current

at a foil thickness x (including the thickness of the chamber window).

(b) c^^-y, the beam-divergence correction, is made necessary by the

relatively short source-detector distance. During extrapolation measurements

the entrance foil remains at a fixed distance Dq from the source, and the

collecting electrode is at a distance Dg+d (see fig. 3). If it is assumed

that the beam intensity obeys the inverse square law over the short distance

Dg+d, it is readily shown that the correction for beam divergence is given by

^div ~ 1 <^Yd, where d is the plate separation in millimeters and yq = 1/Dq

= 0.005/mm for I'+^Pm and 0.0033/mm for both 20^71 and ^Osr+goy (Data Book

873/36).

9
<

20

LJ

q:

o

I A 1

1 1 1 UJ

10

<

o

UJ

h-O

LlI (T

CC

o 0

o

PLATE SEPARATION, mm

Fig. 4. Current Measurements in the Extrapolation Ionization Chamber as a Function of Nominal Electrode-Plate Separation (Micrometer Reading of

Chamber Depth). The curves shown are referred to as extrapolation curves

since they may be used to determine ionization currents for close-to-zero

chamber depths (air gaps). The extrapolation curves go through the origin

only when the micrometer readings coincide with the true chamber depths. If

they do not, true chamber depths may be read off the graph after the origin

is shifted to the intersection of the curves with the "plate separation"

axis.

10
Table 2, Typical Examples for Correction Factors to Measured Ionization

Currents (from Data Books 863 and 868)

Value of Correction Factor for

Symbol Correction to Reference Conditions 2oni 90sr+90Y

A. Chamber Correction Factors

^foil

entrance foil attenuation^ 1.76 1.000 0.982

di V beam divergence*^ 1.013 1.008 1.008

^atten chant)er air attenuation^ 1.023 1.000 1.000

c.t ,p density of air in chanter volume 1.014 1.013 1.007

^back

chamber back scatter 1.010 1.009 1.006

SI de

chamber side scatter^ 1.005 1.002 0.992

^recom

ion recombination and di f f usi on*^ 1.001 1.001 1.001

^phot photon contribution^ 0.989 1.000 0.999

B, Environmental Correction Factors

dec source decay 1.213 1.145 1.059

k mass altitude variations

close to sea level 0.941 0.998 1.001

at ~ 1500 m above sea level^ 2.05 1.002 0.984

'^hum humidity changes 1.001 1.000 1.000

^Factor to correct from a depth of 2.6 mg/cm^ of PIP, the chairber foil

thickness, to zero depth.

^Corrections computed for plate separations of 2.5 mm.

^Corrections computed for applied potentials of 100 V.

^Correction factor required when a source that had been calibrated at sea level is to be used at the indicated high altitude.

11
(c) ^atten' correction for attenuation in the chamber air gap, was determined by measuring chamber current as a function of distance y between

source and chamber using a fixed air gap d = 0.5 mm. It was found that, after

multiplying the current I by the square of the distance y, in order to compensate for beam divergence, the product ly^ could be represented as a linear

function of the distance y. It is then readily shown that the correction for

air attenuation in the chamber is given by Cg|.^gp = 1 + dy/^, where d is in

millimeters and

ya

is 0.009/mm for ^'*^Pm, and zero for ^^^1} and ^Osr+goy

(Data Book 873/36).

(d) c^ p, the correction for air density changes inside the chamber,

arises because of temperature and pressure variations of the ambient air,

since the chamber is of a type that is open to the atmosphere. It corrects to

the reference temperature and pressure using the ideal -gas law. The

correction factor is given by p/p^ = c^ p

= [101.325 (t + 273. 15) ]/295. 15 p,

where p/pq is the ratio of the air density during the measurement of the

ionization current to the density of dry air at the reference temperature and

pressure, t is the temperature in degrees Celsius, and the pressure p is in

ki 1 opascal s.

(e) ^back» backscatter correction, takes into account the difference

between backscatter from water and backscatter from PMMA, the collecting

electrode material. The correction factor can be evaluated from published

backscatter probabilities [4] as ^\)Qr\( = 1 +

yb> where yb is 0.010 for ^"^^Pm,

0.009 for 20^T1, and 0.006 for ^OSr+^Oy.

(f) ^side» sidescatter correction, has been measured and published by

the PTB for this particular type of chamber, using the reference distances

specified by the source manufacturer. [5] The correction is small (necessary

only when an accuracy of better than 1 percent is required). Therefore, it was decided to use the published value for the correction factor, given by

^side " 1'^'''S' whereys

'•s tabulated below for the three types of sources and for selected plate separations, d:

Source d (mm) Ys (mm-i)

0.5-2.5 0.0019

0.5 -0.0016

1.0 -0.0016

1.5 -0.0012

2.0 -0.0003

2.5 +0.0006

90sr+90Y 0.5-2.5 -0.0032

^101. 325 kPa = 1013.25 mbar = 760 mmHg = one standard atmosphere.

12
(g) ^recom' correction for ion recombination and diffusion in the air

inside the chamber, is usually quite small for protection-level measurements.

It only becomes appreciable for large ion currents and/or small voltages. It was calculated by means of published relationships [6] as Cp^j-Q^^ = (RyRiRdf)"^

where the volume recombination = 1 - [5.05 x 10^^ Ic|3/(av^)j, the initial

recombination R^ = 1 - [4.4 d/V], and the diffusion R^j^ = 1 - [(17.24 x IQ-^)

(t + 273.15)/V]. In these expressions, I is the current in amperes, d the

plate separation in meters, A the area of the collecting plates in square

meters (or the beam cross section at the reference point, in square meters, if

it is smaller than the plate area), V the potential in volts, .and t the

temperature in degrees Celsius.

(h) ^phot» correction for contributions to the ionization current due

to photons from material intervening between the source and the chamber air

volume (chamber window plus added filtration, if any, of total thickness x),

may be obtained from an ionization current-versus-depth curve in the

particular material as Cp^^^ = 1 - I(Xp^Q^ )/I(x) , where I(Xphot)

corrected ionization current measured at a depth Xp^Q^ whicn is equal to or slightly greater than the maximum range of the beta particles, and I(x) is the

current measured at the depth x of interest. Values of Xp^^^ for which

I(Xp^Q^) were measured in PTP or polystyrene for the three types of sources

are shown below (Data Book 873/39):

Source

'^phot

^'"^/cm^)

46 (Measured in PTP)

204J1 320 (Measured in polystyrene)

90sr+90Y 1370 (Measured in polystyrene)

(i) '^dec» correction for source decay, depends both on the time

interval At and on the half life of the radionuclide. It is given by

'^dec ~ exp(XAt), where At is taken in the sense measurement date minus

reference date (in days), and the decay constant X in reciprocal days is given

by 7.234 x lO""* for ^''^Pm, 5.022 x 10"'* for 20'+xi, and 6.659 x 10-5 for

90Sr+90Y.

(j) kjj^ggg is a correction for a variation in the total mass of air

between source and phantom, caused by variations in air temperature and

barometric pressure. While the correction is largest at high altitudes (say

1500 to 2000 m above sea level), it may also be appreciable at sea level under

certain weather conditions. The total mass correction was studied in an environmental chamber in which the air pressure could be varied at will. [8]

It was found that the effect was largest for ^"^^Pm and much smaller for both

^O'+Tl and ^OSr+^^Y. The correction factor is given by k^^ggg = ( l+ah+gh2)-i

where h = 1 - (p/pg)* p/pq being equal to (c^. p)"^» the temperature/pressure

13
correction factor (see correction (d)), and a and Bare constants. Table 3

shows empirical values for a and 3 for all three types of sources, with the

beams attenuated by selected thicknesses of PTP. These corrections were checked by actual measurements on a ^'*^Pm source at an elevation of ~ 1500 m above sea level (see Appendix 2). The absorbed-dose rate measured at this

altitude and then corrected to the altitude of the PTB was found to agree to

within 3 percent with the dose rate that had been measured for this source at

the PTB, which is located near sea level. (See publication [8] for a fuller

discussi on.

Table 3. Values of Parameters a and g for Air Density Correction Factor

Source with

flattening filter

Depth in PTP

(mq/cm^)

a B

A'+/Pm 2.6 4.5 12.7

6.3 3.5 19.8

10.1 0.69 31.2

2.6 0.165 -0.50

6.3 0.131 -0.33

46.2 0.143 0.48

90sr+90Y 2.6 -0.085 -0.027

6.3 -0.054 -0.18

46.2 0.040 -0.28

(k)

'^hurn' correction for relative humidity, arises from the fact that

the air mass correction is a function of air density and air density depends

to a small extent on the relative humidity of the air. The correction is

usually quite small and is required only for ^"^^Pm, for which it is given

approximately by k^^^^^ = [exp(4.40 x lO"** H)]/1.020, where H is the relative

humidity, in percent (Buchler Source Manual).

2,4.3,3 Computation of Absorbed-Dose Rate to Water

The absorbed-dose rate to water on the reference date — in grays per

second — is calculated from the slope Al^-ZAd of the extrapolation curve by

Bragg-Gray cavity theory [7] as

6 = (W/e) S^f^^' (Al^/Ad) [l/(poA)] , (2)

14
where W/e — energy required to produce an ion pair in air, in joules per

coulomb

^air^"" " '^^^''O* average mass stopping power of water to that of dry air,

dimensionless

pQ — density of dry air at reference ambient conditions (22°C and

101.325 kPa), in kilograms per cubic meter;

A — area of collecting electrode, in square meters;

Al^/Ad — slope of extrapolation curve, in amperes per meter.

The slope Al^/Ad is determined from several independent current measurements carried out on several days. Reproducibility of the results was found

to be independent of whether they were obtained from measurements done on any

given day or on several days.

2.4.3.4 Comparison of Magnitude of Corrections for the Three Types of NBS

Sources

Table 2 shows that the corrections for ^OSr+^^Y are mostly insignificant.

At sea level, only two of them are larger than one percent, and of these, the

correction for source decay can be calculated easily and accurately, and that

for entrance foil attenuation is usually not required. For ^'+^Pm, the situation is worse. Eight of the eleven corrections are greater than or equal to

one percent, and of these eight, two are larger than ten percent. By far the

largest correction for ^'^''Pm is the one for entrance-foil attenuation, which

however is usually not required. (See sec. 2.4.3.2.) It may be obtained with

a high degree of accuracy since the dependence of current on absorber depth seems to be well represented by an exponential function. As could be

expected, the magnitude of most of the corrections for 20i4j-| y^g^g found to be

somewhat larger than for ^^Sr+^^Y, but considerably smaller than for ^^^Pm.

2.5 The Paral lei -Plate Transfer Ionization Chamber

2.5.1 Descri pt i on

This is a rugged paral lei -plate ionization chamber manufactured by PTW

(PTW model 2047) for use in checking the x-ray exposure rate at a radiographic

intensifying screen or image intensifier. It has a relatively large fixed

volume (about 120 cm^) and thin PTP windows (nominal thickness: 0.05 mm),

making beta-particle measurements from roughly 300 yGy/h to more than

30 mGy/h possible. Figure 5 shows a schematic drawing of the chamber. The

solid aluminum body of this chamber makes it possible to ship it to other

institutions in measurement -assurance studies designed to compare measurements

of ionization currents at other institutions with those of NBS, in identical

geometries. At the reference distance, the beta-particle beam has a sufficiently large cross section to strike the aluminum body of the chamber. But as long as measurement geometry is the same at NBS and the other institutions,

the fact that electron scatter from the solid aluminum body contributes to the

current measurements does not interfere with these studies.

15
->

1

in

<-

14.5mm 1 o 13 mm

t 1 1

Fig. 5. Design of PTW Para1 lei -Plate Transfer Ionization Chamber. With a potential of 100 V across the chamber, absorbed-dose rates to water from

roughly 0.3 to over 30 mGy/h (0.5 to over 50 mrad/min) can be measured.

During transport, the 0.05-mm front and rear windows are covered with PMMA,

about 1 mm in thickness. The diameter of the internal collecting electrode

is 12.4 cm.

16
2.5.2 Principle of Use

For measurements, the front surface of the chamber is placed in the

calibration plane, after removal of the extrapolation chamber. Ionization

current is measured with an electrometer. The calibration factor of the

transfer chamber is then given by the quotient of absorbed-dose rate

determined at the reference point from extrapolation-chamber measurements,

and ionization current corrected to reference values of temperature and

pressure of the air inside the chamber.

3. The Nearly Monoenerget ic Electron Facility

3.1 Introduction

Studies of the response of instruments in beta-particle beams of

different spectral characteristics aid in judging the relative merit of the

instruments for certain field applications. But because of the strong

influence of spectral characteristics on instrument response, it is important

to have the capability of measuring instrument-response functions with essen- tially monoenerget ic electron beams, from which the response in any

beta-particle field may be deduced if the beta-particle spectrum is known at

the reference point. Also, because the response of some survey instruments to

beta particles is rate dependent (see, e.g., sec. 7.3), it is important to

have means for performing rate-dependence studies on instruments over a wide

range of absorbed-dose rates without a change in the spectrum — which is

difficult to do in beta-particle beams. This is why NBS has adapted two of

its electron accelerators to supplement — and in part replace — the function

of the beta-particle sources for the study of the properties of beta-radiation

protection instruments.

3.2 Facility Design [9-11]

The facility consists of:

(1) Two Electron Accelerators, Equipped with Beam Scanners . The NBS

500-keV cascaded-rect i f ier and 4-MeV Van de Graaff accelerators were adapted

for use in the calibration of radiation-protection instruments with electrons

in the range from 0.2 to 2.5 MeV by (a) magnetic scanning of the electron

beams in a two-dimensional raster pattern in order to produce suitably large

beam cross sections; and (b) having the beams exit from the vacuum through

thin, low-atomic number windows. In order to prevent unnecessary spectral

degradation the air path is kept as short as possible.

(2) Beam Monitors . Because of spatial and intensity fluctuations of the

electron beams, continuous beam monitoring is required as a part of the

routine measurement procedure. Four fast plastic-scintillator-plusphotomult ipl i er detectors placed about the beam periphery are used for this

purpose.

(3) An Extrapolation Ionization Chamber . A chamber of essentially

identical design to that employed in the beta-particle dosimetry setup is used

for the determination of absorbed-dose (rate) to water in the

accelerator-produced electron beams.

17
Operation of the facility is described in some detail in the subsequent

parts of section 3. For general safety precautions in the operation of

electron accelerators see the pertinent literature listed in section 11, which

is on file in the office of the Dosimetry Group.

3.3 The Electron Accelerators

The two electron accelerators have been owned, maintained, and operated

by the Ionizing Radiation Division of the Center for Radiation Research for a considerable length of time. Therefore, discussions in this Document will be

limited to special features added for their use in calibrations and other

studies related to radiation-protection dosimetry,

3.3.1 Beam Handling

One of these features is concerned with beam handling. The beam handling

arrangement is shown schematically in figure 6, Aligned and focused electron

beams of energies from 200 keV to 2.5 MeV are passed through a set of

deflection coils which cause the beam to be scanned in a two-dimensional

raster pattern. The scan signal has a horizontal frequency of 416 Hz and a vertical frequency of 12,5 Hz; the amplitudes in the two directions are independently variable.

3.3.2 Beam-Exit Window

Another feature concerns the choice of beam-exit windows. These windows

must be thick enough to withstand atmospheric pressure over a IG-cm^ area, yet

thin enough not to degrade the electron beam energy appreciably. A thin foil

(25 pm) of commercially available polyimide is well suited for electron

energies up to 500 keV. At higher energies, for which energy degradation does

not pose a large problem, either the same type of foil or a 0.10-mm aluminum

foil is used.

3.3.3 Frequent Remote Beam Viewing

Because of the interference of fluctuations in beam location ("beam

wandering") with the reproducible use of the beams for calibration purposes,

the electron-beam position is checked frequently, especially at the lower

energies. This is done by viewing the beam remotely on a light-emitting

phosphor screen placed at the exit window. The unscanned beam is centered on the exit window and the scanning-signal amplitudes are adjusted to spread the

scanned beam over the window area.

3.4 The Beam Monitors

At both accelerators, beam intensity is continuously monitored with four

fast plastic-scintillator-plus-photomultiplier detectors fixed in off -axis

locations (60° off-axis and 20 cm from the exit window at the low-energy

accelerator, and 30° off-axis and 40 cm from the exit window at the

Van de Graaff accelerator). In order to limit counting rates, the monitors

are equipped with plastic collimators with circular apertures of various

sizes, depending on the sensitivity desired, located near the front surface of

the scintillator. The fast (10-ns) pulses from the detector allow counting

rates up to lO^/s with no significant pile-up and with very low background

18
Figure 6. Monoenergetic Electron Beam-Handling Arrangement.

19
rates (less than 2/s). When only one monitor was used, the magnitude of beam

wandering and fluctuations in beam intensity were large enough to limit

dosimetry reproducibility to about 15 percent when the beam was turned off

between individual measurements. Therefore, the four plastic-sci nti 1 latorpl us-photomult ipl i er monitors, fixed in a circular drray about the beam axis, were installed to replace the single monitor. Their signals are being fed

back to additional beam-steering elements for dynamic control of beam

location. With this monitoring control, measurement reproducibility is better

than 5 percent.

3.5 Use of Extrapolation Chamber to Measure Absorbed Dose to Water

Because basic construction and principle of operation of the extrapolation ionization chamber has been discussed in some detail in section 2.4, this

section is limited to features of operation special to the application in

accelerator beams.

3.5.1 Requirements of Limitation in Beam Intensity

The particular type of ext rapolation-ionization chamber was designed

mainly for operation with relatively low-activity radionuclide sources, as a rule producing ionization currents not exceeding a few nanoamperes. When beam

intensities are such that about 1 yA is exceeded, strong insulator leakage

prevents the use of the chamber for at least one day. Around accelerators, it

is difficult to avoid at all times beam intensities that produce large ionization currents in the chamber. When the accelerator beam is operated without a beam shutter, excessive chamber currents may occur during beam tuning. For

this reason, the chamber is removed from the beam for the time of beam viewing

and adjustment. Even with this precaution ~ and even when a shutter is available—excessive chamber currents cannot be entirely ruled out since current

surges may occur during actual irradiations, i.e., with the shutter in the

"open" position. For this reason, NBS has two essentially identical

extrapolation ionization chambers — one of them for use solely in calibration

work with radionuclide sources, and one for use in accelerator beams.

3.5.2 Dose (Rate) Measurements and Study of Radiation-Protection Instruments

Because of fluctuations in accelerator current, a ratio must be

established between the ionization current in the extrapolation ionization

chamber at the reference point and the indication of the monitor(s). Only if

this ratio is constant to within an acceptable limit, can the extrapolation

chamber be replaced by the instrument to be calibrated, and a calibration

factor obtained for the instrument. Constancy of the ratio is checked before

and arter each instrument -calibration run. With the new monitor setup, it is

possible to maintain the monitor reading sufficiently constant to yield the

calibration factor with adequate reproducibility.

4. Characterization of NBS Beta-Particle Beams

4.1 Determination of Absorbed-Dose Rates to Water; International Comparisons

Absorbed-dose rates to air and water were determined for the four sources purchased from Buchler-Amersham at the reference distances from measurements

of ionization currents in the ext rapolation-ionization chamber. The methods

20
used were those described in detail in section 2.4.3. The results are shown

in table 4. Since the sources had been initially calibrated at the PTB, it was possible to compare the NBS calibrations with those obtained at the PTB in

a similar geometry at the same reference distances. Table 4 shows a comparison for absorbed-dose rates to water. The results agree to 1 percent

or better for the ^^Sr+^^Y source and to 2 percent for ^^^1]. A 4-percent

disagreement for ^'^'^Pm is not surprising, considering the size of the overall

uncertainty in the measurement of the ionization current (see table 5, sec. 4.5) and the relatively strong dependence of the measurement results on calibration geometry (see sec. 4.2 and 4.3 below).

4.2 Influence of Measurement Geometry on Source Calibration

In order to arrive at the uncertainty to be expected in NBS source calibrations, measurements were made with the NBS sources of the variation of

dose rate with source-to-detector distance and with detector alignment.

Following is a discussion of the results of these measurements made with the

extrapolation ionization chamber as the detector.

Table 4. Comparison of Absorbed-Dose Rates to Water, at the Surface, Obtained

by NBS and PTB; and Absorbed-Dose Rates to Water at Depth of

Interest in Radiation Protection for Three of the NBS Sources.

Source

Sourceto-detector

distance

(cm)

Absorbed Dose Rate^

Type

Nomi nal

act i vity

(mCi)

to water, at the surface

at

of

to water

a depth

7 mg/cn\^

(yGy/s)

NBS

(y6y/s)

PTB

(pGy/s)

Ratio

NBS/PTB

14 20 0.267 0.257 1.04 0.06

20m 0.5 30 0.341 0.333 1.02 0.33

90sr+90Y 2 30 1.93 1.90 1.01 2.01

II 50 11 513 506 1.01

II II 30 70.6 69.9 1.01

II II 50 25.2 25.1 1.00

^Referred to January 1, 1983 and 20°C at 1 standard atmosphere (101.325 kPa).

'^From column 4 (NBS data).

21
4.2.1 Dependence of Dose Rate on Off-Axis Shift of Detector

For this study, the detector was moved horizontally within the calibration plane so as to cover the entire beam cross section. Typical results are shown in figure 7. Along the horizontal line passing through the reference

point, the points of equal dose rate are seen to be distributed symmetrically

about the beam center for ^^Sr+^^Y and for ^o^j]

^ (jQgg rates varying by

less than ± 0.5 percent and ± 2 percent, respectively, for points not more than 10 cm from the reference point. The troughs in the center are to be

expected because of the presence of the flattening filters.

For ^'^"^Pm the dose-rate distribution is asymmetric, the variation in dose

rate amounting to about ± 10 percent for points up to 10 cm removed from the

central beam axis (± 6% for points removed by not more than ~ 8 cm). Measurements with the shutter removed demonstrated that this asymmetry is caused by

the shutter. When the shutter is in the open position, it is located on the

side of lower dose rate. Presumably the relatively low-energy beta particles

of ^'+"^Pm are scattered at short distances at a wide enough angle to impinge on the open shutter, even though it is mounted close to the source, and this

results in a strong deficit of electrons on that side. The higher energy

electrons of the other sources are presumably not scattered sufficiently at

short distances to be significantly attenuated by the open shutter.

4.2.2 Dependence of Dose Rate on Precision of Source-to-Detector Distance

Setting

Source-to-detector distance was increased and decreased by up to about

3 cm from the reference distance selected for the calibration. Figure 8 shows

that positioning of the detector is not too critical for any but the ^'*^Pm

source, for which a change in distance by 5 mm causes a change in dose rate by

close to 10 percent. As a consequence, there also will be a significant

dose-rate gradient over the depth of a transfer instrument calibrated in the

^'^'^Pm beta-particle beam if this depth extends over more than just a few

mi 1 1 i meters

In figure 8, the dose rates have been multiplied by the square of the

reference distance so that a horizontal line in this graph represents a dependence on the inverse-square of the distance. It can be seen that none of

the sources shows such a dependence. For ^^Sr+^^Y and ^o^j] ^ ^he deviations

from the inverse-square dependence probably are caused by a combination of the

angular spread of the scattered-electron buildup and the use of flattening

filters. The strong variation exhibited by I'+^Pm is due to a large fraction

of the ^^'^Pm beta particles having ranges comparable to the reference

distance.

22
10 -5 0 5 10 DISTANCE FROM BEAM AXIS, cm

15

Fig. 7. Degree of Non-Uni formi ty of Dose Rate along a Horizontal Line through

the Reference Point, Perpendicular to the Beam Axis. Reference

distance: 30 cm for ^osr + soy and 20kji^ 20 cm for ^'^'^Pm,

DISTANCE

-2-10123

FROM CALIBRATION PLANE, cm

Figure 8. Dependence of Dose Rate on Source-to-Detector Distance.

23
4.3 Spectral Characterization by Means of Measurements as a Function of Depth

in Plastic

Measurements were made of ionization current in the extrapolation ionization chancer as a function of thickness of polyethylene terephthalate (PTP)

added to the 2.6-mg/cm2 window of the chamber. The results are shown in

figure 9 for all three types of beta-particle sources. For ^'^Pm, the

decrease in current with PTP thickness is roughly exponential from the onset,

and absorption is essentially complete after an addition of ~ 20 mg/cm^ of

PTP. For larger thicknesses, the ionization current remains at ~ 1 percent of

the maximum current, and probably is due to a combination of bremsst rahlung

produced by the ^'^^Pm beta particles in the PTP and 121-keV gamma rays from

I'+'^Pm. For ^°Sr+^°Y, there is an ionization buildup over the first ~ 40 mg/cm^ of added PTP before attenuation starts to dominate, while for

^O'+Tl the attenuation in PTP seems partially offset by buildup over the first ~ 15 mg/cm2 of PTP.

Because PTP is not readily adaptable to measurements at larger thicknesses, attenuation curves for ^Osn+soy were also obtained with added polystyrene, covering a range of thicknesses from ~ 100 mg/cm^ to beyond 2 g/cm^.

The results are shown in figure 10, demonstrating that any bremsst rahl ung

background (i onizati on-current contribution by the source for polystyrene

thicknesses beyond the beta-particle range) is less than 0.06 percent.

4.4 Spectral Characterization by Means of Pulse-Height Distributions

Because of the emphasis on the change in spectral characteristics with

changes in certain experimental parameters rather than on spectral characteristics per se, these studies were confined to a comparison of pulse-height

distributions, measured with a silicon surface-barrier detector and converted

to an absolute energy scale, ignoring certain spectral distortions introduced

by the detector. Figures 11 through 13 show these spectral distributions for

the three types of sources. (No difference was found between the spectra of

the two 90Sr+^°Y sources used in the same geometry.) The spectra were obtained with a "S-mm deep silicon surface-barrier detector operated at room temperature, in air, under a cover of 0.013 mm of aluminized PTP for light

protection. A low-scatter 2073-j point source (NBS Standard Reference Material

4240) was used for the energy calibration (pulse-height-to-energy conversion).

The pulse-height distribution obtained with this source using the surfacebarrier detector is shown in figure 14. For all but the I'+^Pm source, a lead

collimator with either a 1.6-mm or a 3.2-mm diameter circular aperture was employed near the detector, since collimation was found to remove a considerable amount of scatter (see fig. 11 and 12, bottom). Following is a detailed

discussion of the results for the ^^Sr+^^Y sources:

(a) Influence of Flattening Filter on ^QSr+^°Y Pulse-Height Distribution . Figure 12 shows that the flattening filter causes a shift in the energy

endpoint. This should be kept in mind when the 80-MBq source (used with

flattening filter) is replaced by the 2000-MBq source (used without flattening

filter) in the course of an instrument calibration.

24
Figure 9. Ionization Current as a Function of Depth in PTP.

25
0 800 1600 ^ 2400

DEPTH IN POLYSTYRENE, mg/cm^

Figure 10. Attenuation Curve in Polystyrene for the 2-mCi ^osr+soy Source.

26
ENERGY. HaV

Figure 11. top: Spectrum of I'+^'Pm Source, Used with Flattening Filter.

Distance: 20 cm. Detector uncol 1 i mated,

bottom: Spectrum of 20i+j-i source. Used with Flattening Filter,

Showing Influence of Lead Collimator with Circular

Aperture, 3.2 mm in Diameter. Distance: 30 cm.

27
1000

distance: 30 cm aperture: 1 .6 nm

ENERGY. lleV

ENERGY. HcV

Figure 12. Spectrum of ^Osp+soy Sources, Showing Influence of Flattening

Filter and of Detector Collimator. The theoretical spectrum of is shown

for comparison,

28
_ ;ji distance: 50 cm

ENEKGY, HeV

Figure 13. Spectrum of ^Osr+soy Sources Used without Flattening Filter,

Showing Influence of Source-to-Detector Distance. Detector collimator with

circular aperture, 1.6-mm in diameter.

29
UJ

z

z

<

X

o

a:

UJ

Q.

CO

3

o

o

10' z—r—r~n—r—i—i—i—-r—i

-]—i

t—\—r—

0.98 MeV

10^

10'

1—\—

T—I—I—I—

/1. 05 MeV

J L«J I I I I I L ' ' ' ' ' L 111 , J L 1.0 2.0

ELECTRON ENERGY, MeV

Figure 14. Energy Calibration Spectrum of 20731 Obtained with the 5-mm Silicon

Surface-Barrier Detector. Assignment of pulse-height peaks to known

conversion-electron energies below 1.1 MeV [12] took into account energy

losses in the source covering, in the detector covering, and in the 2.54 cm

of air between source and detector.

30
(b) Influence of source-to-detector distance for ^QSr+^Qy . Figure 13

shows the spectral distributions at the three distances specified by the source manufacturer. The effect of distance is seen to be relatively small.

Therefore, a change in distance, necessitated by considerations either of

required absorbed-dose rate or of required size of uniform beam cross section,

is not expected to introduce an appreciable change in i nst rument-cal ibrati on results.

(c) Comparison of the measured pulse-height distribution with theory . For this comparison, one must consider (1) that the theoretical spectrum shown

in figure 12 was computed for a bare ^^Y source; and (2) that the measured

spectral distribution is for a ^^Sr+^^Y source encapsulated in 50 mg/cm^ of

silver and 0.1 mm (77 mg/cm^) of iron, at a distance of 50 cm (61 mg/cm^) in

air, with a layer of about 1 mg/cm^ of aluminized PTP over the detector, or a composite total of roughly 190 mg/cm^. But for the average and maximum

beta-particle energies given in table 1, one arrives at respective approximate

average and maximum ranges for the ^°Sr beta particles of 60 and 270 mg/cm^.

This means that there are relatively few ^^Sr beta particles reaching the

detector, making the measured pulse-height distribution mainly that of ^°Y,

slowed down by the intervening absorbers. Figure 12 (bottom) shows that the

general shape of the pulse-height distribution obtained with the collimated

detector compares relatively well with the theoretical ^^Y spectrum. The peak

is roughly at the same location. The high-energy endpoint of the pulse-height

distribution is at a lower energy, which is qualitatively compatible with the

energy loss in the intervening absorbers. The difference in shape in the

low-energy region may be the result of a composite of the residual ^^Sr beta

particles reaching the detector (see figure 2 for the theoretical ^^Sr+^^Y

spectrum) and of the distortions introduced by detector-edge and encapsulation

effects, for which no corrections were made.

(d) Change in spectrum with depth of absorber . Figure 15 shows the

change of the ^"Sr+^^Y spectrum with added thickness of polystyrene absorber,

in a semi -logarithmic representation which enhances the visibility of the

effect. The results are of interest since shallow and deep absorbed-dose

rates to water (or plastic) of the type shown in figure 10 often are obtained

from measurements with paral lei -plate ionization chambers whose response

depends on spectral composition. There is seen to be a radical change in the

spectrum with increasing polystyrene thickness, the thickest shown

(0.58 g/cm^) corresponding to a reduction by a factor of ten in the measured

ionization current. These results emphasize the need to know the response

function of the ionization chamber employed for such measurements, over a wide

energy range.

4.5 Uncertainty in the Calibration of Beta-Particl e Sources and Transfer

Ionization Chambers Submitted to NBS

The method of uncertainty assessment used here follows the recommendation

of the Comite International des Poids et Mesures (CIPM) [13]. The uncertainty

estimates are of two kinds. 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 extensive experience of the calibration staff. The Type B uncertainties

are estimated so as to have roughly the character of standard deviations.

31
Figure 15. Effect of Added Polystyrene Absorbers on sosr+^oy Spectrum.

Detector collimator with circular aperture, 1.6 mm in diameter.

32
The estimates of each type are combined in quadrature, and then the two

results are combined in quadrature to give a combined uncertainty. The

confined uncertainty in turn is multiplied by two (or three), to give an overall uncertainty.

Table 5 gives a breakdown of the uncertainties in the determination of

absorbed-dose rate to water at the 7 mg/cm^ depth of interest from the ionization current measured with the extrapolation chamber. The data in this table

are based on the detailed presentation of measurement procedures in

section 2.4.3. The uncertainty in the determination of absorbed-dose rate to

water and the results presented in section 4.2 on the influence of measurement

geometry on source calibration then are used in table 6 to arrive at combined

uncertainties in the calibration of beta-particle sources and transfer

ionization chambers submitted to NBS. The overall uncertainty is quoted as two times the combined uncertainty.

5. Characterization of Nearly Monoenergetic Electron Beams

5.1 Beam Cross Section

The degree of uniformity of the electron-beam cross section was determined radi ographical ly over a range of electron energies and distances from

the exit window. Industrial x-ray film was used, in light-opaque paper

envelopes for electron energies of 300 keV and above, and bare in a darkened

room for lower energies. Densitomet ric scans were obtained of the optical

densities of the radiographs along two perpendicular diameters of the essen- tially circular images of the beam cross sections. Figure 16 shows plots of

the Gaussian fits to representative data obtained for several electron

energies from 200 keV to 2.5 MeV at several distances from the exit window.

Plots of this type permit a choice of distance from the exit window suitable

for covering an instrument of a given size with a beam of a cross section of a pre-determined degree of uniformity. A synopsis of typical results prior to

the improvement in the monitoring system is given in the first three columns

of table 7 where usable field size is shown over the covered energy range.

5.2 Determination of Range of Absorbed-Dose Rates to Water Available for

Instrument Calibration

The method of using the extrapolation chamber in conjunction with a beam

monitor for obtaining monitor indication per unit of absorbed-dose rate to

water (the ratio R) was described in section 3. Because the beam properties

were not sufficiently reproducible when the machine was turned off between

measurements, it was not possible to measure definitive values for R in the

NBS setup prior to installation of the new beam-monitoring and control system.

All that could be done was to establish an approximate range of the absorbeddose rates that were attainable with the initial monitor setup at each of the

chosen accelerator potentials and exit window-to-detector distances. The

results are shown in the last two columns of table 7. The second-to-last

column of the table represents the dose obtained in 10 seconds at a nominal

monitor counting rate of 20 per second, which was the minimum rate that could

be monitored with a sufficient degree of accuracy. The maximum dose rates

33
Table 5. Uncertainties in the Determination of Absorbed-Dose Rate to Water

from Measurements with the Extrapolation Ionization Chamber

Parameter SyrTt)ol Percent Uncertainty

ype A'

204T1 90Sr+90Y

Type b'^

i'^'Pm 90Sr+90Y

A. Corrected Ionization Current (Extrapolation Chamber) Ionization current measured

at a depth of 7 mg/cm^ in PTP I 0.4 0.3 0.1 0.1 0.1 Correction for: Entrance foil thickness*"

"^foil 1.5 0.2 attenuat i on*^ 1.0 0.1 Beam Divergence c div 0.05 0.05 Chanter air attenuation c atten

0.5 0.05 Chamber air not at reference conditions

^t.p

0.1 0.1 Chamber backscatter r back

c . . side

r recom

dec

0.3 0.3 Chamber sidescatter 0.2 0.2

Ion recombination and diffusion 0.3 0.3 Source decay 0.05 0.05

External air mass

near sea level

k mass

1,0 0.5

flf hiGh flltitiifip^ 3.0 1.0 Relative humidity k hum

0.1 0.05

Corrected ionization current at a depth of 7 mg/cm2 in PTP^

c near sea level 0.4 0.3 0.1 1.9 0.7

at high altitudes 0.4 0.3 0.1 3.4 1.1

B. Absorbed-Dose (late to Water at a Depth of 7 mg/cm^

Corrected ionization current per

unit air gap (slope) Al^/6d

near sea level 0.4 0.3 0.1 1.9 0.7

at high altitudes 0.4 0.3 0.1 3.4 1.1 Air density p 0.05 0.05

Area of collecting electrode A 0.1 0.1 Energy expended per unit charge W/e 0.5 0.5

in air Stopping-power ratio jwater

air 1.0 1.0

Absorbed-dose rate to water at

a depth of 7 mg/cm^ 1n PTP^

near sea level 0.4 0.3 0.1 2.2 1.3

at high altitudes 0.4 0.3 0.1 3.6 1.6

^Conventional statistical estimates (standard deviations of the means), obtained from repeated measurements.

''Subjective estimates (see text). A normal error distribution was assumed.

^For radiation-protection measurements, ionization current is usually measured

at or near the depth of interest (i.e., with an entrance foil thickness of

7 mg/cm^ of tissue-equivalent material). Cf^iy then refers only to the uncertainty in the thickness (area density) or the entrance foil used.

^This additional uncertainty in Cf^^^ refers to the correction for attenuation

in the entrance foil, which is required only for certain special procedures

in which extrapolation to zero-entrance foil is necessary. It is not included in the total uncertainties given in this table. (See also

sec. 2.4.3.2.)

^Square root of the sum of the squares of all uncertainties of one type.

^Since, in some instances, the current (I ) is determined for not more than

two plate separations (d), the uncertainty in the slope (fil /Ad) was set equal to that of the determination of at a given value of d — a procedure

which, as a rule, will result in an overestimate of the uncertainty.

34
Table 6. Uncertainty in the Calibration of Beta-Particle Sources and

Transfer Ionization Chambers Submitted to NBS

A. Source Calibration

Percent Uncertainty ^ Parameter Type A'* Type

20'*T1 i-7pm 2oni 90Sr+90Y 90sr+90Y

Positioning of extrapolationionization chamber and sources 1.0 0.2

Absorbed dose rate to water at depth t=7 mg/cm2^

near sea level 0.4 0.3 0.1 2.2 1.3

at high altitude 0.4 0.3 0.1 3.6 1.6

2om and 90sr+90Y

Overall Uncertainties near sea at high near sea at high

level altitude level altitude

Combined uncertainty, %^ 2.5 3.7 1.4 1.6

2 times combined uncertainty. %^ 5 8 3 3

3 times combined uncertainty. %e 8 11 4 5

B. Calibration of Transfer-Ioni zation Chamber

Percent Uncertainty.

Parameter Type A^ Type B^

2om

20ni 90sr+90Y I'^^Pm 90sr+90Y

Positioning of transfer

ionization chamber and sources 1.0 0.2

Measured ionization current 0.4 0.3 0.1 0.1 0.1

Correction of chamber air to reference conditions 0.1 0.1

Absorbed dose rate to water at

a depth of 7 mg/cm^^

near sea level 0.4 0.3 0.1 2.2 1.3

at high altitude 0.4 0.3 0.1 3.6 1.6

Source Decay Correction 0.05 0.05

i-^^Pm 20tTl and 90Sr+90Y

Overall Uncertainties near sea at high near sea at high

1 evel al titude 1 evel al titude

Combined uncertainty, %^ 2.5 3.7 1.4 1.6

2 times combined uncertainty, %^ 5 8 3 3 3 times combined uncertainty, %® 8 11 4 5

^Conventional statistical estimates (standard deviations), obtained from

repeated measurements.

'^Subjective estimates (see text). A normal error distribution was assumed.

""From data in table 5.

^The square root of the sum of the squares of all the uncertainties.

^Rounded.

35
1 I 1 1 1

cm 11 1 n r> r> i 1 1 \— 1 / / e - \ / / " o

ca 1 / / o / / «»

nee ' / yi. " 1 / X^o «* '/ / O o / / 1- o

1/1 5

// ^ — c / / p — o 1 o joo f ' 1 c 1 ^ 1 _ V 1 - €> - O \ \\

«< ^ oo

CM I8\\ 1 1 1 1 1 1 I 1 1 i\ \ I'll o

onUJ

<Jo

u.UJoz<

t-(no

AllSN3a nVDIldO od o

cvi AilSN3a "IVOIidO ob

0) 0)

(_

CT> 3

t_ T3

<U <U

c=

O

LU OJCL • O

0) o

(U 1—1 c c

0) (O Q£ M JC

fO 4-> U

<U T3C o

t_ E > +-> >> o

o>

•r— (Uto

(O rQ <u

0) t_o o o (U -o o 0) +-> a) o

+-> UJ

+-> to

O)o

4-> -O o o

CO •1" 0) to

<4-o +-> de c c o X Mu Ol <*- <u c

to x:u

»/> •r— T3 JC o 0) ^ 1-

(O O) T3 > o

OJ o

<u

•-> 1—N

•1—E to

(- • o T3

to

(1)

C t_ •r" Z5 0) 4-

4J %- C <u O t—

th

<u o o>

t- c

(/) •r— O) 0) o ac E

(O i_ • (U CO 4-> f—

O)

O T3

(D

(- TZI C_ 3 C o

AllSN3a "IVDIldO AiisN3a nvoiido

36
shown in the last column of the table correspond to a monitor counting rate of

10^ per second. At the time of writing (February 1988), representative values

for R and for minimum absorbed-dose levels are being obtained for the new monitor system. No assessment of uncertainties in possible future calibration

procedures with the nearly monoenerget ic electron beams is feasible before

this work is completed.

5.3 Determination of Spectral Composition of the Electron Beam

The 5-mm deep silicon surface-barrier detector equipped with the 1.6-mm

aperture lead collimator (see also section 4.4) was employed. Beam currents

were limited so that count rates did not exceed 2000 s-^. Measurement

procedures were similar to those employed for beta-particle spectrometry.

Therefore, the same caveats expressed in section 4.4 about absolute spectral

shapes apply here. The importance of making the exit windows (polyimide for

the low energies, aluminum for the high energies) as thin as possible is

brought out in figures 17 and 18. Figure 19 shows the spectral distributions

as a function of the distance between detector and exit window. At low

electron energies, degradation in air is seen to be not negligible, but the

spectra are still fairly narrow at distances at which the beam cross section

is sufficiently large and uniform for the study of survey instruments. The

most probable energies shown in these figures were used in column 4 of

table 7, relating these energies to accelerator potential, exit window-todetector distance, and usable field size.

Table 7. Examples of Pertinent Electron Beam Parameters Obtained Prior to

Improvement in the Monitor System

Absorbed Dose to Water

Accelerator Distance Usable Most Probable Minimum Maximum

Potential From Window Field Size^ Electron Energy Del i verable Rate

in 10 s (kV) (cm) (cm) (keV) (pGy) (mGy s-i)

200 20 8.3 130 140 70

300 30 11 220 30 15

300 20 6.7 250 70 35

400 30 8.7 340 90 45

400 20 5.7 360 190 95

1500 100 22 1280 13 7 1500 50 8.9 1370 90 45

2500 100 14 2230 60 30

2500 50 5.7 2320 350 175

^Defined as the diameter of the beam cross section over which flux density

does not decrease by more than 10%.

37
13NNVH3 U3d SlNnOO

>-o

a:UJz

lUzoa

t-o

13NNVH3 U3d SINHOO

13NNVHD «3d SlNflOD

O

to S

a; o

<U -r- c 5 O 4-> •f— jc X. t— <u i \ r i c c

L.<U £_

<4-

O

+-> •r- O O OJ

•-J

•r- -O

«/> c c OJ O 0)

t- 2

+-> (-> O

OJ J3 r—

UJ o;a > c ^ +j 1 iA O -i- O Q

T3 • c 5

fO o

T3

«>

C

5> •- 1 -M O -r- O X

CO UJ •

•> 0) > > -a

O) .r- i^ E 1 •!- o OO 1—

CM

O

a.

'<o E >> c E •1- 1 {- E Lf> (U O CVJ c

z: o <u

<4-

O 14- o

0) o1 O 4-> Ij o o a t_

(C -o o c a> 4-> 0>T3 o

O) T3 0) o < -M

0) <u -o • -a Eu

OJ >>

ir> 3 "o • CM

Ll_

38
0.09 mm

ALUMINUM

uj 9000 - z

z

<

I

o

lU

a.

0 0.5 1.0 1.5 ELECTRON ENERGY.MeV

0.09 mm

ELECTRON ENERGY.MeV

Figure 18. Degradation of Nominal 1.5-MeV and 2.5-MeV Electrons in Different

Thicknesses of Aluminum Added to the 0.09-mm Aluminum Exit Window. Distance

between detector and exit window: 2.0 cm for 1.5-MeV electrons, 50 cm for

2.5-MeV electrons.

39
gure 19. Degradation in Air of Nominal 200-keV to 2.5-MeV Electrons in

Different Thicknesses of Intervening Air Layers.

40
6. Examples of Practical Studies Performed with the Beta-Particle Beams

6.1 Compatibility of Different ^OSr-t-^Oy Sources with Existing Standards

Since spectral characteristics of ^Osr+^Oy beta-particle sources may vary

widely with source geometry, a certain degree of standardization of source construction is required if sources from different manufacturers are to be

used successfully in programs for testing personnel -dosimeter performance.

Three sets of criteria are described below, each designed to limit the

spectral variations allowed for test purposes. The American National

Standards Institute (ANSI N13.il, 1983) specifies a filtration thick enough

(~ 100 mg/cm^) to remove the ^^Sr component, but of a sufficiently low atomic

number {< 26) not to cause excessive bremsst rahlung production [14]. The

International Organization for Standardization (ISO 6980) specifies a residual

maximum energy (Ep^g) of > 1.80 MeV [15]. The Department of Energy

(DOE/EH-0027) specifies the limits on absorption characteristics in PMMA of

the beta-particle beam emitted by the source, requiring the ratio of

absorbed-dose rates to PMMA at depths of 100 mg/cm^ and 7 mg/cm^ to be equal

to 1.01 ± 0.05, and the corresponding ratio at 1000 mg/cm^ and 7 mg/cm^ to be

less than 0.01 [16].

The relatively high-activity (~ 5 GBq, 140 mCi) source used up to late

1985 by the National Voluntary Laboratory Accreditation Program (NVLAP)

testing laboratory for dosimetry performance was built to ANSI specifications.

The source that is used in the Department of Energy Laboratory

Accreditation Program (DOELAP) for testing dosimetry performance by DOE

contractor laboratories was manufactured by Buchler-Amersham according to

specifications given by members of the National Physical Laboratory in

England, who also were instrumental in developing the standard for betadosimetry sources of the ISO. It is of the same type and geometry as the

sources owned by NBS.

At the time this matter was studied (1984), it was considered of interest

to compare the DOELAP source and the source then used for NVLAP testing,

employing the different criteria. In this comparison, we also included an experimental source produced by Amersham to meet the ANSI specifications but

designed to have less internal scattering than the NVLAP source. (As a consequence, its activity is too low for convenient use as a calibration

source.) Table 8 shows the results of this comparison. Also shown is some pertinent information on source construction. Only the experimental Amersham

source is seen to meet all three criteria. The DOELAP source does not meet

the ANSI criterion since it contains silver. The NVLAP source does not meet

the ISO criterion, because the residual energy is too small.

Further elucidation of these results is possible if one examines the

response-versus-depth curves on which the DOE criterion is based (fig. 20) and

the associated pulse-height distributions (figs. 21 through 23) which give

equivalent information but can be obtained with less effort than attenuation

curves. They show that the experimental Amersham source indeed has the most

energetic spectrum and that the present NVLAP source is degraded most. They

also reveal that (a) an operational criterion of the DOE or ISO type may be

better suited for obtaining desirable source characteristics than a criterion

of the ANSI type, which specifies material properties and dimensions; and

41
COo

13o

>- oO

0) ooo

CO

C-«Jo.Eoo

no

COcoO

Q.

00

O)

(J

Q.EOo

CMoOoO00O

CO

00

CO

CO ooo

CTiO

Or-I A Voo OVoooo

(T3

Uoo o00A

CO

00

00

O) oo

4-> O

CMAI

C7) <c o

c:O • • A Voooo OV

«^ Qooo

CLEOa

cu o00V

CM

•r- II O

CO

O)

u

c:EOo

CM

CM

CM

^sl

CO >5 cu

C_) 1 E X +-> CU o E C cu t_ o TO •1

TO CJ o 4-> £_ 4- cu o TO <_) •r" cu •r— O o (C TO o LO o E TO ^- • > d O ID c: CM x: t. TO -M (_ E E E

CO E CO o >> • 4-) CM 13

O E en :3 TO s 13

•r— CU _Q CU t_ I—cr> E o CM -M CO I- CM

CO o CM £_ t_ cu c: cj 00

M e 1—

1 CU o C

<D Q. •r- fD j:: cu \ (_ £_ T3 CJ cu TO o c +-> U TO Q. t_ 1— cr> CU E OO CU t_ -M

•r— fO t_ >> t_ •r— O! E • > Z2 >> o N E cu >>

-t-> 0) +-> -o t_ TO c_ o o cu

M CD TO E > -l-> +-> Q. (V 13 > LO CD. cu c c o > TO •1— o •1

-M • > o o CTL •P" TO

O cu

C > c CJ>LO

u 4- >

L- TO CI. •r- r— .—

cu Z2 l+- 'r— <u • 0 O cu +-> C_ c ">) s_ O O M -M CO o +-> CO o o cu u +J 4- o u <U >> CO (- O (U M C CSJ (J

CL)

C CM

M n3

X

O »\ -M JO TO t_ • <: E r— >^ TO E < Q 1- O) E CO cu E on E CU E CU TO s

<o E -O CO 4-> CU o E (T3 cu C O c: TO CU •1

CUC

a TO o cu "O TO cn E C > •r— •I- t- -M CU u CU O E

(- E "1— f +J E CO E "O O) TO TO o •r— E s_ CM 00 I—

E cu CO :3 > 4-> -M E E Z3 u 0 E o o o • O c c o •p— O c o o TO •< 0 0

4- tn

O 1— CO TO TO •I- o +-> l_ o en CO 1—

+-> E • OJ cu TO oo Ol cu • Q.

CO E c:

O o CO o 4-> -t-J t_ 1—

0) o o O TO cu 00 o o CO o -M E ;s • c CO CU 1—

I- (—

c «=C <:

m E +-> 1/1 o OO o E CU 1—

0 o >>>+-> +J TO E 1—

Q. JC • O TO o: (_ sz

X JO cu t_ <: t/) o 13 £_ cu CO i_ cu 0) 1 CU £_ <c CU o O -M -M c_ CU cu CU E -M o O) o 1 o O CU cu CL CL a o IT3 E oo > (_ TO CU E X >1 TO 0 t_ Q < f—

2r a_ _J E O eC LU 1— 2: 4J

U

cuo

TOcEoo

CTiO A Voo OVQoo I O I

cu

CJOO

CU o00A

00

II

cu

c_ LU

cu

TOou

CM

CM

CM

hvl

42
Figure 20. lonizati on-versus-Depth Curves for Three Different types of

90sr+90Y Sources. Solid line: Buchler-Amersham source; dashed line:

Amersham experimental source; dash-dotted line: special high-activity source

used by the NVLAP Testing Laboratory until 1985. The curves in PMMA are indistinguishable from those in polystyrene. No line could be drawn for the

bremsst rahl ung background of the Amersham experimental source because of the

wide scatter of the associated data points (some of them off-scale,

representing relative ionization currents < 0.001). This behavior probably

was due to the erratic high background noise introduced in the ionization

chamber by a preceding irradiation in a high-intensity electron beam.

43
Electron energy, mcv

Figure 21. Comparison of Spectrum of the Buchl er-Amersham and the NVLAP

Testing Laboratory's ^Osr+^oy Sources, on Semi -Logarithmic Scale. The

ordi nates were adjusted for equal total number of counts.

44
10^

ELECTRON ENERGY, MeV

Figure 22. Comparison of Spectrum of the Amersham Experimental and the NVLAP

Testing Labortory's ^osp+soy Sources, on Semi -Logari thmi c Scale. The

ordi nates were adjusted for equal total number of counts.

45
Figure 23. Comparison of Spectrum of Amersham Experimental ^^Sr+^^Y Source,

Bare and Shielded. Adding 82 mg/cm^ of aluminum to the Amersham experimental source makes its spectrum similar to that of the Buchler-Amersham

source (fig. 21).

46
(b) that the DOE criterion is too loose, since it does not discriminate

between the operationally very different NVLAP and Amersham experimental

sources, while the ISO criterion does. The high background noise shown in

figure 20 for the experimental Amersham source resulted from the use of the

extrapolation chamber after it had been irradiated with an electron beam

producing an absorbed-dose rate to water of the order of kilorads per second.

This alerts one to the need for special care in the use of such a chamber.

6.2 Comparison of ^Q'^Tl and ^QSr+^Oy Source Calibrations at NBS and at a DOE

Laboratory

This study was undertaken because the DOE requires traceability to NBS of

all measurement standards used in the DOELAP program for testing personneldosimetry performance. The calibrations of the transfer ionization chamber in

the ^°Sr + ^^Y beams were found to agree to within 1 percent, while the calibration by the staff of the DOE laboratory in their 20itj-| 59^^ proved higher

by almost 8 percent than that by NBS in the NBS beam. Test Report DG 8555/86

covering these results is reproduced in Appendix 3. The study also confirmed

that, in spite of its thin entrance window, the PTW ionization chamber

number 2047 is sufficiently rugged to be used as a transfer instrument. (See

also sec. 2.5).

7. Examples of Studies Performed with the Nearly Monoenergetic Electron Beams

7.1 Predicting Beta-Particle Response of Instruments from Response to Nearly

Monoenergetic Electrons

This study was considered important since its results are intimately

related to an assessment of the role of the nearly monoenergetic electron

beams in a contemplated type-testing program for beta-particle survey instruments. In spite of the relatively poor reproducibility of the electron-beam

dosimetry prior to improvements in beam stabilization and monitoring, the

results were encouraging. They are covered in a recent publication [11],

which is incorporated in this Document as Appendix 5.

7 . 2 A Study of Rate Dependence of the Response of Radiation-Protection Survey

Meters

At the time of writing (February 1988), C. G. Soares is studying the

response to nearly monoenergetic electrons of beta-particle survey meters over

a wide range of electron currents [17]. The accelerator-produced electrons

lend themselves well to this type of work because currents (and therefore

absorbed-dose rates) can be readily changed without a change in irradiation

geometry and thus in electron spectrum. This is not the case with betaparticle beams.

An example of an instrument being studied is the Eberline Ion Chamber

Type Survey Meter R0-2A, SN 953, which is used in the field for photon and

beta-particle surveys. In an earlier study [18], some rate dependence of its

photon response had been demonstrated. Soares finds that, in the range from

about 5 mGy/h to 2 Gy/h of absorbed-dose rate to water, the decrease in the

response of the instrument with increasing dose rate is more than twice as

47
large with 350-keV electrons as with ^^'^Cs gamma radiation. "Response" here

is defined as scale reading (indication) of the instrument divided by

absorbed-dose rate.

7 . 3 Role of Nearly Monoenerget ic Electron Sources in the Development of a New

Generation of Survey Meters

During the past several years, instrument designers have given thought to

avoiding the difficulties in beta-particle surveys arising from the strong

energy dependence of survey-meter response. Instruments are now under

development that incorporate tissue-equivalent scintillation detectors and

associated mult i -channel analyzers for spectrometry, and microprocessors for

data analysis. Monoenergetic electron beams over wide ranges of energy are required in the development of such instruments (a) for arriving at resolution

and efficiency functions of the detector, needed for spectral unfolding of the

pulse-height distributions, and (b) for obtaining instrument-response

functions needed to develop suitable algorithms that lead to instrument indication in terms of absorbed-dose rate, independent of electron energy. We

have assisted groups from both Los Alamos National Laboratory and from E6&G

Idaho, Inc., with such instrument developments. Excerpts of pertinent

publications by members of these institutions [19, 20] are incorporated in

this Document as Appendix 6.

8. Safety Considerations

It is likely that many of the procedures associated with the calibration

service for which this facility was established will be carried out by junior

personnel, who may start without previous experience in radiation work. The

senior staff member in charge of the facility must take all steps necessary to

ensure that each person working with this facility is thoroughly familiar with

the pertinent radiation protection literature, and is aware of the potential

hazards, the importance of the safety features built into the facility, the

procedures to follow for safe operation, how to detect potentially dangerous

malfunctioning of any part of the system, and what steps to take in case of an accident

8.1 Radiation Hazards

All sources arriving at NBS, and periodically all sources kept at NBS, are examined by a member of the NBS Health Physics Group for radioactive

surface contamination, including shipping and storage containers. This is

considered to remove the hazard of ingestion of or other body contamination by

radioactive materials.

48
The radiation hazard is only moderate for sources with the activity level

that will be calibrated in the facility in question, since even a direct

exposure of the skin for a brief time would not be serious, with the possible

exception of the eyes. However direct exposure will never be necessary, since

protection from a beta-particle source is readily provided by materials such

as a f ew mi 1 1 imeters of plastic, or by simply directing the source away from

the body. Handling tools are provided for removing the source capsules from

their storage containers and placing them on the irradiation stand, without

exposure of any part of the operator's body to a significant fraction of the

maximum permissible exposure. A survey meter is on hand for checking

radiation level s' whenever there is any doubt about when a faulty procedure or faulty equipment has created a potential radiation hazard. This could be the

case, for example, if the shutter failed to close at the electrical command.

Light beams are used to establish a safety perimeter around the measurement area, such that breaking a beam causes the shutter to close. Such a safety device could be defeated by simply crawling under the beam, so the

cooperation of all personnel is required to make the device effective, and it

becomes particularly important to instruct all personnel in the radiation

hazards involved.

If a source is involved in an accident, e.g., if a source is dropped

during transfer, the NBS Health Physics Group will be notified, and they will

test the source and the area involved for radioactive contamination.

8.2 Electrical Hazards

The electrical hazards are of the same type as in other dosimetry

laboratories, and require the same level of skill. Since the collecting

voltage used on the beta-particle ionization chambers is never more than one hundred volts, this hazard is somewhat less than with other ionometric

procedures. As in other dosimetry laboratories, it is essential to verify all

power-supply settings before applying the voltage to the ionization chamber,

and to avoid contact with the off-ground exposed part of an ionization

chamber.

8.3 The Nearly Monoenergetic Electron Facility

This facility, described in section 3.2, can involve high-energy, highintensity beam operation, and may present greater radiation hazard than any

mentioned above. The facility is not involved in the calibration service

described here, but has been limited to research on and development of betaparticle calibration services. If in the future it becomes desirable to

employ accelerator-produced electrons in routine calibrations, a manual of

safe operation procedures will be drawn up and made available to all

operators

49
9. References

[1] Mantel, J. The beta ray spectrum and the average beta energy of several

isotopes of interest in medicine and biology. Int. J. Appl . Radiat,

Isotopes. 23, 407-413 (1972).

[2] Cross, W. G.; Ing, H.; Freedman, N. A short atlas of beta-ray spectra.

Phys. Med. Biol. 28, 1251-1260 (1983).

[3] Boehm, J. Standardization and calibration in beta dosimetry.

Proceedings of the International Beta Dosimetry Symposium; 1983 February;

Washington, D.C., U.S. Nuclear Regulatory Commission NUREG/CP-0050,

73-97, 1984.

[4] Tabata, I.; Ito, R.; Okabe, S. An Empirical Equation for the

Backscatteri ng Coefficient of Electrons. Nucl. Inst. Meth. 509

(1971).

[5] Boehm, J. The perturbation correction factor of ionization chambers in

3-radiation fields. Phys. Med. Biol. 65-75 ( 1980).

[6] Boehm, J. Saturation corrections for plane-parallel ionization chambers.

Phys. Med. Biol. 2J., 754-759 ( 1976).

[7] Burlin, T. E. Cavity chamber theory; Chapter 8, Radiation Dosimetry,

Volume 1, Fundamentals; Second Edition; F. H. Attix and W. C. Roesch,

editors. Academic Press; 1968.

[8] Pruitt, J. S. The effect of altitude on beta-ray source calibrations.

Rad. Prot. Dosim. JJ., 151-157 (1985).

[9] Soares, C. G; Dick, C. E.; Pruitt, J. S.; Sparrow, J. H. Development of

monoenerget ic electron beam sources for radiation instrument calibration.

Nucl. Inst. Meth. Phys. Research; Section B: Beam Interactions with

Materials and Atoms 10/11 , 937-941 (1985).

[10] Ehrlich, M.; Pruitt, J. S.; Soares, C. G. et al. Standard beta-particle

and monoenerget ic electron sources for the calibration of beta-radiation

protection instrumentation. NUREG/CR-4266, U.S. Nuclear Regulatory

Commission, Washington, D.C. 20555; also as NBSIR 85-3169, U.S.

Department of Commerce, Gaithersburg, MD. 20899; 1985.

[11] Soares, C. G. Predicting beta particle response of instruments from

their response to monoenergetic electrons. Rad. Prot. Dosim. 14 , 113-115 (1986).

[12] A handbook of radioactivity measurements procedures. National Council on Radiation Protection and Measurements. NCRP Report No. 58, 1978.

[13] See, e.g., Giacomo, P. Meeting of experts on the subject of expression

of uncertainties, in News from the BIPM. Metrologia 12, 69-74 (1981).

50
[14] American National Standard for dosimetry — personnel dosimetry

performance — criteria for testing. Am. Nat. Standards Institute ANSI

N13. 11-1983.

[15] Reference beta radiations for calibrating dosimeters and dose-rate meters

and for determining their response as a function of beta radiation

energy. International Organization for Standardization. ISO 6980,

1984.

[16] Standard for the performance testing of personnel dosimetry systems,

DOE/EH-0027. Department of Energy; Washington, D.C., 1986.

[17] Soares, C. G. Unpublished data.

[18] Colle, R.; et al. Characterization of Survey Instruments; in Quality

Assurance for Measurements of Ionizing Radiation, Eisenhower, E. H. editor; NUREG/CR-3775; U.S. Nuclear Regulatory Commission, Washington,

D.C. 20555, 1984.

[19] Erkkila, B. H.; Brake, R. J.; and Waechter, D. A. Los Alamos Portable

beta-ray spectrometer. Proceedings of the International Beta Dosimetry

Symposium; 1983 February 15-18; Washington, D.C, U.S. Nuclear Regulatory

Commission NUREG/CP-0050, 1984.

[20] Martz, D. E.; Rich, B. L.; Johnson, L. 0.; David, S. H. III. Field tests

of a portable tissue equivalent survey meter for monitoring mixed

beta/gamma radiation fields. Idaho National Engineering Laboratory;

NUREG/CR-4553; EGG-2448. EG&G Idaho, Inc.; Idaho Falls, ID 83415; 1986.

51
Appendices

Appendix 1. Sample of Calibration Report Form (See sec. 1.3)

DG 8751/87 O. S. DEPARTMENT OF COMMERCE Page 1 of 2 DB 873/170 NATIONAL BUREAU OF STANDARDS

TEN 240032 GAITHERSBURG , MD 20899

1987 Sep 14

REPORT OF TEST

0.5 mCi 204 Beta-Particle Source Amersham-Buchier Source #- RS #87-0068

Submitted by

Received

Calibrated:

1987 Jul 21 1987 Aug 03

Beta-particle sources are calibrated in terms of the absorbed-dose rate to water. Absorbed-dose rate is determined from current measurements with an extrapolation ionization chamber that has a polymethylmethacrylate

collecting electrode. The water absorbed-dose rate is given in SI units

by:

D= (W/e)(S„,,„/S3i,)(p,i,A)-i(di,<,„/ds) Gy/s

where (W/e) = 33.7 J/C is the mean energy expended per unit charge in

ambient air, (S„ater/^ai r ) ratio of the mean mass stopping power of

water to that of air, Pgij. = 1.197 kg/m-' is the density of air at the

reference temperature and pressure (22°C and 1 standard atmosphere) , A = 7.083 xlO"'' m^ is the area of the collecting electrode, di^3^j./ds is the

rate-of -change of corrected current with air-gap spacing in amperes per

meter

The corrected current is given by:

^corr ~ ^ ^TP ^side ^back ^recotn ^atten ^phot ^div ^decay ^humid ^uass

where i is the measured current and the f's are, respectively, corrections

for air density variations inside the chamber, for scatter from and/or

shielding by the side walls of the chamber, for excess backscatter from the

collection electrode, for recombination and diffusion of ions inside the

chamber, for attenuation by the chamber air, for the presence of photons

accompanying the beta particles, and for beta-particle beam divergence; and

the k's are, respectively, correction factors for source decay to the

listed calibration date, for humidity variations, and for variations in the mass of air between source and chamber.

For the source submitted, one calibration was made. The rate of change of

corrected current with air-gap thickness was determined from measurements

with air-gap thicknesses of 0.5, 1.0, 1.5, 2.0, and 2.5 mm. The water

absorbed-dose rate is given in the table in SI units (grays per second)

The tabulated dose rate applies to the date of calibration, to the source- phantom distance and phantom depth given in the table, and to the reference

52
Appendix 1, continued

DG 8751/87 Page 2 of 2 1987 Sep 14

temperature and pressure of 22°C and 101.325 kPa (1 standard atmosphere).

If the air density differs significantly from the density at the reference

temperature and pressure, it may be necessary to apply a correfction as given in: J. S. Pruitt, "The effect of altitude on beta- ray source calibrations," Radiation Protection Dosimetry li, 151-157 (1985).

The f factors associated with the extrapolation chamber, and the k factors

associated with the ambient conditions, are given in the table in the same order that they are listed in the above equations; if a factor varied

during the measurements, the maximum and minimtom values are given.

The overall uncertainty in this calibration is estimated to be ±3%; it is

considered to have the approximate significance of a 95% confidence limit.

The overall uncertainty is two times the square root of the quadratic sum

of the component uncertainties, which were estimated and treated as if they were standard deviations.

Information on the technical aspects of this report can be obtained from

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

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

Calibration performed by J . S. Pruitt

Report approved by R. Loevinger

For the Director

David M. Gilliam, Associate Chief

Ionizing Radiation Division

Center for Radiation Research

National Measurement Laboratory

Source S,water

Phantom Phantom

/Sg^j Correction Factors Distance Depth

k f (cm) (mg/cm^)

Water

Absorbed- Dose

Rate

(Gy/s)

0.5 mCi 1.132 1.00

1.00

1.00

1.01-1.03

1.00

1.01

1.00

1.00

1.00

30 7 1.82 E-7

1.00-1.01

53
Appendix 2. Report of a Trip to a DOE Laboratory by J . S. Pruitt

The calibration at this laboratory of the Buchler ^ Pm source is

considered important because its results provide a test of the validity of

the recently developed high-altitude air-density correction factor. (See

section 2.4.3.3.)

1985 Jan 10 Page 1 of 4

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

GAITHERSBURG, MD 20899

REPORT OF

TRIP TO IDAHO NATIONAL ENGINEERING LABORATORY

by

John S. Pruitt

Dosimetry Group

Center for Radiation Research

Introduction

In November, 1984, I traveled to the Idaho National Engineering Laboratory (INEL)

with several objectives. The primary objective was to provide traceability to the

National Bureau of Standards (NBS) for beta-particle dosimetry, and this objective

was met by performing (1) a calibration of their electrometer used for ionization

chamber current measurements. Secondary objectives were (2) to calibrate a 200-MBq

(5-mCi) thallium-204 source, recently purchased from the University of Lowell, and

(3) to calibrate a promethium-147 source purchased in 1980 from Buchler in West

Germany. Calibration (2) was a service for INEL employees, and calibration (3) was

an indirect comparison with the original PTB calibration, by way of my own measurements of the effect of altitude on beta-particle calibrations. The INEL is

located on the Snake River plain at an altitude of about 1500 m (4900 ft.) above

sea-level

(1) Electrometer Calibration

The electrometer calibration was performed with the University of Lowell thallium204 source irradiating the INEL extrapolation chamber purchased from Nuclear

Associates. For a fixed source distance of 300 mm, a fixed air gap setting of 2.5

mm, and a fixed chamber potential of ± 25 volts, the chamber current was measured in

two ways. Measurement A consisted of the digital readout of the Keithley 642

electrometer to be calibrated, used in the "Charge" mode, divided by the irradiation

time. Measurement B used the Keithley 642 in the "External Feedback" mode, with a calibrated 100-pF General Radio capacitor as the feedback element, and the feedback

voltage measured with a calibrated digital voltmeter (DVM). The results are:

(A) Indicated current =

( l»164Q-(-1.237Q)

^ ^ 1^ ^ ^^^2005 pA

where 1.1640 and -1.2370 are averaged Keithley 642 readings for opposite chamber

potential polarities, on the 10"^^ coulomb scale, and 100 is the irradiation time in

seconds. In addition:

(B) Measured current = 0.9987[ Q'Q^Q^Q-^-Q-Q^^^S^

) x m,V^^lO-^' -_ q. 12021 pA

54
Appendix 2, continued

Page 2 of 4

where 0.05848 and -0.06168 are averaged DVM readings for opposite chamber potential

polarities, 0.9987 is the DVM calibration factor, 100.17x10"^^ is the capacitance of

the 100-pF capacitor in farads, and 50 is the irradiation time in seconds.

These data show that the Keithley 642 readings in the "Charge" mode should be

multiplied by the factor:

Measured

Indicated current = 1.001 ± 0.002

where 0.002 is the standard deviation of the measurements.

(2) Thallium source calibration

The University of Lowell thallium-204 source was calibrated with the INEL

extrapolation chamber, using 50-second runs, a fixed source distance of 300 mm, and

several different air-gap settings and chamber potentials. Current measurements

were made using system B, described in section (1), with the calibrated capacitance

and DVM. The currents were calculated with the formula:

, 0.9987x100.17x10-12 v+.y- . . ^ „

similar to equation (3) in section (1) except for the inclusion of three

correction factors: fj corrects the chamber air to a reference temperature

and pressure, chosen in this case to be 22°C and 635 mmHg; fc is a perturbation correction to account for the presence of the chamber (taken from

J. Bohm, Physics in Medicine and Biology 25_, 65 (1980)); fn is a correction

for ion recombination and diffusion inside the chamber (taken from J. Bohm,

Physics in Medicine and Biology, _21_, 754 (1976)). V*" and V" are averaged DVM

readings measured with positive and negative chamber potential respectively.

The data used are:

d , mm

2.5 2.0 1.5 1.0 0.5

± HV

V+

Vii

25

0.05848

-0.06168

1.0045

1.0015

1.0025

25

0.04690

-0.04976

1.0041

0.9994

1.0024

25

0.03446

-0.03772

1.0038

0.9982

1.0023

Z5

0. 02242

-0.02574

1. j036

0.9984

1.0022

5 0.01000

-0.01326

0.9958

0.9992

1.0107

I,pA 0.12123 0.09727 0.07252 0.04838 0.02340

55
Appendix 2 , continued

Page 3 of 4

where d is the air-gap setting in millimeters and ± HV is the chamber potential in

volts.

The listed values of I were fitted to a least-squares straight line as a function of

air gap setting, d, and the slope of this line was used to determine the absorbed

dose rate at the reference temperature and pressure, ZZ'C and 635 mmHg, on 84 Nov 28

at a depth of 7 mg/cm^ of polyethylene terepthal ate:

Absorbed dose rate = f^QOQx yoy^Sxio-^'^ Q-Q^^^^^ ^^"^ " ^-^^ ^^^^^

where 33.7 is the mean energy expended per unit charge in air, in

joules/coulomb; 1.139 is the ratio of the mean stopping power of water to that

of air; 0.978 is a transmission factor correcting from the 2.61 mg/cm^ ion

chamber depth to a depth of 7 mg/cm^ ; 1.000 is the density of air at the reference

temperature and pressure, in kg/m^; 707.3x10"^ is the collecting electrode area, in

m^; and 0.04891x10"^ is the least-squares straight line slope in A/m. The

standard deviation of this measurement is about 0.3%. It should be noted that

correction of the above absorbed dose rate to sea level reference temperature and

pressure, 22'*C and 760 mmHg, would require multiplication by a factor of 0.987

(taken from J. Pruitt, The effect of altitude on beta-ray source calibrations, to be

published in Radiation Protection Dosimetry).

(3) Promethium source calibration

The Buchler #33 promethi um-147 source was also calibrated with the INEL

extrapolation chamber, using 50-second runs, a fixed source distance of 200

mm, variable air gap settings and chamber potentials. Current measurements

were made with system A described in section (1), using the Keithley 642 calibrated

readout. The currents come from:

I =

similar to equation (A) in section (1) except for the calibration factor 1.001

and the four corrections. These are the three ion chamber corrections already

described, plus f/\, the altitude correction, which corrects the interactions

in the air between source and chamber to the reference temperature and

pressure, 22*'C and 635 mmHg (taken from J. Pruitt, op. cit. ). Q"*" and Q" are averaged Keithley 642 digital readouts for positive and negative chamber potentials

respectively. The data used are:

56
Appendix 2 , continued

Page 4 of 4

d ,mm

2.5 1.5 0.5

± HV

QI;

25

0.02477

-0.02601

0.9964

1,0048

1.0025

1.0125

15

0.01339

-0.01603

0.9963

1.0029

1.0038

1.0131

5 0.00524

-0.00580

0.9960

1.0010

1.0107

1.0139

I,fA 5.166 2.992 1.129

where d is the air gap setting and HV is the ion chamber potential, as before.

The listed values of I were fitted to a least-squares straight line as a function of d, and the slope of this line was used to determine the absorbed

dose rate at the reference temperature, pressure, and humidity, 22°C, 635 mmHg,

and 20%, respectively, on 84 Nov 29 at a depth of 0 mg/cm^ of polyethylene

terepthal ate:

Absorbed dose rate = -pg^g^y^l^l^^^ 2. 019x10-^2 = o.209 pGy/s

where 1.150 is the ratio of the mean stopping power of water to that of air,

1.886 is a transmission factor correcting from the 2.61 mg/cm^ ion chamber

depth to a depth of 0 mg/cm^ , and 2.019x10'^^ is the least-squares straight

line slope in A/m. The standard deviation of the measurement is about 3%.

This calibration may be compared with the original PTB calibration giving the

absorbed dose rate at the reference temperature, pressure, and humidity of 20°C, 760

mmHg, and 45% respectively, on 80 Dec 15 at a depth of 0 mg/cm^. Corrected to 22°C,

635 mmHg, and 20%, the PTB calibration becomes:

Absorbed dose rate = 0.2729x0.3517x2.143x1.011 = 0.208 pGy/s

where 0.2729 is the original PTB calibration, in yGy/s; 0.3517 is the decay

factor (using X = 7.234x10"^ dayM; 2.143 is the altitude factor (taken from

J. Pruitt, op. cit. ); and 1.011 is the humidity factor (taken from the Buchler

Handbook). The agreement with the absorbed dose rate measured at Idaho Falls is

well within the statistical standard deviation of the Idaho Falls measurement.

57
Appendix 3

Measurement Assurance Study by.Means of PTW Transfer

Ionization Chamber, Model 20A7"'' (See section 6.2)

DG 8555/86

DB 868/107

TFN 236381

1986 JUN 19

Page 1 of 2

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

GAITHERSBURG , MD 20899

REPORT OF INTERCOMPARISON

PTW Ion Chamber #2047

Measured at NBS in 1985 APR and 1986 MAY

Measured at --- in 1985 NOV

This report concerns the intercomparison of beta-particle source

calibrations, using sources of ^^^Ti and ^°Sr+^°Y. The two laboratories

involved, the National Bureau of Standards (NBS) and - - - possess

similar sets of Amersham-Buchler sources, which have been calibrated in

terms of tissue dose rate by the PTB in Braunschweig, Federal Republic of

Germany. The intercomparison was performed with a thin-walled parallelplate ionization chamber as a transfer instrument. This chamtber, PTW

#2047, was first calibrated in terms of tissue dose rate per unit current

at NBS, then shipped to - - - for a similar calibration, then returned to

NBS for a final calibration. The primary purpose of the comparison was to

determine if the calibrations performed by the two laboratories were in

adequate agreement

It was agreed that the chamber calibrations would be made with the Buchler

source alignment rods just touching the center of the outer surface of the

chamber entrance window (on the side of the chamber labeled FOCUS) , and

with the plane of that surface perpendicular to the rods. Current

measurements were to be made with the high-voltage electrodes at 100 V, and

the calibration results were to be averages of measurements with both

polarities

The measured chamber currents, after background subtraction, were normalized to 22"'C and 101.3 kPa to account for variations in air density

inside the chamber. The tissue dose rates generating these currents were determined from the source-calibration dose rates, corrected by

multiplicative factors for:

(1) Source decay.

(2) Variations in air density outside the chamber

*This report pre-dates a full uncertainty assessment.

58
Appendix 3, continued

DG 8555/86

1986 JUN 19

Page 2 of 2

Each chamber calibration (in Gy//iC) is the ratio of a tissue dose rate (in

y.Gy/s) and a corrected current (in pA) . The reported results are listed

below in Gy/nC:

Calibration Factor

absolute ratio

Source NBS(i) NBS(f) - -/NBS

0.5 mCi 0.3336 0.3580 0.3317 1.076

2 mCi Sr+Y 0.2917 0.2901 0.2907 0.996

50 mCi Sr+Y 0.3006 0.2978 0.3006 0.991

(1) The corrections for source decay from the source calibration date to

the chamber calibration date made use of conventional half- lives.

(2) The two laboratories used different formulas for calculating correction

(2) . - - used the formulas supplied in their Buchler instruction

manual, while NBS used formulas derived from an NBS study of the effect

of altitude on beta-ray source calibrations. The corrections

calculated by both methods are shown in the following table:

Correction for Variations in

Air Density Outside the Chamber

Source - - NBS

0.5 mCi TJl 1 0.998

2 mCi Sr+Y 1 1.002

50 mCi Sr+Y 1 0.998

Information on the technical aspects of this report can 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

59
Appendix 4. Predicting Beta-Particl e Response of Instruments from their

Response to Nearly Monoenergetic Electrons (See sec. 7.1.)

Kadialion Hroteciiim Oosimclry

Vol 14 No 2 pp 1I A-I15(I'«6)

Nuclear Technology Publishing

PREDICTING BETA PARTICLE RESPONSE OF

INSTRUMENTS FROM THEIR RESPONSE TO

MONOENERGETIC ELECTRONS

C. G. Scares Center for Radiation Research, National Bureau of Standards

Gaithersburg, MD, USA

Abstract — Accelerator-produced, nearly monoenergetic electron beams are being studied for use in obtaining the response of beta particle dosimetry instrumentation as a function of electron energy. Three detection systems were studied:

a thin-window ionisation chamber, a thin thermoluminescence dosemeter, and an electron spectrometer. For each of the systems studied, broad spectrum response was measured with standardised sources of '^'Pm, ^"*n, and '"Sr+'^'Y. The

response of these instruments was also measured in the nearly monoenergetic electron beams. From the monoenergetic

electron responses, a function was constructed of response versus electron energy. Broad spectrum response was then

calculated using this function and pulse-height distributions of the three beta particle sources, measured with the electron spectrometer. This calculated response was then compared with the measured response. For '"'Tl and *Sr-i-**Y, calculated and measured responses agree to within experimental uncertainties.

INTRODUCTION

The National Bureau of Standards (NBS) is in the process of establishing a beta particle cahbration

facilityusing standardised sources of '^'Pm , ^°*T\, and

*'Sr+^(', making it possible to determine broad

spectrum response of beta particle dosimetry

instrumentation under standard conditions^ However, for a full characterisation of the response

of beta particle dosimetry instrumentation, a determination of the energy dependence of 'nstrument response is required. Because of the )road sjjectra of beta particle sources, such sources

ire inadequate for this purpose. For this reason, NBS

has developed a set of accelerator-produced nearly

monoenergetic electron beams, the calibration and

characterisation of which are described in an earlier publication*^\

The object of this work is to determine the degree

to which the broad spectrum response of beta particle

detecting instrumentation can be predicted from

response to monoenergetic electrons. For each of the detection systems studied, broad spectrum response was measured with the standard beta particle sources. The resf>onse of these instruments was also measured in the nearly monoenergetic electron beams. From the monoenergetic electron responses,

a function is constructed of response relative to electron energy. Broad spectrum response is then

calculated using this function and pulse-height

distributions of the three beta particle sources measured with the Si(Li) detector. This calculated response is then compared with the measured

response.

60

DESCRIPTION OF THE DETECTION

SYSTEMS

Ion chamber survey meter (Eberline Model RO-2A)*

This instrument incorporates a thin-window

ionisation chamber for beta particle detection. It was

calibrated in terms of exposure in a '"Cs photon

beam, and its electron and beta particle responses are expressed in terms of this calibration. For both

irradiations, the point of reference was the front surface of the ion chamber.

Thermoluminescence personnel dosemeter

(Panasonic Model 801 AQ)*

This dosemeter contains four sensitive elements 1

mg.cm'^ thick under different filters. Only one of the elements is sensitive to beta particles. It is lithium tetraborate under 23 mg.cm"^ of plastic. The

dosemeter was calibrated in a '"Co photon beam,

again in terms of exposure.

Beta particle spectrometer (Si(Li) detector)

This system was chosen to simulate the sensitive element of state-of-the-art survey meter/

sfjectrometers now becoming available. These

detectors rely on pulse-height analysis and use

*In this report, certain commercially available products are referred to by name. These references are for information purposes only, and do not imply that these products are the

best or only products available for the purp>ose, and do not imply endorsement by NBS.
Appendix 4, continued

( . a Si

internal algorithms for deriving absorbed dose from

the measured pulse-height spectra.

DISCUSSION OF EXPERIMENTS AND

RESULTS

Accelerator-produced, nearly monoenergetic

electron beams were used to measure the response of

the three detection systems as a function of electron energy. The responses measured are shown in Table

1. As in the beta particle measurements, response is defined as instrument reading per unit absorbed dose

to water. The ion chamber and TLD responses are expressed relative to their exposure calibrations. The

Table I. Measured monoenergetic electron responses.

Response to monoenergetic electrons

Electron energy Ion chamber* TLD" Spectrometer***

(keV)

130 0.21 0.01 0.24 220 0.30 0.72 0.40 250 0.35 1.03 0.59 320 0.35 1.13 0.68 350 0.38 1.34 0.83 1310 0.89 1.51 1.69 1390 0.90 1.67 1.54 2280 1.03 1.32 3.12 2360 0.90 1.17

•Expressed relative to response to '"Cs photons.

* 'Expressed relative to response to ""Co photons.

* * * Response here was calculated as a quantity prof>ortional to the mean stopping power of watqr.

Table 2. Comparison of measured and calculated broad- spectrum instrument response.

Response to beta particles

Radionuclide Ion chamber* TLD** Spectrometer

• * •

Meas. Calc. Meas. Calc. Meas. Calc.

'*'Pm 0.06 0.16 0.00 0.01 0.08 0.18 ^"n 0.31 0.33 0.82 0.87 0.40 0.52

'"Sr-l-'*^' 0.80 0.58 1.70 1.53 1.06 1.24

'Expressed relative to response to '"Cs photons.

** Expressed relative to response to '"Co photons.

• '

* Response here was calculated as a quantity proportional

to the mean stopping power of water.

AHhS

response of the spectrometer was calculated as a quantity proportional to the mean stopping power of water:

2n,(T,)s,„,,(T,)/5:n,(T,)

where n|(T,) is the number gf pulses in the ith pulseheight interval corresponding to an average electron energy T,, and Sv,i„j,(T,) is the stopping power in water

in this interval, restricted to electron energies greater than in kcV*''. Both the radionuclides and the accelerator beam monitors were calibrated with an

extrapolation chamber. The response, R, averaged over the broad spectrum is given by

R = 2n,(ti)r.(T.)/2n.(T.)

where n,(Tj) is the number of pulses in the ith interval with average energy T,, and rXTJ^ is the instrument response evaluated at energy T, from the fitted monoenergetic response functions. The comparison

between this calculation and the directly measured

broad spectrum response is shown in Table 2. The principal sources of uncertainty in these measurements are: (1) calibration of the monoenergetic electron beams, which are subject to variations in intensity and direction; (2) use of pulseheight spectra instead of fully corrected beta-particle

spectra; and (3) random uncertainties in instrument

readings. The uncertainty for the points on the response function curves is estimated to be ±10%,

while the uncertainty of the calculated broad

spectrum responses is estimated to be ± 15 to 20%.

For the ^^^Tl and ^^r-l-'^Y data, measured and

calculated resf>onses are in agreement to within the experimental uncertainties. However, for the '*'Pm

data, the calculated response is much greater than the measured response, especially for the ion chamber

and the spectrometer. A possible explanation for this

is a lack of consideration of a low energy cut-off in response due to entrance windows in these detectors. The results of this study indicate that the method of predicting instrument response to beta particles from

response to monoenergetic electrons is valid.

ACKNOWLEDGEMENTS

I wish to thank Margarete Ehrlich, who suggested

this study, and whose comments and encouragement

are greatly appreciated; also Eric L. Bright for able

assistance in performing some of the measurements.

This work was supported in part by the Office of Nuclear Regulatory Research of the US Nuclear

Regulatory Commission.

61
Appendix 4, continued

I'RIDH TINd lihIA I'ARIICl.H HI SI'ONSi:

REKERENCTS

1. Ehrlich, M., Pruill, J. S. and Soares, C. G. Standard Beta-Particle and Monoenergelic F.lectron Sources for ihi- Calihraiion of lieia- Radiation Protection Instrumentation. NUREG/CR-4266 and NBSIR K.S-3169 (US Nuclear

Regulatory Commission and US Department of Commerce, National Bureau of Standards, Washington, DC) ( IW5>).

2. Soares, C. G., Dick, C. E., Pruitt, J. S. and Sparrow, J. H. Development of Monoenergetic Electron Beam Sources for Radiation-Instrument Calibration. Nucl. Instrum. Methods Phys. Res. 810/11,937-941 (1985).

3. Bcrgcr, M. J. and Seltzer, S. M. Stopping Powers and Ranges of Electrons and Positrons. 2nd ed, NBSIR 82-2.550- A (US

, Department of Commerce, National Bureau of Standards, Washington, DC) (1983).

62
Appendix 5. Role of Nearly Monoenergetic Electron Sources in the Development

of a New Generation of Survey Meters: Excerpts from two Publications

NUREG/CP-0050

Proceedings of the

International Beta Dosimetry

Symposium

Held at Washington, D.C.

February 15-18, 1983

Manuscript Completed: December 1983

Date Published: January 1984

T. F. Gesell, Executive Chairman

USDOE Radiological and Environmental Sciences Laboratory

Sponsored by

The U.S. Department of Energy

U.S. Nuclear Regulatory Commission

Office of Nuclear Regulatory Research

Health Physics Society

Proceedings published by the

U.S. Nuclear Regulatory Commission

Washington, D.C. 20555

63
Appendix 5, continued

LOS ALAMOS PORTABLE BETA-RAY SPECTROMETER

B. H. Erkkila, R. J. Brake, and D. A. Waechter

Los Alamos National Laboratory, Lob Alamos, NM 87545

*This work was performed under the auspices of the U. S. Department of Energy

Abstract: The integration of a beta-ray detector to multichannel analyzer (MCA) and computer has resulted in a portable spectrometer for studying beta rays in the field The present detector is a 5 cm diameter by 2 cm thick plastic scintillator manufac- tured by Bicron, Inc. Other detectors can easily be integrated into the package The integral instrument package is 15 cm wide by 15 cm high by 25 cm long and weighs

less than 10 pounds. Internal rechargeable batteries for 8 hours of field operation

are included. The instrument contains a detector, an amplifier, a multichannel

analyzer, and a liquid crystal display (LCD). A microprocessor controls all the functions of the instrument and is programmed to display all necessary information

and 128-channel spectra on the LCD.

5000

5000

c

c

«Xu

\

M

4*c

3

Ou

2000

Fig. 3. Detector calibration measurements for three energies

(375 keV, 1.0 MeV, and 2.5 MeV electrons).

64
Appendix 5, continued

NUREQ/CR-4a«

EOG-244*

Ototributton Catagonr: AN

FIELD TESTS OF A PORTABLE TISSUE EQUIVALENT

SURVEY METER FOR MONITORING MIXED

BETA/GAMMA RADIATION FIELDS

D. E. Martz

B. L. Rich

L. O. Johnson

S. H. Daniel. Ill

Published May 1386

EGEfG l^aho. Inc. Idaho Falls, Idaho 83415

Prepared for the

Office of Nuclear Regulatory Research

Division of Radiation Programs and Earth SciancM

U.S. Nuclear Regulatory Commission

Washington. D.C. 20666 Rn No. A-«834

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

+ NBS MONOENERGETIC ELECTRONS

0 RESL BETA PLAQUE SOURCES (E avg)

M BUCHLEfl PTB BETA SOURCES (E avg)

I- N * m

+0 +4

J I I I I I I I I I I I I L J I I L J I I 1_ 500 1000 1500 2000 2500

ENERGY (KEV)

Figure 16. Measured energy dependence of tlie INEL TE survey meter to mooo-energetic electrons and standard beta sources.

65
11 . List of Pertinent Tables and Reports on File in the Office of the Dosimetry

Group

11.1 Pertaining to the Corrections Leading from Ionization Measurements with

The Extrapolation Ionization Chamber to Absorbed Dose Rate to Water

Berger, M, J. and Seltzer, S. M., Stopping Powers and Ranges of Electrons

and Positrons (2nd Ed.). NBSIR 82-2550-A; U.S. Department of Commerce,

National Bureau of Standards, Gaithersburg, MD 20899; 1982.

Handbook of Chemistry and Physics, 67th Edition; Robert C. Weast, Editor;

Chemical Rubber Publishing Co., Cleveland, OH; 1986-1987.

Hubbell, J. H. , Photon Mass Attenuation and Energy Absorption Coefficients

from 1 keV to 20 MeV. Int. J. Appl . Radiat. Isot. 33, 1269-1290; 1982

ICRU Report 31; Average Energy Required to Produce an Ion Pair.

International Commission on Radiation Units and Measurements; Bethesda,

MD 20814; 1979.

11.2 Pertaining to Radiation Protection

American National Standard: Radiological Safety in the Design and

Operation of Particle Accelerators. NBS Handbook 107; National Bureau

of Standards, Gaithersburg, MD 20899; 1970.

ANL-7291, Revision 1; Moe, H. J., Lasuk, S. R., and Schumacher, M. C.

The Health Physics and Radiological Health Handbook; Radiation Safety

Technician Training Course; Argonne National Laboratory. U.S. Department

of Commerce, National Technical Information Service, Springfield, VA

22161; 1972.

ICRU Report 33; Radiation Quantities and Units. International Commission

on Radiation Units and Measurements; Bethesda, MD 20814; 1980.

NCRP Report No. 39; Basic Radiation Protection Criteria. National Council

on Radiation Protection and Measurements; Bethesda, MD 20814; 1971.

NCRP Report No. 51; Radiation Protection Design Guidelines for

0.1 — 100 MeV Particle Accelerator Facilities. National Council on Radiation Protection and Measurements; Bethesda, MD 20814; 1977.

NCRP Report No. 82; SI Units in Radiation Protection and Measurements.

National Council on Radiation Protection and Measurements; Bethesda, MD

20814; 1985.

66
NBS-IMA (REV. 2-ao) U.S. DEPT. OF COMM.

mm mi^DADLIIO RATA BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBS/SP-250/21

2. Performing Organ. Report No. 3. Publication Date

April 1988

4. TITLE AND SUBTITLE

NBS Measurement Services:

Calibration of Beta-Particle Radiation Instrumentation and Sources

5. AUTHOR(S)

John S. Pruitt, Christopher Soares, and Margarete Ehrlich

6. PERFORMING ORGANIZATION (If joint or other ttian NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE

GATTHERSBURG, 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

Library of Congress Catalog Card Number 88-600514

I I Document describes a computer program; SF-185, PIPS Software Summary, is attached.

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

bibliography or literature survey, mention it here)

In a project funded jointly by the National Bureau of Standards (NBS) and the Nuclear Regulatory Commission (NRC), NBS has developed a calibration

facility for beta-particle Instruments and sources used In radiationprotection dosimetry. Central to this facility are beta-particle and nearly

monoenergetic electron beams characterized in terms of absorbed-dose rates to water and 1n terms of pulse-height distributions of beta-particle and electron

spectra.

This documentation utilizes material previously published or otherwise

disseminated by members of the staff of the Ionizing Radiation Division of the Center for Radiation Research (CRR) of the National Bureau of Standards (NBS).

After a description of the facility and the procedures, results are given of

(1) studies that led to the determination of absorbed-dose rates to water for the NBS beta-particle and nearly monoenergetic electron beams, and to the uncertainties entering Into these determinations; (2) studies of the influence

of source configuration on beta-particle and nearly monoenergetic electron

spectra; and (3) an estimate of the overall uncertainties associated with the

proposed calibration service. Included also are the results of some calibrations similar to those that will be performed under the calibration

service employing the beta-particle sources, and of samples of studies

demonstrating the benefits of using the nearly monoenergetic electron beams in special instrument studies and in a future extension of the calibration

service.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

accelerator-produced, nearly monoenerget ic electrons ; beam characterization; beta-particle

sources; documentation; dosimetry; instrument calibration; radiation protection; safety; source calibration; uncertainties . 13. AVAILABILITY

g3 Unlimited

I I For Official Distribution, Do Not Release to NTIS

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

75

15. Price

USCOMM-DC 6043-P80

.1
U.S. Department of Commerce

National Bureau of Standards

Gaithersburg, MD 20899

Official Business

Penalty for Private Use $300

Stimulating America's Progress 1913-1988