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 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 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 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 Introduction 1.1 General 1.2 Scope and Structure 1.3 Need and Intent 1.4 Review of Current Efforts/Recommendation 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 Calibration Facility 3.1 General 3.2 Background Radiation 3 Scattering 3.4 Equipment Requirements 3.5 The Physical Facility 3.6 Staffing 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 CONTENTS 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 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 ix 6.5 Accuracy and Acceptance Criteria 102 6.6 Calibration Frequency 103 6.7 Calibration Examples 103 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 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 - Calibration of a Thin End Window G-M Counter with a Reference Point Source in a "Weightless" Source Mount 162 D-1.2 Example - 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 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 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 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 / 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; D-1.2 CALIBRATION OF A THIN END WINDOW / 167 was 5309 min-l Thus it was calculated that 5309173,320 = 0.072 of the beta particles (0.294 MeV maximum energy) emitted from a "point" source are detected a t a source-detector distance of 1cm The detected particles include backscattering particles, of course, which serve to increase the apparent efficiency of the detector D-1.2.3 Application to Monitoring of a Surface The calibration would be most directly applicable to the monitoring of a stainIess steel (for example, sink) surface contaminated with a radionuclide which emitted 0.3 MeV(max) beta particles If a plastic or wood bench top were monitored, the backscattering would be considerably less A figure of percent is taken from the curve (Figure D-1) of backscatter as a function of atomic number for 60Co beta particles, where the atomic number of a plastic or wooden surface is taken as (for carbon) In applying the calibration data above to a plastic bench, the 0.072 efficiency figure should be corrected by the factor 1.0611.22 or 0.87, to give a n efficiency for plastic or wooden surfaces of 0.87 x 072 = 0.063 In monitoring a plastic surface, a spot of contamination gave a reading of 9100 clmin with the G-M detector discussed above Dispensing with dead time and background corrections, which are not necessary a t these counting rates for radiation protection purposes, the activity on the surface is 910010.063 = 1.44 x lo5 min-' 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 E-1 General Calculations were carried out for right circular cylindrical detectors and spherical detectors irradiated by point isotropic sources in geometries common in the calibration process For the cylindrical detector, the point source was, in one case (geometry I), located at selected distances along the longitudinal central axis of the cylinder and, in the second case (geometry 2), along the transverse central axis In both cases, the source was outside of the detector volume; for the first case radiation was incident on the flat face of the cylinder and, for the second case, radiation was incident on the curved wall of the detector For the spherical detector (geometry 3), the source was located outside the detector volume on a line through the volume center In all cases, fluences were calculated under the assumption E-2 MEAN-VALUE CALCULATIONS 169 of no radiation attenuation and the entire internal volume of the detector being available (i.e.,no corrections for volume possibly occupied by internal electrodes) E-2 Mean-Value Calculations For the cylindrical detector, height and diameter dimensions relative to the source-to-detector center distance ranged from 1to 0.02 For the spherical detector the diameter relative to the source-todetector center distance covered the same range (1to 0.02) The average value of the fluence rate in a detector volume was evaluated via a mean-value calculation by solving the following equation: where S is the radiation emission rate from the source, x is the distance from the source to a differential volume element dv within the detector volume, and the denominator is simply the actual internal volume of the detector The fluence rate a t the detector center at distance L from the source, c$= is given by: The values of the geometry factor G, which is the ratio of the average fluence rate throughout the volume to the fluence rate a t a point a distance L from the point source a t the center of the volume, as given in Table 2.1 are obtained by dividing $ by &: (Note that for geometry 1, the G-values are the reciprocals of the respective values of E given by Langrill's equation (Langrill, 1984) in Section 4.3.3.)Thus, ifthe fluence rate (or a fluence rate dependent quantity) is calculated a t distance L from the detector center, the actual fluence rate, as seen by the detector, will be G times the calculated value 170 / APPENDIX E The values of $ for the three specific cases were obtained from the following equations by numerical integration See Figure E-1 for appropriate dimensions Geometry (Radiation incident on flat face of cylinder) Geometry (Radiation incident on curved surface of cylinder) Geometry (Radiation incident on sphere) - S / r TT 22P s 4.rrR3/3e=o +=o r=o $, = -1 12 d r sine d0 d+ + L2-2 Lr cos+ sine' E-2 MEAN-VALUE CALCULATIONS r 2) C % "2 % d' u i - * a , w Fig E-1 Geometries used for mean value calculations / 171 APPENDIX F Systematic Uncertainties in the Calibration Process F-1 General Systematic uncertainties may be present in several steps of the calibration process and will contribute to the overall uncertainty in the results Uncertainties in various parameter values that enter into the calibration may be associated with the instrument being calibrated, the transfer standard used to establish the true calibration quantity, the radiation source, various measuring instruments required in the calibration process, environmental influences, and other possible factors Some of these systematic uncertainties are discussed below, and an example is given showing how several of these might affect a calibration result F-2 Systematic Uncertainties Associated with Specific Aspects of Calibration F-2.1 The Instrument being Calibrated Regardless of the instrument type there is always uncertainty associated with the instrument reading Part of the uncertainty is of a random nature associated with the physical processes occurring in the detector and part is systematic related to the ability of the operator to read the scale accurately andlor the ability of the electronics to interpret a given detector signal in exactly the same way each time such a signal appears Those uncertainties associated with the instrument itself (random fluctuations plus systematic variations F-2 SYSTEMATIC UNCERTAINTIES / 173 associated with the detector andlor electronics) are normally evaluated by collecting multiple readings and estimating the standard deviation as in Section 2.7.2 If detectable operator reading biases have been removed (eg., parallax errors), there remain systematic reading uncertainties (these may be biased but undetectable) that may affect results These reading uncertainties might be expected for analog display instruments but not for digital display instruments The magnitude of the reading uncertainty depends on the number and spacing between scale markings on the display Scale divisions are equally spaced on a linear display but are unequally spaced on a log-scale display In the latter case the magnitude of the error in reading the scale will vary from the start to the end of a display decade If instruments are calibrated by model type for a particular radiation i.e., it is assumed that instruments of the same model will respond to that radiation in the same fashion as the calibrated instrument, systematic uncertainties in responses may be significant For example, beta-dose-measuring instruments of the same model may have all been calibrated in a photon field, but complete beta calibration factors may have been determined for only one instrument Because of slight variations in instrument fabrication, other instruments of the same model may exhibit slightly different responses to beta radiation The magnitude of such variations may be estimated from experience with several instruments of the same model exposed under similar conditions F-2.2 The Transfer Standard Instrument An instrument used as a transfer standard will have been calibrated by a n accredited laboratory and, typically, a calibration factor will be supplied Associated with the calibration factor will be a n uncertainty which is the result of both random and systematic uncertainties incurred in the calibration process These uncertainties may be expressed as a single combined uncertainty representative of the maximum uncertainty in the calibration factor In such a case, it is reasonable to divide the quoted uncertainty by a number such as or to estimate a reasonable "standard deviation" to apply in error propagation Depending on how the output of the transfer instrument is measured, a systematic reading uncertainty may be present, a s discussed above for the instrument being calibrated 174 / APPENDIX F F-2.3 The Radiation Source If a radionuclide source is being used, and corrections are being made for source strength based on the half-life of the radionuclide, some systematic uncertainty may result from uncertainty in the published value of the half-life If the activity of the source is a quantity used directly in the calibration, as it might be for calibration of surface-contamination-measuring instruments, uncertainty in the initial activity will also be a source of propagated error in calibrations Machine-produced-radiation fields may vary in intensity during the calibration process If such fields are not monitored with a reference instrument, as is often done, estimates of the magnitude of the variations must be made and included in assessing overall uncertainty F-2.4 Associated Measuring Instruments In addition to the transfer standard instrument, a number of other measuring instruments may be required in the calibration process These include distance-measuring devices, time-measuring instruments, angle-measuringdevices, temperature and pressure-measuring instruments, and other possible instruments Such instruments or devices are not absolutely accurate and information may be available, particularly from the manufacturers, as to the maximum uncertainty associated with use of a particular instrument For example, a mercury thermometer may be specified as being accurate to within + 1"C There may also be a systematic uncertainty associated with reading the scale of some measuring instruments; meter sticks, thermometers, barometers and other devices would often be subject to reading uncertainties F-2.5 Environmental Influences Environmental factors such a s air temperature, pressure, and humidity may influence the readings of instruments Changes in the values of these parameters duricg the calibration process may be a source of systematic uncertainty It is sometimespossible to measure changes that occur and to estimate the effects on the calibration process Variables such as those noted above should be controlled sufficiently during calibration so that resultant uncertainties are very small Other influencing factors such as electric and magnetic F-3 EXAMPLE OF THE INFLUENCES 175 fields and gravity (geotropic effects) may also be sources of systematic uncertainty in some situations F-3 Example of the Influences of Systematic Uncertainties in the Calibration Process In this example, a transfer standard instrument (open-to-atmosphere-air-ionization chamber) is used to establish the true air kerma rate in a gamma radiation field a t a fixed point from a point isotropic source, and an air ionization chamber instrument is then calibrated in the field a t the same location The parameter values and associated systematic uncertainties required to propagate errors are shown in Table F-1 Table F-1-Systematic Uncertainties in the Calibration Exumple A Calibration distance D = m Measurement device: Steel metric tape Maximum uncertainty inherent to measuring device: 0.2% = 0.002 m Maximum reading uncertainty in measuring device: 0.001 m uD = ?4 (0.002, + 0.0012)112= ~t 7.5 x m B Transfer Instrument (Air Ionization chamberldigital electrometer) Calibration factor F and associated maximum uncertainty: 0.96 3% @ 22 "C, 1atm; uF = !4 (0.96) (0.03) = ? 9.6 x l W Mean reading T' = 1.027 mGy h" * C Instrument Being Calibrated Mean reading I' = 1.03 mGy h" Maximum reading uncertainty: t 0.04 mGy h-I u-I - + - 45 (0.04) = f 0.013 mGy h ~ ' D Temperature t = 23 "C Measurement device; Mercury thermometer Maximum uncertainty as reported by manufacturer: f "C Maximum reading uncertainty: f "C Maximum variation in room temperature during calibration: u, = ?4 (2' + l2+ 12)" = 0.82 OC * Tt "C E Pressure P = 100.2 kPa (75.2 cm mercury) Measurement device: Mercury barometer Maximum uncertainty as reported by manufacturer: 0.13 kPa Maximum reading uncertainty: up = ?4 ([(0.01) (100.2)12 + 0.133~' = 0.34 kPa * * 1% * F Source Strength, A (not explicitly used in example) A, = Activity a t initial calibration ('37Cssource, TI, = 30.17 y) A (t) = Current activity = A, e tIn2 T1,2 Maximum uncertainty in T,,,:2 0.03 y h1,2 = ? % (0.03) = f 0.01 y 176 / APPENDIX F n~, The quantity of interest in assessing the calibration is R = where I is the mean instrument reading and T is the presumed true value of the air kerma rate The values of I and T used are the same as those in the example of Section 2.7.2 Errors will be propagated under the assumption that the individual uncertainties are uncorrelated Thus, for any function z, dependent on variables x y , , the standard deviation in z is estimated from As was described in Sections 2.7.1 and 2.7.3, the symbol u shall be used to denote the "standard deviation" associated with systematic uncertainties Since air kerma rate varies inversely with the square of the distance from a point isotropic source, the uncertainty in the kerma rate a t distance D, given a "standard deviation" of u in the distance measurement, may be estimated, recognizing that when the error in the ratio K D2/(D + u,)' is propagated, ub = K is obtained The relative uncertainty, %,would also apply to D an instrument reading a t distance D The value of the true kerma rate T and its associated "standard deviation", u,, determined from systematic uncertainties in independent parameters may be shown as The value of u, calculated from error propagation is For the values given in Table F-1, we obtain Similarly, for the instrument being calibrated, / F-3 EXAMPLE OF THE INFLUENCES 177 and for the values of Table F-1, uf = 0.014 mGy h-l f The resultant "standard deviation" u, in the ratio R = - is then T Note that the instrument being calibrated was a n air ionization chamber, open to atmosphere, and the reading was corrected to 22 "C and atmosphere, the temperature and pressure commonly accepted as the reference values for calibration In general, temperature and pressure corrections are not used to adjust instrument readings (except for the transfer instrument) in routine calibrations This may be reasonable in view of the fact that ambient temperature will vary somewhat in field use, although not making such corrections introduces additional error (bias) in the calibration The systematic and random uncertainties have been combined in quadrature as discussed in Sections 2.7.1 and 2.7.3 If the same source is to be used for future calibrations a t the same distance, it is not always necessary to reassess the true air kerma rate using the transfer instrument Commonly, the value T would simply be adjusted to account for radioactive decay of the source If To is the value of air kerma rate determined originally, the value of T a t some later time t is T = T , e-"ln2"T1/2 07-71 For a given value of u,,,, (Table F-l), the "standard deviation" in T, UT,T,,~, resulting from standard error propagation is For example, for a n elapsed time t = y and for a 137Cshalf-life of 30.17 years and a value of = 0.01 y, we would obtain This uncertainty would be propagated with any other uncertainties in the value of To to obtain the overall "standard" deviation in T APPENDIX G Glossary accuracy: A measure of the extent of agreement between the measured value and the true value angular response: The response of the instrument detector to particles or photons which impinge on the detector at angles deviant from a normal to the facial plane of usual incidence ambient dose equivalent: Symbolized H*(d), this quantity represents the dose equivalent a t depth d i n the ICRU tissue equivalent, 30-cm diameter sphere and along the radius opposed to the direction of the radiation field when the field has been expanded and aligned An expanded radiation field is one in which the directionality and energy fluence distribution at a point of interest are maintained constant and expanded throughout the volume of interest An aligned radiation field is the same as an expanded field except that the field is made monodirectional throughout the volume calibration: For an instrument intended to measure dose or dose rate related quantities, calibration is the determination of the instrument response in a specified radiation field delivering a known dose (rate)a t the instrument location; 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 or the reading per unit radiation emission rate per unit surface area, or the reading per unit activity charged particle equilibrium: An equilibrium condition under which the energies, number, and directions of charged particles leaving a mass element of material are equal to the energies, number, and directions of charged particles entering the mass element contaminating radiation: Radiation different from that expected1 desired from the source GLOSSARY / 179 directional dose equivalent: SymbolizedH'(d)this quantity represents the dose equivalent at depth d along any selected radius of the ICRU tissue-equivalent, 30-cm diameter sphere when the sphere is placed in an expanded radiation field (See definition of ambient dose equivalent for definition of an expanded radiation field) distributed source: An area or volume source with a t least one dimension large compared to the dimensions of the detector and which may produce radiations over a wide range of angles of incidence on the detector dose equivalent: (H)Absorbed dose multiplied by the quality factor effective dose equivalent, HE: The summation, over all the significantly irradiated tissues of the body, of the products of the individual tissue stochastic risk weighting factors, W T ,and the dose equivalent to the respective tissue, HT, i.e., HE = ZTwflP effectiveenergy: In reference to bremsstrahlung radiation from an x-ray machine, the effective energy is the monoenergetic photon energy which exhibits the same first half-value thickness in a given material as the x-ray beam end-pointenergy: Maximum energy (normally applies to beta emitters) energy dependent detector: Detector system which has a different response to different energy radiations, all other factors being equal equilibrium thickness: A thickness of material, impinged upon by primary radiations, sufficient to produce a condition of secondary charged particle equilibrium in the material Em,,:The maximum beta particle energy emitted by an unattenuated source E,,, residual maximum beta energy:The maximum energy of the beta spectrum from all beta decay branches of a radionuclide at the calibration distance Em is less than the corresponding Em,, as the spectrum is modified by absorption and scattering in the source material itself, the source holder, the source encapsulation and other media between the source and the calibration position free air exposure: Exposure to an unconfined, uncollimated source in air under conditions in which scattered radiation makes up an insignificant proportion of the total intensity geotropism:The degree of instrument reading change as a function of the physical orientation of the meter half-value thickness: The thickness of a given material required to reduce the radiation intensity by a factor of two intensity (of radiation field): For purposes of this report, field intensity is defined as radiation fluence (rate), radiation energy 180 / APPENDIXG fluence (rate) or quantities derived from these, such as absorbed dose (rate) and dose equivalent (rate) intrinsic background: The contribution to the instrument reading from the instrument itself, independent of any external radiation kerma: The mathematical product of the mass energy transfer cross section for indirectly ionizing radiation (photons or neutrons) in a material and the energy fluence of such radiation at the point of interest in the material kPa: Abbreviation for kilopascal; the pascal (Pa) is the SI unit of pressure and Pa is equal to newton per square meter; one standard atmosphere = 1.013 x lo5 Pa = 101.3 kPa linearity: The extent to which the instrument reading is proportional to the true quantity being measured as the intensity changes mixed field: Radiation field composed of more than one type of radiation performance check: Following calibration, a source check carried out to ensure that the instrument response to radiation from a known source has not changed beyond acceptable bounds photon: For purposes of this report photon refers to ionizing electromagnetic radiation, specifically x rays or gamma rays point source: A source whose dimensions are small compared to the distance from the source to the detector If radiation emission from the source is also isotropic and unattenuated, the radiation fluence rate varies inversely as the square of the distance from the source With respect to sources used in calibration of beta dose responsive instruments, a source is considered a point source when the sourceto-detector distance is greater than two times the largest dimension of the source or the largest dimension of the detector, whichever is greater precision: The extent of reproducibility of the measurements, commonly quantified by the standard deviation of a group of measurements about the mean random error: An error associated with random (statistical) fluctuations inherent to or associated with the determination of a particular quantity Such errors may be evaluated using standard statistical techniques REM meter1REM counter: An instrument whose response simulates the dose equivalent response of the human body R,,, residual maximum beta range: The residual maximum beta range, R,,,, is the range in an absorbing material of a beta spectrum of residual maximum energy Ere, response: For purposes of calibration, response is the quotient of the instrument reading by the true value of the quantity being measured GLOSSARY / 181 secondary calibration laboratory: A laboratory which maintains and uses standards whose calibrations are directly relatable to primary standards The National Institute of Standards and Technology (NIST) is the primary standards and calibration laboratory in the U.S.A Secondary laboratories participate in a routine cooperative program with NIST to assure the quality of their techniques, procedures, and equipment systematic error: An error of a non-random nature and associated with one or more biasing influences in the measurement process Individual systematic errors may produce either high or low results Systematic errors may a t times be eliminated by proper correction of an observed defect in the procedure or evaluated by careful analysis sometimes involving comparative measurements with other laboratories/facilities transfer standard: An instrument or radioactive source which has been standardized (calibrated) in terms of response (for an instrument) or radioactivity content, radiation emission rate or dose rate (for a source) by measurements made against a national (NIST) standard or a standard maintained by a secondary calibration laboratory uncertainty: A measure of how much confidence one has in the accuracy of a measurement Both random fluctuations, associated with statistical variations inherent to the measurement process, and systematic errors commonly associated with technique or judgment, may contribute to uncertainty weightless mount: Material on which source material is mounted which results in negligible scattering from the mount ... advantage of the historical performance records of instruments in deciding upon calibration frequencies If accurate records are maintained for instruments in use, the individual(s1 in charge of the calibration. .. 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. .. uncertainty For example, in determining the accuracy of a calibration, evaluate the reading of an instrument relative to the estimated true value of the quantity being measured by dividing the reading