NCRP report no 112 calibration of survey instruments used in radiation protection for the assessment of ionizing radiati~1

NCRP REPORT No. 112

CALIBRATION OF SURVEY
INSTRUMENTS USED IN
RADIATION PROTECTION
FOR THE ASSESSMENT OF
IONIZING RADIATION
FIELDS AND RADIOACTIVE
SURFACE CONTAMlNATION
Recommendations of the
NATIONAL COUNCIL ON RADIATION
PROTECTION AND MEASUREMENTS

Issued December 31,1991
National Council on Radiation Protection and Measurements
7910 WOODMONT AVENUE 1 Bethesda, MD 20814


LEGAL NOTICE
This report was prepared by the National Council on Radiation Protection and
Measurements (NCRP). The Council strives to provide accurate, complete and useful
information in its reports. However, neither the NCRP, the members of NCRP, other
persons contributing to or assisting in the preparation of this report, nor any person
acting on the behalf of any of these parties: (a)makes any warranty or representation,
express or implied, with respect to the accuracy, completeness or usefulness of the
information contained in this report, or that the use of any information, method or
process disclosed in this report may not infringe on privately owned rights; or (b)
assumes any liability with respect to the use of, or for damagesresulting from the use
of any information, method or process disclosed in this report, under the Civil Rights
Act of 1964, Section 701 et seq. as amended 42 U.S.C.Section 2000e et seq. (Title VZZ)
or any other statutory or common law theory governing liability.

Library of Congress Cataloging-in-PublicationData
National Council on Radiation Protection and Measurements.
Calibration of survey instruments used in radiation protection for the
assessment of ionizing radiation fields and radioactive surface contamination:
recommendations of the National Council on Radiation Protection and
Measurements.
p. cm.-(NCRP report; no. 1123
"Issued December 31, 1991."
Includes bibliographical references and index.
ISBN 0-929600-23-1
1. Nuclear counters-Calibration., I. Title. 11. Series.
TK9180.N37 1991
539.7'7-dc20
91-38019
CIP
(NCRP report; no.
Bibliography: p.
Includes index.

)

Copyright 0 National Council on Radiation
Protection and Measurements 1991
All rights reserved. This publication is protected by copyright. No part of this publication may be reproduced in any form or by any means, including photocopying, or
utilized by any information storage and retrieval system without written permission
from the copyright owner, except for brief quotation in critical articles or reviews.


Contents

.

1 Introduction .........................................................................
1.1 General .............................................................................
1.2 Scope and Structure ........................................................
1.3 Need and Intent .............................................................
1.4 Review of Current Efforts/Recommendation .................
2 Considerations i n the Calibration Process ....................
2.1 General ...........................................................................
2.2 Level of Calibration ........................................................
2.2.1 General ...................................................................
2.2.2 Full Characterization ................................. .........
2.2.3 Calibration for Specific Acceptance .....................
2.2.4 Routine Calibration ..............................................
2.3 Performance Check .........................................................
2.4 Precalibration Check ......................................................
2.5 Qchnical Considerations of Source Selection ...............
2.5.1 Radiation Type ......................................................
2.5.2 Field Intensity and Source Strength ...................
2.5.3 Source-Detector Geometry ....................................
2.5.4 Traceability of Source Calibration .......................
2.5.5 Accuracy of Calibration Source for Field

.

..

...................................
....................
Instrument Response Considerations ............................

2.6.1 General ...................................................................
2.6.2 Energy Dependence ...............................................
2.6.3 Directional or Angular Response .........................
2.6.4 Detector Wall Effect ..............................................
2.6.5 Geotropism ..............................
.............................
2.6.6 Environmental Effects ..........................................
2.6.7 Influence of Other Ionizing Radiations ...............
2.6.8 Linearity Measurements in Calibration .............
Intensity Determination

2.5.6 Incidental and Spurious Radiations

2.6

2.6.9 Calibration on Selected Scales and Limited

Ranges

................................................................

2.7 Uncertainty in the Calibration Process ........................
2.7.1 Genera.1 ...................................................................
2.7.2 Uncertainty Associated with Random

Variations

...........................................................

vii
2.7.3 Uncertainty Associated with Systematic Errors
2.7.4 Instrument Stability .............................................
2.7.5 Applying the Accuracy Criterion in the

Calibration Process ............................................

.

2.8 Frequency of Calibration ................................................
2.9 Record Requirements ......................................................
2.10 Summary of Recommendations ....................................

3 Calibration Facility .........................................................
3.1 General .............................................................................
3.2 Background Radiation ....................................................
3 3 Scattering .........................................................................
3.4 Equipment Requirements ...............................................
3.5 The Physical Facility .....................................................
3.6 Staffing .............................................................................
4 Calibration of Photon Measuring Instruments for

.

External Radiation Field Evaluation

..........................

4.1 Introduction .....................................................................
4.2 Source Selection ..............................................................

4.2.1 General ...................................................................
4.2.2 Energy Requirements ...........................................
4.2.3 Source Strength .....................................................
4.2.4 Source Output Characteristics .............................
4.2.5 Source Geometry ...................................................
4.2.5.1 Sources in Free Air ..................................
4.2.5.2 Collimated or Enclosed Fields .................
4.2.5.3 Calibration Boxes .....................................
4.3 Characterization of Radiation Field ..............................
4.3.1 General ...................................................................
4.3.2 Selection and Use of Transfer-Standard

Instruments ........................................................
Field Uniformity Over Detector Volume .............
Energy Spectral Quality .......................................
Effects of Scatter ...................................................
Incidental and Spurious Radiations ....................
Instrument Response Considerations ...........................
4.4.1 General ...................................................................
4.4.2 Energy Dependence ...............................................
4.4.3 Mixed Radiation Fields .........................................
4.4.4 Pulsed Radiation Fields ......................................
4.4.5 Time Constant .......................................................
Accuracy and Acceptance Criteria .................................
Frequency of Calibration ................................................
Calibration Examples .....................................................

4.3.3
4.3.4
4.3.5

4.3.6
4.4

4.5
4.6
4.7

32
33


viii

1

CONTENTS

.

5 Calibration of Beta Dose Measuring Instruments for

External Radiation Field Evaluation ...............................
5.1 Introduction .....................................................................
5.2 Source Selection ..............................................................
5.2.1 Energy Requirements ...........................................
5.2.2 Source Strength .....................................................
5.2.3 Source Geometry ...................................................
5.3 Characterization of Radiation Field ..............................
5.3.1 Dose Rate ...............................................................
5.3.2 Field Uniformity ...................................................

5.3.3 Energy Spectral Quality and Incidentall
Spurious Radiations ........................................
5.4 Instrument Response Considerations ............................
5.4.1 Linearity and Stability .......................................
5.4.2 Energy Dependence and Geometry Effects .........
5.4.3 Mixed Radiation Fields .........................................
5.5 Accuracy and Acceptance Criteria .................................
5.6 Frequency of Calibration and Conditions of
Recalibration ................................................................
5.7 Calibration Examples-Determination of Point Source
and Distributed Source Calibration Factors .............
5.7.1 Calibration with Point Sources ............................
5.7.2 Calibration with Distributed Sources ..................
5.7.3 Calibration Factor Application for Field
Measurement Geometries .................................
6 Calibration of Portable Instruments for the
Assessment of Neutron Radiation Fields ...................
6.1 Introduction ....................................................................
6.2 Source Selection ........................
..............................
6.2.1 General ...................................................................
6.2.2 Energy Requirements .........................................
6.2.3 Source Strength .....................................................
6.2.4 Source Geometry ...................................................
6.3 Characterization of Radiation Field ..............................
6.3.1 Fluence Rate and Dose Equivalent Rate ............
6.3.2 Field Uniformity over Detector Volume ..............
6.3.3 Energy Spectral Quality .......................................
6.3.4 Effects of Scatter ...................................................
6.3.5 Incidental and Spurious Radiations ....................

6.4 Survey Instrument Response Considerations ...............
6.4.1 General .................................................................
6.4.2 Energy Dependence ...............................................
6.4.3 Mixed Radiation Fields .........................................
6.4.4 Pulsed Radiation Fields ........................................

.

.
.
.


CONTENTS

1

ix

6.5 Accuracy and Acceptance Criteria ................................. 102
6.6 Calibration Frequency .................................................... 103
6.7 Calibration Examples ..................................................... 103
7. Calibration of Field Instrumentation for the
Assessment of Surface Contamination ....................... 105
7.1 Introduction ................................................................... 105
7.2 Source Selection .............................................................. 106
7.2.1 General ................................................................ 106
7.2.2 Energy Requirements ........................................... 107
7.2.3 Source Strength ..................................................
108

7.2.4 Source Geometry .................................................. 108
7.3 Characterization of Radiation Emission ....................... 109
7.3.1 Particle Emission Rates ........................................ 109
7.3.2 Energy Characteristics ......................................... 109
7.3.3 Effects of Scatter .................................................. 111
7.3.4 Incidental and Spurious Radiations .................... 112
7.4 Instrument Response Considerations ............................ 112
7.4.1 Stability and Linearity ......................................... 112
7.4.2 Energy Dependence ............................................... 113
7.4.3 Geometry Effects ................................................... 114
7.4.4 Mixed Radiation Fields ......................................... 115
7.5 Accuracy and Acceptance Criteria ................................. 116
7.6 Calibration Frequency .................................................... 117
7.7 Calibration Examples ..................................................... 117
Appendix A-1 Photon Source Related Considerations ... 118
A.l.1 Energy ..........................................................................118
A.1.2 Source Strength ........................................................... 118
A.1.3 Air Attenuation ........................................................... 122
Appendix A-2 Photon Measuring Instrument
Calibration Techniques ............................ 124
A.2.1 Low Level Instruments ............................................... 124
A.2.2 Mid-Range Instrument ............................................... 126
A.2.3 High-Range Instruments ........................................... 128
Appendix A-3 Examples of Calibrations in Photon
Radiation Fields ........................................ 130
A-3.1 Calibration of an Eberline R02 Using Automated
Cs-137Calibration Wells ........................................ 130
A.3.2 Free Air Calibration ................................................... 134
A.3.3 Calibration Using a Collimated Source .................... 138
Appendix B-1 Calibration of a Source Using an

Extrapolation Chamber ............................ 141
Appendix B-2 Example of E,, Determination ................... 144
Appendix B-3 Example of Instrument Calibration for
Beta Dose Response ................................. 146


X

1

CONTENTS

Appendix C-1 Neutron Source Measurements .................. 151
(2.1.1 Manganese Sulfate Technique .................................... 151
(2.1.2 Long Counter Application .......................................... 151
C.1.3 Activation Techniques for Thermal Neutrons ......... 152
Appendix C-2 Estimation of Dose Equivalent Rates
from Moderated 23aPu-Beand
Moderated 252CfSources .......................... 154
Appendix C-3 Calibration of an Anderson-Braun Type
Neutron Survey Meter .............................. 157
C.3.1 General ....................................................................... 157
(2.3.2 Example ....................................................................... 158
Appendix D Examples of Calibration of a Thin Window
G-M Detector for Assessment of Surface
Contamination ............................................... 162
D-1.1 Example 1 - Calibration of a Thin End Window
G-M Counter with a Reference Point Source in a
"Weightless" Source Mount ..................................... 162
D-1.2 Example 2 - Calibration of a Thin End Window

G-M Counter with a Reference Point Source on a
Thick Disc Mount .................................................... 165
Appendix E Determination of Average Fluence Rate in
a Detector Volume Relative to the
Fluence Rate at the Center of the
Detector Volume for Unattenuated
Radiation from a Point Isotropic Source 168
E-1 General ........................................................................... 168
E-2 Mean-Value Calculations ............................................... 169
Appendix F Systematic Uncertainties in the
Calibration Process ...................................... 172
F-1 General ............................................................................ 172
F-2 Systematic Uncertainties Associated with Specific
Aspects of Calibration ................................................ 172
F.2.1 The Instrument Being Calibrated ..................... 172
F.2.2 The Transfer Standard Instrument ................... 173
F.2.3 The Radiation Source ......................................... 174
F.2.4 Associated Measuring Instruments .................. 174
F.2.5 Environmental Influences .................................. 174
F-3 Example of the Influences of Systematic
Uncertainties in the Calibration Process ................. 175
Appendix G Glossary .......................................................... 178
References .................................. ............................................... 182
The NCRP ............................................................................... 190
NCRP Publications ................................................................ 198
Index ..........................................................................................209


1. Introduction
1.1 General

The NCRP has provided recommendations for the protection of
workers and the public from the harmful effects of radiation from
occupational or other sources. Implementation of these recommendations as well a s demonstration of compliance with the requirements
of regulatory agencies requires instrumentation and techniques for
the measurement and evaluation of radiation fields and radioactive
contamination.
Instruments designed to detect and evaluate radiation andlor to
assess radioactivity in the workplace provide information necessary
to control the radiological hazards. For situations in which personnel
dosimetry is not available to provide acceptably accurate estimations
of dose equivalent, evaluations based on portable instrument measurements may be helpful. The major applications ofportable instruments, however, are for purposes of radiation dose control. (In this
Report the phrases portable instruments and survey instruments
are used synonymously to refer to hand-held instruments used for
the assessment of radiation fields and/or radioactive surface centamination.)
Proper calibration procedures are an essential requisite toward
providing confidence in measurements made for these purposes. This
Report provides guidance and includes recommendations with
respect to the calibration of portable instruments used in dose equivalent assessment and evaluation of surface contamination. For an
instrument intended to measure dose equivalent or dose equivalent
rate related quantities, calibration is the determination of the instrument response in a specified radiation field delivering a known dose
equivalent (rate) at the instrument 1ocation;calibration normally
involves the adjustment of instrument controls to read the desired
dose (rate) and typically requires response determination on all
instrument ranges. For instruments designed to measure radioactive
surface contamination, calibration may be the determination of the
detector reading per unit surface activity (uniformly distributed) or
the reading per unit radiation emission rate per unit surface area,
or the reading per unit activity. Because of the NCRP's concern

2

1

1. INTRODUCTION

with accuracy in the radiation measurement process, and in light of
discussionswhich follow, some elaboration of this topic is appropriate
in this introduction.
With respect to accuracy appropriate to instrument calibration,
this Report provides discussion of a number of influencing factors
and includes a number of recommendations. These recommendations
are made in consideration of both the problems inherent in certain
aspects of evaluation of the calibration field (e.g., effects of scatter in
neutron radiation fields) and the problems associated with responses
of portable instruments currently available for radiation measurements (e.g., the discrepant responses of thin end window detectors to
point and distributed sources of beta radiation). References to, or
discussions of, the operational use of instruments are included, and
observations are made that an acceptably accurate laboratory calibration does not guarantee the same level of accuracy operationally.
In view of these considerations, some recommendations with respect
to the accuracy required of calibrations differ from earlier recommendations of the NCRP and other groups. In addition, it is noted that
it may not be possible to achieve the level of accuracy in operational
measurements sometimes recommended by such groups. None of
this is intended to excuse any reasonable attempt at eliminating
controllable sources of error in the calibration process, but only
to recognize that real and difficult problems do exist in radiation
measurements, and these necessarily affect our ability to make accurate measurements.
The Report provides considerable discussion of various problems,
complicating factors, and uncertainties in the calibration process.

Awareness of such considerations is necessary in order not only to
understand the impact of various influencing factors on the calibration process but also to encourage attempts to reduce sources of error
and uncertainty.

1.2 Scope and Structure
This Report is concerned with the calibration of radiation survey
instruments. The objectives are to establish the technical guidance,
the techniques and the procedures to characterize the desired
responses of various types of survey instrumentation through appropriate calibration techniques. Dosimetry and techniques for radiological hazards control in the workplace are not discussed.
For purposes of this Report, instruments will be categorized according to intended measurements, as follows:


1.2 SCOPE AND STRUCTURE

/

3

1) radiation field measuring instruments-values are generally
reported in terms of dose equivalent rate with units, e.g., Sv h-l,
rem h-' or in terms of units of absorbed dose rate, air kerma rate or
exposure rate that can be related to dose equivalent rates. In order
to facilitate the use of the international system of units (SI) , the
quantity air kerma can be substituted for exposure. The quantity air
kerma is used in the discussions that relate to calibration of photonmeasuring instruments, although the quantity exposure is commonly used in the United States, and it is referred to at times.
Appendix A provides details on photon-measuring instrument calibrations and in the examples the quantity exposure rate is used in
relation to instruments that read out in exposure rate units. Air
kerma is the product of the photon energy fluence and the average
(weighted accordingto the photon energy spectral distribution)value
of the mass energy transfer coefficient in air at a point of concern.

Under conditions of secondary charged particle equilibrium and
insignificant electron energy loss by bremsstrahlung, one roentgen
of exposure corresponds to an air kerma of about 8.7 mGy (NCRP,
1985). The instruments dealt with are those the readings of which
provide a direct measure of, or may be used to determine, absorbed
dose or dose rate or dose equivalent or dose equivalent rate in radiation fields comprised in whole or in part of x and gamma rays, beta
particles and neutrons.
2) instruments for measuring surface-distributed radioactivityvalues are generally reported in Bq [disintegrationsper second (dpsll
or [disintegrationsper minute (dpm)]commonly referred to a specified surface area. The instruments discussed are those intended for
measurement of alpha, beta and gamma contamination levels on
personnel, accessible surfaces and/or equipment.
The uses of portable instruments can be categorized as follows:

detectionlsearch

for this use, instruments are designed with
maximum sensitivity in order to permit
detection of low levels quickly; response priorities in order of importance are sensitivity,
precision, and accuracy;
relative response this use requires evaluation of existing radiation fieldsto determine changes from previous survey values; response priorities in
order of importance are precision, sensitivity, and accuracy;
exposure control for this use, survey instrumentation must
provide accurate results which are consistent with personnel dosimetry results;


4

1

1. INTRODUCTION

response priorities in order of importance
would typically be accuracy, precision, and
sensitivity.
This Report is intended primarily for those who deal with applied
radiation protection. Therefore, portable survey instruments of the
hand-held type are emphasized. I t may be useful to instrument
designers and manufacturers/suppliers as well as to dosimetrists and
metrologists. Much of the discussion also applies to calibration of
fixed monitors for detection of external radiation with some modifying considerations a s discussed briefly in Section 2.1. There are no
discussions or recommendations regarding calibration of field-use
spectrometers for the assessment of the energy distribution associated with photons, neutrons, or charged particles.
Sections 2 and 3 include subject matter applicable to calibration
of most portable instruments. The remaining four sections relate
to concerns and recommendations specific to the particular type of
calibration being performed. In order to provide an appreciation of
the actual implementation of these concepts in the calibration process, specific examples of selected calibrations are noted a t the end
of each section and are presented in detail in the appendices.

1.3 Need and Intent

Characteristics of the ionizing radiation fields in work places vary
depending upon the radioactive materials being handled, radiationproducing devices in use, and the facility design. The radiation field
can consist of particles and photons, individually or in combination.
The energies present are characteristic of the particular radionuclides or devices that produce the radiations and can be modified by
radiation interactions.
Each instrument has a response characteristic for the various
types of ionizing radiation that is determined by its design. However,
this response may be different for each instrument design. In addition, a given design may show variable response with radiation
energy as well as with radiation type. As a result, there may exist

a n inconsistency of response among instruments and uncertainty
regarding the response of a given instrument. This produces a number of concerns, which can be summarized as follows:
1) limited ability to relate the reading of a survey meter to that of
a n alternative dose-measuring instrument or device; proper calibra-


1.3 NEED AND INTENT

/

5

tion of the instrument and a thorough understanding of its response
characteristics can reduce such discrepancies;
2) different responses of differently designed instruments in the
same radiation field;
3) inconsistent response of a given instrument in fields of different
intensity (see Section 2.5.2 for definition of intensity)
4) energy and geometry dependence, and
5) the limited ability to repeat accurately surveys for comparative
purposes due to inappropriate changes in response with changing
field conditions, including intensity and radiation type.
Thus, the selection and use of radiation detectors zind instruments
require detailed knowledge of their response characteristics as well
as judgment in their application. Traditionally, radiation protection
personnel, on the basis of their experience, have developed "rulesof-thumb", "favorite instruments", and unique techniques for specific
situations. However, because instrument responses can vary widely
with radiation type or energy and with source-detector geometry,
it is not unusual in complex, mixed-field situations for personnel
dosimeter results to differ considerably from what is expected on

the basis of instrument measurements. This uncertainty may lead
protection personnel to apply the most dose-restrictive interpretation
to instrument readings, and this results in significant conservatism
in the application of radiation exposure control techniques. Recent
recommendations of the American National Standards Institute
(ANSI, 1989a;1989b)deal with performance specifications for instrumentation and should have a beneficial impact on the design and
operation of portable instruments.
In view of the large number and variety of instruments available
and the sometimes specialized applications of these instruments,
there will be situations in which the recommendations given in
this report will not apply or will not be inclusive, or will require
modifications. Absolute calibration requirements are not recommended. This is to recognize specialized needs and to allow for the
fact that, with due attention to the response characteristics of a
particular instrument in a particular situation, acceptable calibrations can be performed using approaches different from those recommended in the Report.
This Report provides means for achieving greater consistency in
the evaluation of instrument response. Improved calibration should
provide improved knowledge of instrument response, which will
allow for a better choice of instrument, better determination of effective dose equivalent, and reduction of unnecessary exposure.


6

1

1. INTRODUCTION

Various groups and organizations have made recommendations
regarding instrument calibration; their work forms the basis for
many of the recommendations given.

1.4 Review of Current Efforts/Recommendations
Various national and international standards and handbooks have
been written to establish performance specifications and calibration
requirements for health physics instrumentation; among those cited
a s references for this Report are the following:
1) ANSI Report No. N323, Radiation Protection Instrumentation
Test and Calibration, 1978;
2) ANSI Report No. N320, Performance Specifications for Reactor
Emergency Radiological Monitoring Instrumentation, 1979;
3) ANSI Report No. N42.17A, Performance Specifications for
Health Physics Instrumentation-Portable Instrumentation for Use
in Normal Environmental Conditions, 1989;
4) ANSI Report No. N421.17C, Radiation Znstrumentation Performance Specifications for Health Physics Instrumentation-Portable
Instrumentation to Use in Extreme Environmental Conditions, 1989;
5) IAEA Technical Report No. 133, Handbook on Calibration of
Radiation Protection Monitoring Instruments, 1971;
6) IAEA Technical Report No. 285, Burger, G. and Schwartz, R.B.,
Guidelines on Calibration of Neutron Measuring Devices, 1988;
7) IS0 Report No. 4037, X and Gamma Reference Radiations for
Calibrating Dosimeters and Dose Ratemeters and for Determining
their Response as a Function of Photon Energy, 1979;
8) I S 0 Report No. 6980, Reference Beta Radiations for Calibrating Dosimeters and Dose Ratemeters and for Determining Their
Response as a Function of Beta Radiation Energy, 1984;
9) I S 0 Report No. 7503-1, Evaluation of Surface ContaminationPart 1:Beta Emitters (MaximumBeta Energy Greater than 0.15 MeV)
and Alpha Emitters, 1988;
10) IS0 Report No. 8529, Neutron Reference Radiations for Calibrating Neutron-Measuring Devices Used for Radiation Protection
Purposes and for Determining Their Response as a Function of Neutron Energy, 1989;
11) IS0 Report No. 8769, Reference Sources for the Calibration
of Surface Contamination Monitors-Beta Emitters (Maximum Beta
Energy greater than 0.15 MeV) and Alpha Emitters, 1989; and

12) Lalos, G. (Ed.), Calibration Handbook: Ionizing Radiation
Measuring Instruments, 1983; Calibration Coordinating Group,


1.4 REVIEW OF CURRENT EFFORTS/RECOMMENDATIONS

/

7

Department of Defense Joint Coordinating Group for Metrology and
Calibration (The Lalos reference is a comprehensive treatment of
many aspects of calibration. Unfortunately, as of this writing the
document is no longer in print, and only a limited number of copies
are available.)
The literature contains many additional papers and reports applicable to various aspects of radiation monitoring and calibration.
NCRP Report No. 57, Instrumentation and Monitoring Methods for
Radiation Protection, pertains to personnel monitoring and the use
of radiation survey instruments (NCRP, 1978). It includes some
recommendations regarding measurement accuracy and survey
instrument calibration. NCRP Report No. 47, Tritium Measurement
Techniques (NCRP, 1976),relates exclusively to techniques for measuring tritium and provides guidance on the calibration of tritium
monitors.
The International Commission on Radiation Units and Measurements (ICRU) has published a large number of reports that relate to
measurement and evaluation of ionizing radiation dose. Many of
these pertain to various aspects of calibration. Among these are the
following:
1) ICRU Report 12, Certification of Standardized Radioactive
Sources, 1968;
2) ICRU Report 14, Radiation Dosimetry: X Rays and Gamma

Rays with Maximum Photon Energies Between 0.1 and 50 MeV, 1969;
3) ICRU Report 20, Radiation Protection Instrumentation and Its
Application, 1971
4) ICRU Report 26, Neutron Dosimetry for Biology and Medicine,
1977;
5) ICRU Report 34, The Dosimetry of Pulsed Radiation, 1982;
6) ICRU Report 39, Determination of Dose Equivalents Resulting
from External Radiation Sources, 1985; and
7) ICRU Report 43, Determination of Dose Equivalents Resulting
from External Radiation Sources-Part 2, 1988.

Details of the above references can be found at back of Report.
The latter two reports provide useful information not only on
characteristics of radiation protection instrumentation and some
considerations in calibration, but also on the relationships among
quantities important in dose assessment. Some of the new quantities
(e.g.,ambient dose equivalent and directional dose equivalent) which
ICRU has defined for monitoring purposes are reviewed, and particular interrelationships among quantities are described. The information is important to individuals who are calibrating instruments in
accordance with the ICRU recommended quantities. These quanti-


8

1

1.

INTRODUCTION

ties are discussed to some extent in Sections 2.6.1 and 6.3.1 of this

Report and are defined in the glossary (See Appendix G).
Recent interest in improvingpersonnel dosimetry performance has
resulted in the implementation of major calibration and certification
programs through the National Voluntary Laboratory Accreditation
Program (NVLAP) and the Department of Energy Laboratory
Accreditation Program (DOELAP). Performance specifications and
evaluation procedures are being emphasized in a number of categories. Similar programs are being considered to address certification
of instrument calibration laboratories. The American Association of
Physicists in Medicine has been concerned with instrument calibration for many years and oversees calibration accreditation of participating laboratories. The Health Physics Society has also initiated a
calibration accreditation program that should provide needed services to the radiation protection community. Requirements on dosimetry and survey instruments to provide information from which
organltissue doses can be estimated are becoming more severe. Such
information, obtained from instrument measurements, may constitute the only substantial basis for implementing sound radiation
dose control procedures. This serves to emphasize the need for better
calibration and more complete knowledge of survey instrument
responses to all radiations encountered in the workplace.


2. Considerations in the
Calibration Process
2.1 General

The technical issues related to calibration and evaluation of instruments used for radiation protection purposes are dealt with in this
Report. Certain factors such as the type of display (e.g., analog vs.
digital) and human factor design features (eg.,weight, balance, size)
which affect the selection or desirability of particular instruments,
while important, are not covered here.
Most of the considerations in the Report apply to fixed radiation
monitors as well as to portable instruments. Fixed area detectors are
frequently located on walls or other surfaces and may be mounted
in proximity to sources of radiation or in areas of generally high

background radiation. It may be difficult or impossible to carry out
calibration of a fixed monitor in-situ;the detector may have to be
removed from its normal location to a more convenient one for calibration. If such a detector is normally cable-connected to a remote
readout station, the same or equivalent cable and readout system
should be used in the calibration process. Because of the presence of
potential radiation scattering materials close to a fixed monitor in
the field, such a monitor may be exposed to both primary and scatterdegraded radiations during actual use. If the detector in question
exhibits an energy-dependent response, calibration in a laboratory
setting may not assure accurate performance in the field if the energy
or angular distribution in the two situations are different. Other
features specific to the calibration of these monitors are not elaborated in this Report.

2.2 Level of Calibration

2.2.1 General
Calibration refers to the determination and adjustment of instrument response in a particular radiation field of known intensity.


10

/

2. CONSIDERATIONS IN THE CALIBRATION PROCESS

Some obvious factors which affect response, such as meter zero
adjustment and battery condition, are necessarily considered in the
overall calibration procedure. Additional influencing factors, such
as energy dependence and environmental conditions, may require
consideration in the calibration process, depending on the conditions
of use of the instrument. Thus, the procedures required for calibration may be more or less complex, depending on the need to assess

the impacts of these influencing factors. Three levels of calibration
are defined; these are discussed below and identified as full characterization, characterization for specific acceptance, and routine calibration.
2.2.2 Full Characterization

Full characterization of an instrument involves more than what
is normally required by users of instruments. Routine calibration
(SeeSection 2.2.4) often requires simply the determination of reading
linearity when an instrument is exposed to a single radiation type
of specified energy. Manufacturers of instruments and others may,
however, have the need to characterize fully an instrument being
supplied to users in the field. Such characterization should include
the following:
1) evaluation of the energy-dependence of the response of the
instrument to the radiation types to which the instrument is
intended to respond; note that response, as it applies to instrument
calibration, is the quotient of the instrument reading by the true
value of the quantity being measured;
2) evaluation of linearity of instrument readings;
3) evaluation of the effects of other ionizing radiation types which
may be encountered in field use on the instrument reading;
4) evaluation of the effects of environmental influences, such as
temperature, pressure, and humidity, on the instrument reading;
5 ) evaluation of the effects of nonionizing radiations, particularly
RF radiations, on the instrument reading;
6) evaluation of geotropic effects;
7) evaluation of the ability of the instrument to survive mechanical shock as might be encountered in field use;
8) evaluation of the dose rate-dependence of the response andlor
dead-time characteristics; this is particularly important to avoid
significant exposure when an instrument's response is depressed at
high dose rates;

9) evaluation of the effects of other influencing factors, such as
magnetic and electrostatic fields, and


2.2 LEVEL OF CALTBRATION

/

11

10) evaluation of the angular response of the instrument, preferably at an energy close to the minimum useful energy for the instrument.
Presently, most manufacturers provide information relating to
item (1)above for portable instruments used in air kermaldose measurements and for some instruments used in assessing alpha- and
beta-emitting surface contamination. A user may have to arrange
for characterization with respect to additional items from the list
given above.
2.2.3

Calibration for Specific Acceptance

It may be necessary to use an instrument under specific conditions
of a non-routine nature, and calibration specific to that objective may
be required. An example would be the intended use of an instrument
at temperatures higher than those encountered in general use. Such
an application would require evaluation of the instrument response
at the anticipated temperatures. Calibration might be carried out at
the elevated temperature and, if the adjusted response is acceptable,
the instrument approved for such use. As an alternative to calibrating the instrument at the elevated temperature, if the temperature
dependence of response is known, the calibration reading at a lower
temperature may be used to adjust to what would be expected at the

higher temperature. In these cases, a label should be applied to the
instrument noting that it may not be suitable for other uses if this
is the case. Alternatively, the instrument may be calibrated for
routine use and its response then evaluated under the proposed use
conditions. If responses under routine and proposed use conditions
are significantly different, a correction factor or chart should be
supplied with the instrument for use under the proposed conditions.
ANSI, in report number N42.17C (ANSI, 1989a), discusses performance specifications for portable instruments that are to be used
under extreme environmental conditions.
2.2.4

Routine Calibration

Routine calibration refers to calibration of an instrument for normal use. Normal use is characterized by the following:
1) use of the instrument for radiation of the type for which the
instrument is designed;
2) use of the instrument for radiation energies within the range
of energies for which the instrument is designed;


12

1

2. CONSIDERATIONS IN THE CALIBRATION PROCESS

3) use under environmental conditions for which the instrument
is designed;
4) use under influencing factors, such as magnetic and electrostatic fields, for which the instrument is designed;
5 ) use of the instrument in an orientation such that geotropic

effects are not a concern, and
6) use of the instrument in a manner that will not subject the
instrument to mechanical stress beyond that for which designed.
Routine calibration commonly involves the use of one or more
sources of a specific radiation type and energy (e.g., 137Csor 6 0 C ~
photon-emitting sources for many photon air kerma- or exposure- or
dose-measuring instruments) and of sufficient activity to provide
adequate field intensities for calibration on all ranges of concern.

2.3 Performance Check
Calibrations need to be carried out periodically as discussed in
Section 2.8. In the interval between calibrations, however, the
instrument user should validate acceptableoperation by carrying out
a performance check. This is merely intended to establish whether or
not the instrument is operatinglfunctioning within certain specified,
rather large, uncertainty limits. Although the performance check
may range from a crude determination that the instrument is
responding to a source, to a more detailed determination, deviations
of + 20 percent from the expected reading are generally considered
acceptable for a performance check. The initial performance check
should be carried out in the calibration laboratory following calibration; the source should be held at a fixed and reproducible location
and the instrument reading recorded. The source should be identified
along with the instrument, and the same check source should be
employed in the same fashion to demonstrate the instrument's operability on a daily basis when the instrument is in use.
Beta- or gamma-radiation-emitting radionuclides are commonly
used in sources for performance checking of beta- and/or gammaradiation-measuring instruments. The sources are often no more
than a few hundred kBq in activity and produce a reasonable reading
on the instrument when held very close to the detector. Some instruments use internally mounted sources that can be moved close to the
detector by means of an external control. Alpha-emitting radionuclides are used as check sources for alpha radiation detectors. Portable neutron sources in fixed geometries or, at times, well-characterized beams at reactor facilities, are useful as check sources for neu-

2.4

PRECALIBRATION CHECK

1

13

tron-measuring instruments. Tissue-equivalent proportional
counters (TEPC) often use an internally mounted alpha-emitting
source which serves as both a check source and a calibration source.
It is sometimes convenient to have available more than one check
source for use with a given instrument or with several instruments
of the same type. In such situations, the reading of the instrument,
when exposed to each such check source,should be evaluated in the
calibration laboratory. As above, the specific source must be identified along with the appropriate reading of a given instrument.

2.4

Precalibration Check

Before an attempt is made to calibrate an instrument, a series of
simple operations should be completed to ensure proper condition of
the instrument for calibration. Although the exact checks to be made
will vary with the design of the particular instrument, a number of
these are common to most instruments. These include checking for
radioactive contamination, condition of the batteries, loose or broken
parts, proper operation of the switches, and that the instrument zero
can be adjusted in accordance with the manufacturer's instructions.

2.5 Technical Considerations of Source Selection
2.5.1

Radiation Type

All instruments are energy dependent to some degree and are
designed to respond specifically to one or more of the various types
of radiation. Therefore, it is important that the source used for calibration emit radiation which is representative of that expected in
the field. Typical fields in the workplace can be "simple", such as
those associated with a single radionuclide in a contained configuration, or "complex", such as mixed radiations from a combination of
sources in a variety of configurations. Development of energy
response curves for a particular radiation type andlor evaluation of
responses to other radiation types may require a variety of calibration sources.
Photon sources of the required energy spectra are provided by xray machines with specified filters or K-fluorescence radiators (below
300 keV) and isotopic sources, e.g., 137Csand 6 0 Cfor
~ energies greater
than a few hundred keV. Beta radiation fields are not monoenergetic,


14

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2. CONSIDERATIONS IN THE CALIBRATION PROCESS

and the calibration sources are generally radionuclides mounted
with thin coverings. Recently, electron accelerators have been used
in an attempt to provide mono-energetic electron calibration fields
for defining better the instrument response characteristics. Neutron

fields of particular energy distributions may be difficult to obtain,
and the selection of sources may involve a combination of neutron
generators, fission sources,and isotopic sources. Sources appropriate
for calibration of instruments to be used in surface contamination
assessment include a variety of beta- and/or alpha-emitting radionuclides.
Specific sources and their characteristics are discussed in the sections of the Report treating source selection.

2.5.2 Field Intensity and Source Strength
For purposes of this report, field intensity is defined as radiation
fluence (rate), radiation energy fluence (rate), or quantities derived
from these, such as absorbed dose (rate) and dose equivalent (rate).
Radiation field intensities necessary to evaluate instruments in the
calibration process may require sources that yield absorbed dose
rates or kerma rates from less than 0.1 Gy h-' to greater than 100
Gy h-'. Source activity may range from about 10 MBq to more than
10 PBq. While a source as large as 10 PBq would likely not be applied
to portable survey instrument calibration, it may be required for
calibration of fixed area monitors intended for use in accident dosimetry. Calibration sources for instruments intended to measure surface
contamination commonly range in activity from 100 Bq to greater
than 10 kBq. Choice of the source andlor the calibration facility
arrangement must take the intensities into account. In addition,
high enough intensities must be provided to evaluate instrument
linearity and saturation characteristics.

2.5.3 Source-Detector Geometry

A number of considerations must be taken into account in choosing
a source either to reduce or evaluate geometrical dependencies.
These considerations include whether to select point or distributed
sources, the significance of angular response variations of the instrument, and the ability of the source to provide uniform irradiation

over the detector volume. With regard to the latter point, calibrations
are often performed using sourcesthat produce penetrating radiation
fields whose intensities decrease with the inverse square of the dis-


2.5 TECHNICAL CONSIDERATIONS OF SOURCE SELECTION

1

15

tance from the respective source to the point of interest. The question
commonly arises as to how close a given detector may be to such a
source and still yield a response equal to that estimated from the
fluence rate at a point in the center of the detector volume. In order
to provide a t least a partial answer to this question, the data of Table
2.1 should be useful. The geometry factor G, given in the last column
of the table, represents the ratio of the average radiation fluence rate
throughout the detector volume to the fluence rate at a point a
distance L from the point isotropic source and at the center of the
detector volume. Both the diameter and detector height for the cylindrical detector, and the diameter, for the spherical detector, are
expressed in units of L. The calculations done to obtain the table
values are described in Appendix E; no radiation attenuation was
assumed in the calculations. The factor G represents a correction by
which the fluence rate (or fluence rate-dependent quantity such as
dose rate) at distance L should be multiplied to obtain the fluence
rate (or fluence rate-dependent instrument reading) averaged over
the detector volume, the latter result being the true value appropriate for the calibration. The variation of the value of G from unity
provides an estimate of the magnitude of the systematic error
expected in the calibration process if the fluence rate at distance L

is assumed to be representative of the fluence rate throughout the
detector volume. The tabulated G-values would apply to typical ionization chambers. They would not apply to certain detectors that use
spherical or cylindrical shells for purposes of modifying the incident
radiation so that an enclosed detector would yield a particular
response (eg., neutron dose-equivalent-measuringinstruments with
spherical or cylindrical moderators surrounding a thermal neutron
detector).
The data in Table 2.1 show that for a right-circular-cylindrical
detector irradiated with penetrating radiation from a point isotropic
source on the central longitudinal axis of the detector so that radiation is incident on the flat detector face, the average fluence rate
over the detector volume will be within 1 percent of the fluence
rate at the detector center, if neither the detector diameter nor the
detector height is more than 20 percent of the distance from the
source to the detector center. Similarly, for the cylindrical detector
irradiated on its curved surface by a source on the transverse central
axis, the average fluence rate and that a t the detector center will not
differ by more than about 0.5 percent if neither the detector diameter
nor height exceeds 20 percent of the distance from the source to the
detector center; about the same agreement exists for the spherical
detector.
Additional corrections may be appropriate in the calculations for
particular detectors (e.g., corrections for volume occupied by the


16

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2. CONSIDERATIONS IN THE CALIBRATION PROCESS

TABLE2.1-Ratio of average primary radiation fluence mte in detector volume to
primary radiation fluence rate at center of detector volume. (Distance from source to
center of detector volume = L)."
Results for Cylindrical Detector
Detector surface of
radiation incidence

Flat
Flat
Flat
Flat
Flat
Flat
Flat
Flat
Flat
Flat
Flat
Flat
Flat
Flat
Flat
Flat
Flat
Curved
Curved
Curved
Cwed
Curved
Curved

Curved
Curved
Curved
Curved
Curved
C U N ~ ~
Curved

Cylinder
DiameteriL

1
1
1

Cylinder
heightll

=

Average fluence rate in volume
Fluence rate at diatance L

1
0.5
0.5
0.5
0.2
0.2
0.2

0.1
0.1
0.1
0.02
0.02
0.02
0.02

1
0.5
0.2
0.1
0.5
0.2
0.1
0.5
0.2
0.1
0.5
0.2
0.1
0.5
0.2
0.1
0.02

1.099
0.934
0.890
0.895

1.029
0.979
0.972
1.060
1.005
0.998
1.064
1.008
1.001
1.066
1.010
1.002
1.000

1
1
1
1
0.5
0.5
0.5
0.2
0.2
0.2
0.1
0.1
0.1

1
0.5

0.2
0.02
0.5
0.2
0.02
0.5
0.2
0.02
0.5
0.2
0.02

1.034
1.116
1.144
1.150
1.010
1.029
1.032
0.985
1.002
1.005
0.981
0.998
1.001

Results for Spherical Detector
Sphere DiameterlL

1

0.5
0.2
0.02

1.056
1.012
1.002
1.000

"See Appendix E for additional description.

collecting electrode in some detectors). It is recommended that the
source-to-detector-centerdistance should be at least five times the
maximum dimension of the detector for calibrations using sources of
primary penetrating radiation whose intensity follows an inverse
square relationship with distance from the source. If a source dimen-


2.5 TECHNICAL CONSIDERATIONS OF SOURCE SELECTION

1

17

sion is larger than the maximum detector dimension (as might be
the case when dealing with high activity planar sources used for
some high-level calibrations) , the uniformity of the field over the
detector volume depends on both the detector dimensions and the
source dimensions. For these situations the calibrator may have to
make measurements to demonstrate acceptable uniformity over the

detector volume. However, if a given detector is placed at a fixed
distance from the surface of a distributed source of unattenuated
radiation, the ratio of average fluence rate in the detector volume to
the fluence rate at the fixed distance will be closer to unity than the
same ratio for a point isotropic source exposing the same detector at
the same fixed distance. (This assumes usual calibration sources and
detector geometries; the statement would not hold for an unusual
source configuration such as a curved surface, concave toward the
detector.) This observation is based on the fact that for distributed
sources and detectors of common geometries, the distributed source
has relatively more of its activity further removed from a reference
point in the detector volume (e.g., the center point) compared to the
point source. The greater such distance is, the smaller will be the
difference between the fluence rate to that point and any other point
within the detector volume.
2.5.4

Traceability of Source Calibration

It is common practice to make use of a recognized standards laboratory to provide necessary references for establishing the calibration
fields. This is accomplished in several ways:
1) sources are sent to the National Institute of Standards and
Technology1 (NIST) for calibration:
2) instruments are sent to NIST for calibration; these instruments
are then used to calibrate the facility sources/fields;
3) sources or instruments are sent to a Secondary Calibration
Laboratory for calibration.

NIST is the Primary Calibration (Standards) Laboratory in the
U.S.A. Other countries maintain and operate similar laboratories.

Secondary Calibration Laboratories are laboratories which participate in formal programs involving comparative measurements with
the primary laboratory; these programs are used to establish and
demonstrate an acceptable degree of quality and consistency of performance on the part of the secondary laboratories. Secondary cali'Formerly known as the National Bureau of Standards


18

/

2. CONSIDERATIONS IN THE CALIBRATION PROCESS

bration laboratories may exist among the private, federal, and state
sectors and may offer sewices to various groups within their respective domains. It is likely that tertiary calibration laboratories will
also be established in the near future. Such laboratories would be
accredited through cooperation with secondary calibration laboratories and would have demonstrated a satisfactory level of competence and equipment to perform valid instrument calibrations. Naturally, the further traceability is removed from the Primary Calibration Laboratory, the greater will be the uncertainty associated with
calibration accuracy. Figure 2.1 is a schematic diagram of the trilevel measurement support system common in the U.S.A. (Eisenhower,1982). Tertiary-level laboratories would lie between secondary-level and field-use level on the figure. Figure 2.lb includes a
description from Lalos (1983) of the hierarchy of standards.
The International Atomic Energy Agency has discussed the establishment, development, status and future trends of Secondary Standards Dosimetry Laboratories in the IAEAIWHO network (IAEA,
1985).

2.5.5 Accuracy of Calibration Source for Field Intensity
Determinution

Measurement uncertainties may be introduced at every step in
the calibration. NIST typically provides standards of radioactivity,
calibrated in terms of radioactivity or radiation emission rate, with
uncertainties on the order of one to two percent. Similar uncertainties apply to NIST sources of x rays and gamma rays calibrated
in terms of exposurelair kerma rate. Uncertainties in NIST betaemitting sources calibrated in terms of absorbed dose are typically
5 to 15 percent. Uncertainties in calibrations made at Secondary
Standards Laboratories will likely be two or more times greater

than those of NIST. A facility laboratory dependent on a secondary
laboratory for calibration will commonly operate with uncertainties
in its standards which are greater than those of the secondary laboratory. Thus, uncertainties on the order of 10 percent are not uncommon for such facilities although uncertainties on the order of 4 to 8
percent may be achievable (See Figure 2.lb).
Except for national standard sources maintained by the NIST, all
other standard sources or instruments fall into a category denoted
as transferred standards. This implies that standardization (calibration) has been performed through a transfer process in which the
instrument or source of concern has been standardized through a
measurement made using a standard maintained by NIST. Second-