NUREG/CR-4266

NBSIR 85-3169

Standard Beta-Particle and

Monoenergetic Electron Sources

for the Calibration of

Beta-Radiation Protection

Instrumentation

Jointly Sponsored by the U.S. Nuclear Regulatory Commission

and the U.S. Department of Commerce

NAT 1 INST OF STAND & TECH

AlllOb 3 4 fi 4 5

Reference

NBS

PUBLICATIONS

Prepared by M. Ehrlich, J. S. Pruitt, C. G. Soares

Contributions made also by C. E. Dick, H. T. Heaton, II, and R. 6. Schwartz

U.S. Nuclear Regulatory Commission

-QC100

.056

85~31b9

1983

U. S. Department of Commerce

National Bureau of Standards
NOTICE

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NUREG/CR-4266

NBSIR 85-3169

Standard Beta-Particle and

Monoenergetic Electron Sources

for the Calibration of

Beta-Radiation Protection

Instrumentation

Jointly Sponsored by the U.S. Nuclear Regulatory Commission

and the U.S. Department of Commerce

NATIONAL BUREAU OF STANDARDS

LIBRARY

Manuscript Completed: May 1985

Date Published: August 1985

Prepared by

M. Ehrlich, J. S. Pruitt, C. G. Soares

Contributions made also by C. E. Dick, H. T. Heaton, II, and R. B. Schwartz

U.S. Nuclear Regulatory Commission

Washington, D.C. 20555

SOf>SAU O*

U.S. Department of Commerce

Washington, D.C. 20234

Prepared for

Division of Radiation Programs and Earth Sciences

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, D.C. 20555

NRC FIN B8271

and

Center for Radiation Research

National Bureau of Standards

U. S. Department of Commerce

Gaithersburg, MD 20899

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. The facility consists of beta-particle and nearly

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

plastic and in terms of beta-particle spectra. A second phase of the project

was concerned with establishing secondary calibration laboratories for

radi at ion -protec ti on instruments.

This final report includes a detailed discussion of (1) the determination

of absorbed-dose rates to plastic for each beta-particle and nearly monoenergetic electron beam, dose-rate dependence on altitude above sea level, and

an estimate of the overall uncertainties in dose-rate measurements; (2) betaparticle and nearly monoenergetic electron spectra and their dependence on

source configuration; and (3) degree of achievable uniformity of beam cross

sections. Included also is a review of the results of a first attempt to

predict instrument response to realistic beta-particle environments from their

response to monoenergetic electrons and knowledge of the approximate betaparticle spectra. Attached to the report are proposed guidelines for

establishing secondary calibration laboratories for radi ati on-protecti on

i nstruments.

TABLE OF CONTENTS

Abstract iii

Preface. xi

1. Introduction 1

2. The NBS Beta-Particle Calibration Facility 3

2.1 Measurement of Absorbed-Dose Rates 3

2.1.1 Absorbed-dose rates to air...... 3

2.1.2 Uncertainty in the Determination of Absorbed-Dose Rate

to Air 7 2.1.3 Determination of Absorbed-Dose Rate to Plastic as a Function of Depth in Plastic 15

2.2 Beta-Particle Spectral Composition at the Reference Point 18

2.3 Comparison of Different Types of 90 Sr+ 90 Y Sources 27

3. The NBS Cl ose-to-Monoenergeti c Electron Calibration Facility 28

3.1 Experimental Setup 34

3.2 Beam Character! zation 36

3.2.1 Beam Cross Section 36

3.2.2 Beam Dosimetry 39

3.2.3 Spectral Composition of the Electron Beam at the

Reference Point 39

4. Prediction of Instrument Response to Beta Particles from Response to

Close-to-Monoenergetic Electrons 40

5. Guidelines for Establishing Secondary Calibration Laboratories for

Radiation-Protection Sources and Instruments 49

6. References 52

Appendix 1 Beta-Ray Source Calibration Procedure and Error Analysis 53

Appendix 2 Proposed Guidelines for Establishing a Secondary Calibration

Laboratory for Beta-Particle Source Standardization and

Calibration of Beta-Particle Protection Instruments 57

Appendix 3 Proposed Guidelines for Establishing a Secondary Calibration

Laboratory for Neutron-Protecti on Instruments 65

Appendix 4 Proposed Procedure for Carrying out Measurement-Assurance

Studies for a Secondary Calibration Laboratory

v

LIST OF FIGURES

Page

Figure 1 Beta-Particle Calibration Facility 4

Figure 2 Theoretical Beta-Particle Spectra for the Radionuclides

Employed . 6

Figure 3 Ionization Current as a Function of Electrode Separation in

the Extrapolation Ionization Chamber 8

Figure 4 Dependence of Dose Rate on Detector Positioning 10

Figure 5 Variation of Dose Rate with Air Density Close to Sea Level... 12

Figure 6 Degree of Non-Uniformi ty of Dose Rate over the Horizontal

Beam Cross Section 14

Figure 7 Ionization Current as a Function of Depth in Polyethylene

Terephthal ate (PTP) 17

Figure 8 Attenuation Curve in Polystyrene for the 2-mCi 90 Sr+ 90 Y Source 19

Figure 9 top: Spectrum of 147 Pm Source, Used with Flattening Filter,

bottom: Spectrum of 204 T1 Source, Used with Flattening

Filter, 20

Figure 10 Spectrum of 90 Sr+ 90

Y Sources, Showing Influence of Flattening Filter and of Detector Collimator 21

Figure 11 Spectrum of 90 Sr+ 90 Y Sources Used without Flattening Filter,

Showing Influence of Source-to-Detector Distance 22

Figure 12 Effect of Added Polystyrene Absorbers on 90 Sr+ 90 Y Spectrum... 23

Figure 13 Energy Calibration Spectrum of 207 Bi Obtained with the 5-mm

Silicon Surf ace-Barrier Detector 25

Figure 14 Ioni zation-versus-Depth Curves for Three Different Types of

9 o Sr+9 o Y Sources 30

Figure 15 Comparison of Spectrum of Buchl er-Amersham and University of

Michigan (NVLAP) 90 Sr+90 Y Sources, on Semi -Logar i thmi c Scale. 31

Figure 16 Comparison of Spectrum of Amersham Experimental and

University of Michigan (NVLAP) 90 Sr+90 Y Sources, on Semi -Logari thmetic Scale... i?

vii
Page

Figure 17 Difference between Buchler-Amersham and Amersham Experimental 90 Sr+90 Y Sources, on Semi -Logar i thmi c Scale 33

Figure 18 Beam-Handling Arrangement, Monoenergetic Electrons 35

Figure 19 Degree of Uniformity of Cross Section of Scanned Electron

Beam over the Range of Energies and Distances of Interest.... 37

Figure 20 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 41

Figure 21 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 42

Figure 22 Degradation in Air of Nominal 200-keV to 2.5-MeV Electrons

in Different Thicknesses of Intervening Air Layers 43

Figure 23 Response Function of Ionization-Chamber Type Survey Meter

to Monoenergetic Electrons Relative to Response to Cs - 1 37

Gamma Radiation 46

Figure 24 Response of Thermol umi nescence-Type Personnel Dosimeter to

Monoenergetic Electrons Relative to Response to Co-60 Gamma

Radi at ion 47

Figure 25 Response of Silicon Surf ace-Barrier Detector to Monoenergetic

Electrons Relative to Beam-Monitor Signal...., 48

vi ii
LIST OF TABLES

Table 1

Table 2

Table 3

Table 4

Table 5

Table 6

Table 7

Table 8

Table in

Appendix

Page

Characteristics of Amersham-Buchl er Beta-Particle Sources.... 5

Comparison of Absorbed-Dose Rates to Air Obtained by NBS

and PTB 9

Air Density Correction Factor, (1 + ax + 3x 2

) 13

Estimate of Uncertainty in Calibration of Beta-Particle

Sources 16

Comparison of Different Types of 9 °Sr+ 90 Y Sources 29

Pertinent Electron Beam Parameters 38

Response Characteri sti cs Measured for Three Types of

Beta-Particle Detectors 45

Comparison of Directly Measured and Calculated Instrument

Response to Beta-Particle Spectra 50

Systematic and Statistical Uncertainties 56

i x

PREFACE

During the past few years considerable concern has been expressed within

the health physics community as to whether measurements of doses and dose

rates from beta radiation are being made with sufficient accuracy to assure

protection of workers. One important aspect of such measurements is the

calibration of the measurement devices; and accurate calibrations are

dependent, among other things, on how accurately the strength of the

calibration source is known. Staff personnel at the NRC tend to rely on

evidence that the calibration of radiation sources is "traceable to the NBS."

When we learned that beta radiation standards were not available at the NBS,

we were pleased to provide partial funding to assist the NBS in the purchase

and characterization of such standards. This report of subsequent NBS

activities toward providing beta radiation standards was prepared by

Dr. Margarete Ehrlich, who has been an outstanding contributor to the science

and literature of radiation physics for many years. This contribution is no

exception.

Division of Radiation Programs

and Earth Sciences

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

xi

1. Introduction

This is the final report on a project established (1) to create at the

National Bureau of Standards ( NBS) a facility for the calibration of the type

of beta-particle sources used in radiation-protection dosimetry and for the

study of radiation-protection beta-particle survey and personnel instruments;

and (2) to propose guidelines for a secondary calibration laboratory for

radiation-protection instrumentation. This project was jointly funded by NBS

and the U.S. Nuclear Regulatory Commission (NRC).

The new NBS facility created as a result of this project consists of:

(1) a set of wel 1 -characteri zed beta-particle and near-monoenergetic

electron sources covering an energy range from a few hundred keV to above

2 MeV;

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

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

sources appropriate for the study of radiation-protection instruments; and

(3) a rugged paral 1 el -pi ate ionization chamber, calibrated in the betaparticle beams in terms of absorbed-dose rate to water, and suited for use as

a transfer instrument that is to establish the traceability to NBS of the

beta-particle measurements performed by a secondary-standards laboratory.

One of the reasons why protection measurements for beta particles are

considerably more difficult than for photons is the beta particles' vastly

greater scattering and absorption in the media intervening between the source

and the point of measurement (the "reference point"), and the resulting

greater change in beta-particle spectrum — and consequently in detectorresponse characteri sties — with the measurement configuration. As a consequence, calibrations of radi ati on-measurement instruments in beta - p

f

beams can be considered strictly valid only for the particular conf <j , r r

1
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.

For this reason, the influence on spectral characteri sties of the beamflattening filter and other absorbing materials in the beta-particle beams and

of changes in source-to-detector distance were investigated, and changes in

absorbed-dose rates with barometric pressure were given special attention.

Studies of instrument response in beta-particle beams of different spectral characteristics aid in judging the relative merit of these 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 essentially

monoenergetic-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. 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.

In the following, the various parts and functions of the new NBS facilities are described in detail; the results of a study of the relationship

between the beta-particle fields provided at NBS and those in practical use

elsewhere are reviewed; and an attempt is made to predict instrument response

to a particular beta-particle spectrum from the instrument's response function

obtained with cl ose-to-monoenergetic electrons. Finally, an outline is given

of the requirements for secondary standards laboratories through which these

and other NBS facilities may contribute to the improvement of the state of

radi ation-protection dosimetry.

2
2 . The NBS Beta-Particle Calibration Facility

NBS purchased the Amersham-Buchler 1 beta-particle sources, initially

standardized by the Physi k a 1 i sch-Techni sche Bundesanstal t (PTB), and the

extrapolation ionization chamber designed by the PTB and built for NBS by

Pychlau Technical Works. Figure 1 shows a picture of the setup. Information

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

structure and nominal activity is given in table 1. Figure 2 shows the approximate theoretical beta-particle spectra of the three radionuclides [1,2] 2

.

Following is a discussion of the measurements made to obtain some further

properties of interest in the application of these sources in radiationprotection dosimetry.

2.1 Measurement of Absorbed-Dose Rates

2.1.1 Absorbed-dose rates to air

The extrapolation ionization chamber was used to measure the absorbeddose rate to air for each beta-particle source. Chamber current was found to

be a linear function of the separation of the chamber electrodes for separations between 0.1 and 2.5 mm, after suitable corrections had been applied.

Four of the five corrections applied (those for ion recombination and

throughout this Report, reference to commercial products is made for info "nation only and does not imply that these products are the best or only ones available for the purpose, or that they are endorsed by NBS.

2 Numbers in brackets indicate the literature references at the end of th-s

report

.

3
Figure 1. 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 147Pm source, and is designed for use at the pictured source-to

chamber distance of 20 cm. The source shutter is electrically interlocked, with

photocells defining a safety perimeter as long as the shutter is open, as shown.

4
Table 1. Characteristics of Amersham-Buchl er Beta-Particle Sources

(a) Source Structure and Activity

Radionucl ide Half Life Structure Source

Encapsul ati on

a Nominal Activity

MBq(mCi); date

90 Sr+ 90 Y 28. 5y 90 Sr carbonate

pressed*3 into

Ag foil

50 mg/cm 2 Ag

+0.1 mm (77 mg/cm 2

) steel

1850(50)

;

May 24, 1982

90 Sr+ 90 Y 28. 5y 90 Sr carbonate

pressed*3 into

Ag foil

50 mg/cm 2 Ag

+0.1 mm (77 mg/cm 2

) steel

74(2);

Dec 16, 1982

204 T1 3.78y 204 T1 pressed 13 into Ag foil

20 mg/cm 2 Ag 18.5(0.5)

;

Dec 16, 1982

14 ; Pm 2.62y 147Pm pressed*3 into Ag foil

5 mg/cm 2 Ag 518(14)

;

Dec 16, 1982

(b) Beta-Particle Energies and Ranyes

Radionucl ide Avg. and Max.

Beta-Parti cl

e

Energies (MeV) c

Range in

ai r polystyrene

cm mg/cm 2 cm mg/cm 2

E = 0.196 41 49

90 Srd

^max " 0.546 187 225 0.19 204

9 0y E = 0.935 375 452

Emax

= 2.284 1037 1249 1.10 1167

204ti E = 0.244 58 70

Emax " 0.763 291 351 0.30 319

147 Pm E = 0.062 5.9 7.2

Emax

- 0.225 51 61 0.052 55

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

90 Sr+ 90Y source is listed as 50 mg/cm 2 Ag plus 1 mg/cm 2 Au.

b During the rolling stage.

c Average and maximum energies are for ideal point sources.

^Essential ly , none of the beta particles from 90 Sr penetrate the combined

filtration of (1) the source encapsulation, (2) the air between source <m i detector, and (3) the beam-flattening filter.

5
£ Beta-Particle Energy, MeV

Q_

Beta-Particle Energy, MeV

Figure 2. 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.

6
diffusion, for air-density variations inside the chamber, for beam scattering

by the side walls of the chamber, and for beam attenuation by the chamber's

beam-entrance window) were taken from the literature [3-5]. The correction

required for variations of air density outside the ionization chamber was

measured [6]. (See also 2.1.2b.)

Figure 3 shows the fully corrected ionization currents as a function of

electrode-plate separation. The slopes of the curves were used to determine

the corresponding absorbed-dose rate to air at the front surface of the

chamber. Table 2 gives a comparison of the results with those obtained by the

PTB for the same sources in similar geometries.

2.1.2 Uncertainty in the Determination of Absorbed-Dose Rate to Air

In order to arrive at the uncertainty in the absorbed-dose rate determinations, measurements were made of the variation of dose rate with sourceto-detector distance and with air density, and of the degree of non-uniformity

of the dose rate over the beam cross section. For each of the parameters

investigated, the variations were found to be largest for 147Pm, indicating

that 147Pm calibrations will be the least reliable. This conclusion is

supported by a comparison of the magnitude of the correction for beam attenuation in the entrance window of the chamber, taken from the literature. While

for 90 Sr+ 90 Y and for 204T1 the correction is less than 2 percent, it is almost

a factor of two for 147Pm, i.e., only about one-half of the beta particles

penetrate the window of the extrapolation chamber. The results of the

measurements are discussed below in grater detail:

(a) Dependence of dose rate on detector positioning. Figure 4 shows tn^r

positioning of the detector is not too critical for any but the 14/ Pm source,

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

7
EEo

h-<

cn<

CL

LiJ

CO

LUI

<

CL 8

Figure

3.

Ionization

Current

asa

Function

of

Electrode

Separation

in

the

Extrapolation

Ionization

Chamber.
Table 2. Comparison of Absorbed-Dose Rates to Air Obtained by NBS and PTB.

Source Absorbed Dose Rate

Nomi nal At source-to- yGy/s

a

Type acti vi ty detector distance Ratio

(mCi

)

(cm) NBS PTB NBS/PTB

147 Pm 14 20 0.234 0.227 1.03

204T 1 0.5 30 0.293 0.293 1.00

90sr+9°Y 2 30 1.707 1.685 1.01

II 50 11 451 449 1.01

II II 30 62.4 61.9 1.01

II II 50 22.4 22.2 1.01

deferred to January 1, 1983, 20°C, and 760 mmHg.

9
10

DISTANCE

FROM

CALIBRATION

PLANE,

cm
to 10 percent. As a consequence, there will be a significant dose-rate

yradient over the sensitive volume of a survey meter in the l4/Pm betaparticle beam, if this volume extends over more than just a few millimeters.

(b) Variation of dose rate with air pressure . Figure 5 shows the variation of dose rate with the reciprocal of the air density for the three types

of beta-particle sources measured at NBS, located at an altitude close to sea

level. For 147Pm, there is a 10-percent change in dose rate for a change in

atmospheric pressure of less than 2 percent. No appreciable change in dose

rate was observed with the 204T1 and 90 Sr+ 90Y sources for a change in

barometric pressure amounting to 4 percent. The air-density corrections

measured by the PTB and provided with the sources compensate adequately for

this effect, but they are not suited for use with sources at a laboratory

situated at a much higher altitude. For this reason, NBS carried out

measurements over a wide range of barometric pressures in an environmental

chamber. [6] Table 3 shows the resulting correction factors for calibrations

carried out at sea level for sources to be used at higher altitudes. These

corrections were checked by actual measurements on a 147Pm source at an

elevation of ~ 1500 m above sea level. The measured absorbed-dose rate,

corrected for the difference in altitude, was found to agree to within the

experimental uncertainty (± 3%) with the dose rate that had been measured for

this source at the PTB, located near sea level.

(c) Degree of non-uniformity of dose rate over the beam cross section .

Typical results of this study are shown in figure 6. Along the horizontal

line passing through the center of the beam cross section, the points of equal

dose rate are seen to be distributed symmetrical ly about the beam center for

90 Sr+ 90 Y and for 204T1, the dose rates varying by less than ± 1 percent and

± 2 percent, respecti vely , for points 10 cm off the central beam axes of these

11
147

3 IV H 3S00 3AllV13y

12

1.00

1.02

1.04

1.06

(RELATIVE

AIR

DENSITY)'

1

Figure

5.

Variation

of

Dose

Rate

with

Air

Density

Close

to

Sea

Level.
Table 3. Air Density Correction Factor, (1 + ax + 3 x

2

)

The variable x stands for the quantity (1 - p/p Q ), where p Q is the reference

air density and p is the air density at the point of measurement. The coefficients a and 3 are given below for the four sources and three depths in a plastic absorber, from which the correction factor for the depths and environmental conditions of interest may be computed.

Nucl ide Nomi nal

Acti vity

(MBq)

Absorber

Depth 9 (mg/cm 2

)

No. of

Data

Sets

a 3 a (%)

500 2.6 20 4.8 12.6 3

147 Pm

b

6.3 9 3.5 19.8 4

10.1 9 0.7 31.3 10

20 2.6 30 0.17 -0.52 0.9

204y] b

6.3 9 0.13 -0.33 0.4

46.2 11 0.14 0.48 2

80 2.6 14 -0.08 -0.03 0.6

90 Sr+90 Y b

6.3 11 -0.06 -0.18 0.2

46.2 10 0.04 -0.28 0.4

2000 2.6 11 0.12 -0.50 0.3

90 Sr+ 90Y

c 46.2 10 0.07 -0.18 0.2

318.0 21 0.25 -0.39 0.2

a Thickness of polyethylene terepthalate (PTP) absorber in front of the

extrapolation chamber air gap for all but the 318 mg/cm 2 depth, for which

315 mg/cm 2 of polystyrene was used in front of the 2.6 mg/cm 2 extrapolationchamber window.

b With flattening filter, at the prescribed source-to-detector distance (see

Table 2).

c Without flattening filter, at a 30-cm source-to-detector distance.

13
o

E

Cl — r3 IV H 3S0Q 3A11V13H

-MO

0> OO

(/)

</> O

i_ o

fO

d) CD

<T3 -MC

O) o

co

0)

:3

CT>

14

DISTANCE

FROM

BEAM

AXIS,

cm
two sources. For 147Pm, the points of equal dose rate are asymmetrically

distributed, the variation in dose rate amounting to about ± 10 percent for

points up to 10 cm removed from the central beam axis (±6 percent for points

removed by not more than ~8 cm). The asymmetry evident in the 147Pm beam

cross section may be caused by the shutter of the source holder, and probably

can be reduced.

(d) Estimate of overall measurement uncertainty . Table 4 shows

estimates for the uncertainties in the calibration of the NBS beta-particle

sources. Since these uncertainties are mainly associated with the measurement

process proper, they also hold for the calibration of sources submitted to NBS

by others, provided that the degree of non-uniformity of the beam cross

section of these sources is similar. Appendix 1 gives a detailed discussion

of how the values given in table 4 were obtained. Further uncertainties arise

in the study of radi ation-protection instruments mainly because of limitations

in the reproducibility of the response of these instruments and the larger

size of their sensitive volumes.

2.1.3 Determination of Absorbed-Dose Rate to Plastic as a Function of Depth

in Plastic

In order to arri ve at the shallow and deep absorbed-dose rates to plastic

(which, for protection purposes, may be set equal to absorbed-dose rates to

tissue) measurements were made of ionization current in the extrapolation

ionization chamber as a function of thickness of polyethylene terephthal ate

(PTP) added to the 2.6-mg/cm 2 window of the chamber. The results are shown in

figure 7 for all three types of beta-particle sources. For 14/Pm, the

decrease in current with PTP thickness is roughly exponential from the onse'

,

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

15
Table 4. Estimate of Uncertainty in Calibration of Beta-Particle Sources

Quantity

Uncertai

Systematic a

ity

Statistical b

Corrected ionization current per

unit air gap, measured for four

separate setups and five air gaps

per setup

0 0.6%

147 Pm: 1.2%

204T1 : 0.9%

"Sr+ 90 Y : 0.3%

Absorbed dose to water

(computed from ionization current) 1.6% —

TOTAL 1.7% —

Components combined as the square root of the sum of their squares.

^Estimated from a plot of ionization current-versus-ai r gap (19 degrees of

freedom, on the average) as three times the standard deviation computed from

the residuals of the least-squares fit of a straight line to the data points.

Q Includes uncertainty in positioning the ionization chamber relative to the source and installing the flattening filter, if any.

16
IONIZATION

CURRENT,

RELATIVE

ADDED PTP THICKNESS, mg cm

-2

Figure 7. Ionization Current as a Function of Depth in Polyethylene

Terephthal ate (PTP).

17
PTP. For larger thicknesses, the ionization current remains at ~ 1 percent of

the maximum current, probably indicating the level of bremsstrahl ung produced

by the 147 Pm beta particles in the PTP and of the 121-keV gamma rays from

14/ Pm. For 90 Sr+ 90 Y, there is an ionization buildup over the first ~ 40 mg/cm 2 of added PTP before attenuation starts to dominate, while for

204 T1 the attenuation in PTP seems partially offset by buildup over the first

added ~ 15 mg/cm 2 of PTP.

Because PTP is not readily adaptable to measurements at larger thicknesses, attenuation curves for 90 Sr+ 90y were also obtained with added polystyrene, covering a range of thicknesses from ~ 100 mg/cm 2 to beyond 2 g/cm 2

.

The results are shown in figure 8, demonstrating that any bremsstrahl ung

background (ionization-current contribution by the source for polystyrene

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

2.2 Beta-Particle Spectral Composition at the Reference Point

Because of the emphasis on the change in spectral characteri sties with

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

distributions, obtained with a given detector and converted to an absolute

energy scale, ignoring certain spectral distortions introduced by the detector. Figures 9 through 12 show these spectral distributions for the three

types of sources. (No difference was found between the spectra of the 2-mCi

and the 50-mCi 90Sr+ 90Y sources used in the same geometry.) The spectra were

obtained with a 5-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 207Bi point source (NBS Standard Reference Material

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

18
IONIZATION

CURRENT

ARBITRARY

UNITS

lOO

Figure 8. Attenuation Curve in Polystyrene for the 2-mCi 90 Sr+

J 'V Sou^-.'.

19
counts

per

channel

COUNTS

per

channel

0 0.10 0.20 0.30

ENERGY, MeV

Figure 9. top: Spectrum of 147 Pm Source, Used with Flattening Filter

Distance: 20 cm; Detector Uncollimated.

bottom: Spectrum of 204T1 Source, Used with Flattening Filter

Showing Influence of Lead Collimator with Circular

Aperture, 3.2 mm in Diameter. Distance: 30 cm. 20
ENERGY, MeV

Figure 10. Spectrum of 90 Sr+ 90 Y Sources, Showing Influence of Flattening

Filter and of Detector Collimator. The Theoretical Spectrum of °Y is Shown

for Comparison.

Zl
Figure 11. Spectrum of 90 Sr+ 90 Y Sources Used without Flattening Filter,

Showing Influence of Source-to-Detector distance. Detector Collimator:

Circular Aperture, 1.6-mm in Diameter.

22
COUNTS

PER

CHANNEL

Figure 12. Effect of Added Polystyrene Absorbers on 90 Sr+ 90Y Spectrum.

Detector Collimator: Circular Aperture, 1.6 mm in Diameter.

23
The pulse-height distribution obtained with this source using the surfacebarrier detector is shown in figure 13. For all but the 147Pm source, a lead

collimator with a 1.6 -mm diameter circular aperture was employed near the

detector, since collimation was found to remove a considerable amount of

scatter (see fig. 9 and 10, bottom). Following is a detailed discussion of

the results for the 90Sr+ 90Y sources:

(a) Influence of Flattening Filter on 90 Sr+ 9Q Y Pulse-Height

Pi stri bution . Figure 10 shows that the flattening filter causes a shift in

the energy endpoint. This should be kept in mind when the 2-mCi source (used

with flattening filter) is replaced by the 50-mCi source (used without

flattening filter) in the course of an instrument calibration.

(b) Influence of source-to-detector distance for 90 Sr+ 90 Y . Figure 11

shows the spectral distributions at the three distances specified by Buchler.

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, should not introduce an

appreciable change in i nstrument-cal i bration results.

(c) Comparison of the measured pulse-height distribution with theory .

For this comparison, one must consider (1) that the theoretical spectrum shown

was computed for the bare 90Y radionuclide; and (2) that the measured spectral

distribution is for a 90 Sr+ 90 Y source encapsulated in 50 mg/cm 2 of silver and

0.1 mm (77 mg/cm 2

) of iron, at a distance of 50 cm (61 mg/cm 2

) in air, with a

layer of about 1 mg/cm 2 of aluminized PTP over the detector. Taking into

consideration of the average and maximum beta-particle energies given in

table 1, this leads to respective approximate average and maximum ranges for

24
COUNTS

PER

CHANNEL

Figure 13. Energy Calibration Spectrum of 207 Bi Obtained with the 5-mm Silicon

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

con versi on-el ectron energies below 1.1 MeV [7] took into account energy

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

of air between source and detector.

25
the 90 Sr beta particles of 60 and 270 mg/cm 2 in the composite total of roughly

190 mg/cm 2 of absorber which these beta particles must traverse to reach the

detector surface. As a consequence, there are relatively few 90Sr beta

particles reaching the detector, making the measured pulse-height distribution

mainly that of 90 Y, slowed down by the intervening absorbers. Figure 10

(bottom) shows that the general shape of the pulse-height distribution

obtained with the collimated detector compares relatively well with the

theoretical 90Y 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 90 Sr beta particles reaching the detector (see

figure 2 for the theoretical 90 Sr+ 90 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 12 shows the

change of the 90 Sr+ 90Y spectrum with added thickness of polystyrene absorber,

in a semi-logarithmic presentation 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 8 often are obtained

from measurements with paral 1 el -pi ate 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 2

) correspond!

- ng 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.

26
2.3 Comparison of Different Types of 90 Sr+ 90 Y Sources

Since spectral characteri sties of 90 Sr+ 90 Y beta-particle sources may vary

widely with source yeometry, a certain degree of standardization of source

construction is required if sources from different manufacturers are to be

used successfully in instrument-cal ibration activities. ANSI N13.ll [8]

specifies a filtration thick enough (~ 100 mg/cm 2

) to remove the 90Sr component, but of a sufficiently low atomic number (< 26) not to cause excessive

bremsstrahl ung production. The international standard, ISO 6980 [9] specifies

a residual maximum energy of > 1.80 MeV. The DOE performance-testi ng standard

[10] will specify the limits on absorption characteri sties in polymethylmethacrylate (PMMA) of the beta-particle beam emitted by the source, requiring the

ratios of absorbed-dose rates to PMMA at depths of 100 mg/cm 2 and 7 mg/cm 2 to

be equal to 1.01 ± 0.05, and the correspondi ng ratios at 1000 mg/cm 2 and 7

mg/cm 2 to be less than 0.01.

The relatively high-activity ( ~ 140 mCi ) source used by the University of

Michigan in the National Voluntary Laboratory Accreditation Program (NVLAP)

for dosimetry performance by NRC licensees was built to the University's

specifications. It is in compliance with ANSI N13.ll. The source that will

be used in the Department of Energy Laboratory Accreditation Program (D0ELAP)

for dosimetry performance by National Scientific Laboratories was manufactured

by Buchl er-Amersham according to specifications given by members of the

National Physical Laboratory in England, who also were instrumental in

developing the ISO standard.

It was considered of interest to compare the D0ELAP source and the

present NVLAP source by the different criteria. In this comparison, we also

included an experimental source produced by Amersham to meet ANSI N13.ll

specifications but designed to have less internal scattering (but much lowe27
activity) than the present NVLAP source. Table 5 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.

Further elucidation of these results is possible if one examines the

response- versus-depth curves on which the DOELAP criterion is based (fig. 14)

and the associated pulse-height distributions (figs. 15 through 19) 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 certain critical information about the selection of criteria for

source specification: They reveal that an operational criterion of the DOELAP

or ISO type may be better suited for obtaining desirable source characteristics than a criterion of the ANSI N13.ll type which specifies material

properties and dimensions. They also reveal that the DOELAP 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 (only slowly decaying) background noise shown in figure 14 for results

obtained after irradiation of the extrapolation chamber with a high-intensity

electron beam alerts one to the need for special care in the use of such a

chamber.

3. The NBS Cl ose-to-Monoenergetic Electron Calibration Facility [11]

The NBS 500-keV cascade-rectifier accelerator and 4-MeV Van de Graaff

accelerators were adapted for application in the calibration of radiationprotection instruments by (a) magnetic scanning of the electron beams in a

two-dimensional raster pattern in order to produce suitably large beam cross

28
Table

5:

Comparison

of

Different

Types

of

90

Sr+

90Y

Sources

29
Figure 14. Ioni zati on- versus-Depth Curves for Three Different types of

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

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

source.

The curves in PMMA are undistinguishable from those in polystyrene. No

line could be drawn for the bremsstrahl 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.

30
COUNTS

PER

CHANNEL

Figure 15. Comparison of Spectrum of Buchl er-Amersham and University of

Michigan (NVLAP) 90Sr+ 90 Y Sources, on Semi -Logarithmic Scale. The ordinates

were adjusted for equal total number of counts.

31
COUNTS

PER

CHANNEL

Figure 16. Comparison of Spectrum of Amersham Experimental and University of

Michigan (NVLAP) 90 Sr+ 90 Y Sources, on Semi -Logarithmic Scale. The ordinates

were adjusted for equal total number of counts.

32
COUNTS

PER

CHANNEL

Figure 17. Difference between Buchler-Amersham and Amersham Experimental

90sr+90y Sources, on Semi-Logarithmic Scale. Adding 82 mg/cm 2 of aluminum

to the Amersham experimental source is seen to make its spectrum similar to

that of the Buchler-Amersham source (fig. 15).

33
sections; and (b) having the beams exit from the vacuum through thin, low-Z

windows and keeping the air paths as short as possible in order to prevent

unnecessary spectral degradation. In the following, the experimental setup

and the beam characterization in terms of beam cross section and uniformity,

electron spectrum, and absorbed-dose rates to plastic will be discussed.

3.1 Experimental Setup

The beam handling arrangement is shown schematically in figure 18.

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. The exit windows must be thick enough

to withstand atmospheric pressure over a large area, yet thin enough not to

degrade the electron beam energy appreciably. A thin foil (25 ym) 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.

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. At both accelerators, beam intensity is

continuously monitored with a fast plastic scintillator-photomultiplier

detector placed in an off axis location (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

34
DEFLECTION

COILS

DEFLECTION

COILS

Figure 18. Beam-Handling Arrangement.

35
window at the Van de Graaff accelerator). In order to limit counting rates,

the monitor is equipped with a collimator with a circular aperture, 2 mm in

diameter, located near the front surface of the scintillator. The fast (10-ns

wide) pulses from the detector allow countiny rates up to 10 5 s -1 with no

significant pile-up and with very low background rates (< 2 s -1

). Because of

fluctuations in beam position (beam "wandering") which limits dosimetry

reproduci bi 1 ity, plans for the future include the installation of a circular

array of four or more fast plastic scintillator-photomultiplier monitors,

eventually to be coupled with the beam-steering system.

3.2 Beam Characterization

3.2.1 Beam Cross Section

The degree of uniformity of the electron-beam cross section was

determined radiographically 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. Densitometric scans were obtained of the optical

densities of the radiographs along two perpendicular diameters of the

essentially circular images of the beam cross sections. Figure 19 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-determi ned degree of uniformity. A synposis of the results is shown in

the first three columns of table 6 where usable field size is shown over the

covered energy range.

36
o

+->

A 1 ISN 30 3 VDIldO All SN 3 Q OVDIldO

ac c

03 o

4—

to •r—

<L) c_ e

•r—

O 3

CD 4-

C_ CO

CD CD -r- c E

UJ 3 * “O • 4- CD CD

O o O • • O *r- or CD t_ >

UJ CD CL

f—

C **

*— 03 CD

O cel x: ^ UJ 4-> o CD c

-C CD 03

4-> 4-> -C o 03 4-> cr U. a

u. O) •i~ CD > “O

UJ o c o o • r— £ E < 03 4-> i— a) JZ -Q

GO GO cr> — T- O) Q c= to o 03

O) 4-> "O SZ O

O) 4->

C cC 1/1 /o >0 (1) o o

I/O 4-> "O o

*«-

— >)

O CL 4-> C 4/ LA o _c c

•l— 4-> Ol

4-> -O u c

a) >-X

OO 3

LA .

LA <1) <4_ c

I- -Cu

LAoL

c_> T3 -C

>t- 0)

J

O -C

LA L. >,'0(1/

*J T3 >

•iO E Oj LCD O H N

•i— LA C • o u -o

LA #—

n_ Oi <D O l- •>— *_

0j 4-1 <U C 4)

L.

— XT

4-1 0J (*- o o cn

T3 l

37
Table 6. Pertinent Electron Beam Parameters

Accelerator

Potential

Distance

From Window

Usable

Field Size 3

Most Probable

Electron Energy

Mi nimum

Dose Level

Maximum

Dose Rate

(kV) (cm) (cm) ( keV

)

( yGy) (mGy s _1

)

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

a Defined as the diameter of

uniform to within ± 5%.

the beam cross section over which flux density is

38
3.2.2 Beam Dosimetry

The method for determining absorbed-dose rates to plastic at the reference point was similar to that used with the beta-particle sources except that

a correlation had to be established between the signal from the extrapolation

chamber at the reference point (referred to reference temperature and pressure) and the signal from the fast plastic scintillator used to monitor the

fluctuating beam. Measurements of the ratio of absorbed-dose rate to monitorcounting rate were made for each of the chosen accelerator potentials and exit

wi ndow-to-detector distances. The second-to-1 ast column of table 6 represents

the dose obtained in 10 seconds at a nominal monitor counting rate of 20 per

second, which is the minimum rate that can be monitored with a sufficient

degree of accuracy. The maximum dose rates shown in the last column of the

table correspond to a monitor counting rate of 10 5 per second.

The ratio of absorbed-dose rate to monitor counting rate was reproducible

to within about ± 1 percent unless either the machine was turned off between

measurements or the accelerator potential was changed. These procedures

caused changes of ± 10 percent in the ratio, although no beam shift was

detectable by direct viewing of the beam spot. Therefore, until a more

effective beam control is installed, it will be necessary to establish the

ratio of the absorbed-dose rate at the reference point to the monitor counting

rate before and after each run at a particular accelerator potential.

3.2.3 Spectral Composition of the Electron Beam at the Reference Point

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

aperture lead collimator (see section 2.3) was employed. Beam currents we n e

limited so that count rates did not exceed 2000 s _1

. Measurement procedures

39
were similar to those employed for beta-particle spectrometry . Therefore, the

same caveats expressed in 2.3 about absolute spectral shapes apply here, as

well. 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 20 and 21. Figure 22 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 6,

relating these energies to accelerator potential, exit wi ndow-to-detector

distance, and usable field size.

4. Prediction of Instrument Response to Beta Particles from Response to

Cl ose-to-Monoenergetic Electrons

In order to assess the practical value of a study of radiation-protection

instruments in cl ose-to-monoenergetic electron beams, knowledge is required of

how well one can predict instrument response in realistic beta-particle fields

from their response to monoenergetic electrons. (Note that the term

"response" is used here to signify instrument-scale indication per unit of

absorbed dose to water or plastic.) In principle, this prediction is

straightforward, involving the folding of the beta-particle spectrum of

interest into the instrument response to monoenergetic electrons (i.e., into

the instrument's "response function"), over the energy range covered by the

beta-particle spectrum. In practice, both the spectrum and the response

function may not be known in great detail and with great accuracy. The

question then arises what level of detail and accuracy in the knowledge of the

beta-particle spectrum and the instrument's response function to monoenergetic

40
o

lO

33NNVH0 U3d S1NOOD

13NNVH3 d3d SiNflOO

C\J o

13NNVH0 U3d SiNflOO

CO 3

CD

O

(/> "O

uo c: CD •i

c 3 o 4-J • f— i—

jc:X

h- <r

-*-> -a a c

CD -a

a; 4- o

4— 4-> • — o O oj c Va

CO c

Cl CL) o a>

L_ 3

4-> -t-> a V

cdO i

LU 0Jo

r> c

CD a) 1 </> o • r——>

“O • c 3

HD

Oac > • p- a» 3i 4—) o • p- o X

ro UJ

0> 0) > o CD F

E i • p- o >> o r—

CNJ

O • Q. > ,

(D

<n P * c P

•r— 1 O e= lD o CSJ z O * • '4— o o >>

0)

c: JC L. O 4_> CD

r~

C

4_> o <D

4_> •O 4-

<T3 o t. 1) o

-o 1 CD a w O c z> u

a; • o L o •p- O

C\J E 4-> u

a> >4 a;

L- . 4J 3 O CD cn cl T3

41
ELECTRON ENERGY,MeV

Figure 21. 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.

42
3oo

ELECTRON ENERGY. keV

0 100 200 300 400 ELECTRON ENERGY. keV

ELECTRON ENERGY. MeV ELECTRON ENERGY. M«V

igure 22. Degradation

Different Thicknesses

in Air of Nominal 200-keV to

of Intervening Air Layers.

2.5-MeV Electrons i n

43
electrons are sufficient to compute instrument response to practical betaparticle fields within acceptable limits of uncertainty. Following is a

report on a first attempt to answer this question.

In the course of a related NRC project, the response of a number of

radiation-protection instruments was studied both in the beta-particle beams

and the cl ose-to-monoenergetic electron beams. Among these instruments was a

thermoluminescence personnel dosimeter (TLD); a survey meter of the

ionization-chamber type with a promising response to beta particles; and a

silicon surface-barri er detector used to simulate the sensitive element of

state-of-the-art survey-meter/spectrometers , which rely on internal algorithms

for obtaining absorbed-dose rates in the material of interest from measured

pulse-height distributions. Preliminary response characteristics for these

instruments are shown in table 7 at a number of discrete electron energies.

In order to simplify the folding procedure, an attempt was made to obtain

approximate response functions in closed form by fitting the response data of

table 7 to simple functions. Figures 23 through 25 show the results. Two

functions were used for one of the instruments which exhibits a pronounced

low-energy cut-off. Response, R , to the radiation fields of the three betap particle sources at the reference points then was computed by forming:

V l

r dV n

i8( V ' |

n f<Vwhere r^(T^) is the response of the particular instrument to close-to-monoenergetic electrons of average energy in the ith electron energy interval,

and n^(T^) is the number of beta particles of average energy in this

interval (see figs. 9-11). Table 8 shows the results of this computation for

44
Table 7. Response Characteristics Measured for Three Types of Beta-Particle

Detectors

Response to Beta Particles

Radionuclide

Ion Chamber 3

TLD

b c

Si Surface-Barrier Detector

147Pm 0.05 0.00 0.12

204T1 0.31 0.82 0.33

90y 0.70 1.70 1.08

Response to Monoenergetic Electrons

Electron Energy

(keV) Ion Chamber 3

TLD

b

Si Surface Barrier Detector

0

130 0.24 0.01 0.17

220 0.31 0.91 0.40

250 0.34 1.16 0.54

340 0.43 0.98 0.60

360 0.38 1.05 0.58

1280 1.59 3.37 0.98

1370 1.61 4.23 1.24

2230 1.19 -- 2.15

2320 — 3.44 1.88

Expressed relative to response to Cs-137 photons.

Expressed relative to response to Co-60 photons.

c Response here was calculated as a quantity proportional to the mean stopping

power of water: ^n^(T^) s^ g(T^) / I

n -j(T-j)> where n

^

( T ^

) is the number of

pulses in the ith pulse-height interval correspond!

- ng to an average electron

energy T^ , and s^ q(T^) is the stopping power in water in this interval,

restricted to electron energies > 10 keV [12]. The 10-keV cutoff corresponds

to the lowest pulse-height measured with the silicon surface-barrier

detector.

45
RESPONSE

Figure 23. Response Function of Ionization-Chamber Type Survey Meter to

Monoenerget i c Electrons Relative to Response to Cs-137 Gamma Radiation.

46
RESPONSE

Figure 24. Response of Thermoluminescence-Type Personnel Dosimeter to

Monoenergetic Electrons Relative to Response to Co-60 Gamma Radiation.

47
RESPONSE

ELECTRON ENERGY, MeV

Figure 25. Response of

Electrons Relative to

Silicon Surface-Barrier Detector to Monoenergeti

c

Beam-Monitor Signal.

48
the three instruments and the three types of beta-particle sources, along with

the measured responses from table 7. The results are fair. It is felt that

improvements can be achieved (a) by obtaining definitive values for instrument

response, which — among other things — would require an improvement in

electron-beam control; (b) by refining the functional fits or by working with

point-by-point interpolations; and (c) by fully correcting the beta-particle

spectra

.

5. Guidelines for Establishing Secondary Calibration Laboratories for

Radiation-Protection Sources and Instruments

During the past several years, efforts have been made to find a professional society or some other independent organization interested in improving

radi ati on-protecti on measurements and instrumentation, and willing to sponsor

one or more secondary calibration laboratories. These laboratories would

assist NBS in disseminating the pertinent national measurement standards and

in improving the understandi ng among user groups of how to select and employ

radi ati on-protecti on sources and instrumentation with adequate results.

It was envisaged that the sponsoring organization would require a set of

guidelines for the physical facilities and criteria for the operation of a

secondary calibration laboratory under its sponsorship, and that such guidelines probably would be developed by a consensus among the sponsoring organization, NBS, and other interested Government agencies and user groups. Therefore, the NRC requested that NBS develop draft guidelines that could function

as a basis for further discussions once the sponsoring agency was found.

Appended to this report are (a) proposed guidelines for establishing secondary

calibration laboratories equipped with standardized beta particle, and neutrun

sources, 3 and with the facilities and know-how for studying the response of

Guidelines covering photon sources are being developed by NBS in coope^atior

with the Conference of Radiation Control Program Directors.

49
Table 8. Comparison of Directly Measured and Calculated Instrument Response

to Beta-Particle Spectra

Expressed relative to response to Cs-137 photons.

Expressed relative to response to Co-60 photons.

c Response here was calculated as a quantity proportional to the mean stopping

power of water: ^n^T^) s H 0(T\) / ^(T^), where n^T^) is the number of

pulses in the ith pulse-height interval corresponding to an average electron

energy T. , and s u n (T.) is the stopping power in water in this interval,

1 H 2'J 1 restricted to electron energies > 10 keV [12]. The 10-keV cutoff corresponds

to the lowest pulse-height measured with the silicon surface-barrier

detector.

50
radiation-protection instrumentation in the radiation fields of these sources;

and (b) proposed guidelines for a measurement-assurance program that would

establish and maintain traceability to NBS of the measurements performed by

such secondary calibration 1 aboratories. Recently, the proposed guidelines

have been made available to the ad hoc committee of the Health Physics Society

charged with studying the feasibility of establishing secondary calibration

laboratories under the auspices of this Society.

51
6. REFERENCES

[1] Mantel, Joseph. 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, William G.; Ing, Harry; Freedman, N. A short atlas of beta-ray

spectra. Phys. Med. Biol. 28(11): 1251-1260; 1983.

[3] Bahm, JCfrgen. Saturation corrections for plane-parallel ionization

chambers. Phys. Med. Biol. 21(5): 754-759 1976.

[4] Bahm, Jcrrgen. The perturbation correction factor of ionization

chambers in 3-radiation fields. Phys. Med. Biol. 25(1): 65-75 1980.

[5] Bahm, Jcrrgen. Standardization and calibration in beta dosimetry.

Proceedings of the International Beta Dosimetry Symposium; 1983 February

15-18; Washington, D.C., U.S. Nuclear Regulatory Commission NUREG/CP-0050

73-97.

[6] Pruitt, John S. The effect of altitude on beta-ray source calibrations.

Radiation Protection Dosimetry. 1985; in press.

[7] A handbook of radioactivity measurements procedures. National Council on Radiation Protection and Measurements. NCRP Report No. 58; 1978

November 1. 506 p.

[8] American National Standard for dosimetry — personnel dosimetry

performance — criteria for testing. Am. Nat. Standards Institute ANSI

N13. 11-1983. 24 p.

[9] Reference beta radiations for calibrating dosimeters and doseratemeters

and for determining their response as a function of beta radiation

energy. International Organization for Standardization. ISO 6980;

1984 July. 8p.

[10] DOE laboratory accreditation program for personnel dosimetry systems.

Part II: Standard for the performance testing of personnel dosimetry

systems. Department of Energy; Draft. 24 p.

[11] The material covered in this section was presented by Christopher G.

Soares at the Eighth Conference on the Application of Accelerators in

Research and Industry; Denton, TX; 1984 November 12-14. It will be

published as: Soares, Christopher G; Dick, Charles E.; Pruitt, John S.;

Sparrow, Julian H. Development of monoenergetic electron beam sources

for radiation instrument calibration. Nucl . Inst. Meth. Phys. Research;

1985; in press.

[12] Berger, Martin J.; Seltzer, Stephen M. Stopping powers and ranges of

electrons and positrons (2nd ed.). Nat. Bur. Stand. (U.S.) NBSIR 82-

2550-A; 1983 February. 169 p.

52
Appendix 1

Beta-Particle Source Calibration Procedure and Error Analysis

John S. Pruitt

Absorbed dose rates in water are determined from current measurements

with an extrapolation ionization chamber and are calculated from the

equation:

(W/e) x x k x k T

x k n

x k ux k n q _

a S I U h A

p a

* A

( 1 )

where:

(W/e)

p a

A

( di

c /ds)

is the energy expended per unit ionization charge in air

is the ratio of the average stopping power of water to that of air

is the backscatter correction to make PMMA approximate water

is a transmission correction to arbitrary depth in water

is a correction for source decay

is a correction for humidity variations

corrects the incident beam flux density to that of air at reference

temperature and pressure

is the density of air at the reference temperature ( 22° C ) and

pressure (760 mmHg)

is the area of the charge collecting electrode

is the rate of change of corrected current with air gap spacing.

53
Each measured current is corrected to a set of reference conditions by

the equation:

c

1 f

T

f

S

f R

( 2 )

where:

i is the measured current

fj corrects the air density inside the chamber to reference temperature

and pressure

f<, corrects for scatter and/or shielding from the chamber walls

fp corrects for ion recombination and diffusion inside the chamber.

Corrected currents are determined for five different air-gap settings

(s = 0.5, 1.0, 1.5, 2.0, and 2.5 mm). These are fitted to the straight line:

and the coefficients c

Q and c1 = (di

c /ds) are determined by least squares fit.

Estimated systematic uncertainties plus statistical uncertainties

measured for our own Buchl er sources are listed in the table. The systematic

uncertainties are taken to be 67-percent estimates; therefore, they can be

treated like standard deviations. The first four have been combined in

quadrature to produce systematic uncertainty #5. If systematic uncertainties

#5 through #14 are combined in quadrature the total systematic uncertainty for

the calibration becomes 1.3%.

The statistical uncertainties are given as standard deviations calculated

for the constant c

x = ( di c /ds) from the least-squares fit

(3)

54
a( c - a

Q /N/ A

A = N ES 2 - ( ES ) 2

/

e(c

0 +cx s -i c ) 2

°o ' / N - 2

The degrees of = freedom (N - 2) for the calculations with the 147 Pm, 204T1 , and

90 Sr+ 90 Y sources are 149, 111, and 39 respectively.

55
Systematic and Statistical Uncertainties

Quantity Symbol

Uncer

Systematic

:ai nty

Stati stical

1 . Measured current i 0.1%

2. Interior air density correction f

T

0.1%

3. Chamber wall correction f

S

0.2%

4. Saturation correction f

R 0.4%

5. Corrected current per unit gap ( di

c /ds

)

a. 147 Pm source 0.5% ± 0.4%

b. 204T1 source 0.5% ± 0.3%

c.

90 Sr+ 90 Y source 0.5% i+ o i—* 3^

6. Air ionization constant (W/e) 0.4%

7. Stopping power ratio 0.7%

8. Backscatter correction k

s 0.7%

9. Transmission correction k T

0.4%

10. Source decay correction k D 0.05%

11. Altitude correction k

A

0.3%

12. Humidity correction k

H 0.1%

13. Reference air density p a 0.05%

14. Collecting electrode area A 0.1%

56
Appendix 2

Proposed Guidelines for Establishing a Secondary Calibration Laboratory

for Beta-Particle Source Standardization and Calibration of

Beta-Particle Protection Instruments

Margarete Ehrlich

1. Purpose of Secondary Calibration Facility . To provide routine calibrations,

traceable to the National Bureau of Standards (NBS), for U.S. users of betaparticle survey and personnel dosimetry instrumentation; and to provide

standardizations of beta-particle secondary-standard sources employed in

radiation protection, by means of a comparison with reference-cl ass ionization

chambers calibrated at NBS.

2 . Organizational Structure and Staff

2.1

The secondary beta-particle calibration facility in general may be a part of a larger organization carrying out radiation-instrument calibrations

for other types of radiation.

2.2 The facility shall be under the full-time supervision of a radiation

physicist with an advanced degree on at least the master's level in physics or

in a health-physics-related subject, and at least five years of practical

experience in radiation-protection dosimetry. The staff devoting its full tire

to radiation-instrument calibration should consist of at least one physicist

with two or more years of experience in radiation measurement, and one technician.

2.3 The staff shall have ready access to the services of a machine sho: , an electronic-instrument repair shop, a computer, and a secretary.

57
3. Physical Facility

3.1 The physical plant shall consist of at least one irradiation room (preferably there should be two), and suitable storage, setup and office space,

all designed to meet applicable Federal, State, and local safety codes and

regulations

.

3.2 The physical plant shall be sufficiently shielded from extraneous

radiation sources so that the radiation background due to such sources is

restricted to a level comoarable to natural background, at all times.

3.3 The plant shall be provided with an air-conditioning system adequate

to furnish controllable conditions of ambient temperature and relative humidity

throughout the plant.

3.4 The plant shall be provided with an adequate grounding system and

should be free of undue vibration, shock, acoustic noise, debris and dust.

3.5 Each irradiation room shall be partitioned into an irradiation area

and a control area that is completely shielded from the beta particles in the

irradiation area but preferably permits a view of the irradiation setup. The

size and shape of the irradiation rooms shall be such as to permit the betaparticle sources to be positioned in the irradiation areas so that the contribution to the dose at the point of instrument calibration arising from room scatter

(including bremsstrahl ung) is less than five percent.

4. Beta-Particle Sources

4.1 The laboratory shall be equipped with at least one BuchlerAmersham "Complete Beta-Particle Inactive Plant" and one set of four

associated beta-particle sources (two 90Sr+ 90 Y sources, one 147 Pm source,

and one 204 T1 source), standardized at NBS.

4.2 There should be access to additional standardized betaparticle sources of the same and of a different type (different

radionuclide, different geometry).

58
5. Beta-Particle Measurement Equipment and Accessories . The laboratory shall own at least the following equipment and accessories, dedicated to the use in

beta-particle calibration work, with redundant equipment of dissimilar type to

that in actual use:

5.1 Two plane-parallel thin-wall shallow ionization chambers of referenceclass* quality, for use (a) as transfer instruments in source-standardization

activities and (b) to check on the constancy of the instrument-calibration

setups. The chambers shall provide a useful operating range for all betaparticle sources at the prescribed distance (or distances) of operation. They

shall have high stability and should be ruggedly constructed of material suitable to minimize change of response with age, temperature, humidity, and with

exertion of moderate mechanical force.

5.2 Two electrometers to measure charge. They should be of the feedback

type, with an open-loop gain of at least 10 4 and an input offset current of

less than 10" 14 A. The electrometer circuit shall be electrically guarded at

the potential of the input-contact point. The charge-measurement system shall

retain a charge with a decay time constant of at least 10 6 s.

5.3 Two capacitors for electrometer calibrations, with a stability of at

least one part in 10 3 per year and a time constant of at least 10 5 s.

5.4 Two 4 1/2-digit (or more) voltmeters. One should be capable of

measuring at least 600 volts. Their accuracy shall be ±0.1 percent.

5.5 Plastic filters of appropriate size, and of a thickness range to permit

the measurement of depth-dose curves.

*A reference-class instrument is an instrument exhibiting a performance and

stability sufficient for it to be used for calibrating other instruments.

( IEC 62 C- 12)

59
5.6 At least one calibration range, consisting of an instrument support

(or supports) of adjustable, measurable, and reproducible height, traveling

along a rail that, preferably, is rigidly connected to the source support and

is equipped with a tape permitting the determination of source-to-support

distance. The instrument support(s) shall be suited for positioning the survey

meters to be calibrated, the personnel dosimeters to be irradiated, the planeparallel ionization chambers, and the sources to be standardized to a level of

accuracy and reproducibility consistent with the respective calibration-accuracy

goals

.

5.7 It may be advisable to have a calibration range in each of two

irradiation rooms, usinq one for instrument calibrations and irradiations

and the other for simultaneous source standardizations.

5.8 A device for testing atmospheric communication of ionization chambers.

5.9 At least two high-quality barometers.

5.10 At least two high-quality thermometers, each with a resolution of

0.5 °C or better.

5.11 A device to measure relative humidity to meet the requirements of

Section 6.

5.12 At least one source of electric potential accurate to ±5 percent

and with a short-term stability of ±0.1 percent, suitable for ionization-chamber

polarity and charge measurements.

5.13 At least two thin-window survey instruments.

60
6 . Laboratory Operation

6.1 The physical plant shall be maintained to meet all applicable safety

codes and regulations.

6.2 Ambient temperatures should be kept in the range of 22 ± 3 °C and

ambient relative humidities in the 15-to 65-percent range. The ambient conditions shall be sufficiently uniform throughout the physical plant to ensure

adequate stabilization of both the laboratory's measurement equipment and

the equipment submitted for calibration and/or other services.

6.3 All laboratory equipment shall be maintained in working order and

shall be calibrated periodically in accordance with the following plan:

6.3.1 At least one of the laboratory's plane-parallel ionization

chambers shall be calibrated by NBS over the full range of energies for which

it is used. The calibration schedule should be established in accordance

with NBS policy for this type of instrument.

6.3.2 At least one of the laboratory standard voltmeters and one of the capacitors used for charge measurements shall be calibrated at least

biennially at another facility; these calibrations shall be documented as being directly traceable to NBS.

6.3.3 At least one barometer and one thermometer shall have a calibration documented as directly traceable to NBS.

6.4 The standardization of the beta-particle sources at the specified

distance(s) and the delivery of prescribed values of absorbed dose to personnel dosimeters at these distances shall be in agreement with the National

Standard to within ±3 percent at the 95-percent confidence level. The cal-r - tion factors obtained for radiation-protection instruments shall agree wit*

those obtained by NBS to within ±5 percent at the 95-percent confidence level.

61
6.5 Internal qual i ty-assurance measures shall be designed to discover

changes in equipment performance that would adversely affect the quality of

the laboratory's operation. The laboratory's performance shall be tested by

NBS according to a schedule developed in consultation with NBS , and corrective

actions shall be taken promptly, if required, in order to maintain compliance

with the accuracy requirements stated in 6.4.

6.6 The laboratory shall have a written operations protocol that includes

6.6.1 a statement of the scope of its work, including the types of

beta-particle sources accepted for standardization, the type of radiationprotection instruments accepted for calibration or irradiation, and the range

of dose-equivalent rates and source-to-detector distances over which these

services are performed;

6.6.2 an inventory of all pieces of equipment used in the betaparticle calibration and standardization work, including model and serial

numbers, where applicable;

6.6.3 an outline of equipment-maintenance and instrument-calibration

pol icy;

6.6.4 the operational steps outlined in sufficient detail to permit

a knowledgeable person new to the operation to carry out each step reproducibly

to a precision consistent with the goals of the laboratory;

6.6.5 the accuracies of the calibrations and standardizations performed;

6.6.6 an outline of how records of the entire operation are accumulated and stored, and how the security of proprietary information is ensured;

6.6.7 the procedure for auditing measurement results and approving

reports

;

62
6.6.8 an outline of internal and external qual i ty-control procedures

and schedules.

7. Records

7.1 A comprehensive and readily accessible record system shall be maintained. It shall include at least the following information:

7.1.1 a full history of all laboratory equipment and instrumentation,

including dates of purchase, calibration and maintenance, certificates and

operating instructions;

7.1.2 date of receipt of instrument or source, serial number, details

of required tests, certificate and invoice (or other accounting) numbers, date

of completion of work, of return shipment, and of test report;

7.1.3 actual measurement data for the standardization of sources, for

the calibration of instruments, and for measurements associated with personneldosimeter irradiations;

7.1.4 copies of all reports issued.

7.2 All records shall be kept for a period of time sufficient to comply

with all pertinent local. State, and Federal legal requirements.

8. Test Reports

8.1 The test reports shall consist basically of a statement giving:

8.1.1 for source standardizations, absorbed-dose rate at a specified

source-to-detector distance at a specified depth in the material of interest;

8.1.2 for instrument calibrations, the correction or calibration

factors for use of the instrument at a specified distance from a specified

63
8.1.3

for personnel -dosimeter irradiations, the absorbed-dose to the

material of interest administered to the dosimeters at a specified distance from

the source.

8.2 The test report shall include further:

8.2.1 name and address of the calibration laboratory;

8.2.2 report date and number;

8.2.3 person or institution submitting the instrument or source;

8.2.4 type and serial number of item submitted;

8.2.5 pertinent parameters regarding geometry, ambient conditions,

radiation fields, instrument settings and instrument orientation applying to

the reported test results;

8.2.6 initials or signature of the person who perfarmed the tests,

and signature of the person responsible for the laboratory operations.

8.3 The test results shall not be disclosed to anyone other than the

individual or institution submitting the item for test, except as designated

in writing by the individual or institution, or as required by law.

64
Appendix 3

Proposed Guidelines for Establishing a Secondary Calibration Laboratory

for Neutron-Protection Instruments

Robert B. Schwartz

1. Purpose of Secondary Calibration Facility . To provide routine calibrations, traceable to the National Bureau of Standards (NBS), for U.S. users

of neutron survey and personnel dosimetry instrumentation.

2. Organizational Structure and Staff

2.1 The secondary neutron instrument calibration facility in general may

be a part of a larger organization carrying out radi at ion- i nstrument

calibrations for other types of radiation.

2.2 The facility shall be under the supervision of a radiation physicist

with an advanced degree on at least the master's level in physics or

in a health-physics-rel ated subject, and at least five years of

experience in radiation protection dosimetry or in neutron radiation

measurements. His staff shall consist of at least one other physicist

and one technician.

2.3 The staff shall have access to the services of a machine shop, an electronics shop, a computer, and a secretary.

3.

Physical Facility

3.1 The physical plant shall consist of at least one irradiation room and

suitable storage, setup, and office space, all designed to meet

applicable Federal, state, and local safety codes and regulations.

3.2 The physical plant shall be sufficiently shielded from extraneous

radiation sources so that the neutron background due te such sours*- 1

is <0.1 mrem/h.

65
3.3 The plant shall be provided with an air-conditioning and heating

system adequate to furnish controllable conditions of ambient temperature and relative humidity throughout the plant.

3.4 The plant should be free of undue vibration, shock, noise, debris and

dust

.

3.5 The irradiation room may be either "open" or "enclosed". An "open" room is one in which the walls and ceiling are generally of low mass,

non-hydrogenous, material essentially transparent to neutrons. Radiation protection is provided by means of a sufficiently large exclusion

area. An "enclosed" room is one in which the walls and ceiling are sufficiently massive (usually concrete) to provide adequate shielding

for personnel in the vicinity. The enclosed room must be as large as possible to minimize the room-scattered neutron contribution, and, in

any case, the inside linear dimensions must be >6 m. The size of an open room may be considerably smaller. In either case, a control and

data-taking area must be provided which is completely shielded from

the irradiation area, but which permits viewing the irradiation setup

either directly or by means of closed-circuit television.

4. Neutron Sources and Moderator

4.1 The laboratory shall be equipped with a 15-cm radius heavy water

sphere, covered with cadmium, for producing moderated neutron spectra.

The design of the sphere should be patterned after the NBS design, or

be of similar construction which allows convenient insertion and

removal of a californium source into the center of the sphere, and

minimizes air spaces and extraneous material in the vicinity of the source.

4.2 The laboratory shall be set up so that calibrations may be performed

with either "bare" (i.e., unmoderated) californium, or moderated californi urn.

66
4.3 Since it is necessary to calibrate remmeters over a range of approximately 4 decades in dose equivalent rate, at least two californium

sources of different strengths will be required. The stronger source

should be sufficient to produce a dose equivalent rate of at least 3

rem/h, or 75 percent of the instrument's maximum scale reading, whichever is lower. For a bare source and spherical detector, the minimum

source-detector center-to-center distance shall be greater than 3

sphere radii; for a cylindrical detector the minimum distance must be

greater than 5 cylinder radii. For a 9-inch spherical remmeter, the

minimum emission rate of the "strong" size should thus be ~ 4 x 10 8

n/s

.

The weaker source should be chosen to produce a field of 3 mrem/h or less at a distance such that the ratio of the instrument's response to

the room-scattered neutrons to its response to the di rect-source

neutrons is less than 40 percent. For a bare californium source irradiating a nine-inch spherical remmeter in a cubical concrete room, the

maximum emission rate, Q, of the weak source will be roughly

Q

» 1.5 x 10 5 a

2 n/s,

where a is the length in meters of one of the sides of the cubical

room.

The exact emission rates of the sources should be worked out in con- junction with NBS, taking account of room scatter, minimum source-todetector distance, allowance for source decay, and the differences

between bare and moderated calibrations. All sources must be calibrated by NBS.

5. Equipment and Accessories

5.1 The laboratory shall own at least one of whatever types of remmeters

it will be called upon to calibrate regularly. Specif ical ly , this means the Eberline 9-inch remmeter, but may also include a commercial

version of the Andersson-Braun remmeter. Where possible, the di ;it T

version of the instrument should be chosen, rather than analog.

67
5.2 Closed-circuit television equipment, so that the instrument under test

can be read from the control room.

5.3 A thermometer and a device for measuring relative humidity.

5.4 A calibration range consisting of a low mass instrument support stand

of adjustable height traveling along rails. It would be highly preferable to have the instrument stand motor-driven, with remote control

and remote read-out of its position.

5.5 A 40 cm x 40 cm x 15-cm thick acrylic (polymethylmethacrylate) phantom.

6. Laboratory Operation

6.1 The physical plant shall be maintained to meet all applicable safety

codes and regulations.

6.2 Ambient temperatures should be kept in the range of 22 ± 3°C and

ambient relative humidities in the 15- to 65-percent range. The

ambient conditions shall be sufficiently uniform throughout the

physical plant to ensure adequate stabilization of both the laboratory's measurement equipment and the equipment submitted for calibration and/or other services.

6.3 The laboratory shall measure the effects of room-scattered neutrons

for the more commonly calibrated instruments. These will include, as

a minimum, the 9-inch spherical remmeter, the 3-inch Eberline sphere,

and albedo dosimeters. Room scatter effects for other instruments may

be obtained by calculation and extrapolation from other measurements,

although it is preferable that they also be measured directly. NBS

shall assist in the analysis of the room scatter data.

6.4 Corrections for air scatter and for departure from the 1/r 2 law shall

be provided by NBS.

68
6.5 The laboratory remmeter (section 5.1) shall be carefully calibrated

by measuring its response as a function of dose equivalent at at

least several points per decade, for both the bare and moderated

californium sources. The instrument's dead time should be carefully

measured, and it should be checked for any drift or warm-up problems.

The laboratory should also determine the response of the remmeter to

room-scattered neutrons, as required by 6.3. The instrument should

then also be calibrated by NBS, and the calibrations compared.

Once the instrument's behavior is well understood and it has been

carefully calibrated, it becomes the laboratory's "standard remmeter".

It is then periodically i ntercompared with other remmeters being calibrated. Note that calibrations are always done in terms of a dose

equivalent rate calculated from the source emission rate (measured at

NBS) and the distance, with allowances made for wall scatter and air

scatter. Calibrations are not to be done by comparing with the

laboratory standard remmeter. Rather, the periodic checking of the

laboratory standard is part of the SSL's own quality control, done by

recal ibrating the standard instrument at periodic intervals, along

with other instruments requiring calibration. Each time this is done,

the calibration factor for the laboratory instrument shall be recorded,

and deviations from the expected value shall be investigated.

7. Records

7.1

A comprehensive and readily accessible record system shall be maintained. It shall include at least the following information:

7.1.1 a full history of all laboratory equipment and instrumentat ion

,

including dates of purchase, calibration and maintenance, cer- tificates and operating instructions;

7.1.2 date of receipt of instrument or source, serial number, det : of required tests, certificate and invoice (or other accounting) numbers, date of completion of work, of return s h

; - • , and of test report;

69
7.1.3 actual measurement data for the standardization of sources, for

the calibration of instruments, and for measurements associated

with personnel-dosimeter irradi ations;

7.1.4 copies of all reports issued.

7.2 All records shall be kept for a period of time sufficient to comply

with all pertinent Federal, state, and local legal requirements.

8. Test Reports

8.1 The test report shall include the following information:

• Name and address of calibration laboratory

• Report or calibration number

• Name and address of person or institution submitting the instrument

to be tested

• Type and serial number of item submitted

• Date of irradiation

• Identification numbers of the californium sources used, together

with their emission rates at the time of the irradiation, and

whether used bare or moderated

® Average temperature and relative humidity during the irradiation

• Signature of person performing the irradiations, and of the person

responsible for the laboratory operation

8.1.1 For remmeter calibrations, the report shall include the following information for each calibration point:

(a) Source number

(b) Source-to-detector center-to-center distance

(c) Calculated free field dose equivalent rate at remmeter

(d) Instrument reading

(e) Instrument reading corrected for scattering effects

(f) Calibration factor ( i . e . , (c)/(e) of 8.1.1)

(g) Estimated uncertainty in the calibration factor

70
8.1.2

For dosimeter irradi ations, the test report shall include the

following information:

(a) Source number

(b) Source center-to-phantom front face distance

(c) Calculated free field dose equivalent rate at phantom front

face

(d) Irradiation time

(e) Total free field dose equivalent (i.e., (c) x (d) of

8 . 1 . 2 ) (f) Total dose equivalent corrected for room and air scattering

(g) Estimated uncertainty in the total corrected dose equiva1 ent

8.2 All test reports shall also include a brief narrative description of

the physical arrangement for the irradi ations, and a brief discussion

of the uncertainties. The report shall also include, if appropriate,

a discussion of any unusual behavior exhibited by the instrument under

test

.

8.3 The test results shall not be disclosed to anyone other than the individual or institution submitting the item for test, except as designated in writing by the individual or institution, or as required by

1 aw.

71
Appendix 4

Proposed Procedure for Carrying out Measurement-Assurance Studies

for a Secondary Calibrati on Laboratory

1. Purpose

Margarete Ehrlich

The purpose of measurement-assurance studies is to demonstrate that

instrument calibrations and source standardizations performed by a secondary

calibration laboratory agree with national measurement standards, within the

accuracy specified in the operating protocol of the secondary laboratory.

2. Mechani sm

All measurement-assurance studies shall be carried out by the National

Bureau of Standards (NBS) on a periodic basis, following a schedule agreed upon

between NBS and the secondary calibration laboratory (SCL). The following

tasks shall be included:

2.1

Photons

The SCL shall calibrate two NBS-cal ibrated reference-cl ass ionization chambers in all the photon beams to be used in the calibration work. The

results shall be evaluated by NBS.

2.2

Beta Particles

The SCL shall calibrate one or more NBS-cal ibrated paral 1 el -pi ate

ionization chambers with the sources and at the distances to be used in the

calibration work. The results shall be evaluated by NBS.

2.3

Neutrons

2.3.1 For measurement-assurance studies related to the calibration

of personnel dosimeters , (a) The SCL shall irradiate, on phantom, albedo TLDs

provided by NBS or by another laboratory designated by NBS, at five distances

72
from a bare and a moderated 252 Cf source. A total of 25 dosimeters shall be

used at each source, five each at each of the five distances ranging from 0.3

to 2 m. (b) If the albedo TLDs originate at a laboratory other than NBS, this

laboratory shall simultaneously send another ~10 of their dosimeters to NBS,

for irradiation at the SCL calibration distance in the fields of the NBS bare

and moderated 252 Cf sources, (c) After the return of all TLDs to their respective places of origin, they shall be read out, and the results transmitted to

NBS for evaluation. If required, special procedures may be devised for SCL

personnel-dosimeter calibration with other types of neutron sources.

2.3.2 For measurement-assurance work related to the calibration of

neutron-survey meters (remmeters) . The SCL shall calibrate an NBS-cal i brated

remmeter in terms of dose-equivalent rate in the radiation field of its bare

and moderated 252 Cf sources. The results shall be evaluated by NBS. If

required, special procedures may be devised for SCL survey-instrument calibration with other types of neutron sources.

73

N 8 S-l i 4 A I«v. 2-80

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See Instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR-85/31 69

2. Performing Organ. Report No. 3. Publication Date

4. TITLE AND SUBTITLE

STANDARD BETA-PARTICLE AND MONOENERGETIC ELECTRON SOURCES FOR

BETA-RADIATION PROTECTION INSTRUMENTATION

THE CALIBRATION OF

5. AUTHOR(S)

Ehrlich, M. , Pruitt, J.S., Soares, C.G.

6. PERFORMING ORGANIZATION (If Joint or other than NBS. see Instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No. Interagency Agreement

No. 82-009

S. Type of Report & Period Covered

Final, Sept '82-Dec '84

9. SPONSORING ORGANIZATION NAME ANO COMPLETE ADDRESS (Street. City. State, ZIP)

US Nuclear Regulatory Commission

Washington, D.C. 20555, jointly with NBS

- -

•

.

'

10. SUPPLEMENTARY NOTES ~

| | Document describes a computer program; SF-185, FIPS Software Summary, Is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant Information. If document Includes a significant

bibliography or literature survey, mention It here) t

. 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 radiation-protection dosimetry. The

facility consists of beta-particle and nearly monoenergetic electron beams characterized in terms of absorbed-dose rates to plastic and in terms of beta-particle spectra.

A second phase of the project was concerned with establishing secondary calibration

laboratories for radiation-protection instruments.

This final report includes a detailed discussion of (1) the determination of absbrbed^dose ; rates- to plastic for each beta-particle and nearly monoenergetic electron

beam, dose-rate dependence on altitude above sea level, and an estimate of the overall

uncertainties In dose-rate measurements; (2) beta-particle and nearly monoenergetic

electron spectra and their dependence on source configuration; and (3) degree of

achievable uniformity of beam cross sections. Included also is a review of the results

of a first attempt to predict instrument response to realistic beta-particle

environments from their response to monoenergetic electrons and knowledge of the

approximate beta-particle spectra. Attached to the report are proposed guidelines for

establishing secondary calibration laboratories for radiation-protection instruments.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon t) A^'orbedcdosecto’.plastdc; beta-particle sources; electrons (nearly monoenergetic) ; energy

spectra; guidelines for secondary calibration labs; instrument calibration; prometheum147; radiation protection; strontium-90 + yttrium-90; thallium-204.

13. AVAILABILITY 14. NO. OF

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