Si Units in Radiation Protection and Measurements: Recommendations of the National Council on Radiation Protection and Measurements (N C R P Report)

NCRP REPORT No. 82

SI UNITS IN RADIATION PROTECTION A N D MEASUREMENTS Recommendations of the NATIONAL COUNCIL O N RADIATION PROTECTION AND MEASUREMENTS

Issued August 13,1985 First Reprinting February 28,1994 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). T h e 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 discloeed in this report may not infringe on privately owned rights; or (b) assumes any liability with respect to the use of, or for damages resulting from the use of, any information, method or process disclosed in this report.

L i b r a r y of Congress Cataloging i n Publication D a t a National Council on Radiation Protection and Measumments. SI Units in radiation protection and measurements. (NCRP report ; no. 82) "Issued March 13, 1985." Bibliography: p. Includes index. 1. Radiation-Measurement. 2. Units. 3. Metric system. I. Title. 11. Series. QC795.42.N37 1985 616.07'57'0287 85-3052 ISBN 0-913392-74-X

Copyright O National Council on Radiation Protection and Measurements 1985 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.

Preface In 1948, the General Conference on Weights and Measures (Confkrence Ginirale des Poids e t Mesures, CGPM), a diplomatic conference responsible for the "international unification and development of the metric system," instructed its International Committee for Weights and Measures to develop a set of rules for the units of measurement. The "International System of Unitsn (SI) was developed under this charge, adopted by the CGPM in 1960, and accepted by all signatories to the Meter Convention in 1977. The special units, curie, roentgen, and rad are not coherent with this system and were listed among those units to be used for a limited time. The corresponding SI units are reciprocal second, coulomb per kilogram, and joule per kilogram, respectively. Recognizing that this shift to different units might cause difficulty, particularly in radiation therapy, the International Commission on Radiation Units and Measurements (ICRU) solicited comments on this matter in 1973 and 1974. From these comments, it appeared that the majority of workers in the field would find SI units acceptable if the transition period was sufficiently long and if there could be special names a t least for reciprocal second and joule per kilogram. Therefore, the ICRU proposed special names for these units and the 1975 meeting of the General Conference adopted, for us'e with ionizing radiation, the special names of becquerel for reciprocal second and gray for joule per kilogram. The ICRU will use SI units and, where pertinent, special names. Currently, it will also include the relevant special units, but it plans to drop such usage by 1985; i.e., after a 10 year transition period.' This action of the ICRU was followed by a joint action of the ICRU and the.Internationa1 Commission on Radiological Protection (ICRP) which resulted in the approval by the CGPM in 1977 of a special name-sievert-for the SI unit of dose equivalent. The ICRP has used only SI units in all of its reports since 1977. In view of these actions internationally, but mindful of the sometimes special problems in the U.S.A., the National Council on Radiation Protection and Measurements felt it appropriate to consider its own position with respect to the adoption of SI units in radiation uses iii

' For further details see ICRU Report 33 (1980).

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PREFACE

and applications, in radiation protection and measurement and indeed in NCRP reports themselves. To assist the Council in this matter, an Ad Hoc Committee on Policy in regard to SI was formed. This report is the result of that Ad Hoc Committee's deliberations, suitably modified in the Council's review and approval process. The report therefore states the Council's position on this matter. Basically, the Council has decided to recommend to all that the SI be used and the special names for the SI units be employed where indicated. To accomplish this, the Council recommends (noting that the ICRU's 10-year period has essentially elapsed) a further transition period ending in five years, December 1989. For approximately a two year period through 1986, the NCRP recommends simultaneous use of SI and the present units, the present units being reported first and SI in brackets. During 1987-89, SI units will be quoted first with present units in brackets. Thereafter, only SI will be used. The NCRP recognizes that some will find the transition more difficult than others. It solicits the cooperation of all in making this transition, which experience indicates is less formidable than many suppose. The report was draftRd initially by the following Ad Hoc Committee: Randall S. Caswell, Choirman National Bureau of Standards Gaithenburg. Maryland

Edward R. Epp Massachusetts General Hospital Boston, Massachusetts

William A. McCarthy Cleveland Clinic Foundation Cleveland, Ohio

Fred A. Mettler, Jr. Veterans Administration Hospital Albuquerque, New Mexico

IWph H. Thomas Lawrence Berkeley Laboratory Berkeley, California

Harold 0. Wyckoff International Commieaion on Radiation Units and Measurements Bethe&. Maryland NCRP Seeretariot--Thomas M. Koval Thomas Fearon James Walker

Henry N. Wagner. Jr. The Johne Hopkins University Baltimore, Maryland

The Council wishes to express its appreciation to the members of the Committee and reviewers for the time and effort they devoted to the preparation of this report. Bethesda, Maryland 8 December, 1984

Warren K. Sinclair President, NCRP

Contents Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Concepts of Quantities and Units . . . . . . . . . . . . . . . . . 1.2 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 SI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Base Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Supplementary Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Derived Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Special Names for Units . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Units for Use with SI . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Units Accepted Temporarily . . . . . . . . . . . . . . . . . . . . . . 2.7 Coherence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8 SI Prefixes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 . Relationship Between Conventional and SI Units for Selected Quantities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Specific Energy. Absorbed Dose. and Kerma . . . . . . . . 3.1.1 Specific Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2 Absorbed Dose . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3 Kerma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Dose Equivalent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 SI Units for Nonionizing Radiation . . . . . . . . . . . . . . . . 4 Considerations Concerning Adoption of SI Units . . . . . . . . . 4.1 General Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Radiation Science . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Radiation Physics . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Radiation Chemistry . . . . . . . . . . . . . . . . . . . . . . 4.2.3 Radiation Biology . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Radiation Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Radiation Therapy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Diagnostic Radiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6 Nuclear Medicine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1 3 3 4 8 8 9 9 9 9 10 10 10 12 12 12 12 13 14 16 18 18 20 20 22 22 23 23 23 25 25 26

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vi

CONTENTS

4.7 Environmental Radiation Measurement . . . . . . . . . . . . 4.8 Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.8.1 General Commercial Considerations . . . . . . . . . 4.8.2 Radionuclide Assay Devices . . . . . . . . . . . . . . . . 4.8.3 Radiation Therapy Instrumentation . . . . . . . . . 4.8.4 Instrument Modification . . . . . . . . . . . . . . . . . . . 4.9 Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.10 Regulatory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.11 Training . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Discussion and Recommendations . . . . . . . . . . . . . . . . . . . . . APPENDIX A. Definitions of the SI Base Units . . . . . . . . . . . . . APPENDIX B . Conversion Between SI and Conventional

.

Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . APPENDIX C. Conversion Tables for Activity, Absorbed Dose. and Dose Equivalent Between SI and Conventional Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The NCRP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NCRP Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Executive Summary The National Council on Radiation Protection and Measurements recognized the need to consider its position with regard to the use of SI units in the fields of radiation protection and measurement and in NCRP reports. The present report is the result of its examination of these matters. The report considers the concepts of quantities and units, and gives a brief history of the International System (Le Syst6me International d'unitks, SI). The structure of the SI is then discussed, including base units, supplementary units, derived units, and some special categories of units. The meaning of coherence, which is an advantage of the SI, is explained, as well as the use of prefixes in the SI. The relation between conventional units and SI units for some quantities used in radiation measurement is discussed. Examples are given of calculations in both customary units and SI units. Next, arguments are considered for and against the adoption of SI units. Such questions as the experience with the adoption of the SI in other nations, safety considerations in the adoption of new units, and the impact of SI units on various fields of radiation protection and measurement are considered. A chief argument in favor of the adoption of the SI is that this system, with the desirable feature of coherence, is the consensus candidate for a single system of units to be used for all branches of science and engineering throughout the world. If we are to join the move toward a single system of units in the world, then at some time it will be necessary to change to SI units. Although it is difficult to estimate the economic costs of switching to the SI, it does seem clear that the cost of switching in the near future will be less than the cost of switching at any later time. The gradual adoption of SI units over a transition period will permit familiarization with the new units, provide time for necessary education and training to take place, and is generally consistent with the practice followed in other countries. The Council recommends the gradual adoption of SI units over a transition period beginning immediately and ending in about five years (December 1989). Experience indicates that, with proper educational processes in place, it is not difficult for individuals to change 1

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EXECUTIVE SUMMARY

to a new system of units. Available evidence does not indicate that safety is jeopardized by the introduction of a new unit system. For an approximately two-year period through 1986 the NCRP recommends simultaneous use of SI units and present units. During this period the Council recommends reporting of measurements in conventional units followed by the value in SI units in parentheses. During the period 1987-1989, it is recommended that measurements be reported with the value in SI units given first followed by the value in conventional units in parentheses. After 1989 it is recommended that SI units be used exclusively. In tables, graphs, and radiation records one system of units would be used with a footnote containing conversion factors to the other system. The NCRP recognizes that some organizations may find it more convenient for administrative or other reasons to make an abrupt change to SI units. In some cases it may not be practical to effect a transition in the recommended five-year period. In such cases the NCRP recommends that a carefully considered plan be developed to carry out the transition in an appropriate time period.

1. Introduction The subject of units for measurement is an important, complex, and sometimes emotional one. With the adoption of the modern consensus metric system, called SI for Systime International, throughout the greater part of the world and by many international organizations, the National Council on Radiation Protection and Measurements has recognized the need to consider its position with regard to the use of SI units in the fields of radiation protection and radiation measurement and to the adoption of SI units in NCRP reports. This report considers the concepts of quantities and units, discusses the International System, develops the relationships between conventional and SI units for radiation quantities, considers arguments for and against the adoption of S I units in terms of their use in various fields of activity, and provides recommendations for action. Emphasis in this report is given to ionizing radiation with some discussion of the use of the SI for nonionizing radiation.

1.1 Concepts of Quantities and Units

I t is necessary to distinguish between a physical quantity and a physical unit. " Aphysical quuntity characterizes a physical phenomenon in terms that are suitable for numerical specifications" (ICRU, 1980). " Aphysical unit is a selected reference sample of a quantity" (ICRU, 1980). The magnitude of a specified physical quantity can be expressed as a product of a pure number and a unit.' For example, let the symbol for length be L and the unit for length be meter (symbol, m).' If the 'Thus, if the quantity is divided by its unit, a pure number is obtained. For this reaeon the axes of graphs and the heading of tabular data are frequently given by the quotient of the symbol or name for the quantity and the symbol or name for its unit. In the following example the notation would be length L or - or L/m. meter

m

'Many international organizations use the spelling metre. 3

4

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1. INTRODUCTION

length, L, is measured and d is the symbol for the number of meters in L, Neither the physical quantity, nor the symbol used to denote it, implies a particular choice of unit. Note that symbols for quantities are italicized but those for units are not. As an example, consider the physical quantity called the wavelength, A. A particular measurement might lead to the specification of the "sizen of a wavelength as X

=

5.896 x lo-' m.

This may equally well be written X/m = 5.896 x lo-'. Given these definitions of a physical quantity and of a unit, one needs to set up a system of units so that the magnitude of the quantities of interest can be specified. One possible system might be to specify a unique unit for each quantity. However, some quantities are defined in terms of the product or quotient of other quantities. Thus a different name for the unit of each quantity leads to redundance because the unit for these quantities can also be expressed as the product or quotient of the units for the "other quantities." With the development of the field of quantities and units, the units for a selected few "other quantitiesn have been specified as "base quantities," and their units as "base units." The-unit for each additional quantity is then specified as the product or quotient of suitable powers of the base units. If this "product" or "quotient" contains no numerical values other than unity, the unit system is said to be coherent (see section 2.7).

1.2 History The sophistication and rigor associated with physical quantities and units has advanced over the centuries as commerce expanded and as the needed type and accuracy of measurements increased. Local units of well recognized quantities such as length and mass sufficed when commerce was restricted to local exchange. In early times, the standards for units of length were frequently defined in terms of portions of the human body or by agricultural products. According to a review by Zupko (1968), the inch as a unit of length was brought to England by the Romans and was commonly associated with a thumb's breadth. In the Middle Ages, the inch was defined as

1.2 HISTORY

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5

the length of three medium-sized barley corns placed end to end. The relatively wide possible variations in such a standard would indeed cause serious confusion in commerce. There were many other units of length in vogue in England in the Middle Ages. These included the "nail" which was the length of the last two joints of the middle finger and was later taken to be equivalent to about 2% inches. The "palm" was a length corresponding to a hand's breadth and was later made equal to a distance of 3 inches. The "hand" (equal to 4 inches) is still used for measurement of the height of horses. The "finger" was equal to two nails and therefore corresponded to a distance of about 4% inches. The "span," the distance between the tip of the small finger and the tip of the thumb of an outstretched hand, corresponds to a distance of 9 inches. For larger distances, units such as foot, yard, rod, perch3, furlong and mile were, and some still are, used. In Europe during the 17th and 18th centuries, a very large variety of weights and measures e.xisted. Weights and measures not only differed between countries-in 1742 the French "piedn and "livre" were found to be larger than the English "foot" and "poundn by six and eight percent, respectively (BIPM, 1975)-but many systems of measure existed in individual countries. For example, more than 50 different "livres" existed in France before the introduction of the metric system (Moreau, 1975). The industrial revolution increased the need for accurate and reproducible measures. Many proposals were put forth to resolve the difficulties resulting from varied measures. In 1791, the French General Assembly adopted the principle of a system of measures founded entirely on one base unit of length, the "meter," defined to be equal to one ten-millionth of the length of the quadrant of the earth's meridian. The unit of mass was to be the mass of a cubic decimeter of water at the temperature of maximum density (BIPM, 1975). Further development led to a system based on two artifact standards: a meter bar for length and a kilogram weight for mass. Following further spread of the system, the "Convention du Mitre" or Treaty of the Meter was A "perch of land" was a unit of land area "of no standard dimensions but usually the square of the linear perch common in the region" (Zupko, 1968). The size of a perch (of length) depended upon the region or type of measure for which it was used. For example, one perch was 12 feet when called a Tenant right or Court measure; was 18, 20, or 24 feet when called a Woodland Measure, and sometimes waa 21 feet when called a Church Measure. Perches of 16% feet or smaller were usually agricultural measures while perches of more than 16'/z feet were usually used in forest regions and by town craftsmen.

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1. INTRODUCTION

signed in 1875 by 20 countries including the United state^.^ Since that time both the metric and conventional English systems of units have been legal in the United States. The English units of length and weight are defined in terms of their metric equivalents in the United States and some other countries. Since 1893,the inch has been legally defined in the United States in terms of the meter. For a critical review of the metric system, see Danloux-Dumesnils (1969). Even with the adoption of the Treaty of the Meter, marked variations in the systems of units used throughout the world continued to exist. In fact, various metric systems of units were developed. The units systems known as esu (electrostatic units), emu (electromagnetic units), cgs (centimeter-gram-second) and mks (meter-kilogram-second) are all metric systems. They were evolved for use in limited branches of science. Giorgi in 1901 (Moreau, 1975; BIPM, 1975) made recommendations which led to the mksa (meter-kilogram-secondampere) system, a coherent unit system covering the whole field of electrical, magnetic, and mechanical phenomena. As an example we compare several units in the esu, emu, and mksa systems. Quantity charge potential difference force

esu 2.998 XYOO ' statcoulombs6 1 statvolt 1 dyne

= =

=

emu 1 a b z comb 2.998 X 101° abvoltss 1 dyne

=

mksa 10 c o x b s

=

299.8 volts

=

lo-' newtons

'Three organizations which have been established under the Treaty of the Meter should be mentioned: (1) The Conference Generale des Poids et Mesures (CGPM, General Conference of Weights and Measures) is a diplomatic-scientific body which now meets every four years, provides the budget for the BIPM (see below), and makes final decisions concerning the metric system; for example, names and values of units. (2) The Bureau International des Poids et Mesures (BIPM, International Bureau of Weights and Measures) is the central laboratory of the international metric system, located in SBvres, near Paris, France. It was set up for conservation of the international artifact standnrds and for their comparison with national standards. At present it ia the central focus for comparison of national and international standards of measurement. (3) The Comiti International des Poids e t Mesures (CIPM, International Committee of Weights and Measures) has scientific and management responsibility for overseeing the metric system and the BIPM on a continuing basis. Reporting to the CIPM are seven Consultative Committees in various technical areas. The Comiti Consultatif pour lee Unitis (CCU) advises CIPM on questions of quantities and units. For ionizing rahations the Comiti Consultatif pour les Etalons de Mesure des Rayonnements Ionisants (CCEMRI) has been established with Sections on X and Gamma rays, and Electrons; Radionuclide Measurements; and Neutron Measurement. In the United States the National Bureau of Standards participates in a continuing series of measurement intercomparisons of radiation units through the BIPM and CCEMRI. 'The numerical factor is the velocity of light in cm s-I.

1.2 HISTORY Quantity

esu -

mass energy

1 gram

1 erg

1 gram 1 erg

7

mksa

emu = =

/

= =

lo-' kilograms 10-'joules

The International Union of Pure and Applied Physics and its Symbols, Units and Nomenclature (S.U.N.) Commission expressed the need for the international adoption of an "international practical system of units" for international communication, to be based on the meter, the second, the kilogram, and an electrical unit of the absolute practical system. A similar request was made by the French government (BIPM, 1975). This led the General Conference, during its 1948 meeting, to instruct the CIPM: "to study the establishment of a complete set of rules for units of measurement"; "to find out for this purpose, by official inquiry, the opinion prevailing in scientific, technical, educational circles in all countries"; and "to make recommendations on the establishment of a practical system of units of measurement suitable for adoption by all signatories to the Meter Convention" (NBS, 1981). The 10th General Conference decided in 1954 that such an inter. national system (a modernization of the mksa system) was to be basec on six units: meter, kilogram, second, ampere, kelvin, and candela. 11 1960, the 11th General Conference adopted the name "The Interna tional System of Units" (le Systeme International &Unit&) and issuec rules for prefixes. In addition, the conference divided SI units int three classes: base units, supplementary units, and derived units. As science evolves and develops it is expected that the Internationr System of Units will concurrently develop. The system has bee designed as a dynamic system. For example, in 1971 the Gener Conference added a seventh base quantity, with the unit mole, as measure of the "amount of substance." It is apparent that the use the SI will increase over the years, and will gradually supplant 0th systems. Most countries have adopted or are adopting the Intern tional System as their exclusive legal system.

2.1 Base Units T o form a system of units, a limited set of base units is arbitrarily chosen, in general, to enhance accuracy and simplicity. Once the choice of these base units has been made, an entire system of units can be constructed logically. From a scientific viewpoint, the division of these units into the three classes (base units, supplementary units, and derived units) is arbitrary. During the development of the present metric system of quantities and units the number of base units has gradually increased. The metric system probably came into being because of the suggestion that three concepts-length, mass, and time could be expressed in terms of a single unit-meter. It can be used to specify a unit of time by means of the period of a pendulum of special length. This unit of length can also be used to define a volume and therefore a mass of water (at a given temperature). Probably because of variations of water density with temperature and variations of the period of a pendulum with location on the earth, the three base units (for length, mass, and time) have been defined independently. When the base units for length, mass, and time were used to provide a derived unit for electrical and magnetic quantities, fractional exponents of some of the three units were necessary. Id 1901 Giorgi pointed out that such fractional exponents could be eliminated by adding another base unit-i.e., one for an electrical quantity (BIPM, 1975). Such a base unit-ampere-was adopted in 1948. For treatment of thermal phenomena another base unit-kelvinwas adopted in 1948 and its scale defined in 1954. The inclusion of a base unit-candela-for luminous intensity was approved in 1948. In calculations of the amounts (masses) of different chemical species involved in a given chemical reaction, the ratio of the masses of the two species is needed. As the mass ratio of quantities of each of two different species can be determined from chemical or atomic reactions much more accurately than by means of the separate determinations of their respective masses, it is useful to provide a base unit for the amount of substance of a chemical species, the "mole." Thus, presently, 8

2.5 UNITS FOR USE WITH T H E SI

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9

there are seven base SI units: meter, kilogram, second, ampere, kelvin, candela, and mole (see also Appendix A).

2.2 Supplementary Units There are two supplementary units, the unit of plane angle, the "radian," and that of solid angle, the "steradian."

2.3 Derived Units In addition to base units and supplementary units, there are derived units which are the products or quotients of powers of the base and supplementary units. Examples of SI derived units are given in Appendix B, which is based on National Bureau of Standards (NBS) Publication 330 (NBS, 1981). The base units, derived units, and supplementary units are all termed "SI units."

2.4 Special Names for Units Several of the derived units have special names and symbol8 restricted to use with specified quantities. For example, for absorbed dose, the special name gray (Gy) is given to joule per kilogram (J kg-'). For activity of a radionuclide the special name becquerel (Bq) is given to reciprocal second (s-I). The SI derived units, becquerel, gray, and sievert (for dose equivalent), have been "admitted for reasons of safeguarding human health" (NBS, 1981).

2.5 Units for Use with the SI Some units exist outside the International System that are used with the system. Examples of these are minute and hour of time, liter, and barn. The combination of such units with SI units results in the loss of coherence inherent in the International System. Additionally, some other units continue to exist outside the International System simply because their value in terms of SI units must be obtained by experiment. An example of such a unit is the electron volt (eV).

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2. SI

2.6 Units Accepted Temporarily

In 1969, the CIPM accepted the use of some special units on a temporary basis. Of importance for the present document is that four of the units accepted temporarily are roentgen, rad, curie, and barn. No definition of "temporary" is given. However, many nations have already discontinued their use. 2.7 Coherence A major advantage of the SI is its coherence. A system is coherent when no conversion factors other than unity are needed for the formation of units derived from the base and supplementary units. For example, in the coherent SI for absorbed dose (see Table B.11, 1J 1 Gy = -= 1 J kg-' = 1 J/kg? 1 kg

(2.1)

Whereas, in the conventional system, 1 rad

100 erg

= -=

1g

100 erg g-'

=

100 erg/g.

(2.2)

Similarly, in the coherent SI, for activity of a radionuclide, 1 1s

1 Bq = - = 1 s-'.

(2.3)

In the customary system,

The conversion factors 100 and 3.7 x 101° which occur in Equations 2.2 and 2.4 by definition cause the conventional system to be incoherent. For other examples of calculations in coherent and non-coherent systems of units, see Section 3. 2.8 SI Prefixes Prefixes may be used for convenience to form decimal multiples and submultiples of SI units (NBS, 1981). The currently adopted prefixes are given in Table 2.1. 'The notations such as J/kg and J kg-' are equivalent and both are accepted.

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2.8 SI PREFIXES

11

TABLE 2.1-SI prefixes

-.

Factor

Prefiw

Symbol

Factor

Prefix

E P

lo-'

deci

d

Pta

tera

T

centi milli micro

c m

nano

n P

10'" 1Ol6 10l2 loB

exa

gigs

G

lo6

mega kilo hecto deka

M

103

lo2 10'

k

h da

lo4 lo3 lo-'* 10-l6 lo-''

pic0 femto

atto

Symbol

P

f a

Use of prefixes is indicated by the following example:

Compound prefixes, formed by the juxtaposition of two or more SI prefixes, are not to be used. For example: 1 nm but not: lmpm.

When prefixes are used, there is lack of coherence. The magnitude of a quantity is most simply expressed as the product of a pure number and unit (see Section 1.1).When a prefix is used, part of the pure number is included in the prefixed unit. The advantages of coherence in calculations may be retained simply by converting from prefixed units to coherent units, performing the calculation, and converting back to prefixed units, if desired. Example: What is the power, P, on the target of an accelerator operating at 3 MV terminal potential, E, with a beam current, I, of 10 mA?

P = (3 MV) x (10 mA)

3. Relationship Between Conventional and SI Units for Selected Quantities Some definitions of radiation quantities and sample calculations are presented in this section as examples of the use of SI units in radiation problems. The quantities treated here are widely used in radiation work.

3.1 Specific Energy, Absorbed Dose, and Kerma Specific energy, absorbed dose, the kerma are all physical quantities whose units are given with dimensions of energy per unit mass. For convenience, the definitions of these quantities as given by the ICRU are reprinted here. The reader is referred to ICRU Reports 25 (ICRU, 1976) and 33 (ICRU, 1980) for more precise details and explanations.

3.1.1 Specific Energy The quantity specific energy (imparted), z, is the quotient of c by m, where t is the energy imparted by ionizing radiation to matter of mass m,

3.1.2 Absorbed Dose The absorbed dose, D, is the quotient of dS by dm, where d t is the mean energy imparted by ionizing radiation to matter of mass dm,

NOTE: The absorbed dose is the limit of the mean specific energy as the mass in the region under consideration approaches zero, i.e., 12

3.1 SPECIFIC ENERGY, ABSORBED DOSE, AND KERMA

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13

D = lim 2. m-0

This equation, which indicates the relationship between D and z, could serve as an alternative definition of D.

3.1.3 Kerma The kerrna, K, is the quotient of dE,, by dm, where dE,, is the sum of the initial kinetic energies of all the charged ionizing particles liberated by uncharged ionizing particles in a material of mass dm,

The coherent unit of specific energy, absorbed dose, and kerma in the cgs system of units is the erg g-' and in the International System of Units is the J kg-'. The unit rad is not coherent in either the cgs or the International System of Units, 1 rad = 100 erg g-' = lo-* J kg-' The historical reason for the choice of the size of this special unitthe rad-is that, under conditions of charged particle equilibrium, a specific energy absorption of 100 erg g-' (to within about 10 percent) results from the exposure of a small volume of soft tissue to 1 roentgen (see Section 3.2). This is true over a wide range of photon energy and was a useful numerical equivalence in radiation protection. This numerical equivalence may, however, have led to misunderstandings of the radiation quantities and units used in radiation protection. Such equivalence does not hold in the SI between the units for exposure and absorbed dose, but does hold for the units of air kerma and absorbed dose. The coherent unit in the International System of Units for the quantities discussed in this section is the J kg-' which is given the special name "gray" (symbol Gy). Thus it follows that, 1 Gy = 100 rad. Exampk The absorbed dose, D, in a small mass of tissue resulting from irradiation by charged particles, is given by the formula,

14

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3. CONVENTIONAL AND SI UNITS

where 9 is the charged-particle fluence, (dE/dz),,l is the linear collision stopping power of the charged particles in tissue, p is the density of tissue, and k is a constant depending upon the units used. Assume the following magnitudes for the quantities involved:

(3col

= 2 MeV cm-' = 3.2 x lo-" J m-'

The results in three systems of units are: SI (non-coherent)

1.602 x lo-' rad g MeV-' 45 108cm-~ p Ig~ m - ~ d E 2 - MeV cm-'

10" m-' 103 kg m-3 2 MeV cm-'

dx D

3.2 x

k

3.2 rad

Since 1 rad =

1.602 X lo-"

Gy

SI (coherent)

1.0 Gy kg cm MeV-' m-I 10" m-2 103 kg m-3 3.2 x lo-'' J m-' 3.2 x lo-' Gy

Gy, all results are equivalent.

3.2 Exposure The quantity exposure, X,is defined as the quotient of dQ by dm where dQ is the absolute value of the total charge of the ions of one sign produced in air when all the electrons liberated by photons in a volume element of air having mass dm are completely stopped in air,

3.2 EXPOSURE

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The special unit of exposure in the cgs system of units is the roentgen (R) defined by the International Committee for Radiological Units (ICRU, 1938) as "the quantity of X- or gamma-radiation such that the associated corpuscular emission per 0.001293 gram of air produces, in air, ions carrying 1 e.s.u. of quantity of electricity of either sign." "0.001293 gram of air" is the mass of 1 cm3 of air a t 0°C and 760 mm mercury pressure (standard temperature and pressure). This definition of the roentgen therefore leads to a non-coherent unit in the cgs system of units. In more recent definitions of the size of the unit roentgen, it is usual to convert units and write (ICRU, 1980): 1 R = 2.58 x

C kg-'.

In the International System of Units, the coherent unit for the quantity exposure is the coulomb per kilogram (C kg-'). No special name has been given to this unit. Example

The exposure rate due to photons with energy greater than 6, X 6 ,a t distance L from a radioactive nuclide of activity A is given by ICRU (1971):

In the conventional system Xbis in roentgens per hour (R h-') when A is in curies, and L is in meters. r s , the exposure-rate constant, is in units of R m2 h-' Ci-'. Calculation in conventional units Calculate the exposure rate 3 meters from a 10-curie cobalt-60 source. The value of r, for 60Co (NCRP, 1974) is: where contributions from low-energy gamma-ray photons and x rays below a 6 of 11.3 keV have not been included. Substituting into Equation 3.2 we have:

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3. CONVENTIONAL AND SI UNITS

Calculation in International System units Calculate the exposure rate a t 3 meters from a 1 TBq source.

rd= 2.53 X

lo-''

C kg-' m2 s-' Bq-'

= 2.81 x lo-" C kg-' s-'.

3.3 Dose Equivalent T h e quantity dose equivalent is defined by the ICRU (1980) as follows: The dose equivalent, H, is the product of D, Q, and N a t the point of interest in tissue where D is the absorbed dose, Q is the quality factor and N is the product of all other modifying factors,

H = DQN. T h e special name sievert, symbol Sv, has been adopted for the SI unit of dose equivalent in the field of radiation protection. For a given irradiation, the numerical value in joules per kilogram for the two quantities D and H may differ, depending on the values of Q and N. T o avoid any risk of confusion, the special names for the respective units should be used; i.e., D should be expressed in grays and H should be expressed in sieverts, 1 sievert = 100 rem.

Further ICRU reports which are helpful in understanding the definition of the quantity "dose equivalent" are ICRU Report 33 entitled Radiation Quantities and Units (ICRU, 19801, and ICRU Report 25 entitled Conceptual Basis for the Determination of Dose Equivalent (ICRU, 1976), and ICRU Report 39 entitled Determination of Dose Equivalents Resulting from External Radiation Sources (ICRU, 1985). An absorbed dose of 1 rad from photons results approximately in a dose equivalent of 1 rem. This leads to the numerical relationship that an exposure of 1 R t o soft tissue produces approximately an absorbed dose of 1 rad and a dose equivalent of 1 rem. This convenient "rule of thumb" associated with conventional units, however, can lead to

3.3 DOSEEQUIVALENT

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confusion since it may obscure the differences among the quantities exposure, absorbed dose, and dose equivalent.

Example of dose equivalent calculation The fluence rate of neutrons a t middle latitudes and sea level, produced by the iilteraction of cosmic radiation with the earth's atmosphere may be taken as 7 x cm-2 s-'. Calculate the maximum dose equivalent in a semi-infinite tissue-equivalent slab from a year's exposure t o cosmic ray neutrons.

Calculation i n conventional units For the cosmic ray spectrum the fluence-to-dose equivalent conversion is taken as (Shaw et al. 1969): H/