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Symposium on Measure/Measurements for th QC100 .U57 N0.456, 1976 C.2 NBS-PUB-C 19

Q NBS SPECIAL PUBLICATION 456

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

Measurements for the

Safe Use of Radiation
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901. The Bureau's oveiall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for

Basic Standards, the Institute for Materials Research, the Institute for Applied Technology,

the Institute for Computer Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system

with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry,

and commerce. The Institute consists of the Office of Measurement Services, the Office of Radiation Measurement and the following Center and divisions:

Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Center

for Radiation Research: Nuclear Sciences; Applied Radiation — Laboratory Astrophysics" — Cryogenics" — Electromagnetics' — Time and Frequency".

THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized

materials needed by industry, commerce, educational institutions, and Government; provides

advisory and research services to other Government agencies; and develops, produces, and

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Reference Materials, the Office of Air and Water Measurement, and the following divisions:

Analytical Chemistry — Polymers — Metallurgy — Inorganic Materials — Reactor

Radiation — Physical Chemistry.

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the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations leading to the development of technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice and services to Government agencies upon request. The Insti- tute consists of the following divisions and Centers: Standards Application and Analysis — Electronic Technology — Center for Consumer

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THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

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- Located at Boulder. Colorado 80302.
1 J^i^easurements for the

Safe Use of Radiation

Proceedings of an , \BS 75th ANNIVERSARY Symposium

held at the National Bureau of Standards

Gaithersburg, Maryland, March 1 -4, 1976

Edited by

Sherman P. Fivozinsky

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

Sponsored by:

National Bureau of Standards

American Association of Physicists in Medicine

Conference of Radiation Control Program Directors

Health Physics Society

f U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secrefary

Edward O. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson, Assisiani Secretary for Science and Technology N ^ . NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acfing Direcfor

Issued November 1976
Library of Congress Catalog Card Number: 76-600060

National Bureau of Standards Special Publication 456

Nat. Bur. Stand. (U.S.), Spec Publ. 456, 437 pages (Nov. 1976) CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1976

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

(Order by SD Catalog No. C13. 10:456). Stock No. 003-003-01682-9 Price $5.15
FOREWORD

The explosive growth in the use of radiation in the major sciences

and technologies has created an increasing need for Measurements for the

Safe Use of Radiation. While medicine and industry place the greatest

demands upon measurements at the present time, it is essential to recognize that radiation in all of its forms, non-ionizing as well as ionizing, is impacting our daily lives and our environment in more and more ways everyday. Our concern for safety in the use of such radiation

is a matter of greatest importance.

It is most fortuitous that the Symposium on Measurements for the

Safe Use of Radiation could be identified with the commemoration of the

75th Anniversary of the National Bureau of Standards. Thereby it places

emphasis upon the new and urgent challenges faced by our institution in

the years ahead because of the ever evolving increases in the social uses

of advancing technology. NBS is gratified by the excellent results of

the Symposium and pleased to have shared the sponsorship with the

American Association of Physicists in Medicine, the Conference of Radiation

Control Program Directors, and the Health Physics Society.

Arthur 0. McCoubrey, Director

Institute for Basic Standards

National Bureau of Standards

i i i
PREFACE

These are the proceedings of the Symposium,

"Measurements for the Safe Use of Radiation"

held March 1-4, 1976, at the National Bureau

of Standards, Gai thersburg , Maryland. The

meeting was attended by about 250 scientists

and engineers representing a wide range of

private and government institutions with

interests in radiation measurement.

The purpose of the Symposium was to draw

attention to the rapidly increasing need

for high quality measurements of ionizing

and non-ionizing radiation, and to provide

a forum for discussion of methodology of

measurement, quality assurance, standards,

and measurement assurance activities.

The program consisted of four invited paper

sessions, three contributed paper sessions,

three tutorials, and a panel discussion.

The sessions divided the program into three

broad areas: (1) Standardization and

Measurement Assurance, (2) Therapeutic and

Diagnostic Radiation Measurements, and

(3) Environmental and Personnel Protection

Measurements. A total of 54 papers were

presented. The tutorials were offered by

NBS staff in parallel with the three

contributed paper sessions. They were:

1. Radioactivity Standards and Measurements

2. Dosimetry Standards and Calibrations

3. Laser Measurements

All three were very well received, and I wish

to thank Dr. W. B. Mann, Dr. R. Loevinger,

and Mr. A. Sanders for their cooperation in organizing and presenting, respectively, the

three tutorials. In addition, tours of NBS' Center for Radiation Research, and FDA's

Bureau of Radiological Health took place the

morning of March 3. I wish to thank

Dr. J. E. Leiss and the staff of CRR, and

Mr. R. W. Schneider and the staff of BRH,

for making available facilities and time for

these acti vities

.

The speech on U.S. Energy Policy delivered

by Congressman Mike McCormack of Washington

following the Symposium banquet was one of

the high points of the meeting. An excerpted version of his talk has been

published in the May 1976 issue of

"Dimensions," an NBS magazine.

The opening remarks, papers, and panel

discussion are printed in the order in which

they were presented. To speed publication

of these proceedings, all papers were submitted by the authors in camera ready form.

No attempt was made to record comments or questions following each oral presentation,

nor were the tutorials recorded.

When commercial equipment, instruments, and

materials are mentioned or identified in

these proceedings, it is intended only to adequately specify experimental procedures.

In no case does such identification imply

recommendation or endorsement by the

National Bureau of Standards, nor does it

imply that the material or equipment identified is necessarily the best available for

the purpose.

The editor gratefully acknowledges the

assistance of the National Bureau of

Standards, Office of Technical Publications,

and the extensive secretarial assistance of

Mrs. Lynette Helfstein in the publication of

these proceedings.

S. P. Fivozinsky

National Bureau of Standards

iv
ABSTRACT

These proceedings are the compilation of papers presented at the

Symposium, "Measurements for the Safe Use of Radiation," held in Gai thersburg , Maryland, March 1-4, 1976. The symposium addressed the

present status and future requirements for measurements of ionizing and

non-ionizing radiation. Emphasis was placed on the needs for standardization and measurement assurance activities in medical, environmental,

and personnel protection applications.

Keywords: calibrations, environment, measurements, medical, radiation,

standards.

TABLE OF CONTENTS

Foreword .

.- i i i

Preface i v

Abstract v

Table of Contents vii

Symposium Personnel xv

WELCOME

H.E. Sorrows 1

THE NATIONAL MEASUREMENT SYSTEM, THEN AND NOW

D. T. Goldman 3

INVITED SESSION I GENERAL

G. S. Parker, Chairman

ISl-1 Historical Development of the Need for

Radiation Protection and Measurements in

the United States

L. S. Taylor 5

ISl-2 The Standards Laboratory and the Radiation

Measurement System

R. S. Caswell 13

ISl-3 The Federal Regulatory Agency and the Radiation

Measurement System

R. H. Schneider 25

ISl-4 State Responsibilities and the Radiation

Measurement System

M. W. Parrott 31

CONTRIBUTED SESSION I MEASUREMENT SYSTEM

R. Loevinger, Chairman

CSl-1 Ionization Chamber for Absorbed-Dose Calibration

J. S. Pruitt and R. Loevinger 37

vi i
CSl-2 The Development and Application of Monoenergeti

X-Ray Sources

J. H. Sparrow and C. E. Dick 41

CSl-3 The Design of a Mobile Radiation Calibration

Laboratory

R. C. Placious and J. C. Humphreys 47

CSl-4 Automated Calibration and Recordkeeping System

for X-Ray Survey Instruments

T. R. Ohlhaber, F. J. Prevo and

T. L. Miller 53

CSl-5 Standardization of a Nationwide Gamma Radiation

Calibration System

E. F. Williams, Jr., P. V. Barrans and

G. H. Brodie 59

CSl-6 A Reliable Calibration Program for Ionizing

Radiation Measurement

A. Jacobson 65

CSl-7 AIF-NBS Radioactivity Measurements Assurance

Program for the Radiopharmaceutical Industry

R. Coll^ 71

CSl-8 ANSI-Quality Assurance Program in Radioactivity

Measurements

C. W. Seidel and J. M. R. Hutchinson 77

CSl-9 Filtered Beams at the NBS Reactor

R. B. Schwartz, I. G. Schroder and

E. D. McGarry 83

^' CSl-10 Neutron Source Calibrations at NBS for Calibration

^/^" Checks of Neutron Radiation Instruments

V. Spiegel .........87

viii
CSl-11 Ultrasonic Measurements and Calibration

H. F. Stewart, M. E. Haran and

B. A. Herman 91

CSl-12 Methods for the Evaluation and Calibration

of Microwave Survey Instruments

M. L. Swicord, H. I. Bassen

and W. A. Herman 99

CSl-13 NBS Ultraviolet Radiometric Standards

W. R. Ott 107

INVITED SESSION II THERAPEUTIC AND DIAGNOSTIC

RADIATION MEASUREMENT

J. R. Cameron, Chairman

IS2-1 Quality Assurance for Measurements in

Therapy

R. J. Shalek, P. Kennedy, M. Stovall,

J. H. Cundiff, W. F. Gagnon,

W. Grant, III and W. F. Hanson Ill

IS2-2 Quality Assurance Measurements in

Diagnostic Radiology

M. P. Siedband 119

IS2-3 Quality Assurance for Measurements in

Nuclear Medicine

B. A. Rhodes, W. B. Hladik

and J. H. Gallagher 131

IS2-4 State and Federal Regulatory Measurement

Responsibilities Around Medical Facilities

L. H. Lanzl 139

IS2-5 N.E.X.T. Program--and Radiation Measurements

in the State

J. W. Shaver 147

ix
CONTRIBUTED SESSION II ENVIRONMENT AND PERSONNEL

PROTECTION

J. M. Matuszek, Chairman

CS2-1 Low Level Environmental Radiation Measurements

with a Plastic Scintillation Detector System

A. P. Hull and G. S. Levine 153

CS2-2 Precise Environmental Gamma-Ray Measurements

Utilizing Standard Ge(Li) Detectors

R. M. Keyser, G. V. Walford

and J. A. Cooper 159

CS2-3 The Minimum-Detectable-Activity Concept

J. C. Lochamy 169

CS2-4 use of NBS Mixed-Radionucl ide Gamma-Ray

Standards for Calibration of Ge(Li) Detectors

Used in the Assay of Environmental

Radioactivity

B. M. Coursey 173

CS2-5 The Safety and Reliability of Decentralized

TLD

Carl-0 Widell 181

CS2-6 Error Analysis of Environmental Radiation

Measurements Made with Integrating Detectors

G. De Planque Burke and T. F. Gesell 187

CS2-7 Radio-Frequency Electromagnetic Radiation From Portable and Mobile Telecommunication

Transmitters

J. F. Thiel 199

CS2-8 An Automated Measurement System for Determining

Environmental Radiofrequency Field Intensities II

R. A. Tell, N. N. Hankin, J. C. Nelson,

T. W. Athey and D. E. Janes, Jr 203

X
CS2-9 The Measurement of Optical Radiations:

Selected Practical Considerations

R. W. Peterson, J.. M. Coakley, K. Mohan

and R. James 215

CS2-10 X-Ray Leakage from Conventional and High

Voltage Electron Microscopes

D. F. Parsons, V. A. Phillips and

J. S. Lally 223

CS2-11 Intercompari son of Personnel Dosimeters

H. W. Dickson, W. F. Fox and

F. F. Haywood 225

INVITED SESSION III ENVIRONMENTAL AND PERSONNEL

PROTECTION MEASUREMENTS

W. A. Mills, Chairman

IS3-1 Environmental Radiation Measurements

J. E. McLaughlin 233

IS3-2 Environmental Measurements and Regulatory

Responsibilities

J. M. Matuszek 249

IS3-3 Environmental Radioactivity Standards

A. N. Jarvis and D. G. Easterly 263

IS3-4 Personnel Monitoring Measurements

R. V. Griffith 267

IS3-5 Personnel Monitoring Performance Standards

B. H. Weiss 277

xi
CONTRIBUTED SESSION III MEDICAL

N. Suntharal ingam, Chairman

CS3-1 Optical Interferometric Measurements of

Ultrasonic Radiation and Its Applications to Medicine

M. E. Haran and H. F. Stewart 285

CS3-2 Photon Mass Energy-Absorption Coefficient

Air/Medium Ratios for lonometric Dosimetry

0.1 keV to 20 MeV

J. H. Hubbell 299

CS3-3 Electron Dosimetry with Ionization Chambers

M. J. Berger, S. M. Seltzer, S. R. Domen

and P. J . Lamperti 307

CS3-4 Dose Profiles in Tissue and in Air for

Therapeutic Electron Beams

G. Osman 313

CS3-5 Calorimetric Dose Measurements in Fast Neutron

and Cobalt-60 Gamma-Ray Fields

J. C. McDonald, J. S. Laughlin and

L. J. Goodman 327

CS3-6 Scattered Radiation From a Neutron Collimator

F. H. Attix, L. S. August and

P. Shapiro 335

CS3-7 Dose Control in Radiation Therapy

K. R. Kase, B. E. Bjarngard, B. J. Maddox

and G. K. Svensson 343

CS3-8 Dosimetry Procedures at the Northwest Medical

Physics Center

D. Jones, D. Schumacher and

J. Washington 351

xii
CS3-9 Intercompari son of Prescription Dose Delivery

in the New England Region

J. S. Blinick, J. Spira, D. Cail and

P. Plazeski 359

CS3-10 Electron-Therapy Dosimetry

M. Ehrlich and P. J. Lamperti 365

CS3-11 Thyroid Monitoring and Minimizing 1-125 Uptake

M. M. Gabel , K. W. Price and

G. R. Holeman 371

CS3-12 Attenuation Phantoms for Patient Exposure

Measurement During Radiographic Examinations

Utilizing Phototiming Techniques

Pei-Jan Paul Lin 381

CS3-13 Effects of Measuring Apparatus on X-Ray

Attenuation Measurements

T. J. Morgan, L. B. Smith and

Z. R. Hashmi 389

CS3-14 kVp Measurement for Dental X-Ray Units

M. S. Moroz and J. A. Vukan 395

INVITED SESSION IV STANDARDIZATION AND MEASUREMENT

ASSURANCE

R. S. Caswell, Chairman

IS4-1 National Ionizing Radiation Standards

J. E. Leiss 403

IS4-2 Non-Ionizing Electromagnetic Radiation

Standardi zation

R. C. Baird and A. A. Sanders 417

xiii
PANEL DISCUSSION

E. D. Trout, Moderator

G. S. Parker

J. R. Cameron

W. A. Mills ' . R. S. Caswell 427

List of Registrants 435

Author Index 449
SYMPOSIUM PERSONNEL

SYMPOSIUM CHAIRMAN

James M. Wyckoff

National Bureau of Standards

PROGRAM COMMITTEE

Sherman P. Fivozinsky, Chairman

National Bureau of Standards

Richard Bluemle

South Carolina Department of Health

Paul Carson

University of Colorado Medical Center

Paul Eastvold

Illinois Department of Health

J.M. Robin Hutchinson

National Bureau of Standards

Thomas L. Johnson

Naval Research Laboratory

Robert Loevinger

National Bureau of Standards

Aaron Sanders

National Bureau of Standards

N. Suntharal i ngam

Thomas Jefferson University Hospital

Edward J. Vallario

U.S. Energy Research and Development Administration

XV

WELCOME

H. E. Sorrows

Associate Director for Programs

National Bureau of Standards

Ernie Ambler, the Director of the Bureau of Standards, sends

his regards and regrets that he can't be with you. His response

to the wishes of higher authority is my good fortune for it is certainly a privilege and an honor for me to welcome you.

This conference kicks off a series of special events designed

to commemorate and to celebrate the 75th anniversary of the Bureau. Conferences like this one keep the Bureau as youthful

and timely as ever. During the remainder of 1976 we are holding

a number of symposia, lectures, and events which are appropriate

for our birthday celebration and are designed to be beneficial

to society in general and to the science and technology

communities in particular.

In addition to being the keynote conference for our birthday

celebration, this meeting presents an opportunity to provide

a forum for the transfer of science and technology, one of

the most important services of the Bureau of Standards. We are particularly pleased to join with the American Association

of Physicists in Medicine, the Conference of Radiation Control

Program Directors, and the Health Physics Society in sponsoring

this meeting.

There is a third and even more self serving reason for our

interest in this conference and that is our desire to seek

better ways to provide our services to the nation. I only

need observe that the theme of this conference "Measurements

for the Safe Use of Radiation" is the objective of our radiation

research and to remind you that the National Bureau of Standards

would like to work with you in providing a system of measurement

to the accuracy required, including the means for measurements

to be made correctly and to be traceable to national standards,

if necessary.

You have a tremendous responsibility and opportunity because

the government and the informed public are looking to nuclear

physics and to radiation technology for solutions to many of

the nation's most pressing problems. Energy and health care

are center stage; however, there are many other major problems

and in all cases the solutions must be both safe and economic. A practical system of measurements will go a long way toward

achieving optimum solutions.

I wish you a successful Symposium. You have all the ingredients.

You have a timely topic, an excellent agenda, an inspiring banquet

speaker and you even have beautiful weather. It's a pleasure to have you here.

THE NATIONAL MEASUREMENT SYSTEM, THEN AND NOW

D. T. Goldman

Deputy Director, IBS National Bureau of Standards

As an introductory talk to this conference, I would like to take

the opportunity to introduce the concept of the National Measurement

System which had been introduced by Dr. Robert Huntoon about ten years ago. This is a concept by which we attempt to analyze the

relationship of our activities at the National Bureau of Standards

with needs of our user community on the outside. A reasonable

definition of the National Measurement System is the network of

measurement standards, techniques, procedures and standardizing

laboratories in all of the measurement communities which generate

the base on which day to day measurement operations are conducted

at all levels of accuracy in the United States. This includes

the international agreement on the basic SI units, maintained at the national standards laboratory, the National Bureau of Standards,

and the various mechanisms for transferring the measurement

capabilities maintained at the Bureau of Standards to the users who constitute the major portion of the measurement system.

Since this is the initial conference of the Bicentennial series

at the National Bureau of Standards and is also the first of

the symposia associated with the NBS 75th anniversary, I thought

that perhaps a little historical review of standards might be

in order. At the building of the pyramids the cubit, the biblical

standard of length, was defined as the length between the tip of

the middle finger and the elbow. Skipping over several millennia

we find another lawful definition of unit of length, the rod,

which was defined as the total length of the left feet of 16 men

lined up as they left church on Sunday morning. The rationalists

of the 18th century chose to define the meter as a new unit based

on the decimal system. In fact, Jefferson tried to introduce

this concept into the United States 200 years ago. However, as late as 1830, the incipient national measurement system of the

United States was somewhat limited in its extent and limited

to weights and measures for commerce and taxation, especially

for customs. The concept of utilizing the metric system of

units was examined several times between the founding of the

country and about 1830, and in each case for a variety of

different reasons it was found not acceptable to the body

politic at the time. Not only was the metric system not accepted but even the idea of compatible measurements although

mentioned and featured in the Constitution appears not to have

been used in practice. In fact, it's reported that in the city

of Brooklyn during the 19th century there were several different

standard feet and when you measured a lot you chose the unit of

length that was most convenient for you.

In 1830, there was no international agreement as yet, a national

standards organization did not exist, and national standards

didn't exist. However, to some extent there was a national

agreement in terms of use of different instruments, especially

weighing and length measuring instruments. Seventy years later

as a result of the success demonstrated by our sister organization,

the Physikalisch Technische Reichsanstalt, the German national

standards laboratory which had been founded about 25 years earlier,

3
a movement to establish the Bureau of Standards was undertaken.

The Office of Weights and Measures was the building block upon

which the National Bureau of Standards was established. Twentyfive years earlier the International Bureau of Weights and

Measures had been established, and the idea of the metric system

had been agreed upon formally by the United States (1875), so at least we were on our way to defining a national measurement system

which could be used in practice. Need for it could be demonstrated

as the electric industry grew up. It really made a difference if

different voltages were measured by different things. No doubt,

potential safety hazards in the uncertainty in the measurement of

voltage was a factor, but commerce in the supply of electricity

and electrical equipment was also important. However, there still was no accepted measurement system concept for many different units.

Randy Caswell will be talking later this morning on that specific

portion of the system which we would call the radiation measurement

system. In 1901 radiation had just been discovered, but after the

founding of the National Bureau of Standards, a significant fraction

of the initial publications of the Bureau of Standards which appeared

in the form of handbooks was indeed devoted to the measurement of

radiation quantities.

The National Bureau of Standards functions as the primary mechanism

for linking the international standards generally as maintained

at the International Bureau of Weights and Measures in Paris to the users in our country. We transfer that measurement capability

by a variety of different methods: instruments, calibration

services, measurement techniques or, in some cases, providing

the numerical information which is needed for the transfer. The

measurement system is composed of a variety of different users, including consumers, industrial organizations and the government.

It is our responsibility to meet the needs of the system now and

in the future.

To summarize, our job is to make sure that those who need measurement services, those who perform measurements, can do so with

accuracy and known precision, reliably and compatibly and at reasonable cost. Measurements in one part of the country can

be transferred to another part with assurance as to what the

uncertainties are. In other words, our job at the Bureau of

Standards is to service the National Measurements System and

to make use of this expertise in the solution of specific

national problems. With this introduction I am looking forward

to hearing the discussion of one particular portion of the

measurement system which will be the major topic of the

remainder of the conference. Thank you.

4
NBS SP456 (1976)

HISTORICAL DEVELOPMENT OF THE NEED FOR RADIATION

PROTECTION AND MEASUREMENTS IN THE UNITED STATES

Lauriston S. Taylor

National Council on Radiation Protection and Measurements

Washington, D. C. 20014

Early means of radiation measurement included photographic film,

chemical compounds, selenium and ionization of air, and all were uncertain and inaccurate until about 1920. By 1930, ionization

measurements between different laboratories and countries agreed

within 2 or 3%. Also, quantitative limits for the exposure of

radiation workers had been proposed and based on air ionization

measurements - the roentgen. Accuracy requirements are + 5% for

radiation therapy but to maximize the benefits to patients greater

attention must be given to measurement assurance or traceability

.

Accuracy for protection purposes will vary with the dose levels

involved, for example, + 10% at maximum permissible dose levels for

radiation workers to perhaps + 200% at the level of 10 mrems in a year for the population.

Since the purposes of this Conference

center around problems of the measurement of

ionizing radiation, I shall deal rather

briefly with some of the areas of radiation

measurement and the conditions which lead to their need. In this connection, one cannot

avoid some speculation or even argument

about the required accuracy for particular

applications, but to put our present-day

problems and our present-day thinking into

some kind of mutual perspective, I believe

it will be desirable to go back to the

earliest steps taken in the radiation

measurement field. (Incidentally, when I use the word radiation by itself, I refer to ionizing radiation.)

I . The Purpose of Measurement

Any kind of measurement, whether it be

physical or biological, must have a purpose,

and this may be very specific or fairly

abstract but one just shouldn't run around

measuring things for the sheer joy of it.

Any measurement implies the definition of

appropriate quantities which are to be

measured and the appropriate units in which

to express those measurements. Again if

there were no specific purpose in a measurement, you would not need to worry about

proper quantities and units. In research, in

the clinical laboratory or in medicine, there

may be any or all of a number of purposes in

performing measurements. First, one has to be able to describe his own results so that

others can determine what had been done, and

how they could use it. Secondly, would be

the opposite, where someone else had done

some work and you wish to compare yours with

it. Thirdly, would be the use of measurements to tie experiment to theory or the

other way around. Physical research is at

its best when an experiment suggests a theory that in turn suggests more experiments that test and improve the

theory, labile at times, some very crude

experiments can be importantly indicative,

the new offshoots of some of our theoretical

developments require an accuracy that taxes our measurement capabilities to the extremes. In many instances, it is the

ability to "measure the next decimal place"

that distinguishes between two theoretical

positions

.

Measurements of any kind must preferably be made within some unified

system of quantities and units. We have in

the past used multiple systems and up to a point this was satisfactory since their

interrelationships were known precisely.

The main problem with different systems is

likely to be one of convenience rather than

fundamental significance. In the early

history of radiation measurement, there were

systems of measurement which defied the

establishment of clear interrelationships,

but these were supplanted by better systems

as our scientific understanding of the

problems developed along with the associated

technology

.

In the medical field, we have the

current situation where quantities and units

have been developed over the years and for

the most part are technically sound but do not fall within the SI system of quantities

and units. Therefore, steps are being taken

to eventually phase them out. However, this

is a matter centering mainly around

uniformity of the system and certainly not

the desire or convenience of the medical

profession. There is an established

relationship between the units used in
radiology and those in the SI system and I suspect that nobody outside of the biomedical field would suffer anything but some small inconvenience in allowing continuance

of the old system. But we must not disturb

the purists.

The required accuracy of measurement of

radiation depends to a considerable extent

upon the purpose to which the measurements

will be applied. Our technology places

limitations upon accuracy, but it is usually

found that by the time greater accuracy is

required, our technology is on hand to accomplish it. It is usually the

improvements in technology that create the new needs and so in most instances they go hand in hand

.

It is all very well to be able to measure a quantity with high accuracy in,

let us say , a central standards laboratory

such as the NBS. However, it would serve no purpose if the NBS standards could not be

intercompared with the standards in other

laboratories and could not be applied

through secondary instruments to applications in the field. Since there may

be a number of steps between the measurement

in a standards laboratory and a measurement

in, let us say, a hospital, it is important

that there be a high level of assurance that

the end measurements carry the required

accuracy. We thus have a direct necessity

for what would be called "traceability" or "measurement assurance". Measurement

assurance can be critical for a variety of reasons including commerce and manufacturing,

safety or protection of people, legal

compliance with regulations , medical

applications, research - just to name a few.

The purposes of this conference cover

all of these points and probably more, but to see where we are, it may be helpful to review some of the early background to see our progress: where we have been, where we

are now, and what may be some of the

problems that lie ahead.

This exercise will certainly help us preserve our sense of humor about the

measurement problem and give us some feel

for some of the things that are unrealistic. The program of this conference

should constantly keep in mind what the real

problems are and what are the problems that are of little or no significance and hence

may be weeded out.

II . Early Starts in the Measurement

of Ionizing Radiation

On the question of early radiation

measurements, I shall deal primarily with

problems in this country and more especially

with those centering around the work of the

National Bureau of Standards. That seems

not inappropriate in view of the nature of

this conference. Moreover, it might be

pointed out that the Bureau of Standards has

had direct involvement to one degree or another for over 60 years and, beginning

nearly 50 years ago, has played a dominant

role in the field of ionizing radiation

protection and radiation measurement.

Measurement programs, relating

primarily to the evaluation of sealed

sources of radium, began essentially

simultaneously in the period of 1912 to 1913

at the world's three major National Standards

Laboratories - the National Physical

Laboratory in England, the Physikalischtechnische Reichsanstalt in Germany, and the

National Bureau of Standards in the USA. The trigger for these starts was probably

the Brussels Conference on Radiation

Research held in 1910. As a result of this, an international standard of radium was deposited at the Bureau International des

Poids et Mesures. Prior to this, "accurate"

measurements of radium had been made in

Vienna and comparisons between there and

the BIPM provided the first traceable tie

between the two.

In all of the national laboratories,

the initial and primary purposes of the

radiation measurements had been to evaluate

the amounts of radioactivity in sealed

radium preparations either in terms of

milligrams of radium element or millicuries

of activity. This was primarily a service

to commerce, since the radium was then so expensive and so easily susceptible to deceptive practices. Of secondary interest,

was medical dosimetry which for many years,

beginning around 1915, was described in terms of milligrams or millicuries of

radioactivity with a given filtration, a given distance, and a given time.

The first radium measurements at the

National Bureau of Standards were carried

out by N. E. Dorsey who continued with the

work until the early 1920 's, by which time

he was suffering noticeable injury to his

hands from handling the preparations. About

1925, a more extensive radioactivity program

was started under Dr. L. F. Curtiss. This

6
also consisted primarily in the calibration

of sealed radium sources for commerce and

medicine, but the work was on a very much

stepped up scale because of the increased

availability of radium. At that time, the

radium program was a part of the Atomic

Physics Section which in turn was in the

Optics Division (after all, it was all

radiation). Various researches with radioactivity were carried out at the same time.

X-Ray programs started at all three of

the National Laboratories during the war

years, but these were limited mostly to the use of X rays for inspecting objects and

materials, rather than studies of the x rays

themselves. Probably the first program of

x-ray dosimetry was undertaken at the PTR in

the early 1920 's. The NPL and the NBS began

their programs in 1927. All three of these

programs were directed toward development of

standards and procedures for measuring x- rays in connection with their therapeutic

applications. Preceding this there had been

a considerable amount of research on the

measurement of x rays carried out for the most part in universities.

The first proposal for a physical unit

of measurement of x rays was made by Villard

in 1908, and his proposal was essentially

the same as the definition of the roentgen

adopted twenty years later. Friedrich and

Kroenig proposed a physical unit of

measurement in 1918, which was also

essentially the same as Villard 's proposal.

In 1914 and 1922, Duane, in this country,

using the same description for a unit, built

a parallel plate ionization chamber which

was proposed as a standard for x rays. In

1921, Solomon in France described a unit

known as the "Solomon R" which was

essentially a description of an ionization

chamber system calibrated with a given

radium preparation.

Behnken, in 1924, at the PTR, built a cylindrical pressure ionization chamber

designed to measure in terms of the unit

proposed by Villard and later by Friedrich.

This chamber was abandoned because of

measurement difficulties and replaced by a much larger cylindrical ionization chamber

operating at atmospheric pressure which

continued indefinitely in use. Glasser, in

this country, in 1925, built a simple

parallel plate ionization chamber and also

developed a thimble chamber system, the

outgrowth of which is still the primary

medical x-ray measuring device in use in

the United States. That was the general

state of affairs preceding the x-ray

measurement program started at the NBS in

late 1927.

III. Measurements for X-Ray

Standards of Dose

It has already been noted that by about

1925 there were at least three systems of

x-ray standards which were beginning to become recognized. There was the Friedrich

Standard, a free air ionization chamber of

cylindrical design; the Duane Standard, a free air chamber of parallel plate design;

and the PTR chamber which operated under

considerable air pressure and was also

cylindrical in design. The first attempt to inter-compare standards was made by Behnken

in 1927, at which time he brought to this

country a thimble chamber and an electroscope which had been calibrated in Berlin

against his pressure chamber. As a result

of his measurements an agreement of 0.7

percent was given for the comparisons with

Duane. A "large" discrepancy apparently

existed between Behnken and Glasser. The

NBS Standard was just being constructed at the time, and no comparisons were made.

By 1928, the NBS had completed its

first standard free air ionization chamber

of a parallel plate type. This was substantially different from the one proposed earlier by Duane, from which, on

its face, one would expect substantial

errors. Comparisons made between the NBS

and Duane chambers in Boston in 1928 showed

that indeed there were large differences - of the order of 25 percent, but the comparison

system was so indefinite that attempts were

not made to track down the sources of

difficulty. During the same year comparisons

between the NBS and the Glasser standard

showed close agreement. Both of these

comparisons were just part of the day's work

and have never been published.

In 1931, a more sophisticated comparison

of standards was made by Taylor between the

NBS, NPL, PTR, and Solomon. These employed

for the first time the very much smaller

parallel plate free air ionization chamber

using a guarded electric field so as to reduce its size and weight. The initial

difference between the NBS and NPL standards

was some six percent. Most of the

discrepancy was introduced because of an error in the NPL diaphragm. After correcting

for various established differences, the

agreement was of the order of 0.4 percent.

7
At the Physikalisch-technlsche

Reichanstalt , Behnken had his new

cylindrical open air ionization chamber and

agreement between that and the NBS chamber

was .35 percent. These agreements look

deceptively good, because such accuracy at that time was only barely within the limits

of our technological competence.

It is interesting to note here that in

about 1927 or '28, Behnken had discovered

that their pressure ionization chamber was

in substantial error because of ion

recombination at high pressures. They

quietly changed from the pressure chamber

to a free air chamber without any official

announcement. This probably accounted for

the original large difference between the

PTR and the Glasser Standards, and the

agreement with the Duane Standard.

Comparisons with Solomon could not establish levels of agreement but were in essence comparing the magnitude of his

measuring unit with that of the other

laboratories. It turned out that the ratio

varied from 2.1 to 2.3 over the energy range

studied. Following these comparisons,

Solomon adopted the same chamber that was

used by the NBS.

The next major intercomparisons were carried out by H. 0. Wyckoff in 1953 between

the NBS and NPL. Considerable sophistication of the instrumentation and overall

technology had taken place during the

intervening years, but basically it was

still a comparison between free air

ionization chambers. Whereas in the earlier

comparisons and measurements there had been

talk about agreements within fractions of a percent, no one really believed that the

measurements were that reliable. However,

by the 1950' s, the situation was different

and when agreements were established, there was much higher confidence in their

significance. Also, it might be pointed out that between 1931 and 1953 it had been found

that the energy range of the NBS chamber at the earlier time was not sufficiently

extensive and later measurements showed

degrees of energy dependence which could not have been predicted in 1931 but which were clearly established by 1953.

IV. Measurements for Radiation Therapy

The main drive for the development of

radiation measurement standards came about

through the recognized need for accurate

dosimetry in the applications of radiation

to patients. The problem became more acute

with the advent of the Coolidge hot-cathode

x-ray tube, which by the early 1920 's promised to permit therapy up to 200 kV.

Therapy at lower energies had been practiced

for some time and the clinical attempts at dose control involved a variety of

measurement means. For example, bariumplatino-cyanide pastiles were used for many

years; these change color in accordance with

the amount of radiation received. Another

technique involved the use of photosensitive

film or paper. Both of these techniques

involved matching of color or darkening.

Another system employed selenium cells which

were photosensitive but had peculiar

operating characteristics, at best.

Along with these older methods was a wide variety of ionization chambers which,

while suitable for control purposes within a given clinic, defied any kind of real

comparison because of the lack of a central

comparison point, let alone a recognized

standard. It is surprising, in looking back

over this period extending up until about

the middle of the 1920's, to see how

relatively closely all of these various

measurement systems were to each other in

spite of the wide variations that existed

relative to their fundamental operation.

Attention was directed to all of these

problems at the First International Congress

of Radiology held in London in 1925, and it is largely for that reason that activity in

the area of radiation measurement sprang up quickly all over the world during the

following period.

The big push in this country started in

1925 through action of the Radiological

Society of North America that organized an x-ray standardization committee. After

reviewing the current situation, one of its

first recommendations was to urge the start

of a major program in the field of x-ray

standards and measurements at the National

Bureau of Standards. This was promoted

through congressional pressures and by

considerable correspondence with Mr. Hoover

who was then Secretary of Commerce. The

fruits of their efforts were shown by a line- item appropriation in the budget for the

National Bureau of Standards which was to become effective in July 1927.

The next major step took place at the

Second International Congress of Radiology

in 1928 at which time a definition of the

roentgen was adopted. In the meantime,

standards programs were well underway in the

three major National Standards Laboratories

8
as well as at numerous other points.

Although several minor modifications in

the wording were made later, particularly

to insure its adoption to higher energy

radiation, the definition of the roentgen

remained essentially the same for the next

30 years.

With these steps having been taken,

and accompanied by the development of

reliable portable instruments, mostly using

thimble chambers, it was then possible to rely on a central standard system for the

uniformity of procedures in radiation

therapy. The practical accomplishment of

this has depended to a major degree upon

the traceability of the measurements from

the standards laboratory to the individual

clinic, back to the standards laboratory,

and then out to other clinics.

As far as clinical dosimetry is

concerned, the overall system has been

fairly good for many years - better in some areas than others. However, the system has

not been good enough because in therapy, more than all other applications, high

accuracy and traceability is essential to the best welfare of the patients involved.

As of today, while the problems remain

somewhat the same in principle, they are

often technically more difficult and

complicated. To begin with we are dealing

with higher energy photon as well as particle radiation. To be of ultimate

clinical value there has to be a capability

of comparability between the clinics and

between the measurements with various other

kinds of radiation.

In addition to the different radiations

themselves there are now many different

measurement techniques that may be used,

such as those involving photoluminescence

and thermoluminescence in addition to ionization chambers having special energy

responses deliberately built into them.

To be used most effectively in the

clinic, all of the radiations and all of

the measurement systems will have the same accuracy requirements as the best we have

achieved under the simpler systems in the

past. This means that their errors must be

no greater than five percent at the surface

of the patient and probably should be better

than that. The achievement of this level

of accuracy requires a high degree of

measurement assurance or traceability.

V . Measurements for Radiation Protection

The first tolerance dose for x-ray

workers - a dose then considered to be

without harm - was proposed by Mutscheller

in 1925. It was described as one tenth of

a skin erythema dose to a worker in a year.

In turn, the erythema dose was described as the amount of radiation required to produce

a defined reddening of the skin when

delivered in one treatment. The treatment

was described in terms of tube current and

voltage, distance, time, and size of

irradiated field. This was all based on Mutscheller ' s observation that a few x-ray

workers who had been exposed for several

years to radiation calculated to be about

1/100 of an erythema dose over a one month

period, showed no effects. He then

estimated that since it would take some 8 years to accumulate an erythema dose, a tenth of an erythema dose in a year could be

tolerated with a comfortable margin of

safety

.

In 1927 a survey established that a skin erythema dose was equivalent to 550 + 150 roentgens (1927 PTR roentgens);

thus 1/10 SED/yr amounted to 55 r in a year.

In 1934 the NCRP rounded this figure down to 0.1 r/day or about 25 r in a year. Later,

using essentially the same data, the ICRP

rounded the value down to 0.2 r/day. No deleterious effect has ever been found for

any such exposures. In 1949 the NCRP again

lowered the maximum permissible dose for

radiation workers (formerly tolerance dose)

to 0.3 r in a week (15 r in a year). At the same time it recommended a level of 1/10 of

that for the general population. In 1956

the ICRP and NCRP further lowered the

maximum permissible dose for radiation

workers to 5.0 rem in a year and 1/10 of

that or 500 millirems for the general

population

.

Note two things: (1) no effect on man

has ever been observed for exposures at any

of the permissible levels used since 1925,

and (2) there can be no ascribable accuracy

to a permissible dose level.

Today there are discussions leading to still further reductions of the dose limits

for the population; dose limits in the range

of 5 - 25 millirems in a year are being

considered. The question: what would be

reasonable requirements for the accuracy of

measurements made at these very low doses?

Another question: to have some meaning,

would not everyone need to be monitored

continuously?

9
Bear in mind that people who live in

Bethesda - altitude 400 feet - receive at least 5 mrem a year more than do people

living in downtown Washington. With such

large normal variations in environmental

radiation, is it necessary that measurements

of the dose to the population be measured

with an accuracy of say + 10%? Or perhaps + 200%? Let us not completely lose our sense of humor.

A different set of accuracy criteria

may well apply in a civil defense situation.

Action decisions will almost certainly

depend to a major extent upon radiation

levels where people are and where they

might move to. Yet the biomedical

consequences of any of the exposures are not

known and for all practical purposes must be

assumed. Once assumed, they must be treated

as real and measurement accuracy assigned

accordingly; when the expected doses are relatively small - say a few r in a day - some laxity in accuracy may be tolerated if

necessary. But at high doses where triage

must be practiced to save the maximum

number of people within the resources

available, maximum accuracy is called for.

Yes, even though you do not know the lethal

dose limits.

VI . Measurements for Regulatory Purposes

Measurements and measurement accuracy

for regulatory purposes seem to fall outside

of the normal scheme of laboratory

practices. Actually, it would seem that

regulatory practices should be for one or both of two situations: (1) a strict

control over the design intentions for a facility that may have the potential for

causing injury to persons, and (2) a control

program to assure that injuries are not being caused to persons.

The first is a rational approach to the

problem provided it is carried out reasonably. Unfortunately (1) and (2) are rarely separated in spite of the fact that

in the field of ionizing radiation the

control levels that have been used for the

past 40 years have been so low that no deleterious effects have ever been found for

persons exposed within those limits.

I sometimes think that the real

regulatory purposes may, consciously or unconsciously, be to assure compliance for

compliance sake; surely not just to protect

people.

The traceability and accuracy requirements for regulatory compliance programs

must engender the strictest of all measurement programs. Regulations carry the force

of law and associated penalties. But the

law has no sense of humor; a radiation

worker exposed to 4.995 rems in a year is

deemed to be "safe" whereas another exposed

to 5.005 rems in a year is presumed to have

been injured. Now this can, of course, be

regulated. All that is needed is instrumentation that can measure the workers' dose

with an accuracy of + 0.1%. The situation

becomes a bit more sticky when compliance

necessitates a similar accuracy for

measurement of, say, 5 or even 50 millirems

in a year.

I have obviously exaggerated the

situation to make a point. On the other

hand, I have indeed encountered "inspectors"

in the past who would see no difficulty

with the problem I have posed. If they had

a meter that they thought they could read

to 5.005 rem they would accept the reading

without question. "That's the Law" and I repeat, the law rarely, if ever, has a sense of humor.

Let us consider what may be reasonable

and rational requirements for accuracy and

traceability. The following may be suitable

for discussion purposes but I do not pretend

to necessarily recommend them; they are based mainly on the significance of the

exposures involved:

(1) For occupational exposures near

the upper limit of 5 rem in a year + 10 to 20%.

(2) For occupational exposures of less

than 1/2 the upper limit +

25 to 50%.

(3) For non-occupational exposures of

less than 25 mrem in a year

factor of 10.

(4) For non-occupational exposures of

the order of 100 mrem in a year factor of 3.

(5) For non-occupational exposures of

the order of 500 mrem in a year + 50%.

10
My message:

1) Assure the existence of basic

standards of the necessary

accuracy

.

2) Assure the existence of transfer

standards of the necessary

accuracy

.

3) Develop adequate and reliable

field instruments.

4) Develop adequate means for

calibrating field instruments.

' 5) Assure that at all times any

instrument reading anywhere can be traced to its basic calibration

source

.

6) In measurements anywhere and at any time, don't lose your sense

of humor.

11

NBS SP456 (1976)

THE STANDARDS LABORATORY AND THE RADIATION MEASUREMENT SYSTEM

R. S. Caswell

National Bureau of Standards

Washington, D.C. 20234

In connection with a broad NBS study of the National

Measurement System, the measurement system for ionizing

radiation has been investigated. Eight classes of

radiation users were considered: medical, nuclear

power, industrial radiation processing, defense,

environmental, science, chemical analysis, and

miscellaneous radiation applications. System structure

including regulators, standards laboratories, manufacturers,

as well as final users of radiation was studied.

Several common threads were observed in these studies

for the various classes of users: (1) While there is often a need for new NBS standards where no standards

exist, where NBS standards do exist their accuracy is generally sufficient for present needs. (2) There is

a great need for measurement assurance, especially where

regulatory requirements are involved. (3) A need exists

to help users with training, convenient laboratory

standards, and handbooks for guidance. In other cases,

new or augmented institutional structures within the

system (e.g. secondary standards laboratories) are

needed.

(Ionizing radiation; measurements; standards, uses of radiation)

This paper will be primarily concerned with

the measurement system for ionizing radiations.

In the paper by Dr. Leiss, "National

Measurement Standards for Ionizing Radiation",

the questions of standards laboratory

standards, dissemination of standards, and

measurement assurance will be considered at greater length. The present contribution is based on a study carried out as one of a number of studies of various aspects of the

measurement system by groups in many parts

of the National Bureau of Standards. In this study of the measurement system for

ionizing radiations, I should like to acknowledge the great help given by many

members of the staff of the Center for

Radiation Research, to whom I am much

indebted.

Purpose of the Study

The objectives of the study were: (1) to understand how the radiation measurement

system works in the U.S.; (2) to perform a kind of market research, contacting the customers of NBS services, learning their

needs, and evaluating how well NBS or other

organizations are filling those needs;

(3) to identify the needs of the measurers

of radiation, be they calibration services.

training, measurement assurance round

robins--whatever ; and (4) to develop

National Bureau of Standards program

responses to those needs.

Ionizing radiation is a convenient (but not precisely accurate) term used to describe a class of energetic radiations

such as x and y i^ays , electrons, radioactivity, neutrons, and numerous other

nucleons, nuclei, and mesons. In the

measurement of ionizing radiation we are concerned with a rather large number of

radiation quantities, some of which are listed in Table 1 with the traditional

unit and the customary SI unit. However,

the study was done, not on the basis of

radiation quantities and units, but

rather on the basis of radiation user communities, some of which are listed

in Table 2. In each one of these main

categories of radiation users, different

groups of concerned organizations are grouped into the activity classes shown

for the purposes of the study. The

largest user categories are the medical

community and the nuclear power community.

Using the medical community as an example,

we can list four direct users (or uses)

of radiation: (1) radiation therapy.

13
Table 1 RADIATIONS, QUANTITIES, AND UNITS

Radiation

X and gamma rays

Quantities

Exposure

Absorbed dose

Energy spectrum''

Energy flux density

(beam power density)

Tradi tional

Unit

roentgen

rad

-2 cm MeV-1

MeV cm'^ s"^

Customary

SI Units

coulomb kg

1

joule kg'

-2 . , -1 cm joule

watt cm~^

Electron beams

Radioactivity

Absorbed dose

Energy spectrum*^

Beam current and power

Activity (nuclear trans- formation rate)

Emission rate (particle

or photon)

Power

rad

cm'^ MeV'' ampere, watt

curi e

-1

s

watt

joule kg ^ ^ -2 . , -1 cm joule

ampere, watt

3-1 b

-1

s

watt

Neutrons Emission rate

Fl uence

Flux density (fluence

rate)

Energy spectrum^

Absorbed dose

Kerma Cross section

-1

s -2 cm

-2 -1 cm s

cm MeV

rad

rad

barn

-1

cm

-2 -1 cm s

-2 -1 cm joule

joule kg'^

^

joule kg ^ ^

cm^

Unit gray (Gy) adopted for 1 joule/kg. One rad equals one cJ/kg.

b 1 Unit becquerel (Bq) adopted for 1 s" . ^ Differential distribution of fluence with respect to energy.

(2) diagnostic x-rays, (3) dental x-rays,

and (4) nuclear medicine.

Similarly, manufacturers can be categorized

into instrument makers, radiation source equipment manufacturers, and radiopharmaceutical manufacturers. The major

regulators involved in this area are listed

in Table 3. A very major part of the

regulatory job falls on the state and local

radiation control offices since, unless

particular radiation responsibility is assigned to some Federal agency, the

responsibility falls on the state or local

government

Some of the important standards and

calibration laboratories are listed in Table 4, the NBS being one of them. Other

national standards laboratories such as the National Physical Laboratory in England,

the Physi kal isch- Technische Bundesanstal

t

in West Germany, the Laboratory for the

Measurement of Ionizing Radiation (LMRI)

in France, and the Central Bureau for

Nuclear Measurements (BCMN) of Euratom

located in Belgium are important laboratories

with which we compare standards on a bilateral basis. The International Bureau

of Weights and Measures (BIPM) is a central

laboratory and secretariat for tying all of

the national standards laboratories together

into an international measurement system.

14
Table 2 ORGANIZATION OF THE IONIZING Table 4 STANDARDS AND CALIBRATION

RADIATION MEASUREMENT SYSTEM STUDY LABORATORIES

Main Categories of Users

1. MecMcal

2. Nuclear Power

3. Industrial Radiation

Processing

4. Defense

5. Chemical Analysis

6. Science

7. Environmental Radioactivity

8. Miscellaneous Radiation

Appl ications

Activity Classes

1. Direct Users of Radiation

2. Manufacturers

3. Regulators

4. Standards and Calibration

Laboratories

5. Other Interested Groups

Table 3 MAJOR REGULATORS OF RADIATION

Nuclear Regulatory Commission

Energy Research and Development

Administration

Environmental Protection Agency

Bureau of Radiological Health,

Food and Drug Administration

Occupational Safety and Health

Administration, Department of

Labor

Department of Transportation

State and Local Radiation Control

Offices

National Bureau of Standards

Bureau International des Poids

et Mesures (BIPM), Sevres, France

(International Bureau of Weights

and Measures)

Other National Standards

Laboratories

National Physical Laboratory,

England, UK Physikal isch-Technische

Bundesanstalt, West Germany

National Research Council,

Canada

Bureau National de Metrologie,

France (radiation laboratory

known as LMRI)

Central Bureau for Nuclear

Measurements, Euratom

(in Belgium) , etc. Regional Calibration Laboratories

(recognized by the American

Association of Physicists in Medicine)

Other Calibration Laboratories

and Consultants

State and local laboratories

Certified radiological physicists

Table 5 IMPORTANCE OF RADIATION THERAPY

One person in four will get cancer

50-60% of cancer cases treated

with radiation

Try for cure in 20-60% of cases depending upon patient population received by hospital

Equi pment

280 high energy electron

accelerators (1973)

574 Co-60 (1970)

30 Cs-137 (1970)

5800 x-ray sets (1972)

Accuracy required: 5% in dose to tumor

3% in beam

calibration

15
other standards laboratories are located in various government agencies in state and

local governments and in regional calibration laboratories such as those sponsored

by the American Association of Physicists

in Medicine. Many calibrations are also

provided by radiological physicists

certified by the American College of

Radiology.

Other interested groups include professional

societies, such as the American Nuclear

Society and the Health Physics Society,

standards-setting organizations such as the National Council on Radiation

Protection and Measurements (NCRP), the

International Commission on Radiation Units

and Measurements (ICRU), the International

Organization for Standards (ISO), the

International El ectrotechnical Commission

(lEC), and the American National Standards

Institute (ANSI). Educational institutions

and many government agencies are also very

interested in the measurement system for

ionizing radiation.

We now propose to take a more detailed look

at the measurement system by locjking at some of the user communities. The first

community we shall consider is the Medical

Community.

Figure 1. Modern electron linear

accelerator (linac) used for radiotherapy.

The first user group is that of radiation

therapy. In Table 5 are shown some of the

important characteristics of this field.

The use of high energy electron accelerators is increasing very rapidly whereas the

use of the x-ray sets is tending to decrease.

What makes the measurement problem particularly difficult here is the need for

measurements of significantly higher

accuracy than the overall 5% dose to the

tumor generally accepted as needed. A modern electron linear accelerator used for

radiation therapy is shown in Figure 1. Figure 2 is an example of an "interaction

diagram" for the medical radiation measurement system for x- and gamma rays. Note

that the standards and calibration

laboratories are shown at the top (they

probably should be at the bottom in support

of the system), the manufacturers are on the left, the regulators on the right, and

the users shown at the bottom. The diagram

gives us an indication of where more activity

is needed. For example, the "fair" link

is shown between the AAPM Regional Calibration

Laboratories and the radiation therapy

community because there are not enough

such laboratories to service the whole

community.

STANDARDS AND CALIBRATION LABS

I BIPM I

SOURCE

MANUF,

RADIO- PHARM.

MANUF.

OTHER NATIONAL] STANDARD LABS I

RADIATION DIAGNOSTIC DENTAL NUCLEAR

THERAPY X-RAYS X-RAYS MEDICINE USERS GOOD WEAK

REGULATORY FUTURE

Figure 2. Medical radiation measurement

system for x and gamma rays, not including

nuclear medicine.

16
The measurement link between NBS and the state and local governments is shown as "future," but in fact, NBS now has a program

in cooperation with the Conference of

Radiation Control Program Directors aimed

at establishing measurement links with this

regulatory community. Figure 3 gives an indication of the need for accuracy in radiation therapy. The optimum dose will

depend on the seriousness of the side effect.

If the side effect itself is lethal, the

optimum dose will, of course, be lower than

if the side effect is tolerable, permitting

the radiotherapist to opt for a higher

chance of tumor cure. Figure 4 shows a study by the Radiological Physics Center

located at M.D. Anderson Hospital in

Houston, of the x-ray calibrations of 70 of 200 hospitals which are performing

clinical trails for the National Cancer

Institute. Note that 80% are within the

5% limit in the x-ray calibration. However,

in dose delivered to the tumor (Figure 5) only 65% were within + 5%.

X-rays are used both in medicine and

dentistry for diagnosis. The widespread

use of x-radiation is indicated in Table 6. Diagnostic x rays are also important

because over 90% of the radiation dose to the population is from diagnostic x rays. However, new successes with techniques

such as computerized tomography are making

x-rays an even more powerful diagnostic

tool. Unfortunately, however, the

radiation dose given by these new diagnostic

tools tends to be rather high. The needs

of the medical radiation measurement system

for diagnosis and therapy are listed in

Table 7.

0 3000 6000 9000 12000 RADIATION DOSE, rods (^"Co) Need for accuracy in RADIATION THERAPY

Based on HEAD and NECK TUMORS

Figure 3. Illustration of need for

accuracy in radiation therapy. Determination of optimum dose is determined by the

radiotherapist's judgment of the seriousness

of complications. Too small a dose will

produce fewer tumor cures than optimum.

Too large a dose will produce a high

probability of serious complication.

Nuclear medicine using radiopharmaceuticals

is a very rapidly growing field as summarized in Table 8. An interaction

diagram for the measurement system for

nuclear medicine is shown in Figure 6. Since the diagram was made, the interactions

between NBS and the manufacturers, including

measurement assurance programs, have been

greatly increased. A summary of the needs

of the nuclear medicine community are:

(1) quality assurance system, (2) dose

calibrator improvement and check kit,

and (3) new radioactivity standards.

Nuclear Power Plants

Many nuclear power plants are now being

constructed such as the one shown in Figure 7 at Three Mile Island (belonging

to the Metropolitan Edison Company in

Pennsylvania)

J_L I

I I

RPC/INSTITUTION Figure 4. Comparison of radiation therapy

machine calibrations by the Radiological

Physics Center with those in use at institutions. Frequency refers to number

of institutions; an average comparison is given if an institution had more than one radiation machine.

17
15-

I

RPC/INSTITUTiON Figure 5. Comparison of tumor dose

delivered (RPC) with that prescribed

(institution). Radiation machines and

types of treatment are counted separately

in determining frequency.

Table 8 THE FIELD OF NUCLEAR MEDICINE

10 million applications of

radiopharmaceutical s/year ( 1 974)

98% diagnostic

Growth rate 25%/year

Number of physicians doubled in five years

5000 U.S. Hospitals ^ use 2500 Private Clinics radionuclides

Radiopharmaceutical sales

70 M$/year (1974)

Medical nuclear equipment

140 M$/Year (1974)

Accuracy required: + 10% (USP-FDA)

Table 6 USE OF X-RAYS IN DIAGNOSIS

Diagnostic Medical

129 Million Persons, 210 Million

Examinations (1970)

Examination Growth Rate 2%/Year

133,000 X-Ray Sets (FY 1974)

Dental

225 Million X-Rays (1970)

143,000 X-Ray Sets (FY 1974)

Table 7 NEEDS OF THE RADIATION DIAGNOSIS

AND THERAPY MEASUREMENT SYSTEM

Calibration network using regional

calibration laboratories for

X and gamma rays £ 1.25 MeV Calibration transfer instruments

for high energy radiation

Primary standards for absorbed

dose

High-energy x-rays (linacs,

betatrons, ...) Fast neutrons

Pions (future)

Standards for imaging

STANDARDS AND CALIBRATION LABS

GOOD —

FAIR WFAK

REGULiTORY —

FUTURE

RADIO - PHARM MANUF

OTHER NATIONAL STANDARD LABS

USERS

Figure 6. Interaction diagram for the

measurement system for radioactivity

standards and radiopharmaceuticals used in nuclear medicine.

18
About 60 nuclear power plants are licensed

to operate and currently provide about 8'i of the electrical power in the U.S. In Figure 8 a distribution of the operational

and planned nuclear power plants in the

U.S. is shown. Note the concentration

of nuclear power stations in the East and

specifically in the Middle Atlantic area. The public is certainly clear on the

question that nuclear power, if it is to exist, must be used safely. Some of the

needs of the measurement system for nuclear

power are summarized in Table 9. The NBS

Neutron Standards Program v;as greatly

augmented in 1971 essentially as a response

to the needs listed in Table 9. Another

area of extreme concern related to radiation safety has to do with the nuclear

fuel cycle shown in Figure 9. A study of

the nuclear fuel cycle and its measurement

needs was carried out in 1974 by John

Bartlett, a Presidential Intern at NBS.

A summary of the results of this study is given in Table 10. It is interesting to note that the study was initiated by a concern for questions of measurement

standards for "equity-in-trade" in dealing

in nuclear fuel, but it turned out that

the high accuracy needed v/as for safeguarding

nuclear materials against diversion for

unlawful purposes.

Figure 7. Modern nuclear power plant.

Three Mile Island, No. 1, in Pennsylvania

Figure 8. Nuclear power reactors in the

United States according to Energy Research

and Development Administration, December 1975.

19
Table 9 NEEDS OF THE MEASUREMENT SYSTEM

FOR NUCLEAR POWER (DESIGN DATA

AND REACTOR OPERATIONS)

Neutron cross section measurement

standards for:

Liquid-Metal Fast Neutron

Breeder Reactor (LMFBR)

Light-Water Reactors (LWR)

Fusion Power (controlled

thermonuclear and laser

fusion)

In reactor measurement standards

Performance vs. calculations

Fuel performance

Reactor lifetimes

Reactor control

Safety

Table 10 RESULTS OF THE NUCLEAR FUELS

MEASUREMENT SYSTEM STUDY (BARTLETT)

High accuracy needed for safeguards

(not equity in trade)

Major Need:

Nationally-based measurement

assurance system for fuel

material

Measurements (for demonstrably

viable measurement capability)

Other Needs:

Regulations, guides, consensus

standards

State-of-art performance in

field with sophisticated

measurement technology

(approaches NBS capability)

Real-Time SNM control

(replacing periodic inventory)

Table 11 SOME INFORMATION ABOUT THE INDUSTRIAL RADIATION PROCESSING

FIELD

0. 5 G$ per year

Growth rate 30$/year (recent yrs.)

Highly proprietary

Strong foreign competition

(Japan, France)

Successful areas: Wire and cable insulation curing

Sterilization of medical

disposables

Plastic automotive part curing

Electron beam welding (Activation^

Flame-resistant fabrics

Polyethylene foam Heat shrinkable plastic film

MINING AND MILLING

CONVERSION

FUEL ELEMENT FABRICATION

REACTOR OPERATIONS

SPENT FUEL ELEMENT STORAGE

FUEL REPROCESSING

(waste )

HASTE MANAGEMENT

Figure 9. Nuclear fuel cycle.

Table 12 INDUSTRIAL RADIATION PROCESSING

NEEDS AND RELATED NBS PROGRAM

Needs (some companies):

Calibrations of ki lorad-megarad

dosimetry systems

Dosimeters designed for

specific tasks

Improved read-out methods

NBS Program:

Specific measurement services

for processing industry

Preparing high radiation

dosimeter calibration service

(e" + 60co)

NBS sources available to users

NBS program limited

20
Industrial Radiation Processing

Some of the characteristics of the

industrial radiation processing field are summarized in Table 11. Some of the needs

of this industrial field as well as a modest size NBS program response are summarized in Table 12.

Defense

A summary of the needs of the Defense

Department agencies in the area of

electron, x- and gamma ray measurement

is given in Table 13. NBS response has

been to provide monoenergetic radiation

sources for calibration as well as absolute

and relative dosimetry instrument calibration fields.

Chemical Analysis

Some of the characteristics of chemical

analysis by activation and detection of

the subsequent radiations are listed in

Table 14. Although much activation

analysis is done with neutrons, usually

from a nuclear reactor, protons and

charged particles are often advantageous

for precise analysis of certain elements.

In this field the need is for standard

reference materials (SRM's) and no new specific radiation program need was identified.

Radiation Measurement System for Science

Many needs of scientists working with

radiation are not radiation measurement

as indicated in Table 15. Those needs

that are radiation measurement are generally for source characterization,

detector characterization, standard

reference data, and assistance in theoretical

interpretation of measurements--almost

independent of whether the scientific

field is geology, chemistry, biology and

so on. NBS programs do impinge on these

scientific needs. One need was identified

for a data center for provision of charged

particle range, stopping power, straggling,

and delta ray production data especially

below 10 MeV relative to biology and such

fields as ion implantation.

Environmental Radioactivity

The structure of the environmental

radioactivity measurement system is indicated in Figure 10, while the needs

and NBS response are summarized in Table 16. A very vigorous measurement assurance

Table 13 DEFENSE RADIATION MEASUREMENT

SYSTEM FOR X AND GAMMA RAYS

Interested agencies:

Defense Nuclear Agency

Defense Civil Preparedness

Agency

Advanced Research Projects

Agency

Air Force Tactical Command

National Security Agency

Army Materiel Command

(Natick Laboratory)

Needs (electrons, x and y i^ays)

High-flux, low energy pulsed

x-ray calibrators (for

hardness, survivability,

simulators, plasma)

High-flux, D.C. fluence and

dose measurements

NBS response:

Pulsed monoenergetic x-ray source ( .1-10 keV, 1.5 keV

now)

D.C. monoenergetic sources

(.1-70 keV)

Twin microcalorimeter + dye

films

Standard electron irradiation

field, 4 MeV Van de Graaff

Table 14 CHARACTERISTICS OF CHEMICAL

ANALYSIS

Ultimate sensitivity excellent

Non-destructive analysis often

possible

Several elements can be determined

in single sample

Avoids contaminated reagent

probl ems Carrier techniques can be used

post-irradiation

Can distinguish different

isotopes of same element

Neutrons, charged particles,

photons all used

21
program is now being carried out with the

Nuclear Regulatory Commission, the

Environmental Protection Agency and

concerned state agencies to insure the

reliability of environmental radioactivity

measurements

Miscellaneous Radiation Applications

Some of the characteristics of this field

are summarized in Table 17. The key

problem, as indicated, is in safe use of

the equipment and an appropriate response

seems to be to vigorously support the

voluntary standards system.

Personnel Monitoring

Need for personnel monitoring is common

to all users of radiation. As indicated

in Table 18, the importance of personnel

monitoring is not because it is a large

business (it is quite modest in size) but

because personnel monitoring is a chief

factor making possible the safe use of

radiation for its many desirable benefits.

The structure of the radiation measurement

system for personnel monitoring is indicated in Figure 11. Note the rather

weak links indicated between NBS and the

National Sanitation Foundation (NSF) and

the suppliers of the commercial services,

as well as the absence of a link between

NBS and NSF, a chief tester of commercial

services. This problem is being intensively

studied by a number of government agencies.

The needs of personnel monitoring are indicated in Table 19.

Table 15 SOME INFORMATION ON THE RADIATION MEASUREMENT SYSTEM

FOR SCIENCE

Many needs from NBS are not "radiation" measurement:

Atomic masses Frequency

Length

Proton magnetic moment

Volt, etc. Radiation Measurement Needs:

Source characterization

Detector Characterization

Standard reference data

Theoretical interpretation

of measurements

NBS Program:

>70 calibrated radioactive

sources Data--atomic interactions of

photons

Data--nuclear interactions of

photons

Benchmark measurements

a

12 tot C; proton form factor)

Instrument development and

characterization (Faraday

cup, absorbed dose calorimeter,

large Nal spectrometer)

Reference radiation fields

Radiation transport

Summary

Some of the common threads found throughout

this study, rather independent of the

radiation user group being studied are:

(1) a need for measurement quality assurance (especially in areas where

radiation is regulated); (2) a need to reach out to measurers in the field with

assistance, training, and regional

calibrations of some kind; and (3) some new measurements standards are badly

needed where such standards do not exist.

A much more complete discussion, with

references, of the measurement system for

ionizing radiation should be available

this year as an NBS internal report.

NBSIR 75-946.

Figure 10. Structure of the environmental

radioactivity measurement system.

22
Table 16 ENVIRONMENTAL RADIOACTIVITY

MEASUREMENT

Table 17 MISCELLANEOUS RADIATION

APPLICATIONS

Regulatory problems are important,

NRC, ERDA, EPA are the interested

Federal, agencies

Needs

NBS standards

Traceability to NBS (measurement

assurance)

Training and consultation

NBS Response:

Standards (mixed radionuclide,

natural matrix, gaseous,

low-level a and e ) Lead laboratory measurement

assurance (NRC, EPA)

Round-robins to nuclear power

industry

Low-level counting capability

International inter compari sons National radiation measurement

calibration system (States)

Appl i cations

Radiographic equipment

Gauges

Irradiators

Oil well logging apparatus

Self-luminous products

Smoke detectors

Static eliminators

Heat sources

>70 M$ annual ly

Needs

Adequate safety measures in use of this equipment

NBS Response:

Increase support for voluntary

standards activities (NCRP,

ANSI N43)

PERSONNEL MONITORING MEASUREMENT SYSTEM

Figure 11. Structure of the measurement

system for personnel monitoring.

23
Table 18 MEASUREMENT SYSTEM FOR PERSONNEL Table 19 NEEDS OF THE PERSONNEL MONITORING

MONITORING MEASUREMENT SYSTEM

14 commercial companies

$10 M/year

Growth Rate ~- 10%/year

" 800,000 individuals monitored

(commercial and government

labs and military services)

Film badges and TLD are chief

methods used

Makes possible benefits of

ionizing radiation

Satisfactory quality assurance

Improved personnel monitors

(especially for neutrons)

Monoenergetic neutron calibration

faci 1 ity

Beta-ray source calibration

facil ity

24
NBS SP456 (1976)

THE FEDERAL REGULATORY AGENCY AND THE RADIATION MEASUREMENT SYSTEM

R. H. Schneider

Food and Drug Administration

5600 Fishers Lane Rockville, Maryland 20852

The official actions of a federal regulatory agency can have unique

health and economic impacts. It is often not feasible for other

parties to review the processes leading to such actions in detail

and appeal is tedious and expensive. As a matter of equity it is incumbent on the agency to assure that the errors associated with

facts on which actions are based are sufficiently small as not to affect the course of such actions. When radiation measurements are

used as part of the factual basis for regulatory action this means

that these measurements must be an integral part of the national

radiation measurements system. The manner in which this has been

achieved within the Food and Drug Administration as part of the

implementation of the Radiation Control for Health and Safety

Act of 1968, (P.L. 90-602) is described.

In the abstract of this paper I discussed the

philosophical underpinnings of the connection

between the Federal Regulatory Agency with

responsibilities in the radiation area and

the radiation measurements system, which is indeed a fundamental and necessary connection. I stated that "as a matter of equity

it is incumbent on the Agency to assure that

the errors associated with facts on which

actions are based are sufficiently small as not to affect the course of such actions."

That is indeed the basis of the connection.

In this paper I will discuss, in general,

some of the problems that have arisen and

the solutions to them which we have devised

in the course of developing the regulatory

machinery within the Food and Drug Administration for implementation of the Radiation

Control for Health and Safety Act of 1968.

The details of this process in the four

areas of acoustic energy, microwave

radiation, optical radiation and x-radiation

will be discussed by members of my staff at later sessions of this symposium. I will

then go beyond the bounds established in my

abstract and discuss some of the other

implications which radiation measurements

and therefore the radiation measurements

system can have for a regulatory agency.

The most surprising thing in my experience,

in the eight years since the Radiation Control Act was passed, has been the realization

that the national radiation measurements

system was not ready for the Radiation Control for Health and Safety Act. There were,

I believe, two reasons for that, the first

a technical reason, is simply that the

measurement technology which had been

developed for many of the types of energy

encompassed by the Act such as acoustical

energy, radiofrequency and microwave energy,

and optical energy, while very well developed.

had been developed for use under

circumstances quite different from those

which apply when radiation measurements

are needed for health protection purposes.

I will illustrate this by one simple and

obvious example. In a case where it is desired to evaluate the hazard potential

of a product incorporating a radiofrequency or microwave generator, such

as a microwave oven or a diathermy machine,

it is quite often necessary to make

measurements in the near-field or a radiator. The microwave instrumentation

existing in 1968 consisted primarily of

field strength meters which were used by

antenna engineers to measure antenna

patterns and gains in the far-field,

because their only interest was in the

antenna's ability to concentrate energy

at great distances. Partly as a result

of the Radiation Control Act a whole new science and technology of near-field

measurements has blossomed within the past

eight years. This has been stimulated

further by the passage of the Occupational

Safety and Health Act. There are many

similar examples of new scientific and

technical requirements being imposed on the radiation measurements system by the

creation and implementation of regulatory

authori ty

Because of the state of unpreparedness of

the radiation measurement system a considerable burden of measurement development was

placed squarely on our shoulders and I believe that we have carried it well. The

Division of Electronic Products of the

Bureau of Radiological Health has, in the

past eight years, made many substantial

contributions to the radiation measurements

system. It is appropriate to acknowledge

here that many substantial contributions

25
have also been made by various groups in the Institute for Basic Standards of NBS to assist us in developing the radiation measurements system to the point where it can meet the requirements of radiation measurements for health protection.

A second reason for the lack of preparedness

of the radiation measurements system was essentially non-technical but rather administrative and logistical. I am not an expert

on radiation law but I would like to offer

the following suggestion for your consideration. Though previous acts of Congress had

established various regulatory bodies having

cognizance over radiation, the Radiation

Control Act of 1958 was the first such act under which radiation measurements per se became the sole facts upon which regulatory

actions were predicated. That means that an agent of the Food and Drug Administration in

performing a radiation measurement can develop a datum which will determine whether

or not a formal regulatory action will ensue. Some such actions in the recent past have

had economic impacts tolled in the millions

of dollars in situations where the risks

to the nations health could have been substantial. Hark back to the matter of equity,

which I raised at the beginning of this presentation. In such a situation, as a matter

of equity, the nations health deserves that

measurement to be as accurate as technically

and economically feasible so that regulatory

action is indeed pursued when a limit established in a regulation is in fact exceeded.

It is also a matter of equity that the potential target of the regulatory action deserves

that the measurement be as accurate as technically and economically feasible so that he does not suffer economic loss from a regulatory action which is pursued when in fact

his product was in compliance with the limit

established in the regulation.

The practical impact of this matter is profound. We are maintaining an instrument

population now numbering in the hundreds

scattered across the entire United States in active, if not always continuous, use with

each and every one of these instruments

having a non-zero probability on any given

working day of turning out data which will

have to be defended in a legal action. I submit that the radiation measurements system

was not ready for this. We have, therefore,

in the last eight years devoted substantial

energy and resources to the development of

equipment and systems for the logistical

support, including calibration, of large

numbers of radiation measuring instruments

in the field. One of our later papers in

the symposium discusses one of these systems.

We are confident that we have successfully

integrated our regulatory programs with the

national radiation measurements system, that

we adequately treat the matter of equity

and that we are prepared to make measurements of whatever, wherever, health

protection demands.

Now I would like to talk a little bit about

some of the other applications of radiation

measurements in the programs of a Federal

regulatory agency.

Radiation measurements offer the federal

radiation control agency a means by which to assess the effectiveness of its own programs

both regulatory and non-regulatory. At

times the measurements which are most useful

in assessing program effectiveness are not

the same sort of measurements as are made

for determining compliance with regulations,

because, after all, the intent of regulations

is not to reduce the radiation insult to radiation measuring instruments but, rather

to control, or reduce the radiation insult

to health. In the design of regulations and

the testing conditions which are used in tests of compliance with them we often, in the interest of efficiency, design tests and

measurements which are not direct measurements of health insult. Therefore, when we want to assess the effectiveness of one of our programs we often face another measurements problem.

I now would like to describe to you a system

which we have developed to assist us in meeting such needs in the area of medical radiation exposure. You will hear no more about

this system at this conference so I will go into some detail. Publications describing

it and its use are in preparation. This is

a system for estimating absorbed doses in human organs by calculation rather than by

measurement. There are situations where

estimating by calculation is more cost- effective than estimating by measurement

and such calculated estimates are properly

a part of the radiation measurement system.

A number of years ago we began a collaboration with members of the Health Physics

Division of the Oak Ridge National Laboratory

to apply a system developed by them for

estimation of organ doses from internal

radionuclides to the problem of the

estimation of organ doses from external

radiation sources such as medical x-ray

machines.

The system consists of a detailed mathematical description of the geometry of a phantom which rather closely approximates

26
reference man. The set of equations which

constitutes the phantom includes descriptions of the phantoms' external surface and

descriptions of many internal organs including

a skeleton, lun'gs, thyroid, both male and

female gonads, a uterus, and many others.

Mammaries are presently under development.

The second component of the system is a Monte Carlo radiation transport code. The

equations of the phantom define the geometrical regions in which the radiation transport

code is active. The conversion of the

system for estimation of doses from external

sources was easily accomplished.2>3 The

development of phantoms representing children

of different ages is well underway.

Using this mathematical model as a tool, we

have developed yet another dose estimation

system for more routine applications. It was immediately obvious that routine use of

the transport code to generate dose esti- mates for a large number of specific

clinical situations would be redundant and

expensive. We therefore set out to generate

a data base of reference organ dose data

from which specific clinical situations

could be simulated. This data base is

organized according to the following scheme:

The projections of the external surface of

the phantom in each of the three views,

anterior-posterior, lateral, and posterioranterior are segregated into 4cm x 4cm

square regions by a grid system and these

resulting square regions are ordered and

numbered. The photon energies of interest

were considered to span the region between

approximately 20 keV and 150 keV. Seven

photon energies, more or less uniformly

spaced across this region were chosen.

Transport calculations v;ere then done for

monoenergetic photon beams of each of the

chosen energies into each of the surface

regions of the phantom. Each such calculation produces a tabulation of dose factors

in units of rad per incident roentgen for

every organ in the phantom. The dose

to any organ under given conditions of x-ray

exposure can then be simulated by a weighted

sum of selected cells of this data base.

The weighting factors across the energy

dimension are chosen to represent the

photon spectrum used in the particular

situation of interest and organ dose factors

are selected from the tabulations for those

surface regions included in the radiation

field of the examination. The organ dose

estimates are then obtained by normalizing

with the exposure used in the examination

under study.

The entire data base is maintained on a digital computer and routines have been

written to generate the sums and weighting

factors automatically from simple descriptions of the geometry and technique factors

of the examination. These routines include

provisions for assigning fractional weighting factors to those surface regions which

are not entirely within the examination

field.

The existence of this second system obviates

the need for complete radiation transport

calculation for the evaluation of organ

doses from every clinical situation of

interest. Good estimates of organ doses

are now available quickly and at minimal

cost in computer or user time. We have

already made extensive use of this system.

As can be readily imagined, the system

makes it very easy to investigate effects

on organ doses of variations in technique

factors or size and location of radiation

fields. It permits the user to quickly and

inexpensively perform a large number of

dosimetry experiments by computer simulation

rather than by many painstaking hours of

laboratory work. We have, for example,

found the system to be an invaluable tool

in producing estimates of program efficacy

for environmental assessment reports. The

system was used to evaluate the impacts

of the performance standards for medical

x-ray equipment and our recent policy

statement on gonadal shielding.

To date we have been able to perform several

comparisons of estimates from this system

with dose estimates in the 1 i terature

Such comparisons are of course always

difficult because pairs of dosimetry

experiments performed under identical

situations are difficult to find. The

estimates of doses to directly irradiated

soft tissue produced by our system in general

vary less than 15% from those in the

literature which were obtained under similar

irradiation conditions. Our estimates of

doses to tissue not directly irradiated

vary as much as 30% from values in the

literature. These comparisons have con- vinced us of the adequacy of the system

for evaluation of the effects of variations

in technique factors and irradiation

geometry on absorbed doses in internal organs.

The activities of the federal regulatory

agency in the radiation area need not all be regulatory, in fact, the Congress does not confer regulatory authority for the sake

of regulations, but rather for the sake of some goal related to the nation's welfare.

27
We continue to support, as we have over the

years, non-regulatory programs in the field

of radiological health because we have

determined that certain problems are more effectively attacked by education, consultation, and persuasion, than by regulation.

Non-regulatory programs also can benefit

by integration into the national radiation

measurements system, because wherever

measurements are used credibility enhances

their effectiveness. Two such programs

in which the Bureau participates with the

States are known by the acronyms DENT and

NEXT.

DENT - Dental Exposure Normalization

Technique - is a program for measurement of

the radiation exposures used for radiography

by individual dental practitioners within

a state and reduction of the exposures

delivered by those individual practitioners

whose values lie toward the upper end of the

population distribution.^ Post cards con- taining lithium fluoride thermoluminescent

dosimeters are mailed to each practitioner

within a State with instructions for exposure under the same conditions the practitioner uses for bitewing radiographs. These

are returned by mail and read and the

population distribution generated from the

resultant values. A point on the distribution is then selected which divides the

population of practitioners into those who

will not receive a follow-up visit and those

who will. Those practitioners whose values

are above this point are then visited by

members of the State Health Department.

Their radiographic techniques are reviewed

and consultation and guidance for improvement is given. The subsequent reduction in radiation is demonstrated to the practitioner.

The results of this program have been quite

dramatic. In those states where follow-up

studies have been done it has been demonstrated that lasting significant reductions

in both population mean and extreme have

been achieved. At the present time, Bureau support for the DENT program consists of

maintenance and calibration of the dosimeters

and readers that are used, processing of the

data collected and consultation with State

radiological health programs on operation of

the individual projects.

NEXT - The National Evaluation of X-ray

Trends - is a program about which you will

hear a paper later in this conference,

therefore I will mention it here only to state that the Bureau's participation on the topic of measurements has been to evaluate, by measurements of energy

dependence, and calibrate the instruments

which are used by the States in this program.

In closing I would like to mention briefly

a new project within the Bureau of Radiological Health which at present is in the

pilot stage. Because of the current

interest in mammography we decided that it

would be useful to have a program similar

to the DENT program to evaluate the exposures

being used in mammographic examinations at individual facilities, in an attempt to identify excessively high exposures resulting

from poor technique, such as have been

reported. We then were faced with the

problem of developing a simple, mailable,

inexpensive system capable of producing

adequate estimates of exposure from the low

energy x-ray spectra used in mammography.

We began by investigating the photon energy

dependence of the exposure response of

lithium fluoride chips to low energy x-rays - with both nearly monoenergetic fluorescent

sources and broad spectrum low energy x-ray

sources. We determined that when calibrated

with an appropriate low energy x-ray

spectrum the exposure response of the lithium

fluoride chips did not vary more than plus or minus 20% over the range of beam qualities

which we expect to encounter in a survey of

mammography facilities.^ A pilot study of

this system produced distributions of

values for the various mammographic

techniques which compare extremely well with

reported values for the same techniques.

We are continuing to develop the logistical

support for this system and a second pilot

study is underway.

In this presentation I have tried to give

you some appreciation of both the scope

and depth of the relationship between one

Federal regulatory agency and the radiation

measurements system by discussing the

philosophy underlying that relationship

and describing a variety of examples of the

spectrum of activities of which the

relationship is composed.

References

1. Snyder, W.S., M.R. Ford, G.G. Warner

and H.L. Fisher, Jr., Journal of Nuclear

Medicine , Supplement Number 3, Pamphlet 5,

Society of Nuclear Medicine (August 1969).

and

Snyder, W.S., M.R. Ford, G.G. Warner and

S.B. Watson, ORNL-5000, Oak Ridge National

Laboratory (November 1974).

28
2. Poston, J.W., G.G. Warner, M. Rosenstein

and R.H. Schneider, FDA-73-8029, Government

Printing Office (March 1973).

3. Jones, T.D., J. A. Auxier, W.S. Snyder

and G.G. Warner, Health Physics , 24, 241 (1973).

4. Rosenstein, M. , Organ Doses in Diagnostic

Radiology (in press).

5. DENT: A Quality Assurance Program in Dental Radiography, Gross, R. , R. Bunge and

C.L. Crabtree, Health Physics Mid-Winter

Symposium, Denver, Colorado 2/9/-2/12, 1976.

6. Energy Response Curves for Various

Dosimeters, T. Ohlhaber (to be published).

29

NBS SP456 (1976)

STATE RESPONSIBILITIES AND THE RADIATION MEASUREMENT SYSTEM

M. W. Parrott

State Health Division

Radiation Control Section

PC Box 231 Portland, Oregon 97207

The states' radiation control programs have attempted to respond to public health need through a radiation measurement system that requires much more than instrument

calibration. Few federal agencies can cover their responsibilities without state assistance. This paper explains

these areas from one Radiation Control Director's point of

view which may be shared by several others. Recommendations

for some improvements are made.

(Radiation; regulations; ineffectiveness; measurements;

compromise; states; abstract)

Traditionally, the states have had

public health and safety responsibilities

for their citizens which are not shared

with the Federal government. Although 16 federal agencies attempt to regulate

various health and safety aspects of radiation, they have been unable to adequately

regulate these activities due, in part, to the magnitude of the enforcement problem.

Therefore, a number of federal laws passed

to "protect the public health" fall upon

the states for enforcement. The states accept their traditional duties by assumption of responsibility (i.e.. Nuclear

Regulatory Commission/Agreement State and

Occupational Safety and Health Act), by

default by the federal agencies (i.e.,

U.S. Department of Transportation), or by

the simple statement in a regulation that

"certification" by state agencies is re- quired of certain types of installations

(i.e., Federal Aviation Administration

baggage inspection by x-rays). Unfortunately and all too frequently, this "inspection

and certification" may be accomplished with

anything from a "wet finger" to equipment

which represents the state of art in radiation measurement.

Unlike the ideal conditions present in

a national standards laboratory, the radiation measurement system at the state level

is not confined to the calibration of

instruments alone. To make radiation

measurements, a state inspector must carry

any number of pieces of equipment many

thousands of miles, under severe climatic

conditions to any number of installations,

all of different character. The con- ditions of transport may vary from bus and

foot travel in a large metropolitan area,

or by car or airplane in some of the

larger, less populated states. The inspections require different kinds of measure- ments of diagnostic, therapeutic and

industrial byproduct, naturally occurring

and cyclotron-produced radioactivity,

diagnostic, therapeutic and industrial

x-rays, lasers and microwaves. Usually,

each measurement must be carried out in a short time with such accuracy as can be

achieved under these somewhat hazardous con- ditions--hazardous in that each measurement

may have to be defensible in court.

For clarification, perhaps it is appropriate to show what developed from the

Workshop on "Criteria for an Adequate State

Radiological Health Program for Radiation

Sources not Covered under the Atomic Energy

Act" at the 7th Annual Conference of

Radiation Control Program Directors.^ This

program is outlined in Table 1.

To adequately protect the public health

by attaining a well-balanced radiation

measurement system, one state agency, and

probably a single federal agency should have

the authority, responsibility, capability

and demonstrated performance over all forms

of radiation. Instrument capability may be

present in any number of agencies as well as emphasis on a particular form of radiation.

However, performance may be a totally

different matter in the hands of people who

are not technically competent to know the

limitations of themselves or their instruments.

Unfortunately, the range of authority

governing radiation measurements and

regulations varies widely among the states

from voluntary compliance to complete

31
TABLE I. ELEMENTS FOR A MINIMUM STATE RADIOLOGICAL HEALTH PROGRAM

FOR NON-ATOMIC ENERGY ACT SOURCES

I . X-Ray Sources

A. Machine Registration/Licensing

B. Standards

C. Compliance - Machine Inspection

- Facility Inspection

D. Enforcement

E. Constant Alertness to New Sources

1 1 . Non-Ionizing Sources

A. Registration/Licensing

B. Standards

C. Compliance - Machine Inspection

D. Enforcement

E. Constant Alertness to New Sources

III. Natural Sources (Manufacture, Byproducts, Tailings, Phosphates, Radium,

etc. )

A. Registration/Licensing of Radioactive Material

B. Radioactive Material and Environmental Standard

C. Compliance - Environmental Monitoring

- Radioactive Waste Control (Effluents, etc.)

- Surveillance over Product Manufacturing and Quality

Control

- Facility and User of Radioactive Material Inspections

D. Enforcement

E. Constant Alertness to New Sources

IV. Cyclotron-Produced Radioactive Materials

A. Licensing Users and Equipment

B. Standards

C. Compliance for Production and Users

D. Enforcement

V. Other

A. Public Information and Technical Training

B. Program Effectiveness Evaluation

C. Emergency Response (Fixed Facility and Transportation)

Planning

Training

Response to Incidents

D. Legislation - Writing and Testifying

E. Budget

F. Personnel

G. Update and Publish Regulations

32
legislative authority over all forms of

radiation, ionizing and non-ionizing.

Other variables among the states include

personnel, available equipment, inspection

procedures, travel and climate as well as the frequency, type and method of calibration. There is a total lack of uniformity

in the radiation measurement system. This

was pointed out very well in the beginnings

of the States' participation in the Nationwide Evaluation of X-ray Trends (NEXT). NEXT will be presented in more detail later

in this symposium. The data that was sent

in by the various states in that program

was scattered throughout the spectrum of

results. For several years, the procedures in this measurement system were

followed rigorously while the instrumentation varied grossly. This has been some- what rectified since the Food and Drug

Administration Division of Electronic

Products will not allow data to be incorporated into the overall system unless

the survey instruments have been calibrated

at FDA's Division of Electronic Products at

no cost to the states. Automatically this

introduces some other errors into that data

bank. Some smaller states may not be able

to send in their "only" survey instrument.

Those states may also have the "worst" con- ditions in their state which will not show

up in the overall evaluation of NEXT. Regardless of the bias that may be introduced into the NEXT data, the idea of

having the procedures standardized and the

instruments calibrated by competent

physicists in the FDA is certainly a motion in the right direction.

What is the present world in the State

programs? When it comes to environmental

surveillance, the radiochemical analysis

can be carried out to near perfection. The

sample collection procedures have been

standardized, the equipment has been

developed to provide exceptional accuracy

through sophisticated computer programs

allowing simultaneous analysis of large

numbers of radionuclides and the persons

utilizing the equipment and doing the procedures are frequently highly skilled and

motivated. They can obtain an ever growing

list of radionuclides that have been pre- pared with meticulous accuracy at the

National Bureau of Standards or indirectly

by NBS through the two National Environmental Research Centers. This differs

markedly from the support, personnel,

equipment and procedures in all other parts

or radiation control programs except

possibly that part related to the Agreement

Materials Program. All of this development

is related directly to the nationwide interest in nuclear power while all other elements of a State Radiation Measurement

Program suffer miserably. It should be

noted that it may not be possible to maintain this high degree of accuracy in environmental surveillance. The Environmental Protection Agency Las Vegas facility

has been furnishing a cross check program

among a large number of laboratories in the

U.S. to provide quality assurance to these

laboratories. Many of the state environmental surveillance programs are doing the

only work in this field around nuclear

facilities. Now it appears that, as usual,

as soon as federal funds are curtailed, the

first programs that are eliminated are those

that assist the states. This is going to happen to both the Northeastern and

Las Vegas Laboratories. EPA has decided to cut back these programs. Consequently, one

of the few ongoing Quality Assurance programs for the states is doomed.

This realistic world is such that the

States' radiation measurement system is a compromise. A state inspector must do what

is reasonable and possible. He must be

reasonable. Interpretations of regulations,

if there are any regulations, must be

reasonable. This requires judgment. The

judgment of an intelligent, wel 1 -trained

,

experienced individual who is always aware

of the fact that it is the protection of the

public health that is of prime importance.

The usual inspection of a hospital x-ray

department is a pressure situation. How much time can a given x-ray room be out of

business while the inspector is making

measurements? The survey meter, more than

likely, was calibrated with radium or cobalt

which in no way can be accurate for 70 KVp

x-rays when related to mr/hr. These items

severely curtail the accuracy with which

the measurements can be made. The inspector

must make measurements as to beam size and

alignment, energy, scatter, adequacy of

shielding and timer. In no way can the

accuracy of any of these measurements be

compared to, or attempt to emulate, those

made under laboratory conditions. An x-ray

inspector must inspect 400 to 500 machines

over a large area of several thousand miles

in a single year, go through temperature

extremes, altitude changes and many other

difficulties not encountered in a standards

laboratory. So, how sophisticated must

equipment be? In order to purchase any piece

of equipment, he must go through a complete

scrutiny by a budget analyst to prove the

old equipment is too archaic or unusable to continue to be in service. Then he must.

33
out of necessity, buy the 1 ightest (remember he has to carry this under his arm along with any number of other items with

which to make measurements), the strongest

(remember the rough transport problem)

,

most reliable (who can find a 67 volt

battery in a town of 300) and above all,

cheapest available instrument. There has

yet to be any mention of accuracy. With all

of this, any inspector would be delighted

with + 5% and why not? If shielding design

requires a maximum of 2 mr/hr in an uncon- trolled area, is this inspector going to complain to a facility if he gets 2.1 mr/hr?

Most states show real concern for the

necessity of a complete and adequate radiation measurement system. This can be demonstrated by a comparison of data published

for 1970 by the Bureau of Census reported by

DeVore and data collected during the same period in the State of Oregon. Medical

x-ray examinations were performed on 38 per- cent of the population in the United States

during that year, or 77 million persons. In that same period 65 percent of the population of Oregon, or 1.35 million people,

received medical x-ray examinations. Dental

x-ray exams in the U.S. population totaled

59 million, or 29 percent, while they made

up 62 percent of the examinations in Oregon

for a total of 1.3 million persons. This,

simply stated, shows the population of Oregon will have dental or medical x-rays an average of 1.27 times per year while the

overall U.S. population would average .67 times per year. While the regulations

enacted for Public Law 90-602 have provided

for new equipment to be installed to assure adequate protection against radiation leakage from the overall system and positive

beam limitation (automatic col 1 imation )

,

our experience during inspections has been

that more than 99 percent of the units with

positive beam limitation devices are operated in the manual mode. The positive

beam limitation is cut out because the only

facilities that can afford this kind of

equipment have been using manual coll imation

for years and want to split film for

economy and comparison, and many want to see cone cutting which is not permitted in the

automatic mode.

Filtration, of course, is another area

of concern. Although this, too, is pro- vided for in the law, measurements must be

made on each unit to determine if the

filtration is adequate to eliminate the soft

x-rays which are absorbed by the tissue

rather than contributing to the radiograph.

It has been our experience that two-thirds

of the installers fail to make the actual

measurements to provide for adequate filtration due to lack of instrumentation. Consequently, they must go by a chart and add or subtract filters based on numbers. It should be mentioned that some suppliers have

provided their installers with adequate

instrumentation at considerable cost. Our

studies indicate the lack of adequate

filtration contributes on the average

500 mi 11 i roentgens of unnecessary radiation

per film per x-ray machine. Of all patients

receiving medical x-rays in the State in 1970, approximately 10 percent received un- necessary radiation due to inadequate

filtration. About 11 percent received

unnecessary radiation due to inadequate

collimation. If each overexposure due to inadequate filtration alone is 500 milliroentgens and since there were about

135,000 patients overexposed in this manner

in 1970, then there were 67,500 man roentgens of unnecessary radiation administered. It is much more difficult to quantify patient overexposures due to other

noncompliances, such as inadequate collimation although an attempt is being made with

the NEXT study. X-ray retakes due to poor

techniques should also be considered when

estimating the total amount of unnecessary

radiation administered. The amount of un- necessary radiation described here is quite

high when one considers that the maximum

permissible exposure to any member of the

general population is 500 mr per year. It

is our opinion that it is absolutely necessary to inspect and make the appropriate

measurements on all of these x-ray machines

on a regular basis because of the great

impact these have on the overall radiation

exposure burden.

X-ray inspection frequencies are usually set on the basis of kind of use and

possibility of malfunction almost directly

related to health vs. risk. The desired

frequency of inspection is: dental every

three years, general practitioners every

two years, radiologists and hospitals every

year. There are virtually no states able to maintain this kind of inspection frequency

due almost entirely to the lack of public,

practitioner and legislative awareness that

these devices present a public health hazard

several million fold greater than a nuclear

power plant. Consequently, sufficient funds

are rarely allowed to be directed toward

this rather imposing public health problem.

In Oregon, (the only state with which I am completely familiar) there are 3,588 registered x-ray machines. Nearly one-half of

34
these are dental units. The rest are medical and a few industrial. In order to maintain the aforementioned inspection

frequency, we would be required to inspect

approximately 1,794 units per year. This

would take between 4 and 4.5 inspectors.

Oregon has authorized 4 inspectors but

funded only 2; consequently, less than

one-half of the required number of inspections will be made, mostly on recently

installed units or at hospitals and other

facilities requiring Medicare or hospital

accreditation. It should be mentioned that

there are some other critical areas of an inspection which take time but are very

significant in reducing public exposure to unnecessary radiation. Fluoroscope output

and film processing techniques must be

thoroughly explained to those persons who

are not adequately trained in the use of

x-rays, which we find is usually the case.

Radioactive materials inspections differ significantly from other types of

radiation inspections in the diversity of

use of materials especially in industry and

research, as well as the very rapidly

growing medical uses of radioactive

material. Safety practices are generally

better than with other forms of radiation.

One saving factor is that the degree of

training required for the user far exceeds

the training received by other users of

radiation except perhaps radiologists.

This can also be attributed to the fact

that applications for a radioactive

materials license are completely and

thoroughly reviewed in the licensing process. Source strength ranges from kilocurie quantities in medical therapy and

industry through research uses to microcurie quantities for in vitro medical

diagnostic tests. The inspectors themselves have received extensive training,

both on the job and by special courses

offered to Agreement States by the U.S. Nuclear Regulatory Commission. The range

of instrumentation necessary includes

neutron, alpha, soft beta and beta-gamma

survey meters plus instruments for air

flow measurements on hoods used for

non-sealed sources. Our program for radioactive material has 184 licenses. This

requires more than 200 amendments, 80 inspections and 30 investigations each year.

This requires two men and the assistance of

several others in the program during investigations.

Lasers and microwaves are almost

totally foreign to state radiation inspectors. Although considerable discussion has

35

been had on the subject and several state programs have legislative authority, they

lack adequate background to adjudge some- thing safe or unsafe. A number of guidelines have been set, but they differ

considerably from one federal agency to another. One of the few positive approaches

was made by the FDA Bureau of Radiological

Health when they made microwave survey

instrument comparisons and came up with two that were acceptable. Fortunately, these

differed widely in price. We have yet to see any such work on lasers and can only

guess at the appropriate instrumentation or maximum permissible exposure. Perhaps these

non-ionizing sources are better off left to any other kind of regulatory entity than state radiation control programs.

The individual states' radiation control programs are the only method the

United States has for making the measurements necessary to assure radiation protection.

What should the states have to make

the radiation measurement system operate

properly?

1. An independent evaluation of available

instrumentation to determine relative

merits for different uses and designs

for instruments if none of these meet

all the needs.

2. National calibration facilities for all types of sources and instruments and

provision of transfer standards and

Qual ity Assurance.

3. Assistance of equipping an inexpensive

in-house "quick check" facility to determine reliability of in-use

equipment.

4. Standardized procedures for all types

of radiation measurements.

5. An annual peer review on all facets

of the State's radiation program like

those currently done by the NRC to help upgrade each program.

6. Totally funded continuing education

like those provided by NRC which aid

the new employee and sharpen the

experienced employee.

7. A single federal agency responsible

for radiation protection to provide

the assistance cited in 1 through 6 above.
These recommendations are currently

the only visible means of assuring

uniformity of radiation measurements from state to state.

This data from the real world of the states could be utilized to reach national

decisions on radiation protection. Isn't that what the radiation measurement system

is all about?

References

1. Conference of Radiation Control Program

Directors 7th Annual Meeting, Hyannis,

Massachusetts, April 28 - May 2, 1975.

2. DeVore, R.T. "Diagnostic X-Rays: How

Safe Are They?" FDA Consumer,

June 1973.

3. Boothe, G.F., K.L. Johnston, and

E.A. Ranieri, unpublished data.

36
NBS SP456 (1976)

IONIZATION CHAMBER FOR ABSORBED-DOSE CALIBRATION

J. S. Pruitt and R. Loevinger

National Bureau of Standards

Washington, D.C. 20234

A cavity ionization chamber has been constructed for in-phantom

calibration in terms of absorbed dose. Its principle characteristics are the follov/ing: a smooth external shape, allowing

insertion in the phantom without unnecessary air gaps; a large

collecting electrode and a small internal air gap (1.3 mm), to meet Bragg-Gray cavity requirements; a uniform collecting potential

gradient, allowing high potential gradients and low recombination

loss; a guard electrode design which results in a stabilization

time of a few seconds; construction of all three electrodes from

pure graphite; and a day-to-day precision of about 0.1%. The

chambers have been used to make the high-precision depth-dose

measurements in water and graphite needed to transfer calorimetric

absorbed-dose calibration in graphite to absorbed-dose calibration

in water.

(Absorbed dose; ionization chamber; graphite chamber; cavity chamber;

calibration)

Introduction

This paper concerns cavity ionization

chambers designed specifically for in-phantom

measurements in high-energy beams. They have

been successfully used as transfer instruments, to facilitate the transfer of calorimetric absorbed-dose measurements from a graphite phantom to a water phantom.

The two types of chambers described are

3-terminal graphite chambers, differing

principally in the shape of the air cavity.

The first has a thimble-shaped cavity and

constitutes the chief justification for this

report. It is a stable, high-precision instrument, capable of measurements with a long

term precision of 0.1%. The second type has

a simpler cavity, a cylindrical annulus.

Its performance has been equally satisfactory

in some respects, less satisfactory in

others.

The Graphite Thimble Chamber

Figure 1 is a schematic cross-section of

the thimble-type chamber. The polarizing,

collecting, and guard electrodes are all made

of graphite with a density of 1.79 g/cm^.

The high-potential and guard insulators are made of acrylic plastic, with a 0.08 mm Mylar

washer separating the guard and collecting

electrodes in the air cavity. This small

separation minimizes distortion of the electric field around the edge of the collecting

volume, and minimizes accumulation of surface

charge during irradiation. The components

are assembled with epoxy resin, which fills

extra-cameral volumes except for air bubbles.

37

The resin forms an intimate bond between

electrodes and insulators, removing the

need to coat insulator surfaces with a con- ducting material

.

Electrodes

[\^ graphite

Qcrylic Dimensions are in mm

I Mylar washer

Fig. 1. Schematic cross-section of a PLl

thimble ionization chamber.

When inserted in a cylindrical hole in

the phantom, the hole is filled with a graphite rod of the same diameter as the

chamber. The chamber walls are not quite

thick enough to provide electron equilibrium

for cobalt-60 gamma rays. About 2% of the

chamber current comes from electrons generated outside the cylindrical surface of the
chamber, while about comes from electrons

generated in the matching graphite rod.

The guard electrode in the thimble

chamber serves two purposes. It prevents

current leakage between the polarizing and

collecting electrodes, and it prevents the

electrostatic field in the collecting volume

from being influenced by changes in the

charge distribution on the high-potential

insulator. This feature reduces the chamber

stabilization time to a few seconds and makes

alternate measurements at both polarities

more convenient. The difference between

currents measured with positive and negative

polarities depends to a small extent on the

experimental situation, but is usually be- tween 0.1 and 0.5% of the average. It can

be made to disappear if all parts of the

chamber except the active volume are shielded

from radiation, but this is hardly feasible

in a phantom. In our practice all measurements with these chambers are averages of

positive and negative currents.

The collecting volume is 0,3 cm^ and the

air gap is 1.3 mm throughout, making it a good approximation to a Bragg-Gray cavity.

The collecting electrode has a relatively

large diameter, permitting the use of large

polarizing potentials with a resulting high

ion-collection efficiency. The ratio of the

potential gradient to the polarizing potential varies from 10.7 cm~^ to 5.8 cm~^ at the inner and outer surfaces, respectively,

of the hemispherical part of the cavity.

When the polarizing potential (j) = 60 V, the

minimum gradient is 350 V/cm, and when

(}> = 500 V, the maximum is 5400 V/cm. Within

this range of polarizing potentials, the

potential gradients are large enough so that

corrections for general recombination are negligible up to at least 10^ rad/s in a steady beam. These gradients are not large

enough to cause sparking, which usually

occurs at (j) = 1000 V. Sparking does not seem to damage the chambers, probably because

the position of maximum potential gradient

is far from the insulators.

where r and I^n are the chamber currents or

(p bU

the measured dose rates for polarizing potentials of is> and 60 V, respectively. The

constants have the values

A = 0.069 V

B = 1.3 X 10"5 V"i

C = 0.20 V2/nA

= 0.020 V2/(rad/s) = 2.0 V2/(Gy/s)

K is a normalizing factor which does not differ significantly from unity for dose rates up to 100 rad/s = 1 Gy/s. Recombination measurements with one of the thimble

chambers are compared with Eq. (1) in Fig. 2

1.00 »J>-^ rod/

r(<^) 1(60)

.99

1 % \\ 80

98

1

i

The three thimble chambers in operation

at present all have input impedances larger

than 10^^* and leakage currents of the order

of 10'^^ A, independent of polarity. Their

calibration factors in terms of absorbed dose

to water in a cobalt-60 beam are all close

to 10 rad/nC = 0.10 Gy/nC at STP. Their re- combination characteristics are identical and

can be summarized by the equation:

I

60

K(l A

+ B<t.) I 1 - 1)

Fig. 2. Recombination measurements for

thimble chamber PLl-10. The solid lines are predictions of the depicted equation,

The form of Eq. (1) is reminiscent of

equations used to calculate recombination

corrections to exposure determinations''', in

which the first bracketed term, which is

independent of exposure rate, is identified

with initial recombination, and the second

38
bracketed term with general recombination.

The new feature in Eq. (1) is the linear

term in <^ in the first bracket, which describes the rise at the left side of Fig. 2. It is only significant for (j) > 100 V, and is smaller than 1% for <i> as large as 500 V.

This linear term could represent the beginning of ion multiplication in the chamber,

enough to increase the current above the ex- pected value, but not enough to sustain a chain reaction leading to a spark. The in- crease in current appears with both polarities, but is more pronounced for a negative

polarizing electrode, as would be expected

from the higher mobility of electrons, which

probably constitute a significant fraction

of the negative charge-carrying particles.

For this chamber, Eq. (1) cannot be used

to describe recombination corrections in terms of I^, the current at infinite potential , as is usually done with exposure

measurements^, unless the potential is so low that B({) can be neglected. However, the

first bracket of Eq. (1) is not relevant to a calibrated chamber if it is used at the calibration potential. The correction for different dose rates is given by the second

bracket in Eq. (1).

The thimble chambers have been used to make extensive depth-dose measurements in

graphite and water phantoms which will be

reported elsewhere.

The Cylindrical Chamber

Three chambers have been constructed as shown in Fig. 3, with cylindrical symmetry

and two guard electrodes. The purpose of

this change was to simplify the shape of

the air cavity and make it more amenable

to mathematical analysis. The results to date are somewhat equivocal, probably because

the assembly of a cylindrical chamber is con- siderably more complicated than for a thimble

chamber. The input resistances are lower

(10^°-10^^ q) and the leakage currents are larger ( lO'i't-lO'i^ a). Nevertheless, the

leakage currents are independent of polarity,

and the averaging of positive and negative

polarity measurements appears to correct

accurately for leakage. The depth-dose mea- surements made with one of these cylindrical

chambers, which had a leakage current of

3 X 10"^ 3 A, are indistinguishable from those

made with the better-behaved thimble chambers.

The recombination characteristics also appear

to be similar, at least at low dose rates.

Electrodes

\\ \j graphi+e

\//(/\ acrylic

I Mylar washer

Dimensions are in mm

Fig. 3. Schematic cross-section of a PL2

cylindrical ionization chamber. Dimensions

not shown are the same as for a PLl thimble

chamber.

References

1. See, for instance, T. P. Loftus and J. T. Weaver, "Standardization of ^°Co and is'^Cs Gamma-Ray Beams in Terms of Exposure," J. Res. Nat. Bur. Stand. 78A, 465-476 (1974).

Acknowledgement

This work was supported in part by the National Cancer Institute.

39

NBS SP456 (1976)THE DEVELOPMENT AND APPLICATION OF MONOENERGETIC X-RAY SOURCES

J. H. Sparrow and C. E. Dick

National Bureau of Standards

Washington, D. C. 20234

High flux standard x-ray beams have been developed in the energy

range from 1- to 100-keV. These beams which are produced by di- rect excitation of characteristic x rays in elemental targets

are useful for the investigation of phenomena associated with

radiological imaging and the calibration of radiation detectors.

The design and performance of the sources and some applications

to radiation measurement systems will be discussed.

(Detector calibrations; fluorescence x-ray sources; K x-ray sources; pulsed x rays; radiographic imaging; steady-state x rays)

With the current emphasis on product evaluation and safety, nondestructive examination,

and the need to develop new energy sources,

the use of x rays is increasing rapidly. They

are being employed not only in the conventional industrial and medical radiographic

applications but in new techniques such as x-ray fluorescence analysis and as diagnostic

tools in plasma fusion research. With all of

these applications, however, a common need

arises for a source of variable energy mono- energetic beams of x rays for the examination

of the energy response of systems and for

calibrations of x-ray detector responses.

Historically, this need for monoenergetic

x-ray sources has been filled by the use of

radioactive isotopies or filtered bremsstrahlung spectra. Radioactive sources, however,

are severely limited in their usefulness be- cause of the limited number of lines available (particularly, in the energy range from

1 keV to 100 keV) and because of the limited

intensity range in small sources due to selfabsorption effects. Filtered bremsstrahl ung spectra do not suffer from the drawback of

limited intensity, but do not in many instances

provide the degree of monochromaticity necessary for the characterization of energy re- sponse required.

Monoenergetic x-ray beams from 1-100 keV

can be produced by the excitation of characteristic x-ray lines. These lines can be ex- cited by the interaction of either photons or charged particles in elemental targets. The

photon excitation or fluorescence method has

been widely utilized and is shown schematically in Fig. 1. Electrons produced by a heated tungsten filament are focussed on a cooled anode assembly. X rays and bremsstrahlunq generated in the anode are then allowed

to strike a secondary fluorescent radiator.

Because of the nature of the photoelectric

process, the x rays fluoresced in the secondary target are very nearly monoenergetic

X-RAY TARGET

POST FILTER

Fig. 1. Fluorescence method

(typically, for these sources, greater than

90°^ of the intensity is in the characteristic

line of interest).^ The principal drawback

of this source lies in the two-step process

involved. To achieve high output intensities,

the primary anode requires cooling which com- plicates changing of the targets and altering

of the x-ray energy. Because of these limitations, the intensities of fluorescent sources

are limited to values below about 10^° K

X rays per steradian per second.

In order to obtain greater flux densities and facilitate changes in the x-ray

energy, we have developed a source which

utilizes only the first step of the fluor- escence procedure, direct electron excitation

of characteristic lines in elemental targets.

Figure 2 illustrates the characteristic K line

and bremsstrahl unq angular distribution for

a thin silver target for two different electron energies. Note that the bremsstrahl ung

is emitted preferentially in the direction

of the incident electron momentum vector,

while the K x rays are emitted isotropically.

This suggests that with a proper choice of observation angles (near 180° with respect to the

electron direction) the ratio of K x ray to
SILVER

THIN TARGET

d. The X-ray beams can be optimized

for either maximum output or maximum purity

by a simple adjustment of the experimental

parameters

e. The output can be extremely well

characterized by a measurement of the electron

beam current and energy.

Figure 3 illustrates the K x-ray output

as a function of the incident electron energy

for a variety of targets whose K x rays range

from 0.1 to 70 keV. These yield curves are used to determine the flux density for a par- ticular combination of electron energy

and target material. Figure 4 gives a schemat

Fig. 2. Comparison of the angular distribution of K x rays (solid

lines) and bremsstrahl ung (dashed

lines) for 50 and 500 keV elec- trons incident on thin silver

targets

bremsstrahl ung output can be maximized. To investigate whether this ratio can be made

sufficiently large for thick targets to provide adequate beam purities, an extensive ex- perimental study of this process was undertaken. In these experiments, x-ray targets

ranging in atomic number from beryllium to gold were bombarded with electrons having

energies between 10 keV and 3 MeV from two

low energy NBS electron accelerators. The

x-ray spectra generated at 180 were recorded

as a function of the x-ray target thickness

with various photon spectrometers. The re- sults of this study have been published^ and

can be summarized as follows:

a. The ratio of the K x ray to bremmsstrahlunq intensities can be made large enough

to provide beam purities in excess of 60% for

the entire range of energies from 0.1 to 70 keV.

b. This ratio is essentially independent

of target thickness so that rugged easily

handled targets are feasible.

c. The total output is determined by the

current loading capacity of the target and even uncooled targets can provide flux densities of greater than 10^° K x rays per ste- radian per second.

1—1—I I 1 1 1

0.01 0.1 1.0 INCIDENT ELECTRON ENERGY, MeV

Fig. 3. Dependence of K x-ray yields for

thick targets of Z from 4 to 79 on incident electron energy.

TARGET

LADDER ^ ASSEMBLY

APERTURE a VACUUM WINDOW

BERYLLIUM BEAM STOP

X-RAY TARGETS

^TARGET CHAMBER

'ELECTROMAGNET

Fig. 4. Diagram of K x-ray source at 180°

geometry.

42
S^VEEP MAGNET

LEADSHIEIDING- ANODE & TARGET BLOCK - TO VACUUM PUMP

Fig. 5. Pulsed K x-ray source

representation of the steady-state setup used

for the measurements and as an x-ray source. Applications of this source will be described

below.

In order to provide higher flux densities

(up to 10^2 K x rays/sr/s), a pulsed source

has also been developed utilizing the concepts

described above. Figure 5 depicts this source

in which an intense electron beam generated by

a high voltage pulser impinges on an elemental

x-ray target. Typically, kiloamperes of beam

current are delivered to the target in a few

nanoseconds thereby circumventing the target

heat loading problems associated with high

current steady state sources. Four identical

ports in an 120 geometry are equally spaced

radially around the diode axis. This setup

has been designed primarily as a calibration

source for the calibration of radiation de- tectors utilized in high flux rate environments. In operation, a parallel plate ionization chamber developed at NBS to measure

the pulsed x-ray flux density is mounted on one of the four ports to measure the absolute

output for each output pulse. This detector

has been calibrated on the steady-state source described previously and its linearity with

flux density has also been measured.'* Detectors to be calibrated are placed on the other

ports and compared to the standard detector.

At present, this setup provides pulsed x-ray

beams from 1.5 to 8 keV.

With the availability of monoenergetic

x-ray sources with energies from 0.1 to 70 keV

and flux densities up to 10^® K x rays/sr/s,

a number of experiments utilizing these beams

are underway at NBS, These experiments give

some indication of the utility of such sources

in a number of areas. The pulsed source has

been used to calibrate a number of detectors.

The calibration of a PIN solid state detector is shown in Fig. 6. These diodes are —I 1 1 1 1 1 r

~ SOLID SIfiTE DETECTOri RESPONSE

X RAY ENERGY, kcv

Fig. 6. The energy dependence of a PIN silicon diode as compared to calculated responses from energy

absorbed in silicon.

utilized in the measurement of soft x rays below 10 keV in the diagnostics of plasma

fusion devices. The shape of the x-ray spec- trum can be related to the electron temperature in the plasma, a parameter of prime

43
Fig. 7. Experimental arrangement for scatter measurements

importance in these experiments. Figure 6 illustrates the behavior of the diode as measured (open circles) and calculated

(solid line) in this energy range. The calculated values are based on the mass attenuation coefficients and are corrected for deadlayer effects. Notice the variation in the

response of this device from the calculated

curve especially at low energies. This can lead to a significant error in the shape of

the x-ray spectrum. This diode is designed

for operation in high intensity environments

and has a large steady-state leakage current

which makes calibration in steady-state x-ray

beams an extremely difficult procedure.

The steady-state x-ray sources are particularly well suited for measurements in radiographic imaging. Typical of these experiments

is the measurement of the amount of scatter

introduced into radiographic image signals as the x rays pass through the object to be radiographed. This scatter is the principal source

of loss of contrast and unsharpness in radiographs. ^ Figure 7 shows the setup utilized

to measure this quantity. A beam of mono- energetic X rays impinges on a scatterer of

polystyrene. The x-ray spectrum which is transmitted through the phantom is recorded

by a scintillation spectrometer. The amount

of scatter present in the image signal can be

determined from measurements of the spectra

recorded as a function of the solid angle, fi^. Figure 8 shows the buildup of the scatter con- tribution to the image signal for a 10 cm thick and for a 21 cm thick phantom for x-ray

energies of 32 and 69 keV. These data have

been reported fully elsewhere.^

1 1 — W • 25 cm

1 1 1 1 ^.32 keV —* * *

— 21 em

69 keV /V ^

S

32 keV_j-^__j3-Q

10 cm 69keV„

1

I 2 3 4 S SCATTER SOLID ANGLE. STERADIANS

Fig. 8. Dependence of the build up factor,

B, on scatter solid angle, f2 , for

a 21- and 10-cm. phantoms ana for

incident x-ray energies of 32 and

69 keV.

In conclusion, we have developed highly

monoenergetic x-ray sources in the energy

range from 0.1 to 70 keV and with flux densities up to 10^^ x rays/sr/s. These x rays are extremely useful in the calibration of

detectors as a function of energy and flux

rate and for any system whose energy dependence in this energy range must be ascertained.

44
Reference

1. J.L. Gaines, P.J, Ebert, and G.R. Leipelt, 4. J.H. Sparrow and C.E. Dick, Pulsed High

Lawrence Livermore Laboratory Memorandum Intensity Monoenergetic Low Energy X-Ray

UOPAD 67-107 (1967). Source and X-ray Monitor, to be

publ ished.

2. O.W. Motz, C.E. Dick, A.C. Lucas, R.C. Placious, and J.H. Sparrow, J. Appl . Phys.

42 2131-2133 (1971 ).

3. C.E. Dick, A.C. Lucas, J.W. Motz, R.C. Placious, and J.H. Sparrow, J. Appl. Phys.

44, 815-826 (Feb 1973).

5. Examples of such studies, which include

references to other pertinent investigations, are given by G. Hettinger, Acta

Radiol. 54 129 (1960) and E.D. Trout

and J. P. Kelly, Radiology 104, 161 (1972).

6. J.W. Motz and C.E. Dick, Medical Phys.

2, No. 5, 259-257 (1975).

45

NBS SP456 (1976)

THE DESIGN OF A MOBILE RADIATION CALIBRATION LABORATORY

R. C. Placious and J. C. Humphreys ' : National Bureau of Standards

Washington, D.C. 20234

A mobile x and y-ray standards laboratory with facilities

for calibration and testing of radiation instruments has

been proposed as an effective method for propagating the

national standards for exposure directly to state agencies

responsible for local radiation control. NBS studies indicate that this can be accomplished by equipping a trailer

truck with various sources of radiation, reference class

instrumentation, and power generating equipment. The

trailer described here incorporates radiation shielding

which will be optimized in relation to beam collimation,

direction, scattering, and maximum weight. The design

provides all proper safeguards prescribed by federal or state agencies for transport over the nation's highways.

(Mobile laboratory; radiation calibrations; radiation control; radiation measurement assurance; radiation standards)

Introduction

A mobile ionizing-radiation calibration

facility has been proposed for calibrating

instruments for state and local agencies at their laboratories. This mobile laboratory,

designed by and constructed under National

Bureau of Standards (NBS) supervision,

would further the national goal of radiation

measurement uniformity throughout the United

States. According to recent NBS surveys few states at present maintain a calibration

range for field or laboratory type radiation

measuring instruments. An instrument to be

checked or calibrated is sent to NBS, BRH (Bureau of Radiological Health), or private

calibration laboratories. Because of costs

and time delays, this generally means that

only instruments classed as reference or laboratory standards are calibrated. The

"turn around time" for these calibrations

can be quite long.

As presently viewed by NBS, based on needs expressed by a number of states, the more immediate goals of the mobile laboratory are:

1. To calibrate the principal radiation measuring instruments of the various

states.

2. To provide a tutorial or training

service on calibration and measurement procedures, and the establishment of suitable

calibration ranges.

3. To assist the states in solving unusual or unique radiation measurement problems that may arise in the course of their

v/ork.

A long range goal of the laboratory is to test the states' abilities to calibrate

their own radiation instruments as part of a voluntary measurement assurance program.

All of these goals can be most effectively accomplished with the aid of the mo- bile laboratory.

Design Considerations for a Mobile

Calibration Facility

General Requirements

At this stage the criteria established

for the mobile laboratory are:

a. The laboratory must meet all federal

and state safety requirements for transportation and operation of the facility.

b. The laboratory must be capable of

functioning without tying into the electrical

service and other facilities of the local or state agency.

c. Insofar as practical, the laboratory

should be designed to use commercially available equipment in its construction. The

laboratory itself should be engineered and

built by private contractors according to NBS specifications.

The Radiation Sources

Of primary importance is the decision as

to which radiation sources the laboratory

will require for calibrating radiation instru- ments. Based on available survey information

and personal contacts, it appears that an x-ray system capable of operating in the 25

to 300 kVcp range, and a higher energy, longlived gamma ray source will be needed.

47
The energy range of the x-ray system

places it in the same region as is used for

most medical diagnostic and radiographic pro- cedures. This selection is not merely coincidental since monitoring the radiation

levels of medical and dental x-ray units is one of the most important functions of the state radiation control agencies.

The x-ray source will have continuously

variable, well-regulated tube potential and

current, and would, if necessary, use both

end-grounded and center-grounded tube heads.

The tube current range should be from about

0.5 to 10 mA at 300 kV and 20 mA at 150 kV. These specifications are within the limits

of presently available systems. Water cooling for the tube heads would be necessary.

Based on measurements at NBS, such an x-ray

source will provide an exposure- rate range

from about 25 mR/h to 1000 R/h, using the same filtration presently used at the NBS x-ray standards laboratory and the distance

variation permitted by the length of the

calibration range.

Of the commonly used sources for higher

energy instrument calibrations, generally

^'^h'.s and ^"Co, the former was selected be- cause of its longer half life and reduced

shielding requirement. In addition, the

energy dependence of instruments does not change significantly in the region between

0.6 and 1.2 MeV. The specific exposure rate for '"Cs is about 0.32 R/h per curie at 1 meter. A source with activity of about 3.1

Ci is required to produce an exposure rate of 1 R/h at one meter and about 60 mR/h at

4 meters.

The x-ray tube head (or heads) and the

shielding cask holding the '^'Cs source

would be mounted "piggy-back" style on the same vertical axis and they would be alternately raised or lowered into position for

a calibration. (See Fig. 1.) A single track

MINIMUM-MAXIMUM

CALIBRATION POSITIONS

6 m

would extend down the center of the trailer

for the length of the measurement room. The

track will be designed so that the distance

between a source and the reference point on

an instrument being calibrated can be easily

determined with high accuracy.

Shielding Requirements and Beam Collimation

The geometry described above and shown

in Fig. 1 is designed to minimize scattered

radiation in the calibration area. The useful beam for each source must be collimated

so that no part of the primary beam strikes

the side walls, floor or ceiling. The rear

wall of the laboratory on which the direct

beam impinges is expected to be 6 meters

from the sources. The transverse crosssectional area of the laboratory is expected

to be about 2.5 x 2.5 meters. Therefore,

walls parallel to the beam axis will be about

1.25 meters from the center of the tra.ck. The maximum beam diameter at 6 meters from

the source with good collimation should not exceed about 1.5 meters. This requirement

indicates a beam cone of about 15° total

angle is required. Since most detectors

used in radiation measuring instruments do not exceed 10 cm diameters, the maximum solid

angle intercepted by the instrument being

calibrated is at most 6 degrees at 1 meter and

1.5 degrees at 4 meters from the source. Beam uniformity must be such that over the useful

beam size of 15° the variation in flux must

not exceed 1%.

Where shielding is required lead has been

chosen since it has the best shielding to volume ratio. Lead is also desirable since the

radiation albedo decreases with increasing

atomic number (see Fig. 2). In addition to the radiation shielding it may be necessary

to provide magnetic and radio frequency

shielding in the "skin" of the trailer itself.

Sound and thermal insulation will also be

needed. Fluorescence from the shielding material will be investigated and if significant, the proper material to reduce it without increasing albedo can be applied as an internal lamination to the lead wall.

The shielding thickness is graduated,

with the heaviest shielding on the back wall

opposite the source and on the wall separating

the control area from the radiation area. The

details of the radiation shielding are shown

in Fig. 3. The wall between the control area

and measurement area requires a minimum of 3 millimeters of lead to reduce the radiation

to the same level (2.5 mR/h) as the exterior

surfaces

Fig. 1. Source and detector arrangement in the mobile laboratory.

48
\ \ '

~

"i. w w

A \\ V K\

i O £5=0.? VeV a £>. = 0-='-'^V

A fj,= 2.0V.eV

1

1

1 ^CONCRETE (Z.,„,

1

1 50 70

Z, ATCMiC NUMBER

SO

Fig. 2. Energy albedo of monochromatic photons

with initial energies from 0.2 to 2.0

MeV normally incident on semi infinite

media of different atomic number (from

Berger and Raso)^

WORK

AREA

CONSOLE

[motor fiFN.'~' '(SLIDES OUT)'

STORAGE / '1

1

Laboratory Facilities and Services

Electrical service vill be required in the lab to operate the x-ray set, general

lighting, instrumentation, and air conditioning. The power required should be provided

by a self-contained motor-generator system

with the alternative provision that power may

be obtained by cable where adecuate local ac service is available. The air-conditioning

system would provide both cooling and heating

capabilities as well as control of the relative humidity. The entire laboratory should

be thermally wel 1 -insul ated to minimize the

power required for air conditioning. The

system should be capable of maintaining the

entire lab at a given temperature within ±1 C and the relative humidity to ±5%. The x-ray

unit will require a closed water cooling

system for the tubehead with provision for

heat exchange using the laboratory air con- ditioning unit or its own water cooler.

The following table indicates the estimated electrical power needs.

Table 1. Estimated Electrical Service Needed

AC Power Required

240 V ac, 7 kW

240 V ac, 15 kW

Item X-ray Unit

Air Temperature Control

(Heating & Cooling)

Motorized Tube Head-Source 240 V ac, 1 kW Elevator & Shutter Mechanism

Lighting, Electronics, etc. 120 V ac, 5 kW

Item 2 includes the 3 kW of power needed

for cooling the x-ray tube head. Total power

needs are, therefore, about 28 kW. This can

1.5 m m Pb

.3mm Pb m Pb

H.V. GEN.

RADIATION

AREA

SLIDING.

DOOR

BEAM SIZE

( m ox.)

\0^^^

Fig. 3. Plan view of the mobile laboratory showing the

shielding arrangement surrounding the radiation

area. 49
best be handled (perhaps) by two motor generators rather than one large one. It is assumed that most states will not be able to supply this much power from their laboratory

lines so the motor generator system should

have a high usage factor.

Overall Layout and Weight Estimates of

Laboratory

The overall layout is shown in Fig. 3. The radiation room occupies an area of about

7 by 2.5 meters wide x 2.5 meters high and is expected to house the x-ray generators, the

water cooler and circulator, the x-ray tube

head and the ^^^Cs source with its container.

Access to this area is through a shielded

sliding door properly interlocked and monitored for safety. The control area of about

3 x 2.5 meters wide x 2.5 meters high will

contain all controls for operating the x-ray

unit or the shutter for the source as well as

a data acquisition and processing system. A cable rack properly shielded to prevent radiation leakage into the control room area will

provide the means for sending and receiving in

formation between control room and radiation

room.

Some of the estimated weights and sizes

of the equipment to be installed in the mo- bile facility are given in table 2. Noise

and vibration from the air conditioning units,

the water cooler or the motor generator set must be reduced to a very low level because

of the sensitive electronic equipment used in this facility. This may require, for example,

that these items be so designed that during

operation they can be acoustically and mechani

cally insulated from the trailer (e.g., by out boarding on tracks)

Table 2. Size and Weight of Laboratory Equipment and Shielding

Item

1. X-Ray Units

(a) Generators (2)

(b) Control Unit

(c) Tube Heads

(d) Water Cooler

2. Isotope Source

3. Lead Shielding

Forward Wall (2.5 cm)

Side Walls (2)

Back Wall

4. Electrical Generator

5. Air Conditioning System

6. Control Console

w/Instrumentation & Work Bench

Estimated

Floor Space

Needed

^ (Meters)

2 1/2

1 1/4

1/2

See Figure 3

1

1

2

Estimated

Weight

560

40

90

80

180

4100

700

300

600

Total 6650 kg

50
Choice of Vehicle for Laboratory

Consideration v^as given to two types of

vehicles for the mobile laboratory; the

i recreational van such as many states and

I federal agencies use for environnental monitoring and other functions or a tractor-

] trailer type truck. The recreational type

van comes close to meeting the length requireI ments but the largest has a load-carrying caI pacity that is a factor of three too lov;. It would appear that a truck is the only practiI cal solution. A representative of one of the

major tractor manufacturers suggested that

the tractor-trailer combination vyould be the

best arrangement for the load range (20,000

lb.) the lab would require. One standard

size trailer in this range is 12 m (40 feet)

long, 2.5 m (8 feet) high, and 2.5 m (8 feet)

j wide. This trailer has a mass of 5000 kg

' (11,000 lb.), so a suitable tractor would need

a "gross vehicle weight rating" of at least

14,000 kg (31,000 lb.). This is no problem

j for existing tractors. Because of the delicate nature of the equipment in the trailer

an air suspension shock prevention system

will be prescribed.

Calibration Standard

The mobile laboratory calibration standard

will be a transfer quality ionization chamber

instrument, of high precision, calibrated

against the national standard maintained at NBS.^ As indicated in reference 2, this instrument would probably be of guard ring design with low leakage and high consistency in

exposure rate measurements. In all likelihood, two transfer quality instruments would

I', be provided to cover both the high and low

[ energy range. The redundancy necessary in a mobile laboratory dictates that each transfer

quality instrument be provided with a back-up,

v/hich may or may not be of the same type.

All standards maintained in the mobile laboratory will need to have a careful check on

I their energy dependence over the range from

I 25 keV to 1.2 MeV.

Operational Considerations

It is expected that two people will be

required to man the mobile laboratory in the

I field - one a physicist-supervisor-instructor,

( the other a driver-technician-laboratory

[ operator. They would spend an average of

three days out of the workweek at one site

calibrating and the other two days in travel, set-up and routine checks and maintenance. Considering servicing, vacations,

illnesses, etc., it seems reasonable to assume 40 places visited in one working

year. This means all states could be

covered about once every two years. Most

likely Hawaii and Alaska would be required

to send or bring their instruments to the

nearest calibration facility on the

American continent--perhaps California

or the state of Washington.

Concl usion

A properly designed mobile radiation

calibration laboratory, useful in the

national radiation measurement assurance

program, is most effectively realized in

the tractor-trailer truck design described

here

While the present operational scheme

has this facility traveling from state to state on a regularly scheduled basis, it

could also serve as the means for tying

"regional" calibration facilities to the

national standards should such facilities

be created. Also, if and when the states establish their own calibration ranges, the

mobile laboratory could not only serve as

an adequate design model but could be used

to check the effectiveness and accuracy

of the state calibration ranges.

On the basis of these potential

benefits to the nation, the utility of the

mobile facility seems to be proven.

References

1. M.J. Berger and D.J. Raso, Radiation

Research U, 20 (1950) Also, NBS

Report 5982 (Feb 8, 1960)

2. NBS private communication with T.P. Loftus

51

NBS SP456 (1976)

AUTOMATED CALIBRATION AND RECORDKEEPING SYSTEM FOR X-RAY SURVEY

INSTRUMENTS

Thomas R. Ohlhaber, Frederick J. Prevo, Ph.D., and Thomas L. Miller

Bureau of Radiological Health

Division of Electronic Products

Rockville, Maryland 20852

A system for monitoring and controlling the calibration process for

x-ray survey meters has been designed. A minicomputer with a realtime operating system and on-line disk storage provides the

acquisition, reduction, and control parameters for the process and

also maintains the recordkeeping for instruments dispersed over the

country. The system is designed to rapidly provide information to multiple terminals about the calibration and availability status of

survey instruments in order to facilitate the use of large numbers

of instruments.

(Calibration; Recordkeeping; X-ray; Computer; Instruments; Automation)

Introduction

The administration and enforcement of

Public Law 90-602, the Radiation Control for

Health and Safety Act of 1968, is a primary concern of the Bureau of Radiological Health.

The implementation of performance standards

for electronic products which produce

x-radiation has been a result of Bureau

activities under the Act in addition to general requirements for equipment which is not specifically covered by a performance

standard.

The Bureau has also initiated cooperative proqrams which strive to reduce un- necessary exposure to manmade radiation

(Nationwide Evaluation of X-ray Trends,

Dental Exposure Normalization Technique).

These programs monitor the pooulation exposure and also aim at reduction of exposure

through assistance offered to the users of

x-ray equipment.

The support of the Act and the cooperative proqrams require large numbers of x-ray

measurement instruments consisting mainly of

battery operated portable survey meters.

Since these meters are used in the enforcement of a Federal law, the measurements

performed with these meters must be defensible in leaal actions and possibly court

actions. Therefore, the meters must be

calibrated against recognized standards and

all documentation of this process must be

available. The care exercised in the Bureau

is probably excessive for many of the meters

calibrated; however, all instruments must

possess well documented calibration records

if they are to be used in possible legal

actions

Typically, in a six month period, the

Bureau is responsible for calibrations on over 100 Victoreen Model 666's, 30 Victoreen

Model 555's, 30 Victoreen Model 440 RFC's,

20 MDH Model 101 5' s, 30 Victoreen Condensor

R-meter sets, assorted Low Energy Pocket

Chambers, a large number of thermoluminescent

dosimeters, and assorted other ionization

chamber type instruments. Where applicable,

each instrument is given an electrical check

of the readout electrometer, gross radiation

check, leakage check of integration

capacitor, response time check, and finally

is calibrated against a radiation standard.

It is obvious that a large number of instru- ments are involved and many parameters must

be recorded. In order to continue providing

calibration services for increasing numbers

of instruments and to maintain records and

inventory for these, it became necessary to computerize as much as possible of the

routine work.

Automated Calibration

The Calibration Process

All x-ray survey meters exhibit

readings which are affected in a non-linear

fashion by the energy of the incident x rays.

In addition, most meters are also affected

by temperature and pressure variations since

they are open air ionization chambers. For these reasons, the calibration process must

not only include information which correlates

the reading of the meter with true Roentgens but must also correlate the

response with specific energy, temperature,

and pressure conditions.

A typical x-ray beam contains photons

distributed over all energies up to the

maximum applied x-ray tube potential

53
(kilovol tage) . In our facility, the

combination of kilovoltage, first half-value

layer and second half-value layer is being

used for the x-ray beam quality (photon

energy distribution) specification.

A meter must be compared to a known

standard to determine the calibration correction. The standard used in this laboratory

is a Victoreen Instrument Co., Model 41 5A Tranfer Chamber with a readout composed of a Keithley Instruments Inc., 216 Electrometer

and a General Radio Co., type 1403 Standard

Air Capacitor. Since this system is designed to measure integrated charge from the

transfer chamber, exposure time information

must also be obtained. To secure this

information, an air operated shutter was designed to turn the beam on and off to provide accurate time information.

Since the comparison of the standard

and the meter may be performed in sequence,

the x-ray tube output must be stable. The

best indicators of stability of the tube

(without perturbing the radiation beam with

another instrument) are the high voltage

(kV) and the x-ray tube current (mA)

Table 1 lists the parameters entering into a cal ibration

The first five items are essentially

constants. Kilovoltage and tube current are monitored and controlled. Temperature and

pressure readings are used to correct the

instrument readings. The three remaining

items are the primary variables used to compute the correction factor.

The simplest calibration which can be

performed is a single point calibration.

This entails the comparison of the meter to the standard with a single x-ray beam. In practice, most meters must be calibrated

against a variety of x-ray beams since they

are used at more than one energy.

By changing the kilovoltage and the

filters in the x-ray beam, the entire process can be repeated for different sets of

beam parameters.

Choice of Control System

It becomes evident that a large number

of parameters are involved in the calibration of a meter. If all of these quantities

are recorded and monitored by hand, the

possibility of error increases while throughput decreases. Because of this, it was decided to automate as much of the process

as feasible.

Table 1. Parameters of Importance for a Ca1 ibration

First HVL at calibration position

Second HVL at calibration position

Capacitance of standard capacitor

Correction factor of transfer chamber

Correction factor of electrometer

Kilovoltage of x-ray tube (kV)

Stability of tube current (mA)

Temperature

Atmospheric pressure

Time of exposure

Electrometer reading

Survey meter reading

Calibration control systems in the past

have usually been run by talented and methodical individuals who monitored and controlled

the complete process. Our intent was not to replace these people but rather to provide

them with help in their more mundane tasks as well as to provide more salient information

than was available previously. A competent

operator can observe many types of errors in

a system but is usually incapable of recognizing slow changes in the quality of his

calibrations. For these reasons we decided

to provide a system capable of monitoring the

entire calibration process. After a review

of available systems, it was decided that a system operating on a minicomputer was appropriate. Table 2 lists the equipment in this system. Since the system was intended

to provide meaningful information for the

operator and to speed up the collection and

reduction of data, it was designed to operate

in real time. The power of the computer is used to monitor the process and advise the

operator of trends or errors.

Equipment Changes Needed for Automatic

Contro l

Since the choice was made to use a minicomputer for control of the process, all equipment in the facility had to be made

compatible with the computer. A decision was also made to utilize equipment which could be

used manually if for any reason the automated

system would fail. Thus, the power supply

was fitted with digital kV and mA meters.

The shutter controller was built with digital

timers and comparators and with the ability

for computer control of the exposure time.

The electrometer is a digital device with a remote readout at the control panel

Temperature, pressure, and humidity are also

displayed digitally at the panel. The meter

under test is usually an analog type

instrument that cannot be interfaced directly

to the computer. However, television

54
Table 2. Digital Equipment Corporation

PDP 11/40 Computer System

Implementation of the Computer

Controlled System

PDP 11/40 processor with floating point

and extended instruction set 48 K core memory

Cartridge disk drives, 4.8 million

words total storage

Line printer, 300 lines per minute

Assorted interfaces for data

acquisition and control

Printing terminals, one at each

calibration facility, one at the

computer console

CRT terminals, located at convenient

locations for inventory update

RSX-IIM real time operating system

monitors have been installed to read the

instrument and a controller was designed to turn the function knobs of the instrument

remotely. A terminal which has printing

capability is also provided for manual entry

of the meter reading.

We are presently modifying an instru- ment incorporating a digital electrometer

(MDH model 1015) to provide a digital

output that can be connected directly to the computer. If this survey meter is adopted for routine use, the necessity of

manually entering the reading will be

eliminated, therefore, providing additional

time savings.

Concept of the Calibration Process under

Computer Control

The calibration of a meter can now be

performed without manual intervention. For

each beam used the kilovoltage, first halfvalue layer, and second half-value layer

are stored in the computer. The

capacitance of the standard capacitor and

the correction factors of the electrometer

and transfer chamber are also stored in the

computer. The room temperature and pressure

can now be checked at each exposure, and more importantly, the stability of the x-ray

tube can be continuously monitored. The

computer can also check the sequential steps

of a calibration and therefore standardize

the process. The operator is free to observe the function of the meter under test

and needs only to read the instrument after

the exposure terminates. This savings of

time results from the fact that no numbers

are entered into the system except the

actual meter reading.

The flexibility of the computer based

system allows many modes of operation.

However, since the process of calibration

must be accurate and reliable, caution

must be exercised to insure that the

increased complexity of equipment does not degrade the performance of the system.

Therefore, the automation of this facility

is being performed in stages with extensive

tests performed at each stage to verify the

performance of the system.

The first stage is to use the computer

as a manual entry system assuring that each

of the systems are performing accurately

under manual control. At this stage, the

computer merely contains a program which

calculates the correction factors and types

a calibration report. Since calibrations were previously performed in this manner,

this provides a valid check on the

individual items of the system. At present, our old manual facility also uses this

program.

The second stage is to interface the

computer so that it can read all the

parameters of the process. The operator

still manually initiates all functions,

chooses beam parameters, and basically

controls the process. The computer is the

data accumulator, but minimal additional

tests are performed on the data. This

stage tests both the hardware and software

of data accumulation.

The third stage is to allow the

computer to perform an analysis of the process as related to its input and also as related to previous work. This quality

control function is probably the most important function of the system. The

operator now has information which informs

him on the quality of the work today as compared to all previous work.

The final stage is to program the

computer to control the actual process of

calibration. Caution must be exercised

here since some equipment (such as high

voltage supplies) must be very carefully

controlled because of the potential for

harm to the equipment or the operator. At

this point, the largest time saving is realized because all the routine button

pushing is performed by the computer while

the operator need only enter the survey

meter reading and monitor the quality of

the work. The goal of this system will be

realized when the computer can read the

55
meter as mentioned previously. Table 3. Electrometer Records

It is planned to provide for multiple

simultaneous calibrations. This has been

designed into the system along with auto- matic positioning of instruments in the

beam. Tests must be performed on beam

uniformity and cross-scatter before the

first system could be used. In any case,

the system will allow rapid change from one meter to the next with reduced calibration

time resulting.

Inventory Control

The use of large numbers of survey

meters in the field necessitates a system

of recordkeeping that provides accurate

information on the location and status of

any meter. It was decided to incorporate

this information on the same computer

system, since the calibration of the meter

and its status are pertinent to the use of

the meter.

Design of System

Several factors should be accessible

from a recordkeeping system if it is to be

used. Information about a single meter

should be retrievable by entering the model

number and serial number and information on the status of a class of instruments should

be available. Searches of this data base

should be possible for any parameter stored.

The updating of the information should be a simple, interactive process. Terminals

should be located in convenient places where

the information is generated, otherwise

updates will tend not to be entered.

Implementation of the Inventory System

Since this system is designed for

different types of survey meters, the form

of the records may vary by type of instrument. In general, meters are either selfcontained or are composed of a readout

(electrometer) and one or more sensors

(chambers). Usually, the chambers are interchangeable and thus must have their

own records. Table 3 shows a typical record

format for a survey meter, and Table 4 shows

the individual chamber records.

Cond usion

The system has been partially

implemented at present. All equipment is functional and tlie interfacing of sensors

to the computer is in process. The

programming of the computer to handle

acquisition and reduction of calibration

Model number of instrument

Serial number of instrument

Purchase date

System entry date

Date of last repair

Number of repairs on this instrument

Name of user issued to Address of user

Status Code

1. Returned for repair (to

manufacturer

2. Undergoing electrical check

3. Undergoing calibration

4. Ready for issue

5. Uncalibrated storage

6. Inhouse repair

7. Inhouse use

8. Issued

Use Code Number

1 . BRH Inhouse

2. Compliance EDRO

3. Compliance state contract

4. NEXT program

5. State

6. Other

Owner Code

1 . BRH

2. Proaram indicated in use code

Number of chambers mated with this

electrometer (0=Self-contained)

Model and serial number of each

mated chamber

Table 4. Chamber Records

Model number of chamber

Serial number of chamber

Purchase date

System entry date

Date of last repair

Number of repairs on this chamber

Last calibration date

Model and serial number of electrometer

mated with

Status Code

1. Mated with electrometer

2. Returned for repair with

el ectrometer

3. Returned for repair without

electrometer

4. Storage (unmated)

56
data is complete. The inventory control

phase of programming has been initiated.

Substantial time savings have already been

noted. As rapidly as possible, the

remainder of the system will be completed.

The system makes feasible the maintenance

of a population of instruments numbering in the hundreds, in active continuous use, geographically distributed across the

nation. It would be impractical to attack

this task with conventional techniques.

57

NBS SP456 (1976)

STANDARDIZATION OF A NATIOm^IDE GAMMA RADIATION CALIBRATION SYSTEM

E.F. Williams, Jr., P.V. Barrans and G.H. Brodie

RADEF Instrumentation Test Facility

VJashington, D.C. 20374

A standardized y radiation calibration system has been developed

and is presently being used to provide uniform y radiation fields

for the calibration of 500,000 y measuring, civil defense survey

instruments. The instrumentation, procedures, data handling techniques and secondary standards developed to make the system directly traceable to National Bureau of Standards are described and discussed. A simple test and evaluation of the total system is described.

The test data obtained appears to show that the true roentgen is

probably being provided to 48 maintenance locations with an accu- racy of better than ±6% and that the instruments can be calibrated

to within ±14% of the true exposure rate at one point on each detection range.

(y radiation, roentgen, exposure rate, instrument, Y response, ion

chamber, standard, geometry, calibration)

Introduct ion

The Defense Civil Preparedness Agency

(DCPA) of the Department of Defense has in

readiness a nationwide radiological monitoring system designed to measure significant

levels of radioactive fallout from a strategic nuclear attack on the United States.

The primary radiation measuring instrument

developed for this purpose is the CDV— 715

survey meter. This instrument uses a hermetically sealed, non-pressurized ion chamber for a detector and has four ranges of y exposure rate sensitivity; 0-0.50, 0-5.0,

0-50 and 0-500 R/h. The current from the

ion chamber develops a voltage across one of

four high megohm grid resistors, one for

each range. This voltage is applied to the

grid of an electrometer tube. A 50ijA indicating meter measures the current through

the electrometer tube and is calibrated

against the known y exposure rate with a separate variable shunt resistor for each

range on the instriament. There are %

500,000 of these instruments distributed

throughout the United States in monitoring

stations, fallout shelters and other emergency locations under the direct control of

the local and State Civil Defense Authorities. The individual States are responsible

for the maintenance and calibration of these

instrijments within their boundaries. This

program is implemented by 48 separate State

maintenance shops located within the 50 States and Puerto Rico. The device used to calibrate these instruments is the CDV-794

Y Radiation Calibrator with one or more

located in each shop. The calibrator contains ^ 130 Ci of IS^cs - 137b3 ^.n a sealed

source. The y radiation from this source is

collimated into a lead lined box containing

the instrument in a fixed geometry. By the

59

use of three tungsten attenuators, four dis

Crete Y exposure rates of ^ 0.40, 4.0, 40

and 400 R/h are produced at the instrument

ion chamber. The shunt resistors on the

instruments are adjusted by remote, mechani

cal means to produce the proper indication

on the 50yA meter of the CDV-715 instrument

on each range. The methods and instrumenta

tion used in this system to calibrate the

four Y exposure rates produced in each CDV794 Y Radiation Calibrator against the true

roentgen as determined by the National Bu- reau of Standards (NBS) is subject of this

report

Methods and Instrumentation

The primary secondary standard y radiation field against which this system is standardized is produced by a 490 Ci 13'^Cs

-'-37Ba sealed source. This source is collimated by a shield which produces a y beam in

free air 10 meters in useful length and

3.0 meters in diameter at its widest useful

portion. This Y radiation field is calibrated by feeding the ionization current

collected from a Shonka high energy ionization chamber^ into the input of a vibrating

reed electrometer and a standard air capaci

tor with a low capacitance cable. As the

capacitor charges, the electrometer meter

current closes a sensitive relay starting a counter connected to a 1 kc square wave fre

quency standard. The voltage on the capaci

tor is nulled with a precision voltage

source until the total voltage reaches a predetermined value and the relay turns off

the counter. The voltage collected is 10

times full scale to further reduce timing

errors due to variations in the relay closing current. The total count gives the
Shonka Chamber

High <-

Voltage

Sensitive

Relay

Vibrating

Reed

Electrometer

Start

Stop

Counter

Feedback

/

1 kc

Frequency

Std.

Standard Precision

Air <

Voltage

Capac iter Source

FIGURE NO. 1: BLOCK DIAGRAM OF THE SHONKA CHAMBER SYSTEM

exposure time in seconds and milliseconds.

A block diagram of the basic system is given

in Figure No. 1 above.

2 NBS has calibrated the Shonka chamber,

the voltage source, and the capacitors used

in this system. Point by point calibration

is accomplished by positioning the Shonka

chamber in the collimated y radiation field

and collecting ionization current at a number of points along the range. Radiation

intensity is calculated by the equation:

I = CV . CF . CF

t NBS STP

where: I = radiation intensity

CV = capacity X voltage

t = time in seconds

CFnbs = NBS Calibration Factor

CFsTP - Standard temperature

and pressure correction factors

To eliminate misinterpreting data from

graphs, the radiation intensities are con- verted to program cards for a programable

calculator. The date and desired exposure

rate are entered into the calculator and the

distance is calculated. The calculator used

is a rather basic one and in order to make

the program usable, some simplifying assumptions must be made. The first is the as- sumption that the y range follows inverse

square law. After correcting for air atten- uation and finding the effective source to detector distance for the source detector

geometry, inverse square law is good over a limited range. From 50 cm to a, 6 meters

this program gives distances to better than

1% agreement with measured exposure rates.

At a distance of 5 meters scatter becomes

significant and the program distances are

not used for this application. Overall

precision and accuracy for this total sys- tem which serves as the primary secondary

standard for all DCPA y calibrations is within +4% of the NBS values for the roentgen. To calibrate the exposure rates from

the CDV-79t+ calibrator against the 490 Ci

137cs - iSVjjg standard y field the CDV-765

Transfer Standard"^ was developed for DCPA. This device consists of an hermetically

sealed, unpressurized , ionization chamber

detector, electrometer integrator, and a digital readout, plus a logic and switching control system to permit automatic or semi-automatic operation. The ion chamber

detector has an identical geometry to the

standard CDV-715 ion chamber and is mounted

in a standard CDV-715 instrument case along

with a constant current tube which is used

as an operational check for the instrument

system. The ion chamber current is fed to

60
the input of a vibrating capacitor electrometer connecxed as a feedback integrator.

The chamber is connected by means of a 10

foor low capacity triaxial cable. The electrometer output voltage is digitally displayed at the end of the operational cycle.

The logic and sv.-itching circuits control "che electrometer input reed switches, select the

integration period and range capacitor,

operate the readout and provide an automatic

repeating cycle. Figure No. 2 below, is a block diagram of the CDV-755 system showing the interrelationship of the electrometer, digital readout, and control system.

It should be noted that the operation of

the standard consists of selecting the

desired sensitivity range and actuating the start button. The resx of the operational

cycle is fully automatic and cannot be

altered except by using the reset button

to interrupt an integrator cycle in progress. The time delay circuits must op- erate sequentially before a new integrator

start is possible, and any disruption of

their normal cycle only increases the total

delay time. This was programmed into the

control system to insure that the polystrene

integrator capacitor is completely discharg

ed before each new cycle is started to obtain consistent readings.

The CDV-765 instrument ma^' be operated

either by a single cycle with manual pushbutton restart , or may be operated with a completely automatic repeating cycle with a continuously updated reading.

If the master control switch is set on the low range and the exposure rate is 40.

0

R/h or greater, an over-voltage detector

circuit acts as soon as the electrometer

output voltage exceeds full scale and short

out the electrometer input circuit, dumping

the charge on the integrating capacitor and

protecting the circuits from jamming. The

control circuit then disables itself and

locks up, with an over range trip light

(ORT) to show what has occurred. Until the

reset button is pressed, no restart is possible. The CDV-755 is equipped with an alu

minum clad, styrofoam lined, plywood shipping case which holds the console, detector

cables and instruction manual and protects

it from rough handling during shipment.

Clock Enable - Disable

Clock

Control

Circuits

50 Hz ^

Synch

Sig

11:

Vac

60 Hz

JL

- Enable

Regulated

DC Power

Supplies

1

+12V

200

Binary

Counter

Ion Chamber

Sig

and In Check Source

Sto: Sig

Counter- Reset

Start - Stop

Control

Circuits

Preset

Count

Select St:3rt

Master Control

Sv;itch

and

Status Indicator

LariDS

Integrate

Capacitor

Select

Reed

Svjitch

if Controls

Modified

Vibrating Capacitor

Electrometer

Auto

Reset

Recycle

Time

Delay

Circuits

Ort

Loc Electrometer

Over Range

Trip

Circuit

S ig

Out

Digital

Voltmeter

Readout

R^ad

and

Hold

FIGURE NO. 2: BLOCK DIAGRAM OF THE CDV-765 TRANSFER STANDARD

61
TABLE I: VARIATION IN CALIBRATION BETWEEN TWO TRANSFER STANDARDS

Exposure

Rate CDV-794 Calibrator Serial Nos. Range 002 004^ 035^ 400 R/h 0 89% 1 27% 0 23%

40 R/h 1 35% 1 73% 0 73%

4.0 R/h 1 37% 1 89% 0 9 5% 0.4 R/h 1 57% 1 85% 0 03%

The CDV-765 is calibrated in the

standard -'-'^'^Cs - -'-^'''Ba Y field at exposure

rates of 0.40, 4.0, 40 and 400 R/h and the

output voltage recorded as a calibration

factor at each rate in the instruction

manual. This instrument is then shipped to

a State maintenance shop where it is placed

in the CDV-794 y Radiation Calibrator. The

four discrete y exposure rates provided by

the calibrator are measured with the transfer standard and are recorded. The exposure

rate values determined by the transfer

standard factors are then used by the State

shop to set the response of the CDV-715 ion

chamber instruments using the calibration

potent iometers

.

Evaluation of the System

In order to determine the maximum possible precision for transferring the NBS roentgen to these CDV-715 instruments, our Facility conducted a test and evaluation of each

step in the system. Two CDV-765 Transfer

Standards were calibrated in the 490 Ci

-'-'^'^Cs - 13'^Ba Y field using the procedure

described above. These two transfer standards were then used to calibrate the four

discrete y exposure rates from three CDV-794

calibrators. The variation between the ex- posure rate for each transfer standard was then compared. The results of this comparison in terms of percentage is given for each

discrete exposure rate for each calibrator

in Table I above. The maximum variation re- sponse between the two transfer standards was 1.89% and the smallest difference was 0.03%

for the 12 exposure rates evaluated. The mean variation between the determinations of the two standards was 1.15%.

Twenty CDV-715 instruments were then

calibrated against the exposure rates determined by one of the transfer standards

in CDV-794 y Radiation Calibrator, Serial

No. 035. These 20 instruments were then

placed in the other two CDV-794 calibrators

and their response was compared against

the exposure rate determined by the same transfer standard for the particular calibrator used. The variation between the

CDV-715 instruments y response and the ex- posure rate in the calibrator was determined. The mean y response of the 20 instruments and the maximum and minimum response for the 20 instruments for each

discrete exposure rate for both calibrators

is given in Table II. As given in the

data the largest variation for the 20 instruments over any of the eight discrete

exposure rates was 7.1%. The largest single

variation observed for one instrument was

+3.8%.

The 20 instruments were then placed

in the standard -^'^'^Cs - 13'^Ba y radiation

field at the exposure rates obtained in CDV-794 Serial No. 035 and their y response

compared with the primary exposure rate

value. The percentage of variation from

the primary exposure rate value is given

in Table III. The maximum variation in response for

the 20 instruments over any of the four

discrete exposure rates is 8.0% with the

largest single variation for one instrument

being -7.5%. It should also be noted that

in general the response of the CDV-715 instruments is less than the primary exposure

rate to which they were exposed; particularly at the 0.40 R/h and the 4.0 R/h rates.

TABLE II: VARIATION IN RESFONSE OF 20 CDV-715 INSTRUMENTS IN CDV-794

Instrument CDV-794 Calibrator Serial Nos. Calibration 002 ^ ^ 004

Exposure Mean Extremes of Mean Extrem.es of Rate Response Response Response Response

400 R/h + 0 s% + 3 3% to -3 0% + 1.0% + 3 8% to -3 3% 40 R/h + 0 3% + 2 6% to -1 3% + 0.7% + 2 9% to -1 9% 4.0 R/h + 0 4% + 2 6% to -1 3% + 0.7% + 2 5% to -2 5%

0. 40 R/h + 0 3% + 2 7% to -2 8% + 0.1% + 2 5% to -3 7%

62
TABLE III: VARIATION IN RESPONSE OF 20 CDV-715 INSTRUMENTS IN PRIMARY "y FIELD Primary Field

Exposure Rate Mean Response Extremes ot Response

400 R/h -0.9% +2.6% to -3.8%

4-0 R/h -1 . 4% +2.6% to -3.9%

4.0 R/h -3.5% +0.8% to -5.8%

0.40 R/h -4.3% +0.5% to -7.5%

This effect appears to be the result of

the CDV-765 Transfer Standard being somewhat less sensitive to low energy scatter

radiation than the CDV-715 instruments.

However, further investigation of this

phenomenon is necessary to determine the cause of this anomaly.

The variations that are additive

from this evaluation are the maximum variation betv/een transfer standards (1.9%),

the maximum variation in CDV-715 y re- sponse in the primary y field (7.5%) and

the accuracy and precision with v-fhich the

primary y beam compares with the NBS

roentgen (±4%). This would give a maxi- mum observable error overall of 13.4%.

The statistical validity of using 20

instruments to represent 'b 500,000 instruments and the use of three calibrators to represent 50 is somewhat open to question. However, these 20 instruments

were normal stock and typical of what is found in the State maintenance shop. It can be reasonably assumed that an accuracy

of +14% can be achieved at the calibration point on each range of sensitivity

with these instruments. The two transfer

standards represent a significant number

of instruments since eight instruments

are actually used in this program. Therefore, the variation of 2% betv/een standards

and the ±4% for the precision and accuracy

of the primary y field should permit the

transfer of the NBS roentgen to the CDV-794

calibrator within +6%.

References

1. F.H. Attix and W.C. Roesch, 1966

Radiation Dosimetry, Second Edition,

Volume II pp 54-55 (New York . London

:

Academic Press)

2. T.P. Loftus, Aug. 1973, Report of

Calibration, Shonka Ionization Chamber,

DS 1725/73, National Bureau of Standard

3. Victoreen Instrument Company, Final

Report for Contract No. OCD-PS-65-182

,

Defense Civil Preparedness Agency Pub.

63

NBS SP456 (1976)

CALIBRATION PROGRAM

RADIATION MEASUREMENT

Ahren Jacobson

University of Louisville Radiation Center

Louisville, Kentucky 40201

A RELIABLE

FOR IONIZING

The recent availability in this country of the N.P.L.

#2560 Secondary Standard instrument has afforded a new dimension to instrument calibration procedures. A

description is presented of the accuracy, stability,

and utility of the N.P.L. #2560 for calibration of

various instruments to measure a wide range of ionizing

radiation energies.

(Dosimetry; calibration; ionization chamber; secondary

standardization)

A principal reason for convening a symposium on "Measurements for

the Safe Use of Radiation" arises

from the acknowledged variability of

such measurements being made throughout the country. Although these

measurements are especially critical

in clinical applications of ionizing

radiations, there is also need for more reliable dosimetry in other

areas, such as radiation safety and

environmental monitoring procedures

.

The foundation of any calibration methodology must employ a reliable primary standardization whose

data can be utilized for secondary

and tertiary instrument calibration.

Primary standardization on a local

basis can be demanding, both cost

and ability-wise. To obtain this

precise work at national , regional

,

or commercial standardization laboratories involves a host of problems.

Turnaround time; fragility and bulk

management by suitable transportation

methods; and high cost are some of

the more difficult problems frequently encountered.

It appeared prudent to choose

the recent approach used in the

United Kingdom for assurance of uniformity of radiation therapy dose

measurement in all of their Regional

Therapy Centers. Their method involved use of an ionization chamber

specifically designed as a secondary

standard by the National Physical

Laboratory in England .

^ ^ This instrument has now become available

commercially* and we were fortunate

to have acquired one using grant

funds of the Regional Medical

Program**

.

* NPL Secondary Standard Therapy Level X-Ray Exposure Meter (#2560)

fabricated by Nuclear Enterprises Ltd., Reading, England.

** Ohio Valley Regional Medical Program project #42C: "Regional Dosimetry

Standards Development."

65
FIGURE 1

The 2560 dosimetry system con- sists of three basic components

shown in Figure 1: (1) the ionization

chamber, (2) measuring circuit,

(3) and stability check source. Long

term stability and excellent reproducibility and energy dependence

characteristics are claimed for the

precisely fabricated ionization chamber. It has a wall thickness of only

0.5 mm extremely pure extruded graphite that is remarkably durable. No

conducting coating on the inner collecting surface is necessary, thereby

eliminating the troublesome flaking

problem found in conventional ion

chambers. The chamber volume is

0.325 cm-^ and it contains an ultrapure hollow Al collecting electrode.

Having a hollow, low atomic number

central electrode minimizes "shadow"

effect at lower X-ray energies.

Another outstanding feature of the

ion chamber assembly is the incorporation of an amber insulator in the

stem. Besides its exceptionally high

electrical resistance quality, amber

is essentially air-equivalent. Ionization current is integrated by a highly stable capacitor in the feedback path of an electrometer operational amplifier, which is part of a Townsend balance voltage and is meas- ured potentiometrically by a 10 turn

heliopot. The compact Sr-90 stability check source includes a thermometer for temperature determination.

A reproducibility of ± 0.03 percent

is claimed for readings on this check

source. Such data is possible be- cause of the uniform radiation field

obtained from two concentric circular

foils of Sr-90 and a precision chamber positioning mechanism that assures reproducible irradiation geometry.

66
Nuclear Enterprises Ltd. (the

fabricator of the 2560) provided a primary standardization with the

instrument. In addition to this,

arrangements were made at NBS ^ ' to

have an independent primary standardization performed there before the

instrument was put into use. This

was accomplished for two X-ray

energies and Co-60. Also included

in the NBS standardization were

serial measurements on the Sr-90

stability check source supplied with

the 2560. Although the Nuclear

Enterprises primary standardization

dealt with a variety of X-ray energy

points and no Co-60 measurement, a comparison can be made of two sets

of data for similar X-ray energy

points from both laboratories. This

comparison is shown in Table 1:

TABLE 1

ri.E.. Ltd.

760mm Hg 20°C

2.0mm 7^1 H.V.L. !? 100 kVp

1.035

3.25min Cu H.V.L. (§ 250 kVp

1 .040

*************************************

fl.B.S.

760mm Kg 22°C

1. 86mjn Al H.V.L. g 75 kVp

1.04

3. 25mm Cu H.V.L. @ 250 kVp

1.05

Excellent agreement between

these standardizations, separated by

four months and the Atlantic Ocean,

bear out the manufacturer's claim of

reproducibility and long-term stability. Between primary standardizations, intermediate verification is

conveniently accomplished by use of

the Sr-90 stability check source. Although we have only had the

2560 at our institution since November 1975, it has been instrumental in

clearing up several dosimetry and

calibration problems. A low-energy

ion chamber, recently obtained from

a well-known manufacturer, was found

to have a substantial error in its

primary standardization by intercomparison with our 2560. The particular error resulted from the use of an erroneous correction factor in

the supplier's laboratory. This error was present in forty other instruments standardized for 30 other

customers in this country and overseas. During the four month period

that this error was propagated, none

of the other 30 customers was aware

of its presence until subsequent

notification by the supplier. Undoubtedly, we would have accepted the

low-energy standardization provided

to us as well, were it not for the

availability of the newly acquired

2560 system with its recent standardizations. Prior to acquiring the

2560 our reliance for constancy of

ion chamber response was on the use of a radium constancy source, which

of course, offers no absolute calibration. Coupled with this important

deficiency, is the fact that the

bulky radium constancy pot is not

sent along with the ion chambers for

primary standardization. It has been

essential to virtually "hand-carry"

the ion chambers to and from the

standardizing laboratory to minimize

chances of damage to the chambers.

The rugged and reasonably portable NPL 2560 system is expected to serve as an effective link between

the primary standardization laboratory and our institution. Intercomparisons can be made with reasonable facility. Our recent visit by

the Radiological Physics Center team

found the Co-60 intercomparison bet67
ween our 100 R high energy ion chamber and their instrument to be within 0.3% for several Co-60 exposures.

The calibration factor for our chamber had been determined by simultaneous exposure of it and the 2560 ion

chamber with build-up cap to large

field Co-60 gamma rays. Minimal stem

effect was determined for both chambers. Figure 2 shows a Co-60 beam

intercomparison of a Victoreen mediumenergy ion chamber with 4.5 mm lucite

build-up cap and the 2560 ion chamber

with its build-up cap. We use medium

energy ion chambers in snug fitting

cavities of polystyrene calibration

phantoms for routine calibration of

high energy machines. Therefore, it

is imperative that a Co-60 calibration of the medium energy chambers

be performed since we use C^ factors

on ion chamber data to determine

absorbed dose in rads . Another useful application of this dosimetry

system has been in the calibration of our Cs-137 and Ra-226 sources used

for survey instrument calibrations.

FIGURE 2

68
FIGURE 3

Figure 3 shows this use for a low

intensity beam irradiator that is

employed for some survey meter calibrations. In this instance the 2560

ion chamber and PTW chamber are both

measuring the Cs-137 beam for precise

calibration of the irradiator at a

20 cm S.S.D.

These examples of the versatile

application of the N.P.L. Secondary

Standard demonstrate its reliability

in bridging ionizing radiation dosimetry between regional user and the

primary standardization laboratory.

The promising initial results of the

finely designed and fabricated 2560

dosimetry system offer great hope for

accomplishing uniform instrument

calibrations locally. Intercomparisons on the local level are conven- iently accomplished, thereby ameliorating the time consuming primary

standardization problems regional

users have encountered in the past.

a) L.A.W. Kemp, Brit. J. Radiol., 4_5, 775 (1972).

(2) Dr. Robert Loevinger and T. R. Loftus were very cooperative in arranging

for primary standardization of this new instrument.

69

NBS SP-456 (1976)

AIF-NBS RADIOACTIVITY MEASUREMENTS ASSURANCE PROGRAM

FOR THE RADIOPHARMACEUTICAL INDUSTRY

R. Colle

Atomic Industrial Forum, Inc. Washington, D. C. 20014

and

National Bureau of Standards

Washington, D. C. 20234

The National Bureau of Standards supervises and administers on behalf

of the Atomic Industrial Forum (AIF) a radioactivity measurements

technology quality assurance program for the radiopharmaceutical

industry. The program provides for a monthly distribution of both

millicurie- and microcurie-level Standard Reference Materials to each

of eight participating commercial radiopharmaceutical manufacturers.

A number of the standards are distributed as "blind" samples to establish traceability to the national radioactivity measurements

system. In addition to identifying measurement discrepancies, assistance is provided to the participants in eliminating the sources of

difficulties, and in correcting errors in their measurement techniques.

In this report, results for the "bl ind "-sample traceability exercises

are given, and a discussion of identified sources of measurement errors

and recommendations are also presented.

(Assurance; measurement; radioactivity; radiopharmaceutical; standards;

traceability)

In August of 1973 at a s)miposium quite

similar to the present one, Seidel and

Brantley''' reviewed a number of serious questions and problems which faced commercial

radionuclide manufacturers as related to the

needs for radioactivity standards. Some of

the basic difficulties they cited were:

1) the lack of direct standards for approximately 757o of the more than 100 radionuclides produced by the industry at that time;

2) the sometimes inconvenient physical form

or activity levels of the available standards; and 3) the failure to have industrywide adopted decay-scheme data upon which to base derived standard instrument calibrations

in the absence of direct calibrations based

on national standards. Although the American

National Standards Institute (ANSI) Quality

Assurance Program which was described at that

time''' (the current status of which will be

the subject of the next paper^) addresses a number of these difficulties, it is concerned,

however, only with microcurie-level measurements and its participants include hospitals,

nuclear-power plants and industry. The

Atomic Industrial Forum (AIF) and the National Bureau of Standards (NBS) therefore

entered into an agreement on a research

associate program whereby NBS will supervise

and administer on behalf of AIF a measurements technology quality assurance program

which caters more specifically to the needs

of the radiopharmaceutical industry. Eight

71

radiopharmaceutical suppliers are participating in this program. Concurrently,

on the authority of an interagency agreement, NBS is providing the Food and Drug

Administration (FDA) with services similar

to those being given the AIF participants.

These participants are listed in Table 1. The purpose of the program is to insure the

continuous availability of national radioactivity standards at appropriate levels of

activity for use by the radiopharmaceutical

industry, and thus to establish a degree of

uniformity in the measurements throughout

the industry.

The major effort of the program consists of a monthly distribution of both

millicurie- and microcurie-level Standard

Reference Materials (SRM) to each of the

participants. With the exception of a 133 future Xe gaseous standard, all of the

SRMs are solution standards contained in

"5-ml" flame-sealed borosilicate glass

ampoules. Those which have been already

issued under the program are listed in

Table 2. The choice of radionuclides,

their activity levels as well as an order

of priority for them are selected by a steering committee representing each of the

AIF participants. Obviously these selections must be made in cooperation with NBS,

taking into consideration the compatibility
Table 1 a Parti c ipant s

Standards Program for the Radiopharmaceutical IndustryFood and Drug Administration

Bureau of Drugs

Division of Drug Chemistry

Washington, D.C.

Atomic Industrial Forum Participants

Atomic Energy of Canada, Ltd.

Ottawa, Canada

General Electric Co.

Pleasanton, California

Mallinckrodt Nuclear

St. Louis, Missouri

Me di-Phy s i c s , Inc.

Emeryville, California

Nev England Nuclear

North Billerica, Massachusetts

E.R. Squibb & Sons, Inc.

New Brunswick, New Jersey

Union Carbide Corp.

Tuxedo, New York

Amer sham/Searle Corp.^

Arlington Heights, Illinois

The order in which participants are listed in

this table does not correspond to the order in

which the results are listed in Table 3.

'Participation terminated November, 1975-

with their scheduling capabilities and

existing standard developments. This does

however provide a viable mechanism in which

the radiopharmaceutical industry can direct

requests to NBS to establish standards for

those radionuclides which are of prime

interest and major concern to them. As seen

in Table 2, the trend is certainly toward

shorter-lived radionuclides, reflecting the

needs of the nuclear-medicine community

which is served by the radiopharmaceutical

industry. Although the trend is "healthy,

particularly for patients, it does in itself

present difficulties in the preparation and

distribution of the standards. The millicurie-level standards made available by this

program, compared with the typically microcurie-level standards normally handled at

NBS, are of greater convenience and direct

value for instrument calibrations at activ

ity levels more closely matching those

found in the manufacturing process.

A second major aspect of the program

attempts to satisfy the increasing demands

of both consumers and regulatory agencies

to demonstrate "traceabillty " to the

national radioactivity measurements system

embodied in the NBS. Satisfaction of this

requirement also simultaneously fulfills

the manufacturers' goal of achieving an industry-wide uniformity in measurements.

Although the monthly distribution of stand

ards serves to insure a regular supply of

radioactivity standards to calibrate meas- uring equipment, the mere use of them does

not constitute traceabillty. Despite the term's increased usage lately, a clear

72
Tatle 2 a Standard Reference Materials

Issued Under the Standards Program for the Radiopharmaceutical Industry

RadioNuc li de

131-r

113 Sn

125-

57

85

75

198

59,

Co

Sr

Se

Au

Fe

99m

Tc

99Mo

Half-Life

(Days

)

27.7

8. 0l+

115 .

60.

1

271

.

61+.

9

120 .

Ik.

3

2 . 70

0.25

2.75

SRM NO

1+1+OOH kkOOL

4U01H

i|l+01L

1+1+02H i|ii02L

khOJE

hkO-JL

U1i08H

Itl+OBL

1+1+03H 1|1+03L

ltli09H 1+1+09L

iti+06H U1+o6l

1+1+05H

itl+05L

1+1+llH liUllL

Ul+10

1|1+12H

1+U12L

I s s ue d

April, 1975

May, 1975

June, 1975

July, 1975

Aug. , 1975

Sept. ,1975

Oct. , 1975

Nov. , 1975

Bee. , 1975

Jan . , 1976

Feh . , 1976

March ,1976

Approx

.

Act i vi ty

(mCi )

50.

0. ^

20 . 0 . 1

20 . 0 .

1

20.

0 . 1

10.

0 . 2

30.

0 . 1

10 . 0.1

ho.

0 . 2

80.

0.8

30.

0.2

20.

25.

0.1

Future SRM's to be issued are: ^^Xe ( 5 . 3d ) ; ''"^'''Hg ( 2 . 7d ) ; 123 1(0. 55d); ^°^Tl(3.0d); "'Ga(3.3d) and ^^^In(2.8d) 67 111.

Contained in 5-ml flame-sealed horo s i li cat e glass

ampoule s

.

73
understanding of exactly what constitutes

such traceability does not appear to be universal. A working definition has been given

as "competence that can be periodically demonstrated."-^ This can be accomplished by participation in a round robin distribution of

calibrated, but "blind" samples of unknown

(to the participant) quantity for assay by

the participant. At the present time, approximately one-third of the monthly distributed

SRMs for this program are issued as "blinds."

These are selected by the steering committee

with the understanding that participants may

take substitutes and exempt themselves from

the traceability exercises involving those

radionuclides which they do not market. Upon

receipt of their result for the "blind, " the

participant receives a Certificate for the

SRM. A detailed reporting form and questionnaire issued with the "blind" aids NBS in

helping to identify errors or measurement

discrepancies. To satisfy a request by the

participants of this program for proof of

demonstrated traceability, each participant

in the "blind" round robin will receive a Certificate of Traceability to the National

Measurements System. Participation in the

"blind" round robins, although voluntary for

the most part, is really of great advantage

to the manufacturers in not only continually

monitoring their measurements performance

(i.e. assurance) but also documenting this

performance (i.e. traceability). The NBS en- courages a greater participation in this as- pect of the program and would like to see the

program gradually proceed from a distribution

of standards to a distribution of "blinds."

The results for the "blind" round robin

distributions for 1975 are summarized in

Table 3. They include measurements on both

the H-level (multi-millicurie) anc^^L-level

multi-mi^^ocurie) standards for Sn, ^^-'1, ^Sr and P as well as a few additional

Table 3 Standards Program for the Radiopharmaceutical Industry

Results for the Standard Reference Material "Blinds"

Values Reported are X/NBS

113

Sn

1(1*02L ltU07H 125,

i*l>07L

85

1*1*03H Sr

l*it03L

32

i*i*o6h p

ltl*06L H Others

L

0.8978^ 0

0 .9822^

.9980''

0 .91*16 0

0

. 8832

. 9102

0 . 8717 0 .881*6 0

0

0

.9015 .9371

. 9609

0

1

. 9292

. 0185

0 8517^ 0 .8633^

0 . 93l»0 0 . 9628 0

1 .9380

. 012U

0

1 .9581

.OU8I

0 .9520 0 . 91*51 0

0

0

.9511* .951*9 .9786

00 .9612

.9987

0 9998 11

. 0768

. 0626

0 . 9678 0 .9638 0 . 9881 1 . 021*5 0

1 .9813. 0018

00. 9706

.9905

0 . 9727 0 .8985"^ 1 0006 1 .0056

0 . 9826 0 . 9669 0 .9886 0 9873 1 . 0052 1 . 0226 0 .9905 0 .9686 1 0261 0 . 9221*^

0 . 98UI* 0 .9728 1 . 0105 1 .0037 1 . 0182 1 .0579 1 .0021* 0 .9877 0.9882^ 1 .0030 1 . 0805 1 .0599 1 . 0182 1

1

. 0080

. 0280

. 0061* 0 . 31*10

1 .1805 C 11

. 0053

.0562

0

1 .9771* .0733

1 .1363 1 01*82

*One each for Co(1*1(08H and UltOSL) and Se(l*l*09H and 1*1*09L) and two each for ^ Au(l*lt05H and 1*1*05L).

Revised values after increasing by 5h.% ^Revised value after increasing by 0.8^.

c Not measured.

"^Revised value after decreasing by 1*91.%.

e On retest with M-level activity SRM, X/NBS=1.0282

f Revised value after decreasing by 219.%.

The results in each column of this table are listed in order of increasing values of X/WBS (for the H-level SRM) and does not correspond to the order in which the participants are listed in Table 1.

74
0.80 0.90 1.00 1.10 1.20

x/nbs

FIGURE 1 Distribution of reported results (data of Table 3 using revised values when applicable) for the 1975

SRM "blinds."

One outlier not plotted: "^^P, 4406L (X/NBS = 0.3410).

75
results on Co, Se and Au. In most

cases, the reported values were activity per

unit weight, and after correcting for decay were divided by the NBS values to obtain

X/NBS. There is a total of 78 reported

values on 61 blind samples. Several participants reported the results of measurements

by different methods on the same sample.

Close inspection of Table 3 indicates that

there are no major systematic trends. With

the possible exception of '"''^Sn, where the

reported values tend to be slightly low, all

the radionuclides appear to have been measured about equally well. Similarly, there

are no major differences between the measurements of the H-level and L-level samples.

Although not provided in the table, there

was also no apparent evidence that one assay

method or instrument was superior to another.

In general, the results indicate a fairly

satisfactory agreement with NBS values.

There are, however, a number of mistakes in

data handling which should be distinguished

from systematic or random errors in the

measurements. These mistakes included a 547„ error because of the incorrect use of a branching ratio (gamma-ray abundance), a

2797o error due to a miscalculated decay correction, and a 491% reporting error resulting

from the failure to distinguish between total

activity and activity per unit weight. Oftentimes these can be detected directly from the

questionnaire responses, when sufficient

information is provided, and can be revised

(as has been done in Table 3) to reflect the true measurement capability. Unfortunately

these cases are not that rare, occurring

nearly 107o of the time. A distribution of

the results (data of Table 3 using the re- vised values when applicable) is provided in

Figure 1. As can be seen, the reported

values are distributed about the mean value

of X/NBS = 0.986 with a standard deviation

of 0.058. The values on the wings of the

histogram, which are of the order of 10 - 207„ deviation from the NBS values, are usually

attributable to either unaccountable systematic errors such as in dilution or dispensing,

or systematic errors such as in decay corrections, detection efficiency and instrument

calibration factors which can be adjusted

after receiving the NBS value and Certificate.

A number of participants have been aided in

identifying sources of discrepancies in their

measurements and have been provided with

suggestions for eliminating them and/or with

information for improving their procedures.

A good example of this is shown in the first

values listed in the last column of Table 3 where measurements on both the H-level and

L-level samples showed an approximately 157o

discrepancy. On pursuing it with the participant, a long-existing systematic error

in an instrument calibration factor (arising

from an incorrect decay correction in a reference source) was discovered. On adjusting the calibration after receiving the

NBS value, the participant was "retested"

with an intermediate-level activity (M-level) sample of the same radionuclide. The

reported value on this retest was within 37o of the NBS value.

I have tried to give an overview of the

entire AIF-NBS Radioactivity Measurements

Assurance Program for the Radiopharmaceutical Industry and hope you now have a better

understanding of its objectives and progress

After spending considerable time discussing

errors and discrepancies, I would, in con- clusion, like to emphasize that the radioactivity measurements made by the radiopharmaceutical industry are now, after eliminating data-handling mistakes, generally

within ±207c, of the NBS values. The program

has been successful in meeting many of the

needs of the radiopharmaceutical industry

and I believe we all look forward to its

continued success.

Acknowl edgement

It is a pleasure to acknowledge the

extensive efforts of Dr. W. B. Mann,

Miss L. M. Cavallo, Mr. R. W. Medlock and

the many other personnel of the Radioactivity Section at NBS who have contributed to the success of this program.

References

1. C. W. Seidel and J. C. Brantley,

"Activities of Commercial Radionuclide

Producers" in Activities and Needs

Related to Radioactivity Standards for

Ehvironmental Measurements, Environmental Protection Agency Rpt. No. EPA-670/4-75-006, 39 (U.S. Govt. Printing Office, Washington, D.C., 1975).

2. C. W. Seidel and J. M. R. Hutchinson,

"ANSI Quality Assurance Program in

Radioactivity Measurements" in proceedings of this symposium (1976).

3. L. M. Cavallo, B. M. Coursey, S. B. Garfinkel, J. M. R. Hutchinson and

W. B. Mann, "Needs for Radioactivity

Standards and Measurements in Different

Fields," Nucl. Instrum. Methods, 112 , 5 (1973).

76
NBS SP455 (1976)

AMERICAN NATIONAL STANDARDS INSTITUTE

QUALITY ASSURANCE PROGRAM IN RADIOACTIVITY MEASUREMENTS

C. W. Seidel

New England Nuclear Corporation

Boston, Massachusetts 02118

and

J. M. R. Hutchinson

National Bureau of Standards

Washington, D. C. 20234

The ongoing ANSI N42 . 2 program of writing procedural standards for

radioactivity measurements with Nal(Tl) and Ge(Li) detectors, liquid

scintillation counters and ionization chambers is described.

(Detectors; germanium; ion chamber ; liquid scintillation; sodium iodide;

standard)

In 1972 the Atomic Industrial Forum (AIF)

appointed a subcommittee of Manufacturers of

Radioactive Reference Standards with the objective of obtaining a high degree of consistency and reliability in commercially available radioactive reference standards and

their accompanying Certificates of Calibration. Representatives of all major commercial

manufacturers of radioactivity standards and

the National Bureau of Standards (NBS) served

on this committee. Other companies and interested persons were listed as observers and

attended our meetings or were informed by

mail of our discussions and decisions. This

committee has now become a subcommittee,

N42.2, of the American National Standards

Institute (ANSI).

Several things were accomplished by

the original committee that I feel are worth

mentioning. As a result of meetings held in

1972 and 1973, the manufacturers of radioactive reference standards agreed to follow

ICRU Report 12 (1968) in terms of the format

for the Certificate of Calibration and, in

reporting accuracies, to quote the random

error at the 997o confidence level. In addition, they agreed to refer to this format in

their product literature. Now purchasers of

radioactive standards are able to make an intelligent choice of products by comparing

specifications listed in a common format.

The committee also began a blind round-robin

; measurements program to establish traceji ability to NBS.

I The AIF committee also sought to help

f the users of radioactive reference standards

by promulgating guidelines or methods for

the calibration of nuclear-radiation detectors that are in common use.

the use of radioactive materials in diverse

scientific fields by people with non-nuclear

backgrounds, there was a serious need for

proper guidelines or written standards that

would allow a knowledgeable person to set up

the proper quality assurance program for his

equipment. Many of us have heard of people

who never calibrate their equipment, who

try to measure weak beta-particle emitters

through thick glass or 1-125 through lead,

who know little of decay schemes and abundances of particular radiations, and those

who count samples too hot for their detector

without knowing it. A survey in New England

a few years ago uncovered a number of dose

calibrators (ion chambers) that were out of

calibration by a significant amount and on which a daily calibration check was rarely

performed. Some of the participants in

past NBS round robins completely misidentified the radionuclides, sometimes listing one whose gamma-ray spectrum bore little

similarity to the actual radionuclide.

The Subcommittee believed that the

much needed guidelines or methods of calibration would be more readily accepted by

the multidisciplined users of radioactivity

if they were formulated under the auspices

of a national standards organization. It was also felt that the AIF as an industry

oriented group was not the appropriate

organization to sponsor such an effort.

Therefore, we sought a home in the ANSI

program.

In 1973 we began to work under the

ANSI N43 and N44 committees, but since 1975

have functioned within the N42 committee,

which is chaired by Lou Costrell of NBS. We are presently called the "N42.2 Subcommittee on Standard Methods to Calibrate

We believed that, with the increase in Nuclear Detection Equipm'ent" and are working
on procedural standards to calibrate the

dose-calibrator type of ionization chamber,

NaI(Tl) detector, Ge(Li) detector systems

and liquid-scintillation counters. Table I lists the task groups and their chairmen.

A number of task groups have been

created to draft the written standards, and we have attempted to choose the members of

these so as to be representative of industry,

government and university areas, and any

other groups that we find have an interest

in our objectives. While we have limited

these task groups to 6-8 people in each, we

do allow observers to participate in the

discussions or to supply us with written

suggestions. Currently, our subcommittee

consists of 31 members and 71 observers.

Many observers represent other standards-writing organizations involved in

similar areas such as the American Nuclear

Society, Society of Nuclear Medicine,

College of American Pathologists, American

Society for Testing of Materials, National

Committee for Clinical Laboratory Standards

and others. Regulatory agencies such as the

Nuclear Regulatory Commission (NRC) and

Food and Drug Administration (FDA) are represented in either our active or observer

membership. The observers also include

organizations outside the United States such

as the International Atomic Energy Agency

and the National Laboratories in Canada,

England and France.

For whom are we writing these standards?

You might say that most of our interested

clientele are included in the groups listed

in Table II.

We want to provide laboratory technicians and their supervisor with a guide on how to calibrate and to check the continued

performance of their nuclear radiation

detectors. We want to work with the interested regulatory agencies to provide meaningful written standards to which they can refer in their regulatory guide, or require

their licensees to incorporate in their

quality assurance procedures. And we want

to aid the instrument manufacturers in the

writing of their instrument manuals so that

they can provide acceptable procedures for

the setting up, the calibration and the

checking of the operation of their instruments .

Our objective is to write a concise,

clear guide that will serve as a standard

from which an individual or laboratory may

develop a program for the acceptable operation of its particular measurement systems.

78

Our standards will cover six basic

items as listed in Table III. The standards we are writing recommend both the method

and frequency of calibrating the instruments,

the types of standards that should be used,

the minimum daily (or other interval) checks

on instrument performance which should be

made, the correct procedure for making the

measurements and assessing errors, and a check list of potential problems. Such

standards, if useful, are likely to be

adopted or recommended by regulatory agencies such as the NRC or FDA. One section of our standard is designated as a Precaution section. Here we list common problems and state how to avoid them

or cite references that report on how they

may be properly handled. Examples of some precautions concern recombination effects

in ion chambers at high activity levels, or

the problems of counting high energy betaemitting radionuclides in ion chambers,

coincidence losses in Nal(Tl) detectors,

and quenching or luminescence in liquidscintillation counting. We feel this is one of the most valuable sections of our

standards

An outline of our ion-chamber standard

(N713) is shown in Table IV.

The standards for the ion-chamber

detector and the germanium detectors

recently went out for ballots, and we are now in the process of resolving the comments

we received. We hope the other two standards will be ready for their first ballot

later this year.

Our committee looks forward to a rapid

completion of our effort and the adoption

of the written standards by the multidisciplined users of radioactive material.

We would also like to see greater

participation in the blind round-robin

program begun by the AIF committee and now organized, promoted and directed by NBS

and ANSI N42.2.

Under the blind round-robin program

4 to 6 radionuclide test samples are issued

each year. The program was implemented to accomplish NBS traceability for the participants. For a nominal fee ($100 to $150

per standard) any interested party can obtain one of these "unknown" standards

and a questionnaire. Each participant in

the round robin is expected to complete his

questionnaire within a few weeks stating

(1) his measurement of the activity, (2) any

detectable radioactive impurities, (3) method of measurements and (4) the determination
of the accuracy of their measurements. Upon

receipt of the questionnaire, Lucy Cavallo,

Coordinator of the program at NBS, will send

the participant a Certificate of Calibration

and collate the results. Summaries of the

results of the program for 1973, 1974 and

1975 are shown in Tables V, VI and VII. Several participants in these round robins

have been aided by the staff at NBS in

correcting their measurement systans if

their response has been in error.

The ntnnber of participants in this

program is not large. Most of the major

manufacturers of radioactive material, some National Laboratories, a hospital,

and power reactor groups participate because

of a need to be traceable to NBS. A low

number of participants may also be noted in

the College of American Pathologists testing

program (which began before ours) although

they have had some recent increases in the

number of participants.

Another similar round-robin program

oriented to specific requiranents of the

Radiopharmaceutical manufacturers and the

FDA is now supported by the AIF to provide

these manufacturers with NBS traceability

This program grew out of our initial roundrobin program and has been described by the

previous speaker.

Increased participation in these roundrobin programs by various users such as hospitals and clinical laboratories is

desirable and beneficial to the user. We look forward to your support and participation in our program.

TABLE I

ANSI N42.2 Task Groups and Chairmen

Writing Groups

N42.2.1 Germaniim Detector Standard

Gerald Martin, Jr. , General Electric

N42.2.2 Ion Chamber Detector Standard

Frank Masse, M. I.T.

N42.2.3 Liquid Scintillation Counter Standard - Roger Ferris, Amersham/Searle

N42.2.4 Sodium Iodide Detector Standard

Ron Coley, Commonwealth Edison

Round-Robin Program

L. M. Cavallo, National Bureau

of Standards

TABLE II

For Whom Are We Writing

I. The technical user

II. The regulatory agencies (i.e., NRG,

FDA)

III. The instrument manufacturer

TABLE III

What Are We Writing ?

Standards that will provide a guide to:

I. Setting up instrument

II. Calibration

III, Performance checks

IV. Measurement procedures

V. Estimate of errors

VI. Potential problems (precautions)

TABLE IV

Ion-Chamber Detector Standard (N713)

I. Introduction

II. Scope and Purpose

III. Definitions

IV. Text of Standard

a. Operation

b. Calibration

c. Use

d. Standard Sources

e. Performance Monitoring

f. Accuracy and Reproducibility

V. Sources of Error

VI. Precautions

a. Non-linearity Effect

b. "Simulated" sources

c. Low-Energy Photon Elmitters

d. Beta-Particl e Bnitters

e. Gaseous Radionuclides

f. Plate out of Radionuclides

g. Assay of a Radionuclide for

- which no standard is available

79
1973 ROUND ROBIN

HotV ^^Co - 5/73 ^^^1 - 8/73 ^°^Hg - 10/73 ^^C - 12/73

A 1.091,1.07 9*

B 1.032,1.024

C 0.992

D 1.009,1.054 0.476A

E 1.017,1.022 0.980,0.992,1.005

F 1.006 0.941

GH

I 1 . 010 • ^ 0.951 1 . 029

J 1.043 1.000 1.004 1.011

K 1.059,1.053 0.844 1.003

L 1.000

M 1.018,1.046,1.033 0.943,0.906

N

0 , P 1 . 208B , 0 . 983A_

QR 0.856

ST . U

VW

X

Y

Z

Where more than one number is given,

the laboratory reported more than one measurement

B_ refers to the originally reported

measurement; A refers to the value

after correcting for computational

or other data handling error.

80
1974 ROUND ROBIN

Lgbora- 51^^ _ 3^^^ 75^^ _ ^^^^ 10^7/74 ^^\e-9/74 ^^Fe - 10/74 - 11/74

A

B 1.015 0.919,0.951*0.970 0.996,0.987 1.011,1.014

C 0.938

D

E 0.994 1.016 0.960 1.035

F 1.016 0.961 0.995 0.968

G 0.990,0.982, 1.026,1,028,

0.962 1.051

H 1.068,1.064

I 0.957 1.010 1.018

J 0.993 1.018 0.995 0.982 0.976 0,986

K 0.993 1.187 0.994 0.955

LM 1.034,1.166, 0.970,1.284

1.097,1.030 1.143

N0 1.001

pQR

S 1.006 1.033 1.004

T 1.064 1.25,1.19

U

VW

X

Y

Z

* Where more than one number is given

the laboratory reported more than one measurement.

81
1975 ROUND ROBIN

tory Co - 6/75 Sr-1 /15 Sr - 8/75 P-10/75 Se-11/75 Au - 12/75 Sn-6/75

A

B 1.001,10006*

C 0.996,1.012

D

E 1.000 0.995 0.948,0.996 1.004,1.007 0.981

F 1.033 0.995 57.8B,0.963A

G

H

I 5.099B, 0.03B, ^ . 0.994A 0.990A

J 1.007 0.974

K 1.001

LM

N 0.977 0.974 1.154 0.894

0 ' ^ 1.009,0.988

P

Q

R - S . .

. TU

V 1.001 w

X

Y

Z

Where more than one number Is given,

the laboratory reports more than one measurement

_B refers to the originally reported

measurement; A refers to the value

after correcting for computational

or other data handling error.

82
:bs SP456 (1976)

FILTERED BEAMS AT THE NBS REACTOR*

R. B. Schwartz and I. G. Schroder

National Bureau of Standards

Washington, D. C. 20234

and

E. D. McGarry

Harry Diamond Laboratories

Adelphi, MD 20783

Using a combination of resonant scatterers and filters, we are producing

high intensity, very pure beams of 2 keV, 25 keV, and 144 keV neutrons

at the NBS reactor. These beams have low background, are well calibrated

and very stable, and are intended for use as calibration standards.

(Monoenergetic neutrons; neutrons; neutron beams; neutron filters; reactors; resonant scatterer.

Introduction 2 keV Beam

One of the missions of the NBS Center

for Radiation Research is the development

and maintenance of standard neutron fields.

These fields are to be used for the calibration of radiation instruments and of personnel

dosimeters, as well as for other CRR programs.

In this paper we shall be primarily concerned

with those fields developed by means of filter

techniques at the NBS reactor.

Our particular concern is with the energy range below 30 kev (sometimes referred to

as the "neglected energy range.") ^ Our concern arises from the fact that a very large

fraction of the neutron dose—typically 30%-

40%—in the working environment around a nuclear reactor is due to neutrons in this

energy range. ^ Despite the importance of

these "intermediate" energy neutrons, very

few instruments are ever directly calibrated

anywhere in this range, and for a very good

reason: it is very difficult to produce useful monoenergetic neutron fluxes of these

energies

At the NBS reactor, we have used filter

techniques to produce high intensity neutron

beams with energies of 2 keV and 25 keV, and

are soon to install a 144 keV beam. Neutron

filtering occurs at energies where there are very deep minima in the neutron total cross

section. (These occasional minima are caused

by interference between resonance and potential scattering.) An appropriate thickness

of a material whose cross section shows such

behavior will transmit essentially all neutrons whose energies correspond to the minimum in the cross section; but will attenuate

neutrons of all other energies. The production of high intensity filtered beams was pioneered at the Materials Testing Reactor;^

unfortunately the MTR was shut down before

the development of the beams could be completed. The filters themselves were subsequently transferred to NBS.

The 2 keV beam is produced by a scan- dium filter: the minimum (or "window") at

2 keV results from overlap of the interference minima from the resonances at 3.24

and 4.27 keV, producing a cross section with

a minimum value of only 50 mb . ^ (See Figure

1.) Unfortunately, there are also several

other windows in scandium at higher energies.

These windows are responsible for a relatively

high transmitted neutron flux in the energy

region between 8 keV and 800 keV. These

higher energy neutrons form an unwanted background to the desired 2 keV beam. In the

work of Simpson et al. at the MTR,^ this

background flux was equal to 2/3 of the 2 keV

flux. This tvpe of background can be tolerated in certain measurements, but would be

100 pz0.01

1.0 10 100 NEUTRON ENERGY. eV

1000

Fig . 1 Total neutron cross section

scandium from 1 eV to 8 keV. The d

10,000

of

eep

scandium'

minimum at 2 keV is responsible for

functioning as a filter. Other minima at higher energies are not shown.

83
disastrous for the calibration of dosimeters.

Unfortunately, the problem is inevitable in

any facility in which a scandium filter looks

at a reactor core.

This background is essentially eliminated

in the NBS installation by the use of a through tube in conjunction with a resonant

scatterer. The through tube passes 10 cm outside of the edge of the reactor core, and

the collimating system containing the filter

only sees a scatterer at the center of the

tube. (See Fig. 2) This design eliminates

unwanted core neutrons and gammas. We use manganese to scatter the neutrons, making

use of the large scattering resonance at 2.375 keV. Although the resonant energy does

not exactly match the energy of the 2 keV

window, the resonance is sufficiently broad

(r=400 eV) that at 2 keV the scattering

cross section is still > 100 barns."* Physically, the scatterer is a 3 mm thick Mn-Al

alloy, containing 57 atomic percent manganese.

REACTOR

SHIELD

WALL

NBS REACTOR

FUEL

ELEMENT

SCANDIUM

FILTER

MANGANESE

SCATTERER

Fig . 2 Schematic representation of the NBS

Reactor showing the filter, colli:uator , and

scatterer in the through tube (not to scale.)

In terms of total flux (i.e., the area

under the peaks) , the higher energy contaminants amount to approximately 6% of the

2 keV flux without the titanium; the addition

of the titanium; the addition of the titanium

reduces their contribution of ^3% of the 2 keV

flux.

The neutron intensity, as measured with

a calibrated BF^ counter is 6 x 10^ neutrons/

sec. The beam diameter at the reactor face

is 1.7 cm, diverging to 6 cm at 3m from the reactor. The gamma ray background is '^1 Mr/

hr.

25 keV Beam

The 25 keV beam is produced by an ironaluminum filter, in another through tube in

the reactor. For this case, the higher

energy background problems are not so severe

and it suffices to use a "flat" scatterer

(i.e., graphite). The higher energy flux

amounts to 1% of the 25 keV flux. The 25 keV

flux is 2 X 10^ n/ sec, with a gamma background of '^1 mR/hr.

144 keV Beam

A 144 keV beam, using a silicon filter,

will be installed later this month. The

flux should be at least equal to that in the

2 keV beam, with essentially no neutrons

other than those at 144 keV.

Summary

The NBS filtered beams provide a very

important calibration facility. All three

will have low background, be very stable,

and will be well calibrated. The three

beams can be run simultaneously with no interference or cross-talk, and will serve

as calibration standards. It is our intention that the "neglected energy range" be

much less neglected in the future.

The advantage of this design may be seen

in the spectra shown in Fig. 3 (These data were taken with a 1 atm. hydrogen proton-recoil counter) . The solid line shows the

spectrum obtained with a 110 cm. long Sc filter; the dashed curve shows the effect of

adding one cm of Ti to the Sc. It can be seen that even without the Ti, the main se- condary peak at 29 keV has an area of only 3% of the 2 keV peak. The addition of the Ti

reduces this peak (as well as the ones at 7,

15, and 40 keV) by a factor of about 2-1/2,

at the cost of only 17% of the 2 keV peak.

84
References

/ fO fOO

Neutron f^nergy (kev)

Fig. 3 Neutron spectrum through scandium

filter. The solid curve represents the

spectrum with a 110 cm scandium filter alone,

and the dashed curve the spectrum observed

with the addition of 1 cm of titanium.

*Work supported in part by the Energy

Research and Development Administration

1) J. R. Harvey and S. Beynon, Proc. First

Symposium on Neutron Dosimetry in Biology

and Medicine, Neuherberg/Munchen (Germany)

1972 p. 955.

2) O.D. Simpson, J. R. Smith and J. W. Rogers

Proc. Symp. Neutron Standards and Flux

Normalization; p. 362, CONF-701002, U.S. Atomic Energy Commission (1971); also,

Proc. Third Conf. Neutron Cross Sections

and Technology, p. 598, CONT"-71031 (1971) .

3) W. L. Wilson, "Neutron Cross Section

Measurements and Gamma Ray Studies of

'*^Sc"; M.S. Thesis, University of Idaho

Graduate School, Dec. 1966 (unpublished).

4) M. D. Goldberg, S. F. Mughabghab , B. A. Magurno , and V. M. May; "Neutron Cross

Sections, Volume IIA, Z=21 to 40;" BNL

325, Second Edition, Supplement No. 2 (Physics-TID-4500) February 1966.

85

NBS SP456 (1976)

NEUTRON SOURCE CALIBRATIONS AT NBS FOR CALIBRATION

CHECKS OF NEUTRON RADIATION INSTRUMENTS

V. Spiegel

National Bureau of Standards

Washington, D. C. 20234

The manganous sulfate bath method of neutron source calibrations at the NBS Is described together with

the corrections applied for neutron capture In the source Itself, capture by competing nuclei of either

fast or thermal neutrons, and thermal neutron leakage. The advantage of commercially available Am-Be

(cif,n) neutron sources as a calibration check for

radiation Instruments In the MeV range Is stressed.

(Neutron source; calibration; manganous sulfate;

radiation Instruments)

Introduction

This paper describes the manganous sulfate bath method used at the

NBS to calibrate the emission rate of

neutron sources. For a more thorough

review of axid references to the meas- urement of neutron emission rates and

neutron spectra see reference (1) by

Caswell.

Neutron sources which are used

for the calibration of neutron radiation Instruments are selected on the

basis of their size, stability, predictable and long half life, energy

spectrum, and emission rate. The

emission rate Is used to calculate

the strength of the neutron field at

the position of the neutron radiation

Instrument. Neutrons emitted from

these sources are not of uniform

energy and therefore require fluencedose conversion factors to give a true calibration of a neutron dosimeter. It is not the purpose of this

paper to describe such a complicated

procedure for any given Instrument,

but to recommend that, once such a relationship is established, the

operation is most easily checked over

I long periods of time by an Am-Be ( a, n)

1 neutron source. Calibration of Neutron Emission Rates

At the NBS the manganous sulfate

2 bath method is used to compare the

neutron emission rate of an unknown

source to the standard radium-beryllium, photo-neutron source, NBS-I.

The principle of the method is shown

In Figure 1. The source is placed in

a thin-walled, air-filled, teflon

beaker at the center of a 125 cm diameter sphere of manganous sulfate

solution. Due to the large slse of

the bath very few neutrons escape.

The remainder are captured by any of

the following processes: (1) fast

capture in the fluorine of the beaker,

(2) fast capture in "'^O or ^^S, (3)

55 slow neutron capture in Mn or hydrogen, or (4) slow neutron capture in

the beaker or the source itself. The

slow neutron capture in ^^Mn produces

Mn, which decays to Fe with a half life of 154 minutes. It is the

gamma activity of the ^^Fe which is

measured at the sodium iodide crystal.

The Induced activity of the bath is

measured for the unknown source and

for the standard source, NBS-I. The

ratio of the source emission rates

is determined from the ratio of the

two count rates after corrections

have been applied for escape and capture in other nuclei.

Fig. 1 Manganous Sulfate Bath Source

Calibration (Schematic)
A circulating manganous-sulfate

bath system, as shown In Figure 2,

Is used at the NBS. The circulating

pump thoroughly mixes the bath solution by maintaining a flow rate of

about 70 liters per minute through a dispersive nozzle. About 6 liters

of solution per minute flow to a remotely located gamma-ray counting

system, where counting can be con- tinued and statistics accumulated as long as desired. The Sodlum-Iodlde

crystal, located at the center of the

Marlnelli beaker. Is used for neutron

sources with emission rates between

5 8 10 to 10 neutrons per second. The

second crystal, located above the

Marlnelli beaker, views a small fraction of the activated solution

through the lead shield and Is used

for neutron sources with an emission

8 10 rate between 10 and 10 per second.

The second crystal Is needed for more Intense neutron sources, because the

main crystal counting system would be

paralyzed by the resulting manganese

activity. The relative efficiency of

the two counting systems Is measured

by using an Intermediate neutron

source with an emission rate of about

10^ per second, which can be counted

simultaneously by both systems.

BEAKER lO'dIa i 10'

Fig. 2 Circulating Manganous Sulfate

Bath and Shielded Gamma-Ray Detector

The uncertainties in a calibration are listed in Table I. The most

recent absolute calibration of NBS-I

by Noyce, et al."^ was quoted with an error of ^1%. Escape from the 1.25

meter diameter bath is quite small even for the higher energy (Q;,n)

neutron sources and essentially non- existent for low energy {y ,n) neutron

sources. The error in the correction

depends upon the size of the correction. A correction is applied for

capture in the source Itself. In the

Table I Uncertainties in the Calibration of

Neutron Sources

Systematic Errors n) (Y,n

Emission Rate of NBS -I 1% 1^

Escape 0. Qffo Absorption in Source 0.1-0. 0.1%

Past Neutron Loss to 1%

Reactions

Random Error

Counting of Activated 0.2^ 0.2-^

Bath

Total Error 1.6-1.7^ l.M

case of sources containing fissionable

material, such as plutonium, neutron

multiplication by thermal capture in

the fissionable material may partially cancel or be larger than losses due

to capture in the source encapsulation. This correction is minimized

by locating the source in an air

cavity with thin teflon walls to re- duce the thermal neutron flux. The

correction is calculated^ for each

individual source and its accuracy

again depends on the size of the cor- rection. The least well understood

correction, which is very Important

for (a,n) sources because they have

many neutrons with energies above 3 or 4 MeV, is the loss of fast neutrons due to reactions such as

0(n,Q') and S(n,p). Calculations

of this correction by Murphey^ are used for our calibrations, but, be- cause these disagree with some ex- perimental measurements"*", there still

appears to be an uncertainty of about

1% in the correction for (cif,n) sources. The total errors listed in Table

I result from the linear addition of

the quadratic sum of random errors and quadratic sum of systematic errors.

2 26Ra-Be(a,n) sources increase

in neutron source strength due to the

growth of ^-"-^Po. ^'•^Po decays with

a half life of 138.38 days and is

210 formed by the alpha decay of Pb

in the radium decay chain with a

88
half life of 22.3 years. The relative source strength as a function

of time in years from the date of

separation of ^^^Ra is given by the

following relationship^,

F( t )=1+. 143( 1 . 01437-1 . OSieee'''^/^-'-

* +.01728e-^/-^^^^). (1)

239Pu-Be(a,n) sources increase

in neutron source strength due to the

241Am is formed by 241 growth of Am

241, the beta decay of ""Pu with a half

life of 14.89 years. If the atom

241 percent of Pu in relation to the

other neutron emitting Plutonium

isotopes are known at the time of

manufacture the growth of a Pu-Be

(a,n) source can be calculated by a rather complicated formula, which will

not be included here, but may be

found in an article by Caswell^.

Instrument Calibration Checks with

Commercially Available Sources

The sources commonly used for

instrxoment checks are ^'^"'Am-Be(a,n),

^2^Ra-Be(a,n), and '^^^P\i-Be{a ,n)

Their average neutron energies are approximately 3.9 MeV, 2.8 MeV, and

3.4 MeV respectively. Only the

Americium sources have a single well

known half life (433 yr), which is

predictable over long periods of

time. If great care is taken in determining isotopic composition of

the source material before manufacture, an accurate effective half

life can be determined for the other

two sources. However, this information is not always available and is

•ften lost or forgotten.

Another popularly used neutron

source with emission rates up to lO-'-'-' per second is ^^^Cf spontaneous fission. Its spectrum has been well

g Studied and it can be manufactured

to approximate, very closely, a point

source. However, it does have an inconveniently short half life of about

? so 2.64 years and contains Cf and

254Cf, which further complicates the

determination of the effective half

life.

References

1. R.S. Caswell. "Review of Measurements of Absolute Neutron Emission Rates and Spectra from Neutron Sources." Neutron Sources

and Applications , Proc. ANS Nat'l

Topical Meeting, April 19-21,

1971, Augusta, Ga., C0NF-710402,

Vol. I, p. 1-53 (1971).

2. K.W. Geiger. "Recent Improvements in the Absolute Calibration of Neutron Sources."

Metrologia 4, 8 (1968).

3. R.H. Noyce, E.R. Mosburg, Jr.,

S. Garfinkel, and R.S. Caswell.

"Absolute Calibration of the

National Bureau of Standards

Photoneutron Source III. Absorption in a Heavy Water Solution

of Manganous Sulphate . " J, Nucl.

Energy a/B 17, 313 (1963).

4. V. Spiegel, Jr. and W.M. Murphey.

"Calculation of Thermal Neutron

Absorption in Cylindrical and

Spherical Neutron Sources."

Metrologia 7, 34 (1971).

5. W.M. Murphey. "The Correction

Factor for Fast Neutron Reactions on Sulfur and Oxygen in

the Manganous-Sulfate-Bath Calibration of Neutron Sources."

Nucl. Instr. & Methods 37, 13

(1965).

6. W.M. Murphey, National Bureau of

Standards, Washington, D. C.

20234. "The Growth of '^"'•^Po in Ra-Be(a,n) Neutron Sources."

Internal Report, January 1967.

7. R.S. Caswell, R.H. Noyce, S.B.

Garfinkel and E.R. Mosburg, Jr.

"Recent Developments in Neutron

Source Standardization." Neutron

Dosimetry II , 547, IAEA, Vienna

(1963).

J. A. Grondl and CM. Eisenhauer.

"Fission Spectrum Neutrons for

Cross Section Validation and Neutron Flux Transfer." Conf. on Nucl. Cross Sections and Technology, Washington, D. C. 1, 250

(March 1975).

89

ms SP456 (1976)

ULTRASONIC MEASUREMENTS AND CALIBRATION

H.F. Stewart, M.E. Haran , B.A. Herman

Bureau of Radiological Health

Food and Drug Administration

Rockville, Maryland 20852

Introduction

In the past few years ultrasound techniques have become well known and are re- ceiving widespread acceptance in clinical

practice. For example, diagnostic ultrasound is rapidly finding use as a compliment

to other techniques and often is the modality of choice for visualization of structures within the body supplanting procedures

involving the use of ionizing radiation.

Some of the main areas of application at the

present time are visualization of abdominal

organs, pregnant uterus, and the heart. It

is estimated that approximately 35% of the

pregnant women in the United States currently under go ultrasonic insonations and that

by 1980 this number will increase to about

90%. Along with the increasing use of this

modality come the responsibility for the

assessment of the risk that may be involved

with its use. This requires the measurement and characterization of ultrasonic

fields by researchers engaged in the evaluation of biological effects associated with

ultrasonic exposures. There is also the

need to determine parameters of the ultrasonic field from fields produced by ocnmercial

ultrasonic devices in order to evaluate the

potential risk their use may present on the

basis of biological effects data. These

measurements are especially necessary in

view of the wide proliferation of such devices. In addition the establishment of

any necessary performance criteria requires

the quantification of the ultrasonic fields

involved and calibration of measurement devices used.

Ultrasonic Quantities and Measurement

Techniques

The ultrasonic field parameters which

generally are of the greatest concern include total output power, intensity, and

field distribution. In the case of ultrasonic intensity it is important that peak

temporal and spatial values as well as average spatial and temporal values be measured. The need for peak intensity measure- ments is evident when one considers that

peak intensities from ultrasonic diagnostic

pulse echo equipment are on the order of

10^ times the spatial and temporal average

intensities (1,2). Other ultrasonic parameters which are of interest include acoustic pressure, particle velocity, speed of

91

particle displacement, frequency spectrum

and attenuation coefficients. Table 1 shows most of the quantities used along with

appropriate SI units and quantity symbols.

Relationships exist among the quantities in Table 1. For traveling plane waves

the Langevin radiation force (F) is related

to the power by the relationship F = Pd/c

where P is the acoustic power, c is the propagation velocity of the wave, and d is a factor determined by the type of interface

an object presents to the ultrasonic field,

values for d are shown in Table 2. For a more detailed discussion of the topic the reader

is referred to the following references (3, 4). The ultrasonic intensity (I) for progressive plane waves is given by I = pv

where p is the rms value of the particle

pressure and v is the rms value of the

particle velocity. The relationship among

acoustic pressure, particle velocity and

specific acoustic impedance is given by the

following formula p = Pocv. Unfortunately

in the near field of ultrasonic transducers

(applicable to most medical applications)

the structure of the ultrasonic field is

complex and measurement and calibration

methods must properly take this into account Several methods for measuring acoustic field

parameters exist. These may be grouped into

six general techniques: (1) radiation force,

(2) thermal as by calorimeters, thermal

couples and thermisters, (3) electrical, i. e.

equivalent circuits, (4) electroacoustic

conversion as by piezoelectric, magneto

strictive transducers and capacitance trans- ducers, (5) optical, including both optical

diffraction and interferometry , (6) reflection as by reciprocity calibration. The

parameters measured by the different techniques are shown in Table 3.

Measurements using one or more of these

techniques are being carried out in some university laboratories (usually associated

with medical schools or hospitals) , by cer- tain manufacturers, and by the Bureau of

Radiological Health and the National Bureau

of Standards. The Bureau of Radiological

Health maintains an inhouse capability to measure the acoustic field parameters from

medical ultrasonic equipment in order to conduct biological effects investigations

and evaluate ultrasonic products. This

work is being carried out as part of the

Bureau of Radiological Health's responsi-
bility under the requirements of the Radiation Control for Health and Safety Act of

1968, P.L. 90-602. The law specifically

directs the Bureau of Radiological Health to conduct a wide range of studies into possible conditions of exposure to electronic

product radiation i to develop practical

measurement techniques for the evaluation

of radiation emission, and to investigate

possible biological effects associated with

such radiation.

Investigations by the Bureau of Radiological Health and reports from others have

shown inadequacies in the calibration of

ultrasonic medical devices (5) . There has

also been an inadequacy in the reporting of

parameters used in biological effects investigation (6) . This makes it difficult

or impossible to correlate the results airong

investigators. These deficiencies have been

due to not only to the lack of recognition

of the importance of characterizing ultrasonic fields but also the unavailability of

commercial equipment' with which adequate

and reliable measurements can be made. The

lack of available instrumentation is cur- rently a major limitation in the evaluation

of ultrasonic devices. Currently there is no portable commercially available equipment

which is capable of measuring ultrasonic

quantities from pulse echo diagnostic equipment. Only one U.S. manufacturer is marketing a measurement device to determine total

power from ultrasonic therapy transducers.

The remainder of this paper will discuss some of the techniques that can be used

to measure the output from ultrasonic medical

devices. A comparison of measurements made

by the Acoustics Branch, Bureau of Radiological Health and the National Bureau of

Standards are shown.

Radiation Force Techniques

The radiation force technique is cur- rently the most widely used method for

measuring ultrasonic power. It measures the force produced by an ultrasonic field impinging on a target. The force produced is

independent of frequency and is proportional

to the total time averaged ultrasonic power

in the field. For a progressive plane wave impinging on a perfectly absorbing target

the radiation force is equal to the acoustic

power divided by the speed of sound. One

advantage of this technique is that it is an absolute method that can easily be calibrated by the use of weights. For example,

an acoustic power of 1 m.illiwatt exerts a force of 0.068 milligrams weight on a totally absorbing target. The system used by the

Bureau of Radiological Health to measure the

output from diagnostic ultrasonic devices

uses a sensitive micro-balance from which a target suspended in water is attached. This

system and other similar systems have been

reported in the literature (7,8). Table 4 presents some measurements made of the output from some pulse-echo diagnostic ultrasonic transducers. This data as well as that reported by Carson indicates that the

temporal and spatial average power levels

from pulse echo diagnostic equipment is

usually less than 10 milliwatts per sq/cm

(2) . These low ultrasonic levels also

illustrate that very sensitive systems must

be used to measure the output from such

equipment. For a totally absorbing target

10 mW corresponds to a force of only 0.68

mg weight. Although such measurements can

be made in a laboratory environment using

an analytical micro-balance system, reliable

commercial radiation force systems have not yet been developed and marketed for the

measurement of these diagnostic levels. Output levels from ultrasonic therapy equipment

is usually in the range of 1 to 20 watts (5). The forces produced by these levels as a result of radiation pressure, are about 3 orders of magnitude larger, than those produced by diagnostic pulse-echo devices and

are readily measured. Unfortunately the

availability of commercial instruments to measure the output of ultrasonic therapy

equipment is very limited. There is a definite need for the development of a portable

measurement system that can be used to check

the calibration of such equipment.

A technique which has been used for the measurement of radiation pressure involves

the measurement of the deflection of a target suspended in a sound field in a pendulum

arrangement. This system has been reported

in the literature and is used in our laboratories for calibration purposes (7,9).

Thermal Detectors

Thermal methods of measuring ultrasonic

field quantities take advantage of temperatures that rise due to the absorption of

acoustic energy in a suitable material. The

most widely used type of calorimeter is

called a substitution calorimeter. The ultrasound is directed into an absorbing medium and the subsequent temperature rise

measured. The system is calibrated by dissipating a known amount of energy in the

absorbing medium by means of a resistance

heating coil and measuring the resulting

temperature rise. If the thermal loses of

the calorimeter are the same in both cases

then a direct determination of absorbed

92
ultrasonic energy can be obtained. This may

also be considered an absolute measurement

technique. Calorimeters provide a unique means for the measurement of ultrasonic

energy since they are least effected by the

ultrasonic field shape and pulse duration

and are not dependent on plane wave assumptions. Such a system developed by the

Bureau of Radiological Health has a dynamic

range from about 10 milliwatts to 50 watts

(7) . This calorimetric system was used in

making comparative measurements by the radiation force float and acousto-optic methods

using National Bureau of Standards sources.

The results of this intercomparison are shown in Table 5 and 6.

Electrical or Equivalent Circuit Method

A method employed by the National Bureau of

Standards for the determination of ultrasonic power is the evaluation of the parameters of the equivalent circuit model of

resonant transducers under loading conditions

(11) . This method involves calculation of

the resonant radiation conductance of quartz

transducers from admittance measurements

made with a twin-tee circuit of NBS design.

(12) . Measurements of susceptance and con- ductance were made at the terminals of the

transducer under three conditions ; unloaded

(transducer in air) , loaded (transducer in

water) , and effectively clamped (at approximately twice the resonant frequency) . The

data is treated to yield values of conductances G' (unloaded) , and G (loaded) , and G" (determined from measurements at 2W) , and

from these, the diameters, D'=G'-G" and

D=G-G" , of the circles of motional admittance

of the transducer can be calculated. Accurate values of the resonant frequencies (unloaded

and loaded) are also obtained. From the

circle diameters the resonant radiation con- ductance is calculated, Gj-=D (1-D/D').

Related methods for determining efficiency

have been discussed by others (13,14,15).

The National Bureau of Standards has

recently announced the availability of these sources for calibration of measurement equipment. Continuous wave output levels in the

range of microwatts to 1 watt are available.

An overall accuracy of 5% or better is

claimed at frequencies of 1,2,3 and 5 megahertz. The maximum beam diameter is 2.5

centimeters. These sources are transportable and may be taken to customers' facilities for calibration of their measurement

equipment. The availability of these sources

in addition to an ultrasonic calorimeter

measurement system to measure ultrasonic

power directly is a significant advance in

the capability to calibrate and measure the

93

output from medical ultrasonic equipment.

Piezoelectric Method

A parameter of importance other than

total acoustic power is the distribution of

the energy in the ultrasonic field, both

spatially and temporally. To be able to follow variations of the energy in both time

and space a method is needed with a fast

response time and small spatial dimensions.

An appropriate technique involves detecting

an electrical output from a small piezoelectric crystal place in the ultrasonic

field. The voltage output from such a crystal (i.e., hydrophone) is proportional to the acoustic pressure and may be squared to obtain a signal proportional to the acoustic

intensity. An absolute rather than a relative measurement of intensity can be obtained by calibrating the hydrophone using one of the other absolute methods. By scanning

a hydrophone across the beam the intensity

distribution across the field can be found

and plotted.

OPTICAL METHODS

Acousto-Optical Diffraction Techniques

The literature in the area of acoustooptical diffraction techniques is well developed, consisting of two general categor

ies (15) . Schlieren visualization and

pressure measurement. The study of ultrasonic fields by means of optical methods

has the advantage of providing a means of

measurement while avoiding the disturbance

of the ultrasonic field. One of the measurement systems used by the Bureau of Radiological Health employs the Raman-Nath theory

of light diffraction in which it is assumed

that the ultrasonic field presents a phase

grating to a normally incident beam of mono- chomatic light (17,18). The irradiance of

each of the diffraction orders of the re- sulting Fraunhofer pattern can be used to calculate the acoustic pressure within the

ultrasonic beam. One of the disadvantages

of this system is that it requires a knowledge of the ultrasonic field size and

shape and thus is more applicable for work

with sources of known configuration. The

results of comparative measurements made

with the radiation force and calorimetric

methods using National Bureau of Standards

sources is shown in Tables 4 and 5. An extension of this method for measurement of

pulsed ultrasonic fields is currently being

investigated by the Bureau of Radiological

Health and offers possibilities as a portable measurement system. Results using

this method were reported at the recent
meeting of the Acoustical Society of America

(19) .

Interferometric Optical Method

A newly developed optical method that

detects the movement of a thin membrane in

a sound field but that does not require

prior information with regard to ultrasonic

field pattern or size has recently been developed (20) . This is a scanning optical

interferometer and as used in our laboratory

can provide either qualitative and quantitative information about the spatial distribution of particle displacement amplitude

of fields emitted by diagnostic ultrasonic

transducers. It promises to provide a con- venient and rapid method for the evaluation

of ultrasonic transducers. A discussion of

this system and its application to the evaluation of medical ultrasonic transducers is

given in another paper in this symposium

(21).

Results of Intercomparison Measurements

Comparative measurements taken with

the radiation force, calorimetric and

acousto-optic methods used by the Bureau

of Radiological Health were made with two National Bureau of Standards ultrasonic

transducers. The ultrasonic output power

from each transducer was determined by NBS

using measurements of the impedance of the

generating ultrasonic transducers and the

applied voltage. These predicted outputs

were compared with values measured by the

Bureau of Radiological Health.

Tables 4 and 5 present the data collected for measurements on two air-backed

quartz crystals, each with a resonant frequency of 2 MHz. The quartz crystals were

circular disks with active surface diameters

of 1.905cm (3/4 inch) and 1.270cm (1/2 inch).

The variations in the radiation force

readings in Table 4 at an applied voltage

of 203.0 volts have not been explained.

Previous intercomparisons between the radiation force and acousto-optic techniques

have shown no statistical difference between the two measurement methods at the

95% confidence level (22). For the measure- ments reported in Table 5 using the acoustooptic system, the highest possible power

generated by the 1/2" crystal was too low

to allow the conditions necessary to precisely align the sampling light beam in the

acoustic field. The result is that the

optical data in Table 5 is not as reliable

as is possible with the present system.

Alignment was not a problem with the 3/4"

crystal. The limits of error in the rms voltage applied to the ultrasonic transducer

are believed to be +_ 1% of the voltage. The

maximum intercomparison differences were within +_ 20% of the mean, and for most

measurements the difference was much within

+ 5%.

CONCLUSIONS

Methods exist that can provide reliable means of measurement of ultrasonic field

parameters. However, at the present time

there is almost no commercially available

ultrasonic measurement instrumentation. In addition there is a lack of availability of

a low cost transfer standard which can be

used by non-technical personnel to calibrate

such equipment. The problems in both of

these areas should be corrected in the

future. Some commercial companies are beginning the development of instrumentation

to measure ultrasonic field parameters, and

the National Bureau of Standards is working

toward the development of transfer standard sources

Some measurement areas not discussed

in this paper needing work include, invivo

dosimetry, calibration in relation to frequency content of ultrasonic pulses, finite

amplitude wave effects and methods for the

characterization of vibrators, such as used

in dentistry or the surgical removal of

cataracts

References

1. Carson, P.L.; Acoustic Exposure Determination in Diagnostic Ultrasound - Responsibilities of Researchers Studying Biological Effects of Ultrasound

Proceedings of the Society of PhotoOptical Instrumentation Engineers, Vol.

47, Application of Optical Implication

in Medicine III: 16-117, Aug. 1-2, 1974.

2. Carson, P.L.; Exposure Measurements on Diagnostic Ultrasound Instrumentation

Presentation to the Ultrasound SubCommittee of the Obstetrical Gynecological Device Classification Panel

(FDA) Jan. 22, 1976.

3. Rooney, J.A. , and Nyborg , W. L. ; Acoustic

Radiation Pressure in a Traveling Plane

Wave, AJP, Vol. 40, 1825-1830, Dec . 1972

4. Hueter, T.F., B.H.R.H. Sonics Sec. 2.12

Radiation Pressure, Wiley, N.Y. 1955,

43-53.

94
5. Stewart, H-F. et al; Survey of Use and

Performance of Ultrasonic Therapy Equipment in Pinnelas County, Florida, DHEW

Publication (FDA) 73-8039, May 1973.

6. Woodward, B. and Allen, A.J. ; The

Importance of Calibration in Medical

Ultrasonics, British Journal of Radiology, 47, 707-711 , October 1974.

7. Stewart, H.F.; Ultrasonic Measuring

Techniques, Seventh Rochester International Conference on Environmental

Toxicity Fundamentals and Applied

Aspects of Non-ionizing Radiation,

June 5-7, 1974.

8. Rooney, J. A. ; Determination of Acoustic

Power Outputs in the Microwatt-Milliwatt Range, Ultrasound in Medicine and

Biology, Vol. 1, 1-4, 1973.

9. Rooney, J.A. ; Acoustic Radiation Pressure and its Use in Power and Intensity

Determining.

10. Hasegawa T. , Yosioka K. ; Acoustic Radiation Force on a Solid Elastic Sphere,

J. Acoustical Soc . America 46, 1739-

1143 (1969).

11. Zapf, T.L.; Calibration of Quartz Transducers as Ultrasonic Power Standards by an Electrical Method 1974 Ultrasonics

Symposium Proceedings, IEEE Cat. No.

74, CH0896-isu Nov. 11-14, 1974.

12. Huntley, L.E. ; A Self-Calibrating Instrviment for Measuring Conductance at Radio Frequencies, J. Res. National

Bureau of Standards, 696, No. 2. pp. 115-126 (1965).

13. Bobber, R.J.; Underwater ElectroAcoustic Measurements, Naval Research

Laboratory, Washington, D.C. July 1970

pp. 90-99.

14. Hueter, T.F. and Bolt, R.H.; Sonics,

John Wiley and Sons, N.Y. pp. 99-122,

1955.

15. Kinsler, L.E. and Frey, A.R. ; Fundamentals of Acoustics, Second Addition,

John Wiley and Sons, N.Y. pp. 360-364,

1962

16. Kline, W.R. and Cook, B.D.; Unified

Approach to Ultrasonic Light Diffraction, IEEE Transactions Sonics and

Ultrasonics, SU-14, 123-134 (1967).

17. Raman, CD. and N.S.N. Nath; The Diffraction of Light by High Frequency

Sound Waves , Pare Indian Academy of

Science 3, p. 406 (1935); 2, p. 413

(1935); 3, P. 75 (1926); 3, p. 119

(1926) ; 3, p. 459 (1936)

18. Kline, W.R., Cook, B.D. and Mayer, W.G,

Light Diffraction by Ultrasonic Gradings, Acoustica, 15, p. 57 (1965)

19. Cook, B.D. ; Investigation in Measurement of Pulse Ultrasonic Waves by

Optical Techniques, University of

Houston Contract No. FDA 74-39.

20. Mezrich, R. , E.T. Zold, K.F., Vilkomerson, D.; Ultrasonovision ; Interferometric Means for Measurement in Vizuilization of Ultrasonic Wave Front

Interactions with Biological Tissue,

Proceedings of the Society of PhotoOptical Instrumentation Engineers, Vol.

47, 69-74, Aug. 1-2, 1974.

21. Haran, M.E. and Stewart, H.F.; Optical

Interferometeric Measurements of

Ultrasonic Radiation and its Application to Medicine. Presented at the

National Bureau of Standards 7 5th

Anniversary Symposiums - Measurements

for the Safe Use of Radiation, March

1-4, 1976.

22. Haran, M.E., Cook, B.D. and Stewart,

H.F.; Comparison of an Acousto-Optic

and a Radiation Force Method of

Measuring Ultrasonic Power, Journal of

The Acoustical Society of America,

1975.

95
Table 1. Ultrasonic Quantities, Symbols and SI Units

Quantity Symbol SI Unit

Power P w Intensity • I W-m~^

Speed of sound c m-s"-'- Force F N Pressure P Pa=N-m"2

Particle displacement y m

Particle velocity (vector) u m-s~l

Specific acoustic impedance for N-m~3. s=k:g-m~2' s"-*- progressive plane waves. Pq is

the equilibrium density p^c

Amplitude attenuation

coefficient Np/m

NOTE: Notation in the table above: m = meter, s = seconds,

kg = kilogram, N = newton. Pa = pascal, W = watt, Np = neper

Table 2

Value of Constant D for Various Physical Situations

Physical situation D Perfect absorber, normal incidence

*r = 1 1 Perfect reflector, normal incidence

*r = 0 or «> 2 Perfect reflector, incident at angle

0 to sound beam, *r = 0 or 2 cos^S

*r = Z2/Z]^ the impedance ratio at an interface

where Z = pc

c is the velocity of sound in the medium

p is the density

Table 3. Ultrasonic Quantities Measured by Different Techniques

Quantities Detected

Techniques Power Intensity Pressure

Particle

Displacement

Particle

Velocity

Calorimetry X(l)

Thermal Probe X(l)

Optical

Diffraction X (2) Radiation Force

(Total field) X(l)

Interferometr ic X(2)

Piezoelectric ' X Radiation Force

(Sphere deflection) ' X(l) . Equivalent Circuit X . Rec iproc ity X ( 3

)

NOTE: Notation in the above table: X = basic quantity measured

1 = time average only, 2 = temporal peak, 3 = peak pulse power

96
Table 4

Measurements of Ultrasonic Output Use Radiation Force Method

Generator # of Transducers

Manufacturer Measured Range (mW) Mean

Picker 57 <1 to 13.6* 5.3

Unirad 2 <1 <1 Smith Kline 3 <1 to 5.7 3.1

* Only two of the 57 measurements were above 10 mW. These two values were 10.5 and 13.5 mW.

Table 5. Comparative Measurements of Acoustic

Power from 1/2" Quartz Transducer

Power (milliwatts) (See note b below)

Electrical Rad. Force Acousto- Calorimetric

Voltage^ Method Method Optics Method Method

(volts) (NBS) (BRH) (BRH) (BRH)

203.0 204(+0.4) 200(-1.6) 219(+7.7) 190(-6.5)

100. S 50.3(-1.9) 52.6(+2.5) 51(-0.6)

30.26 4.53(-4) 4.9(+4)

a —rms voltage measured across the transducer.

b—numbers in ( ) indicate percent deviation from the mean

(measured-mean/mean) . Where the mean is determined by averaging

the values obtained for all techniques at one voltage setting.

Table 6. Comparative Measurements of Acoustic

Power from 3/4" Quartz Transducer

Power (milliwatts) (See note b below)

Voltage^

(volts)

Electrical

Method

(NBS)

Rad . Force

Method

(BRH)

AcoustoOptics Method

(BRH)

Calorimetric

Method

(BRH)

232.6

203.0

100.8

50.54

30.26

603 (-4.4)

459(-1.0)

113 (-0.5)

28.5(-3.9)

10. 2 (-3.8)

658 (+4.4)

377 (-18.7)

535 (+15.4)

lll(-2.3)

479 (+3.3)

116.7 (+2.

30.8 (+3.'

11.0(+3.i

469(+l.l)

a—rms voltage measured across the transducer

b—numbers in ( ) indicate percent deviation from the mean

(measured-mean/mean) . Where the mean is determined by averaging

the values obtained for all techniques at one voltage setting.

97

NBS SP456 (1976) ^

METHODS FOR THE EVALUATION AND CALIBRATION OF

MICROWAVE SURVEY INSTRUMENTS

M. L. Swicord, H. I. Bassen and W. A. Herman

Electromagnetics Branch, Division of Electronic Products

Bureau of Radiological Health, Food and Drug Administration

12721 Twinbrook Parkway

Rockville, Maryland 20852

A facility has been established for the evaluation and calibration of

microwave power density instrumentation. It includes an anechoic chamber

with a precisely positionable cart for measuring gain and chamber standing

waves. The operable frequency range is from 900 MHz to 10 GHz. A laser

provides a means of aligning antennas. The absolute calibration of

distance and microwave power are performed by NBS. The radiated electric

field strength is then established by BRH as a primary standard using

precise antenna gain measurement methods. An instrument evaluation

facility includes an absorber-lined temperature chamber, a probe/slotsource positioning system, and devices for measuring the polarization

response, modulation response, and the linearity of the microwave survey

instruments

(Microwave Power Density ; Microwave Radiation Measurement)

Introduction

Since the passage of the Radiation

Control for Health and Safety Act of 1968

and the subsequent promulgation of the

Microwave Oven Standard under that Act, the

Bureau of Radiological Health has found it

necessary to establish means of thoroughly

evaluating and precisely calibrating

microwave radiation instrumentation. These

instruments are used in a wide range of

programs including the enforcement of the

Microwave Oven Standard, radiation hazard

assessments, and radiation measurements

performed on other electronic products such

as microwave diathermy. This paper

describes the facilities and methods

established and used for these purposes.

Plane Wave Calibration Facility, Physical

Characteristics

The main element of the calibration

facility is an anechoic chamber. The

cham.ber has approximate external dimensions

of 3.6 X 3.6 X 9 m.eters. This provides a usable interior longitudinal axis of 7 meters. A fiberglass cart, which is also

partially covered with anechoic material,

can be moved along the longitudinal axis of

Che chamber and remotely positioned to millimeter accuracy. The fiberglass cart rides on two stainless steel rails located

below the anechoic floor material of the

chamber. The rails are located off center

so that no specular reflections can occur.

A chain drive parallels each rail and is

connected to a motor outside the chamber.

A microwave oven survey probe to be

calibrated is placed in a dielectric holder

which, in turn, is mounted on the cart (Fig.

1) . The probe is extended in front of the cart as far as possible on a dielectric rod.

The readout device for the probe is then

placed on the cart, behind anechoic

material. The probe is irradiated by a truncated horn antenna. Standard gain horns

are not used due to increased problems and

uncertainties which occur when horn gains

are experimentally determined in the Fresnel

region or even the "pre-Fresnel" region of an antenna. A high power, rigid coaxial

transmission line is used as a forvrard power

monitor with integral direction coupler

between generator and antenna. While the

probe is exposed to a precisely known plane

wave, the meter readout indication is

observed with a closed circuit TV camera utilizing a telephoto lens. The rear wall

is covered vzith a high performance pyramidal

absorbing m.aterial. Ports in the rear wall

are provided for a camera, a spotlight and a laser. The split beam laser is used for the

alignment of transmitting and receiving

antennas with the central longitudinal axis

of the chamber, and with the cart's rail

system during the antenna gain measurement

procedure. Transmitting antennas can be

centered and aligned by placing a precisely

machined, teflon-coated, insert containing a first-surface mirror in the waveguide throat

of the transmitting horn as shown in figure

2. The reference laser beam is reflected

99
ANECHOIC CHAMBeR

TRAVEL FORWARD POWER

MONITOR

Figure 1: Probe Calibration in "Plane Wave" Faci 1 i ty

100
from this mirror and serves as a means for

providing antenna alignment to better than

one degree. This beam is also used for

alignment of the receiving antenna on the

cart. Cart wobble is minimized by alignment

of the rails to eliminate lateral and

vertical cart position variations during its

travel

In the front, or transmitting end of

the chamber, a connection is made from the

microwave generator to the rigid coaxial

feed system through high power flexible

transmission line. Relative forward power

is read on the coupler's side arm digital

power meter. A single meter reading

determines the absolute radiated power

through precise "power equation" techniques

developed by Dr. Glenn Engen of the National

Bureau of Standards-^. A description of this

specific application has been published by

the Bureau of Radiological Health^. An equipment console provides control signals

to the cart and displays accurate position

data. A shaft encoder is used to provide a digital cart position readout accurate to ± 2 millimeters (calibrated against an NBScalibrated steel tape).

Method of Generating a Precisely Known

Plane Wave Power Density

The procedure for establishing a calibrated plane wave field is as follows.

The standing waves in the chamber are measured by mounting a probe, of the type to be calibrated, on the cart. The cart is

then driven the entire length of the chamber

and the output of the receiving probe vs

distance is recorded, while a constant power

level is transmitted by a horn antenna. From this procedure, a relative "quiet zone"

can be determined near the center of the

chamber, for a particular frequency. Next,

the horn gain is measured in this quiet zone

' via continuous path loss measurements

' between pairs of nominally identical horn

, antennas. The Power Equation methods are

' most appropriate for determining net power

i transfer (path loss) between transmitting

i and receiving antennas, since mismatches

I between these antennas and their respective

source or load are precisely accounted for.

( Antenna gain is then computed as a function of path loss between a pair of

nominally identical antennas by the formula:

„ „ _ \ (4^R) 2 (1)

S^R - p; "~r~

where: G^ = transmitting horn gain, G^^ = receiving antenna gain, R = antenna

separation distance at the aperture planes,

X = wavelength of transmitted signals, and

Pj^/Pr|, = received power/transmitted power

ration (path loss)

Equation (1) assumes that no multipath

reflections are received from chamber

sidewalls or the cart and its supports and

that no mutual reflections may exist between

antennas. These conditions are met by

providing a high performance anechoic

chamber, proper antenna separation, and

sufficient antenna directivity or gain. The

gains G^ and G^ are a function of separation

distance and approach a constant "far field"

value at large separations

Errors in path loss measurements due to antenna skin currents (side lobes) , and

leaking cable connections must be

experimentally detected and minimized. Skin

currents flowing on the outside of the horn

antenna and on microwave feed cables can give rise to radiation side lobes. Since

the method of antenna gain measurement used assumes that reciprocity exists, the gain of an individual antenna must be unaffected by

its mounting structure and surroundings.

These side lobes may be reflected to the

opposite antenna by the materials

surrounding the antenna (i.e. absorber

behind the transmitting antenna, the cart on which the receiving antenna is supported, or the cables feeding each horn) . Their

effects are detected and minimized. No significant changes (< 0.1 dB) in path loss

due to sidelobes occur for any of the

measurements in the final BRH calibration

system.

The use of three pairs of antennas

(with a total of three antennas utilized)

allows the determination of the gain of a single antenna. This gain is position

dependent, since the measurements may not be

made in the true far field, especially at 915 MHz. The antenna separation is

sufficient however, so that little spatial

variation (less than 5 percent) occurs, over

the aperture of the receiving antenna under

worst case conditions. This implies that a small probe will observe the same power

density as the receiving antenna with its

much larger effective aperture.

The calibration of absolute transmitted

power from the horn antenna is accomplished

via a special high-power coupler /bolometer

(calibrated by NBS) . This coupler/bolometer

has a waveguide input flange of the same dimensions as the waveguide horn antennas

101
(Fig. 3). To calibrate the "forward power

monitor" which consists of a directional

coupler, permanently mounted on the rigid

coaxial feed system, the transmitting

antenna is removed and the calibrated

coupler/bolometer is substituted as a termination. The readings of the digital

power meter on the forward power monitor are simultaneously compared with the substituted

D.C. power supplied to the bolometer as measured by a precision bridge. The net transmitted power is absolutely calibrated

using specialized techniques and the

equipment illustrated in figure 4. The

forward power monitor reading is related to transmitted power by leveling power P^,

during all measurements. Waveguide offset

shorts are used as references to provide

accurate matching factor measurements.

Absolute power density is then computed

at a point in the previously determined

"quiet zone" of the chamber where both

quasi-far field conditions and minimal

standing waves (as seen by a small isotropic

probe) exist. Power density is computed as:

. W!^ (3)

where: S = power density, P^j. = absolute

transmitted power, Grp(R) = transmitting horn

gain (a function of distance) and, R = distance from the horn to the point where

power density is computed. The standing waves (due to chamber wall reflections)

which are seen by an isotropic probe under

calibration must be accounted for to remove residual probe calibration errors. Then the

process of calibrating the plane-wave

facility is complete. A worst-case absolute

power density uncertainty of less than ± 0.6

dB at 2450 MHz and ± 0.8 dB at 915 MHz are readily achieved using a very conservative

error analysis.

Probe Calibration Procedures in the Plane

Wave Facility

When calibrating a probe in the known

plane-wave field, precautions must be taken

to avoid additional errors due to reflections from the dielectric cart, the

probe support structure, and the

instrument's cable and readout meter. Reflections from the cart are measured and

reduced to acceptable levels (± 0.2 dB at 2450 MHz and ± 0.5 dB at 915 MHz). A large

amount of additional scattering from the

cable connecting the probe to the readout

meter was detected. Shielding of the cable

with absorber was necessary over portions

where it was oriented vertically (aligned

with the electric field) . Even with this

absorber, significant residual scattering

occurs, particularly at the lower

frequencies. To eliminate the residual

scattering errors a "multi-position

averaging technique" was devised (Fig. 5).

These standing waves had a known period of

one-half wavelength. Therefore, several

calibrations were performed on each probe,

with the probe mounted ahead of the cart at various spacings. The spacing of the

probe's sensor tip to the cart (D2) was varied in quarter wave increments for each

calibration, while the distance from the

probe's sensor to the transmitting horn (Dl) was maintained at a constant value. An average of four such readings was computed.

Cart and support reflection/errors are thus

reduced to a value below 0.05 dB (1 percent)

by this multi-position averaging technique.

Whenever the separation between two antennas is not great, compared to the

parameter (a^/A), where "a" is the maximum

dimension of the transmitting horn, some amplitude and phase variations exist in the

plane of the receiving antenna aperture used

during gain measurement procedures. Spatial

averaging of these fields occurs since the

receiving antenna aperture is large. In contrast, a small antenna such as a radiation hazard meter, will see only the

maximum field intensity if it is on the

boresight of the transmitting antenna. At 2450 MHz separations used exceeded 20 a^/A. At 915 MHz, since separations of 6.5 a^/A were used, the spatial averaging error was experimentally evaluated by continuously

scanning planes normal to the axis of

propagation, with a small probe. Numerical

integration was performed over a rectangular

area with dimensions identical to the

receiving horn aperture. The integrated

power density over the aperture space

differed from the central point value of

power density by less than 5 percent.

Because the effective aperture of the

receiving antenna is much smaller than its

physical aperture, this is a worst-case test

of non-uniformity of the "plane wave" used

to calibrate probes.

Instrument Evaluation

After calibrating an instrument in a known plane wave field, the response of that

instrument to various field parameters

existing during the measurement of a leaking

microwave oven or other near-field source, must be evaluated. The ambient temperature,

102
FIELD

- MEASUREMENT

PROBE

gV -

GENERATOR

Figure 5: Multi-Position Averaging Technique

103

i

i
the polarization of the electric field, its

amplitude modulation, and other parameters

must be simulated and the instrument's

response noted. The test apparatus shoxm in

figure 6 provides a means for some of this

evaluation. More absorber is used in actual

practice than is shown. A probe, supported

by a dielectric holder, is positioned in

front of a slot source. The holder is

mounted on rails as shown. The variation of

probe response, as a function of distance

from the slot source, can be observed and

standing waves occurring in the system may

be minimized by adding microwave absorber.

The "near" vs. "far field" responses of

various probes may be compared by moving

them toward the slot source from the rear of

the apparatus. A probe can be rotated to obtain the angular response (antenna

pattern). During this rotation, the center

of the receiving sensors of the probe is

placed on the axis of rotation. A stable 20 watt source is used to provide field

strengths of appropriate magnitude. Power

to the system is leveled and a modulator,

driven by a "function" or a pulse generator,

allows complete control of the amplitude

modulation im.pressed upon the radiated

fields. This allows the evaluation of

instrument linearity vs. field modulation

while constant average power is maintained.

The average power level may also be adjusted

to determine the instrument linearity for CW

radiation of varying amplitude.

Another system is used for determining

the polarization response of a probe, i.e.,

the response to a linearly polarized E field

as the probe is rotated about its handle

axis (Fig. 7). Leveled power is fed to an absorber-lined box from an open-ended

waveguide, while the probe body is supported

by a plastic holder. The sensor is

maintained at a constant distance from the

waveguide. Rotation of the probe handle

allows one to observe the polarization

response or "polarization ellipticity" of

the instrument.

A specially designed temperature

response evaluation system has been

developed. It consists of an absorber-lined

environmental chamber (60 cm cubicle)

capable of temperature variations from -20°

C to +50° C using liquid nitrogen cooling

and electrical heating. The instrument

under test is placed in a plastic holder at

a fixed distance from a slot source (Fig.

8). The fields near the slot have been

separately tested for temperature

independence. The variation of relative

probe response may be observed while

constant net power is fed to the slot source

at different ambient temperatures. The

entire instrument, including readout, can b placed in the chamber, with the instrument

readout viewed through a small plastic

window in the side of the chamber.

A final evaluation procedure must be

performed to determine the radiof requency

interference (RFI) susceptibility of the

probe body, cable, and readout meter to strong fields at frequencies which the

microwave oven or other sources to be

surveyed, operate. The sensor tip of the

probe is totally enclosed in metal foil.

The probe body, readout meter and cable

(which is aligned with the electric field) are irradiated in the plane wave calibration

facility. The response of the readout meter

is noted and compared to the reading

obtained with the sensor tip unshielded.

Many instruments fail this test, due to faulty RFI gaskets in readout units, or insufficient filtering in the readout or probe body.

Acknowledgement

Much of the electronic and mechanical

design for the various systems was performed

by Mr. J. Duff and Mr. J. Bing. Many of the

measurements involved in the various

evaluation procedures were performed by

Mssrs. D. Witters and J. Duff. Theoretical

analysis of field curvature and other

antenna gain considerations were performed

by Mr. S. Neuder.

References

1. Engen, G. "Power Equations: A New

Concept in the Description and

Evaluation of Microwave Systems." IEEE Transactions on Instrumentation and

Measurement Vol IM 20, No. 1, Feb.

1971.

2. Herman, W. and H. Bassen. "Precise

Microwave Power Density Calibration

Method Using the Power Equation

Techniques." Dept. of Health,

Education, and Welfare publication FDA

75-8028, March 1975.

104


NBS SP456 (1976)

NBS ULTRAVIOLET RADIOMETRIC STANDARDS

W. R. Ott

National Bureau of Standards

Washington, D.C. 20234

A brief review of the standard sources and detectors available as services from NBS will be given. Emphasis will be on the use of

such standards to calibrate the radiant power of unknown sources or the response of radiation detectors and spectral radiometers at wavelengths less than 400 nm. The following standards, listed in

order of decreasing wavelength, are or will soon be available: the

tungsten filament quartz-halogen lamp (above 250 nm) ; the tungsten

strip lamp (above 225 nm) ; the low pressure mercury vapor lamp

(253.7 nm) ; the deuterium arc lamp (165 to 350 nm) ; the argon "miniarc" (115 to 400 nm) ; photodiode detectors (20 to 254 nm) ; and the

S5mchrotron radiation source, SURF-II (5 to 400 nm) . The relative

strengths and limitations of these radiometric standards with

respect to accuracy, reliability, convenience, and intensity and

wavelength range will be discussed.

(Ultraviolet, Radiometry, Standards, NBS, Sources, Detectors, Survey)

Introduction

In the wavelength region between

5-400 nm, NBS offers a variety of standard

sources and detectors which may be used to calibrate the radiant power of unknown

sources or the response of radiation

detectors and spectral radiometers. A

brief review of the available services

will be given and in addition several

current projects which are meant to improve

the accuracy, performance and variety of

ultraviolet radiometric standards will be

mentioned

The ultraviolet wavelength region can

be considered to extend from 400 nm, the near ultraviolet, to about 4 nm where the

soft x-ray region begins. For perspective

some key wavelengths in this region are illustrated in figure 1. The critical

atmospheric ozone absorption occurs between

200-280 nm, the so-called UVC region. The

UVA and UVB regions define the regions

315-400 nm and 280-315 nm respectively.

Molecular oxygen absorption characterizes

the region below 200 nm and thus the

terminology "vacuum ultraviolet" to indicate

the necessity for the absence of air in a measurement system designed to detect or utilize such radiation. Atomic hydrogen

absorption sets in at 100 nm and sets a limit to the range of interstellar astrophysical measurements. However, the real

importance of the ultraviolet wavelength

region is contained in the energy scale

corresponding to the wavelength scale in

figure 1. The ultraviolet energy range

spans from about 3 eV to 300 eV, which is

quite large as compared to the visible,

for example, which extends only from 1.5 eV

107

SOFT X-RAY

''//////'//////// !

; VACUUM OR FAR UVm^^,'^(;,^jVISt^R:

1

FUSIQM PLASMAS

1

•i SOLAR SCIENCE >

UV LASER MERCURY PEAK OF SKIN SHORTEST TO DATE LAMP UV SENSITIVITY

Hi 0, IO3I ABSORPTiONl

1230 iV •.21 ! I 12 eV 1 2 eV Figure 1. The ultraviolet region of the

electromagnetic spectrum defined in terms

of wavelength (nm) , energy (eV) and some key processes.

to 3 eV. This energetic radiation can be

used to drive and control all sorts of

chemical and biological reactions. For example extreme doses can cause skin

cancer; controlled doses can heal.

Continuous exposure to ultraviolet radiation causes the fading of paints and

dyes; controlled exposures in specific

processes are used to produce protective

coatings on materials. Some documented

examples of the various applications of

ultraviolet radiation as both a natural

and artificial element in our home and

work environment are shown in Table 1.
Radiometric Quantities Phocochemlstry

Thin film production

Tooth decay prevention Textile dyes

Paint curing

Industrial finishes

Cast hardening

Instant oscillographs

Material degradiation

UV photosensitive paper Counterfeit money detection

Bacteriological

Germicidal lamps in hospitals,

schools, and offices Gennicidal lamps in industry

Environmental Studies Atmospheric sciences

Ecology and the ozone layer

Oil spill identification

Water purification

Water pollution

Smog Gauge

Medical and Therapeutic Jaundice treacnents Calcium deficiency

Skin disease treatments Wrinkling

Drug detection

Medical research

Fusion Research

National controlled thermo- nuclear fusion efforts

Space Science Communications Astrophysics

Sky lab Space shuttle Reentry and rocket exhaust

VUV and X-Ray Lasers Integrated circuits

Plasma probes

Isotope separation Molecular synthesis High resolution holography

Tumor therapy

Laser fusion

Photobiology

NBS benchmark experiments

Plasma Chemistry

Thin film deposition

Plasma arc steel furnace Recycling of alloys in steel production

Table 1. List of applications requiring

ultraviolet radiation measurements.

Why are accurate radiant power measurements needed in these areas and how does

NBS interact with the measurement system?

Basically, in three ways: (a) on the

research level—new techniques require

accurate documentation for the process or procedure to gain widespread commercial

acceptance; (b) on the sales level—commercially available products need standards in

order to meet specifications and to compete

on a national and international market; and

(c) on the applications level—the user needs standards in order to check specifications, to maintain quality control, and

to monitor the health and safety of workers.

As an example of these impacts, consider an industrial assembly line process in which

certain manufactured items like paper

products, glass, metal parts, microcircuits

,

or textiles are being coated by radiation

curing. (a) For the uv radiation curing

technique to be accepted in the first

place, it would have to be well understood

so that performance could be guaranteed.

(b) The equipment which would enable the

assembly line to operate at the highest

speed is the most desirable (maximum ultraviolet flux in the most effective bandpass)

(c) Periodic calibrations in the field are necessary to accurately monitor the intensity

of bands of uv radiation hazardous to employees and to measure possible deterioration of the equipment due to radiation

damage of the optical components.

In general, there are two ways to determine the radiant power of an unknown

light source: (a) through the use of

standard sources and (b) through the use

of standard detectors. Standard sources

are most useful when it is desired to know

the emission characteristics of an unknown

source. Standard detectors are most

useful, on the other hand, when it is

desired to know the radiant power at the

location of a detector. For example, if

the quantity of int^rest^is spectral

radiance [watts cm nm sr ] , that is,

if one is concerned with the power s^adiated

by a specified emitting surface (cm ) in a certain wavelength band (nm) at a given

solid angle (sr) , then the light source to be investigated as well as the standard source may be set up in such a manner that

the radiation from both passes through the same optical-spectrometric arrangement,

thereby eliminating all geometric and

other specific factors of the instrumentation. In short, the calibration is

effected by a simple substitution of sources in the experimental arrangement

On the other hand, if the quantity of

interest is spectral irradiance [watts

cm~^ nm"-'-] , that is if one is concerned with

the radiant power incident on a specified

surface area (cm^) in a certain wavelength

band (nm) , then ideally a standard detector

and filter arrangement is placed at the

location of the irradiated surface to measure the power. Alternatively, if such

a standard radiometer is not available, a standard source of spectral irradiance may

be used in conjunction with a suitable

diffusing element (to account for variations

in the geometries of the standard and the

unknown sources) to determine the response

function of the user's dif f user-radiometer

system.

At NBS, standard sources of both

ultraviolet spectral radiance and spectral

irradiance are available. In the case of

NBS standard detectors for the ultraviolet, the calibrated quantity is the

absolute quantum efficiency [photoelectrons

per incident photon] as a function of

wavelength.

Standard Sources

The three primary source standards

being used at NBS are the gold point

blackbody cavity for which the spectral

radiance is given by Planck's law, the

wall-stabilized hydrogen arc for which the

spectral radiance is given by accurately

108
known quantum mechanical absorption coefficients for atomic hydrogen, and the

I electron storage ring facility for which

the spectral radiance is given by the

theory of electron synchrotron radiation.

The selection of one of these standards for

a calibration application is influenced

mostly by the specified wavelength region.

The primary standards are most often

used to calibrate secondary or transfer

standards of spectral radiance which can

then be issued to customers. The secondary

radiance standards are also used to generate

spectral irradiance standards. The following ultraviolet standard sources, listed in

order of decreasing wavelength, are available from NBS: the tungsten filament

quartz-halogen lamp (above 250 nm) ; the

tungsten strip lamp (above 225 nm) ; the low

pressure mercury vapor lamp (253.7 nm) ; the

deuterium arc lamp (165 to 350 nm) ; the

argon "mini-arc" (115 to 400 nm) ; and the

synchrotron radiation source, SURF-II (5 to 400 nm) . The relative strengths and limitations of these radiometric standards with

respect to accuracy, reliability, convenience, and intensity and wavelength range

will now be discussed.

(1) Tungsten Lamps

Tungsten strip lamps are used as transfer

standards of spectral radiance, and tungsten

quartz-halogen lamps are used as transfer

standards of spectral irradiance. The

tungsten resistive element is heated to incandescence by a specified dc electric

current. Power requirements are about 8 A

@ 110 V for the irradiance standard and

about 50 A @ 15 V for the radiance standard.

Between 400 and 250 nm, the spectral distribution of the quartz-halogen lamp and the

tungsten strip lamp is approximately that

of a 3000 K and 2700 K blackbody respectivelyStrengths: Techniques are well understood and calibrations based upon the

conventional blackbody cavity are possible

with uncertainties on the order of 1-3

percent

Limitations: Relatively weak sources

in the ultraviolet, especially below 250 nm

where the application of these sources is

limited by both low signal levels and significant scattered visible light. For example,

the spectral radiance of a tungsten strip

lamp at 230 nm is about 1000 times lower

than it is at 400 nm.

(2) Low Pressure Mercury Vapor Lamps

The irradiance of the 253.7 nm resonance

line of mercury can be measured for various

types of low pressure lamps supplied by

the customer. The approximate uncertainty

is 5% for irradiance levels on the order

of 0.5 to 30 pW cm" . Strengths: Calibration technique

utilizes quartz-halogen spectral irradiance

standard and can be extended to other line sources in the region above 250 nm. Limitations: The calibration of the

customer's lamp is only as good as its

stability and reproducibility.

(3) Deuterium Arc Lamps

These are available as transfer standards

of spectral radiance (165-350 nm) and

irradiance (200-350 nm) with calibration

uncertainties of about 10%. A short L- shaped arc discharge is formed between two electrodes set at 90° in the sealed lamp

and excites a molecular Y)^ continuum. The

low pressure deuterium lamp spectrum peaks

at about 200 nm where the spectral irradi- ance of a typical 30 W lamp is about 100

times greater than that of a 1000 W quartzhalogen lamp. Calibrations are based upon

the hydrogen arc primary standard of

spectral radiance. Irradiance calibrations

have just recently become available down

to 200 nm. Plans are to extend this

service down to 165 nm. Strengths: Very strong ultraviolet

output, portable, low power requirements.

Limitation: Alignment is difficult,

and there are uncertain long-term aging

effects

(4) Argon Mini-Arc

The mini-arc is a one atm pressure

wall-stabilized arc discharge operated at power levels up to 1.5 kW depending upon

the intensity desired. The spectrum Is line-free between 194 and 350 nm and

interrupted by only a few very narrow air

impurity lines between 114 and 194 nm. The spectral radiance of the argon continuum is determined in the region 114-350 nm by comparison with the hydrogen arc primary

standard. The mini-arc has just recently

become available. Future plans are to determine its suitability as a far ultraviolet spectral irradiance standard.

Strengths: Continuum is intense (a

factor 1000 stronger than the tungsten

strip lamp at 250 nm) , lamp aging is

negligible, the uv wavelength coverage is

the most extensive of all transfer lamps,

109
and the alignment procedure is more reliable

than for the deuterium lamp.

Limitations : The lamp system and

ignition procedure are slightly more complicated than those previously described; not commercially available.

(5) Synchrotron Ultraviolet Radiation

Facility (SURF-II) .

The theory of synchrotron radiation

from accelerated electrons is on excellent

theoretical grounds, and the continuum

emission can be quantitatively predicted

when the energy of the electrons, the

radius of the orbiting electrons, and the

electron current are known. The radiation

beam from the NBS 240 MeV storage ring is

highly collimated, polarized, and intense^

on the order of lO"*" photons s~"'-nm~"''mrad in the wavelength range 10-400 nm. Uncertainties in the flux determination are expected to be in the 5 to 10% range.

There are no plans at present to calibrate

transfer sources with the synchrotron

radiation, but rather guests are invited to use the facility for their own purposes.

Efforts are currently being made to organize

and streamline the procedures so that such

a collaboration can be carried out most

efficiently.

Strengths : An intense source of

continuum radiation which covers the

entire visible and ultraviolet region.

Linii tation: Major facility, high vacuum requirements, guest worker arrangements must be made.

Standard Detectors

The primary standard for detector

calibrations at NBS is a double-ionization

chamber, a gas-filled detector in which

each photon absorbed produces one electronion pair which is collected by a simple

arrangement of parallel plates used to set up the collecting field. The transfer to the actual photodiodes which are available

to customers is accomplished through the use of a uniformly grey (percent of radiation absorbed is independent of wavelength)

thermopile whose efficiency has been

calibrated with the ionization chamber at short wavelengths and checked with a spectral

irradiance standard source at 253.7 nm. Windowed photodiodes calibrated in this manner have been used to evaluate the

response of other detectors such as photomultipliers, and the response of radiometers such as "hazard meters" which are designed to have a spectral response equivalent to a specified erythemal curve.

(1) Window Photodiodes

This transfer standard is basically a vacuum photodiode in which ideally the

only parameter to be determined by reference to an absolute standard is the

absolute photoelectric yield of the photocathode as a function of wavelength.

The detectors are evacuated and sealed by

an ultraviolet transmitting window to avoid atmospheric contamination of the

surfaces. Calibration uncertainties are estimated to be about 6%. The uniformity

of the quantum efficiency of the cathode

can be excellent, on the order of 1 percent.

Aging effects are likewise negligible.

(2) Windowless Photodiodes

Basically this is the same type of

detector as described above, but the lack

of a window and the consequent occasional

exposure of the photocathode to laboratory

conditions affects the overall reproducibility. Nevertheless, there is evidence

that natural aging effects cause not more

than a 20 percent change in quantum

efficiency over a three year test period.

Strengths of (I) and (2): Wide

wavelength region of applicability (20 nm

to 300 nm) ; relatively high accuracy and

stability

Limitations of (I) and (2): These are

low current devices (no amplification)

If they are used to calibrate more sensitive

photomultipliers, linearity and scaling

problems with the latter introduce additional uncertainties.

Summary

Ultraviolet standard sources and

detectors are available from NBS with

uncertainties in the range of 3-10%

depending upon the type of calibration

required and the wavelength region of

interest. In certain cases, arrangements

can be made to use NBS radiometric facilities; collaborative efforts are also

possible. Requests for services should

either be directed to the appropriate

sections (232.03, 232.04, 232.07, or 232.14) or, in general, to the NBS Office

of Measurement Services.

Fart of the research described here

was supported oy the National Aeronautics

and Space Administration.

110
TSS 3P456 (1976)

QUALITY ASSURANCE FOR MEASUREMENTS IN THERAPY^

R. J. Shalek, P. Kennedy, M. Stovall, J. H. Cundiff,

W. F. Gagnon, W. Grant, III and W. F. Hanson Physics Department, University of Texas System Cancer Center

M. D. Anderson Hospital and Tumor Institute, Houston, Texas 77030

Recommendations regarding quality control in physics relating to radiation therapy are derived from experience with the Radiological

Physics Center which is a national review program, and the Texas

Regional Medical Physicists, which is a regional consultation activity.

(Quality control; radiation measurement; tumor dose; radiation therapy;

medical physicist).

During the last half a dozen years

there has been a growing appreciation for

the need for quality control in physics relating to radiation therapy. A much cited

reference^ demonstrates a steep relationship between tumor control probability and

radiation dose for T2-T3 lesions of the

supraglottic larynx from analysis of clinical

results. A dose change of S% results in a change of tumor control probability from 38

to 72%. This rather sharp dependence on the

absorbed dose may be less critical but still true to some extent for radiation treatment

at other sites. Findings which will be re- ported here include the evaluation of radiation measurement and calculation at 174

institutions visited by the Radiological

Physics Center (RPC).t' The RPC reviews the

measurement and calculation systems employed

by institutions participating in interinstitutional clinical trials involving radiation

therapy; it also reviews the validity of

tumor dose statements for individual patients

entered into the trials. In addition

some of the information reported here comes

from experience with the Texas Regional

Medical Physicists (TRMP)c. TRMP has acted

as a consultant to physicists for about

seven years; it does not serve the medical

community directly. Its activities have

become self-sustaining from fees collected.

Services include: mailed thermoluminescent

dosimeters for therapy machine output

checks, dosimetry review visits, computer

calculation of dose distributions for individual patients, analysis of films for verification of beam uniformity and light localizer coincidence, computer calculation of

families of isodose curves from decrement

line data from individual therapy machines,

and rental of expensive but seldom used

equipment.

A finding of these various experiences

is that in most institutions the medical

physicist is reponsible for the correctness

of fulfillment of dose prescription, usually

with no further check or review of his

activities or conclusions. Three recommendations on quality control, which the data

and findings reported in this paper support,

are listed here to give emphasis to them:

1. One person should review and take responsibility for all steps from radiation

measurement to fulfillment of dose prescription. The person best able to do

this is usually the medical physicist.

2. Routine review of the accumulating radiation absorbed dose to patients under

treatment can reduce random errors.

3. Review of dosimetry systems by outside

individuals is important. These reviews

may be informal

.

Findings of the Radiological Physics Center

and Texas Regional Medical Physicists

A summary of the activities of the Radiological Physics Center appears in Table I. As of February, 1976, the RPC staff had

visited 174 institutions for the purpose of

reviewing dosimetry relating to interinstitutional clinical trials. A number of the

institutions had been visited more than once. The criteria used by the RPC for acceptable

dosimetry are agreement to + 3% for machine

calibration, and agreement to 5% on fulfillment of tumor dose prescription. It is

TABLE I. Summary of activities, February 1976.

Number of institutions visited 174

Number of machines reviewed 352

% calibration within + 3% 78%

(275/352)

Number of machines x protocols

reviewed 768

7o tumor dose within + 5% 88%

(678/768)

seen from Table 1 that at the time of the RPC review 78% of the measurement and dosimetry

systems were within the + 3% criteria for

calibration and that 88% were within the +_5% criteria for the fulfillment of tumor dose

as defined in the protocols. When the RPC

111
finds a discrepancy exceeding the criteria

it works with the institution to understand

and resolve the discrepancy. Small discrepancies, often compensating, were found at all but one or two institutions; these were indicated in the reports to the institutions

but there was no follow-up procedure when

the overall discrepancy was less than 5%.

Insight for quality control can be derived from a review of the information from

the dosimetry reviews. In Figure 1 is shown a bar graph in which frequency of occurrence is plotted vs. the ratio of

machine calibration determined by the RPC to the calibration in clinical use by the institution. It is seen that for the 352

machines measured, the mean of this ratio

is very close to 1.00 which suggests that

the calibration methods employed by the RPC are consistent with those employed generally.

The extreme range of the ratio was .84 to 1.14.

TABLE II. Measurement errors.

60

50

40

30

20

10

-4-1-

mean= 1005

a-= .034

It

,84,9 ,92 94 % 98 1.0 1.02 1.04 1,06 1,08 112 114 RPC/ Institution

Figure 1. Frequency of occiirrence versus

the ratio of RPC calibration to that in

clinical use on 352 radiation therapy

machines reviewed at 17^ institutions.

In Table II is a listing of the types

of measurement errors encountered, together

with the number of instances and the maxi- mum discrepancy for each type. The first

entry, the chamber correction factor , is shown in greater detail by the bar graph

in Figure 2. Of 167 i ntercompari sons be- tween the RPC chamber and the chambers in use at institutions, the mean ratio of the

chamber correction factor derived by the

RPC for the institution's chamber to that

Error # Max. error

Chamber correction factor 47 12%

Timer error 38 9% Field size dependence 34 7%

Source movement mechanism 4

T C 0/ Ifa/o Anomalous source decay 3 6%

Distance indicator 4 5%

Light and radiation field 58 15mm

coincidence

Symmetry of beam 33 20%

in use by the institution was 1.005 with a standard deviation of 2,7%. The RPC measuring system is a Farmer chamber and a Keith! ey 602 electrometer. The constancy

of the RPC chamber and electrometer is measured before and after visits by utilizing a Strontium 90 constancy check source

and a cobalt-60 irradiator. Over the same period of time, the standard deviation of

these constancy measurements was +_0,4%.'* The

range of the derived chamber factor expressed as a ratio of RPC to institution was 0.88 to 1.09, In Table III the chamber

calibration results are expressed in another

way according to the origin of the calibration factor. Chambers calibrated at the

National Bureau of Standards (NBS), the

40

mean -1.005 a -.027

30

20

10

-I- 4- 4-

.88 ,94 .96 ,98 10 102 104 1.06 1,09 RPC/ Institution Figure 2. Frequency of occurrence versus

the ratio of the chamber correction factor

derived by the RPC to that in use at an institution for the instrument used to calibrate radiation therapy machines. There were 16T chamber intercomparisons

112
TABLE III. Chamber Intercompari sons

r ihyA'hinn mean

Agency # ratio 0 = > 3%

NBS & MPL 31 1 002 .017 1 RCL 20 1 002 .015 0 ONE INTERMEDIATE 23 1 004 .015 1 VICTOREEN 42 1 012 .028 11 OTHER 51 1 003 .032 9

National Physical Laboratory (NPL), or Regional Calibration Laboratories (RCL)

shov/ed ratios very close to 1.00 with a standard deviation of about 1.5%. The next category which is labeled "one intermediate"

is tiose instruments which have a chamber

correction factor derived by comparison to an instrument assigned a factor by NBS or NPL.

In this category the intermediate instrument

is not at an RCL or Victoreen Instrument

Company. These also appear to have about

the same mean and standard deviation as those from NBS, NPL or an RCL. Those calibrated at Victoreen Instrument Company had

a ratio 1.01 with a standard deviation of

2.8%. Previous problems at that facility

have been acknowledged" and are thought to

be corrected. Those instruments listed in the "other" category are usually the result

of informal or undocumented intercomparisons. Here, though the mean was approximately 1.00 the standard deviation was over 3%. It is important to indicate that

it is possible for any calibrating laboratory to assign an inappropriate calibration factor or for a field instrument to change sensitivity after calibration. A check of the calibration may be done by

informal intercompari sons and constancy

checks of instrument performance. It is advisable to establish a constancy checking

procedure with a radioactive source. This

procedure should be used regularly but

especially before and after chamber calibrations.

A strong case can be made for monitoring of the calibration of radiation therapy

machines by mailed thermoluminescent dosimeters (TLD), This activity has been

offered by the Texas Regional Medical

Physicists since about 1970. The possibility of reviewing and rectifying discrepancies is an important aspect of the mailed

TLD system. A number of calibration

problems have been uncovered which were

then resolved by telephone conversations or

visits to the physicists at the institution

concerned. In Figure 3 is shown the plot

of the ratio of dose by TLD to the dose

stated by an institution for a 6 MV X-ray

machine and a cobalt-60 irradiator from

0.9

Co -60 6 MV

1973 1974 !975 1976

Date Figure 3. Ratio of radiation measured by

mailed TLD to that predicted by an institution from their machine calibration versus the date of irradiation. The open circles

are for 6 !W X-rays and the solid circles

are for cobalt-60 gamma rays.

the same institution. Prior to 1973 the

X-ray points had considerable variability

although on average would have been close

to 1 (agreement). It is not entirely clear

what caused the early observed variability.

In our hands 3% from an expected value

is taken to be within the experimental

error. When a discrepancy of 5% or greater

is indicated, TRMP requests the initiating

physicist to repeat the TLD irradiation.

In the AAPM protocol for high energy

x-rays and gamma rays^ the recommended

method of calibration is at 5 cm or more depth in water depending upon the energy.

An alternative method in that document is calibration in air for cobalt-60 gamma rays

and 2 MV X-rays. The RPC has collected data

on many cobalt-60 machines indicating that

these tvyo methods of calibration appear to yield different calibration values. In Table IV the results of this study are listed,

The uncertainty indicated is the standard

deviation of the mean. The numbers in parentheses indicate the number of machines

in each category. Thus, machine calibrations

for cobalt-60 machines at 80 cm from the source may be 1 to 2% higher if the calibration is done at 5 cm depth in water than

if calibration is in air. This comparison assumes the use of standard tables for backscatter factors and depth dose®. The reason

for discrepancy is not entirely clear. It could be in the use of standard tables which

do not precisely apply to a particular

machine or there may be a more fundamental

problem involving the use of cylindrical

ionization chambers in air and in phantom.

113
TABLE IV. Calibration in water/calibration in air, 10 X 10 cm2 field, 80 cm SSD.

5 cm Depth:

All Machines 1 014 + .009 (42)*

Picker 1 .015 T .008 (15)

AECL 1 014 T .009 (27)

10 cm Depth:

All Machines 1 028 + .009 (33)

Picker 1 027 .007 ( 9)

AECL 1 028 + .010 (24)

susceptible to alteration by jarring. It is worth mentioning that physicists should be careful that their method of measuring dis- tance during calibration is the same as that

used clinically. The entry, light and radiation field coincidence , refers to the geometric alignment of the localizing and treat- ment beam. The last entry, symmetry of beam , applies particularly to x-ray machines in which the compensating filters, the target,

or the direction of the accelerated electrons

is not on the axis of the collimator.

15 cm Depth:

All Machines

Pi cker

AECL

1.027 + .010

1.025 + .008

1.028 + .011

(31)

( 8)

(23)

* Numbers in parentheses indicate number of

machines in each category.

The other measurement discrepancies in Table II are more or less self explanatory.

The entry, timer error , means the possible

difference between the time or monitor units

set and the effective irradiation span by

the machine. The field size dependence

means the difference in measured exposure

rate or absorbed dose rate with different

field sizes set on the machine. At some institutions the same exposure rate or absorbed dose rate was utilized for all field sizes. Source movement mechanism

refers to a few cobalt-60 machines in which

the source was not centering over the aper- ture and to one machine with a problem

with a mercury shutter. These machines

had been calibrated with the source or shutter in proper position, but due to mechanical fault the source at the time of

the RPC visit was not centering or the shutter not opening completely with a consequent

reduction in dose rate and the appearance

of asymmetry in the beam. These problems

indicate the danger of doing only an initial

machine calibration with subsequent calculation of clinical dose rate from source decay.

At one institution more than two years separated the calibration and the discovery of

the problem. The entry, anomalous source decay , refers to the few instances in which

a cobalt-60 source was apparently decaying

with an abnormal half-life. It is speculated

that this result occurred in sources which were composed of cobalt-60 pellets of different specific activity. The pellets of

greater specific activity were placed initially at the front of the source but as the source was moved the pellets became mixed,

resulting in a changed gamma-ray attenuation

and a decrease in exposure rate. The entry,

distance indicator , applies principally to optical distance indicators which may be

The errors involved in the determination of tumor dose includes the discrepancies which may have occurred in calibration

and, in addition, the use of inappropriate

factors in calculating tumor dose. The

definition of tumor dose by the institution

also enters. The relationship between the

frequency of occurrence and the ratio of

tumor dose determined by the RPC to that

stated by the institution is shown in Figure 4 for simple radiation treatments.

60

50

40

30

20

10

oU.

.7 .8 .85 LlLL

mean-. 996

<r=.046

4-

1.0

RFC/institution

I.I 1.12 1.18 1.23

Figure k . For uncomplicated treatments

the frequency of occurrence versus the ratio

of the fulfillment of tumor dose estimated

by the RPC to that stated by the institution.

Irregular fields (mantle, etc.) and tangential breast treatments are not included.

Each of the 352 treatment machines reviewed

are counted once.

114
Not included are data on irregular fields

(mantle, etc.) and tangential breast treatments. This graph is indicative of the re- liability of the fulfillment of tumor dose

prescription for simple treatment techniques

and would apply to protocol and non-protocol

patients. Again the findings cluster around

1.00 with an extreme range of 0.71 to 1.23.

In Figure 5 the frequency of occurrence

is plotted vs. the ratio of RPC to institution for tumor dose as defined in the

various protocols. If several points of

calculation were called for in the protocol,

the point showing the worst discrepancy

I50r

TABLE V. Calculative errors.

130

no

90

70

50 -

30

10

I -.44

I -.52

I -.57

I - .71 2- .76

I - .80

2- .81

I - .82

I -1.24

mean =0.994

a =5.3%

I 84 88 92 96 1.00 L04 1.08 RPC /Institution

1.12 1.16 1.20

Figure 5 • For all protocols the frequency

of occurrence versus the ratio of the fulfillment of tumor dose estimated "by the RPC

to that stated hy the institution. Each

treatment machine is multiplied "by the number of protocols treated upon that machine

to obtain the number of events. There was

a total of T68 events. Where more than one calculation point is specified in the protocol, the point with the worst agreement

is plotted.

is plotted here. These results are for 352

radiation machines and usually include

multiple protocols per machine. The mean

is close to 1.00 with a standard deviation

of 5.3%. The extreme range of 0.44 to 1.24

renders the distribution non-gaussian. It

is interesting that the + 5% criteria for

acceptable fulfillment of tumor dose is only slightly less than one standard

deviation of the values found in practice.

The types of calculative errors encountered are shown in Table V. A

Error Max, error

Attenuation factor 94 1.5%

f-factor or 43 6%

Tray factor 39 14%

Off-axis correction,

irregular fields 37 13%

Wrong % depth dose 49 20%

Backscatter factor 63 7%

Rad (muscle)/rad (H2O) 134 1%

frequent calculative error was the

neglect of the attenuation factor of 0.985

required for cobalt-50 measurements made in air. The second entry, f-factor or Z\ , is the neglect of or the improper transition

from exposure to absorbed dose with a maxi- mum error of 6%. The larger errors in this

category arose because of confusion of

roentgen and rad units on the part of some institutions. Occasionally, the same numerical value was calculated in roentgen and reported to the clinical

study in rad. The tray factor (attenuation

in the tray for holding secondary blocking)

is sometimes neglected, with errors of up

to 14%. The next entry, off-axis correction

for irregular fields , applies to those in- stances where the dose at selected points

is determined by calculation and not derived

from isodose curves^. This correction is not required in rectangular fields where

isodose curves are available since in these instances the off-axis correction is included in the isodose curves. The entry,

wrong percent depth dose , was usually a systematic mistake in the application of

published data or in a few instances was

the result of experimental depth dose

measurements which were incorrect. The

entry, backscatter factor , means lack of, or improper use of, backscatter factor.

The last entry, rad (muscl e )/rad (H 9O) refers

to the question of whether absorbed dose

should be calculated to muscle or water. The ratio is 0.99 at cobalt-60 and higher

energies. No opinion is expressed here

concerning which is more correct.

Information on the usefulness of an outside review procedure is shown in Table

VI. The RPC made 69 revisits. Table VI is c listing of the number of institutions in which the review concluded that the fulfillment of tumor dose was improved, the same as, or worse than the earlier visit.

The numbers are not strictly valid since the

complexity of the protocols reviewed on different visits may not have been the same.

115
TABLE VI. Impact of RPC visits.

Revisits

Number of revisits 69

Tumor dose ratio, RPC/Institution:

Improved 26 (38%)

Same (within + 1%) 32 (46%)

Worse 11 (16%)

Correction of Major Problems

Number of problems exceeding 5%: Found 62

Resolved 51 Not Resolved 11

Nonetheless, the numbers indicate that tne visits were probably useful to institutions

in tightening control over their techniques.

The fact that 15% were in worse agreement at the time of the second visit was discouraging but may indicate that a continuing re- view effort is necessary. In the RPC reports

which are submitted to the institutions,

apparent discrepancies are discussed and

recommendations to review the problems in question are made. When the total discrepancy at an institution exceeds 5%,

special efforts are made to resolve the

problems. Of 62 such instances 51 have been

resolved to date. Of the 11 unresolved

problems, some are in negotiation, but a few are problems of long standing.

Random Errors - Experience of

RPC and Other s

The measurement and calculation

problems discussed have been systematic in nature. The RPC has not kept a record of

the many random arithmetic errors that

have been discovered in the review of

patient's dosimetry calculations, which it does for each patient entered into most of

the studies. There have been literally

hundreds of these. The RPC findings are consistent with a retrospective review of

patient charts by Sutherland^". He discovered numerous arithmetic errors, or errors in the choice of parameters for calculation. The regular review of the

accumulating patient dose during treatment

is an expensive but worthwhile procedure.

Physicists at M. D. Anderson Hospital have

been doing this weekly for more than 20

years

Another level of quality control is the

reproduceabi 1 ity of treatments from day to day upon patients. A study by Kartha and

his associates'^ involved the passive

monitoring of four machine parameters on

over 5,000 radiation exposures. The

criteria used for declaring an accidental error were rather generous but even so it turned out that accidental errors occurred on the order of 3% of the time as shown in Table

VII. If these were on a random basis, it

is likely that the contributions of such errors to the uncertainty in total dose

was not large. Of course this institution

may be above average in the care taken by

personnel

TABLE VII''. Accidental errors of various

types and frequency of each

in percentage of the total.

Minimum Accicriteria # of dental

Parameter used setups errors

C

Field size 1 cm 2364 78 3 3 Gantry 15° 1064 23 2 2 Col 1 imator

rotation lOo 1136 26 2 3 Treatment

time 0.5 min 1011 36 3 6 Total 5575 163 2 9

(Data are from Kartha, Chung-Bin, Wachtor

and Hendrickson ^ ^ with permission of the

American Institute of Physics)

Uncertainties In The Delivery Of

Absorbed Dose

When one set of measurements and calculations are compared with another it is necessary to consider the uncertainties in each. Is it possible to determine the tumor

dose with substantially less uncertainty

than the +5% which is utilized by the RPC for acceptable fulfillment of tumor dose

prescription? Loevinger and Loftus^^ have

considered this question by estimating the

systematic and random uncertainties in the

measurement chain from national radiation

standards to the delivery of absorbed dose

to a single point in a tissue phantom by

cobalt-60 gamma rays. The steps are:

1) choice of physical constants for the

national radiation standard, 2) determination of exposure in the standard beam, 3)

calibration of the secondary standard

instrument for a regional calibration laboratory, 4) calibration of a field instru- ment by a regional calibration laboratory,

5) calibration of the treatment beam with

the field instrument, 6) calculation of

absorbed dose to a single point in a tissue

phantom. In order to include in the model

the variations in the quality of measurement

116
procedures and instrumentation, starting

with step 4, two models are defined. The

minimal model represents a minimum standard

of practice and instrumentation, while the

optimal model represents the best current

standard of practice and instrumentation.

There is no limit on the magnitude of

mistakes that can be made. Thus the large

systematic errors plotted in Figure 5 would

be outside both the optimal and minimal

models. Large arithmetic errors that can

be made would also be outside both models.

The institution and the RPC usually have

instruments traceable to the same national

standards. Thus it is only in steps 3 to 6 that differences in measurement and calculation may arise. Loevinger and Loftus con- cluded that the 95"^ confidence limit for the

combination of these steps (3 to 6) is +2°^ for the optimal model and +5% for the minimal

model. The limits for steps 3 to 5 (i.e. to calibration of the treatment beam) were about

the same. From these calculations of uncertainty, the choice of +5% for the criteria

for fulfillment of tumor dose is reasonable

for simple treatments. For complex treat- ments such as with irregularly shaped fields

the +5% criteria may be too small. The +3%

criteria for machine calibration is reasonable.

Recommendations for Quality Control

The implications for quality control

from the above information suggests the

following points:

1. One person should review and take

responsibility for all steps from

radiation measurement to fulfillment

of dose prescription. The person

best able to do this is usually the

medical physicist.

2. Routine review of the accumulating

radiation absorbed dose to patients

under treatment can reduce random

errors

3. Review of dosimetry systems by outside individuals is important.

These reviews may be informal.

4. Chamber calibration factors should

be directly traceable to the

National Bureau of Standards. That

is, there should be not more than one intercompari son instrument be- tween the national radiation

standard and the field instrument

used for therapy machine calibration.

5. Mailed thermoluminescent dosimeters

are very useful for checking the

calibration of radiation therapy

machines. Provision for resolving

discrepancies by telephone conversations or visits is important.

6. Monitoring of machine parameters in individual radiation treatment is probably useful. Time or monitor

units set is likely the most important parameter.

7. The most common measurement and calculative errors are listed in Tables II and V.

a. Supported in part by NCI Grant

CA 10953.

b. In 1968 the Radiological Physics

Center (RPC) was established by the American

Association of Physicists in Medicine (AAPM)

upon suggestion of the then existing Committee for Radiation Therapy Studies (CRTS)

which has been succeeded by the Committee

for Radiation Oncology Studies (CROS). The

Radiation Therapy Committee of the AAPM

serves as the steering committee for the RPC. The present membership of that committee is as follows: Peter Wootton (Chairman),

Farideh Bagne, Frederick L. Faw, James R. Goodgame, Mukund K. Kartha, John H. LeVan,

Larry Simpson, Alfred R. Smith, Nagalingam

Suntharal ingam, Kenneth A, Wright, Robert J. Shalek (RPC), Max Boone (CROS), Jack S. Krohmer (ACR), Richard C. Granke (ANSI),

and Robert M. Quill in (ANSI). The operating

staff of the RPC include physicists:

Lawrence W. Berkley, Jack H. Cundiff, William

F. Gagnon, William F. Hanson (Associate

Director), and Robert J. Shalek (Director);

technicians: Jamie Boling and Paula Kennedy;

and secretary: Brenda Truitt. The interinstitutional clinical studies (with

clinical chairmen shown parenthetically)

being served by the RPC include: Acute

Leukemia Cooperative Group B (James F. Holland, M.D,), Brain Tumor Study Group

(Michael Walker, M.D.), Central Oncology

Group (Theodore Grage, M.D.), Children's

Cancer Study Group A (Denman Hammond, M.D.),

Cooperative Hodgkin's Disease Clinical Trial

(James Nickson, M.D.), Eastern Cancer

Oncology Group (Paul Carbone, M.D.),

Gynecologic Oncology Group (George C. Lewis,

Jr., M.D.), National Bladder Cancer Cooperative Group A (George Prout, M.D.), National

Surgical Adjuvant Breast Project (Bernard

Fisher, M.D.), National Wilms' Tumor Study

(Guilio D'Angio, M.D,), Radiation Therapy

Oncology Group (Simon Kramer, M,D,), Southeastern Cancer Study Group (John R, Durant,

M.D.), Southwest Oncology Group (Barth

Hoogstraten, M.D.), Veteran's Administration

(S. Stefani, M.D., E. Humphrey, M.D., R. Byhardt, M.D.), Western Cancer Study Group

(Joseph Bateman, M.D.), Intergroup Ewing's

Sarcoma Study (Mark Nesbitt, M.D.), Inter-
group Rhabdomyosarcoma Study (Harold Maurer,

M.D.)' Studies which are now closed include:

Cooperative Study on Radiotherapy for

Carcinoma of the Prostate. (Juan del Regato,

M.D.), Renal Cell Carcinoma Adjuvant Study

(Clair Cox, M.D.), The Controlled Study to Determine the Value" of Combined Irradiation

and Chemotherapy for Squamous Cell Carcinoma

of the Oral Cavity, Pharynx, and Larynx

(Simon Kramer, M.D. )

c. TRMP council: Louis Deiterman, Jr. (Chairman), Lloyd Johnston (Vice-Chairman)

,

Donald Arnold, Louis Levy, Robert Perry,

and Ann Wright; operating personnel include:

Cathy Kasper, Joye Roll, Robert J. Shalek

(Director), and Marilyn Stovall (Associate

Di rector)

References

1. Shukovsky, L.J. (1970) "Dose, time,

volume relationships in squamous cell

carcinoma of the supraglottic larynx" Am.

J. Roentgenol., Radium Therapy and Nucl

Med. 108, 27.

2. Golden, R. , Cundiff, J.H., Grant,

W.H., and Shalek, R.J., (1972) "A review

of the activities of the AAPM Radiological

Physics Center in interinstitutional trials

involving radiation therapy" Cancer 29,

1468.

3. Grant, W. (1974) "AAPM Radiological

Physics Center: A status report" Medical

Physics 1, 41.

4. Grant III, W.H. , Cundiff, J.H.

,

Hanson, W.F., Gagnon, W.F., and Shalek,

R.J. (1976) "Calibration Instrumentation

Used by the AAPM Radiological Physics

Center" Med. Phys. in press.

5. McCarthy, W.A. (1975) "Importance of

the calibration of all the parameters

affecting radiation exposure measurement"

Med. Phys. 2, 83.

6. AAPM (1971) American Association of

Physicists in Medicine, "Protocol for the

dosimetry of X- and gamma ray beams with

maximum energies between 0.6 and 50 MeV"

Phys. in Med. and Biol. 16, 379.

7. Grant III, W.H., Cundiff, J.H.,

Gagnon, W.F., Hanson, W.F., and Shalek,

R.J. (1976) "Calibration in water versus calibration in air for cobalt-60 gamma

rays" Med. Phys. in press.

8. Cohen, M. , Jones, D.E.A. and Greene,

D., Eds. (1972) Central Axis Depth Dose

Data for Use in Radiotherapy, a survey

prepared under the auspices of the Hospital

Physicists Association, published as Supplement No. 11, Brit. J. Radiol.

9. Cundiff, J.H., Cunningham, J.R.,

Golden, R., Lanzl , L.H., Meurk, M.L.,

Ovadia, J., Last, V. Page, Pope, R.A.,

Sampiere, V.A., Saylor, W.L., Shalek, R.J.,

and Suntharal ingam, N. (1973) "A method

for the calculation of dose in the radiation

treatment of Hodgkin's disease" Am. J. Roentgenol., Radium Therapy and Nucl. Med.

117, 30.

10. Sutherland, W., (Oct. 1973), Oral

presentation at Hospital Physicists Association - National Physical Laboratory

meeting, Teddington, England.

11. Kartha, P.K. I. , Chung-Bin, A.

,

Wachtor, T., and Hendrickson, F.R. (1975)

"Accuracy in patient setup and its con- sequence in dosimetry" Med. Phys. 2, 331.

12. Loevinger, R. , and Loftus, T.P. (1976).

"Uncertainty in the delivery of absorbed

dose." Proceedings of the International

Course on Ionizing Radiation Metrology,

Varenna, Italy, September - October, 1974,

in press.

118
NBS SP456 (1976)

QUALITY ASSURANCE MEASUREMENTS IN DIAGNOSTIC RADIOLOGY

M. P. Siedband

Medical Physics Section

Department of Radiology

University of Wisconsin

Madison, Wisconsin 53706

The concatenation of elements in an x-ray system necessitates

a two level approach to system evaluation. The first level tests determine the weakest links of the chain while the second level tests provide more precise measures of elemental performance.

Several types of first level pass-fail tests are described in terms of their utility in a routine quality assurance program

for use in hospitals and in their relation to the second level

tests. The requirements for accuracy and precision of the

measurement apparatus are discussed.

The Need for Quality Assurance Measurements

The variations of performance of

machines and their operators in obtaining

diagnostic x-rays have always been known,

but only in recent years have the magnitudes

of these variations been studied. The

National Institute of Occupational Safety

and Health (NIOSH) examined about 75,000

chest x-rays as part of a nationwide

program to detect pneumoconiosis in coal

miners.^ These films were about equally

divided into good quality, fair quality

and poor quality, and about three or four

percent of the films were considered useless. A number of surveys of the performance of x-ray apparatus have shown

variations of over 3 to 1 in measured

output for similar settings,^ The

Nationwide Evaluation of X-ray Trends (NEXT)

task force, formed in 1971 of seven state radiological health program representatives

and a representative of the Bureau of

Radiological Health, has developed a method

for conducting surveys of diagnostic x-ray

procedures, and has collected patient

exposure data for a number of diagnostic

examinations. The extremes of measured

exposure varied by a factor of 10 for most examinations and over 100 in the worst

instances.^ These surveys clearly show

the need for the routine use of quality

assurance measurements in diagnostic

radiology

In order to decide what types of

measurements are appropriate, it is

necessary to identify the areas where

problems arise. There are a number of

factors which can cause degradation of

radiographs; we can tabulate the most

common ones: 1) The operator may select

the wrong x-ray exposure factors, kVp

and mAs, or may fail to use the collimator

properly and expose too great or too small

an area, or may have the patient positioned

incorrectly requiring additional exposure.

2) The apparatus may slowly drift from its

correct setting - as the tube filament ages

the anode current will shift from its

calibrated values even though the filament

current remains stable. 3) The automatic

replenishment system of film processors

will add chemicals at an incorrect rate if

a succession of films larger or smaller

than a particular average size are processed. This is especially true when

long lengths of fluoro spot film, 70 mm

to 105 mm, are processed. Improper

operation of automatic film processors can result in underdevelopment of films re- quiring up to 4 times the normal x-ray

exposure to obtain equivalent film density.

4) Component failure, the inadvertent

absence of critical parts such as filters,

or mechanical misalignment between the

x-ray source assembly and the grid

preceding the film cassette will obviously

cause variations in equipment performance.

For these causes of variation of film

quality, the most common procedure is for

the technologist to close his own servo

loop by adjusting the x-ray exposure

factors as a result of his observations of

the average film density.

Besides the causes of degradation of

radiographic quality listed above, there are several other factors which must be

considered in a quality assurance program.

There are significant differences in the

energy distributions and efficiencies of

single phase, three phase, inverter type,

capacitor discharge and constant potential

generators. Generators other than

constant potential and filtered polyphase

machines tend to favor production of lower

energy x-rays, which are almost entirely

119
absorbed by the patient. Incorrect filtration will have little effect on the

appearance of the finished films but a great

effect on patient exposure. The film-screen

combination must be chosen to give the best

compromise between contrast, detail and

noise in the final radiograph, and patient

exposures for the examination being performed.

Normal manufacturing tolerances in the thick- ness of the glass wall of the x-ray tube,

variations in electron guns (beam concentration), anode heel effect, phototimers which

must complete an integral number of half

cycles of operation, and manufacturing

variations in films and screens are also causes of variations in diagnostic

radiographs

One problem of quality assurance measurements in diagnostic radiology is

the subtle nature of image degradation.

Due to the internal impedence of the x-ray

generator, a shift of tube anode current

caused by normal drift or parts failure

will cause a change in the opposite direction of the tube voltage and may result in no perceptible change in the density of the

final radiograph. However, the contrast

scale, that is, the film density change vs. object thickness change, is a function of

the voltage applied to the x-ray tube, as

is the absorption of certain contrast

media, i.e., iodine based contrast agents.

Since most diagnostic x-rays are negative,

the absence of the image of an anomaly may

be interpreted to mean the absence of the

anomaly rather than the failure of the

apparatus to image it.

In simplest terms, to use the

radiographs themselves as quality control

devices is just plain wrong. Since the

radiograph incorporates the results of the

individual elements, the quality of the

power lines, the characteristics of the

x-ray generator, filter, collimator,

patient, grid, film-screen combination,

film processor, x-ray illuminator and the

human viewer, even if the image does not look quite right, it may still not be

possible to determine the cause and take

the correct remedial action. Tests using

standard phantoms may be of value in

assessing overall system performance.

However, objective tests of each element

of the system over the normal range of

operation will permit the determination

of the operating characteristics of each

element, and show whether it is operating

within normal bounds. Such objective

tests would also permit the Intercompa rison

of different x-ray systems and their

optimization to produce the best radiographs and the least patient exposure.

The Need for Three Levels of Quality

Assurance Measurements

The operator or x-ray technologist

taking the radiograph is the individual

who observes the operation of the machine

most closely. The radiation physicist,

if one is used, will only survey the

equipment intermittently. Medical physicists at hospitals often limit their

efforts in diagnostic radiology to radiation protection or to the teaching of

physics to residents so that they can pass

their board examinations. Only rarely are they engaged in the day to day problems of

quality assurance. Moreover the consulting

costs of outside physicists are such that

it is probable that they will not be called

each time the operator suspects that the

apparatus is not performing optimally.

Thus it is important that the operator or x-ray technologists have test tools and a series of procedures that he may use in

a first level quality assurance program.

Since it is possible to have films of

proper density and reasonable appearance

even though the apparatus is not functioning correctly, these procedures must be

performed on a regular schedule. This first

level quality assurance program must include a number of pass-fail tests,

involve test exposures, include quality

control tests of the processor, and be safe

for the operator and the equipment. The

equipment must not be dismantled or operated in an unusual way. Furthermore,

the first level tests should be capable

of being understood by the radiologist

and by the technologist. For example,

a series of test patterns may be radiographed to estimate the focal spot size

of the x-ray tube rather than using a pinhole test for the same purpose. The test pattern may be readily related to anatomical parts so that the effect of

improper focal spot size may be seen in terms of obvious effects rather than

abstract numerical parameters.

During the past several years a program has been underway at the

University of Wisconsin to teach first

level quality control to x-ray technologists and physicists.^ A number of test tools were developed for this purpose.

Many of these tools are modifications or improvements of earlier tools used at other institutions or in industry. The

tools, which are described in more detail

120
in the appendix, include an x-ray test cassette for estimating actual kVp values,

a focal spot test tool which incorporates

a lead pattern comprising eleven groups of

bars, a timing and mAs step wedge device

which contains a motor driven spinning top

and a six step copper wedge, a copper

mesh pattern containing' eight pieces of

mesh ranging from 16 to 60 mesh holes per

inch, an aluminum penetrameter , and a collection of standard devices such as a photographic sensitometer and densitometer

and other devices. This collection of test tools permits the operator or technologist to assess the performance of his

equipment on a scheduled basis or whenever

malfunctioning is suspected. The sensitometer and densitometer would be used for

a daily evaluation of performance of

automatic film processors.

A second level of quality assurance

testing would be that performed by a qualified physicist or engineer familiar

with the inner workings of the x-ray

apparatus and with the use of measurement

devices capable of providing more accurate

numerical results. The engineer developing

new apparatus will find it necessary to determine in some detail whether his new system is better than the old one. The

physicist performing acceptance tests of

a new installation will want to know

whether all of the specifications listed

in the purchase order have been met and

whether each element of the system has

been installed and aligned properly.

Detailed measurements of this type dictate

the use of more sophisticated measurement

apparatus and are far more painstaking

and time consuming than pass-fail quality

control tests. Tests at the second level

would include those described in industry

standards, in international standards, in

the reports of professional societies, and

those prescribed by national and international organizations concerned with

radiation protection and measurements.

At the second level of testing, the physicist responsible for recommending the type

of radiation shielding to be used in an installation would evaluate the completed

room with its installed apparatus to be

certain that his recommendations have

been followed.

A third level of testing^ is con- cerned with verifying that the installation

meets state and federal standards of

performance. Many of these tests are the

responsibility of the manufacturer to certify that his apparatus performs in a

constant and safe way. Compliance with

state and federal requirements, however,

does not necessarily mean that the x-ray

apparatus will meet the user's requirements regarding selection of the tube,

generator, and associated apparatus

to solve the particular diagnostic problem.

What Goes Wrong

A good quality assurance program

should attempt to solve real problems.

To understand the nature of the real

problems, operator and equipment performance must be surveyed and the limitations

of equipment design must be understood.

We will look first at the x-ray tube

and generator system. The emission of

the x-ray tube filament is proportional to the 8th power of filament current

(Richardson-Dushman and Langmuir equations)

,

approximately. Thus a 1% change of

filament current results in an 8% change

of emission. The high voltage transformer

is made small to save costs and has a fairly high impedence to protect the system

in the case of a tube or cable short

circuit. The regulation is of the order of

4 kVp/100 mA (big systems) to 12 kVp/100

mA (smaller systems). X-ray production is

roughly proportional to (kVp)^ and

transmission through thick parts is proportional to (kVp)^ so that film darkening

is roughly proportional to (kVp)^. Thus

a 10% change of kVp will cause a change of up to 50% in film exposure.

Consider then, a generator which is

supposed to operate at 80 kVp/400 mA but

where the filament current drops by 4%

due to temperature and aging effects. The

emission will fall about 30%, the kVp will

rise about 10 kVp , and the film density

may actually increase slightly. Most

important, the contrast (especially of

iodine compounds) will decrease; clearly,

film density is no guide to generator

calibration.

We consider next the x-ray tube

collimator. The tube collimator serves

four purposes: 1) to define the limits

of the x-ray field, 2) to predict the

position of the field with a localizing

light and/or dial scale, 3) to add

necessary beam filtration and 4) to reduce

off-focus radiation. The most common

field problem of col].imators is that the

light and radiation fields may not be

congruent - the bulb may shift, the mirror

may shift, or the entire assembly may

121
shift out of position. The filter may be

incorrect. Rotating anode x-ray tubes

are imperfectly designed, with only 75%

or so of the primary electron beam

impinging on the focal spot. The remaining

electrons are broadcast throughout the

interior of the tube.*' A good collimator

will use lead elements to define a minimum

solid angle to exclude the off-focus

radiation. If these lead elements are out

of position (or absent) off-focus radiation

may exit the tube.

The next stage in the diagnostic x-ray

system is the imaging element. We will

consider first image intensif iers . Most

image intensifiers have a built-in vacuum

pump which will use either an external

power supply or will derive power from

focus electrodes. In any case, when the

tube is gassy, the point of best focus will

be different from that obtained when the

tube is "hard." It is normal for a new

tube to be a bit gassy. When the tube has

not been used for a while it may take

several hours to pump down. The resistive

voltage divider for the focus electrodes

may drift as the parts age. Thus, the

electrical focus may have to be adjusted

from time to time. Limiting resolution

of a high contrast copper mesh or lead

grill pattern may not be sufficient to describe the performance of an imaging

system. Other tests, for low contrast

performance, are also required. A 2% penetrameter test or a measurement of

the veiling glare (a particular type of

contrast ratio test) will be helpful in

measuring the imaging tube's ability

to visualize the soft shadows of low

contrast tumors.

Where films and screens are used for

imaging, the film processor must be

considered. The film processor is the most critical apparatus in an x-ray

department. Variations of temperature

and replenishment rates, contamination,

light leaks (or unsafe safe lights) , or improper set-up and operation are responsible for changes in base fog level, mean density (sensitivity) , and gamma

(contrast) . ^ ' Splashing (fixer to developer) and/or contamination (residues

of cleaning agents, rust of steel wool

fibers) will cause random fluctuations

of performance. Mechanical slippage will

alter the time-temperature relationship

required to obtain good films. The slow

changes of performance which may be

caused by shifts of temperature or replenishment rates must be plotted to

be able to take corrective action. X-ray

test exposures, e.g., exposure of a test wedge, are not valid tests of film

processors because the normal variation of

generator output (2% line, 2% display of

kVp, 1% of filament and 8% of mA) will

total about 20% over long times, and will

not yield values of film exposures, density,

gamma, etc. which are useful for quality

control purposes. A sensitometer and

densitometer must be used.

A First Level Quality Assurance Program

The elements necessary for the implementation of a successful first level

program may be listed as follows:

1) Realization of the necessity and

benefits of the program by the department

head and all subordinates, 2) regular

scheduling of tests with provision for

additional tests when problems are suspected,

and following repairs, 3) daily film

processor testing, 4) availability of

simple tests which give valid test data

when compared with more elegant methods,

and 5) scheduled monitoring by a qualified

medical physicist.

A series of first level tests which

incorporates tests known in industry and in

hospitals for many years has been incorporated into a quality control program developed

at the University of Wisconsin. Similar

programs have been developed by other

universities and a protocol for first level

testing is near completion by a Working

Panel of the American Association of

Physicists in Medicine.

The series of tests can be combined

with visual inspection of the rooms and

equipment. Cables should be examined for

insulation damage. Mechanical devices and

locks should be checked for proper

appearance and functioning. The room

inventory should be listed. Test data

should be recorded and filed for each room

and compared With current data to note

changes. Test tools may be purchased, or tools similar to commercially available

ones, may be built, but they should always

be available for use. For most rooms, tests should be done every month; special

procedure rooms may be tested more frequently. A test form should be developed

for recording test data. Test limits should

be provided by a first survey accomplished

by a medical physicist or, instead of

limits, actual values can be recorded and

monthly variations noted.

122
Some of the test tools and their

applications are described in the following

sections. The test tools permit testing

without dismantling the system. Using the

clamp-ammeter, adjusting electrical focus,

estimating veiling glare and a few other

tests may require some additional skill

beyond that of some technologists but all

are within the capabilities of field service

personnel

The Wisconsin X-ray Test Cassette

provides a means for estimating the tube

potential and filtration of diagnostic

x-ray equipment. The technique used is

a modification of methods described by

Ardran and Crooks

The Test Cassette is a modified

8" X 10" x-ray cassette using a single

intensifying screen covered by an optical

attenuator in five selected areas. Attached to the front of the cassette is

a lead mask containing five pairs of

columns of ten holes each. The mask is

positioned so that the right hand member

of each pair of columns is directly above

one of the gray bars of the optical

attenuator. Over the left hand column of

holes are positioned copper step wedges,

one step per hole. A sheet of copper is

positioned above the first four pairs of

columns for kVp measurements; the fifth

pair is used to estimate the HVL. The

front of the cassette is enclosed by a plastic cover with the various test areas

identified.

The 1/16" copper sheet above the

first four pairs of columns, which are

used to measure tube potential, serves to "harden" the incoming primary x-ray beam

by removing most of the low energy photons.

No beam hardening absorber is used over the

fifth pair of columns because the halfvalue layer depends on the energy distribution of the beam.

The result is a narrower x-ray spectrum

with an energy range over which the

attenuation coefficient of copper is almost

a linear function of energy. There is a particular thickness of copper absorber

which, when placed over the unattenuated

intensifying screen results in an optical

density in the test film equal to that

obtained with no absorber and with the

optical attenuator. For a fixed optical

attenuator the copper thickness required

to accomplish this match increases with

increasing kVp. This "match" thickness is

used to estimate kVp and HVL. For the kVp

measurements, the "match" thicknesses are determined for the hardened beam; for the

HVL measurement the non-hardened beam is

used.

The four copper wedges used for kVp

determination have different thickness

ranges to allow kVp measurement over a

tjide range (50-140 kVp) without sacrificing

accuracy. The wedges are designed so that,

from left to right, the "match" thickness

for 60 kVp, 80 kVp, 100 kVp and 120 kVp are close to the center steps and the kVp

per step is small.

In use, a total of five exposures is

usually made. The cassette is loaded with

any standard x-ray film. The long (10")

sides of the cassette should be parallel

to the anode-cathode axis of the x-ray tube.

The collimator is adjusted so that the

entire length and width of a region are covered. Exposures are made one region

at a time with the other regions of the

cassette covered with lead sheets. The

exposure is not critical. Typical mAs values at 1 meter are given in Table 1 for

single phase x-ray units. However, variations in film type and processing may make

it desirable to adjust these values. This

will not affect the kVp accuracy of the

test cassette. The mAs requirements for

three phase units are about half of those

for single phase units.

UTien exposing the half-value region,

the collimator is opened to include the

rectangular region marked "identification"

where lead characters or other identification markers are placed to identify the

pertinent data, such as machine and date.

After the film is developed the

resultant image consists of 10 columns of

dots in 5 pairs. Of each pair of columns,

the right hand one will consist of dots of

nearly uniform density, while the other

column will show a density gradient. The net density of the uniform reference

column should be between 0.5 to 1.5. If

not, the exposure should be repeated with

an appropriate alteration in the mAs value

in Table 1. The location in the columns

where adjacent dots are of equal density

is a measure of the kVp . The use of a densitometer is preferable, but a good

approximation (+3 kV) may be made by eye.

If an exact density match is not present,

interpolation is used.

Each cassette must be calibrated with

a known kVp x-ray unit. The calibration

123
curve for three phase units lies above the curve for single phase units. The HVL

region is calibrated at 60 kVp against known

HVLs.

The cassette is capable of reading to an overall accuracy of within 3 kVp. It is

suggested that machines within a department

be calibrated with the x-ray test cassette

and monitored from time to time to be

certain that kVp variations of less than

5 kVp for 60 and 80 kVp measurements and

less than 8 kVp for 120 kVp measurements are maintained. Deviation from these values

may indicate defective rectifiers or loss of one phase for polyphase machines. A high

HVL may indicate that the x-ray tube is near the end of life or that the added

aluminum filtration required for most systems has been removed, subjecting the

patients to unnecessary exposure.

Similar test cassettes have been

developed having only a single pair of

columns centered over the 80 kVp match point.

These are useful for quick tests at high mA settings to verify both kVp and mA since an incorrect mA value will cause a shift of kVp

,

the most common type of generator fault.

The Wisconsin Timing and mAs Test Tool

is a plastic box containing a six step

copper step wedge and a 1 rps motor rotating

a brass plate which has four slits spaced

90'^ apart. The timing test tool is used

with an 8" x 10" standard cassette and

two lead blockers so that four exposures

may be made on one film.

The timing device functions in a manner similar to the classical spinning

top technique. In the spinning top test,

which is suitable only for single phase

x-ray generators, the individual pulsations

of the x-ray generator were registered on the film and could be counted to determine

exposure time. This may still be done using

the timing test tool. However, because the

timing test tool is driven by a synchronous

motor, the continuous output of 3-phase and

capacitor discharge machines may also be

measured with respect to x-ray exposure

time. Four exposures are made varying

both exposure time and x-ray tube current

but maintaining constant mAs. Exposure

times may be determined by measurement

of the length of the darkened arcs caused

by the moving slits in the brass plate by

using either a standard protractor or the

special protractor provided with the test

tool. If the exposure time is correct and

if the images of the wedge pattern in the

four exposures are close to one another in

general appearance and optical density, it

may be assumed that the mA calibration of

the generator is also consistent (constant

mAs) .

In use, the timing tool is positioned

on top of an 8" x 10" cassette on the table

top with a 40" FFD and the collimator

adjusted so that the beam just covers the

active area of the timing test tool. The

unused area of the cassette is covered with

pieces of lead sheet. Four exposures are made on four different areas of the cassette

at 80 kVp, 10 or 70 kVp 30. Typical values

might be: Exposure 1: 1/5 of a second at

50 mA; Exposure 2: 1/10 of a second at 100 mA; Exposure 3: 1/20 of a second at 200 mA; Exposure 4: 1/30 of a second at 300 mA. The exposure times can be measured

with a standard protractor. In the case of

single-phase full wave rectified equipment,

one may also count the number of impulses.

An extra impulse at a low brightness level

is permitted at the beginning of each arc

on the film.

All four sets of the wedge pattern

images should appear to be of approximately the same darkening (density) . If the darkening varies by more than one

"step", the machine should be adjusted

by a qualified serviceman. The density

uniformity of the timing arcs should also

be examined; non-uniformity may indicate

mA drift during the exposure or component

weakness. (Exception: capacitor discharge

systems, which should fall off in output

during the on-time)

Suggested acceptance limits of

exposure time are:

Time (3 Phase) (Single Phase)

(sec) Angle pulses

1/5 680-76° 24+1

1/10 340-38° 12+1

1/20 15°-20° 6 1/30 10°-14° 4

A Pen Dosimeter and Clamp-Ammeter can

be used to check system mA calibration. By

checking a generator at 50 to 100 mA at long exposures with the dosimeter-ammeter

method, the timing/mAs tool may be

calibrated and then used to check higher

mA values.

In the case of an ideal single phase

generator, the turns ratio of the transformer will determine the ratio of

124
secondary to primary current. Thus, the turns ratio can be determined by using the

Wisconsin X-Ray Test Cassette to check the

output kVp while reading the high-voltage

transformer primary with the voltmeter

section of the clamp-ammeter. The voltage

drop within the transformer will range from

about 4 kVp/100 mA for large units to

12 kVp/100 mA for small units. UTien testing

at 50 mA, add 2 to 6 kVp to the secondary

reading - then

. ^ {kVp (measured) + 2 to 6} 1000

(1.414) (Primary Voltage)

In the case of a three phase generator, assume that it consists of three single

phase systems operating in parallel;

determine the turns ratio of one of these

"single phase generators" by measuring

from any phase to neutral and comparing

that voltage to the output kVp.

Because the clamp-ammeter is graduated

in units of RMS amperes (0.707 I ) while

, t. • T J max

the current sought is related to average

amperes (0.636 I ), the clamp-ammeter

, , , max -L i reading must be corrected by the ratio

0.636/0.707 = 0.90. Ignoring exciting

current losses, the secondary current into

the x-ray tube is:

(i pri. RMS) (0.90)

1 sec. avg. = 1. ^—

Turns Ratio

The result is multiplied by 3 for 3 phase

units. Except for exciting current losses,

which are usually low for operation at 80 kVp, transformer losses can be assumed

to be entirely in the form of a secondary

voltage drop. Waveform distortions will

also cause some error giving readings up

to 10% high, but usually can be ignored.

A pen dosimeter and a set of aluminum

plates (EC-llOO alloy, 10 cm x 10 cm x 1 mm,

10 total) may be used to measure the output

at 80 kVp at 1 meter and plot the attenuation

to determine HVL. The output of 3 phase

machines should average 6.0 + 2.0 mR/mAs,

1 phase machines about 4.5 + 1.5 mR/mAs

(assuming 2.5 mm Al added). The HVL must

exceed 2.5 mm Al. Of greater importance

will be recording the output of each machine

each month to detect gradual changes.

The Nine-Cent Collimator Test is a simple test to check coincidence of the

light and x-ray fields using pennies and

an 8"x 10" cassette. The coins are placed

in pairs at each edge, one coin at the edge,

the second coin touching the first and

aimed toward the center. The last coin is

placed in the upper right quadrant. The

light field is set to illuminate only the

central coins and an exposure is made

(100 cm, 60 kVp, 2 to 4 mAs) at 1 meter. Ideally, only the central coins are fully

displayed i.e. the light and x-rays are coincident (+ 1/2 coin width is reasonable).

The Wisconsin X-ray Collimator Test Tool consists of a base approximately

20 X 25 cm in which are embedded two rectangular frames made of copper welding

rod, and a brass ring. Mounted to this

base is a Plexiglas plate in the center

of which is a brass screw. The plate is

supported by 15 cm aluminum rods.

The system is used only for testing

overhead collimators. The test tool is

placed on top of an 8" x 10" or larger

cassette on the table top. The focal spot

of the x-ray tube is positioned 1 meter from

the cassette. The light beam device of the

collimator is set so that the edges of the

light beam fall in the middle of the space

defined by the inner and outer rectangles

of copper rods. This should also set the

lead shutters such that the x-ray beam is

confined to the same area if the system

is aligned. An exposure is made at approximately 60 kVp and 2 to 4 mAs. The

small marker rectangle of the test pattern

is placed so that it corresponds to the

position of the patient's right shoulder

so that the direction of collimator errors

may be later determined. Lead numbers or other patient identification may be placed

on the test tool for convenience in film

identification. The film image should

show only the innermost wire frame. Further

the x-ray image of the brass screw should

be confined within the outer border of the

image of the brass ring. The dimensions

of these test objects have been chosen

_

so that a maximum x-ray beam positional

error of + 1.2% of the focus film distance

and an angular displacement of 3° will be

allowed by the test tool.

The Wisconsin Mesh Pattern is

designed to check the focus of fluoroscopic systems. The test tool is an

18 cm square plastic plate containing

8 wedge shaped .pieces of copper mesh of

16, 20, 24, 30, 25, 40, 50, and 60 mesh

holes per inch. . Lead numbers permit the

identification of the mesh.

To avoid the effect of focal spot

size on detector system resolution, the

copper mesh should be placed close to the

face of the image intensifier. Operate the

125
system in the fluoroscopic mode at the

lowest possible kVp setting and at approximately 1 mA.

Because of the limitations of scanning,

there will be a restriction on television

resolution. However, all television systems

should operate up to the limit of this

scanning constraint. A standard television

system will have 480 scanning lines in the

visible field. Because of the random

relationship between the scanning lines and

the mesh position, this corresponds to 200

mesh holes per raster height in the television frame. Thus, a 6" image intensifier

system observed via television must be

capable of resolving 30 mesh in the center

field and a 9" system must display at least

20 mesh. It has become common practice to adjust the television to flatten the top

and bottom of the round image so that most

9" systems should resolve 24 mesh. If the

6" systems are similarly adjusted so that a 5" field is displayed, then they should

resolve 35 to 40 mesh via TV. When 9" systems are in the magnified mode, they must resolve 30 mesh and may resolve

40 mesh via TV. Television systems must

be adjusted for both optical and electrical

focus

.

Using the mesh tool, the systems should

perform as follows (minimum values):

4-6" 6" 9-10" 9" center edge center edge

I. A. (telescope) 40 to 60 30 35 to 50 24

I. A. (viewer) 40 30 35 24 TV 30 to 35 30 20 to 24 20

16 mm

cine 35 30 30 24

35 mm

(and larger) 40 to 60 30 35 to 50 24

The Wisconsin X-ray Focal Spot Test Tool employs a heavy metal pattern which

contains eleven pairs of bar pattern groups.

Each pair of groups consists of three slots

in a group in one direction and three slots

at right angles to the first group. The

spacing of the slots in the eleven grouppairs varies from 0.6 line pairs per mm to 3.35 line pairs per mm. The test pattern

is mounted in the center of a 10 x 10 cm Plexiglas plate which contains a fluorescent screen and a lead shield. This

plate is mounted 15 cm over another thin

Plexiglas plate. The test tool is designed

to be used with any non-screened film.

In use, the focal spot of the x-ray

tube is placed 45 cm from the test pattern.

Film is placed 15 cm below the pattern.

If the image of one bar and one space between bars is exactly the same as the

dimension of the focal spot, then the

x-ray image is incapable of producing a bar

image. False resolution may be sometimes

obtained, that is, instead of seeing three

bars on each axis in the image at the film

two or four bars may be seen. This will

not occur with both sets of a group

because of the asymmetry of the focal spot.

Only when the dimension of the focal spot

is less than the dimension of the bar and

space in the plane of the focal spot can

the image be resolved. X-ray focal spots

are not symmetrical and the standards for

performance of x-ray tubes permit one axis to be 1.4 times larger than the other

axis. In addition, the standards also

permit the actual dimensions of the

smaller axis to be from 30 to 50% larger

than the labeled value! Table 2 lists the

number of groups that can be resolved for

a given nominal focal spot size and the

corresponding maximum actual focal spot

dimensions. The bar patterns are listed

in terms of line pairs per mm.

A group of bars is said to be

resolved when both sets of three bars can

be seen clearly by eye in the x-ray image

on the film. If two or four bars are seen or if the image is just a blur, the

image is said to be unresolved. Obviously,

larger focal spot tubes will resolve only

the larger groups. Reference to the table

indicates what one should expect of focal

spots of given nominal sizes . For example,

a 2mm rated focal spot must resolve the

first four groups, a 1.5 mm focal spot must resolve the first six groups, a 1.0 mm

focal spot must resolve eight groups, and

a 0.5 mm focal spot must resolve all groups.

Smallest Maximum Size

Groups Lines Nominal of Actual

Clearly Pairs/mm Size (mm) Focal Spot

Visible (mm)

1 0 6 2 8

2 0 7 2 5

3 0 85 2 3 4 1 0 2 0 2 60 X 3 64

5 1 15 1 8 2 34 X 3 28

6 1 4 1 5 1 95 X 2 73

7 1 7 1 3 1 50 X 2 18

8 2 0 1 0 1 4 X 1 96

9 2 5 0 8 1 12 X 1 57 10 2 8 0 6 0 90 X 1 26

11 3 35 0 5 0 75 X 1 05

126
The Wisconsin Phantom and Penetrameter

is used to estimate the low contrast performance of image intensifier systems and

to check the performance of automatic

brightness stabilizers and phototiming

systems. The phantom and penetrameter

consist of two phantom plates of aluminum

7" X 7" X 3/4", a plastic coated lead sheet

7" X 7" X 1/16" and a 1/32" thick aluminum

penetrameter plate with holes of 1/16",

1/8" 3/16" and 1/4".

At 100 kVp 2 mA all systems should

display the 1/4" and 3/16" holes clearly

and the 1/8" holes should be just visible.

Better grade systems show the 1/16" holes

clearly.

Automatic brightness stabilizers

operating on fixed mA, variable kV, should

stabilize at 85 to 115 kVp for full phantom^

70 to 95 kVp for 1/2 phantom and go to the

lowest kVp , below 60 kVp for no phantom.

The Wisconsin Veiling Glare Test Set consists of a photometer incorporating an objective lens, aperture plate and a variable gain photoraultiplier tube and

output meter. Two lead strips are used as X-ray test objects, one is 1.5 cm and the

other is 2.2 cm wide.

Image focus or the limiting resolution

of high contrast objects are not sufficient

criteria for describing the performance

of image intensifier tubes. Systems may

display the 4 Ip/mm section of the lead

grill pattern and yet yield poor patient

images. A test such as that using the

Wisconsin Phantom and Penetrameter is of

value only as a pass-fail test. This

veiling glare test yields a numerical

value for contrast.

The main factor influencing loss of

contrast in image intensifier systems is

light scatter, in the input phosphor

assembly, the output screen-window, and

the collimating lens, and is independent

of the radiation level. By using a lead

strip as the test object and measuring

the light level behind the image of the

lead strip and in the area adjacent to the lead strip, a standard can be made

for evaluating the light scatter, i.e.,

limiting contrast or veiling glare of

image intensifier systems.

A lead strip (15 mm wide 30 cm long

for 170 mm or smaller input tubes, 22 mm

wide for larger tubes) is taped to the

center of the face of the image intensifier

assembly. The TV camera is removed to expose the collimating lens. A lead apron

is used to drape the input of the system

and protect the tester. The system is

operated with the shutters open to just

touch the edges of the input screen, using operating factors of 60 kVp and 1 mA. The photometer is positioned to monitor

the light adjacent to the image of the lead

strip (tilted slightly) and the gain set

so that the meter reads 100 mA in the

highlight. The photometer is then

positioned to read in the area of the

strip. Veiling glare is defined as:

VQ = 100 - strip light level

^ ^^^^^ 100 + strip light level

The veiling glare will yield values

of about 50% worse than conventional

disc tests; e.g., a disc contrast ratio of

11:1 will have a veiling glare of about

80%. Reasonable values of veiling glare

for new Csl tubes should be:

80+ Cineradlographic , Special Procedures

72+ Two-Port Use, Fluorospot filming

catheter positioning

65+ Single port TV or viewers, GI examinations

Below 65 reject for low contrast

The Wisconsin Tomographic Test Tool

consists of the following:

5 plastic discs 9 cm in diameter;

three of the plastic discs, 1, 2 and 4 cm thick, serve as spacers;

one plastic disc 1.5 cm thick contains lead numbers (1 to 12) in

a helix, with number 1 1 mm, number 12 12 mm, etc., above the

bottom of the disc;

1 plastic disc 1.3 cm thick

contains 4 tilted copper mesh

pieces of 1.2, 1.6, 1.8, and 2.0

holes/mm (Fig. 3) : and a lead

aperture plate covered with white

plastic. The lead is 1.5 mm thick

with a central hole 2 mm in diameter.

The tomo test tool is used to:

1) determine the location of the fulcrum

(plane of the cut) , 2) determine the

thickness of the cut, 3) test the overall

resolution at the location of the cut and 4) test x-ray exposure uniformity over

the trajectory of the moving source.

127
Quality Assurance for the Measurements References

As can be seen, the first level tests

are not costly or complicated . For each

first level test there is a more precise

second level test: test cassette (4%) - voltage divider (1%), focal spot tool (20%)-

pinhole camera (5%) - pen dosimeter (10 to

20%) - electrometer (3-10%) - coarse tests

of collimation - precision fixtures, etc.

While certain tests at the first level

are either obvious or self calibrating,

e.g. the mesh pattern, the veiling glare

tool, the tomo tool, other tests require a reference or standard base. The Wisconsin

Test Cassette must have a test traceable

to a standard. The "simple" pen dosimeter

is not so simple and must be checked against

an instrument of known (standard) performance. At least one manufacturer's

product fails to meet published claims in most units. The dosimeters of other

manufacturers have been shown to have a rather broad spread of sensitivities.

Even the aluminum alloy used as an x-ray

absorber has occasionally been improperly

labeled so that copper bearing alloys

yielded incorrect values of beam

attenuation

.

Second and third level tests, meaning

those tests performed by physicists using

more precise measurement devices and by state and federal government surveyors,

can be considered as a form of quality assurance for the first level tests. The complete quality assurance program in a department of diagnostic radiology must include the first level tests on a frequent schedule with the detailed tests performed, say, on an annual basis.

The evidence of the surveys shows

that the problem is not one of precision

but of gross error solvable by means of

simple tests and simple understanding

of some rather basic principles. Emphasis

should be placed on frequent testing and

training at the user level to solve these

problems.

1. Trout, E.D., et al. Analysis of the

Rejection Rate of Chest Radiographs

Obtained During the Coal Mine

"Black Lung" Program, Radiol . 109,

25-27, Oct. 1973.

2. Holloway, A.F., Campbell, E.M.

,

Essengerg, A. , Exposure Characteristics

of Diagnostic X-Ray Machines,

Brit. J. Radiol . 45, 48-52, Jan., 1972.

3. Cameron, J.R., Wochos , J.F., Patient

Exposure from Diagnostic X-rays: An Analysis of Two Years of NEXT Data.

(To be published by Bureau of

Radiological Health, 1976).

4. Cameron, J.R., Siedband, M.P., Quality

Assurance in the Hospital, AAPM Summer

School Syllabus, Houston, July, 1975.

5. NEXT Task Force, Suggested Optimum

Survey Procedures for Diagnostic

X-Ray Equipment, Bureau of

Radiological Health, Washington,

July, 1975.

6. Rao, G.U.V., Influence of Focus and

Off-Focus Radiation on Radiographic

Detail. Report to the Bureau of

Radiological Health, DHEW 1966,

(Also, Private Communication - 1971).

7. Lawrence, D.J., A Simple Method of

Processor Control. Medical Radiography

and Photography, Vol. 49 No. 1,

Kodak, Rochester, 1973.

8. Gray, J., Photographic Quality

Assurance in Diagnostic Radiology (In

preparation for Bureau of Radiological

Health, J. Vucich, Project Officer)

Washington, 1976.

9. Jacobson, A.F., Cameron, J.R.,

Siedband, M.P., Wagner, J., Test Cassette for Measuring Peak Tube

Potential of Diagnostic X-ray

Machines. Medical Physics, Vol. 3, No. 1, AAPM, New York, 1976.

128
INFORMATION

HVL FILTRATION

120 kVp REGION " I ^

100 kVp REGION

80 kVp REGION

60 kVp REGION

Fig. 1 X-Ray Test C assette

Fig. 3 Focal Spot Test Tool

1/16 COPPER OVER 60-120 kVp STEPWEDGES

I HVL omroiesm* .-^ COPPER n» cuurrr STEP ^'^'^e-' HEDGES

v".'}/ LEAD

.^v:o\JiVV/ MASK

OUTSIDE CASSETTE

INTENSIFYING SCREEN

OPTICAL ATTENUATOR

X-RAY

FILM

INSIDE CASSETTE

^ig. 2 Diagram of Test Cassette

SB mmm

Fig. 4 Test Images of 1 mm, 2mm Spots

Fig. 5 Radiograph of Mesh Pattern

129
Fig. 6 Tomography Test Tool
NBS SP456 (1976)

QUALITY ASSURANCE FOR MEASUREMENTS IN NUCLEAR MEDICINE

B. A. Rhodes, W. B. Hladik, J. H. Gallagher

Depa rtments. of Diagnostic Radiology and

Pharmacy Practice

The University of Kansas Medical Center

Kansas City, Kansas 66103

This report reviews the measuring systems currently in use to assure safe and effective medical use of radioisotopes. One group of control tests measure the

accuracy and precision of in vitro radiometric assays.

Another group of measurements is applied to in vivo

studies. This group includes measurements of quantity

and quality of administered isotopes, performance of

nuclear imaging devices and even the performance of

physicians who interpret the images. Measures of

overall system performance are also employed. The use of feedback loops to control and improve the quality

of nuclear studies is an area of current exploration.

(Quality assurance; nuclear medicine; radiopharmaceuticals;

SS'^Techneti um; gamma camera)

I n t roduct i on

The taking of measurements has been

basic to nuclear medicine since its inception. Unlike most areas of medicine in which quantitative techniques are a relatively modern addition to the methodology, nuclear medicine began with the use of tracers to measure physiological

functions not quantifiable by other means.

A nuclear medicine measurement is, in fact,

merely counting. Scintigraphy, the imaging

process used in nuclear medicine, is produced by temporally integrating and

mapping these counts. Specialists in nuclear medicine are technologically

oriented and accustomed to the utilization

of quantized ravi data. Thus, it is natural

for those in nuclear medicine to approach

quality assurance via the use of quantitative techniques.

The purpose of this report is to summarize the current status of quality

assurance measurement programs in nuclear

medicine. These programs have been divided

into those dealing v;ith in vitro measurements and those dealing with in vivo

measurements. They are further divided

into internal or in-house programs and

external or mul t

i - i nst i tut iona 1 programs.

Finally, the use of data feedback loops

are discus"sed in terms of those mechanisms

in current use and in terms of those additional mechanisms where automatic feedback looping might provide added quality

assurance for diagnostic measurements in nuclear medicine.

In Vitro Nuclear Studies

One of the most rapidly increasing

uses of radioisotopes is in the quantitative analysis of trace substances in plasma

or serum by a technique usually referred to

as radioimmuno assay. Utilizing substoichiometric amounts of specific molecular reagents such as antibodies to triiodothyronine and the radiolabeled form of

the substance being analyzed, radioiodinated

triiodothyronine in this example, it is possible to measure the level of the

hormone in a small sample of blood. Such

assays have been simplified by the use of

disposible reagents and containers. Auto- matic techniques have been employed whereever possible to facilitate inexpensive,

high volume laboratory service.

Several types of measurement errors

have been identified in radioimmuno

analysis.' These errors range from non- specificity and loss of potency of the

subs to I ch iomet r I c reagent, to radiochemical

impurities, and to presence of unknown

interferring substances in the original

blood sample. These errors are in addition

to the usual analytical errors of volume

variations, and the statistical noise of

nuclear counting procedures.

Two major types of control measurements are employed to assist in quality

assurance. First are the in-house controls.

Frequently this is a high and a low level

131
pooled serum sample which is run along with

each set of standards and unknowns. The

results are plotted versus time to provide

a day by day evaluation of the performance

of an assay. Second are the i nte r 1 aboratory

controls. These are also aliquoted and sent

to various participating laboratories as "un- knowns". The analytical results are returned to a central administration where an interlaboratory comparison of data is made

and reported back to the participants. One such program, used by over 6000 laboratories,

is offered by the College of American Pa- thologists and is supported by subscriptions

paid by the participants.^ The largest ^ Federal Government program originates at the National Center for Disease Control

(CDC) in Atlanta. The Clinical Laboratories

Improvement Act passed by the U.S. Congress

in 1967 gave CDC the authority to impliment

proficiency testing programs for such tests

as Radioimmuno Assays.^

Individual results of interlaboratory

comparison studies are reported in units of

standard deviation intervals (S.D.I.).

The S.D.I, is a multiple of the standard

deviation which describes the frequency

distribution of the particular i n te rcompar i

son. It gives a numerical evaluation of a particular laboratory's response to the mean of peers. A positive or negative

S.D.I, indicates a participants answer

which is above or below the mean respectiveIn Vivo Nuclear Studies

Amount of administered radioisotope

Quality assurance measurements of

radioisotopes administered internally are designed to control the safety of the test

in addition to its accuracy and precision.

The initial basic control test establishes

the amount of radioactivity administered.

Both underdosage and overdosage lead to unnecessary radiation exposure. If too

small a dose is given, the patient is irradiated unnecessarily because insufficient radioactivity prevents the acquisition of the required data. If too large

a dose is given, the patient exposure is proportionally higher and in some cases

the detection instrument may be overwhelmed

by the excess radioactivity, so that the

desired information is either degraded or

1 OS t all togethe r

The usual procedure is to calculate

the volume required to provide the necessary amount of the radioactive tracer. This

is accomplished using data on the label which is provided by the radiopharmaceutical

supplier. The dose is drawn into a syringe

132

and the radioactivity checked using a dose

calibrator. This is usually a large volume

ion chamber, calibrated to give digital

readout of uCi's or mCi's. Direct readings

are available for the radioisotopes routinely used in clinical nuclear medicine.

Radioactivity standards used by industry to provide the assay reported on radiopharmaceutical labels can be traced

back to standards prepared and certified by

the National Bureau of Standards (NBS).'*

The accuracy of dose calibrators can also

be regularly checked using standards

tracable to NBS standards. Thus, the two in-house quality assurance checks are:

1) conparison of the calculated dose

to the measured dose and

2) comparison of the dose calibrator

reading to the value on the label

of a calibration source.

An interlaboratory quality assurance program is provided by the College of

American Pathologists in cooperation with

NBS. Radioactive samples, "unknowns", are distributed to subscribers by NBS. The

radioassay results are collected from the

participants and are compared to the NBS assay. Reports of these studies are published.^ Initial surveys revealed significant inaccuracies in the assay of

certain radionuclides. Subsequent surveys^

indicate improvement. This improvement

probably is directly related to the fact

that the surveys were made and the results

publ i ci zed.

Problems occuring due to the administration of the radiopharmaceutical are

less controlled than the amount of administered radioactivity. These can be classified as follows:

1 ) dose i nf i 1 t rat ion

2) partial loss of dose due to residual radioactivity left In the syringe

3) accidental overdosing

k) purposeful overdosing

A survey of 228 consecutive brain,

lung and liver doses revealed three minor

administration problems: the nature and

frequencies of the problems are listed

in Table I.

i

I
Table 1. Radiopharmaceutical Administration Problems

Type

Repeat Injection Required

Syringe Flushed (Causes bolus

to have tail)

Dose I n f i It rated

Rate Per

1000 Administrations

P roced u re (Scan

)

50

18

Lung

Bone and

Bra i n

A repeat injection is occasionally required

because the initial dose is too low to carry out a study. They occurred in the

dispensing of ^^'^Tc-mi crospheres for lung

scanning. About 5% of the time such a large amount of the dose remained in the

syringe after injection that a second dose

would have to be given. This delays the

procedure and causes the patient the discomfort of a second venipuncture. The

routine procedure for the injection of bolus

doses of ^^n^Tc-OTPA for dynamic studies of

brain flow preliminary to brain scanning is to make a single rapid injection without

flushing the syringe. With almost every

other procedure the syringe is routinely

flushed to increase injection efficiency.

However, for the brain studies flushing is avoided because it causes spreading and

tailing of the bolus which degrades the data,

This was the most frequent error. Infiltration or extravasation of enough of the

dose to require reinjection was the least

frequent problem and occurred i n less than

2% of the administrations. Some infiltration is relatively common, but rarely does

an injection have to be repeated.

The accuracy and precision of actual

radiopharmaceutical administrations for

lung scanning with ^^"^Tc-mi crosphe res , brain scanning with ^^"^Tc-DTPA and bone

scanning with ^^"^Tc-py rophosphate are listed i n Tab 1 e 2

The data was obtained by routinely measuring

syringe residuals, correcting the residual

for radioactive decay back to the time of

administration, and then subtracting to get

the actual dose delivered to the patient.

The underdosage resulted primarily from

occasional large syringe residuals. Overdosage was not a significant problem.

Purposeful overdosage is most likely

to occur in situations where the radiopharmacy is located at a distance from the

clinic. Thus, the radiopharmaceutical must

be ordered in advance. The ordering or prescribing usually occurs the afternoon

before the study. The order includes the

time of study in addition to other pertinent

data. On the morning of the study doses are drawn into the syringes in amounts to give the required number of mCi's at the

specified time. This means that before this

time the dose is an overdose and later it becomes an underdose. The amount depends

directly on the half-life of the isotope.

Values for 1 and 3 hours differences of rep- resentative radioisotopes are given in Table

3. Since approximately SS% of the doses

used clinically are ^^"^Tc- 1 abe 1 ed radiopharmaceuticals, doses of this isotope are

the primary concern. For example, in an attempt to avoid underdoses, the prescription may specify a time later than the

patient is scheduled. If however, the

patient arrives earlier or on time and is

Table 2. Radiopharmaceutical Dosage Errors

mC i 's 59mj^

Number of 1 ntended Actual Maximum Percentage

Agent Observat ions Dose Dose Range Unde rdosage Overdosage

Mi crospheres 60 5.0 A.l 1.19-5.79 76.2 15.8

DTPA 8k 20.0 19.5 15.52-21.35 22.4 6:8

Py rophosphate 8h 15.0 U.7 12.59-15.89 16.1 5.9

133
Table 3. Over and underdosage errors in radiopharmaceuticals

Isotope

Percentage error

d i f fers from t i me

i nofdosage as time of administration

calibration. Time is given in hours.

-3 -1 0 + 1 + 3

113m|n +248.

+51 .5 0 -34.0 -71.3

59"^Tc +41 .4 + 12.2 0 -10.9 -29.3

e^Ga +2.7 +0.9 0 -0.9 -2.6

given the full dose, the patient is overdosed. The most extreme case is probably a 30mCi brain scan injected 3 hours early

resulting in 12.4mCi excess dosage. Data defining the magnitude of this potential

problem is not available.

Quality of the administered radiopharmaceut i cal

Measurement of the quality of radiopharmaceuticals includes several parameters

in addition to those for non- rad i oact i ve drugs. The presence of the radioactivity

within the molecule provides unique problems

and some unique advantages compared to other drugs. In particular there are the

problems of radi onuc 1 i d i c and radiochemical

purities both of which are subject to change with time. There is the advantage

that the radioactivity itself makes it possible to carry out b i od i s t r i bu t i on studies directly. The most common measurements are listed in Table 4.

Table 4. Quality Control Measurements of

Rad i opha rmaceut i ca 1

1. Rad i on ucl i d i c purity

2. Radiochemical purity

3. Appearance

4. Sterility

5. Apy rogen i c i ty

6. B iod i s t r i but i on

Radiopharmaceutical quality control

measurements have been extensively re- viewed in the published proceedings of

national and international conferences such

as the Proceedings of a Panel on Analytical

Control of Radiopharmaceuticals organized

by the International Atomic Energy Agency

and held in Vienna, 7-11 July, 1969^; The

First International Symposium on Radiopharmaceuticals, held in Atlanta, 12-15

February, 1974® and Standardization Per- formance and Quality Control in Nuclear

Medicine, held at the NBS, 12-14, June 1975^,

The stringent demands for more rapid

and more sensitive quality control tests

for radiopharmaceuticals have led to new

test systems that are now finding applications in the testing of non- rad i oact i ve drugs. Two examples of this are the use of

the limulus lysate gelation test for

bacterial endotoxi n

^'^ and the radiometric

test for ster i 1 i ty . The limulus test

has allowed the detection of pyrogen levels

in ci sternographi c agents that are high

enough to cause fever and aseptic meningitis

in patients, but too low to be detected by

the standard rabbit method recommended by

the U.S. Pha rmacopoei a. ' ^ ^ The data in Table 5 shows the reduction in numbers of

adverse reactions to ci sternograph i c agents

that have occurred since the introduction of

this new test.

Table 5. Adverse Reactions to Cisternography

Year Number of Reported

React i ons

1971 30 1972 23 1973 9 1974 4 1975 0

Data from the Adverse Reaction Subcommittee

of the Society of Nuclear Medicine, H.L. Adk ins, Cha i rman

The data in Table 5 is an example of

the effectiveness of radiopharmaceutical

control testing at the present time. The

number of patient administrations

continues to increase while the total

incidence of adverse reactions is de- creas i ng

134
Performance of nuclear instrumentation

Performance testing of imaging devices

is a standard of practice. The tests are usually classified according to the frequency of their use: routine- those done

before each study, daily, or weekly or those done at intervals longer than a week.

These are listed in Tables 6, 7, 8.

Table 6. Routine Tests of Nuclear

Instrument Performance

1. Film check

2. Procedure parameters check

3. Cal i brat ion check

k. Col 1 i ma tor check

Table 7. Daily Tests of Nuclear

instrumentation Performance

1. Field uniformity response

2. Energy Calibration

3. Spatial resolution

h. Sens i t i vi ty

Table 8. Weekly Tests of Nuclear

Instrumentation Performance

! 1. Linearity, size and distortion

' 2. Ancillary equipment function

These procedures for the performance of

these tests are available in handbooks such

as the Manual of Nuclear Medicine Procedures.^'* Quality Assurance of Scintillation Camera (Workshop Manual and Quality

Control in Nuclear Medicine^. The area of

quality assurance testing is still evolving.

Data on the frequency, significance and

costs of nuclear instrumentation dysfunction

has been reported. However, studies such

as the one reported by Ashare and Podikal^^

review some of the kinds of problems

which may be encountered. This study re- vealed that in certain instances the

computer correction of gamma camera field

non-uniformity introduced artifacts which

degraded rather than improved image quality.

The College of American Pathologists

also operates an i nterl aboratory nuclear

instrumentation performance test program

in which imaging phantoms are distributed

to part i ci pants. -^^ Training in the techniques of measuring the performance of detection and image

instrumentation is a current major activity

in nuclear medicine. The Bureau of Radiologic Health (BRH) of the FDA has taken a leading role in this area and is receiving

the support of the Society of Nuclear

Medicine in their efforts. The two day

BRH Training program has been sponsored by

local groups across the country.

Observer Performance

The performance of nuclear physicians

in detecting abnormalities on radioisotope

scans is measurable under certain specified

cond i t i ons . ^ The methodology for taking

this measurement is derived from signal detection theory. An observer is given a set

of images with various signal to noise

ratios and asked to report whether or not

he observes a lesion. His answers are scored as true or false positives and true

or false negatives. A plot of true positives

versus false positives provides a point on

a receiver operator characteristic (ROC) curve. By repeating the test under varying

discrimination criteria data is obtained

from which the ROC curve can be constructed.

This curve is a quantitative measure of how

well the observer Is performing under the

specific test conditions. The observer does not have to be a human, it may instead, be

a computer programmed with a pattern recognition algorithym. These techniques are being used only experimentally at present.

However, we can anticipate that their impact

on nuclear medicine will have a positive

effect on the improvement of physician per- formance .

Overall System Performance

Both information theory and operations

research are providing techniques for

system ana 1 ys i s

. ^ ' ^ ^ Thus, a set of tools

are being developed whereby the overall

performance of imaging systems and their

impact on patient care can be measured.

Two measurements that have been used to evaluate nuclear medicine imagins systems

are figures of merit and modulation transfer functions (MTF) . A f i gure-of-meri

permits a quantitative ranking of different

information detecting or transmitting

systems. The MTF permits a quantitative

measure of the degradation of information

as it is transferred through a system. We have yet to apply these techniques in routine quality assurance programs.

Methods for measuring the diagnostic

usefulness of information provided for

nuclear medicine studies are ava I 1 ab 1 e

. ^'^ One procedure is to obtain from the refer135
ring physician his probable diagnosis,

and his certainty about each, both

before and after he is provided with the

results of the study. Change in his diagnostic certainty is measured by an index

known as a log likelihood ratio. -^^ Another

procedure is to measure outcomes of two procedural paths, one of which includes a nuclear test and the other a control which

does not include the test. Patients with

similar symptoms on admission are randomly

divided between the two paths. Measurements are made of duration and costs of

hospitalization, one-year .survi val s etc. which allow for quantitative comparisons of

the outcomes of the two alternatives. As more reports are generated illustrating the

value of these kinds of analysis we can expect that they will become more widely

used.

Feedback of Quality Control Data

Quality assurance is achieved by the

making of appropriate performance or quality

measurements followed by a prompt response

to correct the situation when indicated.

When used, the feedback of quality control

data is customarily prompt in nuclear medicine. The physician will look at the images

before the patient is dismissed from the

clinic. Radiopharmaceutical or nuclear in- strumentation problems are often immediately

apparent from the pictures. Corrections are ordered before the patient is released and

before the next study is begun. Unlike

regular drugs where altered b i od i s t r i but i on or metabolism is not apparent except by the

use of carefully designed experiments,

defective radiopharmaceuticals give visual

b i od i s t r i b ut i ons within minutes after each

patient administration. Hence, a poor

quality tracer is used only once before it

is recognized, discarded and replaced with

a better formulation or different agent.

It also is common practice for the physician

to have before him a picture of the daily

flood field source as he reads gamma camera scintigrams. This allows for a comparison

between the image of the patient and the

quality control image.

Currently we are exploring the use of

the computer to assist in the collection and

evaluation of control data in hopes of

further improving the quality of nuclear

medicine studies. Two computer assisted

feedback loops are under investigation:

the radiopharmaceutical purity control of

95n^Tc-labeled radioph a rmaceut i cal s and the

performance of the gamma camera. The hypothesis under test is that a prestudy

verification of the current quality of these

two system components will both improve

patient care and reduce operating costs. The approach is to image a field flood, a phantom and a column chromatorgram of the

radiopharmaceuticals before each days work.

The images of these control objects will be

compared to data stored in the computer and

evaluated as to whether radiopharmaceutical

purity and gamma camera performance are within prescribed tolerances. If not, the

computer will be programmed to issue in- structions as to which component is outside

tolerance and to provide data characterizing

the problem.

Radiochemical Purity

Persson^^ has shown that column chromatographic separation of ^^"^Tc-compounds can

be achieved without elution of radioactivity

from the columns. After development of the

columns vMth saline or buffer, the fractionation of the ^^'^Tc- 1 abel ed components are

Figure 1: A set of qe 1 ch r^iyia toq raphy

columns in a special holder for use in the

radiochemical purity testing of ^^'^Tc- labeled radiopharmaceuticals.

136
f

f I

Figure 2: A gamma camera image of the

columns shown in figure 1. Variations in radiochemical purity of the various ^^^^Tc-S Colloid preparations are visualized. The

differences can be quantitatively evaluated

by computer analysis of the image.

visualized by scanning or gamma camera

imaging, Figure 1 and 2. The data contained in the image can be transferred into a digital computer for quantitative analysis

of radiochemical purity. Included in the

computer programs are the routines for com- paring the radiochemical purity to the ex- pected values and for reporting the

d i f fe rences

Gamma Camera Performance

the uniformity of response of the camera.

A recorded daily evaluation of this data

documents the aspect of gamma camera per- formance. The collective record is therefore a reference measure of the quality of

performance of the gamma camera. V/hen a current days data is compared to the data of

record it becomes possible to make a decision

as to whether or not the performance has

deteriorated to unacceptable levels. By

doing this, a tighter quality feedback

control loop is established.

Phantoms can be used to evaluate reso- lution, field size, and linear distortion.

Again, day-to-day recording of these control

values can create a record against which

the current days performance can be measured.

The comparison of the current operation to the record of past operations then provides

additional feedback loops. The quality

control feedback loops can be programmed so

that at the end of data acquisition and

analysis, a 'go' or 'no go' is displayed.

Summa ry

Measurements of the quality of nuclear

measurement is becoming standard operating

procedure in the clinical practice of

nuclear medicine. Methods for quality assurance and performance testing of system

components have been developed. Their use

is providing evidence that quality assurance

programs are effective in nuclear medicine.

Further evidence of the impact of quality assurance programs is the results of the

analysis of intra- and i nter 1 aboratory

control samples. Investigation of quality assurance techniques continues with emphasis

on measuring system performance, on determining appropriateness and cost-effectiveness of clinical nuclear medical measurements, and on establishing more effective

feedback mechanisms.

Refe rences

1. Thorell, J. Quality Parameters of In Vitro Assays in Quality Control in Nuclear

Medicine, B.A. Rhodes ed. , C.V, Mosby Inc.,

St . Loui s (in Press)

In computerized gamma camera applications it is customary to image a field of

uniform radioactivity called a field flood.

The data obtained usually is not uniform

however, because of regional variations in the sensitivity of response of the camera.

Thus, a matrix provides the means of

obtaining images equivalent to that which

would have been obtained if the response of

the camera was truly uniform. The correction matrix contains data vyhich measures

2. Hansel 1 , J.R. Proficiency Testing of

In Vitro Assays _i_n Quality Control in Nuclear Medicine, B.A. Rhodes ed. , C.V. Mosby Inc., St. Louis (in Press).

3. Spierto, F.W. , Knight, M.S. and Shaw,

W, External and Internal Quality Control

Programs for Clinical Radioassay Procedures

in Quality Control in Nuclear Medicine, B.A.

137
Rhodes ed. , C.V. Mosby Inc., St. Louis

(In Press).

h. Mann, W.B., Cal i , J. P. Traceability of

Radioactivity Measurements to National

Standards _i_n_ Quality Control in Nuclear

Medicine, B.A". Rhodes ed. , C.V. Mosby Inc. St. Loui s (In Press)

5. Hauser, W. Radioactivity Measurements

and Their Quality Control _i_n_ Quality Control

in Nuclear Medicine, B.A. Rhodes ed. , C.V. Mosby Inc., St. Louis (in Press).

6. Cavllo, L.M., Coursey, B.M., Garfinkel,

S.B., Hutchinson, J.M.R. and Mann, W.B. Needs for Radioactivity Standards and

Measurements in Different Fields. Nuclear

Instruments and Methods 112 , 5-18 (1973).

7. Analytical Control of Radiopharmaceuticals, International Atomic, Vienna, STI/

POB/253 (1970).

8. Subramanian, G. , Rhodes, B.A., Cooper,

J.F. and Sodd, V.J. Radiopharmaceuticals,

The Society of Nuclear Medicine, Inc., New York (1975).

9. Rhodes, B.A., Quality Control in Nuclear

Medicine, C.V. Mosby Inc., St. Louis (In

Press)

10. Cooper, J.F., Hochstein, H.D. and

Seligmann, E.B.: The Limulus Test for

Endotoxin (pyrogen) in Radiopharmaceuticals

and Biologicals. Bull. Parenteral Drug

Assn. 26:153 (1972).

11. Chen, M., Rhodes, B.A., Larson, S.M. and Wagner, H.N. , Jr.: Sterility Testing

of Radiopharmaceuticals. J. Nucl Med. 15: ]]h2 (197^).

12. Rhodes, B.A. , Kamanetz, G.S. and Wagner,

H.N.,Jr. The Use of Limulus Testing to Reduce the Incidence of Adverse Reactions

to C i s ternograph i c Agents. Neurology 2k: 810 (1974).

13. Cooper, J.F. and Harbert, J.C. Endotoxin as a Cause of Aseptic Menigitis after

Radionuclide Cisternography. J. Nucl. Med.

16:809 (1975).

]k. Keyes, J.W.,Jr., Carey, J.E., Moses,

D.C. and Beierwaltes, Manual of Nuclear

Medicine Procedures, 2nd Edition, The

Chemical Rubber Co.

15. Quality Assurance of Scintillation

Camera (Workshop Manual), Bureau of Radiological Health, DHEW. (In Press).

16. Ashare, A.B. and Padikal, Computer

Correction of Gamma Camera Field Non-Uniformity: Possible pittfalls \n_ Proceedings of

a Symposium on Sharing of Computer Programs

and Technology in Nuclear Medicine held in Atlanta, Ga. , 26 January 1976, B. Howard Ed.

( In Press)

17. 1974 Survey Manual, College of American

Pathol og i s ts

18. Goodenough, D.J. and Knowles, L.G. Measurements of Observer Performance in Quality Control in Nuclear Medicine, B.A. Rhodes ed. , C.V. Mosby Inc., St. Louis

( In Press)

19. Potchen, E.J. Current Concepts in Radiology, Vol. 2, C.V. Mosby Co., St. Louis (1975).

20. McNeil, B.J,, Keeler, E. and Adelstein,

S.J. Primer on Certain Elements of Medical

Decision Making. N. Engl. J. Med. 2^3 :21

(1975).

21. Persson, B.R.R. Gel Chromatography

Column Scanning: A Method for Identification and Quality Control of ^^'^Tc Radiopharmaceuticals \_n_ Radiopharmaceuticals,

G. Subramanian, B.A. Rhodes, J.F. Cooper

and V.J. Sodd eds. The Society of Nuclear

Medicine, New Yo7k (1975).

138
NBS SP456 (1976)

STATE AID FEDERAL REGULATORY MEASL^REMENT' TxESPONSIBILITIES AROUI© MEDICAL FACILITIES

Lawrence H. Lanzl, Ph.D.

Depar-tment of Radiology and Franklin McLean Memorial Research Institute

(operated by the University,' of Chicago for the U.S. Energy Research

and Development Administration under Contract E( 11-1) -6 9)

and

Chairman, Radiation Protection Advisory Council, Department of

Public Health, State of Illinois

Radiation exposure to mian is due chiefly to diagnostic x-ray procedures, in which

radiation is intentionally directed toward a patient. Radiation therapy presents

a lesser problem because a smaller percentage of the population is subjected to

such treatment. Recently, some innovative steps were taken in the State of

Illinois to reduce patient exposure in four diagnostic procedures without reducing the benefits derived therefrom. However, if these procedures are to be car- ried out properly, it is necessary to increase the precision and accuracy of

radiation exposure measurements to the order of ±2%. The usual accuracy and precision of radiation protection measurements are of the order of ±20%. Thius,

should the Illinois radiation protection rules becomie widely adopted, our national dosimetry network will need to upgrade exposure measuremient techniques

(Medical facilities, diagnosis, therapy, regulatory, measurement accuracy, traceability)

Introduction

The main theme of this Symposium concerns the technical communication system

which insures that radiation measurements

are correctly made, sufficiently accurate,

and traceable to the national radiation

standards. The measurements necessary for

the safe use of radiation may be applied

to two patient groups. One group is related to the safe use of radiation administered for the primary purpose of diagnosis to determine the state of an individual 's health. Radiation damage to human

beings in this group is a secondary and

negative byproduct of the use of the radiation.

In the second group, the primary purpose of administering radiation to the patient is to destroy human tissue , i.e., a mlignant tumor. This use is designated as

therapeutic .

In general, the regulatory agencies

of federal, state, county, and city government have been very much concerned with the

first, but not with the second group.

Below, two examples of types of radiation measurements that need to be made in

medical facilities, one for each of the two

patient groups, will be given. These examples will demonstrate the importance of

precision and accuracy, and of traceability

not only to the U.S. national radiation

standards, but also to the world-wide dosimetry network. Some radiation measurements also need to be placed in a third

group. This group involves the measurements

necessary for radiation and non-radiation

workers in a medical facility as well as visitors to the facility.

Regulatory A.gencies

All of the regulatory agencies set limits involving radiation measurements,

and/or determinations which require some measurement. In some cases, the actual

measurement is carried out by the agency

and in others, the medical facility is obligated to carry out the measurements, or at

least to demonstrate that it has the staff

and equipment capable of doing so.

Some of the major regulatory and

other agencies at the federal level that

have an impact on medical facilities in the area of radiation measurements and their

responsible areas of concern are:

1. Deparlment of Transportation

Shipment of radioactive sources and

pharmaceuticals to the medical facility and

shipment of radioactive waste from the facility to the waste disposal site.

2 . Food and Drug Administration , Bureau of

Drugs-21 CFR 361.1, July, 1975

Limits the radiation dose from radioactive material that can be administered to

an adult research subject in a study. The

annual and total dose commitment to the

whole body, active blood-forming organs,

139
lens of the eyes, and gonads shall not ex- ceed 5 rems, etc.

3. Food and Drug Administration, Bureau of

Radiological Health—Radiation Control

for Health and Safety Act. P.L. 90-602,

1968

The purpose of the law is to regulate

and control the emission of electronic product radi at j.on through the promulgation of

performance standards for diagnostic x-ray

systems and their major components.

Nuclear Regulatory Commission

Provides for the licensing of byproduct, source , or special nuclear materials

in quantities in excess of specified exempt

concentrations in gaseous concentrations

and liquid and solid concentrations. Capability of maJ<ing radiation measurements is

obligatory under a license.

5. Environmental Protection Agency

Provides for federal radiation guid- ance in many areas including , for example

guides which govern the prescription of

diagnostic x-rays and which assure the use

of consistent techniques in the taking of

x-rays in federal agencies.

6. Department of Labor—Occupational Health

and Safety Act

Provides for limits of radiation ex- posure to workers.

Most, but not all state governments

have public health laws coupled with rules

and regulations which require radiation

measurements. If a state has a good control programi, the cities usually delegate

radiation regulatory matters to the state. Some cities, however, may have programs

which are strong in themselves. In such

cases, the city hias more regulatory impact

than the state.

Radiation Measurement, Accuracy, and

Traceabiliby to the National Standards

Those persons who write regulations

arid set numerical limits of any sort seldom,

if ever, include in the regulation any

statement about the uncertainty permitted

in the limit due to the uncertainty of the

measurement required by that limit. For

example, we never see a level of uncer- tainty regarding a regulation for automobile traffic speed limits. It is usual,

however, that the operational traffic offi- cer decides on an allowable uncertainty

before he issues a speeding ticket.

Likewise, in the area of radiation

regulation, the various writers do not include information on required accuracy of

the measurement of a limit.

There is an adage in physics that one

should make a physical measurement with the

accuracy and precision required to answer

the question at hand, but not beyond what

is needed. This certainly applies to

measurements for the safe use of radiation.

Generally, an accuracy of ±20% is considered sufficient for operational health

physics purposes. This degree of uncer- tainty reflects our lack of definitive information and knowledge on the radiation

risk to human beings at the generally ac- cepted maximum permissible dose-equivalent

levels (see discussion on radiation protection philosophy in reference 1 ) . However

situations arise where ±20% accuracy is

not sufficient. I will describe an example

from the State of Illinois Rules and Regulations for the Protection against Radiation, which applies to the diagnostic group

referred to above.

Diagnosis i

1 . Dental

Several years ago the Illinois Depe.rtment of Public Health, Division of

Pa.diological Health, began accumulating

specific radiation-patient exposure data

during regulatory inspections of various

radiation facilities throughout the state. In an initial study, measurements were

obtained and collected from all manufactiiT^es of intra-oral dental x-ray machines.

The prime objective of this study, as reported by M. E. Neuweg and P. N. Brunner

centered arou'^d the "technique"

involved in achieving a diagnostic radiograph. "These were the use of slow speed

dental film and improper film development

technique . ... The total m.easured exposures

ranged from less thian 100 mR per radiograph to greater than 7000 mJ^ per radiograph. All exposures were measured at the

distance from the tip of the dental cone

used by each facility in order to simulate

entrance exposure to a patient . ... One

hundred percent of all exposures identified

above 500 mR were conducted with slow speed

dental film. 55% of all incident exposures

above 1000 mR resulted from inadequate film

development technique. . . . Analysis of the

expC'Sure measurements was presented to our governor-appointed Psadiation Protection

Advisory Council, which consists of seven professionals, including health physicists,

140
medical and dental licensed practitioners

and industrial executives. As a result of

this presentation, the follovzing rule was developed by the State Health Department

staff with the advice and approval of the

Council: 'For routine intra-oral radiography, the incident exposure to an average patient shall not exceed 1000 mR per

radiograph and should not exceed 500 mR per

radiograph.' This rule becam.e effective on January 1, 1973."

"After a year of feedback information, we have received strong support from

numerous dentists, professional societies

and other state radiation control programs.

I'^any licensed dentists in Illinois affected

by the rule have not only complied with the

exposure reduction to the patient but have

also indicated to us the production of

higher quality radiographs which are diagnostically beneficial to the practitioner. Use of higher sensitivity film

and/or proper film development techniques

assures compliance, but more importantly,

true radiation hygiene is practiced by

exposing each patient to only the minimum

amount of radiation.

As of July 1973, the radiological

health computer data base consisted of

3020 bitewing exposures. Eighty percent of

exposures were within the 1000-mR-exposure

limit. (See Figure 1.) From July 1973

through March 1974, operators of 221 dental

x-ray machines had reduced the exposure to

within the required exposure limit. Assuming an average patient exposure of 1870

mR before reduction, an average of 700 mR

per exposure after inspections, and an

average workload of 50 exposures per week,

672,000 man-rens of unnecessary radiation

exposure were eliminated the first year as a result of these radiation exposure regulations."

This work by Neuweg and Brunner and

by the staff of the Illinois Department of

Public Health represents a substantial step

in eliminating radiation exposure without a negative impingement upon the practice

of dentistry. It is clear that this work,

leading to a safer use of radiation, would

not have been possible without a measurement responsibility on behalf of a regulatory body .

2 . Medical

In a later v;ork, Neuweg and

Brunner 2 carried out a study to determine the range of radiation exposure

levels used in several common diagnostic

examinations. Radiographic technique

factors such as kilovoltage, milliamperage

,

time, and target-to-film distance were obtained for an "average-sized" adult undergoing these examinations. Subsequently,

using the same factors , they measured the

exposure directly above the radiographic

film with a direct-reading low-energy

dosimeter, with the patient removed from

the beam.. Figure 2 is a plot of the number

of exposures measured as a function of ex- posure in milliroentgens for 866 lateral

lumbar spine exposures. It should be noted

that the exposures varied from 200 to 2,600

ttR per radiograph. This large range should

be understood in terms of the variety of

techniques employed by diagnostic radiologists in a large number of medical

facilities. In spite of this large range,

the examining radiologists apparently are

satisfied with the quality of the radiographic films which they obtain by use of

their various methods. The data show that

75% of the lateral lumbar spine exposures were below 1,400 mR each. Of the 866 ex- posures, 216 were above 1,400 mR.

It was reasoned that, if 75% of the

existing facilities could obtain a clinically acceptable radiograph by ex- posing the patient to a maximum dose of

1.400 mR per radiograph, then all other

facilities should be able to alter their

techniques to reduce unnecessarily high

radiation exposures. On this basis, a new

rule was developed by the State Health

Department staff and subsequently approved

by the governor-appointed Illinois Radiation Protection Advisory Council. This

rule states, "After January 1, 1975, the

exposure measured at the tabletop of the

technique used for an average adult patient

for routine medical radiography will be

the following: 'Lumbar spine (lateral)

incident exposure shall not exceed 1400

milliroentgens per radiograph and should

not exceed 1000 milliroentgens per radiograph. '

This new approach to the reduction

of patient exposure does not interfere with

medical practice, except for requiring

changes for those practitioners whose procedures result in higher patient exposures

than those used by a substantial majority.

We have estimated the impact, in

terms of reduced exposure, to Illinois

patients due to the implementation of this

rule together with companion rules for

abdominal, cervical spine, and skull examinations. The estimates were made on the

141
basis of U.S. census date for 1970 for the

total Illinois population, and of the

number of radiographs per year per person

for a sample of Illinois for the four

examinations in question. The data on the

number of radiographs were taken from a report issued by the State of Illinois .

Using the exposure distribution of Figure 2 for lumbar spine and Figures 3, 4, and 5 for abdominal, cervical spine, and skull

exposures, we estimate that the total ex- posure received by Illinois residents is as follows:

Lumbar spine 465,000 man-roentgens/year

Abdomen 22,800 "

Cervical spine 38,600 "

Skull 20,800 "

With the assumption that all

techniques were modified to reduce the

exposure to the Medical Radiographic

Incident Exposure limits, the following

figures show the amount by which the ex- posure to the Illinois patient population

has been reduced starting on January 1,

1975:

Lumbar spine 60,000 ± 20 , 000 man-R/year

Abdomen 3,600 ± 1,800 "

Cervical spine 1,500 ± 450 "

Skull 2,500 ± 1,500 "

The uncertainty in these figures is

due to the statistical variation of the

number of radiographs in the studies performed .

If the incident exposure had been

reduced less than the limit, a still

greater overall reduction would have been

achieved. We have also calculated exposure

reductions for changes in tecl'inique so that

those exposures above the incident exposure

limits were reduced to the former average

values . These reductions are as follows

Lumbar spine 100,000 man-roentgens/year

Ab)domen 5,300 " Cervical spine 1,700 "

Skull 5,600 "

Further reduction in patient ex- posure may certainly be possible for other

diagnostic procedures as well.

In Implementing these new medical

limits, one cannot think in terms of the

usual accuracy uncertainties of ±20% that are in common use in health physics

Since a given medical facility could be

shut down if the ''sha"!! limit" were to be

exceeded just because of a large measure- ment uncertainty, it would be unfair to

the facility and, in turn, could lead to costly court adjudications. Thus, our

system of accuracy in certain radiation

protection problems like those described

above and our traceability to national

radiation standards should be strengthened.

Therapy

It has only been in the last ten

years that definitive studies were re- ported in the literature on tumor control

probability as a function of total dose

delivered to the tumor. Figure 6 shows

the tumor control probability for two types

of malignancies, squamous cell carcinoma

of the supraglottis and Hodgkin's disease.

The data are from a recent paper by

Goitein^ in which he discusses the work of

ShukovskyS and Kaplan 6, In radiation

therapy, the aim is a high probability of

tumor control with as low a level of harmful side effects as possible. In the case of the supraglottis lesion, we see that an

increase of only 10% in the dose delivered

increases the tumor control probability

from. 20% to 75%. Thus, the treatment dose

needs to be selected with great care and

treatment needs to be carried out with a high degree of precision and accuracy.

There are very few data for other types

of tumors that have been analyzed in detail in terms of tumor control probability

versus dose, as in the examples above.

Complications from such high doses

>!ave also been studied. In 1975,

Svensson et al.'^ from Sweden published a paper on radiation-induced lesions of the

brachial plexus correlated to the dosetime-fraction schedule of a series of

patients . Such complications may arise

when a cancerocidal dose is delivered to

regional lymph nodes in axillary and

supraclavicular field irradiations . The

complications are neurologic in nature. The sym.ptoms wary from slight numbness of

the fingers to total paresthesia of the

arm. The injury to the connective

tissue cells in the plexus region is

thought to be the cause of the lesions

142
Therefore, the Ellis formula is used as the one parameter in the analysis. This

formula gives the Cumulative Padiaticn

Effect (CPE), and takes into account the

daily dose in rad, the repetition parameter, the total time for the treatrr.ent in days, and the total absorbed dose to

the plexus for the whole course in rad.

The results of Svensson et al. are plotted

in Figure 6. This figure shows that the

frequency of lesions as a function of CPE

rises quite steeply.

The uncertainty^ of the shape of the

frequency-CRE curve is dependent on dosim.etric, anatomic, and statistic factors.

The filled squares in the plot in

Figure 6 are from the data of Stoll and

Andrews 8 of Australia, for the same

complication. The shift in the CRE scale

represents a dose shift of 7%. It is

highly likely that a large portion of this

shift is due to the accumulation of errors

in the traceability of radiation standards

from the hospital to the national radiation standards of one country, then to the

Bureau International des Poids et Mesures, and then through a second

country's national radiation standards to

the hospital in that country. As we

develop more such data around the world,

like those presented by Svensson et al .

,

it becomes imperative that our dosimetry

accuracy improve and remain at a high

level, and that the uncertainties

involved in traceability of actual clinical

dose from one center to another become

snaller.

The importance of accuracy and

traceability cannot be stressed enough.

References

4. Goitein, M., and Busse, J., Immobiliza

tion Error: Some Theoretical Consider

ations. Radiology Vol. 117, No. 2, p. 407. November 1975.

5. Shukovsky, L. J., Dose, Time, Volume

Relationships in Squamous Cell

Carcinoma of the Supraglottic Larynx.

Am, J. Roentgenol. Vol. 108, p. 27,

January 1970.

6 . Kaplan , H . S . , Evidence for a

Tum.oricidal Dose Level in the Radiotherapy of Hodgkin's Disease. Cancer

Res. Vol. 26, p. 1221, June, 1966.

7. Svensson, H., '.-'estling. P., and

Larsson, L.-G., Radiation-Induced

Lesions of the Brachial Plexus Correlated to the Dose-Time-Fraction

Schedule . Acta Radiologica, Therapy

,

Physics, Biolog,', Vol. 14, No. 3, p. 228, June 1975.

8. Stoll, B. A., and Andrews, J. T.,

British Med. J., Vol. 1, p. 834,

1966.

1. Review of the Current State of

Radiation Protection Philosophy in

National Council on P>adiation Protection and Measurements. NCRP Report

No. 43, January 15, 1975.

2. Neuweg, M. E., and Brunner, P. N. Radiation Exposure in the Healing Arts.

Applied Radiology, Vol. 3, No. 6, p. 35, November/December 1974.

3. Comprehensive Radiation Assessment

Study, Illinois Department of Health

and United States Environmental Protection Agency, Springfield, Illinois,

1975

143
DENTAL- BITEWING

600-

EXPOSURE READINGS IN MILLIROEISITGEN

Figure 1 (Ref. 2)

LATERAL LUMBAR SPINE

140

200 400 600 800 1000 1200 I400| 1600 1800 2000 2200 2400 2600

DOSE (MILLIROENTGENS) MANDATORY LIMIT

Figijre 2 (Ref. 2)

144
ABDOMEN

500 1

100 200 300 400 5001 600 700 800 900 1000 15002000 2500

DOSE (MILLIROENTGENS) ^MANDATORY LIMIT

Figijre 3 (Ref. 2)

A.R CERVICAL SPINE

55

25 50 75 100 125 150 | 175 200 225 250 275 300 400

DOSE (MILLIROENTGENS) ^MANDATORY LIMIT

Figure U (Ref. 2)

145
A.R SKULL

Figure 6 (Pef. 7)

146
NBS SP456 (1976)

NATIONWIDE EVALUATION OF X-RAY TRENDS (NEXT)

PROGRAM - AND RADIATION MEASUREMENTS IN THE STATE

John W. Shaver

University of California-San Francisco

Radiological Health Sciences Education Project

San Francisco, California 94117

A Task Force of state and Bureau of Radiological Health representatives has

applied the "standard man" concept as a method of evaluating the impact of

government control efforts on medical x-ray exposure. Exposure data are collected using standardized procedures and equipment, and Organ Dose Index values

are obtained for selected critical organs. It is expected that these data can

be correlated with governmental agency radiological health activities.

(Diagnostic, nationwide evaluation, physician, physicist, technologist, x-ray

departments, x-ray trends)

An often quoted statistic is that medical and dental x-rays account for approximately 90S of the man-made radiation exposure

to the U.S. population.^ This statistic,

with which I have no quarrel, is sometimes

used to justify or defend public health

expenditures in x-radiation control activities. Although not generally stated, the

implication is that if we in radiation control are effective in our programs, this

percentage will be reduced. However, the

conclusion can be reached that not only is this ratio reasonable but if we were more

effective in all our radiation control efforts, it would probably increase and not

decrease. Perhaps this needs explanation.

Medical and dental radiation is virtually the only radiation exposure to the

population that is intentional ; the rest is an unwanted by-product of the workplace, or from products or operations which utirfze

ionizing radiations. For example, the exposure to the population from the radioactive

effluents and wastes of nuclear reactor

operations is unwanted. The exposure of

operators to radiation from industrial x-ray

defraction units is unwanted. The exposure

to operators of industrial radiography units

is unwanted and the population exposure from

color television sets is certainly unwanted.

Unproductive

Productive

ACTUAL

However, patient exposure is intentional and

essential if diagnostic radiology is to be

used. Thus, it stands to reason that the

intentional population exposure should far

exceed the unwanted exposure. If we could

ideally control all manmade exposure, medical and dental exposure should account for

100% of the man-made radiation exposure and

not just 90%. Figure 1 depicts this concept

in graphic form. I mention this only be- cause I do not find the use of this particular statistic presented as a "stand alone"

statement a logical argument for defense of

diagnostic x-radiation control budgets or expenditures. Of course, this is not saying

that there is no room for improvement in the

application of diagnostic radiation, but it

is saying that problems must be clearly

stated.

The problem as regards population

exposure from diagnostic radiology is not that it accounts for 90% of the total man- made exposure, but rather that a portion of

the diagnostic exposure is unproductive and

can be reduced. Some have estimated the

unproductive component to be on the order of

30%. 2 If this is the case, then the ar- gument can be made that diagnostic radiology

offers the greatest potential for reducing

total population exposure. Figure 1 also

shows this concept.

IDEAL

Productive

Diagnostic X-ray

Figure 1

Other Source Diagnostic X-ray

POPULATION EXPOSURE FROM MANMADE RADIATION SOURCES

147

Other Sources
Identi fyi ng probl ems ,determiningtrends

,

and evaluating progress towards problem

reduction were all considered in developing

the system that I am about to describe.

Increasing demands have been placed on government programs to precisely identify problems and then develop and efficiently manage

specific programs to deal with the problems.

The Conference of Radiation Control Program

Directors recognized this coming trend

several years ago and in conjunction with the

Food and Drug Administration's Bureau of

Radiological Health authorized the development of a system which would assist in meeting the demands for better data management. 3 It was in this light that the "NEXT" system was developed. Specifically the

Conference called for the formation of a Task

Force, in May 1971, to design a uniform

program for surveys of x-ray facilities. The

Task Force, co-sponsored by the Bureau of

Radiological Health, was appointed in July

1971 and consisted of representatives from

both state and federal radiological health

agencies.

The Task Force adopted the project name

of "NEXT", an acronym for Nationwide Evaluation of X-ray Trends. In reviewing its

charge the Task Force identified four specific objectives.

1. Identify the optimum components of a radiation survey.

2. Provide a system for uniform data

col 1 ection.

3. Provide a system to assign program

priorities.

4. Provide a system to measure the

effectiveness of the program.

To meet these objectives, the Task Force developed two major items:

1 . An Optimum Survey Procedures Manual

designed to meet, in part. Objectives 1 and 2, and

2. An Organ Dose Index System (ODIS)

designed to specifically meet

Objectives 3 and 4.

OPTIMUM SURVEY PROCEDURES MANUAL

The Procedures Manual, entitled "Suggested Optimum Survey Procedures for Diagnostic X-Ray Equipment" was completed by a subcommittee of the Task Force and distributed in May, 1974. Development of the

manual, which took over two years, was

complicated in that the details of the

Federal Diagnostic X-Ray Performance Standard were not finalized, which in turn complicated the revision of Part F, "Use of

X-Rays in the Healing Arts," of the Council

of State Governments Suggested State Regulations for Radiation Control. Since the

procedures contained in the manual were to

be sequenced in parallel with the requirements of Part F, for the convenience of state personnel, the Subcommittee had to lockstep its work with that of several other

groups. As regards the term "optimum," it

is applied to indicate that the procedures

are not necessarily the most accurate but are a blend of the parameters of accuracy,

required equipment, time needed to conduct

the test, ease of performing the test in the

field and overall costs. The tests were specifically developed with the state field

inspector in mind. In many cases alternate

test procedures are provided to give each state program as many options as possible to accomplish its work.

The Task Force does not view the Manual

as an end point but rather as a starting

point. As improved test equipment and

procedures are developed, they will be

evaluated and incorporated into the manual.

As new diagnostic equipment is developed,

new test procedures for its evaluation will

be added. Furthermore, as the Manual is used, inevitable flaws will be found which

will require revision. Nevertheless, the

Task Force does modestly submit that the

Manual may form the basis for a generally

agreed upon "Standard Methods" for the

evaluation of diagnostic x-ray equipment.

The manual is proving helpful as a resource

to train new personnel and as a source to refresh one's memory on tests that are infrequently performed. And finally, the

Task Force feels that the use of this Manual

does assist in meeting Objectives 1 and 2.

ORGAN DOSE INDEX SYSTEM

As put forward in Objective 4, states felt an urgency to have available a specific

parameter to measure program effectiveness.

The days of counting how many facilities

have had filtration added to their machines,

or how many facilities have had proper

collimation installed are over. Budget

people are asking what impact your program

has on the public's health, and asking that

this impact be expressed in meaningful

terms. In an effort to be responsive to this request, the Task Force selected

radiation organ dose as a parameter which

most closely satisfies these requirements.

In particular, it was felt that if patient

148
skin exposure and gonad dose (both ovarian

and testicular), bone marrow and thyroid

doses were obtained from statistically

representative facilities throughout a stat(

and tracked with time that the effecti venes; of the overall radiological health effort

could be monitored. This is what NEXT/ODIS

is trying to accomplish.

The methodology required to calculate

the radiation dose to a specific organ from

diagnostic x-ray examinations is known.

The methodology used in the X-ray Exposure

Studies of the U.S. in 1954 and 1970, with

certain modifications, was adopted by the

Task Force for the Organ Dose Index System.

As previously mentioned, besides skin exposure, the Task Force requested radiation

doses be calculated for the testicles, ovaries, thyroid and bone marrow. Currently,

only gonad doses are being calculated; how- ever, the means to compute the other organ

doses will be available this year.

TABLE 1 - NEXT ODIS - Diagnostic X-ray Projection

and Anthropometric Guidelines

Projection

Body Part of Interest Thickness

(Centimeters

)

Chest (P/A) Thorax 23

Skull (Lateral) Head 15

Abdomen (KUB)

Scout Film (A/P)

Abdomen 23

Retrograde Pyelogram, Scout Film

(Cysto Units) (A/P)

Abdomen 23

Thoracic Spine (A/P) Thorax 23

Cervical (A/P) Neck 13

Lumbo-Sacral Spine (A/P) Abdomen 23

Full Spine (A/P) Chest & Abdomen

23

Feet (Weight Bearing) (D/P)

(Podiatrists Only) Foot 8

Dental Bitewing (Posterior) Left Bicuspids

& Molars

Dental Periapical Central Incisor

(Maxillary)

Dental Cephalometric (Lateral) Head 15

To simplify the overall system, only 12 common diagnostic radiographic examinations

were selected by the Task Force for inclusion

into the system. However, the system is flexible and open-ended and if a need is felt to expand this list, it can be done.

For example, as interest in mammagraphy

continues to grow, it could be added to the

system. The projections included are shown

in Table 1. To simplify the dose calculation, a patient having standard anthropometrical characteristics was preselected

(Table 1). Using this system the organ dose

can be calculated from the following data:

1. Beam quality as measured by the half

value layer and kilovoltage

2. Beam size, shape and location, and

149
3. Machine radiation output using the

facility's technique for the selected

examination. (Technique including

kilovoltage, mi 11 iamperage, exposure

time and target-to-film distance).

HOW DO YOU IMPLEMENT THE SYSTEM?

A statistically representative sample

of the healing arts x-ray facilities in a particular jurisdiction is drawn by the

radiological health program. During the

inspection of a selected x-ray facility, an inspector determines if any of the 12 exam- inations are performed in the facility of

the machine being inspected. If so, he asks

the operator to set the technique (milliamperage, kilovoltage, exposure time, targetto-film distance and collimation) that would

be used for a patient possessing the stan- dard anthropometrical characteristics discussed above. For example, if a chest P/A

is the most frequent examination performed

with the machine in question, the operator

is asked to set the technique that would be

used for a patient with a 23 cm chest.

The inspector measures and records the

needed parameters in accordance with stan- dard procedures that have been developed

using a test stand which standardizes field

measurement techniques. Since the test stand is the critical component of the

survey which fixes geometry and provides for

reproducti bi 1 i ty and uniformity of test

measurements, a brief history of its development may be of interest. The idea of a test stand was presented to the Task Force in prototype form as the then called "North

Carolina Chimney." Since we did not have a shop in North Carolina to construct anything

elaborate, we simply used two different

diameter rainfall collection containers,

stacked them one on top of the other and

surrounded the entire contraption with scrap

lead. A slot was cut out of the bottom to permit entry of the small probe of the

Victoreen 666 instrument which was mounted

on a special jig to provide for consistent

positioning (See Figure 2).

To insure proper credit, the North

Carolina Chimney was designed to simulate as best we could the filtration measurement

geometry described by Dr. Dale Trout. ° We attempted to make his laboratory arrangement

suitable for field use. From there, a more

refined collapsible stand was constructed by

the Bureau of Radiological Health's Northeastern Radiological Health Laboratory in Winchester, Massachusetts which was designed

by Alvis Jordans. Later, a rigid stand was developed by the Bureau of Radiological

Health's Division of Electronic Products

(See Figure 3). I might add that the test stand concept is incorporated in both the

Optimum Survey Procedures Manual and in the Bureau of Radiological Health's Routine

Compliance Field Tests and that the same

test stand can be used for both NEXT and

field compliance surveys.

Figure 2: North Carolina Chimney

150
Figure 3: BRH Test Stand

In the case of NEXT, these data (collected by using the test stand) are used to calculate patient exposure at skin entrance

and organ dose indices. The term index is used because the dose is calculated for a standard patient possessing fixed characteristics. I emphasize that NEXT/ODIS does not provide estimates of population dose nor

does it provide per capita dose information

but it does provide information that can be

used to effect changes in them.

USE OF THE NEXT SYSTEM RESULTS OF THE DATA

The most striking results from the NEXT system are the extreme variability of exposures calculated for the same standard pa- tient undergoing the same radiological examination. For example, representative sample

data collected in fiscal year 1974-1975 in facilities conducting lumbo-sacral spine

examinations shows the following:

1. The Ovarian Dose Index varied from 5 to over 400 mi 1 1 irad,

2. The Testicular Dose Index varied from

less that 0.5 to greater than 1000

mil 1 irad.

3. The Exposure at Skin Entrance varied

from 70 to greater than 3800 milliroentgens with a median value of 560

mil li roentgens.

4. The source-to-film distance varied from

30 to 75 inches.

5. The mil 1 iamperage-seconds varied from

30 to 300.

6. The x-ray beam size varied from a rect- angular beam 5x9 inches to a circular

beam 36 inches in diameter.

The mean exposure for this examination

was recorded as 640 mi 1 1 i roentgens . Although the lumbo-sacral examination is not performed as often as the chest P/A exam, if

a program's objective is to optimize gonad

dose, this particular examination stands out among the 12 as an opportunity for improvement. In this regard, a program may wish to concentrate it activities in those facilities conducting the high dose examinations.

An initial sample can identify the types of

facilities conducting these examinations and

follow-up samples can plot the overall program effectiveness. It is hoped that the use of this system can assist program managers in setting more precise priorities and

provide the needed data to more efficiently

accomplish them. However, I should emphasize the NEXT is simply a planning and

evaluation tool. Unless the problems identified by NEXT data are translated into

action programs, changes will not occur.

151
SUMMARY

By monitoring the Organ Dose Indices over time, trends in patient exposure can be

detected. Comparisons between organ dose

indices of participating jurisdictions and

the nationwide data can also be made. This

system appears to be a useful people-oriented

data item for radiological health agencies to assess program effectiveness and to do program planning. NEXT is certainly not a panacea for evaluating how well we are doing

but it does provide a mechanism that can relate our efforts to our overall goal of

reducing unproductive radiation exposure. ^

Thus, from the State program viewpoint,

the efforts of this symposium are welcomed.

The results will be beneficial if they suggest a means of increasing the accuracy and

reliability under the constraints of cost and

manpower available to State radiation programs. However, these suggestions should

address not only the measurement of radiation

exposure, for the measurement of many other

parameters is involved in evaluating medical

x-ray facilities, and all of these must be addressed to arrive at satisfactory measurements for the safe use of radiation.

REFERENCES

1. U.S. Environmental Protection Agency,

Office of Radiation Programs. Estimates

of Ionizing Radiation Doses in the

United States, 19602000. ORP/CSD 72-1,

Page 168.

2. National Academy of Sciences, National

Research Council. The Effects on Populations of Exposure to Low Levels of

Ionizing Radiation, Page 13, November

1972. Report of the Advisory Committee

on the Biological Effects of Ionizing

Radiations; Division of Medical Sciences;

EPA; and BRH. Superintendent of Docu- ments, U.S. Government Printing Office,

Washington, D.C. 20402. Stock Number

55505-00005.

3. Third Annual National Conference on Radiation Control, DHEW Publication (FDA)

72-8021, BRH/ORO 72-2 (1971), Pages 324-

325, National Technical Information

Service, Springfield, VA. 22151.

4. Population Exposure Studies Section, Bureau of Radiological Health. Population Dose from X-rays U.S. 1964. Washington, D.C: Public Health Service

Publication No. 2001 , 1969.

5. A Panel Presentation by J.N. Gitlin, R.R. Fuchsberg, and R.F. Brown at the 49th

Annual Meeting of the American College

of Radiology, Miami, Florida April 3-8,

1972.

6. Proceedings. Third Annual National

Conference on Radiation Control. Rockville: DHEW Publication (FDA) 72-8021

BRH/ORO 722, 1971.

7. Gitlin, J.N., and P.S. Lawrence. Population Exposure to X-rays U.S. 1964. Washington, D.C: Public Health Service

Publication No. 1519.

8. Trout, E. Dale; J. Kelley, and A. Lucas. Determination of half-value layer.

American Journal of Roentgenology 84: 729, 1960.

152
NBS SP456 (1976)

LOW LEVEL ENVIRONMENTAL RADIATION MEASUREMENTS WITH A PLASTIC

SCINTILLATION DETECTOR SYSTEM

A. P. Hull and G. S. Levine

Brookhaven National Laboratory

Upton, New York 11973

A plastic scintillation detector-photomultiplier combination, operated in the current mode, has been developed at Brookhaven National

Laboratory for the measurement of low level environmental radiation

Its output is digitized and processed by semiconductor circuits,

which also provide for hourly compensation of variation in gain

utilizing a built-in calibration source.

The instrument has been tested in the field for the past two years

to establish its response capability to diurnal and longer term variation in background radiation, as well as small increments

produced by a distant y-field irradiation source. (Supported by

U.S. ERDA.)

(Low level environmental radioactivity measurements scintillation

detector)

Introduction

Life has evolved and exists in a constant low level of external background radiation. Although this has been known since

the discovery of natural radioactivity, the

accurate determination of its levels did not become a matter of widespread interest until

the development of nuclear reactors, and the

consequent possibility of measurable dose

increments to large populations from manmade radioactivity. The adoption by the

Nuclear Regulatory Commission-'- of 5 mrem/yr

dose limit as an "as low as practicable"

design objective for light water-cooled

power reactor effluents has stimulated an interest in the accurate measurement of very

small increments to background levels.

Until recently, ion chamber-electrometer combinations were the most suitable

instruments for making low level measurements of environmental radiation. A simple

6 liter unpressurized bakelite wall ion

chamber with a dynamic condenser electrometer (DCE) was developed at Brookhaven

National Laboratory in the late 1940' s. A number of these instruments were used for

two decades in the field for the continuous

measurements of the dose from the '^'Ar in

the plume from the air cooled Brookhaven

Graphite Research Reactor. ^ They were capable of detecting changes in radiation levels of about 1 (j,R/hr, and of establishing

yearly dose increments (above background)

of about 5 mR. A somewhat more sensitive

pressurized version, with a stainless steel

wall, has been developed by the Health and

Safety Laboratory. A commercial version is

currently in wide use, particularly to establish the small environmental dose increments

occasioned by the short-lived noble gases in

the air effluents from commercial power reactors. Various devices, Geiger-Miiller (GM)

counters, film and thermoluminescent dosimeters (TLD) have also been employed. Unless

specially shielded, the GM tube's variable

energy response complicates the dose interpretation of its signal. Film is not a satisfactory environmental detector, due to its limited sensitivity (~ 20 mR) and vulnerability to temperature and humidity. Although several currently used TLD's are sen- sitive to doses as low as 1 mR, as passive

devices they do not lend themselves readily

to the measurement of plant effluent related

doses of a few mR/yr in the presence of

naturally occurring levels in the order of

100 mR/yr.

At Brookhaven, a plastic scintillator

photomultiplier combination has been developed as a replacement for the ion chamber

DCE.^ It provides for a high sensitivity

with a small detector volume, P + Y or y only response, an air equivalent energy

response, and a high signal to inherent noise

ratio. The D.C. output of the PM is digitized and processed by semiconductor circuits,

hence it is a digital environmental monitor

(DEM). It contains a built-in calibrator,

which is utilized on an hourly basis to com- pensate for the temperature related changes

in the gain characteristic of the photomultiplier .

153
During the past two years the DEM has

been operated side by side with the BNL ion

chamber DCE (Figure 1) and TLD devices,

measuring perimeter doses, and in other

applications requiring great sensitivity.

Results from this experience are described

in the present report.

Instrument

The DEM is a self-contained, semiportable, line-powered low-level radiation detection system, with automatic standardization (Figure 2). The detector portion consists of a 2" X 2" diameter tin loaded plas- , tic scintillator (NE-106) coupled to a low

noise photomultiplier (EMI 9635A or 9750B)

operated in the D.C. mode (Figure 3). The

signal current is converted to pulses which

are counted and recorded, repetitiously, by

a digital printer for selected intervals

between 1 and 16 hr. Internal standardization with an alpha source (^-^^Vu) is performed hourly to compensate for temperature

effects on the gain characteristics of the

photomultiplier. The detector section of

the instrument is separate from the remainder of the instrument. In the DEM' s which

have been field tested to date, the detectors have been housed in an aluminum con- tainer whose wall thickness varies from

0.86 -2.57 g/cm^, thus making them essentially beta opaque.

Calibration

The instruments are periodically calibrated with a ^^Co source mounted in a holder precisely located with respect to the

detector so that the dose rate at the crystal

is 5.42 p-R/min (5/74), significantly greater

than background, but well within the range

capabilities of the instrument. Table 1 summarizes a number of calibrations for

various individual instruments.

Field Measurements

Figure 4 illustrates the hourly dose

recorded by a DEM located in a perimeter

monitoring station adjacent to the BNL Ecology Forest (815 m distant). The local

biosphere is exposed to the "skyshine" radiation of a 6350 Ci 137cs source for 20

hours each day. The source is lowered into

its shield for 4 hours alternate mornings

and afternoons. This near unique situation

demonstrates several of the instrument's

capabilities

The diurnal pattern and the day to day

variations while the source is exposed re- sults from the temperature dependent density

of the air scattering medium between the source and the monitoring station. The

average background level, as established

from the trough of the period when the source is down, was 6.3 uR/hr. It is es- sentially constant except for two days,

12/25 and 12/30 during which precipitation

occurred. The average source radiation increment, as determined from the difference

between the adjacent "source up" and the

"background" periods was 9.1 uR/hr.

Comparisons with the other low level

measurement devices operated side by side over time with the DEM at this location have

revealed some systematic differences which

imply that the DEM underestimates the true environmental dose by about 207o. Attenuation and absorption in the enclosure and

under response to the cosmic ray contribution by the plastic detector are possible

explanations

A DEM was recently used to evaluate the

impact of the installation of a more intense

137cs source in our calibration laboratory.

A collimated 3 Ci source, used to calibrate

personnel dosimetry film, was deemed insufficiently intensive for high dose ex- posures. Measurements were made with the

DEM to delineate the anticipated increase

in facility background expected from a more

intense source. The DEM, being reasonably

portable, was set up, took baseline data

with the source removed over the weekend,

and because of its great sensitivity, provided the necessary information in several

hours. The incremental addition, due to the present source, was found to be 31.1 at the point where personnel operate the free

air sources during instrument calibration,

and at worst 0.5 p-R/hr in the repair area where the occupancy factor is highest.

Conclus ions

The field testing has demonstrated that an instrument utilizing a scintillator,

phototube detection scheme is a useful instrument for the measurement of background

environmental radioactivity and small

increments thereto.

154
i
HOURLY INCREMENTAL DOSE

AT P-9 MONITORING STATION

DECJ974

1

1 I I I r DEM NO. 4 - 7.78 c//xr

MEASUREMENT AT 50min PAST THE

HOUR,0 ENDS 04:50 12/24/74

150

130

Q.

UJ

o

90

70

50

30

10

12:50 PM

r0 20 40 60 80 100 120 140 160 180

TIME (HOURS) .

Figure 4

156
References

1. U.S. Nuclear Regulatory Commission,

"Opinion of the Commission", Docket

RM 50-2 (4/30/75)

2. M.M. Weiss, "Area Survey Manual",

Brookhaven National Laboratory Report

BNL-344 (1955).

3. H.L. Beck, et al .

, "Final Report on Continuous Monitoring with High Pressure

Ionization Chamber", Health and Safety

Laboratory Report HASL-290 (1975).

4. J.D. Chester, R.L. Chase, S. Woods,

"A Digital Monitor", Brookhaven

National Laboratory Report BNL-16922

(1972)

Table 1 DEM Calibrations

Unit Date Source Distance (cm) C /\JjT

1 8/5/75 137-Cs 55 9.9

1 8/5/75 137-Cs 55 9.5

2 12/17/74 60-Co 55

' 7.15

2 1/24/75 60-Co 55 7.36

2 4/5/75 60-Co 55 6.35

4 5/6/74 60-Co 55 9.39

4 5/7/74 60-Co 55 9.50

4 7/2/74 60-Co 200 8.95

4 9/11/74 60-Co 55 8.58

4 9/16/74 60-Co 55 8.60

4 1/24/75 60-Co 55 7.78

4 4/29/75 60-Co 55 8.43

4 11/5/75 60-Co 55 8.71

4 11/18/75 60-Co 55 8.37

4 11/25/75 60-Co 55 8.10

4 11/25/75 137-Cs 55 9.63

4 12/30/75 60-Co 55 8.03

4 12/30/75 137-Cs 55 9.38

4 2/4/76 60-Co 55 8.37

4 2/19/76 60-Co 55 8.69

4 2/19 137-Cs 55 9,70

5 2/7/75 60-Co 55 11.85

5 11/3/75 60-Co 55 14.62

Notes

Additional collar

around scint.

Unit has double scint,

157

NBS SP456 (1976)

PRECISE ENVIRONMENTAL GAMMA-RAY MEASUREMENTS

UTILIZING STANDARD Ge(Li) DETECTORS

R.M. Keyser, G.V. Walford, J. A. Cooper

ORTEC Inc. 100 Midland Rd

Oak Ridge, TN 37830

Introduction

The analysis of environmental samples

for radionuclides is fast becoming a large

segment of the gamma-ray analyses performed.

Many research laboratories and routine

testing stations encounter a large number of

samples. Some of these are relatively complex. In this paper we are concerned with

the physical parameters of the measuring

system; that is the characterization of the

detector, the sample-detector relationship

and the multichannel analyzer (analog to digital converter) and how these characteristics affect the accuracy of the measurement.

The counting sensitivity of a system

can be used as a figure of merit for that

system and can be defined as the minimum

detectable^ limit or minimum acceptable

limit. There are five factors affecting the

sensitivity of a system. These are: the

background, the system energy resolution,

the detector efficiency and sample shape,

the time allotted for the measurement, and

the accuracy required for the result. We

will discuss these parameters and their mea- surement and how they affect sensitivity for

a series of standard germanium detectors.

Definition of Sensitivity

The definition of the sensitivity

limits of a spectrometer have been previously

described ^ ^ in some detail and hence are only outlined here. In brief, two definitions are used. The minimum detectable acti - vi ty is reached when the peak of interest is just observable above the interfering counts.

This point has been previously determined ^ when the peak standard deviation is one third

that of the peak area. However, in order to give clarity to this study, and to present

conservative data, this point was defined

when the standard deviation of the peak area

was one fifth that of the peak area. The

mi n i mum acceptabl e act i v i ty i s reached when

the experimentalist is just able to achieve

the statistical precision required for a specific experiment.

It has been shown that either of the

above sensitivity definitions is described

^ ^ as:

Where Ae= minimum detectable or acceptable

activity (in gamma per min. emitted

from the samp 1 e)

e = absolute full energy peak efficiency

for that sample (count rate of peak

of energy e, divided by gamma rays of energy e emitted per unit time

from the samp] e)

C = Statistical precision of the peak

area (standard deviation of peak

area divided by the peak area).

B = Total interfering counts under peak

(these counts include both environment and sample generated scatter

counts). Energy resolution is also

incorporated in this parameter.

T = Count time.

T and C are set by the experimentor

whereas e, B and the energy resolution are determined by the construction of the spectrometer. Ae Is determined for a spectrometer by substituting values for the counting

parameters into the above equation.

Background

There are two groups of applications

whose background contributions are sufficiently different to require separate con- sideration. These groups are separated by

the origin of the background counts under

the peak. In the first groups, the interfering background is due to the environment

and in the second the interfering continuum

has significant sample generated contrlbut I ons

The natural background can be reduced

by placing the detector in a shield.

159
Figures 1 and 2 sho^-J the background inside

and outside the shield for 60 and 90cc stan- dard detector in a 10 cm lead shield. The

background is characterized in Tables 1 and

2.

The data in Tables 1 and 2 along w\th

the energy resolution of the detector, C=0.2

and T= ^lO.OOO seconds (11.1 hours) yield the

sensitivity for the detector both inside and

outside the shield. This is shown in columns

1 and 2 of Table 3. Column 9 shows the in- crease in sensitivity due to the shield.

Columns h and 5 show the change in sensitivity for lower energies when the sample contains an intense high energy peak.

As expected, the change is most pronounced

at the lower energies (factor of 1.8l reduction in sensitivity), but is also seen at the

higher energies (1.17).

Table 2 shows a list of peaks commonly

found in the background spectrum. If the

peak of interest coincides with one of the

background peaks, then the background peak

area must be accounted for in calculating the

sensitivity. As a general rule, the sensitivity is reduced by a factor of two, if one

of the interfering peaks is encountered.

Energy Resolution

The energy resolution of a system con- sisting of a detector, amplifier and multichannel analyzer is affected by every com- ponent in the system, how the resolution is calculated and how the data are collected.

The energy resolution affects the analysis of

the data in two ways: 1) the ability to separate two adjacent peaks is dependent on the width of the peaks and 2) the minimum

detectable limit is dependent on the background under the peak, which is dependent on the peak base width.

In order to observe the influence of

system energy resolution on the spectral

statistical analysis, the experimental con- figuration in Figure 3 was used. This con- figuration allows the resolution to be

broadened independent of the other counting

parameters. The detector is a germanium

detector with a resolution of 1.65 keV FWHM

at 1.3325 MeV. This resolution was measured

with more than 20 channels in the peak and

the FWHM was calculated from a peak with

more than 10,000 counts in the peak channel.

The following method was used for all the FWHM calculations: the contents of the maximum channel is divided by two to yield the

half maximum. The full width points are

found by linear interpolation between the

contents of the channels surrounding the half

maximum point.

The process of digitization in the ana 1 og- to-d i g i ta 1 converter yields a set of

numbers that represent the total number of

pulses that lie between the lower and upper

bounds of each channel. The numbers do not represent the average amplitude within each

channel. As a consequence of this, the measured f u 1

1 -w i dth-ha 1 f-maxi mum is a function

of the position of the true peak centroid

relative to the channel boundaries. Figure

k shows the variation of measured FWHM as

a function of peak centroid position. As is expected, the curve is symmetrical about the

midpoint between the channel boundaries and

the resolution or FWHM broadens at the midpoint.

In addition, the number of channels or data points in the peak have an effect on

the FWHM measurement. Figure 5 shows the

broadening of the FWHM's as a function of

energy/channel . The curves are steepest for

the lowest resolution detector, that is, the

smaller the FWHM the greater the broadening

with increasing energy per channel. The

energy scale corresponds to 750 keV full

scale in a 4096 channel analyzer at the low

end to k MeV full scale at the high end. As can be seen, the I.65 keV detector gives a resolution of 1.95 at 1 keV per channel,

while a 2.07 keV detector degrades to 2.32

keV and 2.99 keV detector degrades to 3-02

keV. This indicates for many systems the

detector resolution is not the limiting factor in determining the sensitivity of the

system. The effect of energy resolution on the peak area statistical precision is shown

in Figure 6. Figure 7 shows a sample spectrum

used for this determination.

The spectrum in Figure 7 was analyzed

when the energy resolution of the system was

1.7 keV FWHM at 1.33 MeV. At I85 keV, the

statistical accuracy to one a of the peak area was ± 1.67% and at 1.33 MeV was 1.76%. The

energy resolution was successively degraded

in steps using the techniques shown in Figure

3. At an energy resolution of 2.8l keV at 1.33 MeV, the statistical accuracy had been

degraded to 1.80% for the 185 keV peak and

1.96% for the 1.33 MeV peak. These degradations are small compared with the energy

resolution broadening. Similar degradations were observed for other peaks in the spectrum, some of which involved near doublet

s i tuat i ons

.
Peak Efficiency

The sample shape or the sample-detector

geometry contributes significantly to the

detectable limit. Three geometries were used

to show this effect. These geometries were

chosen because they maximize detection capability and are shown in Figure 8. Geometry

a) shows a disc of approximately 3 cm diam- eter having a volume of 5 ml . Geometry b)

shows the measurement made for the disc

paralleling two detectors. Geometry c) shows

the use of a 500 ml Marinelli beaker placed over the end cap. Both disc and Marinelli

beaker geometries of other dimensions are available, but the data in this paper can

be extrapolated to those geometries.

Efficiencies were determined for the

different geometries by filling them with a uranium ore standard which simulated the

efficiencies for a typical soil sample.

Measurement of liquid samples in the same geometries would show improved count rates

due to the lower density and mean atomic

number of the liquid. As a check upon the

calibration of the ore, the disc sample was

cross checked with known Csl37 and Co60

sources. The ore samples provide an energy

calibration from l85 keV to 2(>^k keV. Figure

9 shows a typical spectrum measured for the

Marinelli beaker with a 90cc detector.

Table k shows the efficiencies measured

for 60cc and 90cc for these geometries at several energies. As seen in the Table,

there is a large difference in efficiency

among the geometries.

To compare the effectiveness of the

60cc and 90cc detectors, their efficiency

ratios for the same sample geometry were determined as a function of energy and are shown in Table 5- At l85 keV, the ratio of

the 60cc and 90cc for the disc source is approximately 1.1. Since at this energy 75%

of the incident photon beam interacts primarily within the first 11 mm of the detector,

this ratio represents the difference in solid

angle between the two detectors. At 2204 keV

this ratio is 1.34 and represents the combination of solid angle and detector dimension.

It is important to note that these ratios

represent solid angle differences and not detector quality differences.

It is interesting to note that the

Marinelli efficiency ratios are greater than

disc efficiency ratios, representing greater

utilization of the sensitive germanium

available. The ratio, however, reaches only

1.5 (the sensitive volume ratio of the two detectors) at energies in excess of 2 MeV,

indicating that the volume of a large detector is only fully utilized for one sample

geometry and energy range. In view of this

fact, the optimization of a detector system

requires that attention be placed upon solid

angle considerations as well as detector size.

Referring back to Table 3, we see that

the minimum detectable limit (in gammas/min)

for the Marinelli beaker is higher than for

the disc sample. Column 10 shows the ratio

of Ae for disc sample and the Marinelli sample. Table 6 shows the minimum detectable

limit expressed as gammas/mi n/ml of source

for the disc, Marinelli and for the Marinelli

volume reduced to a 5 ml disc (approximate

100:1 volume reduction). These data indicate

that the most sensitive geometry is the re- duced sample and it is approximately 2.5

times as sensitive as the Marinelli beaker.

However, the Marinelli geometry is approximately kO times as sensitive as the unreduced

disc sample.

Some samples cannot easily be reduced

(soils, ores, ashes), and in some cases the

number of samples prohibits reduction so the

Marinelli beaker geometry represents an easy

method for increasing sensitivity.

Cone 1 us i on

The general conclusion from this work is that standard detectors, shields, electronics

and sample geometries provide sensitivities

adequate for a large majority of the current

environmental uses. In order to improve the

sensitivity for a system, large expenditures

are necessary in order to make small advances

in Ae. For example, in the energy range near 200 keV a 60cc detector degrades in uncertainty

as the FWHM broadens. The uncertainty changes

from 1.67% to 1.71% as the resolution changes

from 1.7 keV to 2.0 keV. There is a significant cost difference between a 1.7 keV and

2.0 keV resolution detector.

Walford, G.V., Al i aga-Kel 1 y , D.T.W., and

Gilboy, W.B., IEEE Transactions on Nuclear

Science, NS-I9, No. 1, Feb. 1972, p. 127-

J. A. Cooper, NIM 82 (1970) 273-

G.V. Walford, and W.B. Gilboy, The Natural

Radiation Environment 11, Houston, TX

(1972) ERDA Conf. 72085-P1 135.

161
TABLE 1

TYPICAL BACKGROUND CONTINUA OBTAINED FOR THE

60cc AND 90cc DETECTORS

60cc No 60cc With Background 90cc No 90cc With Background

Nom i na

Shield Shield Reduct ion Shield Shield Reduct i on Energy cpm/keV cpm/keV Factor cpm/keV cpm/keV Factor

185 k.5 0.053 85 6.1 0.063 97

609 0.013 26 0.53 0.017 33 1238 0. 12 0.0055 21 0. 16 0.0059 29

1764 O.Ol^t 0.0022 7 0.022 0.0030 7 220k 0.012 0.0017 6 0.022 0.0023 9

TABLE 2

TYPICAL BACKGROUND PEAKS OBTAINED FOR THE

60cc AND 90cc DETECTORS

Energy 1 sotope

Peak Count

60cc No Shield

cpm

Peak Count

60 cc Shield

cpm

Peak

90cc

cpm

Count

No Shield

Peak Count

gOcc Shield

cpm

185 2 7 0. 13 3. 1 0.22

295 5 4 0. 08 7. 9 0.11

352 9 8 0. 09 13. 4 0.21

511 3 7 0. 60 5. 8 1 . 14

583 208^,

3 0 0. 237 4. 2 0.36

609

1378

214,.

Bi 214„.

Bi

8

0

2

43

0.081

Not 1

)

Observable

1 1

0.

2

74

0.15

Observab 1

1460 40K 1

9 0 06 21 . 9 0.087

1764 21\i 1 63 0 018 2. 5 Observab 1 e

2204 0 .42 Observab 1

0. 74 Observab 1

2614 208^, 1 .77 0 084 3. 1 0.15

Not observable indicate C > 0.33 for that peak.

162
TABLE 3

MINIMUM DETECTABLE ACTIVITY (in y/min) FOR SEVERAL SAMPLE- DETECTOR CONFIGURATIONS

AND BACKGROUND RADIATION CONDITIONS AS A FUNCTION OF ENERGY

60cc

Energy (keV) Ae Ae Ae Ae 1

1 Ae Ae AeA«7 AeA.^ Ae /Ae Ae /Ae

1 Q r

11.6 4.9 2.9

1

1 4.9 1 6 1 .

3.4 L Q-7r 6.075 • 32 609 12 3.2 9.6 3.5 i 6.3 19.3 2.0 4.3 3.75 .33

1238 16 5.3 15 7.5 i 9.0 25.7 3.4 6.0 3.02 .35 1764 1

6.2 17.4 7.8 8.5 24 3.8 5.5 1 .77 .35 2204 13 7.5 21 8.8 1 8.9

!

, 4.4 5.5 1 .73 .36

90cc

Energy (keV) Ae^ Ae^ Ae3 ^Ae^" Ae5 Ae7 Ae« AeVAe^ 2 , 3 Ae /Ae^

185

1 12.2 1.5 4 2.8 4.3 12 1.0 3.1 8.13 .37 609 13 3.0 7.8 6.7 5.4 14 1.9 3.5 4.33 .38 1238 17 4.4 11 6.6 7.2 18 2.7 4.8 3.86 .4 1764 10.5 4.2 10.5 6.6 6.8 17 3.2 4.4 2.5 .4 2204 12.6 6.0 15 7.1 7.0 17.4 3.6 4.3 2.1 .4

No Lead shield, disk sample natural bg dominates interfering continuum,

disk sample, natural bg dominates.

Marinelli Beaker, natural bg dominates,

disk sample vi'ith 1 nCi of ^^Co and ^^^Cs in disk,

disk sample with 2 nCi activity of U and Th in disk.

Marinelli Beaker with 5 nCi activity of U and Th in beaker,

tv/o detectors in parallel with disk sample, natural bgrd dominates,

two detectors in parallel with disk sample with ^ 2 nCi of U and Th in disk.

T = 40,000 sec, C = 0.2

Ae in y/mln emitted from sample.

1 0cm shield,

10cm shield,

10cm shield,

10cm shield,

1 0cm shield,

10cm shield,

10cm shield.

163
TABLE h

ABSOLUTE FULL ENERGY PEAK EFFICIENCY (e) FOR SEVERAL SAMPLE - DETECTOR CONFIGURATIONS

Detector Con "f i 9 u ra t i on

e At 185 keV At 609 keVe At 1238 keVe At 1764 keVe At 2204 keV

60cc Geometry a) .082 .025 .012 .0084 .0067

90cc Geometry a) .091 .030

.

. -

1 .0151

1 .011 .009

60cc Geometry b) . 16 .050 .02k .017 .013

90cc Geometry b) .18 .059 .030 .022 .018

60cc Geometry c) .025

. .0081 .00k2 .0030 .0024

90cc Geometry c) .03^ .011 .0060 .0044 .0036

TABLE 5

COMPARISON OF 60cc 90cc ABSOLUTE FULL ENERGY PEAK EFFICIENCIES

Energy (keV) Disk Sample - Ratio^^ Mart i nel 1 i Sampl

e

90cc ^ . " 60CC

185 1.11 1 .36

609 1.18 1 .41

1238 1.25 1 .43

1764 1.31 1 .^7

2204 1.34 1 .50

Ratio of absolute full energy peak efficiency for 60cc and 90cc

164
TABLE 6

MINIMUM DETECTABLE ACTIVITY IN y/MIN/ML OF SAMPLE MATERIAL FOR THE MARINELLI BEAKER

AND FOR THE DISK SAMPLE

ONE 60cc DETECTOR

Energy (keV) Ae (Fory/m

Disk Sample)

n/ml Ae (For Mar i ne 1 1y/mi n/ml

Beaker) Ae

by Mar i nel 1 i VolEvaporation to Reduced5ml Disk

185 0.3 8.9 X lo'^ 2.7 X 10"^ 1)

609 0.64 1.7 X lo'^ 5.6 X lo'^

1 9 "2 R 1 1

1 . 1 -2 - 7 ^ . D X I u

1 0 1 . /. -2

^ . 1 X 1 u

1 1 1 n

^ I.I X 1

U

i

i oon/i 1 r -?9 Q V in i • 0 X 1 u -9

1 '3^ V in 1

.

io X 1 u

ONE 90cc DETECTOR

Energy (keV) Ae (Fory/m

Disk Sample)

n/ml Ae (For Mar i nel 1

y/min/ml

Beaker) Ae

by Mar i ne 1 1 i VolEvaporation to Reduced5ml Disk

185 0.3 7.2 X lO'^ 2.7 X lO'^

609 0.6 1.4 X lO"^ 5.4 X lo'^

i 1238 0.9 2.0 X lO'^ 7.6 X 10'^

1764 0.8 1.9 X lO'^ 7.6 X 10'^

2204 1.2 2.7 X 10'^

1 . 1 X 10'^

Expressed as the activity in y/min/ml of original sample volume.

T = 40,000 sees.

C = 0.2

Natural background dominates the total bg contribution.

165
Counting Time 40,000 seconds

Energy

Fig. 1. Log Plot of Background Count Without Shielding

for a 90 cc Detector.

0 0 1 0,2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Channel Number

Fig. 4. Measured FWHM as a Function of Peak Centroid.

Counting Time 40,000 seconds

Energy

Fig. 2. Log Plot of Background Count with 10 cm Lead

for a 90 cc Detector.

NOISE SOURCE

Fig. 3. Block Diagram of Electronic System Used in FWHM Studies.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 KeV/Channel

! I II I 0.8 1.6 2.4 3.2 4.0 MeV/Full Scale

Fig. 5. Measured FWHM as a Function of

Digitization Width.

166
1.33 MeV Peak

(a) Disk Sample with One Detector

Detector Position

Disk

End Cap

(b) Disk Sample with Two Detectors

System Energy Resolution at 1.33 MeV (keV FWHM)

r

Disk

Fig. 6. Variation of Peak Statistical Accuracy (C) as a Function of Energy Resolution.

(c) Marinelli Beaker Placed upon Detector

r ^

550 ml Sample

Fig. 8. Sample-Detector Geometries Used for Measurement

of Weak Activities.

352.6 keV

185.7 609 keV

keV (

"Co

1173 keV1332keV

Detector - 8101-1521W

Counting Time 4000 seconds

Energy Energy

Fig. 7. Sample Spectrum from 1.7 keV Detector with

Uranium Ore and 6° Co.

Fig. 9. Spectrum of a Uranium Ore Sample in a Marinelli Beaker Used for 90 cc Ge(Li) Detector Calibration.

167

NBS SP456 (1976) THE MINBTLm-nETECTABLE-ACTTVITY CONCEPT

Joseph C. Lochamy

Duke Power Company

Charlotte, N. C. ?S242

Various definitions of "minimum detectable activity (T^A) " are discussed and statistically valid representatives are presented. The

methods of Currie are given primary emphasis and include development

of the Critical Level, Detection Limit, Less-'^han Level, and Determination Limit in terms of count-rate.

(Background; Critical Level; Detection Limit: Determination Limit;

Less-Than Level: minimum detectable activity: statistical)

Introduction

A casual review of the literature references to "minimum detectable activity

(MDA)", "sensitivity", and "limits of detection" often gives the reader the feeling

that everyone is going in different directions, without any really-unified effort to define exactly what is being pursued. Out

of all the different mathematical expressions for >?DA used in lov7-level radiation

counting, there are at least four different

types of MDA implied. Ouite often a single

mathematical representation is inappropriately called-upon to satisfy the requirements of several of these MDA concepts.

These four concepts will be defined in terms

of count-rate and, for convenience, will be

called

the Critical Level (L^,) the Detection Limit (L^)

the Less-Than Level (Lt)

and

the Determination Limit (Lq)

Critical Level

Usually, some test is applied to lowlevel counting data to determine if the

sample count-rate is statistically different from the background. This test is often

expressed as a net count-rate equal to some multiple of the background standard deviation (o],) . This test is probably the most

common conception of the MDA, and we will

call it the Critical Level (L^) . In words,

the Critical Level (L^) is defined as that

net count-rate which must be exceeded before

the sample is said to contain any measurable

radioactive material above background.

Rather than choosing an arbitrary mathematical expression, let us define a statistically meaningful Critical Level (L^,) .

Figure 1 shows the normal distribution

which would be expected for a mean net count-rate (Rg) of zero in the presence of

some background count-rate (R^,) . The value

for Lq is expressed as

L(, = kao

where k is the one-sided confidence factor

and is the standard deviation of a zero net count-rate. If k is chosen so that ^57- of the measurements of a true zero mean net

count-rate are less than L^,, that is

k = 1.65, then there is only a 5°A chance

that a true mean count-rate of zero will be

falsely recorded as a positive value.

CRITICAL LEVEL

0 Lc

Figure 1

If the net count-rate exceeds L^, it is

reported as positive with its two-sided

confidence interval. If the net count-rate

is less than L^,, it can only be said that

the total count-rate (Rt) is not statistically different from the background at the

0.05 level (or whatever k-factor level is

chosen)

In the following block, the Critical

Level (L(,) is derived in terms of the background standard deviation (a^)

169
CRITICAL LEVEL (L^)

From Figure 1: L^ = koQ

Since Rg = Rq = Rt - = ^

= ^ = ^

Thus,

Tb

. RL = T^b(Tt + Tb) Tt Tb

And, oq

Therefore

,

Ob

kab

1 + Ik

t J

TbTt

1/2

1/2

where k refers to the one-sided con- fidence level.

In terms of count-rate.

Ik

Tb

1/2

are equal (Tb = Tt = T) , then

L(, = k

,Rk 1/2

(1)

The Critical Level is used only to determine if a measurement is statistically

different from background. It should not be used as a practical MDA reporting-level

and should not be reported as a "less-than"

value

Detection Limit

Often, when you are asked to specify

your MDA, the question really being asked

is "If I send you a sample, what does the

count-rate have to be in order for you to be sure of detecting it?" In other words,

what level can be detected on a routine

basis? If you specify L^, you may as well

flip a coin to determine if the activity

will be detected; that is, a true mean net

count-rate of L^, has one chance in two of

falling below L(, for any single measurement.

The Detection Limit (Lj) is the value

that should be specified. The Detection

Limit (Ljj) is defined as the smallest quan- tity of radioactive material which can be

detected with some specified degree of con- fidence. If you are satisfied with a one- chance-in-two degree of confidence, then

Ld

However, if you do not want the integrity of

your instruments (or your own integrity)

doubted 507, of the ti.me, you should specify

a Detection Limit such that you will be sure

to see the activity at least, say, ^57 of

the time.

DETECTION LIMIT

0 L

times

Figure 2

Figure 2 graphically represents the De- tection Limit (Ld) , and shows its relationship to Lq. The value for L^j is expressed

as

^d = l^c + ^^d

where k is, again, the one-sided confidence

factor and a^j is the standard deviation of

a net count-rate equal to L^j. If k is

chosen so that 95% of the measurements of a true mean net count-rate L(^ (that is,

k = 1.65) are detected, then Ld is a practical detection limit which you are '^57, con- fident of detecting.

In the block on the following page,

the Detection Limit is derived in terms of

the background standard deviation (ab)

Notice that for ab = 0 (i.e. Rb = 0) , the

Determination Limit still has a non-zero

value.

170
DETECTION LIMIT (Ld)

From Figure 2 : L^j = + ka^j

Since L^j = Rd = Rt - Rb

ad^ = Rt + Rli = Rd + + M Tt Tb Tt T-b

But R<i = Ld

Thus .,2 = Ld ,

and

Ld = Lc + k

Since Lc^= k^a^,^

TbTt

It

11/2

Ld = L^ + |k2^ + Lc2

1/2

Rearranging and solving for Ld,

Tt

In terms of a^,

k2

Ld = ^ + 2kab 1 +

Tb 1/2

Again, in terms of count-rate,

k2

Ld = + 2k

Tb

1/2

and for T^, = T^. = T, the practical

form is

k2

Ld = + 2k

1/2

(2)

Less-Than Level

Thus far, we have specified a Critical

Level (Lc) for determining if a measured

count-rate is statistically greater than

background, and a Detection Limit (Ld) which

the true net count-rate of a sample must

exceed if we wish to be sure that a single

determination will detect the activity.

However, nothing has been said about the

quantity of radioactive material which could

be present even though it is not detected

;

that is, the measured count-rate is less

than L(,. In general, neither L^ nor Ld satisfies the requirement for the Less-Than

Level (Lt)

.

The Less-'^han Level (Lj.) is defined as the maximum true count-rate which a sample

could have, based on the measured count-rate

Rg, where Rg is less than L^- The mathematical expression for L^ is given by

Lt = Rg + kag

where k is the one-sided confidence factor.

If P5''^ confidence is desired (k = 1.65),

this expression savs that there is only a

5°' chance that the activity actually present

exceeds Lj..

In the special case where the sample

net count-rate is exactly zero, then

Lf. = kao = L(, But, in general, L^- is somewhere between L(, and Ld. Of course, it is possible

to have Rg < due to the statistical

nature of low-level counting. In this case,

it seems appropriate to let Lj- = L^, for all

Rg < 0.In terms of count-rate

Lt = Rg + k Rb 1/2

t 'b and if Tt = T^, = T, then the practical form

is

T J DETERMINATION LIMIT (L^,)

Lt = Rg + k

(3)

One last MDA concept is defined in the

literature as the lowest net count-rate

which can be detected with a specified re- lative standard deviation. This definition

describes the Determination Limit (Lq) , and

is expressed asS = •^q'^q where fa is the reciprocal relative standard

deviation desired.

In the block below, the Determination

Limit (Lq) is derived in terms of a^,.

DETERMINATION LIMIT (Lq)

L„ = fqOq = fq

Rg + Rb + Rb.1^/^ Tt Tbj

Since Lq = Rq

Lq = fq Lq , Rh(T, + T>,)l l/2 Tt TtTb J

Lq - fq Tt )

(Continued on next page)

171
DETEPMINATION LIMIT (Lq) (Continued)

Solving for Lq

f

2

T = »-q. 1 +

j

In terms of count-rate,

2 -q 2T t L 1 +

The practical case where Tj- = = t gives

Lq - 2^ 1 +

(4)

a. If Rg > Lj,, the result is reported as positive, with the tbio-snlSed

confidence interval desired,

Rg ± kog, where k = 1.96 at the

0.05 level.

b. If Rg - Lc, Lt is calculated using

the one-sided confidence interval

and reported as less than Rg + kog,

where k = 1.65 at the 0.05 level.

ACKNOWLEDGMENT

This paper is designed to he primarily

instructional rather than creative. The

author has freely used material from the

paper by J. K. Hartwell listed below.

REPORTING PRACTICES

The Detection Limit (L(j) and the Deter- mination Limit (Lq) are not used for routine counting and reporting. In those cases where you are required to specify

your minimum detectable activity (e.g. to a regulatory agency) , it is recommended that

the Detection Limit be given then as a practical reporting limit. The Determination

Limit (Lq) is useful when a "sensitivity"

with a specified relative standard deviation is required.

For routine low-level counting, only

the Critical Level (Lq) and the Less-Than

Level (L^-) are of interest. Their use is as follows:

SOITRCES

J. K. Hartwell, "Detection Limits for

Radioisotopic Counting Techniques".

ARE-2537. June 22, 1972.

L. A. Currie, "Limits for Oualitative De- tection and Ouantitative Determination". Analytioal Chemistry, Volume

40, No. 3, 586-693. 1968.'

B. AltshuDer and B. Pasternak, "Statistical

Measurements of the Lower Limit of Detection of a Radioactivity Counter".

Health Physics, Volume 9, 293-298.

1Q63.

172
NBS SP456 (1976)

USE OF NBS MIXED-RADIONUCLIDE GAMMA-RAY STANDARDS FOR CALIBRATION OF Ge(Li) DETECTORS USED IN THE ASSAY OF ENVIRONMENTAL RADIOACTIVITY

B. M. Coursey

National Bureau of Standards

Washington, D. C. 20234

The NBS mixed-radionucl ids gamma-ray emission-rate standards were first

distributed in 1972 for use in calibrating Ge(Li) detectors used for

measuring environmental radioactivity (1). These sources allow the user to prepare full-energy-peak efficiency vs_ yray energy curves for

the detector from measurements made with a single standard. This paper

will discuss the development of these standards and examine their current use in the assay of radioactivity in liquid, gaseous, and soil samples.

(Radioactivity, standards, environment, gamma-ray measurements, Ge(Li)

detectors)

Design of Mixed Gamma-Ray Standards

The five mixed-y-ray Standard Reference

Materials (SRM's) currently available are listed in Table lA. A total of about 300

such sources are prepared annually from a single master solution containing the nine

components shovm in Table IB.

1 . Spectroscopic Properties

The y-ray spectrum of a typical source, obtained with a Ge(Li) detector-multichannel

analyser system, is shown in Figure 1. A

typical "efficiency curve, " full-energy-peak

efficiency vs_ Y"ray energy, for a Ge(Li)

detector is shown in Figure 2. The eleven

principal Y"i^sy lines in the present mixture

were chosen to allow one to determine the

entire efficiency curve over the energy

region from 88 keV to 1836 keV.

Others have suggested the use of single

radionuclides which emit y rays of many

energies for detector calibration (2,3).

Standards of a single radionuclide, such as IS? ^-'^Eu, are clearly easier to use than mixed

standards because only one half-life correction need be made. There are two advantages,

however, in using mixed standard sources.

First, one can choose the spacing of the

y-ray energies and, second, uncertainties in

y-ray emission rates are generally less for

those radionuclides with simple decay

schemes. Some precautions which should be

taken in the design and use of a mixed

standard are given here.

The first consideration in preparation

of the standard is that accurate y-rayemission rate standards of the individual

components be available. The nominal uncertainties in the emission rates for the

eleven principal y rays in the present

standards are given in Table IB. Research

is continuing at NBS with 4Trp-y and x-y

coincidence counting techniques to improve

the accuracy of these standards. Evaluations of nuclear-decay scheme parameters,

such as half lives and y rays per decay, are continually reviewed to insure that the

most recent data are provided on the certificates. In most instances, the decayscheme parameters are those suggested by

the Oak Ridge Nuclear Data Project (4).

Decay schemes for components used in

a mixed standard should be relatively simple

to minimize interference from secondary

events in the measurement of full-energy

peaks. Single- and double-escape peaks

from high- energy y rays, Compton edges, and

weak y-ray transitions may present problems

in the region of the spectrum where other

calibration peaks are present. Three

potential problems of this nature are associated with the present mixed standards

when used with detectors of poor resolution.

The single-escape peak from the 1836 keV

transition occurs at 1325 keV, and may only

be partially resolved from the Co 1333-keV

peak (see Figure 1). The 511-keV annihilation peak from pair production may interfere

with the measurement of the ^^Sr 514-keV

peak. And finally, the Tl x rays from the

703 decay of Hg may not be resolved from the

^•^^Cd 88-keV peak. The extent of these

problems will depend on the resolution of

the detector.

Whether a single radionuclide standard

or a mixture is used for detector calibration, the problem of summing must be considered when the radionuclide measured emits

coincident radiations (5). Summing can obviously occur in measurements of both the

173
standard and the unknown and can only be

reduced by using absorbers or counting at greater distances. Summing problems may be

encountered in the present mixed standards,

for examplgx^ in the full -energy-peak measurements for ""^Co, '^^Ce, and ^^Y. Even vjhen

summing is not severe for the components of

the standard it may be a problem for radionuclides in the sample such as '^'-I and

•'^^Eu. Corrections require a knowledge of

the decay scheme as well as full-energypeak efficiencies and total efficiencies for

monoenergetic y rays.

2 . Physical Properties • \

The point sources, SRM 4215 and SRM

4216, are prepared by depositing an accu- rately-weighed aliquot, usually between 20

and 50 milligrams, of the calibrated radionuclide mixture onto polyester tape, drying

the source, and covering it with another

layer of the same tape. The tape, which is 0.006-cm thick, is supported by a 5.2-cm

diameter aluminum annulus. The source is exposed to hydrogen sulfide during the

drying period to precipitate the mercuric

sulfide and prevent escape of ^'^^Hg by

volatilization.

The solutions are distributed in breakseal glass ampoules or in "standard geometry"

borosilicate-glass bottles fabricated at the

NBS glass shop. Dimensions of the standard

bottles are given in Table 2. It was found

necessary to introduce standard bottles

because the non-uniformity of commercially

available bottles led to serious errors in

the measurement process (1). Snpty containers like the ones used for these standards

are available from NBS.

A solution source, such as SRM 4242,

is prepared by depositing a weighed aliquot

of the master solution into the bottle

containing a weighed quantity of approximately 4 N hydrochloric acid (which has a density of 1.06 ± 0.01 g/cm^). The carrier

content of the master solution is adjusted

so that the ratio of stable carrier atoms to radioactive atoms is about 10^ for each of

the parent radionuclides. Under these

conditions we have found no evidence of

deterioration of the solution, such as precipitation or "plate-out, " over long

periods of time.

The most recent of the mixed standard

sources, SRM 4254, contains approximately

5 ml of a 4 N hydrochloric acid solution in

a break-seal glass ampoule. The solution

also contains approximately 20 ppm cation

carrier for each of the parent radionuclides

in the mixture. It is intended to be opened

by the user to prepare secondary standards

as described in the following section.

Use of Mixed Gamma-Ray Standards in Environmental Laboratories

Table 3 lists the number of mixed Y"ray

SRM's distributed between February 1972 and

February 1976 and gives some indication of

the kinds of users of these materials.

Illustrations of how the sources are being

used in the assay of liquid, gaseous, and

soil samples are given in the following

sections

1 . Detector Calibration for Liquid Samples

SRM 4242 (450 ml) and SRM 4243 (50 ml)

are specifically intended to be used as sealed counting standards. As mentioned

previously empty standard bottles are available from NBS which can be used to count the

environmental liquid sample. Although few

laboratories have adopted these standard

geometry containers for routine use, the

bottles do play an important role in intercomparative measurements between licensees,

environmental consultants, and regulatory

agenci es

It is often necessary to calibrate a detector for a small container, such as a 20-ml liquid-scintillation vial, or a large

volume container such as a 1-liter bottle.

SRM 4254 has a higher radioactive concen- tration than the other two solution sources

and may be quantitatively transfered or diluted to prepare such secondary calibration sources. Dilutions should be made with

carrier solutions which are greater than

1 N hydrochloric acid and contain about

20 ppm of stable carrier for each of the

parent radionuclides.

2 . Detector Calibration for Gaseous and

Airborne Particulate Samples

Mixtures of radioactive noble gases,

such as effluents from nuclear power

stations, are often measured in spherical

glass ampoules. Radioactivity SRM's of

several individual noble gases, namely

^^Kr, 127, 131m, 133^^^ ^^^^ ^^^^ distributed by NBS in similar standard ampoules

(see dimensions in Table 2). Recent

measurements at NBS have shown (6) that

efficiency curves applicable

to mixtures of gases in such glass spheres

may be obtained from measurements made with

a mixed yray point source, SRM 4215, at the center of the sphere. The uncertainty

174
in this method is generally less than 2% for

Ge(Li) detectors with volumes less than

60 cm-^ at source-to-detector distances

greater than 10 cm. The corrections necessary at closer distances are discussed in

the reference.

Gamna-ray measurements are often used

to assay radioactivity entrained on particulate air filters. Palms et al (7) have

prepared secondary standards for use in

detector calibration for such measurements.

They prepared disk-shaped sources of

differing thickness and diameter by introducing aliquots of an NBS solution standard

into a "homogeneous polyester glass resin."

The use of such sources appears justified

providing consideration is given any

difference in absorption or activity

distribution between the filter and standard.

3 . Detector Calibration for Soil and

Sediment Samples

The current solution SRM's are not intended for use in soil measurements. The

activity incorporated in such standards must

be homogeneously distributed and the chemical

composition and density of the standards

should be similar to those expected for the

unknown samples. Laichter et al (8), for

example, have prepared solid y-ray emissionrate standards from evaporated sea water and

tabulated attenuation coefficients to be

used with different tjT^es of materials. In more recent work, Noyce et al (9) have

described the preparation of the NBS river

sediment standard, SRM 4350, which is primarily intended for use in radiochemical

studies. SRM 4350 may be used for Ge(Li)

detector calibration, but the low activities

of the certified yray- emitting radionuclides

limit its use for this purpose. Soil yray

emission-rate SRM's similar to the liquid

standards described in this paper are under

consideration at NBS.

References

1. Cavallo, L. M. , Coursey, B. M.

Hutchinson, J. M. R. , Garfinkel, S. B., and Mann, W. B. , Nucl. Inst, and

Methods, 112, 5 (1973).

2. Seyfarth, H. , Hassan, A, M. , Hrastnik,

B., Gottel, P., Delang, W. , Nucl. Inst,

and Methods, 105 , 301 (1972).

3. Debertin, K. , Schotzig, U. , Weiss, H.M. PTB-Mitteilungen, 85, 187 (1975),

4. Current nuclear-decay scheme parameters

are obtained from Dr. Murray Martin,

Oak Ridge Nuclear Data Project, Oak

Ridge National Laboratory.

5. WcCallum, G. J., Coate, G. E. , Nucl.

Inst, and Methods, 130, 189 (1975).

6. Coursey, B. M., Hutchinson, J. M. R. , Unterweger, M. P., submitted for

publication.

7. Palms, J. M. , Tanner, B. K. , Wood, R. E. Boone, F. W. , IEEE Trans. Nucl. Sci.,

NS-21 , 585 (1974).

8. Laichter, Y. , Notea, A., Shafrir, N. H. Nucl. Inst, and Methods, 1_13, 61 (1973)

9. Noyce, J. R. , Hutchinson, J. M. R.

Mann, W. B. , Mullen, P. A., IEEE Trans.

Nucl. Sci. (1976).

Acknowl edgement

A proper acknowledgement for a paper on

the mixed yray standards should include a list of all 17 members of the radioactivity

section, as a group effort is involved in the

preparation of these standards. Special

acknowledgement must be given, however, to the three individuals most responsible for

the design and development of these standards: Miss L. M. Cavallo, and Drs. J. M. R. Hutchinson and W. B. Mann.

175
TABLE lA - - NBS Mixed Radionuclide Gamma-Ray

Emission-Rate Standards

Approximate Total y-YiajDescription Emission Rate on Date of Issue jy/s)

SRM i+215 Point Source 8 X lo'^ SRM ^216 Point Source k X 10^

SRM h2k2 i+50-ml Liquid Source 1 X 105

SRM k2h3 50-ml Liquid Source 2 X io5

SRM U25^ 5-ml Liquid Source 1 X 105

TABLE IB -- Calibration Lines for the 9 Components in

the Mixed y-Ray SRM's and Nominal Uncertainties

in the y-Ray Emission-Rate Values Reported

a ,b

Parent Radionuclide Y-Ray Energy (keV) Total Uncertainty {%)

88,. 0 3,. 0

5^Co 122 . 1 2 . 3

165..9 2 ,.7 203„

Hg 279,. 2 1 .. 2

391 .. 7 2 ,.9

51^., 0 1 ..7 137 ^ ' Cs 661,.6 2 ,, 1 6°Co 1173., 2 1 ., h

^°Co 1332 ,. 5 1

k 88, 898., 0 3. 0 88, 1836. 1 2 . 3

Includes uncertainty quoted by compilers of decay-scheme

data where appropriate

b Values shown here may differ slightly from one year's

sources to the next

176
TABLE 2 -- Standard Geometry Counting Containers for

ITBS Environmental Radioactivity Standards.

Dimensions are given in centimeters.

CYLINDER CYLINDER SPHERE

Volume ^50 ml 50 ml 31+ cm-^

Intended Use For Waste Water Primary Coolant Notle Gases

1+21+2 I42I+3 ^307^

U308

0 . E . T . 50±0

3 . 80±0 . 02 h . 22±0 . 05

Bottom 0.5 0.3

0 - 275 0 . 2 0.1

Height (Cyl.) 13.0 6.5

Internal Radius

( Sphere )

2 . 0

SRM U307 and 1+308 are gaseous y-raj emission-rate

standards (see text and reference 6).

TABLE 3 — Distrihution of Z^TBS Mixed Radionuclide

Gamma-Ray Emission-Rate Standards

From February 1972 - 1976.

USER CATEGORY 1972 1973 197i+ 1975 1972 - 1976

Nuclear Power Stations 29 1+3 51 67 190

National Laboratories 26 h3 60 183

Other 63 108 100 105 376

Total 118 19I+ 205 232 7^9

177
178
100 200 500 1000 2000

GAMMA-RAY ENERGY (keV)

FIGURE 2 -- Typical efficiency curve obtained with a mixed y-ray point-source standard

(SRM ii215) at 25 cm from a 32-cm3 Ge(Li)

dete ctor

179

MBS SP456 (1976) THE SAFETY AND RELIABILITY OF DECENTRALIZED TLD Carl-0 Widell

AB Atomenergi

Studsvik, Sweden

Decentralized reading of personnel TLD is in Sweden performed at the different nuclear power stations. The TLD results are automatically processed in the central dose register at Studsvik. In order to check and

correct the readers, reference dosed dosimeters are used in each batch

of TLD to be read. The reference dosed results are processed by the com- puter and will show the status of the different TLD readers. The reference dose irradiation is done with a Sr-90 source, which is placed above

a rotating table in a brass shielding. The dosimeters are placed on this

rotating table and will get a dose equivalent to 300 mrem gamma. In addition to this procedure a serie of calibration dosimeters is irradiated at

a central calibration facility each month and read in the local TLD-reader.

Introduction

All nuclear energy installations in

Sweden are performing their o\m personnel

and environmental 3~Y dosimetry themselves.

This is done by an automatic TLD-reader. The

reader is coupled by the punched tape to a central dose register at Studsvik, figure 1. The terminals are connected to the computer

via telephone lines.

In order to check the' calibration of

the TLD-system irradiated dosimeters are

DO^E. R.EjCoRl>S

Figure 1. The Swedish Dose Register System

181

always used and evaluated together with the

other dosimeters in the reader stack. In that manner the computer will get an information about the status of the TLD-reader

and will automatically correct for deviations in the calibration.

TLD-system

The TLD-system consists of a dosimeter,

figure 2, with four pellets on a slide.

This slide is introduced into the reader

and the pellets lifted up into the reading
chamber. Hot nitrogen is flushed on to the

pellets and the light read by a peltier

cooled photomultiplier . The TLD-reader is

shown in figure 3.

If the foreigner in question is not found on that list the computer proceeds

and gives the man a three digit number and

a check letter.

The dosimeter slide is identified by a hole coded number. This number is unique in

this dosimetry system in Sweden. The dosimeter number is in the computer linked to the social security number of the wearer.

Identification of persons

In order to put the doses on the right

person all doses are stored on the persons'

social security numbers. These numbers con- sists in Sweden of ten digits, a six digits

birth date, a three digits birth number and

a check figure.

35 07 Year

28 351 7Check

Month'^^"^ Number

Swedish social security number

35 07

Year

28 - 703 JCheck

Month'^^'^ Number

Foreigners number

The personal identification numbers have

always a check function which allows the

computer to check the correctness of the

numb e r

.

The calibration control of the TLD-system

The results from the TLD appear on a punched tape. This punched tape is given

to the computer via a local terminal at the

nuclear power plant. The computer evaluates

the doses from the TLD results. It is of course necessary to feed the computer with

up-to-date calibration constants.

Swedes are by this system very well identified.

Foreigners are handled in another way. For them the computer is given their birth

date and the letter U. The computer is then

listing all foreigners in the system with

this birth date.

This is done by using special calibration or reference dosimeters which are always used when personnel dosimeters are

read.

The calibration dosimeters are irradiated at the calibration lab at Studsvik.

They are given doses from 20 to 10 000 mrem.

0 0 j 5 ijj

Figure 2. TL Dosimeter

182
Dose mrem m Gy

0 0 0

1 20 0 2 2 50 0 5 3 100 1 4 200 2

5 500 5

6 1 000 10

7 2 000 20

8 5 000 50

9 10 000 100

This serie is used at each installation once

a month.

Together with reading of other batches

of dosimeters reference dosimeters are used.

These dosimeters are irradiated in a refer- ence dose irradiator with a Sr-90 source, figure 4.

Dosimeter

number

10

11

Dose mrem m Gy

annealed dosimeter

300 3

In the reference dose irradiator the dosimeter slide with the pellets is placed on

a rotating table. The table is rotated with

a constant speed by a synchronous motor and

the TLD pellets moved in under a Sr-90 source with a large surface.
The TLD result is compared with the

nominal value of the calibration or reference dosimeter which gives a calibration

factor. This calibration factor is compared

with that in the computer stored preceding

calibration factor. The mean of the two ca- libration factors is used and stored back

for further use in the computer.

If the new calibration factor deviates more than ± 5 % from the old one a signal

is given on the terminal. This signal has

to be cleared before the evaluation can proceed

.

In order to follow up the history of

the calibration and reference dosimeters and

the history of the calibration factors a record of the different monthly

values is produced by the computer.

Conclus ion

In this way a very good control of the

standard of the whole country wide personnel dosimetry system can be obtained.

It is very convenient to use the refer- ence dose irradiator for the day to day

routine handling of the dosimeters. This

reference dose irradiator can be used on

the same table where the dosimeters are handled.

The use of social security numbers is

of great advantage as the system can be

used over the whole country without any

identification problems.

Figure 4. Reference Dose Irradiator

184
Due to this central dose register and n- decentralized TLD-readings personnel can rn.i t u °TM ^° ^ "^^^^ 8°°^

fro. the dose point of view'.ove around IZL u, r " °' radiation. We Sweden without any restrictions or paper nflt ? -^T international system

controls. °^ ^^^^ kind thus making it easier to handle dose records between countries.

185

NB£ SP456 (1976)

ERROR ANALYSIS OF ENVIRONMENTAL RADIATION MEASUREMENTS

MADE WITH INTEGRATING DETECTORS

Gail de Planque Burke

Health and Safety Laboratory

U. S. Energy Research and Development Administration

New York, New York 10014

Thomas F. Gesell

The University of Texas School of Public Health

Houston, Texas 77025

Proper use of integrating detectors (e.g., TLDs) for environmental monitoring, particularly around nuclear facilities, requires a high degree

of measurement accuracy and a meticulous assessment of measurement errors. Mathematical expressions for the dose in air at a site are formulated in terms of factors affecting the measurement including detector efficiency

and calibration techniques as well as characteristics inherent to the

detector such as energy dependence and contributions to the total

during storage and transport of the detector. The assessment of errors

for individual terms in the equations is discussed and errors are combined to determine the accuracy of the estimated dose.

(Error; Environmental Radiation Measurements; Integrating Detectors; TLD)

Introduction

Integrating solid state detectors, for

example, thermoluminescence dosimeters j are now used extensively for environmental radiation monitoring, particularly in the

vicinity of nuclear facilities. Measurements made with a network of integrating

detectors are often used either alone or in

conjunction with other measurements to assure compliance with regulations such as Appendix I of 10 CFR 50" or the proposed

Environmental Protection Agency standard

concerning environmental radiation protection for nuclear power operations. ^ This

use of integrating detectors requires not

only a high degree of measurement precision

and accuracy but also an assessment of

measurement errors.

A recent intercomparison" of measurements made with integrating detectors,

primarily TLDs, demonstrated that reasonably

accurate environmental radiation measurements are feasible. However, it is apparent

that the errors quoted by many of the

participants frequently underestimated the

actual errors. The study involved the

inter comparison of a set of dosimeters given

a known dose in the laboratory and another

set exposed for three months in the environment. In the latter case, the reported

results differed from the mean of all the

results by more than the participants'

stated errors in 37 cases out of 54. Somewhat better results were obtained for the

laboratory-exposed dosimeters, where 22 of

54 cases differed from the mean by more than

the stated error. These results indicate

the need for a method of error assessment.

An ANSI Standard* is available which

designates performance, testing and procedural specifications for thermoluminescence

dosimetry used for environmental measurements

While the standard specifies acceptable

error limits and provides tests for determining individual errors, it does not recommend a method for the assessment of the

total error.

This paper contains mathematical ex- pressions for environmental radiation measure- ments and a method for assessing the total

errors. Our simple treatment provides a practical solution in a situation where quite

often the practice is to report either no error or an error reflecting only measurement

precision. The treatment presented uses thermoluminescence dosimetry as a particular

187
case, however, the methods when suitably

adjusted should be appropriate for most integrating detectors.

Field Measurements

To obtain a measurement of radiation

dose in air at a particular location in the

environment for a designated period, the

ideal integrating detector would be "turned

on", marking the beginning of the measurement

interval , only after arrival at the specified location and "turned off", marking the

end of the measurement interval, before

removal from the location. In this ideal case

where

XpQ = the TL response of the field detector

in units of exposure,

Xj = the exposure rate resulting from

self -irradiation

,

Dp = the average dose rate in air at the

field location during the time the

detector was operating at that

location,

tp = the length of time the detector was operating at the field location,

fp = the fraction of the true exposure

producing TL response which may

differ from unity due to timetemperature-dependence (commonly

occurring in the form of "fading"),

and

1/a = the factor required to convert

exposure to dose in air.

Quantities expressed in units of exposure,

i.e., X, or Xt , indicate that the TL response

of the detector obtained when measuring these

quantities is equivalent to the TL response

to a given exposure during calibration. Thus

the conversion from TL response, expressed as exposure, to absorbed dose in air requires

that the field energy spectrum and angle of

incidence relative to the calibration energy

spectrum and angle of incidence as well as the conversion factor from roentgens to rads,

assuming electron equilibrium, be taken into account. In this case

a = ed/0.869 (2)

where

1 R = 0.869 rads, assuming electron equilibrium,^

e = the fraction of the true dose

producing TL response which may

differ from unity due to the energy

dependence of the detector response,

and

d = the fraction of the true dose producing TL response which may differ from

unity due to the directional depend- ence of the detector response.

The time- temperature-dependence term results from the instability of many TL

phosphors. Most often the detector undergoes a loss of stored signal with time, the rate of loss being dependent on temperature.

Although it does not seem to completely

describe the phenomenon,*^ the RandallWilkins model^ can be used to determine

corrections for time- temperature-dependence

Corrections are applied in two situations.

When the exposure rate is constant during

the measurement interval, the fraction of

the actual or true exposure that results in

TL response is

f = (l-e"^^)/xt

where

X = B exp (-E/kT),

B = the frequency factor (related to the

presence of lattice vibrations in the

phosphor )

,

E = the trap depth,

k = the Boltzmann constant, and

t = the length of the measurement interval

When exposure has occurred before the beginning of the measurement interval and no further exposure occurs during the interval,

the fraction of the true exposure resulting

in TL response is

f'= e-^^

Other methods may, of course, be used in

determining f and f', but these factors will

still affect the measured response of the

detector in a similar fashion.

These terms represent the primary

factors known to affect TLBs. If a different type of integrating detector is used,

some factors may be eliminated, e.g. , selfirradiation, while others may have to be

added. However, the equation should be of an analogous form.

Equation (1) can be solved for Dp or Dptp to determine the average dose rate or the total dose at the field location during

the time the detector was operating at that

location. Unfortunately, this ideal case is impossible practically for TLD unless the

detector can be prepared (annealed) and

analyzed (read out) right at the field

location. In practice, the detector is

prepared in a laboratory, perhaps stored

briefly, transported to the field location,

188
left there for some tiir.e, transported "back to the lahoratory, and perhaps stored again

before readout. Thus the entire measurement

cycle may be composed of several intervals,

of which the field interval is only one, and

the total exposure obtained with the field

detector includes exposures other than the

field dose. Control detectors are commonly

used to estimate the exposures received by

the field detector during storage or transit

or both. V/e now consider a few of the more common situations involving both field and

control detectors for isolating the field

dose

The simplest practical case is that

where the field detector is prepared as close to the field location as possible with essentially no storage or transit prior to exposure in the field. At the conclusion of the

field interval, a control detector is

prepared and transported with the field

detector to the laboratory and both are

i"x.ediately read out. In this case the

exposure for the field detector is given by

Xpc = (Xi + aDp)tpfFf (3)

(Xj + CXD^g )"t'];'2%s

where

Djg - "the average dose rate during return

transit (the subscript T2 indicating post-field transit),

trj^2 - "the length of time of the postfield transit interval,

f^s = the fraction of the true transit

exposure producing TL response

which may differ from unity due to time- temperature -dependence,

f ,j,2 = the fraction of the exposure

received up to the beginning of the

post-field transit interval producing TL response which may differ

from imity due to time- temperaturedependence .

The exposure for the control detector is

given by

XcD = (Xj + 0!D^3)tj3fj3.

Subtracting equation (4) from equation (3^

yields

XfD - = (Xi + aDp)tFfFf (5)

assuming that self -irradiation is identical

for both the field and the control detectors

(if it is not, equation (5) will be more

complex) and assuming that the a appropriate

for the transit situation is the same as the a used for the field interval. The error

involved in assuming the latter will be

discussed below. Equation (5) can be solved

for Dp or Dptp to determine the dose or average dose rate at the field location

during the field interval. These quantities

can thus be determined from two measurements

and estimates of Xj, tp, fp and f 'xsj the

latter being the fraction of the true field

dose producing a TL response which may differ

from unity due to fading during transit.

If the return transit is followed by a storage interval prior to readout, the field

detector then yields

XpD = (Xi + aDp)tpfpf 'xsf S2 (6)

+ (Xj + aDr|n2)tr[.gfrp2f 33

+ (Xi + Xs2)ts2fS2

where the subscript S2 Indicates storage

after the field interval and Xgg is the

exposure rate in the storage area determined

with the type of detector used in the field

and as controls. (IF X3 is determined by

other means, a factor analogous to a may be

required so that this quantity is expressed

in the proper units). In this case, the

exposures received during the field and

transit intervals are subject to possible

fading during storage as indicated by f 'r^g in the first and second terms. The control

detector yields

*^Xj + 0;Dr['2)tr[;.2frpaf S3 (

)

- (Xi + Xs2)ts2fs2.

The difference, given by

XpD - = + aDp)tpfpf 'T2f S2 (8)

is identical to equation (5) except for f gs- A more complicated situation exists

when the field and control detectors undergo

storage and transit prior to the field interval as well as after the field interval. In this case

XpD = (Xj + XsJtsifSif Tif pf Tsf S2

+ (Xj + OiDTi)tTifTif Ff Tsf S2

+ (Xi + aDp)tpfpf 'Tsf S2

+ (Xj + a%2)tx3fT2f S2

+ (Xi + Xs2)ts2fS2 (9)

where the subscripts SI and Tl apply to prefield storage and transit, respectively. The

189
control detector, prepared with the field

detector and accompanying the latter at all

times except during the field interval

yields

^CD " ^ Si^%i%i^ T^^ S^^ T^-^ S2

* * + (Xj + aDr[^;L

\jtipgfrpi f III ggf rjigf gg

+ (Xj + Xg3)tg3fg3f ^gf g2

+ (Xi + aDT2)txsfTsf Ss

viThere

+ (Xj + Xg2)tg2fss (10)

where the subscript S3 indicates storage

during the field interval. This storage

should "be sufficiently near the field location so that transit time between the

storage area and the field location is

negligible and tSs = tp. Subtracting equation (10) from equation (Q) and simplifying

yields

%D - ^CD =

(^I ^ Xsi)%ifSif Tif Tsf Ss(f F

- f Ss)

+ (Xi + aDrJtTafxif Tsf S2(f F

" f'ss)

+ (Xi + aDF)tFfFf T2f S3

- (Xj + Xs3)tFfS3f Tsf S2 • (11)

As in the earlier cases the terms pertaining

to storage and transit after the field

interval cancel. However, the terms pertaining to storage and transit prior to the

field interval remain since the field

detector and the control detector are locatHl

at different places during the field interval. Hence the fading undergone by the two detectors on the exposures received prior to

and during the field interval may be different. The exposure received by the field

detector is subject to fading in the field

while that received by the control detector

is subject to fading in the field storage

area. In all other respects, the control

accurately represents the extraneous

exposures received by the field detector.

Other combinations of field and control

detectors are possible and can be described

by analogous equations. In general, the

exposure received by a detector is given by

X = ^ [(Xj + aiDi)ti . i-1

f . (f • f

^i*-^ i+i ^ i^

f n+i = 1

= the TL response of the detector in

units of exposure, and

= a single interval during the entire

measurement cycle, e.g., the time

spent in storage prior to field

exposure

When field and control measurements are combined, all terms will cancel except those

having factors resulting from the detectors

being in different locations during corre- sponding intervals.

Error Assessment

The field dose can be estimated from

equations (1), (5), (8), (11) or similar

equations depending on the manner in v/hich

the field and control detectors are used

during the entire measurement cycle. For

example, in the case represented by equation

(5) the field dose rate is given by

^FD -^CD ^I

Dp = ; (13)

where Xpp and Xq^ are TLD measurements and

Xj, a, tp, fp and f 'rps measured or estimated separately. The equation is of

the form V = f(xi, Xg, .... x^) • If each

factor in this equation is independent so that the possibility of error compensation

exists, the error for V is®

(I;) <^x.)=] (14)

where S, the standard deviation for Xj_.

Assuming that each factor in equation

(13) is independent, the error for the

field dose rate in this case is given by

SA

F'

df

(15)

190
Analogous equations for detemining the

total error can be derived for equations (1), (8), and (11). To compute the value of Sf)-p, the errors for each individual factor must

be obtained. A thorough treatment of the

methods available for determ.ining each

factor and its respective error is beyond

the scope of this paper. However, some basic concepts should be considered when

assessing the individual errors.

Error Associated with a

V/hile a functions as an independent term

in equation (13) its components e and d given in equation (2) may not be strictly

independent. The energy dependence term, e, arises because the response of the detector

to environmental radiation (including cosmic

rays) representing a broad energy spectrum

is usually not identical to the response to

the calibration source, generally ^ Ra,

-^''Cs, or ®°Co.^ Although the detector

response as a function of energy can be

determined by experim.ent or theory or both,

a correction factor should take into account

the detector response weighted by the particular environmental energy spectrum. Hence

the error for this factor must reflect the

accuracy with which both the detector

response and the environmental spectrum are

known.

The directional dependence term, d, is

required if the detector response is not

spherically symmetric. In this case, the

response of the detector will be different

when the geometry of the incident flux in

the environment differs from that encountered

during calibration. While directional

dependence can be estimated, the application

of a correction for directional dependence

requires that some assumption be made about

the direction of the incident flux. Again,

the error in d must reflect the accuracy

with which both the directional dependence

and the direction of the incident flux in

the environment are knovm.

Since d is actually a function of energy,

e and d are not strictly independent. However, in the interest of simplicity, they

will be ass\imed to be independent in order

to avoid calculating covariance terms for

assessing the total errer. The effect on

the total error of assuming the independence of e and d should be insignificant

relative to the problems involved in

accurately assessing the individual errors

for both e and d.

Error Associated with f and f

'

As indicated earlier, corrections for

time- temperature -dependence may be necessary. Appropriate corrections can be based

on theory, for example, the Randall -Wilkins

model, or determined empirically by means of

tests simulating actual storage, transit and

field conditions. Even if the amo\mt of

fading or the fading rate as a function of

temperature is known, corrections can be

made only if certain assumptions are made

about the actual exposure conditions. When

fading occurs during interval i following

the irradiation interval, the proportion of

the signal retained at the end of interval i is given by f \, which should be determined

for the real temperature profile ( ideally

the temperature of the phosphor) or some approximation thereof. When fading and

irradiation occur simultaneously, the proportional signal retained is given by f, which should also be determined for the real

temperature profile. In the latter case, some assumption must be made about the

irradiation rate; usually it is assumed to

be constant.

Methods for minimizing the effects of

time- temperature -dependence of TLD's and

for determining these effects directly are discussed in the ANSI Standard.* The errors for f and f

' are difficult to assess exactly and must include not only the errors

for the amount of fading or the fading rate

as a function of temperature, but the errors involved in the assumptions made regarding

the actual exposure conditions

As can be seen from equation (11), the

fading factors prevent the cancellation of

otherwise similar terms for the field and

control detectors when both detectors are exposed prior to the field interval. If the errors for these factors are large, it is

probably best to avoid this particular use

of field and control detectors for isolating

the field dose.

Error Associated with t

The error associated with t, which in

equation (13) is the length of time of the

field inteiva], results from the difficulty

in precisely defining the beginning and end

of the field interval. In most practical

situations there is some small but finite

amount of time during which the field

detector and the control detector are separated, but during which either the field

detector is not in the field or the control

detector is not in storage (S3). During

this time the field and control detectors

191
may not be affected identically. Since

determining this difference is often impractical, one way of approximating the possible error involved is by regarding the error as one of timing. However, when assigning this error, it is really the error in assuming

that the two detectors were affected in an identical manner during this brief interval

of separation that must be considered.

Error Associated with Xj

Self-irradiation occurs in TL detectors

when the phosphor itself or its packaging

is radioactive. For example, self -irradiation occurs with most glass -encapsulated

phosphors because of in the glass.

Usually Xi is determined empirically.*

In the equations presented, it was

assumed that Xj is identical for the field

and control detectors. This assumption

permits the cancellation of terms and hence

considerably simplifies the equations

defining Dp. This assumption can be made as long as the two values for Xj are reasonably

similar, if this is true, the error for Xj

should reflect the spread in the values of

Xi for the detectors in question. If the

values differ substantially, then the

assumption becomes unreasonable, the equations must be expressed in the appropriate

more complex form and the errors for Xj in

each case should reflect the accuracy of the

estimated exposure rate due to self-irradiation.

Error Associated with Xg

When storage is necessary prior to or

during the field interval, one or more terms of the form Xs enter the equation(s) for the

field dose, for example, equation (11). In this case, even if f 'p is identical to f 33

and the term including Xsi becomes zero, the errors involved must still be included

in the total error. Hence Xg^ and its error must be determined. Xga cannot be

eliminated f2X>m equation (11) and must also

be determined.

If Xs itself is measured with the

detectors used in the field and as controls,

then the error for this term is simply the

measurement error. If it is determined in

another manner, then corrections may be

necessary to insure that the value truly

represents the TL response in units of

exposure of the field and control detectors

measuring Xs. For instance, if. a different

detector is used to determine Xg, it is

essential to consider the possible difference in the response of the two types of

detectors to the radiation field in the

storage area.

Error Associated with Drp

When transit takes place prior to the

field interval, the term % appears in the

equations for the field dose. Again using

equation (11) as an example, the term

containing D^j will become zero if f ^'p = f g3 . However, as was the case for Xs, the errors must still be included in the total

error.

Unfortunately, Dr|^j^ is usually unknown.

If it were knom, then there would be little

need for using the control detector for

determining this portion of the extraneous

exposure . Not only is it unknown, but it is

extremely difficult to estimate with any

accuracy when the transit conditions are uncertain as would be the case if the

detectors are transported by airplane . The

results of the intercomparison^ mentioned

earlier revealed that variations in the

magnitude of the transit doses could not be

explained by differences in transit distance.

Further, attempts to estimate these transit

doses taking into account typical airplane

speeds and altitudes, appropriate cosm.ic

radiation data, and reasonable estimates of

exposure during ground level transit yielded

poor results

Thus when transit prior to the field

interval is unavoidable and the transit

conditions are highly uncertain, large errors may be introduced and this technique for

isolating field dose may be unsatisfactory.

Errors Associated with Xpp and Xq^

and Xqj) are the quantities determined from the field and control detectors,

respectively. The errors for Xp£) and Xq£) are measurement errors and may be derived

differently depending on the calibration

method. Calibration techniques are discussed

in the ANSI Standard* and fall into two general categories, batch calibration and

individual calibration. Whichever method is

used, the conversion from TL response to exposure is described by

- - \J^y -CAL

where N is the TL response of field or control detectors and Nqj^l '^^^ TL response

of either the same detectors (individual

calibration) or another detector(s) from the same batch (batch calibration) given a known

exposure Xqal f^^om the calibration source.

192
If the detectors are cali"brated individually, the measurement evroT represents

the precision v/ith which the detector

produces identical TL responses givenidentical exposures. Since the calibration

exposures can never be reproduced exactly

due to errors in position and timing, tests

to determine the reproducibility of a single detector will include these errors

If batch calibration is emiployed, the

measurement error represents the precision

with which the detectors in the batch

produce identical TL responses given

identical exposures. Again, since the calibration exposures can never be identical,

due to errors in position or timing or both,

the uniformity of response for a batch of

detectors will include these errors. (Although when determining the measurement

evroT for the batch calibration method,

tim.ing errors can be eliminated if all of

the detectors to be tested are exposed

simultaneously, it is probably more convenient to include this source of error in the

measurement error for reasons which will be

explained later.)

Tests for detenrdning the measurement

errors should be designed to incorporate the errors encountered in actual measurement

situations. These include variations in

detector preparation (cleaning, annealing,

etc . ) and readout ( reproducibility of the

reader heating cycle, stability of the

reader electronics, etc.) as well as human

errors (differences in canying out all of

the procedures involved ) . Variations due to physical differences am.ong detectors or

statistical variations in the physical

processes (trapping efficiency, light emission, etc.) either among detectors or within

a single detector as a function of time will

be included automatically during testing.

If both timing and position errors are included in the measurement error, then the error in X^al represents only the error in

the determination of the exposure rate . from

the calibration source at the calibration

position(s)

Restricting the errors for Xp^ and Y^j) to measurement errors permits the error for

'/CAL "to he considered in isolation. Since

the error of X-cAJ. affects not only Xfd and

but usually Xs and Xj as well, it will

affect all of these terms systematically and

should, therefore, enter into the field dose

equation only once. If done this way, the

term is then independent of all other terms

and the error may be combined with all other

errors in the manner described by equation

^4).

Disregarding Certain Errors

Depending on the usage of the field dose

estimates, it may be possible to ignore

certain errors when assessing the total

error.

For example, if simultaneous measurements at several locations within a small

geographic area are made virith a single

batch of TLDs and the primary quantities of

interest are their relative values, it would

be reasonable to eliminate from the total error the error for Xq^^ since Y^^i affects

all of the measurements systematically. It might also be reasonable to ignore the errors

for a, f and f

' if there is some assurance that the effects of these terms are also

systematic. Also, if measurements made at a single field location at various times are to be examined for time variations, it is again possible to ignore the error for Y^AL

and perhaps reasonable to ignore the error

for o.. For both cases, the errors for the

dose estimates are logically smaller than

they would be if all sources of error were

considered. Several other situations may

permit the elimination of one or more error

terms, but must be evaluated on a case -bycase basis.

If field doses determined from TLD

measurements are to be compared with estimates made with a different detector

calibrated against a different source, then

all errors, including that for XcAL niust be

combined to determine the total error.

Effect of Various Parameters on the Total

Error for the Field Dose

To determine the effect of individual

parameters on the total error, typical values were chosen and then varied for the parameters in equations (5) and (11), and the

total errors calculated. Equation (5)

describes the relatively simple case involving no pre-field transit or storage and only

post-field transit, whereas equation (11)

describes the more complex case involving

both pre- and post-field transit and storage.

Tables I and II contain the values

chosen for nominal cases for equations (5)

and (11), respectively. A percent error, representing one standard deviation, is

assigned for each parameter. Both equations were solved for Dp and the total error for

Dp determined in the manner indicated by

equation (1<4). All errors were considered

in assessing the total error except that for

XcAL which was omitted in order to focus on

the errors inherent to the entire TLD

193
measurement process. An error for y^^i^ can easily be combined with the total error

using equation (14)- For the simple case

(Table I) and the more complex case (Table

II) the errors for Dp, excluding the errors

for XcALj ai'e 6.6 and 7.0 percent, respectively.

Table III illustrates the effects of

changes in individual parameters and their

errors on the total error for Dp. For each

variation considered in Table III, all

values except XpD, X^j) and those listed are the same as the values given in Tables I and

II. XpD and XcD were adjusted appropriately

for each variation so that the estimated

field dose rate, Dp, would be about 7.82

uvad/h as in the nominal case.

The first three variations demonstrate

the effects of changes in either the field

fading or the error for field fading. Unless

the fading is large, as in variation 2, the

results are similar for the simple and

complex cases. As fading itself increases,

the error is greater in the complex case than in the simple one. This occurs mainly

because the fading of the pre-field storage

and transit exposures for the field detector

is different from that for the control

detector.

Variation 4- illustrates the effects of

self-irradiation, assuming self -irradiation

is identical for the field and control

detectors. Variation 5 shows the effect of an increase in the uncertainty of the energy

response. Since e and d appear together in

the equations, the effects of any changes in

d are the same as the effects of identical

changes in e

.

The effects of an increase in the

transit dose rate and an increase in the error of the transit dose rate are illustrated in variations 6 and 7. Although

transit dose rate does not appear in

equation (5), the total error still increases

in the simple case because Xp^ and Xqd are necessarily larger, resulting in larger

measurement errors than in the nominal case. On the other hand, because transit dose rate

is absent in equation (5), changes in the error for this term affect the total error only in the complex case.

Variations 8 and 9 indicate the effects

produced when some of the previous variations are combined. Variations 10 and 11 are analogous to 6 and 7 in that the transit

dose itself is increased and then the error

for the transit dose rate is increased. The

results show that increases in the error for

the transit dose rate produce greater

increases in the total error when the fading

in the field is large. For a factor of five

increase in the error for the transit dose

rate, the total error increases by a factor

of two when field fading, fp, is 20 percent,

whereas the increase in the total error is

only a few percent when fading is only 3 percent.

Only a few of the possible situations

are represented by the variations in Table

III. However, any variation approximating a particular situation may be analyzed in the same manner.

Conclusions

A review of the results of an intercomparison of environmental radiation detectors

indicated the need for developing a method

of error assessment for environmental radiation measurements. Using thermoluminescence

dosimetry as an example, we have formulated

mathematical expressions for the dose in air

for several cases where field and control

detectors are used for isolating the dose at

a field site. Using standard statistical

techniques, these equations can be used to determine the total error of an estimate of

the field dose.

Typical cases were examined to reveal

the effects of changes in the various parameters and their respective errors on the

total error. The results indicate that the

relative advantages of different applications of control and field detectors depend

on measurement conditions.

The method presented here can readily be

employed to estimate the total error for

situations other than those considered.

Also, when suitably adjusted, the techniques

used should be appropriate for most integrating detectors

.

In certain instances the method presented

requires that some approximations be made,

for example, certain parameters are assumed

to be independent when this is not strictly

true. Thus, from a statistical point of

view, a more sophisticated treatment may be

desirable and should be further investigated.

The simple treatment presented is designed

merely to provide an easily applied method

for estimating total errors in a situation

where frequently the practice is to report

either no error or an error reflecting

only measurement precision.

194
Ac>no-,7lenc-p-^,PTn+

.r.^ T gratei-ol to K. 0' Brier, and .1. TMer of KASL, and T. Downs of the Universixy of Texas for mary helpful

discussions. We also thani H. Prichard and R. Lane oi the University of Texas foT- t^enr programming assistance.

'

References

-?fuclear Reg-olatoi-y Coimission '1975) "Radio acxive Material in Light-lVater^ooled

..uclear Po^er Reactor Effluents (10 CFr 50 ---ppendix I)," Federal Register, lo No 8?'

pp. 19439-19443 . ''^>- } £^ i\o

. u /

"Invirosmental Protection Agency '1975)

;tnyiroEiaental Radiation Pratection fo^

proposed Sta-^d- ards," rederal Register, AO, No. 104

pp. 23420-23425. '

^Gesell, T. F. PutVp r ,^0 - . - V (^(Jic\ ' ^' - • a^-a Beciser, K a976) "An international Intercompar-isor of inviromental Dosimeters Under Field "and Lahoratoiy Conditions," Health ^hys^es 30 pp. 125-133.

--^t^-GS, JJ,

-American National Standard ri975) "Perform-

^o^ThJ^^'''^^

Procedural SpecifieatioiB

.or ihermolummescence Dosimetiy (Environ- mental Applications)," ANSI N545 - I975.

^Radiological Health Handbook (1970) U S Department of Health Education and Weifa^e Rock^/ille, Maryland.

-^J-^-te,

'of?^f \ % ^-^^^ "Thermoluminescence'

01 CaF^rMn Exposed at Elevated Tem.peratures

Z °' l^'^-l.^.l RaJiatIo""

PP.^31?-345, m Proc. Fourth Int'l. Conf on ^umanescence Dosimetiy, Krakc^w, Poland.

'^^^^l' ''^-ilkins, M. E. F. (I945) Phosphorescence and Election Traps," Proc of the Royal Society, 18^, pp. 366I389

'o?'?!;.-'

Introduction to the Theoiy of Error, Addison-V/esley, Camhridge, Massachusetts. '

195
TABLE I

TEST PARAMETERS FOR EQUATION (5) NCMINAL CASE

Parameter Value Error«- Error {%)

• 0.00 /iR/h 0.20 uR/h

e 1.00 0 .02 2.0

d 1.00 0 .02 2.0

676 h 2 h 0.3

% 0.97 0.048 5.0

1 T2 1.00 0.005 0.5

%D 7.00 mR 0.140 mR 2.0

^CD 1.10 mR 0.022 mR 2.0

Dp 7.82 urad/h 0.69 urad/h 6.6 total

erro];

for %

><-0ne standard deviation.

195
TABU. II

TEST PARAilETERS FOR EQUATION (11) Na.UNAL CASE

arair.eter Value Errors Error (

0.00 ,aR/ii 0.20 iiR/h

• 2.00 wR/h 0.20 uR/h 10.0

•^3 2.00 uR/h 0.20 uR/h 10.0

20.0 uR/h 20.0 uR/h 100.0

e 1.00 0.02 2.0

d 1.00 0.02 2.0

72 h 0 0.0

50 h 0 0.0

tp 676 2 0.3

-fsi 1.000 0.005 0.5

1.000 0.005 0.5

f 'rj- i 1.000 0.005 0.5

fF 0.970 0.0^9 5.0

f'F 0.950 0.048 5.0

fS3 1.000 0.005 0.5

o3 1.000 0.005 0.5

1.000 0.005 0.5

f S2 1.000 0.005 ' 0.5

^FD 8.10 mR 0.16 mR 2.0

3.60 mR 0.07 mR 2.0

11

Dp 7.82 ^rad/h 0.73 uvad/h 7.0 total error

for D„

r

»tQne standard deviation.

197
TABLE III

EFFECT OF CHANGES IN VARIOUS PARAMETERS AND THEIR

ASSOCIATED ERRORS ON THE TOTAL ERROR

Equation (5)

post-field transit)

Equation fll) (pre- and post-field

transit and storage)

Variation on

nominal case

(see Tables

I and II)

Parameter

Total error Error for Dp Parameter

Total error Error for Dp

7 .00

1.00 5 6.91

f'F 1.00 5 fp 0.80 5 10.80

^ F 0.60 5 % 0.97 1 4.92

^ F 0.95 1

Xl 2.0 uR/h 10 8.24

e 1.0 10 11.98

aDji 100 uR/h 10 10.31

100 uR/h 50 10.52

0.80 5 11.75

f F 0.60 5 Xl 2.0 uR/h 10

nominal case

1. eliminate field

fading

2. increase field

fad ing

3- decrease field

fading error

4. introduce

self -irradiation

5. increase error

in e (or d)

6. introduce a transit dose

equivalent to -10 mR

7. variation 6 plus increase error for

aDxi

8. increase field

fading and

introduce

self -irradiation

9. variation 8 plus increase error for e

( OT d)

10. variation 8 plus introduce

transit dose

equivalent to ~10 mR

11. variation 10

plus increase error for aDiji

fp 1.00

fp 0 . 80

6.63

5 6.63

5 6.67

fp 0 . 97 1 4-48

Xj 2.0 uR/h 10 7.72

1.0 10 11.77

same as 6

fp 0 . 80 2.0 uR/h 105 7.78

same as 8

1.0 10

same as 10

12.46 same as 8

e 1.0 10

14-75

10.74 ccDti 100 uR/h 10 13.67

10.74 aDji 100 AiR/h 50 24.28

^Change occurs only in Xp£) and Xqq.

198
NBS SP456 (1976)

RADIO-FREQUENCY ELECTROMAGNETIC RADIATION FROM PORTABLE AND MOBILE

TELECOMMUNICATION TRANSMITTERS

J. F. Thiel

Texas Department of Health Resources

Austin, Texas T8T56

The increasing number of sources of electromagnetic radiation in the

environment has placed more emphasis on the evaluation of such sources. Several of the more common sources of electromagnetic radiation to which individuals and equipment are exposed have been surveyed in an attempt to assess potential personnel hazards from the radiated fields

Measurements and examples of several typical types of antenna and

transmitter combinations are presented.

Introduction

During the past several years the

levels of radio-frequency electromagnetic

radiation to which individuals have been

exposed has grown dramatically. For example

transceivers used in the Citizens Radio

Service license applical ions d\iring the past

eight (8) months have more than quadrupled

over that received for the same period a year ago: forty thousand applications in

September 197^; l60,000 in the month of

September 1975; and currently ^15,000 in

December 1975. During this same period of

time the number of licensed Citizens Band

Stations has almost doubled from one million

to two million with over 2,658,000 licensed

as of January 1976. It is estimated that

approximately 12,000,000 to 15,000,000

Citizen Band transmitters are presently in use in the United States."*" Reported cases

of electromagnetic interference to other

electronic devices, such as home entertain-

"ent equipment and antiskid braking systems

on trucks has grown from approximately

->5,000 diiring FY7i+-75 to an estimated

75,000 this year. During FY7U-75, 69^ of

^ the complaints received were from operations

I

involving the Citizens Radio Service, a large majority of which were directly

related to over-powered operation. Data

which we have gathered indicates that

1 approximately 0.5^ of all transmitters used

in the Citizens Radio Service are using

power amplifiers to boost power output to

, between 75 and 100 watts radiated.

In 1959, the Department of the Navy

I initiated an effort to establish a national

safety standard on the exposure of personnel

!to potentially hazardous electromagnetic

radiation. The results of that effort is

the present American National Standards

V Institute Committee C95 on Radio Frequency

Radiation Hazards . The ciirrent co-secreta-

\ riates for the committee are the Institute

of Electrical and Electronic Engineers and

' the Department of the Navy-Naval Electronic Systems Command.

The scope of the C95 committee project

is as follows: "Hazards to mankind, volatile materials and explosive devices which

are caused by man-made sources of electromagnetic radiation. The frequency range of

interest extends presently from 10 kHz to 100 GHz. It is not intended to include

infrared. X-rays or other ionizing radiations .

Presently existing standards published

by the American National Standards Institute

include ANSI C 95.1-197^, "Safety Level of

Electromagnetic Radiation with Respect to Personnel." The applicability of this

safety guide is as follows:

1. Normal environmental conditions - higher

temperature or higher relative humidity

requires that personnel exposure be lowered.

2. Doe;; not apply to the expos\ire of

patients by a physician.

3. Frequency band of 10 MHz thru 100 GHz.

h. All possible sources of electromagnetic

radiation in the above range

5. Continuous or intermittent exposure

(time weighted average)

6. Whole-body and partial-body expos\xre

situation.

7. "Healthy people". For example, people

without circulatory ailments.

As good as this standard is, it raises

serious questions as to its applicability

in specific circumstances.

Electromagnetic interference to electronic devices poses two distinct effects on the health and well-being of mankind. The

first and most direct effect is that with

electro-explosive devices and safety
systems. The second and less direct effect

is on home entertainment devices and medical

prostetic devices such as the cardiac pacemaker .

In an attempt to review the magnitude

of exposure of personnel, measurements have

heen made in the near field of antennas on mobile and portable telecommunications

equipment

.

Specifications of Typical Telecommunications

unity to 5 dBi gain in the far field.

Pover Output of Equipment

Mobile arid portable equipment measured

had power outputs ranging from h watts to 150 watts CW and to 200 Watts PEP, Portable

equipment had nominally 2 to ^ watts power

output in the VHF-Hi band with mobile units

using external power packs having up to 25

watts output.

UHF equipment in the hOO MHz thru 500 MHz band had nominal power outputs of 50

thru 110 watts using antennas with 5 dBi

gain. VHF-Hi equipment had power outputs

of nominally 100 watts using antennas with

3 dBi gain. VHF-Lo equipment had power

outputs of nominally 100 watts with antennas

of unity gain. Units used in the 27 MHz band of the Citizens Radio Service were normally in the power output range of 50 thru 100 watts using unity gain antennas

.

It should be noted that the Citizens Radio

Service is limited to h watts by Class D licenses issued by the Federal Communications

Commission under ^7 CFR 95 in the 27 MHz ISM band.

Linear power amplifiers are manufactured

by numerous concerns as broadbanded amplifiers designed for operation in the 1.6 thru 30 MHz range. Due to the high efficiency of the

"broadband" design, it is an excellent amplifier for the 80 thru 10 meter amateur radio

bands. However, many of these amplifiers

are actually being used in violation of

Federal Communications Commission rules and

regulations in the 11 meter Citizens Band.

Several typical examples of these power

amplifiers are given below in the table.

Because of the number of units in the

field and training and background of the

operators of the equipment, units of the

Citizen Radio Service were chosen for measurement purposes. Part of any program for

the purpose of radiation exposure control

must analyze: who is being exposed, knowledge of the individuals being exposed of

the potential exposure hazard, and potential

number of individuals being exposed.

Equipment

Antenna

Common antenna found to be used with

mobile and portable equipment consists of

the following:

1. Base loaded whip antennas whose electrical length is increased by the addition

of inductive reactance at the base of the

antenna. Base loaded antennas have been

generally mounted on trunk decks or the areas surrounding the trunk lid. They are also found mounted on the top center of

roofs and rarely on the front fenders.

2. Top loaded antennas whose electrical

length is increased by the addition of

inductive reactance at the top of the

antenna, have been generally found to be

located on the rear deck or trimk of

'ehicles and sometimes mounted on the

mirrors of trucks.

3. Center loaded antennas whose electrical

length is increased by the addition of

inductive reactance at the center of the

antenna, have been generally found to be

located clipped to the rain gutters and

mounted on mirrors of vehicles.

h. Quarter wave whip antennas have been

generally foimd to be mounted on the rear

fenders and bumpers of vehicles. They have

also been found to be mounted on tool boxes

in pick-up trucks directly behind the

passenger cab.

5. Disguised antennas are generally mounted

in locations similar to that used for normal

car radio reception, with the addition of

inductive load under the fender. These

antennas are used for legal undercover work

and more commonly by individuals who wish

to disguise the fact that they have a radio

in their vehicle to minimize theft of the

radio

.

All of the above antennas exhibit from

200 I
MFG Input Output

Power Power

A. 3.5 W 100 W

B. 2 W 125 W

C. 3 W 80-110 W

D. k V 100 W

E. 3 W 300 W

F. 1 W 100 W

G. 3 W 100 W

H. -1.5 W 100 W

Meas\arements Made

Measurements of the electric field

strength in the proximity of mobile and

portable telecommunications transmitters

were made using a General Microwave RAHAM

Model 2 Power Density Meter. The Model 2 meter uses two diode detectors attached to

orthogonal dipoles to measure the voltage

field strength to power density assuming a free space impedance of 377 ohms.

Experimental results indicate that

when one measures within field strengths

in the near field of an antenna that

reproducible results can only be obtained

if the measuring process does not significantly distort the radiated field.

At distances of 50 centimeters or less

from antennas whose input power is 100 watts

power densities (electric field strength

equivalent of free space) in excess of one milliwatt per square centimeter can be

foiind. As one approaches the loading coils

on antennas shorter than one quarter wave

length, power densities in excess of several

hundred milliwatts per square centimeter

are found.

Direct personal contact with an antenna

whose input power is in excess of 35 watts

may resixLt in a visible radio-frequency

burn.

Measurements were made on a two watt

walkie-talkie operating in the VHF-Hi band

using a borrowed Narda magnetic field probe.

At approximately five centimeters from the

base of the antenna a magnetic field intensity equivalent to a free space power density of 2.5 milliwatts per square centimeter was found. The maximum electric

field strength equivalent to a free space

power density of five milliwatts per square

centimeter was found near the end of the

antenna.

Calculation Technique for

Radiation Hazard Evaluation

A technique for radiation hazard

evaluation was described by Mumford^.

The power density to which individuals are exposed from an electromagnetic radiation

source depends on its radiated power in the

direction of the individual being exposed.

A conservative estimate of this power density in the near field of an antenna may be

calc\ilated assuming an isotropic point

source at the center of radiation and the

power density W calculated from the equation

P

Where

W = power density in watts

per square meter

P = effective isotropic

radiated power in watts

R = distance in meters

7f = 3.lhiG--

In order to have a conservative estimate of the power density in the near field

one must assume 100^ ground reflection,

which doubles the electric field strength

and quadruples the power density, ^ The

equation below provides an estimate of the

distance at which the power density exceeds

the radiation protection guide given in

American National Standard C 95.I-I97I+:

cICp^)= rry/7.72 (2)

If calculations based upon the above two

equations indicates that there may be a problem then the actual antenna system and

duty cycles of the transmitters will have to be considered.

Since "duty cycle" is a term used

throughout the land mobile indviscry, the

appropriate multiplier for the type of

equipment being evaluated should be used to

201
take into account the fact that the radiated

energy from mobile transmitters is not present \mder normal conditions for the entire

averaging period used in the AHSI radiation

protection standard. It is suggested that

any portable or mobile installations that

permit operation by full duplex basis be

considered to have a 100^ duty cycle.

Mobile units that are not capable of duplex

operations but require the transmitter to be turned off before it is possible to hear

the reply from the other party should be

designated as using the EIA design, intermittent, duty cycle of 20^ transmit and 80%

stand-by or receive time, i.e. one minute

transmitter on and four minutes transmitter

off. A similar situation exists for

portable transmitters. However, because of

limitations of the power supply a more reasonable duty-cycle to assume would be 10%

transmitter on and 90% off.

If after taking into account the duty

cycle of the units being evaluated, one finds that individuals are still lining

exposed above the radiation protection guide

as indicated by calculation, one should make

an actual field measurement to determine

the magnitude of potential personnel exposure. Such an evaluation should include

measurements at locations normally occupied

by individuals and measurements at some specific reference distance from the antenna

say, one meter, to evaluate the situation.

Conclusions

Measurements made on a commonly

available mobile and portable telecommiinications system indicate that individuals

may be exposed to levels of electromagnetic radiation comparable to the current

radiation protection guide if they are at distances of less than one meter from the

antenna. With the apparent increase and

authorization of the land mobile radio

spectrum which includes the i+06-512 MHz and

806-960 MHz frequency band as well as numerous bands below 300 MHz. Consideration

should be given to the applicability and

interpretation of the current radiation

protection guide. There exists a need for

measurement standards in order to appropriately evaluate personnel exposure, both

occupational and the general public.

Effects of electromagnetic radiation on safety systems and flammable and volatile

materials needs to be evaluated.

References

1. Private Commiinication , Richard Smith,

Federal Commiinications Commission,

February 1976.

2. Status Report on the Activities of the

American National Standards C95

Committee on Radiofrequency Radiation

Hazards, received from Professor Saul

W. Rosenthal, chairman C95 , December 8,

1975.

3. W. W. Mumford, "Some Technical Aspects

of Microwave Radiation Hazards", Proc

IRE Vol. kg, pp. lt27-i+^7, February I961.

k. Federal Communications Docket No. 19555,

Implementation of the National Environmental Policy Act of 1969.

5. Federal Communications Commission, Report No. 710I+, VHF-UHF Radiation Hazards

and Safety Guidelines , J. Damelin,

July 1971.

6. Draft: Texas Regulations for the

Control of Radio-Frequency Electromagnetic Radiation, Nov. 1975.

7. Private Communications, Homer L. Huddleston, Motorola, Inc., December

1975.

202
NBS SP456 (1976)

AN AUTOMATED MEASUREMENT SYSTEM FOR DETERMINING ENVIRONMENTAL

RADIOFREQUENCY FIELD INTENSITIES II

R. A. Tell, N. N. Hankin, J. C. Nelson, T. W. Athey, and D. E. Janes, Jr.

U. S. Environmental Protection Agency

9100 Brookville Road

Silver Spring, Maryland 20910

A measurement system is described which is used to measure environmental

RF exposure levels. The system uses a spectrum analyzer interfaced to a minicomputer for data acquisition and is contained within a van for

nationwide use. Special antenna systems allow isotropic field intensity

measurements in selected bands between 0.5 MHz and 10 GHz.

(Radiof requency ; spectrum analyzer; antenna; microwave; RF exposure; minicomputer)

Purpose of System

There are several reasons why the U. S. Environmental Protection Agency feels that

data on environmental levels of electromagnetic radiation is needed: the considerable

,

current controversy over the existence of

low-level or nonthermal effects of electromagnetic radiation; ^ the demonstrated

susceptibility of electronic devices to the

existing environment;^''* the gross uncertainties in estimates of population exposure; •' the rapid growth in the number and

power of radiation producing devices;^ and

the need to verify or improve predictive

exposure models. ^'^ In order to obtain the

required data and to determine the need for

standards, EPA is conducting an environmental nonionizing radiation measurements

program. The first phase of this effort was concentrated on field and analytical studies

of selected high power sources. These

sources have included aircraft,^ air traffic

control, 5 and military radar; satellite

communications systems; and UHF broadcast

systems.^ Generally, the survey instrumentation used in the field studies were broad

band, high level, power density detectors.

The purpose of this paper is to describe the

instrumentation developed for the second

phase of the program, a system for measuring

and analyzing the multisource general

electromagnetic environment.

Basic Requirements of the System

On the basis of the desired aspects of

data interpretation for these measurements,

a set of basic requirements was developed.

The frequency range of the equipment should

encompass all bands of interest from an exposure point of view. This means a fundamental detection capability from VLF to approximately 10 GHz or X-Band. Table 1 shows the bands of preliminary interest in our study. These particular bands were

Table 1

INITIAL MONITORING BANDS OF INTEREST

FREQUENCY PRINCIPLE USE

0-2 MHz VLF Communications and AM Standard Broadcast

54 - 88 MHz Low VHF Television Broadcast

88 - 108 MHz FM Broadcast

150 - 162 MHz VHF Land Mobile

174 - 216 MHz High VHF Television Broadcast

450 - 470 MHz UHF Land Mobile

470 - 806 MHz UHF Television Broadcast

1-10 GHz Radar

203
selected primarily on the basis of highest

potential environmental field intensities.

The sensitivity of the system should be

adequate for measuring relatively weak

signals in order to ascertain the relative

contribution to the total exposure picture

from all signals. Frequency resolution was not a particularly critical factor; however,

adequate selectivity should be used to allow

differential evaluation of signal components

in a given band. Stability commensurate

with typical monitoring times was deemed to be sufficient.

Since previous work revealed the

difficulty of making accurate field amplitude measurements ^ ^ '

'^ ^ we have concentrated

on the ability to accurately measure the

signal's field strength or power density,

i.e. , to determine the actual exposure

produced by signals in various bands. The

desired accuracy is obtained with carefully

calibrated antennas, using specially

devised antennas where necessary. Consideration was given to the polarizations of

different signals. Figure 1 illustrates

conceptually the problem of determining the

effective field strength of a signal where

the resultant electric field vector is the

vector sum of three mutually orthogonal

components. Thus in certain cases, the use

of an antenna sensitive to only one polarization is not desirable.

Another important requirement for the

system was that of data analysis. The

system was to incorporate a method for

acquiring spectral amplitude data, correcting this data for system response functions,

and analyzing the data for the pertinent

information desired, providing if possible,

an on-the-spot indication of results after

some appropriate processing. This, in the

field processing feature, would also

minimize the possibility of undetected hard- ware or operator error at the field site.

The last major system requirement was

that of mobility so that environmental

evaluations could be performed at many

locations throughout the country.

A generalized block diagram of the

final measurement system is shown in figure

2. In essence the system is composed of a scanning spectrum analyzer with several

different types of antenna systems, depending on the particular band of interest,

interfaced to a minicomputer data acquisition system. Similar arrangements have been

proposed and are being used in surveillance applications from a spectrum management viewpoint. Analog signal amplitude information from the spectrum analyzer

is digitized and input to the minicomputer

where it is subsequently operated on by

data correction and analysis routines.

Trigger and timing signals are generated and

Figure 1. Relationship

between power density ^'^ and electric field

components

Zn Z(, Zfl

_5_

La

Figure 2. Block diagram of measurement system

HORIZONTAL POLARIZATION VERTICAL POLARIZATION

TURNSTILE DIPOLE

] c

HIGH PASS FILTER

SPECTRUM ANALYZER

ANALOG SWITCHING VOLTAGE

ANALOG AMPLITUDE DATA

SCAN TRIGGER

SCAN TIME SIGNAL

DATA ACQUISITION SYSTEM

204
sensed by the computer to provide effective

control of the spectrum analyzer. Other

signals under program control are also provided for other events such as antenna

switching.

Antennas

Table 2 lists the different antennas

used with the system according to the

different monitoring bands. In each case

the antenna of selection provides, within

reasonable limits, an accurate measure of

the total, resultant field amplitude at any

measurement point. In order to make maximum

use of the spectrum analyzer as a scanning

receiver, the antennas used exhibit an omnidirectional characteristic for those bands

routinely containing many signals. In this

fashion the analyzer may be repetitively

swept over the given band and used to measure the amplitude of all signals in the

band, regardless of the particular direction from which the signal approaches. Also, the antennas have been selected for

being responsive to the significant polarization components in the bands of interest.

For frequencies in the 10 kHz to approximately 60 MHz range, a vertical

monopole antenna with an active impedance

matching circuit at the base is used. This

antenna is referenced to a small square

groundplane and is used to monitor signals

mainly in the AM standard broadcast band.

Because of minimal wave tilt in AM broadcast signals, the vertically polarized

antenna provides in a single measurement,

the true field amplitude. As the frequency

increases above approximately 2 MHz into the

HF band, significant signal depolarization

occurs, and a single measurement of the

vertical field component is no longer

sufficient for an accurate measurement.

Orthogonal dipole arrays are used for

measurements in the VHF television and FM band since FM stations now use circular

polarization with vertical field strengths

in some instances 5 dB greater than the

horizontal values. ^ In actual practice, a complete scan of the band is performed with

one dipole connected, before electronically

switching to the next dipole. Thus measurements of Ex, Ey, and Eg are repetitively

made in a sequential fashion where upon the

resultant electric field strength or power

density is determined. These antenna

systems consist of thick dipolar elements,

cut for resonance at mid-band and fed with

50 Q coax. By using large diameter elements a fairly uniform frequency response Is obtained throughout each band for which the

array is designed. Two sets of these

antennas are used depending on whether the

low VHF TV band or the high VHF TV band is

being measured. In the case of television

signals only two horizontal orthogonal

dipoles are necessary to accomplish the

required task, since depolarization of the

horizontally transmitted TV signal is not significant at relatively short distances

from the station. Using a system of three

orthogonal dipoles, for measurements within

the FM broadcast band, with an average gain

figure, ^8 isotropic response with

approximately 2 dB uniformity has been

obtained.

Two different vertical coaxial dipole

antennas are used for the land mobile bands.

Gains above a A/4 radiator of 0 dB and 3 dB

are used in the VHF and UHF bands respectively.

A commercial type of television receiving antenna is used for measurements in the

UHF TV spectrum. It consists of a horizontally polarized, log periodic array which

. Table 2

MEASUREMENT SYSTEM ANTENNAS

APPLICATION

VLF; AM Standard Broadcast

VHF-TV and FM

Land Mobile

UHF-TV

Radar

DESCRIPTION

Active Vertical Monopole

Orthogonal Dipoles

Vertical Coaxial Dipoles

Directional Log Periodic (Horizontally Polarized)

Vertical and Horizontal Saw-tooth, Planar Log Periodics

205
exhibits a directional gain of about 5 dBi across the UHF band (dB above an isotrope)

This antenna was selected over an alternative of crossed dipoles in order to provide

an adequate gain across the band. It was

found that when the dipole length was shortened enough to allow reasonable operation across most of the band, the effective

area became so small as to adversely affect

overall system sensitivity, except for

exceptionally intense signals. Signal

peaking is accomplished by use of a rotator

on top of a telescoping antenna mast.

A pair of small, saw-tooth, planar, log

periodica are used in the frequency range of

1 to 10 GHz. Two signal cables provide output from both the horizontally and vertically mounted antennas which are also rotatable

from inside the van. A nominal 8 dBi gain

is apparent over their frequency range.

System Calibrations

Each antenna employed for routine monitoring was calibrated across its intended

frequency band by referencing it, indirectly

to a set of National Bureau of Standards

(NBS) calibrated, standard dipoles. This

was accomplished by first using the NBS

antennas to calibrate a set of Singer tuneable dipoles which cover the frequency range

of 30 to 1,000 MHz, and then using the

Singer dipoles at appropriate frequencies to calibrate, by the comparison technique, the

system monitoring antennas. This procedure

required that an area electromagnetically

quiet be found to properly employ the NBS

standard dipoles since they rely on broadband, diode detection of the calibrating

field. The presence of fields, other than

the intended calibration field interfers

with the overall calibration accuracy by

adding in the undesired detection voltage of

the extraneous fields. A calibration trip was arranged to the location of the National

Radio Astronomy Observatory in Greenbank,

West Virginia for this calibration purpose.

Through special permission, intentional

calibration fields were established for

conducting the tests. A least squares method was used to fit

a curve to the Singer dipoles which resulted

in a maximum deviance at measured points of

1,4 dB across the frequency range of 50-

1,000 MHz. With the Singer dipole antenna

factor now known functionally at any

frequency, a comparison approach was used

to calibrate the various systems of antennas. These calibrations were carried out with the antennas in their normally used

position on the van. The calibrating field

was established with a dipole antenna at a distance of 600 feet. Antenna calibration

data was recorded at appropriate frequency

intervals for each system for maximum signal

pickup, i.e., main lobe alignment. In the case of orthogonally constructed dipoles,

measurements were individually performed

for each dipole in the array. These data were then subsequently fit using least

squares techniques so that a functional

expression of the given antenna's frequency

response was developed. Now, except for

the fact that the antenna data were for main

lobe application, the antenna factors were ready for computer implementation. One

final determination remained.

The monitoring van was driven to the

Department of Commerce's Institute for Telecommunications Science's antenna range

facility near Boulder, Colorado. Here, the van was placed on a 30 foot diameter turntable where radiation patterns for each of

the antenna systems were measured. This was accomplished by radiating a stable field

2,225 feet from the van and, as the vanantenna system was rotated, using the monitoring equipment itself to record and

analyze the pattern data. The minicomputer

provided a convenient means for normalizing

and plotting the received signal and

storing the measured data on magnetic tape.

Using this data, average antenna factor

variations were computed and applied to the

previously measured main lobe data to ar- rive at a final antenna factor, which when

employed by the measuring system, would

account for the total uncertainty in the

measurement of resultant field strength or power density. The results of all of this

antenna calibration work are contained in

the application programs which are used by

the system. Table 3 provides a summary of

the total uncertainties measured for the

overall system. The presence of the van

had been presumed to be more significant in

the pattern data than what was actually

observed. The total system uncertainty can be characterized by a number of errors: inherent error in basic calibration accuracy of the NBS standard dipoles, the error

due to using the Singer dipoles at frequencies between calibration points, the error associated with the fitting of a functional

form to the measured calibration curves for

the monitoring antennas, an error due to the angular dependence of the antenna system being used, and a system conversion

error which is a measure of the spectrum

analyzer-computer system's ability to accurately detect, plot, and record RF signal levels. Each of these factors are

206
Table 3

SUMMARY OF OVERALL SYSTEM ERRORS (dB)

Band NBS

Dipole

Singer

Dipole

Curve

Fit

Angular

Dependence

System

Conversion

Total Resultant

RMS Error

AM 1 .5 dB* N/A +0 4 +0.5 ±0 5 +1 .6

Low VHF 1 .0 +0. 6 +0 8 +2.2 +0 5 +2 .5

FM 1 .0 +0. 6 +0 4 +1.8 +0 5 +2 .1

Low LM 1 .0 +0. 7 +0 7 +0.8 +0 5 +1 .6

High VHF 1 0 +0. 8 +0 5 +2.0 +0 5 +2 .3

High LM 1 0 +0. 5 +1 5 +1.1 +0 5 +2 .0

UHF 1 0 +1. 1 +1 2 +1.0 +0 5 +2 .0

*Ref erenced directly to NBS field strength meter (Rhode and Swartz type HFH) . The

system as now configured is limited to discerning changes in RF level no smaller

than 0.25 dB which is a limitation imposed by the resolution of the ADC.

provided in Table 3 and show that the sys- tem has a total uncertainty no greater than

2.5 dB, a feat which is the result of

extensive field work. The angular dependence error also includes a factor for a slight variation of the angular dependence

upon frequency.

Detection Hardware

The principle detection equipment consists of a Hewlett Packard spectrum analyzer mainframe with variable persistence display and a number of plug-in RF sections to provide a total detection range from 20 Hz

to 18 GHz . Normally the narrow band signal

peak height is taken as the received signal's power except in the case of broadband

radar spectra where determination of the

impulse bandwidth, PRF, and pulse width are necessary to accurately obtain the peak and

average exposure levels.

An additional device used in high

intensity radar measurements is the high

speed Biomation transient recorder. The

recorder actually consists of a very high

speed (100 MHz) analog-to-digital converter

(ADC) and its own 2,048 word core memory

and timing circuitry. By using a broadband

diode detector connected to an antenna, a dc

voltage replica of the radar's signal is

obtained which can be digitized and stored

in the memory. By using a dual time base

mode, the pulse width and PRF may be determined directly from a single recorder sweep.

Figure 3 gives an illustrative example of

this type of display where the pulse width

is seen on the left and the PRF is apparent

on the right. The time scale on the left

half of the screen in 4 ysec/division and on the right half is 1 msec/div showing a 10 usee wide pulse at a rep rate of 1 kHz. By

judicious choice of trigger levels, the

maximum amplitude of rotating radar beams

Figure 3. Detected RF pulse with

transient recorder 4 usec/div on left; 1 msec/div on right, 10 ysec

wide pulse at 1 kHz PRF.

207
can be measured. Also, pulse widths from more than one radar may be determined simultaneously.

Data Acquisition Equipment

One of the main selection criteria for

the data acquisition system was the development time necessary for preparing application software by personnel not previously

familiar with real time programming tasks. For this reason a system oriented around an interactive interpreter with extensive hardware control under an extended version of

BASIC (EBASIC) was chosen. It was deemed

more important to get a number of conceptual

acquisition and analysis processes actually

operating on the computer in a fairly short

period of programming time with the relatively long processing time associated with

an interpreter as opposed to spending a long

time preparing few programs which execute

very rapidly.

Figure 4 is a block diagram of the data

acquisition system. The system is con- figured around a Varian Data Machines central processing unit (CPU) with a 750 nsec

cycle time and 32 K of 16 bit word core memory. A 123 K word, fixed head, disc provides high speed mass storage for the system

operating software, programs, and data.

Additional slow access mass storage is provided by a dual transport digital magnetic

cassette unit. A large screen CRT is used

to display alphanumeric and graphic data

while a standard ASR-33 teletype is the main

communication link between the operator and

the system. To facilitate interfacing to other instruments, a console is provided for access to various hardware features of the

system which include a 13 bit resolution

ADC multiplexed to 16 channels, two digitalto-analog converters (one 14 bit and the

other 10 bit resolution) for outputing

voltages, single bit inputs for sensing the

status of various events, and electronically switched, control outputs. Additionally

two channels of digital output and one channel of digital parallel 16 bit input are available. An interface connects the transient recorder for computer controlled high

speed pulse acquisition and digital data

transfer to main memory. Figure 5 is a photograph of the system as it appears in

the measurement van. The RF detection

equipment is in the rack to the left and

the data acquisition system is beyond the

operator. A hardcopy unit provides finished hardcopy output from the CRT display.

Connection to the spectrum analyzer is

accomplished via the interface console. A trigger signal is developed by one of the

digital-to-analog converters, and conversely the analog amplitude signal from the

analyzer is read in via one of the ADC's

multiplexed channels. Since the Hewlett

Figure 4. Block diagram of data acquisition system

DISC, 123K WORDS DUAL CASSETTE TRANSPORT

ANALOG DATA IN

16 CHANNELS

8 STATUS INPUTS, SENSE SINGLE DIGITAL INPUTS

DIGITAL INPUT

1 CHANNEL (16 BIT)

ANALOG DATA OUT

8 CHANNELS

INTEREACE

CONSOLE

COMPUTER

32K WORDS (16 BIT) CORE

750 nS CYCLE TIME

8 CONTROL OUTPUTS, TRANSISTOR SWITCHES

DIGITAL OUTPUT

2 CHANNELS (16 BIT)

CRT DISPUY X-Y PLOTTER

TRANSIENT RECORDER

(HIGH SPEED ADC)

208
Figure 5. Radiof requency measurement system in van

Packard analyzer conveniently allows for

timing purposes, and has an output proportional to beam deflection on the analyzer's

display, a very straightforward interface

is possible.

The key to simplicity in the overall

system, however, is the power of the Varian

extended EBASIC software. Besides the

normal programming instructions in EBASIC,

extentions have been included to operate

all peripheral hardware via calls to assembly language subroutines. This allows

rapid program development through interactive communication with the system and

yet very fast input-output (I/O) operation.

A brief summary of the more useful software

features includes:

• Versatile I/O commands for acquiring

and outputting analog and digital data;

• Program and data file handling on both the disc and cassette; and

• Graphic display routines.

Analysis Techniques and Representative Data

Table 4 lists the main features of the

data acquisition and analysis procedures we

have developed. These features include: an absolute power calibration of the spectrum

analyzer so that received antenna power is

correctly indicated; signal averaging via

software whereby spectral powers represented on a logarithmic scale in dBm are con- verted to absolute power, summed, averaged,

and then converted back to dBm format for

display purposes; peak power retention programs to retain the peak, or maximum,

signal power ever observed over some specified monitoring time; measurement of

elapsed monitoring time via reference to the CPU's real time clock; spectral power

integration where signal powers may be

summed across a given band yielding a measure of the total exposure power density;

programs for measuring band occupancy to determine the percentage of time that signals exist during a given observation

period; antenna calibrations to arrive at proper correction factors to apply to received power for determining field

strength or power density; statistical

treatment of the data to determine the

length of monitoring time required at any

given location to yield the required con- fidence in measurements of mean spectral

power; CRT data display routines for plotting processed data and indicating pertinent

control settings; and routines for storage
Table 4

FEATURES OF DATA ACQUISITION AND ANALYSIS

Absolute Power Calibration

Signal Averaging - Determination of Mean Power Spectra

Peak Power Retention

Elapsed Monitoring Time Measurement

Spectral Power Integration

Per Cent Occupancy Measurement

Antenna Calibration

CRT Data Display

Digital Cassette Data Storage

of spectral monitoring data on digital

cassettes

.

Several of the software developments

include the use of assembly language subroutines which provide for machine speed

execution of various repetitive operations

that would normally be substantially slower

due to the overhead of the EBASIC processor.

Notable in this area are routines for peak

signal retention, signal averaging, and band

occupancy.

Examples of system output for several

bands are shown in figures 6-9. These

represent actual environmental data collected during our first few field trips. Figure 6 shows the FM band for a site in Washington, DC. All system calibration factors

are included so that the absolute field

strength of any signal is obtained. The

total power density from all signals in the

band is calculated and printed near the top

of the graph. The dynamic range capabilities of the system are also illustrated in

Figure 6. FM average field strength - Washington, DC

130 '

129 --

lie —

TOTAL POWER DN5TV 287949 uw/cn/cn

lee oauv/n

le scwNs

^89 DBTR POINTS

1413 HOURS

2 MIN ET

338 DftVS 19?5

BU - 30 KHZ

SITE CODE- 2 OPERflTOR CODE* 4

92 96 193 FREOUEMCV <«H2> 184 100

210
this figure, the highest levels being a factor of 10^ greater than the lowest.

Other pertinent parameters of the measurement are also printed on the graph.

Figure 7 is an example of data collected in the lower half of the VHF-TV band

(channels 2-6). Only channels 4 (66-72 MFxz) and 5 (76-82 MHz) are on-the-air at this

location. Part of the FM band is shown from

88-96 MHz, but these signals are not included in the power density total for the band.

Figure 8 shows the upper half of the VHF-TV

band at another location. The AM standard

broadcast band is sho^m in Figure 9.

Figure 7. Low VHF TV average field strength - Washington, DC

146— 'OTRL POUER DNSTV = 196134

138 i

15 SCftNS see D«Tfl POINTS

UU/Cfl/Cn 1435 HOURS

1 niN ET 338 OflVS 1973

BU > ie» KHZ

SITE CODE - 2 OPERATOR CODE - 1 12e —

M -- M -

79 --

46 4 ^S6 £6 76 FREQUENCV CHHZ)

H—

86

Figure 8. High VHF TV average field strength - Atlanta

2S SCANS see OftTR POINTS

X2i-r TOTAL POWER DNSTV - 2 2e636E-02 UW/CM/CH 1083 HOURS y 2 WIN ET 349 DRVS 1975

BH - 108 KHZ iSITE CODE - 8 OPERATOR CODE > 1

ft

4« I 1 1 1 \ 1 1 1 \ 1 1 178 188 198 288 210 228

FREQUENCV <nHZ)

211
Figure 9. AM average field strength - Atlanta

Measurement Program Implementation

The measurement system described is

installed in a 27 foot Travco van for portable operation and has been used in various

metropolitan area studies since October of

1975. A pneumatically, air-operated 26 foot

telescoping mast attached to the rear of the van is used for elevating the antennas for

monitoring. Two separate 6 kW electric

generators are used to provide on-board

power; one unit powers all utilities such as air conditioning, heating, lights, etc., the

other unit is dedicated to the instrumentation.

Presently the van is being used to collect environmental data in cities on the East coast. As data becomes available more refined methods are being developed for

selecting the total number and location of

monitoring sites within a given metropolitan

area. Initial monitoring locations are chosen on the basis of RF source distributions and population densities. As the monitoring program progres ses, measurement in

high density West coast cities are planned.

electromagnetic radiation," Office of

Telecommunications Policy, Washington,

DC, 1971.

2. Michaelson, S.M. and C.H. Dodge, "Soviet

views on the biological effects of

microwaves - an analysis," Health

Physics, 21: 108-111 (1971).

3. Mitchell, J.C. et al, "Empirical studies

of cardiac pacemaker interference,"

Aerospace Medicine, 45: 189-195 (1974).

[This paper gives references to earlier

work.

4. Vreeland, R.W. , M.D. Sheperd, and J.C. Hutchinson, "The effects of FM and TV broadcast stations upon cardiac pacemakers," In Proceedings of IEEE Electromagnetic Compatibility S3,'Tnposium,

publication no. IEEE 74CH0803-7 EMC.

5. Lynn, J.F., "Sources of electromagnetic

pollution or look what we are doing to ourselves," Workshop on Electromagnetic

Pollution, Society of Automotive Engineers, Rockville, MD (1973).

References

1. Electromagnetic Radiation Management

Advisory Council (ERMAC) , "Program for

control of electromagnetic pollution of

the environment: the assessment of

biological hazards of nonionizing

6. Tell, R.A. and J.C. Nelson, "Calculated

field intensities near a high power UHF

broadcast installation," Radiation Data

and Reports, 15: 401-410 (1974).

7. Athey, T.W. , R.A. Tell, and D.E. Janes,

"The use of an automated population data
base in population exposure calculations," Health Physics Society Eighth

Midyear Topical Symposium (1974).

8. Tell, R.A. and J.C. Nelson, "Microwave

hazard measurements near various airborne radars," Radiation Data and

Reports 15: 161-179 (1974)..

9. Tell, R.A. and J.C. Nelson, "RF pulse

spectral measurements in the vicinity of

several ATC radars," Environmental Protection Agency, (EPA-520/1-74-005)

Washington, DC (1974).

10. Hankin, N.N., R.A. Tell, and D.E. Janes,

"Assessing the potential for exposure to hazardous levels of microwave radiation

from high power sources," Health Physics

Society Annual Meeting, 1974.

11. Hankin, N.N., "An evaluation of satellite communication systems as sources of

environmental microwave radiation,"

Environmental Protection Agency, (EPA520/2-74-008) Washington, DC, 1974.

12. Smith, S.W. and D.G. Brown, "Nonionizing

radiation levels in the Washington, DC

area," IEEE Transactions on Electromagnetic Compatibility, Vol. EMC-15, No.

1, pp. 2-6, February 1973.

13. Envall, K. , R.W. Peterson, and H.F. Stewart, "Measurements of electromagnetic radiation levels from selected

transmitters operating between 54 and

220 MHz in the Las Vegas, Nevada, area,"

USDHEW, PHS, BRH, Rockville, MD 20852

publication number BRH/DEP 72-4.

14. Hagn, G.H., S.C. Fralick, H.N. Shaver,

and G.E. Barker, "A spectrum measure- ment/monitoring capability for the

Federal government," final report on contract OEP-SE-70-102 prepared for

Office of Telecommunications Policy by

Stanford Research Institute, SRI project

8410, May 1971.

15. McMahon, J.H., "Capability of the FCC

mobile monitoring van," in the proceedings of the Vehicular Technology Conference Record, Cleveland, Ohio, December 3, 4, and 5, 1973.

16. Barghausen, A.F. and L.G. Hailey, "Radio

spectrum measurement system - current

applications, experience, results," in

proceedings of IEEE Electromagnetic

Compatibility Symposium, July 16, 17,

and 18, 1974, San Francisco, California,

IEEE publication 74CH0803-7EMC , pp. 140-145.

17. Tell, R.A. , "Signal polariations in the

VHF and UHF broadcast spectrum," IEEE Transactions on Broadcasting, December

1974.

18. Halberstein, "Gain averages as criteria

for antennas EMC-perf ormance

, " in proceedings of IEEE International Electromagnetic Compatibility Symposium, July

1972, IEEE publication 72CH0638-7EMC

pp. 333-336.

19. Schultz, L.D., A.D. Spaulding, and A.F. Barghausen, "Radio spectrum occupancy

signals and noise," Ibid, pp. 42-49.

213

NBS SP456 (1976)

THE MEASUREMENT OF OPTICAL RADIATIONS:

SELECTED PRACTICAL COMS I D ERAT I ONS

R. V/, Peterson, J. Marshall Coakley, Kshitlj Mohan

and Robert James

Electro-Optics Branch, Division of Electronic Products

Bureau of Radiological Health

Food and Drug Administration

Rockville, Maryland 20852

This paper describes the measurement capabilities

for coherent and incoherent optical radiations at

the Bureau of Radiolosical Health and discusses

some problems that arise in developing a program

of measurements required for enforcing compliance

with Federal standards and for related regulatory

respons i b i 1 i 1 1 es

.

(lasers, calorimeters, detectors, calibrations,

rad i ometr y

)

I ntroduct i on The biological effects from certain regions of the optical spectrum

are v/el 1 establ I shed . 1>2,3,4 Many

of these effects constitute a health hazard. For example, retinal lesions, photokerat i t i s, erythema, and skin cancer are some of the

known or suspected effects of exposure to certain spectral regions of

the optical spectrum.

The v/ide use of lasers, UV

sources, and other intense optical

radiation sources and the potential

hazards associated with their, use

has emphasized the need for characterizing them, thereby Increasing

the burden on credible optical

metrol ogy

.

The Bureau of Radiological

Health, by virtue of the Radiation

Control for Health and Safety Act

of 1963 (PL 90-G02) has been delegated regulatory responsibilities

in several areas, among them promulgation of performance standards for sources of optical radiation.

A Federal safety standard for

the performance of all lasers and

laser products has been published

and is due to become effective on

August 2, nyr,. in order to enforce

the compliance of industry with this

standard, a comprehensive program

for laser pov/er measurements has

been established in our laboratory.

This paper. In part, will describe

this measurement capability.

The proliferation of UV sources

such as sunlamps, UV medical or dental devices, high intensity discharge lamps, etc. has resulted In many alleged radiation injuries.

In order to characterize the optical

performance of these sources, capabllities in radiometric measurements

of Incoherent optical radiation have

been developed. Selected technical

problems encountered In this program

wi 1 1 be descr I bed

.

The tieasurement of

Coherent Radiation

At the heart of our measurement

program for coherent optical radiation are tv/o calorimeters referred

to as the C-serles ca 1 or I me ter s ^ >6 developed by the National Bureau of

Standards In Boulder, Colorado.

NBS maintains the national scales

for laser power and energy measurements using a set of three of these

calorimeters as the reference standard. Under an interagency agreement

with MBS, a set of two calorimeters

215
was purchased and is maintained in our laboratory. A joint NBS/BRII

Measurement Assurance Program is

used, to intprcompare the measurement

capabilities at NBS and BRH four

times a year.

The BRH calorimeters are used

In turn to calibrate a variety of

laser power and energy measuring

detection systems, some of which are used for compliance related measurements.

Figure 1 shows a sketch of the

C-serles calorimeter. Each calorimeter consists of a copper cylinder blackened inside by copper

oxide, or a similar material, that

absorbs the incident energy. Surrounding the absorber is a constant

temperature housing that thermally

isolates the absorber from Its

surroundings as well as provides a constant reference temperature for

the thermocouple sensors. The

volume Inside the temperature con- trolled housing Is evacuated to a typical pressure of 10"^ mm Mg or less. A window Is necessary to

allow optical radiation to pass to

the absorber v/hlle maintaining the vacuum. Calibration heaters are attached to the absorber to permit

electrical calibration and direct

reference of measurements to primary

electrical standards.

The C-serles calorimeters are

useful for measuring laser energy

from O.ni J up to 30 J to an overall

accuracy of ±1.2"^. It has a maximum

CI/ pov/er limitation of 1 v.'att, with

an energy density limitation of

0.1 J/cm2. Depending upon the window material used, measurements can

be made over a wavelength range

extending from the ultraviolet up

through the far infrared of C02

lasers.

In practice, the tv/o calorimeters are used first to measure the

ratio of a beam splitter. One of

the calorimeters is then replaced

by an instrument to be calibrated.

By measuring the incident energy

with the remaining C-serles calorimeter and using the beam splitter

ratio, the energy/power input to the

instrument can be determined and the

calibration factor established.

To assure confidence in our measurements, a record of the system's performance with time is maintained. One of the more meaningful

records that may be kept Is a control chart. l.'e maintain a control

chart of the electrical calibration

factor vs. time for the C-serles

calorimeters.

Figure 1 THE C-SERIES CALORIMETER

216
These control charts indicate

a standard deviation of less than

0.2^i for 25 different runs spaced

over several months.

Hov/ever, for the actual optical

measurements taken with the C-series

system, there are other sources of

uncertainty also, such as the accuracy of the estimate of window

transmission, absorption of the

absorber surface, polarization of

incoming beam, etc. The error inherent in the electrical calibrations is only one part of the overall error of the C-series system.

Figure 2 sho\.'s a control chart

of a commercially available laser

power measurement device. The

calibration factor plotted against

the run number. Notice that the

data are in general, randomly

scattered about the mean. If the

plot started indicating a trend. It v/ould mean that the Instrument's

characteristics were changing and

any measurement taken './i th the

instrument in this condition would

have to be carefully evaluated.

Hov/ever, in this case, the points

are scattered in a random fashion.

You v.'i 1 1 notice that the one s i gma

levels for this Instrument are con- siderably greater than they are for

the C-series calorimeter; l.R<; vs 0.2%. Whenever the measurement

system Is a step removed from the

reference system, the uncertainty

i s i ncr eased .

0.90 r2 0.85

0.80

• _ •

Mean=0.8688

Sf ondord Deviation -1 .6 %

_L

0 5 10 EXPERIMENT NUMBER Figure 2 CONTROL CHART FOR A LASER POV/ER INSTRUMENT

Two other points should be made

with respect to measurement of laser

radiation. The first is that laser

beams are extremely intense sources

of light. Even the low powered

lasers may be sufficiently intense

to saturate, or even damage, certain detectors leading to erroneous

results. Silicon detectors are especially susceptible to overloadi ng.

The second point Is that If a window or protective covering is

used for a detector. It should be a wed.ge shape; i.e. have non-parallel

front and rear surfaces. Since

laser light Is coherent, the use of

optical flats would result In errors

due to Interference.

Finally, as with any metrology

program, it Is essential that control charts be maintained to monitor system performance.

Before Instruments for the

measurement of laser power or energy

under noncontrol led conditions are calibrated, they must be evaluated

to characterize their performance

as a function of variations in the

relevant parameters such as temperature, position of the Incident beam

on the detector, etc.

\!e routinely make studies of

the temperature and humidity dependence of detector response for "different detection Instruments. The

details of these measurements are contained In a separate publlcation7, and Figure 3 details the

temperature dependence of response

'for a laser power meter which utilizes a silicon photodiode.

The spatial uniformity of the

response of a detector surface is

quite critical v/hen making laser

pov/er or energy measurements. Measurements of this characteristic are part of the routine evaluation of

detectors In the Electro-Optics

Branch of the Bureau of Radiological

Health. These measurements are made

by scanning a He-Ne laser beam across the sensitive surface of a detector and displaying Its response

on an oscilloscope. Figure if I s a schematic of this measurement

f acl 1 I ty.

217
oa.

140

130' -

120 —

110 -

100 —

90

80

Detector Head and Read-Out

Detector Head only

Normalization Temperature

In addition, all detector

systems should be evaluated with

respect to their stability, reproducibility and linearity over the

wavelength regions and radiation

levels at which they are used.

The Measurement of

Incoherent Radiation

The interest of BRH In the area of incoherent optical radiation is based on tv«/o considerations

first, it has responsibilities and

authority under provisions for

defect actions to regulate any

product that emi ts unnecessary

potentially hazardous radiation and

second, a growing interest In regulating products emitting UV radiation, such as sunlamps and high

Intensity discharge lamps.

In order to fulfill our mission In this area. It is necessary

to maintain credible capabilities

for the measurement of various

parameters related to incoherent

opt I ca 1 r ad I a 1 1 on

.

40 60 80 100 TEMPERATURE (°F)

120

Figure 3 TEMPERATURE DEPENDENCE OF

RESPONSE - SI LI CON

POl/ER METER

"Y" DEFLECTOR

Figure h EXPERIMENTAL SYSTEM FOR MEASURING DETECTOR SURFACE UNIFOR74ITY

218
The parameters of primary

Interest are spectral Irradlance,

spectral radiance and luminous

flux. Our laboratory maintains a set of standards (obtained from

the National Bureau of Standards)

for each of these quantities. A set of working standards Is calibrated against the standard from

NBS and used In the routine measurements. Periodic I ntercompar I- sons v/i th N3S and participation

In Measurement Assurance Programs

(MAP) ensures the continuing validity of all measurements that are made

We will briefly describe nov7 our spectror ad I ometr I c measurement

facility and will give some representative results. Three different

systems for the measurement of

spectral irradlance have been

established. The first Is a rapid

scanning spectrorad i ometer built

around a silicon diode target

vidlcon tube and a minicomputer

with which the spectrum can be

measured In i^OO nanometer sections

from 300 to 1100 nanometers. This

provides a capability for making

nearly Instantaneous measurements

of spectral Irradlance. It is used

primarily f or measur I ng radiation

from sources that cannot be sus- tained for too long a period of

time, such as v/elding arcs and

photographic flash units. The

second system is a set of portable

spectrorad i ometer s each consisting

of a small single grating monochromator, a photomu 1 1 i pi I er tube

and an amplifier, high voltage

supply and readout unit. These

are used with a set of appropriate

cut off filters to reduce the errors due to stray light. The

third Instrument, which is used

for maintaining the scales of

spectral irradlance at BRH, however. Is more elaborate and Is shown In figure 5. It consists

of a double grating Czerny Turner

monochromator vyhich views a BaSOti

plate at a h5° angle. The BaSntt

plate Is in turn irradiated by a standard and the test source to

provide the calibration of the

spectral Irradlance due to the

test source. Measurements can be

made from 250 to 800 nanometers

using either a photomu 1 t I p 1 i er with an S-20 response or a solar

blind photomul tl pi I er tube which

responds only to radiation shorter

than hOO nm. The wavelength is

el ectro-mechani cal 1 y scanned and

data can be obtained almost con- tinuously over the wavelength

scale. The spectral bandpass of

the instrument can be chosen

between approximately 0.5 and

2 nanometers.

HIGH VOLT- AGE

SUPPLY

PHOTOMULTIPLIER

RADIO- METER

DATA

PROCES- SOR

DOUBLEGRATING MONOCHROMATOR

QUARTZ INPUTOPTICS

\

45°

LIGHT INPUT PATH

BaS04

DIFFUSER PLATE

Figure 5 SPECTRORAD I OMETR I C MEASUREMENTS SYSTEM

219
The primary measurement ref- erence standards used are a 1000

v/att FEL type quartz halogen lamp

calibrated by TIBS for spectral

irradlance betv/een 250 and 1500

nanometers and a deuterium discharge lamp calibrated for spectral

Irradlance by a commercial laboratory between 180 and hOO nano- meters. A deuterium lamp standard

calibrated for spectral Irradlance

between 180 and kOO nanometers Is

nov/ available from the National

Bureau of Standards and our future

measurements In that region will

be based on this standard. The

standard and the test lamps are operated at specified distances

from the BaSOtt plate and a data

acquisition system consisting of a Hewlett Packard Model 2570 data

coupler, a model 9810 calculator,

an X-Y recorder and a magnetic

cassette data storage system is

used for collecting, storing and

reduci ng the data

The uncertainties associated

vn th the measurement of spectral

irradlance depend greatly on the sources being measured. The uncertainties with respect to SI inherent In the calibration of our 1000 watt quartz halogen primary

standard lamp are assigned by TIBS and are 2.6'^o at 250 nm and ].2%

above 1+50 nm. Other sources of error that are taken into account are:

a. Uncertainties in the

calibration of secondary standards

b. Error in the calibration

of wavelength scale

c. Error because of temporal drifts in the system

d. Errors because of lack

of linearity in the instrument.

(Uhen test sources emit at levels

vastly different from the standard

this may be a substantial error)

e. Errors because of stray

radiation from wavelength regions

other than that being measured

f. Uncertainties because of

lack of reproducibility, and

S. Uncertainties because of

temperature dependence of Instrument response

All these Lincer ta I nt I es are taken

into account in the final estimation of the uncertainties asso- ciated v/i th a measurement.

Figure 6 shows the spectral

irradlance in '.J cm-2 nm-1 at a distance of 30.5 cm from a device

used for the polymerization of

dental filling material by irradiation with UV radiation. Other

sources that are measured in our laboratories range from units

used in phototherapy for hyperbilirubinemia to sunlamps, fluorescent lamps, high intensity

discharge lamps, etc.

References

1. mils, L, F., C. n. Lytle,

F. A. Andersen, K. B. Hellman,

and L. E. Bockstahler: A Revlev; of Biological Effects

and Potential Risks Associated

with Ultraviolet Radiation as Used In Pentistry. U. S. PHEW

Publication No. (FPA) 76-8021

( n7fi) .

2. Freeman, R. G. : Data on the

Action Spectrum for Ultraviolet

Carcinogenesis. J. Nat. Can.

I nst. 55A ( 1975)

3. Van Pelt, !/. F., '/. R. Payne,

and R. Peterson: A Revlev;

of Selected Bloeffects Thresholds for Various Spectral

Ranges of Light. U. S. PHE'f

Publication (FPA) 7i|-8010,

Rockvi lie ( 1973)

k. PI tts, n. G., and T. J. Tredici: The Effects of Ultraviolet on the Eye. Am. Ind.

Hyg. Assoc. J. 32:235 (1971).

220
SPECTRAL IRRADIANCE OF A DENTAL DEVICE

10

i 10-'

^10-

LUz

< -9 Q 10

<

i

'0-

»—^ 10

10

200 300 400 500

WAVELENGTH (nm)

600 700 800

Figure 6 SPECTRAL ^RRADIAfJCE FROM ^ DENTAL -UV IRRAPIATIOM DEVICE (30.5 CM FROM THE APPLICATOR TIP)

5. Jennings, P. A., E. P. V/est,

K. M. Eyenson, A. L. Rasmussen,

1. R. Simmons: Laser Power and

Energy Measurements. NRS

Technical Mote 382. (Oct.

19R0 ) .

6. './est, E. P.: Pata Analysis

for Isoperibol Laser Calorimetry. MRS Technical Mote

396. (Feb. 1971).

7. Landry, R. J. and R. Peterson: Temperature dependent Response of Optical Radiation fleasurement Instrumentation. To be published.

221

NBS SP456 (1976) X-RAY LEAKAGE FROM CON\^NTIONAL AND HIGH VOLTAGE ELECTRON MICROSCOPES

D. F. Parsons

Roswell Park Memorial Institute

666 Elm Street, Buffalo, N.Y. 14263

V. A. Phillips and J. S. Lally

(Members of the Radiation Safety Committee of the

Electron Microscopy Society of America*)

.

A survey of the x-ray leakages from the electron microscopes used by

members of the Electron Microscopy Society of America has been

carried out. Few, if any, out of several thousand long term microscope users received an exposure detrimental to health. However, a sufficient number of design errors, and accidental over exposures

have occured to make standardization of maximum allowed leakage desirable. The radiation hazards of high voltage electron microscopes

are also discussed.

(Electron microscopes; x-ray leakage; health hazards; high voltage

electron microscopes)

Introduction

A survey was completed in April 1972,

by the above members of the Radiation Safety

Committee of the Electron Microscopy Society

of America.-'- The basic findings have been

published^ and a start made in standardization of x-ray leakage by publication of a Handbook for electron microscopists . This

survey covered conventional fixed beam

electron microscopes (40-200 kV) . Scanning

microscopes and microprobe analyzers were

not covered. Experience is gradually being

acquired with high voltage microscopes (HVEM)

installations of which there are now a sizable number (nine in USA, nine in U.K.

,

four in France, one in the Netherlands, one

in Germany, one in Russia, one in Sweden

and seven in Japan), In the next few years,

it is planned to reconstitute the Radiation

Safety Committee and to make a survey of

HVEM x-ray hazards. A preliminary discussion is given in this report.

Since the publication of the survey,

there has been considerable interest by

State, County, City and Institution radiation safety officials in the results and

recommendations. However, to-date, there

has been no attempt to introduce standardization by a Federal Agency as recommended

by the Committee. We hope that further

discussions of our result at this meeting

will be a first step towards standardization. Recently, there are signs that

several manufacturers of electron microscopes have modified their instruments to conform to EURATOM standards of x-ray leakage (less than 0.1 mR/hr at a distance of

10 cm, from all external surfaces of the

equipment)

.

Results

A survey of 589 electron microscopists

gave the following main results:

1 . Frequency of checking the microscope

for leaks by electron microscopists and

health inspectors . 8% never monitored, 44%

monitored at one year or larger intervals,

10% monitored two or more times a year and

11% monitored continuously. Only 17% of

those monitoring named the monitor and most

monitors used showed wide variation of sen- sitivity with x-ray energy.

2. Use of film badge . Only 17% used badges

on . their person.

3. Frequency of significant leaks. (> 0.5

mR/hr) . Bearing in mind the general infrequency of adequate monitoring, nether theless

,

51% reported detecting x-ray leaks. However, only 18% recorded a figure. Of these,

41% found more than 0.5 mR/hr. Hence, x- ray leakages of > 0.5 mR/hr must occur with

considerable frequency. It was also found

that a casual view of the magnitude of the

leak exists since most regarded leaks of > 0.5 mR/hr as "probably not hazardous".

4. Site of Leaks . Gun 53%, camera and

viewing chamber 27%, specimen chamber and

column 16%, high voltage tank and cable 3%,

luminous dial clock 2%.

*The views expressed are those of the authors only.

223
5 . Variation of leaks with type of micro - scope . The fraction of each type showing

leaks varied markedly with type indicating

a need for design improvement in certain

types. The worst hazard exists with some

old early models which are still in use. A few of these may still not have lead glass

windows. Manufacturing and installation

errors have led to omission of some lead

or lead-glass shielding and serious leaks.

Poor vacuum has led to high gun current and

high x-ray leakage. Manufacturers have had

particular trouble in designing the specimen chamber, viewing chamber and camera to be leak free.

6 . Accidents and deaths attributable to x-ray exposure . Failure to use lead glass

windows caused facial burns and conjunctivitis. A claim was made against a manufacturer with respect to death from leukemia

but the case was not proven. Two long term

electron microscopists have recently re- ported traumatic type cataracts and the

corresponding discovery of unrecognized

high leak rates at the viewing chamber.

A survey of the death certificates

of 37 electron microscopists was inconclusive because of the small sample available.

Discussion

We found only one well-organized institutional microscope monitoring program.

City, County and State inspection appeared

uniformly inadequate or inoperative. Sufficient incidents of over exposure have been

found to justify some improvement in the

x-ray safety situation. The required steps

are: (a) Agreement on maximum permissable

leakage level, (b) Agreement on monitoring

instruments and procedure, (c) Improved education about x-ray safety, (d) Legal implementation. The (Federal) Regulations for

the Radiation Control for Health and Safety

Act of 1968 is in effect, but is not being

applied to electron microscopes (only to television sets). This requires that ex- posure rates not exceed 0.5 mR/hr at 5 cm. from any point on the external surface of

the equipment. This is similar to the Euratom requirement which has already been met

by some (but not all) manufacturers. In actual effect are Radiation Protection Laws

in some states which limit the permitted

body dose. It appears simpler and safer to control x-ray leakage of the instrument than

to limit the dose received, and the Federal

Health and Safety Act of 1968 would be

suitable for conventional microscopes.

cult with the high voltage microscope and more careful shielding will be required to meet Health and Safety Act requirements.

The much higher penetration of the 1 Mev

x-ray is offset at most points by the

thicker shielding used on the column. The

effects of using unusual beam conditions

(misalignment or very high intensity) or of

fitting different accessories with inadequate shielding are, of course, liable to be more drastic than the conventional microscopes .

HVEM's are "conditioned" once or twice

a day by taking the acceleration voltage

20% higher than the voltage to be used for

microscopy. Gas discharges occur in the

accelerator tube during "conditioning" (once or twice a day) which can give rise to trans

lent high levels of X-ray radiation that

require the microscope to be evacuated. The

conditioning procedure is used to prevent

similar discharges from occuring during

normal operation and is therefore, a recommended safe procedure. Hence, in the

design of a HVEM facility, attention has to be paid to the shielding of the walls a- round the microscope so that conditioning

does not affect nearby personnel.

The EMSA Radiation Safety Committee

will make recommendations about this following a study to be made in the near future

References

--D. F. Parsons, V. A. Phillips and J. S. Lally. "Survey of X-ray Safety of Electron

Microscopes". Report of the Radiation

Safety Committee of the Electron Microscopy

Society of America (April 10, 1972) . Copies

available from D. F. Parsons.

^D. F. Parsons, V, A. Phillips and J. S. Lally, Health Physics, 26, 439 (1974).

"^Radiation Safety Committee of the Electron

Microscopy Society of America. "Handbook

of X-ray Safety for Electron Microscopists".

Available (price $1.00) from Dr. George G. Cocks, School of Chemical Engineering, Olin

Hall, Cornell University, Ithaca, N.Y. 14850.

However, the situation is more diffi224
ms SP456 (1976)

INTERCOMPARISON OF PERSONNEL DOSIMETERS*

H. W. Dickson, W. F. Fox, and F. F. Haywood

Oak Ridge NaHonai Laborat-ory

Oak Ridge, Tennessee 37830

The first intercomparison of personnel monitoring dosimeters at Oak Ridge National

Laboratory's DOSAR Facility was conducted during the period May 14-16, 1974. Ten independent laboratories and companies participated in an intercomparison of neutron and gamma-ray dosimeters used for routine personnel monitoring. The

dosimeters, which were sent through the mail, were exposed at the Health Physics

Research Reactor in three "standardized" radiation fields, which have been used

for the past several years for intercomparing nuclear accident dosimeters. In addition, a 14-MeV neutron field was used as a fourth exposure condition. The

results of the intercomparison show widely varying dose estimates; e.g., reported

values of neutron dose equivalent have standard deviations ranging from 47-102%

of the mean. A second intercomparison was conducted in a similar manner on

February 18-19, 1976; however, only a few preliminary results are available at

this time.

(Dosimetry; intercomparison; neutron; gamma; reactor; personnel)

Introduction

For the past ten years, the annual dosim1-3

etry intercomparisons at the Oak Ridge National Laboratory's DOSAR Facility have provided

an opportunity for laboratories in the United

States and foreign countries to test dosimetry

systems in simulated nuclear accident situations. These studies have been successful in developing

guidelines in instrumentation and procedures and

in establishing "standardized" radiation fields whose characteristics such as energy spectrum,

intensity, and uniformity have been measured and

accepted. The Health Physics Research Reactor

(HPRR) has been used as a pulsed radiation source. The unshielded reactor and the reactor used with

either of two shields, a 12-cm-thick Lucite

shield or a 13-cm-thick steel shield, provides

three different neutron and gamma-ray spectra.

Many experimenters over the years have

expressed interest in using the same "standardized"

radiation fields for the comparison of the response

of routine personnel dosimeters used at low radiation levels typically encountered in personnel

monitoring. Recently other groups, including the Nuclear Regulatory Commission (NRC), became

interested in the same project. As a result, the

first Personnel Dosimetry Intercomparison Study

(PDIS) was conducted during the period May 14-

16, 1974, with ten groups participating. The

participants included Brookhaven National Laboratory; Dow Chemical Company, Rocky Flats; Gesellschaft fur Kernforschung (GFK), Karlsruhe,

Germany; Lawrence Livermore Laboratory; Los Alamos Scientific Laboratory; Naval Ordnance

Laboratory; R. S. Landauer, Jr., and Company;

Savannah River Laboratory; and Union Carbide

Nuclear Division (Oak Ridge National Laboratory

and Y-12 Plant).

Radiation Sources

The HPRR and a l4-MeV neutron generator were used to expose personnel dosimeters to mixed

neutron and gamma-ray fields. The reactor was

operated in a steady-state mode at a power level

of one watt for a length of time necessary to produce a radiation dose range commonly encountered in personnel monitoring. Since dose

equivalents of a few hundred millirem are commonly encountered, this order of magnitude was

selected. In order to produce this range of radiation levels, a free air tissue kerma of approximately 40 mrad was selected for the neutron component; and the reactor operating time was

calculated based on this kerma. The neutron generator was operated to produce a similar range of radiation levels. The resultant reactor

runs were performed as shown in Table 1.

Calculations of the unshielded HPRR spectrum and the steel and Lucite degraded spectra

have been made using a discrete ordinates trans- 4

port code. The results of these calculations,

presented in Table 2, represent the reactor spectra used for this intercomparison study.

*Research sponsored by the Energy Research & Development Adm. under contract with Union Carbide.

225
Table 1. Summary of Reactor Operations for Intercomparison

Run No. Shield Power Time (min.) Fissions

1 Unshielded 1 watt

2 Steel 1 watt

3 Lucite 1 watt

During the course of this intercomparison,

the DOSAR Low-Energy Accelerator (DLEA) was

unavailable for the production of neutrons; consequently, the 14-MeV neutron exposures were made using a Phillips, sealed-tube, neutron generator whose radiation field components were not

as well known as those for the DLEA. In addition

to the 14-MeV neutrons, there was a significant exposure due to low-energy x-rays (E < 150 keV)

emitted from the vicinity of the ion source.

Experimental Details

All badges were placed on water-filled trunk portions of Bomcb phantoms. These were placed

at three meters from the reactor in the case of reactor exposures and at one meter from the

Phillips tube in the case of the l4-MeV exposure. When shields- were used, they were placed at two

meters. The placement of dosimeters on the phantoms is shown in Fig. 1, while a typical experimental arrangement with reactor and shields in place is given in Fig. 2.

Generally, the dosimeters were mailed or shipped to the DOSAR a few days in advance of the intercomparison. The dosimeters were returned

in a similar manner the day after the intercomparison exposures were completed. Exceptions to

this procedure were that dosimeters from local laboratories were hand-carried back and forth. Because dosimeters from the GFK laboratory,

Karlsruhe, Germany, arrived late, it was necessary

to make an additional exposure on an independent

but "identical" basis. The types of dosimeters

used by the participants are listed in Table 3. Information provided to the participants included

the reactor operation data shown in Table 1, the position of their dosimeters as shown in Fig. 1, and the calculated neutron spectra shown in Table 2.

5.0 9.25 X 10'

13.9 2.57 X 10^^

26.4 4.90 X 10^^

Reference Dosimetry

Sulfur pellets exposed on the reactor during

the intercomparison exposures were used to esti- mate kerma for the three-meter position. The

estimated kerma values were 36, 42, and 35 mrad

for the unshielded, steel-shielded, and Luciteshielded runs, respectively. The count rates on

the sulfur pellets were quite low, and a standard deviation of ± 20% was expected due to counting

statistics and other sources of error. Based upon

the neutron spectra that have been published"^

for the three exposure configurations used, the dose and dose equivalent can be calculated. Using the dose conversion factors given in Radiation Dosimetry^ for that section of a phantom

designated element 57, the dose conversion

factors for the HPRR spectra were calculated and

are shown in Table 4. Using the fission yield

and the calculated leakage of the HPRR, the neutron fluence was calculated for each reactor

run. By applying the previously determined dose

conversion factors and average quality factors

(QF),*^ the dose and dose equivalent were calculated and are given in Table 5.

In the case of the l4-MeV neutron exposure,

the dose equivalent was monitored by a tissue- equivalent proportional counter used in an integrating mode and placed at the approximate

position of the dosimeters. This monitor indicated

a dose equivalent of 325 mrem for the operation.

Due to variations in the angular intensity of the radiation around the neutron generator tube, it is expected that the actual dose equivalent varied

from phantom to phantom. The high x-ray exposure levels that were encountered were not anticipated, and no provision was made to monitor

them. A summary of the reference values of neutron dose and dose equivalent for the four exposures is presented in Table 6.

226
Table 2. Calculation of HPRR Spectrum for NAD Intercomparison

N(E)AE*

Group Upper Energy (ev) Mid Energy (ev) No Shield Lucite Shield Steel Shield

1 1.49 E7 1.22 E7 9.53 E7 3.31 E7 1.35 E7

oI 1 .U L/ O TO to 1 . lo CO J. 00 COto 1 .0 t/

3 6.7 E6 5.77 E6 3.43 E9 4.29 E8 3.8 E8 4 4.97 E6 3.87 E6 1.44 ElO 2.58 E9 1.57 E9

5 3.01 E6 2.12 E6 3.76 ElO 5.56 E9 7.94 E9

6 1.5 E6 1.16 E6 3.16 ElO 3.19 E9 1.21 ElO

/ y.U/ bo 6.Uo t5 4.6

1

t lU 3.69 COEv 3.34 ElO

8 4.08 E5 2.13 E5 3.39 ElO 3.08 E9 5.02 ElO

9 1.11 E5 9.80 E4 2.60 E9 4.18 E8 2.13 E9

10 8.65 E4 7.64 E4 2.0 E9 3 81 F8 9 91 F9

n 6.74 E4 5.95 E4 1.5 E9 3.49 E8 1.41 E9

12 5.25 E4 4.63 r A E4 1 .21 E9 3.24 E8 1.25 E9

13 4.09 E4 3.61 E4 9.71 E8 3.05 E8 5.61 F8 14 3.18 E4 2 81 E4 • 8.40 E8 7 98 E8 6 64 F8

15 2.48 E4 2.19 E4 7.35 E8 2.76 E8 2.5 F8

16 1.93 E4 1.70 E4 6.37 E8 2.66 E8 1.01 E8

17 1 .50 E4 1 .03 E4 1 .58 E9

"7/0 /.60 E8 1 . 14 E8

18 7.10 E3 4.88 E3 1.39 E9 7.23 E8 1 .02 E8

19 3.35 E3 2.03 E3 1.62 E9 9.48 E8 1 . 16 E9

20 1 .23 E3 8.48 E2 1.04 E9 6.97 E8 4.2 E8

21 5.83 E2 3.54 E2 1.24 E9 9.21 E8 4.47 E8 22 2.14 E2 1 A~J

1 .47 E2 r\ A c 8.45 E8 6.91 E8 3.14 E8 23 1 01 E2 6,96 Fi 7.76 E8 6.90 F8 z.? .uu 88 Fa 24 4.79 El 3 73 El 4.72 E8 4.59 F8 1 69 F8 25 2.90 E

1

2 26 PIU 1 4.54 E8 4.60 E8 1 67 F8

26 1.76 El 1.37 El 4.34 E8 4.61 E8 1.61 E8 27 1.07 El 7.34 6.09 E8 6.93 E8 2.11 E8 28 5.04 3.93 3.82 E8 4.58 E8 1.28 E8 29 3.06 2.18 4.84 E8 6.11 E8 1.71 E8

30 1.56 1.25 3.04 E8 3.79 E8 1.21 E8

31 1.0 8.06 E-1 2.81 E8 3.41 E8 9.16 E7

32 0.65 5.41 E-1 2.42 E8 2.86 E8 7.83 E7

33 0.45 2.12 E-1 1.78 E9 2.67 E9 5.63 E8 34 0.1 2.24 E-2 3.36 E9 1.95 ElO 1.09 E9

5.0 E-3

*This number is the area of the histogram for each energy nterval

.

227
BACK

Fig. 1. Typical Placement of Dosimeters on Phantom Section

228
Fig. 2. Typical Experimental Arrangemeni- for Reactor Exposures

Table 3. Dosimeters Used by Participants

Group

Dosimeter Type

Neutron Gamma

A-1 TL pair TLD

A-2 Thorium

A-3 Standard interpretation

A-4 NTA nim Film

B TLD albedo TLD

C-1 TLD albedo

C-2 NTA film Film

D TLD albedo TLD

E TLD albedo TLD

F-1 NTA film Film

F-2 TLD albedo TLD

F-3 TLD albedo TLD

G NTA film Film

H TLD albedo TLD

I TLD albedo TLD

J Fi m/TLD

229
Table 4. Neutron Dose Conversion Factors and Average Quality Factors for HPRR Spectra

Dose Conversion iipctor

Shield (mrad cm^ x

10"') QF

Unshielded 25.5 9.4

Steel 17.9 9.5

Lucite 14^6 8.9

Gamma exposures varied; but, using previous

intercomparison results, gamma doses of 5.6, 4.7,

and 34.5 mrad were calculated based on the number of fissions that occurred on the three respective runs during the operation of the reactor. In actuality, the dosimeters accumulated additional

gamma exposure from the residual activity in the reactor core. The exposure rate at three meters

from the unshielded reactor was approximately 15 mR/hr. A good estimate of the gamma dose is not possible due to the varying lengths of time

the dosimeters remained in proximity to the reactor and the varying attenuation through the

shields. However, it is reasonable to assume that 15-20 mrad could be added to the dose delivered

during the actual reactor operation. This would

suggest total gamma doses of 20-25 mrad for the unshielded and steel -shielded runs and 50-55 mrad

for the Lucite-shielded run.

Table 5, Absorbed Neutron Dose and Dose Equivalent

Calculated from HPRR Fission Yields

Reactor Run

Fluence

(cm-2 x 10'^) Dose

(mrad)

Dose Equivalent

(mrem)

1 1.82 46.4 436

2 3.11 55.7 529

3 2.60 38.0 338

Table 6. Reference Values of Neutron Dose and Dose Equivalent

Run Spectrum

Dose (mrad) Dose Equiva ent (mrem

Cal culated Measured Calculated Measured

1 Unshielded HPRR 46.4 36 ± 7.2 436

2 Steel -shielded HPRR 55.7 42 ±8.4 529

3 Lucite-shielded HPRR 38.0 35 ± 7.0 338

4 14 MeV 43.9 325

Results

The results of all participants are given in Tables 7 and 8 for the four exposure configurations used during the intercomparison. The averages of the participants' estimates were 453

± 213 mrem for the bare reactor, 554 ±346 for the steel -shielded reactor, and 675 ±687 for the Lucite-shielded reactor. Some of the participants

gave several dose estimates either due to the use

of multiple dosimeters or due to various means of interpreting their results. The average includes

all estimates that the participants claimed to be

valid, even those taking into account actual knowledge of the spectra. The results of the GFK

laboratory (I) are included but not averaged with

the others. This is because the GFK dosimeters

arrived late and had to be exposed at a different time but under "identical" conditions. The operation of the reactor should be reproducible to within a few percent, and the GFK results should

be in good agreement with the others for the three reactor runs. In the case of the 14-MeV

exposure, the conditions for GFK were altered byl

placing a lead shield around the sealed source

tube to attenuate the low-energy photons (^150

keV) from the accelerator. This explains the significantly smaller gamma dose reported by GFK.

230
ible 7. Results of Personnel Dosimeter Intercomporlson Expressed in Dose Equivalent (mrem)

Uns h I e 1 d e d Reactor S te e -Shie Ided Reactor

Phantom

DEn

No. 1 Phantom No. 2 DEn DEyPhantom No. 3 DEn DE^Phantom No. 1 DE^Phantom No. 2 DE, DE^

Phantom No. 3 DEn DE^

582

520

435

350

25—

—

350

—

30

—

350

—

30

612

484

223

330

15

20

—

330

—

20

—

330

—

20

510 22 510 22 550 11 550 n

307

307 20°

378

302

~2°

380 23 350

540

23

20

370 23 410 16 410

690

1720

420 17

140 952

662

3123

23

140 865

594

23

25

26

160

921 512

23

28

19

50

1120

1050

19 24

22

20

1112

1144

1917 2040

1259

870

19 24

21 220

420

30

30 405

383

25

35

395 25

35/14^

400

565

20

20 690

402

20

14 620

28/1

5^"

470 25.9 429 24.0 460 24.3 527 18.6 564 18.1 584 19.4

I l\J I [ 1 1 1 CI 1 '^First number given is ba ;ed on fi m dosimeter; second number is based on a TLD.

3ble 8. Results of Perso nnel Dosimeter Intercom| Dorison Expressed in Dose Equivalent (mrem)

Lu c i t e - Shield ed Reactor 1 4-M eV Neutron Generator

^hantom No. 1 CE, DE,

Phantom No. 2 DEn DE^Phantom No. 3 DEn DE^Phantom No. 1 DE DE^

n Y

Phantom No. 2 DE, DEy

Phantom No. 3 DEn DE^

431

451

434

400 90 400 90 400 90

587

220

505

490

587 505 587 505

383 51 383 51 435 435

443

297 ^3°

308

283 307°

420 76 400

380

70

60

440 69 1600 310 1400

390

160

600

1400 120

100

2515

870

89

95

89

140

2437

1000

89

84

83

90

2422

1272

89

86

97

500

405

85

65 405

418

65

65

425 65

67/54''

100 315700

320 225

341

265

24^

225 270

375

628 80 656 75.5 776 74.2 564 460 532 367 737 341

not mrem First number is based on film dosimeter; second number is based on a TLD.

ce shielded with lead 231
Summary and Conclusions References

A summary of the results is presented in Table 9. It Is reasonable to expect a more fa- vorable agreement between the several participating laboratories if the results of experimental

devices and nonroutine dosimeters are ignored or

if a selective data handling technique is used. For example, if the extreme data points for each

of the phantoms in Tables 7 and 8 are excluded,

the resultant average dose equivalent estimates

are 431 ± 1 12, 539 ± 238, and 501 ± 240 mrem,

respectively. Also, if the upper and lower extremes are excluded from the l4-MeV results, the average neutron dose equivalent becomes 409 ±

154 mrem.

1. F. F. Haywood, 1970 Intercomparison of Nuclear Accident Dosimetry Systems at the Oak Ridge National Laboratory , ORNL-TM3551 (Feb. 1972).

2. J. W. Poston and F. F. Haywood, 1972 Inter - comparison of Nuclear Accident Dosimetry

Systems at the Oak Ridge National Laboratory,

ORNL-TM-4387 (July 1972).

3. H. W. Dickson, F. F. Haywood and K. Becker,

Tenth Dosimetry Intercomparison Study, August

27-September 7, 1973 , ORNL-TM-4566 (Mar.

1975).

This study was found to be valuable to the 4. participants, and the wide range of results indi- cate some problems with the response and/or the interpretation of the dosimeters used. The second

Personnel Dosimetry Intercomparison Study was

held February 18-19, 1976. The reference dosim- 5. etry was upgraded to provide for more accurate

low-level gamma-ray as well as neutron dosimetry.

At this time, only preliminary results are available

and it has not been determined as yet if the re- 6.

suits are in better agreement than those of the

first study. This addition to our dosimetry inter- comparison program was judged to be worthwhile,

and plans are under way to continue these studies

in the future.

J. W. Poston, J. R. Knight and G. E. Whitesides, "Calculation of the HPRR Neutron

Spectrum for Simulated Nuclear Accident

Conditions," Health Phys. 26, 217 (1974).

F. H. Attix and W. C. Roesch (eds.), Radiation Dosimetry (Academic Press, New York,

T968r~

M. S. S. Murthy, R. C. Bhatt and S. S. Shinde,

"Estimation of Quality Factor and RBE for Degraded Fission Neutron Spectra," Health

Phys. 27, 9 (1974).

Table 9. Summary of Results

Spectrum

Neutron Dose Equivalent

(mrem)

Gamma Dose Equivalent

(mrem)

Bare HPRR 453 ± 213° 24.6 ± 5.9

Steel-shielded HPRR 554 ± 346 18.1 ± 4.3

Lucite-shielded HPRR 675 ± 687 75.1 ± 14.2

14 MeV 587 ± 501 384 ± 151

°Error given for one standard deviation

232
MBS SP456 (1976)

EWIROmEmAL EADIATION MKA^UEEMEITTS

James E. McLau-gtilin

Health, and Safety Laboratory

U. S. Energy Research, and Development Administration

New York, K.Y. 10014-

Determinations oi radiation dose rates and radionuclide

concentrations depend on systematic considerations of

detector characteristics and instrument system calihrations

,

as well as the composition, source distributions, and fli:).xenergy and angle distributions of the environmental radiation field. Routine in situ measurements around nuclear

facilities are made primarily to monitor changes in certain

radiation parameters. Even these measurements should be

based on absolute determinations in order to attribute

changes correctly to the source. The systematic considerations required for monitoring environmental sources are discussed and realistic examples of gamma-ray measurements

are presented along '...^ith a suggestion for beta ray measurement . (Environmental gamma-rays, source distributions, instroments, calibration,

field testing)

Introduction

Environmental radiation measurements are made for a variety of purposes, but primarily to aid in as- sessing man's exposure to environmental sources. Measurements made

in connection with nuclear facilities monitoring provide data on measurable radiation and radionuclide

levels. Then relationships between

radionuclides in effluents and the

consequent doses to individuals or population groups can be established

or verified and any radionuclide

concentration trends detected.

Many of the research and measurement efforts in this field have

been described at numerous meetings,

notably at the Natural Radiation

Environment Symposia in 1954 and

1972^, 2_ Whether external radiation

measurements are male because of

scientific curiosity or for practical purposes, e.g., geoscientific

surveys or nuclear facilities monitoring, the problems are complex and

formidable, and should be addressed

in a systematic fashion.

The employment of hand-held

survey instrum.ents, ionization chambers, thermo luminescence dosimeters,

and gamma-ray spectrometers for in

situ measurements of the external

radiation field is reviewed and the

validity of such measurement is discussed. Some of this information

has been developed for the National

Council on Radiation Protection and

Me asurement s .

3

The basic problem is that there are

no environmental external radiation

calibration standards. One must take

account of the instrument and radiation field characteristics that are considered integral parts of a measurement system. The system is then

"calibrated" by relying on a complex

and sometimes tenuous argument supported by theoretical considerations

and independent measurements. This

is often difficult when one wishes to

quantify one of the smaller and often

variable components of environmental

radiation, e.g., that due to manmade

radionuclides.

Theoretical Considerations

The two main contributors to the

penetrating component of an environmental radiation field are cosmic

rays, mostly muons, and gamma rays,

mostly photons from terrestrial

sources. The exact composition and

intensity of the field, in terms of

either the ionization rate in air

233
near the ground or absorbed dose

rate in free air, vary with time and

with location. The detection and

quantification of either these natural changes in environmental "background or changes attributable to

the addition of manmade sources depends on the degree to which one can determine the responses of radiation

detectors to photons and muons. In most applications, one can

subtract the response to cosmic rays

corresponding to the measurement altitude from the total detector response. If the latter is expressed

as a free air dose rate, the values

for the mid-geomagnetic latitudes

corresponding to the continental

United States shown in Figure 1 can be subtracted. 3 ,^ These values were inferred from measurements of cosmicray ionization by several investigators. The data neglect the effect

of changes due to solar modulation

during the solar cycle, solar flares,

and atmospheric temperature and pressure. 3 At or near sea level, i.e. in

most inhabited locations, these

changes are usually less than 10 percent in magnitude and are negligible

for many measurement programs.

T'ae determination of the cosmicray response of a detector is not a straightforward process. The

charged-particle equilibrium in the

detector that exists for terrestrial

photons may not exist for cosmicray muons and electrons. So one must either perform a complex detector evaluation based largely on theory or evaluate the response em- pirically as a function of altitude

as indicated in the following

section. 3,,'4-

The gamma-ray calibration of a radiation detector with a known point

radionuclide source, such as 226Ra, or a known photon beam, is applied to

convert the field measurement to a convenient parameter like dose rate.

This procedure can be seriously in error if the detector response is

strongly dependent on the (photon)

Energy or if the response is anisotropic. The major environmental

sources are the 23Su and the ^^^Th.

series and ^OK in the ground. The

.latter two contributors tend to dominate the gamma-ray absorbed dose

rate in free air about one meter

above ground, the height usually em- ployed for convenience.

ALTITUDE (km) 30 25 20 15 10 0 5 00

20 ' ' <

' ' '

' ' 700 750 800 8 50 900 950 1000 1050 ATMOSPHERIC DEPTH (g crn'M

Fig-ure 1. Absorbed dose rates in

free air from cosmic-ray charged

particles at geomagnetic latitude

50°K (Ref. 4).

The calibration problem can be

illustrated with a comparison of the

energy spectr^um from a sealed source

of 22o;Ra with that in a "typical"

field situation. Typical contributions to dose rate were derived from

many field measurements made in the

eastern United States. 5 The measurements were coupled with theoretical

calculations of the radiation field

above a half-space containing the sources shown in Table 1, under the

assijunption that the series are in

radioactive equilibrium. Disequilibrium can exist for 238tj ^^ot in

any important way for 232t]2^ because

of the short h? If-life of 220Rn.

The energy spectrum for this

typical situation is shown _in Figure

2 as the "boxed" histogram." The

spectrum from a 225Ea source, shown

as the "shaded" histogran, is much

harder, so the dependency of the

detector response on energy cannot

be neglected.
Tablel . Goiitri"butioTis to Pose Rate

' One Meter Above Grouiid

Absorbed Dose Rate

Raiiation Source in Free Air (jirad/h.)

^0,

233^.Tj + daagbters

2.2

1.2

2.6

0.6

^0.1'

P^2

Th. + daugliters

-5^Gs, "^^Zr - ^^ITb

222Rn daughters m axr

TO'TAL

222

a. The Rn daughter contributxon

is vaxiable according to the

soil tjrpe and local atmospheric

conditions.

6.7

ENERGY INTERVALS, keV

226 Figure 2. Comparison of Ra calibration source energy spectrum and

that from enviro2amental natural gamma

rays (Latter from Ref . 5)

The angular distribution of the

photon flux from the ground, the predominant environmental source, is

also a significant consideration in

field measurements. One can often

characterize the half-space source as having a -uniform distribution of natural emitters with depth and an ex- ponential di'stribution of deposited

manmade nuclides or recently deposited natural emitters. The calculations show that a significant

fraction of the photon flux incident

on a detector is at large cLigles from

the normal. Figures 3 and 4 show the

angular distributions from a EO-keV

and a 652-keV source at 30 cm. and

100 cm. above the ground, respectively. The a values represent the

vertical distributions in the ground

and are in effect reciprocals of re- laxation length. So, a = 0.10 cm~l

represents a deeply distributed 60

keV source and a = 1000 cm~l approximates a plaie source.

The angular photon distributions

illustrate the problem of improving

detector response merely by increasing detector size. Only a small

fraction of the flux originates from

sources directly beneath the detector; most of the flux is incident

from angles closer to the horizontal

and hence plane detectors may not be

desirable. This observation, for

example, affects an improvement of

plane, photon detectors designed to

measure actinides deposited on the

ground based on an increase of the

plane detector area. An ideal detector should be one with its sensitive

surface oriented to intercept photons

from angles of 60 - 90°.

The a = 0.33 cm""*" curve for a high energy photon emitter like 137Gs

represents a typical distribution

from world wide fallout in a location

that has not been disturbed. Here

about one half of the flux is incident from angles above about 70°

and, consequently, a detector placed

1 meter above the ground in effect

"samples" a large volume of ground,

perhaps one that is tens of meters in

diameter.

Calculations have also shown

that virtually the entire dose rate

at 1 meter above ground with a uniform distribution of the, natural

emitters is attributable to about the

top 25 cm. This is significant in

selecting a measurement location, because one only needs to test that the

horizontal variation across the 10 - 20 meter area being measured is minimal with a survey instrument. This

simple test and considerations of source distributions in the ground

and flux distributions have indicated

that in situ spectrometry can be em- ployed in environmental and geological survey to quantify soil concen- trations of gamma-ray emitting nu235
elides. The circumstances for tliis

application have heen reportedS and

are revie-^^ed in a later section.

ANGLE, e IDEGREES)

Figure 3. Ang-alar distribution of

photons at 30 cm above 60 keV

source in the ground for two vertical source distributions (Ref .7)-

ANGLE, e (DEGREES)

Figure 4. Angular distribution of

photons at 100 cm above 652 keV

source in the ground for three

vertical source distributions

(Ref. 8).

The calculations we have em- ployed to characterize the typical

gamma-ray field were based on a soil

composition with 10 percent water by

weight. The water content can vary

widely, of course, but a comparison

of mass attenuation coefficients for

dry soil and that containing 25 percent water indicated the resulting

exposure rates or dose rates would

vary by approximately 5 percent, a value that is within the estimated

uncertainty of the calculations and

also the field measurements.

For certain applications, the

effect of airborne radon daughters on detector response is not negligible. Variations of dose rate in

the lower atmosphere can be substantial and should be evaluated in con- nection with measurements on aircraft and around nuclear facilities

Theoretical calculations have been

made to identify the magnitude of

these variations at heights above

the ground between 1 meter and several hundred meters. Figure 5 depicts the relative contributions to

the ionization rate in air for the

principal environmental sources. The

contributions from radon daughters

for the extreme meteorological con- ditions are a small part of the total

but they vary by about a factor of

four, the max:imum being well within

1 [arad/h at ground level.

Theoretical treatments of the

type identified here have been re- ported (see for example Eefs. 5 a^id 8) . The tabulated energy aad angle

distributions can be folded into

measured detector response functions

of energy and angle to obtain cor- rection factors for detectors with

non-flat or anisotropic responses.

Survey and Continuously-Monitoring

Instr'uments

Instruments employing scintillation or G.M. counters have been

widely used for measuring radiation

from environmental sources. However,' one should not be misled by the apparent simplicity of survey instruments. Quantitative measurement re- quires that one account for contributions from all sources of penetrating radiations, interference by

any beta-ray component, and departures from linear energy response.

A practical instrument should:

a. be sufficiently sensitive to

allow the measurement of environmental gamma rays down to about 1

txR/h,

b. have a sufficiently short

time constant to permit the making of

many measurements, but sufficiently

long to minimize rapid fluctuations,

c. have a uniform and nearly

isotropic response over the gamma-ray
rziergy range oi inheres u,

d. "be insensitive to climatoiDgical 3h.a:-iges, e.g., temperature

cnanges, and

e. "be small and liglatv/eigiit

,

i_f it is to 'oe used for hand->ield

r jrveys.

TOTAL (INVERSION)

RADON DAUGHTERS (INVERSION)v^

RADON DAUGHTERS (NORMAL TURBULENCEJv

10 100

HEIGHT A80VE SEA LEVEL(m)

figure 5- Calculated gamma- and

cosmic-ray ionization rate in free

air as a function of iieight alcove

fae ground (Ref. 5).

Th.e responses of organic scintillators have been extended to lowenergy pliotons, as required for measuring many environcaental gamma-ray

fields (see previous section), "by tiie

addition of iiigh.-Z material . 5 » 10 Pqj.

example, coating anthracene with a thin layer of ZnS(Ag) extends its

"flat" response down to a"'oout 50 keV.

Figure 6 illustrates the combined

response of these materials in terms

of exposure rate. The detector response belov; I3OO keV is energy independent v/ithin about ± 15 percent,

nen employed in an instrument having

a D.C., as opposed to a count rate,

output. A similar response and a detection limit of about 1 [iR/h has

been achieved with a tin-loaded

plastic detector; the digital data

readout of this instrument makes it

suitable for aatomatic data recording

and unattended monitoring. 10

Inorganic scintillation detectors, e.g., K"aI(T^.), show enhanced

low energy responses because of their

greater effective atomic numbers.

Interpretation problems resalt from

departures from uniform response of

this type of detector aad the greater sensitivity to low energy photons.

The tacit assumption is often made

that the total counting rate above

some discriminator threshold level

is proportional to exposure rate,

but the reduced low-energy response

should be assured by setting the discriminator sufficiently high so that

any gain shift or zero shift has a negligible effect. H This is importa:at because the environmental low

energy photon flux density is relatively large and is variable (see

Figure 2). 8 Hence, the determination

of exposure rate based on calibrations with radioactive sources such

as 137Cs or 226Ra ca:a be seriously

in error (see previous section).

9 1.0

5 10' INCIDENT PHOTON ENERGY (keV)

Figare 5. Gamma-ray response

function of ZnS-plastic combination

scintillator (Ref. 9).

Field measurements of radioactive areas in Colorado and on Bikini Atoll have illustrated the

problem of directly applying point

source laboratory calibration data to

ITal(Ti) responses. The detector response to one of the usual highenergy point sources per unit ex- posure rate will probably be smaller

than the corresponding value for the

largely scattered and low-energy

gamma-ray field. In the cases mentioned this led to overestimates of

the field exposure rates by factors

of 1.5 to 2.12,13

The NaI(Ti.) detector sizes are often 2.5 X 2.5 cm or only slightly

237
larger and the high-energy response

may, therefore, be energy dependent.

The use of these detectors for special studies, e.g., the environmental

measurement of 6 - 7 MeV photons

from produced in direct-cycle

light-water power reactors, can lead

to significant underestimates, if

they are based on the incomplete,

laboratory calibration described

above

An acceptable calibration is to

determine the response of the survey

instr-oment to known sources in the

laboratory and compare this response

with a well-calibrated ionization

chamber. This procedure should ac- count for the fact that photons from

the point source, unlike those in the

environment, are not generally isotropically incident. A convenient

method is to base the calibration on simultaneous in situ measurements

for different " environmental radiation

fields. In these procedures one needs to account for differences in

response of each detector to cosmic

radiation.

Ionization chambers are attractive because measurements with this

instrument have been made for many

years in connection with cosmic-ray

research ajid the early environmental

monitoring programs. ~ -'-7 These

measurements were historically interpreted in terms of the ionization

rate in free air and then converted

to estimates of exposure rate or absorbed dose rate in free air, based

on reasonable assumptions.

lengthy series of high-altitude,

balloon-borne experiments. 17 The

small, solid-state electrometer was designed for minimal variation with

temperature in the range encountered

in most' field worklS. The system

finally evolved for monitoring environmental gamma rays has the characteristics listed in Table 2.

Table 2 . Nominal Design Specifications of HPIC System.

Chamber wall: 0.30 cm. (2.36

g/cm2) stainless steel.

Chamber volume: 8 H (25 - cm dia.) sphere.

Chamber filling: 20-25 atm.

ultra-high-purity argon.

Collecting voltage: 300 V.

Electrometer: HASL-designed

solid-state electrometer

(Reference 18)

Data recorder: ADC to scaler,

voltmeter to strip-chart or cassette tape.

The pulse-sealer output is used

for rapid situ exposure rate measurements and the cassette tape output

for continuous unattended monitoring.

The detector employed with each

system should be manufactured to the

specifications given, and evaluated

and calibrated to avoid departures

from the desired characteristics.

Problems of insufficient sensitivity and reliability, as well as the unwanted response to internaLl

radioactive contamination, limited the use of air-filled chambers aiad many

low-pressure chambers. These problems are largely alleviated by the

employment of high-pressure ionization chambers (HPIC) of the type

employed by Millikan, Spiers and

others, l^r 16 rptie upjc exhibit suitable signal-to-noise ratio, discrimination against beta rays, greatly

reduced response to radioactive contaunination, and remarkable resistance

to environmentcLl stresses.

Several types of this instrument

were developed and employed in a

The response of this or any

chamber to the environmental gammaand cosmic-radiation field is represented by

E k I + k I

c c Y Y

+ R

a' (1)

where R is the system response in appropriate current, voltage, or pulse

rate units, I and I are the con- tributions from cosmic and gamma

radiation, respectively, R^ is the

response to any internal radioactive

contamination, and the k factors are the conversions to absorbed dose rate

in free air or to exposure rate. A

HIPIC is considered unacceptable if

exceeds a value that is equivalent

238
to about 0.1 ;iR/ii iron an external

gamma-ray source. Analyses of tiiese

and otner unwanted effects have "been

described. 19, 20 q^oe two main calibration problems are described

siiaply in tenns of tJae parameters in

equation (l) . In tiiese non-airliie

detectors, kc and ky are usually not

equal and there is no knovm

"standard", pure, cosmic- radiation

field for a direct determination of

kc- Th.e factor, ky, depends on

energy so the use of a value derived

from a laboratory calibration with

point sources or beams may not be

valid for field measurements.

(s.d.), is due to the small time and

space variations in cosmic-ray ionization rate in the mid-latitude s7 , 10

lihe uncertainty in I^, is similar

although some of the literat"are indicates the error in I^, may be some- what larger. 22, 23 However, Ic is

generally a minor contribution to the

total ionization rate so the larger

error would increase the uncertainty

only slightly for most applications.

For the nominal HPIC in Table 2, k^

equals ky, within 2 percent, though

one should not ass^ome this equality

pertains for other HPIGs or other

detectors

.

The determination of k^^ derives

from a complex combination of both

theoretical calculations of the

interaction of muons and electrons

in the chamber, as v/ell as the con- sistency of the result v/ith many

field meas;irements made at various

altitudes and v^ith data in the literatare (see reviev; in Reference 20).

The theoretical calculation takes

account of the slight attenuation in

the steel wall of the cosmic-ray

electrons and the net buildup of

electrons passing through the chamber because of pair production interactions. The net build-up or transition effect in our chamber was calculated to be about 20 percent. 21

Since the electrons make up roughly

one-half of the total chargedparticle flux in the lov^er atmosphere and the muons are practically

unattenuated by the chamber viall,

the net enhancement of the ionization current from the chamber wall is

about 10 percent. The estimated

value for the chamber in Table 2,

~5xlO~l^ amperes per [iR/h, is consistent v/ith the result derived from

many measurements of nearly pure

cosmic ray and mixed cosmic- and

gamma-ray fields. In these measurements R was determined by subtracting

ly and ky values obtained from in

sxtu spectrometry and laboratory calibrations. By normalizing an experimental kc Ic curve as a function of

altitude to I^ = 2.1 ion pairs/cm3-

s-atm. at sea level for STP, an IqVS.

altitude profile, corresponding to

the dose rate profile in Figure 1,

and values for k^, were derived. 17

The uncertainty of the product kp

Iq, estimated to be about ± 5 percent

The gamma-ray calibration of any

detector is accomplished in a more

direct fashion. First, the response

to primary photons from a known point

source is obtained in the laboratory.

It is necessary to subtract the response to scattered photons in a particular location (obtained by

shielding the detector from the primary photons) from the total response

obtained vjithout a shadow shield.

Then, the resulting ky should be cor'- rected for differences in the calibration source energy spectrum and

that expected in the field. The

field spectrum is "softer" than the

laboratory, so the laboratory ky for

the particular HPIC must be corrected

to obtain a ky for the expected field

spectrum. 20

The overall accuracy of this instrument for measurements of gammaray or gamma plus cosmic radiation is

about ± 5 percent (s.d.). This es- timate derives in part from the con- sistency of ion chamber and gamma-ray

spectrometric measurements in many

different radiation fields, that is

discussed in the next section.

Useful comparisons with calculated responses of real chambers have

been made that take account of the

effect of wall attenuation and buildup, as well as energy absorption in

the argon gas as a function of photon

energy. The comparison is shown in

Figure 7 where the response on the

ordinate, F, is

J(E,.,PY) = B^e—^-7^. (2)

^ . Mr

239
and [J-g/o is the mass energy absorption coefficient, [i the linear absorption coefficient, x the mean

thickness of the steel wall for isotropically incident photons, and By

the ratio of ionization in argon due

to all photons to that produced by

the primary photons from monoenergetic sources. The competing effects

of the vsarious factors in equation

(2) yield a fairly flat energy response for the 2.3& g/cm2 wall chamber, while the thin-wall (0.40 g/cm2)

device is obviously unsuitable.

For the thick-wall chamber in

Figure 7, the ky value for the typical natural gamma-radiation field in

Figure 2 is 2.5 x 10-1'^ amperes per

^iR/h, a value that is only 1.03 times

that obtained from the laboratory

calibration vjith a 226Ra source. Because of the non-uniform response of

the thin-wall chamber, the laboratory

ky must be multiplied by 1.23.

DIA.O^e-g/cm^ THICK STEtL CHiiMeER,

ENERGY (keV) Figure 7. Gamma-ray response

function thin wall and thick wall

high pressure argon ionization

chambers (Eef. 20).

Thermo luminescence Dosimeters

The thermo luminescence phenomenon

has been widely exploited for environmental as well as for geological

measurements. This extensive technology has undergone intercomparison

tests under environmental conditions

and guidance on performance specifications and testing procedures was recently recommended in a national

standard24i25, j\,s for any field instrument, thermoluminescence dosimiters (TLD) should be little affected

by environmental conditions. The TLD

should be protected against moisture

and the response corrected for the

effects of any trace radioactivity in

the phosphor or its packaging and

large temperature changes on the

measured response. This is accom- plished by relying on special control

dosimeters and, in some cases, on temperature profile data for the

usually lengthy exposure times. The

national standard offers partial

g-aidance on a test aimed at developing suitable correction factors for

thermoluminescence fading. The em- pirical determination of fading and

self-irradiation corrections is difficult and time-consuming, so one is

often inclined to rely on approximate

corrections

.

An indication of the attainable

overall accuracy is indicated by a comparison of 7LiF TLD and HPIG measurements obtained from the HASL monitoring site in Lloyd, New York, near Poughkeepsie . The results in

Figure 8 cover about fifteen months

and are reported as the average ex- posure rate for each four-week monitoring period. The measurements

were made and analyzed according to

the procedures described in Section

C-01 and C-04 of the HASL Procedures

Manual and they, of course, resemble

the guidance in the national standard. 25, 26

Figure 8. Average environmental ex- posure rates from TLD and HPIC

measurements at Lloyd, New York.

Tne error bars in the figure

(standard deviations) represent the

accuracy of the field measurements,

if one were to neglect the small

laboratory calibration errors. The

precision of the ionization chamber

measurements is not indicated, but

is about ± 1 percent (s.d,). A meas- ure of agreement between the two de240
1

Tectors '.vas taken to be the diiier-

- zoe "between tiie average exposure

rates divided "by the ionization cliamI ber result. Th.e average agreement

xor tlie total period indicated in

Tigure 8 is within 0.5 percent and

zhe mean diiierence lor ^- - week

I measurement periods is 2.7 percent,

j It is believed that the overall acI curacy for these type of terrestrial

{ gamma ray plus cosmic ray measurej ments is about ± 5 percent (s.d.).

I A part of the uncertainty is attrib-

: uted to the relative responses of

I TLD, LiF in this case, to cosmic-ray

' muons and electrons.

The recent international intercomparison has afforded additional insight into the TLD capabilities as now employed. In this test fortyone organizations provided TLD sets

for expos'ure to a gamma-ray calibration source and to the nat'ural radiation environment for three months

at a location near Houston, Texas.

The agreement indicated by all of the

dosimeters was 28 ± 5

^(s.d.) for

the laboratory exposure and 16 ± -4- hlR (s.d.) for the field. It is interesting to note that about one- fourth of the participants corrected

for fading by relying on either temperature profile data or by controls,

while the majority made no correction.

A general evaluation of error propagation has been developed for

TLD and other integrating dosimeters

and is reported at this symposium. 2?

Thermally-stimulated exo -electron

emission (TSEE) is a detector surface

phenomenon related to thermoluminescence that may be suitable for en- vironmental use. Results from the

intercomparisen and more recent field

tests indicate that TSEE is a promising development, but difficulties

arise during exposure that are attributed to moisture on the detector

surf aces. 23

In Situ. Gamma-Ray Spectrometers

Field spectrometry does not yet

fit the technical and economical re- quirements for routine environmental

monitoring. However, spectrometry is

important, and possibly necessary for

special environmental and geological

surveys. It aids in determining the

radionuclide composition of environmental sources and their relative

contributions to the radiation field.

In some cases, one can quantify

radionuclide concentrations in the

ground and monitor the changing a- mount of enviroimental moisture and

its agricultural importance . 8, 29 , 30

In situ spectrometers usually have

minimum responses to cosmic-ray muons

muons, so the gamma-ray responses can be employed to evaluate the responses

of radiation detectors or to provide

independent verification of these re- sponses (see previous section).

Moderately-large (10 x 10 - cm)

ITal(Ti) and large (60 - IJO cm3)

Ge(Li) detector systems have been

successfully employed by a number of

organizations for radiation measurements in the general environment and

around nuclear facilities. Measurements of nat'ural emitters with uncollimated Nal(T.O detectors take

several tens of minutes, vjhile

Ge(Li) measurements can usually be

accomplished in an hour or less.

For example, the several pulse height

spectra obtained at the same location, but at different times', near

a boiling v;ater power reactor were useful in assuring that the response

of a HPIC monitor was attributable

to the noble gas plume. 3, 30

The areas of the measured photopeaks are proportional to the flux

densities of uncollided photons from

particular radion^aclide emissions.

If the source geometry is reasonably

well known, these photopeak areas can be correlated to individual con- tributions to the exposure rate

above groimd or the absorbed dose

rate in free air. The correlations

depend on the radionuclide distribution in the ground and the energy

and angle spectra above it. An important advantage of in situ spectrometry is that an uncollimated

detector above the ground in effect

samples a very large volume of soil

and comparable counting statistics

are obtained in a small fraction of

the time required for the laboratory

spectrometric anaJLysis of a collected

sample

.

241
A limitation of in situ spectrometry is that the accuracy of determinations of the exposure rate above

the ground or radionuclide concen- tration in the ground depend on the source distribution and compositionP

Determinations of radionuclide con- centrations depend strongly on the

distribution with soil depth, 'while

exposure rate determinations are much less dependent.

The detector response in terms of

total counts in an absorption peak

from the uncollided photon flux density is determined by calibrating

the detector in the laboratory with

small point sources. The analysis of an in situ spectrum and its relation

to exposure rate or concentration depends on the following parameters:

(N /cp) = the count rate in a partic-

° ' ular photopeak per unit flux

density of photons of

energy, E, incident along

the detector axis of synmetry;

(N„/N = the ratio of photopeak count

° rate in the field, where

these photons are not necessarily incident along the

detector axis of symmetry,

to the count rate from an equal flux density along

the detector axis. Eor a uniform angular response

Nf/^Q is unity. This ratio

depends on the source

energy, distribution and

soil properties;

(cd/C) the total uncollided flux

density at the detector per

unit soil concentration

(pGi/g) or mCi/km) of a particular nuclide as a function of energy, source distribution, and soil properties;

(X) = the exposure rate in [xR/h

at one meter above the

groTjnd from all photons

from a particular nuclide

including the secondary

photons produced in the soil

and air; and

(cp/X) = the ratio of the flux

density at the detector due

to photons of energy, E, from

the decay of a particular nuclide and any daughters to the

corresponding total exposure

rate for that nuclide and its

daughters.

Then the photopeak count rate is

related to the exposure rate in air

above the ground by:

(W^/i) = (Nf/N^) (N^A) (cp/X), (3)

where the units are min-l/|j,R h~l,

and, similarly, to radionuclide con- centrations in the ground by

(N^/C) = (N^/N^) (N^/cp) (cp/C), (4)

and the units are min-l/pCi g~l or mCi/km2.

Tne high resolution of Ge(Li) detectors allows one to measure absorption peak areas with little interference from neighboring peaks

as well as several peaks from, the

sane nuclide. However, the counting

efficiency is somewhat lower and

slightly longer in situ counting

times are required to obtain statistical precision comparable to that

of Nal(TjJ).

The first two terms in Eqs. 3 and

4 represent the angular response

correction and counting efficiency,

respectively, of the particular detector. The ratios cp/Z and cp/C depend only on source composition and

geometry and can be used for any detector. Some values for these parameters are given in Ref . 8 and other

values can also be inferred if one employs decay scheme data.

The most important parameter for

determining the exposure rate attributed to a particular. radionuclide or

natiaral series is 9/X. Model calculations have shown that while cp and

X vary over a range of about ten for

different ground source depth distributions, the ratio cp/X is nearly flat

from a plane - to a deeply-distributed source. 5 Thus, even if one has

only a poor knowledge of the actual

(Jepth distribution, the error in the

X inferred from field measurements is

reasonably limited.

242
This lias "been illustrated by the

comparison of spectrometric measurements of nat^j.ral emitters and independent HPIC measurements. The sum- med spectrometric photopeak contributions usually equal the HPIC re- sults within about 5 percent. 8 Such

data have been used to estimate the

overall accuracies of exposure rates

ietermined from spectrometric measurements of sources in the ground. ^2

For Ge(Li) spectrometry, the estimates range from six percent (^Ok,

-32Th series) to twelve percent

(25Su series). The poorer accuracy

of the uraniiim series is related to source geometry -uncertainties caused

by radon migration.

In situ determinations of radionuclide concentrations in the ground

are more difficult and their ac- curacy depends strongly on the ac- curacy of cd/C which in turn depends

directly on the vertical source disI tribution. Test measurements with

NaI(T^.) and comparison with laboratory analyses of collected soil

j samples have indicated agreement

within a factor of two for 137Cs when

the vertical distribution is ass-umed

and within twenty percent when it is

ietermined for the sample collection

site. 8 Field tests in the Livemore

Talley were made with an unshielded

70 cm5 closed-end coaxial Ge(Li) detector. 50 Soil samples were collected in increments to a depth of 25 cm

to determine the vertical distribution and the total activity for the

radionuclides inferred. The results

from one location used for field calibration, shown in Figure 9, indicate as expected that natural radioactivity is distributed uniformly in

undisturbed places, while 137Cs from

world-wide fallout decreases exponi entially with depth. The average

concentrations for the thorium and

uranium series and for ^Og and 137Cs

jj from the in situ and laboratory anI alyses agreed within a few percent.

In cases where the depth distribution

I was not measured, agreement was within 15 percent for the natural, uniformly distributed nuclides, and

within a factor of 2 for 137Cs.

A similaj? experiment was made at

the HASL monitoring site in Lloyd,

New York as part of a long-term regional monitoring effort. 31 Soil

samples were collected in increments

to about 20 cm depth, stones were removed, the soil dried and the con- centrations of ^Ok the 23Su and

232Th series and l57cs determined

with a large field Ge(Li) spectrometer mounted temporarily in the

laboratory. The depth profiles were qualitatively similar to the profiles

in Figure 9. Given the radionuclide

concentrations and assuming the in

situ (soil plus rocks) composition,

the exposure rate above the in situ

soil was calculated to be 8.2 ± O.-^- This value assumed 15 percent 222r^^ migration from the soil,

but accounted for the resulting

radon daughters in the air. The ex- posure rate was determined from an independent HPIC measurement to be

8.3 - 0.4 [iR/li. The former error is

based on laboratory counting statistics (s.d.) and latter is the es- timated HPIC accuracy (s.d.).

:

1 1 1

r.

1 ;

N

0 5 10 15 DEPTH, cm

20 2

Figure 9. Measurement of radionuclide concentration with soil depth

(Ref. 30).

Field Tests of Instruments and

Desirable Developments

The determination of dose to man

is the major one of the three principal purposes of measurements identified in the introduction. While

doses are often calculated, assess- ments for nuclear facilities ought

to depend in some way on measurements

or on experimentally - verified com- putational methods.

243
Table 3. Estimated Noble Gas Plume Exposure Near a BWR Obtained from

HPIC Monitoring Record (Aug. 1975 - Sept. 197^) (Ref. 33).

Distance and Direction % of Time in

(km) Operation^

Background Plume

Exposure (mR) Exposure (mR)

1.3

2.6

^.6

^4.6

5.2

5.8

8.0

a. b.

NNE

ENE

E

NNE

NE

NNE

ENE

97

98

86

52

95

36

65

84

78

71

4^

84

30

58

3.5

5.1

0.86

0.10

2.1

0.1

0.7

Refers to time of successful HPIC data acquisition.

Includes the cosmic-ray contribution equivalent to

about 35 inR. Plant effluent monitoring indicated the noble gas

activity release rate during the period ranged

from 4 to 140 x 103 |aCi/s.

A three-year long experiment to

monitor radioactive noble gases

around a boiling water power reactor

(BWR) was concluded in 1974. The

twofold purpose was to develop and

field-test measurement methods suitable for dociomenting ambient environmental gamma-ray levels and quantifying any contributions attributable

to plant operation, and to obtain

data for verifying commonly-used

methods for computing the diffusion

and dispersion of radionuclides in

the air. Several reports describe

the results, their significance and

comparisons of measurement and calculation. 32-35

In one part of the experiment, seven HPIC monitoring systems were deployed at locations between 1.3 and

8.0 km from the BWR. 18, 20 x variations analysis was applied to the

measurements to separate the relatively rapidly - varying plume con- tribution from the slowly - varying

background. A summary of the results

from the period August 1973 through

August 197'4- is given in Table 3. The

analysis produces an essentially

unique identification and quantification of the noble gas contribution to

the total environmental gamma-ray

field.

As indicated in a previous section, the overall accuracy of this

type of measurement for the total

exposure rate (or absorbed dose rate

in free air) is estimated to be

within 5 percent (s.d.). If the

HPIC produces data for a year, or a large fraction thereof, it was con- cluded from the tabulated and other

data that the exposure attributable

to a nearby nuclear facility can be

determined within 1 mrad/y.

Comparison of these measurements

with several common computational

methods indicated agreement, usually

well within a factor of two. Identification of these models can be obtained from the references.

It should be noted that, given

this type of verification, one can

est.ijuate population doses in terms

of man-rad. It was estimated that

an environmental "cost" of the one year operation of the BWR in question was 12 man-rad within 16 km,

and about 60 man-rad within 80 km. 5^

Of course, those of us who develop

and perform measurements are not

certain we understand the significance of these population dose estimates.

244
In aiio1:h.er part of the experiment, TLD neasurements were made for

monthly periods at ahout two dozen

locations, in.cluding the seven HPIC

locations. A semi-empirical calculation, correlating the soil moisture and exposure rate above ground

was made to estimate the expected

"background value. 32 The difference

between the expected value and the

total exposure rate after correcting

for the cosmic-ray contribution

(equivalent to 3-7 (J-R/ii) '•'''as attributed to the noble gas plume. The

agreement betvfeen the total exposures

derived from measurements at four

locations and calculations of the

type used for the HPIC comparisons

v;as generally within a factor of two.

The agreement at a location km

from the BWE vms poor, because the error in the inferred plijme contribution was about equal to the error estimated for the soil-moisture,

exposure model.

Based on a typical backgro'ond

corresponding to 10 jiR/h, or 88 mR/y

including the cosmic-ray contribution

to the annual environmental exposure

from a nuclear facility v/ould have an error of about ± 1 mR (s.d.). Tnis

is based on twelve successive, monthly TLD measurement periods and ac- quisition of suitable climato logical

data for the soil moisture - exposure

model. Hence, if one were to es- timate the annual increment over

this "typical" background with this

method to be 10 mR, the associated

error v/ould be acceptably small. It

-lould be noted that the error vrould

.5 somewhat reduced in areas v;here

background is relatively lov/ (see

Table 3) . A parallel demonstration v/as

undertaken on behalf of the Nuclear

Regulatory Commission by the Health

Ser^/ices Laboratory of ERDA.36 Five

HPIC detector units were modified to

provide exposure rate vs. time data

to a small computer located in a special trailer facility. Simultaneous recording, processing and transmitting of data from a nearby meteorological tower v/ere performed and

the results were retrieved either

from the local facility or in HSL

located at the National Reactor

Testing Station in Idaho. Thou^

"hardv;ire" transmission v/as employed

in this experijuent , and this tended

to limit the area monitored, a brief

test of EM radio telemetry was demonstrated to be feasible. A siiailar

system has been successfully used at

the 13,000 km2 Idaho site.

The demonstration was conceived

as a possible prototype for a large area monitoring program. The multiple application of a complex system

has recently undergone investigation.

In this effort, remote Nal(TA) detectors are expected to produce data

on environmental levels and on effluents at selected release points

on an automatic and continuous basis.

An important problem for the development of instruments of this

type is the lack of systematic instrument research and development in

the Government and industry. The

gamma-ray monitoring efforts described in this paper relied heavily

on detectors and instruments developed during the last two decades.

If the use of nuclear energy is to

continue, continued development measurement methods is needed. One class

of methods, pertaining to plutonium

and other actinides is being investigated and has been reviewed.

7

Tne development of an environmental beta-ray monitor is underway.

Monitoring of o5Kr released from a reactor fuel reprocessing facility is

also being attempted. Separation of

any contribution from the relatively substantial total beta-ray

levels are based on the analysis

method developed for the HPIC. Though

insufficient data is available for

dose estimation, several months of

operation indicates that the G. M.

count rate monitors are functioning

reasonably well. 57

It seems apparent that combinations of continuous monitors that

have been thoroughly field tested

can be deployed around nuclear power

plants, and possibly reactor fuel re- processing plants, in fairly limited

quantities.10520,33,37 These systems

in conjunction with a larger number

of monthly TLD measurements, in the

experience of HASL are a practical

program for monitoring external radiation. The measurements can be interpreted to form a record of any addi245
tions to the natural environmeiital

external dose rates.

Rapid surveys for olDtaining information on sites for routine monitoring or on long-term deposits of

radionuclides are best accomplished

with suitably calibrated, possibly

flat-response, survey instr-oments,

and in situ spectrometers. The data

on qualitative variations of gammaray levels and on dose rates attributable to specific natural radionaclides and pre-existing mannade nuclides can aid greatly in preparing

a facilities monitoring program such

as may be recommended or required. 38

The costs of equipment procurement for a monitoring program and of

its operation and data processing

are high, but they appear to be defensible and not necessarily extraordinary when one considers the cost

and value of many current monitoring

efforts

Acknowledgments

The author gratefully acknowledges the many efforts of his as- sociates in the Radiation Physics

Division, especially H. L. Beck, G.

de P. Burke, C. V. Gogolak, K. M.

Miller and P. D. Raft, who performed

the work of the Division's program

described herein, as well as the con- tributions of W, M. Lewder, who in a sense was the originator of the first

HilSL investigations in environmental

radiation in the 1950s. In addition,

without the support of the Instrumentation Division, many of the measurements would not have been possible,

Finally, acknowledgment is due P. G. Voilleque, formerly of the ERDA

Health Services Laboratory for the use of unpublished information.

References

1. Adams, J. A.S. and Lowder, W.M.

,

Eds., The Natural Radiation Environment, Univ. of Chicago Press,

.Chicago (196^) .

2. Adams, J.A.S., Lowder, W.M. and

Gesell, T., Eds. (1972). The Natural

Radiation Environment II, Report

CONP-72O8O5 (U.S. Energy Research and

Development Administration, Wash.).

3. Environmental Radiation Measurements, National Council on Radiation

Protection and Measurements (to be

published)

4. Lowder, W. M. and Beck, H. L.

,

Cosmic ray ionization in the lower

atmosphere, J. Geophys. Res. 7I, ^661

(1956).

5. Beck, H. L. , The physics of en- vironmental radiation fields, p. 101

in The Natural Radiation Environment

II, USERDA Report CON-720805 (1972).

6. Beck, H. L. , Gamma radiation

from radon daughters in the atmosphere, J. Geophys. Res. 79, 2215

(197^).

7. Beck, H. L. , McLaughlin, J. E.

,

and Miller, K. M. , In situ determinations of environmental plutoniium

and its related nuclides, IEEE Trans.

Nuc. Sci. (to be published, 197o)

8. Beck, H. L. , DeCampo , J. and

Gogolak, C. V. , In Situ Ge(Li) and

Nal(Ti') Gamma-Ray Spectrometry for

the Measurement of Environmental

Radiation, USERDA Report HASL-258

(1972)

9. Kolb, W. ond Lauterbach, U. , A

scintillation exposure rate meter

for environmental radiation survey,

p. 245 in The Natural Radiation

Environment II, Report CONP-720805

(1972)

10. Chester, J. P., Chase, R. L.

and Wood, S., A digital environinental

monitor. Report BNL-16922 (1972).

11. Wollenberg, H. A. and Smith, A.

R. , Studies in terrestrial y radiation, p. 513 in The Natural Radiation Environment, Univ. of Chicago

Press, Chicago (1964).

12. Gudiksen, Paul H. and Crites,

Thomas R. , A comparison of gamma-ray

exposure rate measurements at Bikini

Atoll, p. 727 in Operational Health

Physics, Proceedings of Ninth Midyear Topical Symposium (Health

Physics Society, 1976).

13. Lowder, W. M. , Private Communication (I97I).

246
1^. Millikan, R. A. and Nelier, H. V., A precision world survey of sea-level

cosmio-ray intensities, Ph.ys. Rev.

50, 15 (1936).

15. Super, J.B.H. and Chase, R. L.

,

Monitor for low intensity gamma rays.

Rev. Sci. Instram. 21, 356 (1950).

16. Spiers, F. W. , McHugh, M. J. and

Appletiy, D. B. , Environnental j-^^J

dose rate to populations, surveys

nade with a portable meter, p. 385 in

The lTatij.ral Radiation Environment,

Univ. of Chicago Press, Chicago (196'^.

17. Lowder, W. M. , Raft, P. D. and

Beck, H. L. , Experimental determination of cosmic-ray charged particle

intensity profiles in the atmosphere,

p. 908 in Proc. of National Symp. on Natural and Manmade Radiation in

Space (E. A. Warman, ed.). NASA rpt.

NASA m Z-24^0 (1972).

18. Negro, V. C, Watnick, S. , Raft,

P. D., A temperature-compensated

electrometer for environmental measurements, IEEE trans. Nucl. Sci.

TTS-21, 805 (197^).

19. Shamos, M. H. and Liboff, A. R.

,

ITew ionization chamber technique for

the measurement of environmental

radiation, Rev. Sci. Instrum. 59, 223

(1968)

.

20. DeCampo, J. A., Beck, H. L. and

Raft, P. D., High Pressure Argon

Ionization Chamber Systems for the

Measurement of Environmental Exposure

Rates. USAEC Report HASL-260 (1972).

21. Beck, H. L. , Monte Carlo calculations of electromagnetic shower

transition effects, Nucl. Instram.

Meth. 91, 525 (1971).

22. Lov/der, W. M. e_t al. Experimental investigations of the environmental radiation field, p. 255 Iel The

Natij.ral Radiation Environment II,

USERDA Report CONP-720805 (1972).

23. George, M. J., New data on the

absolute cosmic ray ionization in the

lower atmosphere, J. of Geophys. Res.

75, 3695 (1970).

24. Gesell, Thomas P., Burke, Gail

de P. , and Becker, Klaus, An international comparison of environmental

dosimeters under field and laboratory conditions, Health Physics 30,

125 (1976).

25. Performance, testing, and procedural specifications for thermoIximinescence dosimetry (environmental

applications), ANSI N 5^5 - 1975,

American National Standards Institute, New York (1975).

26. Harley, J. H. , Ed., HASL Procedures Manual, USERDA Report

HA.SL-300 (1972).

27. Burke, Gail de P. and Gesell,

Thomas P. , Error analysis of environmental radiation measurements made

with integrating dosimeters, proceedings of this symposium (197&).

28. Gamnage, R. B. and Haywood, F.

F. , Experiences in environmental

monitoring with BeD ceramic dosimeters, p. 324 in Operational Health

Physics, Proceedings of Ninth Midyear Topical Symposium (Health

Physics Society, 1976).

29. Kogan, R. M. , Nazarov, I. M.

and Fridman, Sh. I). , Gamma Spectrometry of Natural Environments and

Formations (Atomizdat, Moscow)

(English translation, Israel Program

for Scientific Translations,

Jerusalem, (1971).

30. Anspaugh, L. R. , Phelps, P. L.

,

Gudiksen, P. H. , Lindeken, C. L. and

Huckabay, G. ¥. , The in situ measurement of radionuclides in the environment with a Ge(Li) spectrometer, p.

279 in The Natural Radiation Environment II, USERDA Report CONF-720805

(1972).

31. Miller, K. M. , private communication (1976).

32. Burke, G. de P. , Variations in

Natural Environmental Gamma Radiation

and Its Effect on the Ijiterpretability of TLD Measurements Made Near

Nuclear Facilities, USERDA Report

HASL-289 (1975).

33. Miller, K. M. , Gogolak, C. Y.

and Raft, P. D. , Final Report on Continuous Monitoring with High

Pressure Argon Ionization Chambers

Near the Millstone Point Boiling

Water Power Reactor, USAEC Report

247
HASL-290 (1975).

34. Gogolak, C. v., A Data Set for

Noble Gas Plume Exposure Model Validation, USERDA Rc-'port HASL-295

(1975)

.

35. Gogolak, G. V. and Miller, K.

M. , Determination of gamrna-ray ex- posure in the vicinity of a boilingwater power reactor, p. 207 in

Population Exposure, Report COWF741018 (1974)

.

35. Voilleque, Paul G. , private

communication (1975)

•

37. Beck, H. L. and Preeswick, D.

C,, A continuous monitoring system

for 85Kr in the environment. Trans

Am. Nuc. Soc. 21, 92 (1975).

38. Environmental Radioactivity

Surveillance Guide, USEPA Report

ORP/SID 72-2 (1972)

.

248
NBS SP456 (1976)

ENVIRONMENTAL MEASUREMENTS AND REGULATORY RESPONSIBILITIES

J.M. Matuszek

Radiological Sciences Laboratory

Environmental Health Center

Division of Laboratories and Research

New York State Department of Health

Albany, New York 12201

During recent years, federal and state regulatory agencies have ex- hibited a propensity for issuing environmental radiation regulations

regardless of the quality of the data base available for derivation

of the standards. Particular examples of this trend are the AEC/NRC

Regulatory Guide 4.3, EPA's Federal Safe Drinking Water Regulations,

and some state and local edicts. Though the federal agencies appear

to be improving in their efforts to obtain a sound environmental

data base in certain areas, recent estimates of health effects at- tributable to -'^C effluents show that some interest continues for

the "estimate and speculate" method of arriving at environmental

radiation standards. Some state and local regulatory agencies, more closely attuned to political and media pressure, have rendered decisions or made statements which are not readily supported by the

data obtained from environmental measurements. Environmental radiochemical laboratories, on the other hand, have found that quality assurance requirements ordinarily acceptable for routine surveillance

programs are not always sufficient at low radioactivity levels for

the regulatory needs developed. As even more stringent analytical

requirements are imposed, the staffs of both the laboratories and

the regulatory agencies must be fully cognizant of the statistical

limitations of environmental data developed at very low radioactivity levels and respond accordingly in making regulatory decisions

or setting environmental regulations.

(Regulations; radioactive effluents; environmental samples, laboratories; radioanalytical data; quality assurance)

Introduction

Regulatory actions based on environmental measurements involve a double responsibility: the laboratory must provide the

most reliable data possible within analytical and statistical limitations, and the

regulatory agencies must take care to apply

the data only to the extent permitted by

these limitations. All too often, one or both of these requirements are not met. The

result is adverse public reaction, administrative problems, and/or added expense to resolve questions arising.

During the 1960s environmental radioactivity was caused predominantly by fallout

from nuclear weapons testing. Effluents

from the few operating experimental nuclear

reactors were rarely above ambient fallout

levels, except for samples collected very

close to the point of discharge. Radioanalytical requirements were relatively simple.

Radioactivity levels from fallout in most

environmental samples were well above the

minimum detection limits of ordinary counting systems, and the counting requirements

centered on obtaining satisfactory accuracy

and precision for each measurement.

In the 1970s ambient fallout levels

have decreased, and analytical sophistication has increased to the point where nu- clear reactor effluents can be identified

even at some distance from the source. These reactor-related radioactivity levels

are one or more orders of magnitude below

previous fallout levels, and the corresponding dose levels to the public are extremely

small. Nevertheless, regulatory agencies

have been developing environmental radiation

regulations which tax radioanalytical capabilities technically and financially.

In this brief presentation I cannot

hope to discuss all the radiological environmental regulations and decisions made

during the past few years. Instead, I shall

touch on some of the regulations made by the

U.S. Nuclear Regulatory Commission (NRC) and
the U.S. Environmental Protection Agency

(EPA) which affect the greater body of en- vironmental radiochemical laboratories and

erivironmental regulatory agencies. Some decisions by state and local regulatory au- thorities, though not universal in impact,

will be mentioned as examples of the environmental measurement problems which occur because of the more direct response by these

agencies to public opinion. Finally, I shall offer some examples of the regulatory

problems caused by inadequacies in data re- ported by state regulatory agencies or by

radioanalytical laboratories.

For the sake of time, the scope of this

discussion is limited to topics associated

with radioactivity levels in the environment,

leaving discussion of ionizing environmental

radiation to Dr. McLaughlin, the preceding

speaker, and leaving unaddressed the less

controversial topic of nonionizing environmental radiation.

Regulatory Imposition of New Analytical Tasks

Though the NRC has existed for only one year, much of the staff remains from its

parent, the U.S. Atomic Energy Commission

(AEC). Many regulatory decisions made by

the AECjWhich are still in effect as NRC

Regulatory Guides or as technical specifications at various nuclear facilities, imposed

new or more difficult tasks on radiochemical

measurement laboratories.

The EPA's Office of Radiation Programs

has existed somewhat longer. However, re- cent regulations by EPA have also imposed

analytical tasks which are out of proportion

to anticipated benefits. The situation in

the radiochemical measurement field resembles

that described succinctly by Hirschfeld^ for

analytical developments in general:

"The Delaney amendment and other

masterpieces of legal and bureaucratic regulation formally embody

the principle of today's maximum

analytical capability being tomorrow's minimal legal requirement.

This automatic no-win situation

for the developers of analytical

techniques, besides its eventual

effects on the publication of

analytical research, will keep a substantial fraction of the ana- lytical workload in perpetual

proximity to the technological and

eventually fundamental limits."

New radioanalytical methods are being

developed to meet the sensitivity and precision requirements of the new regulations.

All too often, considerable financial and

technical effort is required of the radioanalytical laboratories to incorporate or develop these new techniques. Such methods — in proximity to technological limits —

are difficult to use for routine analysis

and provide unreliable results when operated

by technicians. Hiring better trained staff

to meet the more demanding technical requirements leads to still another cost burden for

the laboratory. These additional costs,

which are eventually absorbed by the public,

are for the most part not readily justified

when one realistically considers the beneficial effects on health which are likely to accrue from many of the stringent new regulations.

Regulatory Imposition of Unreliable or Techn ologically Limited Analytical Tasks

Certain of the radiological environmental regulations have imposed analytical tasks

which approach the technological limits of

radioanalytical systems. Particularly important among these are NRC s Regulatory

Guide 4.32 (in association with requirements

of 10CFR50, Appendix I,^ and of Regulatory

Guide 1.42^) and EPA's Interim Primary Drinking Water Regulations^ (IPDWR). The IPDWR

also prove difficult to justify with regard

to health benefits which may be derived, but

they impact the measurement laboratory most

heavily through the regulated imposition of

unreliable analytical tasks.

Appendix I and Regulatory Guide 4.3 . At the beginning of this decade the AEC was increasingly pressured by various environmental groups to provide evidence of safe

operation of nuclear power reactors. One

result of this pressure was the development

of 10CFR50, Appendix I. The requirements

expressed in Appendix I have never achieved

formal legal status but are being imposed on operators of new nuclear power reactors

through the license approval process.

Based on dose levels and concentrations

established in Appendix I, the AEC required

laboratories (in Regulatory Guide 1.42) to obtain analytical precision of ±25^ at an un- specified confidence level for 131l levels

of 0,5 pCi/liter at time of sample collection, or approximately 0.2 pCi/liter at time

of counting. The reactor operators complained to the AEC that the analytical requirements were beyond the scope of existing

measurement capabilities. A radiochemist on the Directorate staff was assigned the task

of quickly "developing" a satisfactory method.
1973

1.8 -

1.6 % %

1.4

I I I 1I I I I I I I I

0.45

0.35

0.25

0.15

0.05

- V'

2\

I I I I I I I I I I I I I I I I I I I I I I I I I ] I I I I I I 5 15 25 5 15 25 5 15 25 5 15 25 5 15 25 5 15 25 5 15 25 5 15 25 JAN FEB MAR APR MAY JUNE JULY AUG

1973 1974

1.8

1.6

»

I I I I I I I

I t I ] ] I 1 I I 1 I I I I ] 1 I 1 i I ] ] I I I 1 1 ] I

0.35

H 0.25 -

0.15

>

uioo

<a

1 0.05

vi

J.* 9

5 15 25 5 15 25 5 15 25 5 15 25 5 15 25 5 15 25 5 15 25 5 15 25 SEP OCT NOV DEC JAN FEB MAR APR

Fig. 1. Running 10-point average background and standard deviation values for a lowbackground beta-particle-counting system (from Ref. 8).

Combining analytical methods from several

laboratories, including those of our Radiological Sciences Laboratory (RSL),6 he derived at an untested analytical method which

would presumably suffice. When preliminary

tests of the method were conducted at an AEC

contractor laboratory, that laboratory ap- parently ignored several important statistical parameters which would have precluded use

of this method for the purpose intended. De- spite its technical weaknesses, the method

was promulgated^ as Regulatory Guide 4,3.

251
TABLE I. Example Calculation (from Ref. 8).

Net pCi/liter

(net counts) (factor for protein-bound fraction)

(count time) (chem yield) (counter eff) (sample vol) (decay) (dpm/pCi)

(360±75) (l.05±0.05)

(lOOOiO.l) (0.6±0.l) (0.28±0.02) (4±0.0l) (0.5±0.005) (2.22)

0.5

sp

N 75 \ 2 ^ fo.05 0.l \ 2 /Oi_l

Qaeoj VI. 05 1000 j \0.6 0.28

IQ-l (4.34 + 0.23 + 0.00 + 2.04 + 0.51 + 0.00 + O.Ol)*

10"! (7.13)'^

0.27

^ + (0.02\ 2 ^ ( o.Ol \ 2 ^ /0.005^ 2' 0.5

We have shown in a previous publication^

that the analytical method published in Regulatory Guide 4.3 cannot achieve the sensitivity and precision required by the NRC.

First of all, the background counting rate

of commercial (^-counting systems can differ

significantly from predicted statistical behavior (Fig. l). The fluctuations in background rate occur even though the counting

systems are powered through voltage stabilizers and line filters and even though the

humidity and temperature of the countingroom air are closely controlled. The reason

for periodic behavior of the background fluctuations is not clear. Nal(Tl) systems used

for Y~spectrometric analysis of 131l exhibited similar nonstatistical background fluctuations. Regardless of the source of background fluctuations, this nonstatistical behavior of the counters can result in falsepositive 131j values.

The RSL also found large quantities of

stable iodine in milk from dairy farms in

New York State. Investigation by the New

York State Department of Environmental Conservation (NYSDEC) attributed the high levels

of stable iodide to a fungicide fed to barned

cattle in wet, cold climates. 9 Apparent

chemical recovery factors could be quadrupled

by the stable iodide in the milk, producing

a fourfold underestimate of 131I levels. Addition of carrier sufficient to overcome the

natural iodide content would reduce the p- counting efficiency to unacceptably low values. Uneven distribution of carrier on the

sample mount can also cause large counting

errors.

A mathematical analysis of the several

accountable sources of error (Table l) indi- cates that 'background contributions to the

counting error and chemical recovery errors would be the predominant terms setting the

analytical precision for 131l in milk. The

chemical recovery error could be reduced by

measuring stable iodide in each milk sample

-- at the cost of doubling the number of

analyses.

Periodic nuclear weapons tests were

found to increase ambient ISlj levels to three times the specified level of 0.5 pCi/

liter (Fig. 2). Incorrect attribution of

elevated 131i levels to local nuclear power

reactors can be avoided only by analysis of

several control samples collected remote

from each reactor.

An interlaboratory comparison studylO

conducted by the ABC substantiated the inability of some 15 laboratories to achieve

the precision and accuracy required now by

the NRC.

Regardless of all these problems, re- actor operators are still required by NRC

regional inspectors!! to obtain l^^I data at the sensitivity and precision specified by

Regulatory Guide 4.3.

Interim Primary Drinking Water Regulations . A heavier cost burden to most states

in the way of increased analytical workload

252
will be created by the IPDWR. In 1971 the

EPA Office of Radiation Programs began developing standards for radioactivity in drinking water. The regulations were finally

published in 1975.^ During that period the

draft IPDWR were revised time and again as serious technical errors were discovered,

yet the various versions showed little cognizance of many practical aspects of performing environmental measurements.

Many of the analytical methods, sensitivity requirements for several radionuclides, and cost estimates for certain analyses recommended in the IPDWR are incompatible with normal laboratory practices. For

example, the sensitivity requirement for

gross-alpha measurement was at one time less

than the fundamental limit by a factor of 4. In this case, recommendations from several

laboratories did lead to correction of the

technical impossibility being imposed.

Originally, specific 226Ra limits in

IPDWR were set on the basis of population

served. Such a requirement would have

caused difficult enforcement problems for state water-supply agencies. That requirement was changed only after a concerted

effort was organized by several states and

DATE 7/1

i

1.0

o

Ql

<

q:

UJo

2

O

o

ro

0.

0.01

8/1

1

9/1 lO/l

I 1

T

T

J \ \ L 0 50 100

DAYS AFTER CHINESE BOMB TEST

Fig. 2. Iodine-131 concentration in milk collected during 1974 in areas of New York State

remote from operating nuclear facilities (from Ref. 8).

253
the problem brought to the "attention of EPA's Office of Water Supply.

A new requirement calling for measurement of both 226Ra and 228Ra founcJ its way

into the final version of the interim regulations. The EPA cost estimate for measuring 226Ra and 228Ra is based on a single

separation of Ba(Ra)S04, though counting of

Ba(Ra)S04 for 226Rg technically inferior

to the emanation method of analysis. Fur- thermore, the recommended separation procedure has been found unreliable as an analytical method for 228Ra analysis.

An important incongruity in the IPDWR

permits concentrations of natural radioactivity which will result in dose levels of 60 mrem per year per individual, while only 4 mrem per year per individual is allowed from

man-made radioactivity. Measurement requirements for man-made radioactivity are at or below the current technological limit for

and 90sr. Measurement of 131l at 1 pCi/liter will require p/y-coincidence counting or excessively long counting periods on conventional p-counting systems unless special arrangements are made for rapid transport of the samples to the laboratory, where

they must be analyzed immediately on receipt.

A slower-paced analytical effort would re- quire shipment of large sample volumes (up to 4 liters of water). 90sr measurements at 0.5 pCi/liter will also require large sample

volumes, as will the 134cs and 137Cs measurements. Specially developed counting systems,

field concentration of certain radionuclides,

or transportation of up to 10 or more liters

of water may be necessary to meet all of the

IPDWR requirements. Such stringent sensitivity requirements for man-made radioactivity

are incompatible with the 15-fold greater

levels permitted for natural radioactivity.

The allowances in IPDWR for compositing

samples prior to analysis of natural radioactivity must be carefully examined by state laboratories and regulatory ' agencies. Pre- cipitation and adherence of many radionuclides to sample containers may prove a significant problem if samples are stored over

an extended period. Measurement laboratories

may be required to develop new techniques to assure representative sample transfer when

samples are composited.

The stringent sensitivity requirements

for man-made radioactivity are also incompatible with the monitoring frequency allowed

in the IPDWR. Measurements for environmental

cannot be considered meaningful if obtained on the basis of a single quarterly

grab sample. The IPDWR statement that if

maximum contaminant levels are found, "monitoring shall be continued with monthly sam- ples or the analyses of quarterly composites

of three monthly samples," is inconsistent

with the physical decay processes involved.

Because of its 8-day half-life, the decay of

131j from the time of collection to the time

of measurement will be incalculable for sam- ples composited over a period of three

months. Even 89sr decay will not be readily

calculable for composited samples.

Additional questions concerning the

viability IPDWR may also be raised. For

ex ample:

- Calculation of health risks should not be based on individual consumption of 2 liters of drinking water, but rather about

half that amount.

- The recommendation that laboratories

use outdated radioanalytical methods, such

as those of the American Public Health Association, will cause added cost burdens to the measurement laboratories while providing

inferior results. Some of the radioanalytical methods recommended, including those of

the American Society for Testing and Materials, cannot meet the sensitivity requirements of the IPDWR. .Laboratories will find

it necessary to develop or adapt methods at the limits of current technology for some radionuclides.

- Monitoring of "any community water

system contaminated by effluents from nuclear facilities" presents an interpretive dilemma. Contamination by 129i and HTO in

gaseous effluents from fuel-reprocessing

facilities or 131l and other radionuclides

in liquid effluents from hospitals may also

require consideration.

- The permitted use of utility discharge

data made concentration estimates at the

water intake dependent on water dispersion

models, which may result in serious overor underestimates of actual concentrations.

- Failure to require laboratories to exhibit proficiency through participation in

a nationwide quality assurance program may

jeopardize the reliability of results.

To summarize, the specific analytical

requirements now imposed on the measurement

laboratories by the IPDWR are out of line

with the dose justification, the sampling

and compositing allowances, the costs allowed for analyses, and the quality assurance requirements.

254
Recn_ilator\' rrograms Re'cuirina rurther iechnical ^evelcsment

Certain regulatory programs, including

some of long standing, require further technical development. Though Emergency Re- sponse Planning has only recently achieved

notoriety, state representatives to the

Conference of Radiation Control Program

Directors have attempted for the past few

years to draw attention of responsible federal agencies to deficiencies in the program.

Technical problems have also developed in

NRC s Independent Measurements Program.

Emergency Response Planning . As a con- dition for a license to operate a nuclear

power reactor, the facility operator must

develop with the State an emergency response

plan, under the guidance of the NRC and EPA.

The :!RC, as lead agency for emergency

planning, recently has come under considerable pressure from various groups concerned

about technical and financial inadequacies

in emergency planning for accidents at nuclear facilities. This problem was highlighted

in February 1976 by extensive media coverage

^ f alleged deficiencies at several reactor

sites. EPA, the U.S. Energy Research and

Development Administration (ERDA), the U.S.

Defense Civil Preparedness Agency (DCPA),

the Federal Disaster Assistance Administration (FDAA), state agencies, and facility

operators also have extensive responsibilities in the emergency response planning program.

Current problems with environmental

measurements for emergency planning involve

, plume transport and protective action guides.

! In-depth studies of plume transport are only

'.ow being started. To the credit of the NRC,

.asic research programs are being developed

I to identify gaseous and particulate species

{ which may be released in a reactor accident

I and to resolve the chemical behavior of

these species as a plume moves.

On the other hand, insufficient technical and financial assistance has been provided to state and local agencies which must

eventually administer any emergency-response

action. WASH-1293 provides a checklist for

cadre operations'^ vvhich the states are ex- pected to use as the basis for their own plans. However, there are important deficiencies in the WASH-1293 dose estimates and

in EPA's protective action guides. 1^

, Very little is known concerning radioiodine species which may be released in a

reactor emergency, though l-ilj dose to thyroid is postulated to be the predominant

factor determining the need for evacuation.

Reliable methods for sampling various chemical species of radioiodine must be developed.

Laboratory measurements for the several iodine radionuclides released may prove too complex for Nal(Tl) Y~spectrometry, but few

environmental radiochemical laboratories

have even one Ge(Li) system for high-resolution y-spectrometry. Prediction of radioiodine dose is further complicated by inadequate plume transport models for the terrain

surrounding most reactor sites in the eastern U.S., particularly at distances beyond

5 or 6 km.

Present evacuation plans are therefore

scientifically deficient. Such plans for

densely populated areas are dependent on predictive transport models, but the source- term radioiodine species are not yet adequately identified, and the transport mechanisms are still poorly defined. State and

local authorities may be forced to base

evacuation plans in part on the emergency

response capability of environmental radiochemical laboratories. Few state laboratories, however, are prepared to produce

definitive results within an hour or two following an accident. Even should the

laboratories be prepared, the procedures for

collection, shipping, and handling of emer- gency environmental samples differ markedly

from those used for routine surveillance

programs.

Research requirements for emergency

planning include identification of radionuclide species which may be released in an accident at each nuclear facility and development of site-specific mathematical

models for evacuation planning. Site-specific data can be obtained only by appropriate laboratory and field studies at each

nuclear facility.

Independent Measurements Program . The

Independent Measurements Program evolved

approximately five years ago, when the AEC

began considering environmental data as well

as discharge data for regulatory purposes.

This program appears to have been a response

to criticism from environmentalists and

other antinuclear groups, who charged that a conflict of interest existed in the AEC as it was then structured. The states were to be introduced into part of the monitoring

sequence, which othervjise was performed by

the individual operator or contractor laboratory.

255
Problems immediately surfaced. Firstly,

the individual contracts with the states

were severely underfunded by the AEC. One

goal of the AEC funding may have been to induce the states to provide supplemental

funds, but this is always a difficult proposition. Secondly, most states did not have

the technical or administrative resources to implement such a program. Thirdly, technical support by the AEC developed late in the

program, and even that support was less than

adequate.

The obstacles to technical and administrative support on the state level are not

always realistically appreciated by federal

agencies. State bureaucracies are often

slow to move, whether the need is for added

funds or for changes in job titles to at- tract trained personnel. Several states did

obtain supplemental funds and did begin work.

However, technical capabilities at many of

the state laboratories were impaired because

properly trained nuclear chemists and physicits could not be hired in existing job

titles or at existing salary levels. Even

the reactor operators experienced problems

in obtaining trained laboratory staff be- cause of the limited number of experienced

environmental radiochemists immediately

available.

Laboratory facilities in several states

were not adequate to cope with the variety

of samples collected. These limited facilities, not only in many state laboratories

but also at the reactor sites, made difficult the processing of high-activity inplant samples in the same laboratory with

low-activity environmental samples. In some instances counting equipment was set up in

the same room or in close proximity to where

chemical spearations were being performed.

As for technical support, the samples

provided to the laboratories were often non- representative. Many corrosion-product

radionuclides were found to be fixed permanently on container walls when in-plant sam- ples were shipped or stored for more than a few days. This problem was only discovered

2 or 3 years after the program was initiated.

The shipping problem was solved by adding

paper pulp to the large-volume samples, but

difficulties in aliquoting radioactive material adsorbed to the pulp increased problems

of sample cross-contamination.

Technical support for quality assurance

has also been less than adequate. Results

for intercomparison samples run at state,

reactor, and AEC (now ERDA) laboratories are made available to the laboratories only after

a year or more, so that recognition of any

performance inadequacies is delayed.

Regulations Imposing Impractical Analytical

Tasks

As in the case of the IPDWR, regulations or regulatory decisions have been

issued by various agencies without full con- sideration of health benefits to be obtained.

Analytical tasks in these cases do not nec- essarily approach or exceed technological

limits, but the tasks are likely to set ex- cessive workloads which cannot be readily

justified by cost/benefit considerations.

Control of 85Kr Efflu ents in the

Nuclear Power Cycle. EPA regulations-'-'^ for

control of 85Kr effluents appear to reflect

the agency's philosophical approach to regulatory responsiblities. As a matter of polciy,15 EPA's Office of Radiation Programs

develops all regulations on the basis of a very restrictive interpretation of the BEIR

Committee recommendations. 16

At the 1973 Noble Gases Symposium, ^"^ the EPA made a well-orchestrated presentation designed to convince the technical com- munity that 85Kr releases from the nuclear

fuel cycle must soon be brought under con- trol. Eight papers were presented extolling

the need for separating Q^Kt from gas effluents generated in the nuclear fuel cycle.

Recommendations for ^^Ki control have

appeared again in EPA's most recent proposed

rulemaking action. 1^ Health effects savings

realized by reducing environmental 85Kr effluents are partially offset by increased

occupational health effects from storage of

^^Kr. Yet it appears that radiochemical

laboratories may soon be required to conduct

routine measurements of environmental 85Kr

levels.

Control of ^^C in the Nuclear Power

Cycle. EPA's most recent proposals for

rulemaking actionl4 also suggest that l^c

is the single most important radionuclide in

the nuclear fuel cycle in terms of "possible

health effects." Little substantive evidence is provided in support of this contention. Calculated health effects^^ supporting the proposals are identical to those

presented by Magno et al.^^ at the 13th AEC

Air Cleaning Conference. In that study a series of speculative calculations were derived from an undefined model to obtain

estimates of health effects. No experimental data were submitted to verify the estimates of l^C production at nuclear power

reactors, nor were any environmental measure-
.-.ents presented,

Tne RSL has performed a limited number

of measurements20j21 on in-plant samples.

These values indicate that reactor releases

of 14c may be v/ithin the range estimated by

Magno. However, no data have as yet been

obtained at the fuel reprocessing sites,

•.'here Magno predicted most -"^C would be re- leased to the environment. Lacking a spe-

:ific model description of exchange processes anticipated for various chemical species

cf -^C which may be released, it is difficult to accept the startlingly high estimates

of health effects obtained by EPA.

Again, radiochemical laboratories may

oe required to develop methods for routine

i analysis of l^C released to the environment.

Reactor laboratories may also be required to develop analytical methods for regulatory

control.

Burial Site Effluents. State regulatory

agencies may on occasion yield to intense

public pressure by arbitrarily modifying en- vironmental standards.

Rainwater infiltrating trenches at the

low-level radioactive v/aste burial site in

. West Valley, N.Y. had raised the water level

I in some of the trenches to the point of overi, flow. Prior to overflow, the site operator

I had requested a permit to pump the water from

the three affected trenches and release the

I water at a controlled rate to local streams. Concentrations of radionuclides in such a discharge would have been several orders of

magnitude below corresponding concentrations

in releases that had previously been permit-

* ted from the fuel reprocessing plant on the

same site. After two years of attempting to arrive at a decision acceptable to all state agencies involved, overflow eventually forced

the state agencies to permit pumping and disI charge.

' The NYSDEC environmental radiation code22

includes discharge limits identical to those

I in other state codes and in federal code

10CFR20.23 Hov/ever, state and local environmental groups became actively involved in

the situation, and some of the nev/s releases

were irresponsible. One Buffalo television

i station issued a nevis bulletin that State

I Police would be aligned along the entire 30-

I mile course of Cattaraugus Creek to prevent

access by the public during the period of

discharge.

i Finally, NYSDEC set the discharge limits

I at the levels in the State Health Department's

, drinking water code, even though the nearest

public water supply intake is on Lake Erie,

35 miles and a 500-fold dilution downstream.

Since levels permitted by the drinking water

standards are approximately one-thirtieth of

those for discharge, the RSL was required to perform more sensitive analyses than would

otherwise be necessary. We were also re- quired to analyze more samples than are nor- mally necessary for control purposes in or- der to maintain an extensive public record.

In addition, the site operator was required

to perform a large number of analyses to maintain careful control of discharge rates.

The site operator expended approximately $100,000 in waste treatment and discharge

control. The analytical costs to the state exceeded $50,000. Integral dose savings vrere extremely small, a potential saving of at most one death every 10^ years in a population of 2 X 106.

Environmental Data and Public Relations

Environmental measurement laboratories

dealing directly with state or local regulatory agencies are likely to find stringent

quality assurance a paramount consideration,

even for single samples. This is a conse- quence of the closer public review of data

reported by the state or local agencies.

Reporting Foirnat. The laboratory staffs

normally report all results, including statistical errors. Actual detection limits for

each sample are reported for radioactivity

levels below the limit of detection. On the

other hand, the regulatory agency usually- re- ports the environmental data received from

the laboratory in the simplest format possible. Levels below the limit of detection

are often reported as "nondetectable" (ND),

a term the public tends to view as "zero."

Another common problem area develops

when the regulatory agency attempts to make

specific environmental evaluations from data

for a limited number of samples or from re- sults with inadequate controls. Generalizations which seem innocuous to the reporting

agency may incite strong public reaction due

to the public's sensitivity to radiological

safety. This in turn may create difficult

technical decisions and interpretations for

the regulatory agency and more work for the

radioanalytical laboratory.

The following examples illustrate the care necessary to ensure data reports which

do not induce needless public alarm.

"Hot" Milk. Measurements of HTO concen- tration in milk are performed routinely by the
RSL as part of the NYSDEC s statewide sur- veillance program. A problem developed in

1973 when a single sample was reported24 to have a positive level of HTO, while all previous values for that site had been reported

to the public as "ND," indicating results

below the sensitivity of the analytical method.

For years ^'^Sr levels in milk from this

dairy farm on Long Island had been among the

highest measured in the state, Coincidentally, this farm was close to Brookhaven

National Laboratory, which has long been a thorn to downstate environmental groups.

In 1973 the annual average levels25 of ^^St

in milk from this farm were the highest in

the state, about double the statewide average. An NYSDEC review of farming practices

indicated this farmer fed his cattle scrap

potatoes from nearby farms and did not fertilize his fields as heavily as do other

dairy farmers on Long Island. This contention was supported by the low 90sr levels in

milk from another nearby farm where different feeding and fertilizing methods were

used. Also, analyses of whole and peeled

potatoes (peels and meat analyzed separately)

showed that ^Osr levels in the feed potatoes

were similar to levels from other locations

in the state.

Unfortunately, just at the time state

and local officials were attempting to re- solve the 90sr dilemma, a group of milk sam- ples were analyzed for HTO content on a liquid scintillation counter with poor sensitivity. (The counter normally used was inoperative.) One positive result was obtained, a value of 1,500 ± 1,200 pCi/l, and unluckily

that value was for a milk sample from the

farm with elevated 90Sr levels. All previous

HTO results for milk from that farm had been

publicly reported by NYSDEC as "ND," though

the analytical results reported by the RSL were "<500 pCi/l." Local officials had interpreted the "ND" values to mean "zero," so when the 1,500 pCi/l value was reported by

NYSDEC without an associated analytical error, it appeard to show a huge increase in

HTO activity. A local press release alleged

that releases from Brookhaven had caused the

increased activity.

The RSL and NYSDEC staffs spent much

time and effort explaining the statistical

behavior of radiochemical data to the press,

local officials, the local citizenry, the

operator of the milk cooperative receiving

that milk, and the farmer. A lawsuit was threatened by the farmer until he started

selling his milk again. The issue finally

lost its high public profile, but the farmer

25

removed his farm from the state surveillance

network. This experience illustrates how

important it is for the laboratory to maintain a consistent measurement capability.

Equally important for the regulatory agency

is the inclusion of analytical errors with

reported data.

"Hot" Fish. In its 1973 Annual Report

NYSDEC reported that two fish samples from

the Peconic River collected below the discharge from Brookhaven had 90sr and 137c:s levels higher than those from elsewhere in

the state. NYSDEC attributed the elevated

radioactivity levels to effluents from

Brookhaven. Shortly after the report was

issued, a fish kill occurred in the Peconic

River. The RSL received a large number of

rotten fish collected along the stream bank

for assay because the local environmental

groups had attributed the fish kill to ra- dioactivity discharged by Brookhaven. This

proved not to be true. Radioactivity level

in the dead fish were similar to those obtained statewide.

"Hot" Kelp and Mussels. A delayed re- action occurred following the report25 of

elevated levels of 131l in kelp and ^^'^Cs in mussels off Fishers Island, which lies in

the mouth of the Connecticut River near the

Connecticut coast. These elevated levels were attributed by NYSDEC to the "nuclear

power plants that eventually discharged into

Long Island Sound." Several months later

this statement was relayed by a county health

official to a Connecticut newspaper editor,

who credited the elevated levels to discharges from the Millstone Point reactor in

Connecticut. The press coverage drew an angry response by environmentalists on Long

Island, calling for the congressman representing that district to investigate the

issue. As a result, the RSL, the Connecticut Health Department Laboratory, and Millstone's contractor laboratory became involved

in an analytical round robin, analyzing fish,

mussels, lobsters, and kelp for various radionuclides to find the origin of the elevated

radioactivity levels in specimens from Long

Island Sound.

The Rochester Committee Samples. In 1968 members of the Rochester Committee for

Scientific Information surreptitiously collected water samples within the exclusion

area at the nuclear fuel reprocessing plant

in New York. The samples were sent to an AEC laboratory for analysis with no indication as to the sampling location. The 90sr

levels in these samples were incorrectly

overestimated because a technician inadvertently applied an aliquot factor twice,

258
causing a 1,000-fold error in the reported

result. The Rochester Committee seized on the high values to issue a "report" to the

press alleging inordinately high releases by

the reprocessing plant to offsite surface

waters. 26 The AEC laboratory later issued

correct analytical values, but, the Rochester Committee did not issue a corresponding

correction of its hazard estimates.

The Shippinqport Incident. In 1973 Dr. Ernest Sternglass claimed that the Shippingport reactor had seriously endangered the

population in Aliquippa, Pa. As part of his

evidence he asserted that gross-beta values

in stream samples downstream from the plant

were significantly higher than those for upstream samples. His position was apparently

substantiated when the Pennsylvania Radiological Laboratory reported high "^^Sr levels in

samples from the Shippingport environs.

However, a subsequent intercalibration study

between the state laboratory, an AEC laboratory, and the reactor operator proved that

the state laboratory had incorrectly calibrated the counting system used for measurement of ^^Y, the 90sr daughter. Where most

laboratories obtain a counting efficiency of

approximately 50^ for ^Oy^ the state laboratory had for some time been using a counting

efficiency value of only 15%.

Sternglass' s claim of excess deaths due

to the Shippingport reactor received extensive press coverage, including a series of

special state hearings on nuclear reactor

safety. Subsequent corrections by the AEC

and the state laboratory, showing that downstream levels of 90sr were indistinguishable

from upstream levels, received little public

attention. The absence of specific reactorrelated radionuclides indicated that the

elevated gross-beta activity was probably

due to natural radioactivity (40k) introduced

into the river by sewage from the city of

Pittsburgh, another fact which failed to re- ceive follow-up coverage by the press.

Realistic Measurement/Regulation Relationship

Laboratory/regulator relationships are not always negative, as this survey of problem areas might suggest. The bulk of environmental data are reported with no adverse

public reaction. Environmental results have

been used to formulate regulations, though

not as often as would seem desirable.

Control of Effluents from a Nuclear Fuel

Reprocessing Plant. Responsibility for

performing environmental research is a difficult issue to resolve, though independent

state programs can be useful for the regulatory process. Because of work performed by

the State of New York, controls on nuclear

fuel reprocessing effluents were imposed by

the AEC which now appear realistic and adequately supported by environmental data.

Prior to 1971 radioactivity levels in

stream samples containing liquid effluents

from the fuel reprocessing plant in New York

often were at the limits of 10CFR20.23 The

state's surveillance program provided the

AEC with data concerning concentrations of

90sr, 137Cs, and other fission products in

offsite surface waters. On the basis of

the data provided, the operator of the re- processing plant was required by the AEC to install a liquid waste treatment facility

which resulted in a 100- to 1,000-fold re- duction of liquid effluent radioactivity

levels.

A similar situation arose with 129j

gaseous effluents from the same plant.

Measurements by the RSL and the EPA produced

evidence^ that 129i releases were greater

than plant discharge records would indicate.

Levels of 129j animal thyroids and milk

samples collected offsite indicated that

thyroid doses to individuals could approach

unacceptable levels. Again, presentation of

the evidence to the AEC led to the imposition of new technical specifications for

129i at this and other commercial fuelreprocessing plants under construction in

the United States.

New Environmental Research Programs.

The NRC has apparently increased its efforts

to gather environmental data before issuing

regulatory decisions related to environmental issues. The agency has conducted studies

to identify the chemical species of radioiodines released from nuclear power reactors2'7 and is cooperating with ERDA to develop new plume-transport models. Translation of the ideas into funded programs has

been slow, but the delays are apparently

those to be expected as a new agency begins

implementing its research programs.

The EPA has funded a study to measure

hospital-related radionuclide levels in

sewage treatment plant effluents. With the

U.S. Geological Survey, it is conducting

extensive studies to develop standards for

low-level waste-burial sites. Toward its

responsibilities for regulating nonionizing

radiation, EPA has also performed a number

of field evaluations of environmental levels

of microwave radiation and is preparing to evaluate radiation fields generated by

ultra-high voltage transmission lines.
Pressures from various sources often

make it difficult to take sufficient time to develop substantive environmental data before

preparing regulations. However, such re- search is an inescapable responsibility of

the regulatory agencies and the measurement

laboratories.

Acknowledgement

The suggestions offered by Dr. Alan

Levensohn are very much appreciated.

References

1. T. HIRSCHFELD, Analytical Chemistry 48,

16A-31A (January 1976).

2. "Measurements of Radionuclides in the

Environment: Analysis of 1-131 in Milk,"

Regulatory Guide 4.3, U.S. Atomic Energy

Commission, Directorate of Regulatory

Standards (Sep. 1974).

3. "Numerical Guides for Design Objectives

and Limiting Conditions for Operation

to Meet the Criterion 'As Low As Practicable' for Radioactive Material in

Light-Water-Cooled Nuclear Reactor Effluents," 10CFR50, Appendix I, Federal

Register 36 FR 11113 (June 1971).

4. "Interim Licensing Policy on As Low As Practicable for Gaseous Radioiodine

Releases from Light-Water-Cooled Nuclear

Power Reactors," Regulatory Guide 1.42,

U.S. Atomic Energy Commission, Directorate of Regulatory Standards (June

1973).

5. "Interim Primary Drinking Water Regulations - Proposed Minimum Containment

Levels for Radioactivity," Federal Register 40 FR 34324 (August 14, 1975).

6. J.J. GABAY, C.J. PAPERIELLO, S. GOODYEAR, J.C. DALY, and J.M. MATUSZEK, "A Method for Determining Iodine-129 in

Milk and Water," Health Phys., 26, 89

(1974).

7. B.E. VAUGHN, Pacific Northwest Laboratory Annual Report for 1973, Part 2,

Ecological Sciences, BNWL-1850 PT 2, p. 126 (1974).

8. J.M. MATUSZEK and C.J. PAPERIELLO, Nu- clear Technology 27, 514-522 (l975).

9. J.M. MATUSZEK, J.C. DALY, S. GOODYEAR,

C.J. PAPERIELLO and J.J. GABAY, "En- vironmental Levels of 129i," Proceedings

of the Symposium on Environmental Surveillance Around Nuclear Installations,

Warsaw, Poland, Nov. 5-9, 1973. International Atomic Energy Agency, Vienna,

1974, Vol. II, pp. 3-20.

10. G. HAMADA and B. WEISS, "An Intercomparison of Various Analytical Laboratories

in Determining Low Concentrations of

Iodine-131 in Milk Samples," Conf.

Bioassay, Environmental and Analytical

Chemistry, October 1974, Cincinatti,

Ohio, unpublished. Copies available

from Nuclear Regulatory Commission.

11. U.S. Nuclear Regulatory Commission,

Docket No. 50-220, Inspection No. 75-13,

License No. DRR-63, (September 16, 1975)

12. "Guide and Checklist for the Development and Evaluation of State and Local

Government Radiological Emergency Re- sponse Plans in Support of Fixed Nu- clear Facilities," WASH-1293, U.S.

Nuclear Regulatory Commission, December

1974.

13. "Manual of Protective Action Guides and

Protective Actions for Nuclear Incidents," EPA-520/1-75-001, U.S. Environmental Protection Agency, September

1975.

14. "Environmental Radiation Protection for

Nuclear Power Operators - Proposed Standards," Federal Register 40 FR 23420

(May 29, 1975).

15. "POLICY STATEMENT- Relationship Between

Radiation Dose and Effect," Office of

Radiation Programs, Office of Air and

Waste Management, U.S. Environmental

Protection Agency, Washington, D.C. (March 3, 1975).

16. "The Effects on Populations of Exposure

to Low Levels of Ionizing Radiation,"

Report of the Advisory Committee on the

Biological Effects of Ionizing Radiations, National Academy of Sciences/

National Research Council (November

1972).

17. R.E. STANLEY and A. A. MOGHISSI (editors)

"Noble Gases," CONF-730915 (l975).

18. "Environmental Radiation Protection Re- quirements for Normal Operations of

Activities in the Uranium Fuel Cycle,"

(Draft Environmental Statement), U.S.

Environmental Protection Agency (May

1975).

19. P.J. MAGNO, C.B. NELSON and W,H. ELLETT, "A Consideration of the Significance of

Carbon-14 Discharges from the Nuclear

Power Industry," 13th AEC Air Cleaning

Conference (San Francisco), U.S. Environmental Protection Agency (1974).

20. CO. KUNZ, W.E. MAHONEY and T.W. MILLER,

"I'^C Gaseous Effluent from Pressurized

Water Reactors," Proceedings of the

Health Physics Society's Eighth Midyear

Symposium on Population Exposures, Knoxville, Tenn., Oct. 21-24, 1974.

21. CO. KUNZ,- W.E. MAHONEY and T.W. MILLER,

"14q Gaseous Effluents from Boiling

Water Reactors," Transactions of the

American Nuclear Society, Annual Meeting, New Orleans, Louisiana, June 8-13,

260
1975, Trans. ANS, pp. 91-92.

22. - New York State Code of

Rules and Regulations 6NYCRR Part 380,

"Rules and Regulations Relating to Prevention and Control of Environmental

Pollution by Radioactive Materials,"

New York State Department of Environmental Conservation, Albany, N.Y. (October 1974).

23. Code of Federal Regulations 10 CFR Part

20, "Standards for Protection Against

Radiation," U.S. Nuclear Regulatory

Commission, Washington, D.C.

24. "Radioactivity in Air, Milk and Water

for July-September, 1973," Environmental Radiation Bulletin Number 3, 1973, N.Y. State Department of Environmental

Conservation, Albany, N.Y. (1974).

25. "Annual Report of Environmental Radiation in New York State - 1973," New

York State Department of Environmental

Conservation, Albany, N.Y. (l974).

26. "Strontium-90 in Buttermilk Creek,"

Industrial Radioactive Waste - Bulletin

No. 2, Rochester Committee for Scientific Information (February 24, 1968).

27. B.H. WEISS, P.G. VOILLEQUE, J.H. KELLER,

B. KAHN, H.L. KRIEGER, A. MARTIN and

C. R. PHILLIPS, "Detailed Measurements

of 131l in Air, Vegetation and Milk

Around Three Operating Reactor Sites,"

NUREG-75/021, U.S. Nuclear Regulatory

Commission (March 1975).

261

NBS SP456 (1976)

ENVIRONMENTAL RADIOACTIVITY STANDARDS

A. N. Jarvis and D. G. Easterly

U.S. Environmental Protection Agency

Las Vegas, Nevada 89114

Policy decisions concerning environmental quality must be based on valid and comparable data. The distribution of radioactive reference samples and conducting interlaboratory studies aids in the

critical evaluation of such data and helps assure that measurements are correctly made, accurate, traceable to national radiation standards, and comparable. Standards distribution and intercomparison studies are discussed. Traceability studies between

the National Biireau of Standards and the U.S. Environmental Protection Agency are reviewed. Identification and documentation of

potential radioactive pollutants that could result from expanded

energy programs are also discussed.

(Quality assurance, quality control, radioactivity, calibration,

reference methods, intercomparison studies)

Policy decisions concerning environmental quality must be based on valid and com- parable sample analysis. Interlaboratory

studies and the distribution of radioactive

reference samples aid in the critical evaluation of environmental data and help ensure

that measurements are correctly made , accurate, and traceable to the national measurements system.

Since its inception in 1970, the U.S. Environmental Protection Agency (EPA) has

conducted a radiation quality assurance proI gram designed to encourage the development

and implementation of quality control procedures . Various aspects of this program have

been described in EPA publications . ^

» ^ ' ^ ' Some of those in attendance at this sympo-

, slum may be familiar with, and in some

I instances may be participants in, the Radia-

! tion Quality Assurance Program conducted by

' the Environmental Monitoring and Support

Laboratory-Las Vegas (EMSL-LV) . However,

for the benefit of those neither involved

I in, nor familiar with this program, I will

give a brief overview of our activities.

The radiation quality assurance program

I has three major aspects. These are:

- Standards distribution

- Methods evaluation

- Laboratory intercomparison

studies

The standards distribution program involves the preparation and distribution of

calibrated samples which are used by laboratories for instrument calibration and chemical yield determinations. Currently there

are in excess of 30 different radionuclides

available on request. About 100 samples are being shipped each month. During the second

quarter of FY-76 (October-December 1975)

,

290 samples, comprising 23 different radionuclides, were distributed to laboratories

throughout the co\intry.

Methods evaluation involves the selection and collaborative testing of tentative

reference methods. At present, collaborative tests of methods to determine plutonium

239 in soil are in progress. In addition,

as a result of the EPA's proposed Interim

Drinking Water Regulations^ the tentative

reference method for the measurement of

gross alpha and gross beta radioactivities

in environmental waters^ is being tested.

Tentative reference methods for the analysis

of tritium, strontium-89 and 90, and radium

in water are in preparation and will be

collaboratively tested this year.

The laboratory intercomparison studies

program includes the preparation and distribution of samples (milk, water, food, air

filters, and gases) containing known amounts

of one or more radionuclides. A major objective of this program is to assist laboratories involved in environmental radiation

measurements to develop and maintain both

intralaboratory and interlaboratory quality

controls. Approximately 100 laboratories

currently participate in one or more of the

cross-check programs.

Periodically the data from the intercom

parison studies are summarized and published

as EPA reports. These summarizations enable

us to get some indication of the quality of

environmental radiation measurements . A sum mary of the milk data was recently published

263
and summaries of the water and diet crosschecks are in preparation.

Having briefly described the EMSL-LV

quality assurance program, I will outline

current and future needs, and, in some instances, describe what is being done in an effort to meet these needs.

We see an urgent need for the preparation of additional reference materials,

standards, and quality assurance guidelines,

and for increased efforts in the areas of

traceability and training.

It is imperative that we identify, produce, and make available additional reference materials. Both the EPA and the

National Bureau of Standards (NBS) have

attempted during the past several years to increase the number and kinds of standards

and reference materials required for environmental monitoring. Unfortunately the production and. testing of reference materials

are laborious and time-consuming procedures.

As a consequence, availability often lags

well behind the need. This lag time has

been increased as a result of the rapid

expansion of the energy program, the debate

on the role nuclear power will play in this

expansion, the problems associated with

phosphate mining and processing , and the

promulgation of the Interim Drinking Water

Regulations

There is further need to identify,

quantify, assign priorities to, and then

prepare the reference materials required in

the monitoring of those pollutants which may

arise from the expanded energy program, as well as those required to monitor phosphate

production activities and drinking water. The EPA, through its laboratory in Las

Vegas, is taking action in these areas of

interest and concern. An investigation of

the potential pollutants which may arise as a result of the expanded energy program has

been initiated and should be completed

within a year

In cooperation with the NBS we have

made a mixed gamma standard available to laboratories. A "round -robin" study, involving the measurement of polonium-210 by

30 laboratories, is in progress. Mancos

shale, a widespread and well-characterized

geological type , is currently being analyzed

(primarily for radium) and should soon be

available as a reference material. An aqueous plutonium-2 39 sample should also

become available within the year.

As a result of renewed and increasing

interest in both the nuclear fuel cycle and

naturally-occurring radionuclides, three

types of samples are being prepared which

should be of interest to those involved in

environmental monitoring activities. The

samples include pitchblende, uranium mill

tailings (including a 16-mill composite)

and thorium ore. It is planned to use these

materials both as reference materials and,

diluted with soil, in the intercomparison

studies program.

Negotiations with NBS for the production of aqueous solutions containing radium228 are currently in progress.

As they have indicated, the NBS cannot

maintain direct traceability to every user

of radioactive materials. Consequently,

they have outlined a plan® wherein the

radioactivity laboratories of the EPA, the

Energy Research and Development Administration, the National Radiation Council, the

Food and Drug Administration, and others

would be traceable to the national measurements system. These laboratories would in

turn assure traceability to State laboratories, nuclear power facilities, and con- tract laboratories

Beginning in 1974, the EMSL-LV instituted continuing intercomparison studies

with the NBS--the goal being to establish

the traceability of the EMSL-LV measurements

to those of the NBS. A summary of the

results obtained during 1974 and 1975 has

been published.

Another need is for the publication of

guidelines describing radioanalytical methodology, calibration techniques, and quality

control procedures. I was much encouraged

by Carl Seidel's paper describing the

calibration guidelines being prepared in the

American National Standards Institute quality assurance program. Guidelines such as these

have long been needed by user laboratories

Agency priorities, in particular the

energy program and the drinking water regulations, have required us to turn our attention to the methods and guidelines needed by

laboratories which will be involved in monitoring water supplies and those monitoring

in the environs of nuclear facilities.

Therefore, as described above, much effort

has been devoted to the preparation of

reference methods for the analysis of water. However , recognizing the need for other

guidelines designed for technicians, we

264
began the preparation of guidelines on gamma

spectroscopy and liquid scintillation counting which deal with standards preparation

,

calibration, and quality control procedures.

This overlapping of activities within

the radiation measurements community indicates what I believe to be another of the

major needs. There is a need for better

communication, increased liaison, and more cooperation between Federal, State, and

private agencies. Since we are all working

on the improvement of our only product,

radiation data, it seems only reasonable

that we should assist each other as much as possible. I would, therefore, urge that

during the remainder of this symposium we give thought to better ways to more quickly

exchange information and interact in areas

of common interest.

Still another need in the area of en- vironmental radiation measurements- -an area which has been too long neglected—is that

of sampling and sample-handling procedures.

For years many laboratories have been much

concerned with, and devoted considerable

effort to, decreasing the uncertainty of an analytical measurement by 0.5 percent or 0.25 percent, or even 0.005 percent. But

there has been too little interest in and

too few attempts to determine (much less

report!) the uncertainties associated with

the collection and handling of samples, and

the chain of custody records essential in

legal proceedings

One of my personal concerns , and one which I consider to be a pressing current

problem, is the lack of training received by

the analysts and technicians responsible for

radiation measurements. We are all aware that the quality of the data is dependent

upon the competence and reliability of individual analysts and technicians. As so eloquently stated by L. Cavallo and her

colleagues at the NBS,^^ "... the long-term

credibility of our curie lies in a piece of

apparatus and the skill of its custodian."

Unfortunately the training of technicians

and analysts has been much neglected.

Federal agencies have abolished training

programs and almost eliminated training

grants. A few institutions and agencies

offer short- or long-term training courses

However , high tuition fees and the cost of

travel, coupled with travel restrictions,

have prevented the attendance of those who

are most in need of training. Consequently,

I would suggest that the organizations

sponsoring this symposium make a concerted

effort to initiate the allocation of the resources required to institute training in

radioanalytical techniques and quality

control procedures. Moreover, I would urge

that we all explore ways in which less ex- pensive training can be provided and the means whereby we can assist one another.

References

1. The U.S. Environmental Protection

Agency's Quality Assurance Program,

M72-199, December 1972.

2. Radioactivity Standards Distribution

Program 1975, EPA-680/4-75-002a , April

1975.

3. Environmental Radioactivity Laboratory

Intercomparison Studies Program, EPA- 680/4-75-002b, May 1975.

4. Measurement Methods Standardization

Strategy Document, USEPA, September

1973.

5. Proposed Maximum Contaminant Levels for

Radioactivity, Federal Register, Thursday, August 14, 1975, Vol 40, No. 158,

pp 34324-34328.

6 . Tentative Reference Method for the

Measurement of Gross Alpha and Gross

Beta Radioactivities in Environmental

Waters, EPA-680/4-75-005 , Jxine 1975.

7. Preliminary Milk Report, EPA-680/4-75-

007, June 1975.

8. W. Mann, "Radionuclide Metrology and

Quality Assurance," Activities ^ and

Needs Related to Radioactivity Standards

for Environmental Measurements , ed . by

J, E. Eldridge and B. Kahn , pp 47-54,

EPA-680/4-75-006, June 1975.

9. Radiation Quality Assurance Intercomparison Studies, 1974-1975, EPA-600/4-

75-014, October 1975.

10. C. W. Seidel and J. M. R. Hutchinson,

ANSI Quality Assurance in Radioactivity

Measurements , CSl-8 , Symposium on Measurements for The Safe Use of

Radiation, March 1976.

11. L. M. Cavallo, et al.. Needs for Radioactivity Standards and Measurements in

Different Fields, pp 5-18, Nuclear

Instruments and Methods, 112 (1973).

265

ms SP456 (1976) * PERSONNEL MONITORING MEASUREMENTS

Richard V. Griffith

La^^^:ence Livermore Laboratory

Livermore, California 94550

A personnel monitoring program must include a well integrated com- bination of dose determination methods, and should not depend on a single dosimetry system. Many of the necessary techniques have

become well developed and dependable, such as the personnel gamma

dosimeters in use today. However, other monitoring methods are still not adequate. The two most important personnel monitoring

problems remaining are development of personnel neutron dosimeter

and in-vivo measurement of plutonium at sublung burden levels.

Although there are a few techniques under development to attack

these problems, satisfactory long-term solutions will require much

more work. As the developments in nuclear power and medicine continue, the need for solutions to these problems will intensify.

(Dosimetry; betas; neutrons; gammas; x rays; calibrations; plutonium)

Introduction

|! A successful personnel monitoring program

ij requires a number of well integrated mon-

! itoring techniques . Dependence on the

!j dosimeter or other single system for evaluaI tion of personnel radiation exposure will

paint an incomplete picture, and eventually

'1 will return to haunt those responsible for

the radiation protection program. A

thorough investigation of the working environment before and during occupation by the

worker is essential to prevent surprises.

II An interview and resurvey following an indicated high exposure are also very important, although the scenario leading to an exposure is often difficult to recreate

because of changes in the work area. An

example of the problems created by dependence on the dosimeter is the nimiber of high

exposures indicated following nonoccupational medical treatment with radioj isotopes . This particular problem even

1 1 reaches those responsible for environmental

'. surveillance in the form of elimination into

the facility sewage system.

A more basic need for a total monitoring

program is the inherent inadequacy of a

• single dosimeter to adequately characterize

I the radiation distribution experienced by a human wearer. The dosimeter tells us something about the intensity of the radiation

I at the point of measurement. It tells us little about the distribution through the

body, and in fact, cannot even be used to determine the radiation direction. It does

not know whether the wearer was leaning over

a glove box or working outside a reactor

shield. Only the individual and those con- cerned with his safety can tell us that.

The dosimeter cannot provide data which can

be used to determine individual organ doses,

but it can provide information that must be

used by a trained professional to do that.

It is also important to note that a com- plete personnel monitoring program includes

the need for internal exposure information

as well as that from external fields. Here

again, reliance on a single assay method is

generally inappropriate. Chemical bioassay,

whole-body counts, nose swipes, and airsampling data must be combined to make a complete evaluation. There are certainly

times when one technique is more feasible

than others , but continuing total dependence

on one method is poor practice and unjust

to the employee.

One point that is stressed at LLL , incidentally, is that it is the responsibility

of the health physicists and other safety

professionals to provide a safe working

environment, but that the employee is

responsible for his own safety — that is,

it is his responsibility to use the Laboratory facilities safely. That is as true for

radiation safety as for any other aspect of

safety. In that sense the individual is

part of his own personnel monitoring program.

External Dosimetry

Having said all this, however, the personnel dosimeter must be considered the

Work performed under the auspices of the U.S. Energy Research & Development Administration under contract No. W-7405-Eng-48

.

267
single most important element in a personnel

monitoring program. Its primary value is

that it can be used to identify those individuals that have experienced a radiation

environment in excess of the lower limit of

detection of the system. The dosimeter's

secondary value is that it can be used to provide an estimate of the relative severity

of the radiation experience at the location

of the dosimeter . The degree of confidence

in the accuracy of that estimate is a function of the optimism of the dosimetrist.

The performance of the dosimetry system

depends on three factors:

• Inherent characteristics of the detecting element.

• Quality of the calibration.

• Precision of the readout system.

Confining the discussion to the first two

of these factors, it is helpful to begin by

reviewing the preliminary results of a recent survey on personnel monitoring, including 83 personnel monitoring services in

33 countries."'" The survey questions deal

with a number of aspects of the dosimetry

system, but I shall only touch on the types

of detectors used and their relative merits.

The survey covered monitoring of personnel for X rays and gammas, betas, and

neutrons. Of course, the bulk of the users

are monitored for x rays and gammas only,

with only 15% of all people monitored being

issued a neutron dosimeter. In the case of

gammas, x rays and betas, it is no surprise

that TLD and film are the dosimeters of

choice, with only 3% of the services using

radiophotoluminescent glass. The move is

clearly to TLD. In the U.S., 72% of the

personnel monitored by the services responding to the survey are now or will soon be

monitored by TLD exclusively, with LiF used

by over 75% of the services. Internationally, the trend is the same, although

lagging somewhat. The principle advantage

cited by TLD proponents is an easily automated readout, with the energy response

second. Those preferring film cite the

availability of a permanent record as the

main advantage, with secondary interest in

energy evaluation and the high information

content of the film "image." With regard

to sensitivity, estimates of "lowest meas- urable dose" by TLD service range from 5 to 100 mrem, with an average of 22 mrem. For

film, the range was 5 to 150 mrem with a 40-mrem average.

The survey clearly identifies development

of fast neutron dosimetry as the major problem in personnel monitoring. Over two- thirds of the responses cited a fast neutron

capability as the improvement most needed

in their present systems. Forty- two services reported doing research and development. Of these, the largest number - fifteen — are working on neutron dosimetry.

Others reported work on system improvements

and new or improved TLD phosphors. It is

of interest that only three are working on beta dosimeters and two are doing R and D

to improve calibration and standards techniques .

Dosimetry Development

What is really going on in dosimetry

development? For x rays and gammas we are

in the position of fine-tuning reasonably

dependable systems. People are becoming

quite comfortable with TLD, primarily LiF.

A great deal of information has built up over recent years for both film and LiF, and some of the disadvantages cited for TLD by

dosimetrists are not considered to be as severe as they once were. Few people now

feel the need for the dosimeter itself as part of the permanent record. The energy

determination question is being partially

resolved by use of multiple crystal badges

with built-in absorbers to provide

penetrating-nonpenetrating dose information.

Another approach is to use additional TLD phosphors with different energy response,

such as CaFa. The image aspect of film may

still be an advantage in some cases, how2 ever

.

Minor problems with LiF cause a con- tinuing but low-level search for other

phosphors, but with greater emphasis on use

for environmental monitoring than personnel

dosimetry. As experience with long-term use

and recycling of LiF grows, the dosimeters

are found to be durable. However, changes

in sensitivity with repeated high3 temperature annealing have been reported.

Our own experience is a loss of sensitivity

by as much as 30% after seven years. In practice, of course, the calibration program

compensates for that effect. Another disadvantage is that the glow peak is somewhat

lower ('^'200°C) than for other phosphors,

resulting in some fading at elevated environmental temperatures (up to 8% per

4 month) . The annealing of LiF is complicated and time-consuming.^ It is not as sensitive as other phosphors such as CaFz

268
and MgzSiOi* (Tb) , but sensitivity is not a

I limitation for personnel monitoring and the

i| other problems do not seem to outweigh the

I advantages of LiF, including stability and

flat energy response.

Research continues on other detection

mechanisms, primarily those involving thermally or optically activated chargetransport effects such as radiation induced

thermally activated currents (RITAC) , radiophotoluminescence (RPL) , and thermally

8—13 stimulated exo-electron emission (TSEE)

Of these, RPL is the oldest, having been

I studied for over 25 years. Very little

I current research is being done on it for

personnel dosimetry. RPL has suffered re- jection because of inadequate sensitivity,

but recently a detection level of a few

14

, mrem has been reported. The largest U.S.

i use of RPL was in the DT-60 dosimeters

issued by the Navy. The most attractive

feature is a dosimeter capable of maintaining a permanent integrated exposure.

simpler readout system, and needs no annealing before reuse.

There are, of course, a number of other

methods of gamma detection, but they suffer

from poor sensitivity, high cost, or poor

stability. The likelihood of seeing a gamma dosimeter to replace TLD , film or RPL

in the near future is very small. Any new

unit will not only have to be superior to existing dosimeters, but must overcome the

wealth of experience dosimetrists have with

their systems . A lot of time and money is

invested in a personnel dosimetry system,

so the justification for changing it must

be very solid.

Challengers in the neutron dosimetry

field face a much brighter future. There

is simply not a satisfactory neutron dosimeter available. There are more methods for

detection of neutrons than gammas; however,

the personnel neutron dosimetry problem is more difficult than that for gamma dosimetry :

TSEE involves emission of trapped electrons from the surface of an insulator by

heating. The emission of electrons with

I energies of a few eV has a temperature

I

I dependence similar to a TL glow curve.

I' Both GM and proportional counters have been

used for detection. The attraction of TSEE

has been its potential sensitivity (a few

I

; yR) and the fact that the electrons are

M detected only from a very thin layer on the

surface (a few \xm) . The thin layer emission

is thought to give it a Linear Energy

Transfer (LET) dependent response. The

major obstacle to the use of TSEE has been

the care required in handling and the protection necessary to prevent nonradiationinduced signals from surface damage. TSEE

dosimeters are also sensitive to light and

humidity. '^

RITAC has been investigated only relatively recently. RITAC really includes a

I family of thermally activated current

' phenomena: thermally stimulated conductance

(RITSC) , thermally activated polarization

(RITAP) and thermally activated depolarization (RITAD) . RITAC involves measurement

of thermally induced current with a potenII tial ('V/l-S X 10^ V/mm) applied across a detector such as CaFa or AilaOs (sapphire)

rhe current through the detector is measured

d-th a picoammeter as it is heated, and

recorded as a function of temperature to

'ield glow peaks similar to TL or TSEE. The

ittractive features of RITAC are that it has

I sensitivity comparable with TL but uses a

• The neutron energy range is wider (9 decades as opposed to 3 or 4 for

gammas and x rays)

• The quality factor for fast neutrons

is ten times that for gammas, re- quiring ten times the sensitivity for

the same dose-equivalent level.

• The gamma sensitivity of some neutron

detection processes impairs discrimination between the two radiations.

Three neutron dosimeter types are in use today — NTA film, TL albedo and fission

track etch dosimeters. None of these can

be considered a long-term solution to the

general problem of personnel neutron dosimetry. NTA film has been used most extensively, but has been severely criticized

because of its fading properties, need for

laborious manual counting and gamma sensi4 16 tivity. ' Much of the concern about

fading can be overcome by adequate protection, but the counting is still tedious and

17 18 does not lend itself to automation. ' In addition, it is not well suited for use

in areas where there are a large number of

low-energy neutrons.

TL albedo dosimeters have become popular

in the last few years, primarily because

they are sensitive and use a TLD read20 21

out. ' The dosimeter depends on body

269
1000

100

10

1

0.1

0.01

10

/ ^

Albedo dosimeter response

Dose equivalent conversion _^ \ ^ \ ^ \ 10 " 10 ^ 10'^

Neutron energy — MeV

10^

Fig. 1. Comparison of relative response of TLD albedo dosimeter with neutron dose

equivalent flux to dose equivalent

conversion

.

thermalization and reflection of fast neutrons for its response. The major limitation is that they are severely energydependent. Figure 1 is a schematic

comparison of the calculated response of an albedo system compared with the NCRP38

22,23 dose-equivalent response curve. Below

about 5 X 10~ MeV the responses are roughly

parallel, but for higher energies they in- vert. For fast neutrons where we need doseequivalent information the most, the sensitivity is the least. Use of these dosimeters

requires that either the spectrum be well

known or a thorough investigation of each

positive reading be made to establish a spectral calibration factor. Efforts have

been made to eliminate the spectral dependence of these dosimeters, but they have not been completely successful . '^^

Fission track detectors have found

limited use in the U.S., but have been more

25— 2 8 accepted internationally. Although

they are not as sensitive as albedo dosimeters, the major objection is the need for

personnel to carry radioactive materials

such as ^^^U, ^^^U, 2^^Np or ^^^Th in their

badges. Many health physicists feel it

inappropriate to give a dose of one radiation in order to measure the dose of another.

Perhaps as much in desperation as for any

other reason, a number of potential neutron

detection methods have been considered for

personnel monitoring:

• ^Li(n,a)-^H and ^°B(ri,a)^Li with cellu29 30 lose nitrate track registration. '

nitrate track detectors for fast neu31 tron measurement.

Detection of fast-neutron-induced

direct recoils in cellulose ni-

, , 32,33

trate.

Detection of fast-neutron- induced

direct recoils in polycarbonate film

by spark countxng.

Detection of fast-neutron-induced

direct recoils in polycarbonate sheet

by electrochemical etching .

Formation of tracks from (ll,p) and

37

(r), alpha) reactions in AgCl.

Development of an hydrogeneous TLD

Detection of damage in silicon

39 diodes

.

38

Detection of self-activation in TLD materials

.

Use of LET-dependent differential glow

41 42 peak formation in TL materials. '

TSEE with hydrogenous radiators 43

Be recoil radiators with cellulose

These methods depend either on detection

of the tracks of individual charged particles resulting from neutron-induced nuclear

reactions, or on some bulk effects such as activation, atom displacement or electron

trap formation. Track detection is ^attractive because it allows one to look at individual neutron interactions. A more

important advantage is that the registration process requires a minimum LET for

track formation, so the technique is gen44 erally gamma insensitive. The main disadvantage is that readout is laborious and

not easily automated. Other solid-state

dosimetry techniques are more easily automated, but the response decreases with

increasing LET, making the relative

gamma-to-neutron sensitivity much too high.

The emphasis in track registration is to develop simple readout techniques for detection of tracks, particularly from fast

neutron interactions. The fast neutron

interactions usually involve detection of

direct recoil tracks in plastics such as polycarbonate or cellulose nitrate by

270
chemical etching. Detection methods include

track contrast enhancement by including dye

in the film, semiautomatic detection by

spark counting, or track enlargement by

46-48 electrochemical etching. We have

found that the efficiency of the red dye

I contrast enhancement method is very sensiI tive to film thickness and etching parami 33

I eters . Use of the technique routinely

would require very careful control of

I etching conditions , to a point that might

j not be practical in a large-scale field

program. In addition, automated counting

I would require an expensive image analysis

I system. Spark- counting direct recoils in

polycarbonate film seems to lack sufficient

I sensitivity for routine personnel work, and

a recent study suggests poor reproducibility.3^'^9

The use of high-frequency altemating-

, current etching to enlarge damage tracks is

p one of the more promising track etch methods

i for fast neutron detection. An irradiated

' foil of polycarbonate is placed between two volumes of etchant at room temperature, and

an ac field is applied across the sample to enlarge the tracks to dimensions of tens of

microns in diameter (Fig. 2a). They are visible with the unaided eye, but need some magnification for counting. The plastic

I must be selected and handled carefully because tracks form on scratches and other

artifacts, resulting in a significant background. However, with a few square centimeters of suitable material, it should be

possible to detect fast neutron doses of

100 mrem. One interesting aspect of the

enlarged tracks is that most of them are below the surface, with only a fine

('MD.S-ym) track connecting to the surface.

We discovered this while trying to look at

the tracks with a scanning electron microscope. After having a lot of trouble

locating a track, we found a slight depression in the sample surface (Fig. 2b). As we increased the magnification the depression collapsed, revealing a subsurface

cavity (Fig. 2c). We found a number of

these pits on further investigation, which

also collapsed under the increased dose from

high-magnification electron scans.

Silver halide single crystals have been

used to detect heavy ions and cosmic rays.

The latent tracks are made visible by decoration techniques using UV illtimination or

UV illumination in combination with an electron beam.^^'^^ Although neutron detection has been reported, the application

50

(a) Optical microphotograph of

electrochemically enlarged recoil

tracks in polycarbonate (102. 5x).

(b) , (c) Scanning electron microphotograph (S.E.M.) of electrochemically enlarged recoil track

before and after collapse of poly

carbonate surface (1344x)

.

271
of these crystals to personnel dosimetry

has not been pursued. Unlike NTA track

films, gamma background does not develop in

the crystal.

As we shift attention to other solid

state detector techniques, we find that

silicon diodes have been discussed' for many

years, but still suffer from lack of sensitivity and high cost. TLD autoactivation

is the build-up of TL signal from the decay

of neutron activation products after preliminary readout and annealing. The short

half-life of activation products in TL materials precludes its use for routine

personnel dosimetry. It might, however,

have application in criticality dosimetry.

People have attempted to associate

hydrogenous material with TLD by use of

polyethylene radiators, or by inclusion of

heat-resistant hydrogenous material in the

38 5

1

TLD matrix. ' Neither of these approaches has been particularly successful,

with gamma sensitivity and difficulty in

producing phosphors as the main problems.

Others have looked at LET dependent flow

peaks in materials such as CaFarTm, however,

the success has been limited by lack of

sufficient neutron sensitivity and high

43

gamma response. Finally, Becker has used

TSEE with polyethylene radiators, but the

gamma sensitivity and care required in

handling have prevented use in routine personnel monitoring.

The beta monitoring problem seems to be

of least interest to dosimetrists . Film or

TLD are used routinely for beta dosimetry,

and are satisfactory for most routine

applications. However, certain applications

such as measurement of weak betas from

tritium do require more study. TSEE is a possible solution. It is sensitive and the

thin sensitive layer is particularly well

suited to measurement of low-energy electrons. In addition, the TL signal from a detector like BeO can be used to measure

52 the gamma background. Again, however,

the necessary dosimeter protection and careful handling have prevented routine use of

TSEE.

Calibration

Calibration and quality control are a vital part of a personnel monitoring program. Without careful attention to these

considerations, the effort expended in the

rest of the dosimetry program is wasted.

The following are some points that need to

be stressed in that regard:

• Calibrations must be made in terras of

physical measurements. The method of

conversion to dose equivalent is up to the dosimetrist, but the primary

calibration must be capable of description in physical units. I often

hear people ask for calibration data

in rem because that is what the field

health physicist is obliged to use, but a calibration that cannot be re- produced physically will only lead to problems when subsequent data com- parisons are made.

• Radiation fields must be carefully

characterized. This is a particular

problem in neutron dosimetry where

dose equivalent is often determined

with a remmeter. Measurement of

primary fields should be made with

more than one instrument, and preferably more than one instrument type. At LLL, for example, our neutron

fields are determined by spectrum

measurement, long-counter measurement,

computer calculation, as well as by

calibrated remmeters. In the near

future, we will also be making LET

and absorbed dose measurements.

• Every effort should be made to duplicate the radiation field quality

found in working environments. It is

really upsetting to talk about using

a PuBe neutron source to calibrate

dosimeters for use around reactors

and other fields with low-energy neutrons. The growing availability of

^^^Cf sources makes it quite practical

to provide moderated fission spectra

using spheres of polyethylene, metal

such as aluminum, or metal shells

which can be filled with water or

53 D2O. Of course, spectral duplication implies detailed field surveys,

but simple techniques to do this have

been developed.

• Intercomparisons with other laboratories are very valuable. They can

be done through formal intercomparison

programs, or more easily in many cases informal dosimeter exchanges with

friends at other laboratories.^^

Organized intercomparisons can provide

a rigorous test of the dosimetry system, while informal exchanges have the

advantage that the data need not be

published and the results can be com- pared in a short time

.

272
To finish the question of calibrations,

I want to acknowledge some development in

radiation field production. Monoenergetic

x-ray fields require careful development

The most popular technique is the use of

secondary fluorescent radiators excited by

either machine-produced x rays or isotopic

sources, Isotopically excited sources in

particular can produce high levels of

Compton scatter which make a significant

dose contribution. Even machine-generated

x-ray sources must be carefully filtered so that the dose comes from the fluorescent

X rays and not scatter. The filtration

reduces x-ray intensity severely, and may

require a rather large x-ray generator to make up for the loss in intensity.

Two alternative x-ray production techniques are under study at LLL. In one case, special tubes, referred to as TRansmission

Anode X-ray (TRAX) tubes, have been built

so that the electrons strike the back of the

characteristic target which is part of the

tube. The target filters the radiation,

preferentially transmitting its own characteristic X rays (Fig. 3a). Silver and

copper tubes have been made which produce

characteristic x rays at levels approaching

10 R/min at 10 cm from the tube face. The

cost of these tubes is significantly lower

than that required for a high- intensity

x-ray generator.

Another technique under study is production of X rays by anomalous Borrmann

transmission through single crystals such

as germanium. The energy of x rays

scattered from crystal planes is a function

of the angle between the beam and crystal

planes (Fig. 3b). Rotation of crystal and

second collimator is expected to provide us with a dial-an-energy , monoenergetic x-ray

source. Although the x-ray yield is much

less than from the previously discussed

TRAX tubes, it may be a very useful system

for instrument energy dependence measurements .

Thin foil o) target material

Fig. 3. (a) Production of transmission anode

x-rays (TRAX). (b) Production of

monoenergetic rays by anomalous

transmission through crystals

(Borrmann effect)

I would like to say a word in support of

the NBS filtered beam monoenergetic neutron

58 facility. This facility should be a valuable addition for calibration of neutron

dosimeters and detectors. The availability

of 2-keV neutrons will give us valuable

data, particularly for moderated detectors,

that was not available before.

In-vivo Dosimetry

A complete discussion of personnel monitoring must include internal dosimetry.

Estimates of dose from internally deposited

radioactive material have the advantage that

the material is available for measurement

and sampling for a sufficient length of time

to permit use of sophisticated detector

systems and acquisition of adequate data.

In fact, most radionuclides can now be

assayed quite accurately. However, some isotopes have unfavorable decay schemes with

little or no gamma emissions, and are very

difficult to detect.

The most important example of an unfavorable decay scheme is 2 3^Pu which has no K

X rays and only 0.047 17-keV L x rays per

alpha disintegration. We obtain a count

rate of only a couple of counts per minute

from a lung burden of ^^^Pu in an average

male, using a 300 cm^ chest counter. It is

possible to detect plutonium in vivo at that

level, only with elaborate detection equip59 ment using anticoincidence techniques.

Uncertainty in the geometric distribution

273
in the lung makes accurate measurement even

more difficult.

There is now a concerted effort to improve ^^^Pu measurement capability on a nationwide basis, including work by the

ERDA-established Inter calibration Committee

for Low-Energy Photon Measurements.^'^ The

committee, made up of members from ERDA contractor laboratories, is primarily involved with a program of chest-counting

facility intercomparisons . The intercomparison program includes circulation of a set of heavy-element sources of ^^^Pu,

^^^Pu, ^"^^Am and other transuranic

isotopes. In addition, three solid

tissue-equivalent torso phantoms with re- movable major organs are under construction

to be used in an ongoing ERDA sponsored

mult i- laboratory heavy-element intercom61,62 . ^ parison program. Finally, xn the

future, we hope to conduct an in-vivo calibration study similar to that performed by

the U.K. involving inhalation of aerosols

labelled with low levels of ^°^Pd which has

6 3 an x-ray used to simulate plutonium.

There seems to be a physical limitation

on the lower limit of detection possible

with chest counters. However, techniques

such as solid-state detector arrays or use

of multiwlre proportional counters may be

very helpful for location of material in

the lung and improved estimation of x-ray

region himan backgrounds. An esophageal

probe has been used for truly internal

counting resulting in a lower limit of

detection, but the technique requires

medical attendance and does not have widespread appeal.

Summary

A complete personnel monitoring program

must Include a well integrated set of dose

assessment techniques. This is particularly

true in view of a growing nuclear energy

program and increasing use of radiation in

medicine. I have discussed a number of

techniques for both external and in vivo

dosimetry. There are a number which could

not be included, but must be considered in

the overall dosimetry program. Use of

biological dosimetry, for example, can be

very useful in some applications, and

development of analytical techniques such

as chromosome aberration detection will

certainly become increasingly more valuable.

The backbone of a monitoring program —

the personnel dosimeter — has distinct

limitations, and it must be used with those

limitations in mind. Personnel neutron

dosimetry is currently the most difficult

external monitoring problem. Although a number of techniques are under study, the

most satisfactory solution may require use

of composite techniques such as addition of

fast neutron track detectors to more sensitive, but less accurate albedo dosimeters.

Careful calibration of personnel dosimeters is vital to the monitoring program.

We must recognize the need for calibrations

which can be reproduced in physical terms. We must characterize the calibration fields

carefully and accurately, and use fields

which simulate real life environments. It will be important to have more intercomparisons, particularly on an informal basis

with emphasis on improving our systems

rather than being graded on performance.

The need for internal dosimetry is not

as widespread as for external monitoring,

but the quality of the dosimetry is just

as important. The most difficult in-vivo

measurement problem is reduction of the

^^^Pu limit of detection. There seems to be no single solution, but like neutron

dosimetry, it will require a coordinated

combination of techniques.

Acknowledgments

I wish to thank Ken Crase, George

Campbell, Dale Hankins, Tom Crites, Wade

Patterson, and Dave Myers for their con- tributions to the preparation of this paper

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276
NBS SP456 (1976)

PERSONNEL MONITORING PERFORMANCE STANDARDS

Bernard H. Weiss

U. S. Nuclear Regulatory Commission

Washington, D.C. 20555

Efforts in the United States to provide some measure of uncertainty

to the interpretation of routine personnel monitoring badge results

have received the attention of various groups, individuals and

governmental agencies for over 20 years. However, in general,

health physicists and the users of these personnel monitoring

devices still do not have great confidence in the results. This

paper will review the history of the significant activities in

trying to develop a system for testing the performance of personnel dosimetry services. This will also include a discussion of

the current published standards and those in draft with respect

to their different approaches and objectives. In particular, the

paper will describe activities of the Nuclear Regulatory Commission in considering mandatory performance testing of personnel

dosimetry and the work of a Health Physics Society Working Group

which could be the basis for an NRC proposed rule.

(Personnel dosimeters; film badges, TLD, HPS Working Group)

In most instances, measuring exposures

of individual workers to harmful conditions

is a laborious procedure. In general, one must utilize bulky and complicated sampling

equipment, perform time consuming chemical

analyses or physical measurements; relate

the measurement to a worker's breathing rate

and work habits; and finally, making further

assumptions, arrive at an annual or periodic

exposure

.

Health physicists have considerable

advantage over other health professionals

in making these evaluations . For ionizing

radiation one has available various kinds of

simple personal monitoring devices. The

most common and universally used are the

badge type dosimeter, i.e., either film or thermoluminescent dosimeters (TLD) . Next

to the radiation warning symbol, the public

probably recognizes this symbol of the

nuclear industry most easily. These devices

1) are small and lightweight, 2) require

no power source, 3) are inexpensive to use,

4) have a sensitivity less than one percent

of the regulatory limit, 5) can differentiate between the different types of

ionizing radiation, and 6) have an accuracy

sufficient to meet health protection re- quirements .

The technical literature provides

sufficient evidence as to the accuracy of

both film and TLDs. However, tests of personnel dosimetry suppliers under controlled

conditions have shown that these devices do not always measure up to their capability.

Consequently, the question "How accurate

are the results obtained from personnel

dosimeters processed on a routine basis by

commercial, military, governmental or other

private services?" has been of considerable

import in this country for at least twenty

years. This paper will consider efforts to establish a national personnel dosimetry

testing laboratory and the criteria for

evaluating personnel dosimetry performance.

It will seek to explore why no effective

and nationally recognized personnel dosimetry testing service exists after considerable effort by federal and state agencies,

commercial processors, and other industry

and private groups. Finally, this paper

will attempt to draw some conclusions for

future action from the past effort to avoid

repeating past mistakes.

There have been several attempts to evaluate film dosimeter processor perfor- mance beginning in 1955 and continuing

sporadically through 1975. In addition,

much effort has been expended on establishing personnel dosimetry performance

criteria. In spite of all the previous

efforts, the problem of assuring adequate

personnel dosimetry processor performance

to users of these devices has remained un- resolved .

Since 1955, several studies"*" ^evaluated

routine film and TLD dosimeter performance.

In general, the conclusions that can be

drawn from these studies indicate that some processors demonstrate a capability more

than adequate to satisfy health protection

needs. However, these studies also show

that performance by a substantial fraction of participants is not satisfactory

277
for use by health physicists and the results

are misleading to the users. These studies

have almost universally concluded that there

is a need for an independent, objective

method of periodically evaluating personnel

dosimetry services. The latest study, conducted by Battelle, Pacific Northwest

Laboratories in 1974 indicates that there

has been little improvement in the perfor- mance of personnel dosimetry processors^.

The retracing of any recent history

regarding the attempt to establish a personnel testing laboratory or performance

criteria should begin with the most formal

and significant event in the long history,

i.e., the September 4, 1963 Federal Register

publication by the U. S. Atomic Energy

Commission - Notice Regarding Need for

Establishment of Film Dosimetry Calibration

Laboratory^. This notice did several things:

1. Announced that the AEC was considering

the desirability of issuing a proposed

regulation which would require AEC

licensees to utilize film dosimetry

services which met certain minimum performance criteria;

2. Proposed a minimum set of performance

criteria to provide standards of accuracy for evaluating film dosimetry ser- vices utilized by licensees;

3. Indicated that the Commission was studying the possibility of utilizing a laboratory for Commission contract activities; and

4. Requested expressions of interest of

organizations and other persons desiring

to establish and operate such a laboratory .

A significant number of comments were

received and a large majority of these favored the establishment of the laboratory.

A number of groups expressed interest in

operating such a laboratory although most

indicated their interest in operating such

a laboratory was contingent upon financial

support from the Federal Government. The

AEC was concerned about what official or quasi-official recognition could be granted

to a non-governmental laboratory by a regulatory or other governmental agency. The

Commission was not sure what criteria should

be used by a laboratory in a testing program

and what criteria could be used to evaluate

the testing laboratory. The Commission

wanted to involve industry in evaluating

licensees; also, to have AEC contractors

tested by a group over which they had direct

control, e.g., another AEC contractor.

initiating a program in which the National

Bureau of Standards would develop guides for

the establishment and operation of a film

dosimetry calibration laboratory. Because

of the Bureau's prestige and general preeminence in the standards field, it was felt

that they might play a key role in a testing

or calibration program even though NBS was unwilling to be involved in the approval of

specific film dosimetry services. However,

in view of the impending NBS move to Gaithersburg in 1966 and because of previous

commitments, NBS indicated that they would

have to delay start of the work and completion of the task would take three years.

Because of the priorities that the AEC

placed on this program, the Battelle Pacific

Northwest Laboratory was asked to conduct a similar program. "The primary purpose of the

Battelle study will be to consider ways and means to appraise any evaluation laboratory

that might be established. Performance criteria and procedures for use by evaluation

laboratories also will be drawn up. These

criteria would help to provide standards for

measuring continuing accuracy of service. "9

Independent of these AEC activities, the

National Sanitation Foundation (NSF) was showing interest in the evaluation of film

dosimetry services. The U. S. Public Health

Service (PHS) began supporting a program at NSF in 1963 which aimed at the development

of procedures and performance standards for

evaluation of film dosimetry services. NSF

felt that when procedures and standards were

developed, the film dosimetry industry

would voluntarily support an evaluation program leading to the award of the NSF Seal

of Approval.^ NSF believed that this approval would be recognized by the AEC and

others. The objectives of the NSF program

were consistent with the needs expressed by

the AEC and the program received broad

support from the film dosimetry industry.

The NSF Foundation Council of Public

Health consultants unanimously approved

Standard No. 16, "Standards of Performance

for Film Badge Services" on May 4, 1966.

The NSF advisory committee on film badge

services represented members of government

agencies, professional organizations, AEC

contractors, and industry. The basis used

in the establishment of control limits for

film badge service accuracy was primarily

developed from a study conducted by NSF

involving approximately two thousand film

badges from twenty five different organizations . ^ »

In trying to develop a program, the AEC

then considered, among other proposed actions

278
The NSF standard considers types of

radiation at different energy ranges. The

type of radiation with which a badge is

irradiated is not revealed to the participants. In order to successfully meet the

performance standard the participants' interpretation must be within certain control

limits. As stated above, these control

limits were established on the basis of the

performance of a group of film processors

that participated in an initial study.

These limits are specified in terms of the

width of an error band within which all but one of the participants' results must fall.

After reviewing the objectives of the

NSF program and anticipating its usefulness

in defining performance criteria that reflect the capabilities of film dosimeter

technology the Commission felt it was not in

a position to support NSF financially or approve their standard, although AEC staff

participated in program development meetings.

The Commission considered it desirable to supplement the efforts of NSF by establishing an alternate parallel program to permit

the Commission to determine the adequacy of

any evaluation program that might be established.

The Battelle study begun in 1965 took a considerably different approach to the

evaluation of performance. Battelle utilized two distinct parameters for use in

establishing performance; the systematic bias

and the random variations. These parameters

were considered to some extent to be unique

and that the analysis of performance should

consider them separately. A participant's

results would be corrected for a bias

greater than 10% before the variance criteria would be applied. Performance criteria are specified in terms of the parameters of the equation D^=a+6E-j_+e£^ where:

D-L is the actual delivered dose; E-j^ the

reported dose; a and g are systematic bias

parameters; and eg^^ is a random variable for

which the variance is computed.

The Battelle report^ also concluded

that for its evaluation of the performance

of the services tested that excessive bias

and variance existed among film dosimeter

processors. It concluded that a film dosimeter evaluation program should be implemented but reached no conclusion whether it

should be voluntary or implemented by regulatory requirements. The 1967 Battelle

report has not had a significant effect on

the testing of personnel dosimeters since an evaluation laboratory has not been established.

Following publication of the Battelle

study, the Commission considered requiring

all film badge services for AEC contractors

to meet performance standards compatible

with the standards recommended in the

Battelle report. In addition, commercial

film dosimeter vendors which were not providing a service to an AEC contractor but were desiring to bid on such services would

demonstrate their performance through an evaluation laboratory. The Commission con- sidered soliciting bids on a competitive

basis, for a film dosimetry evaluation laboratory service to judge the performance of

AEC contractor and commercial film vendor

services to AEC contractors. This would

have allowed the AEC contractor and other

private laboratories who could meet the

conditions of the call for bids to compete

on a competitive bid basis. However, a unique situation played a role in delaying

implementation of that plan. Up until 1967,

all AEC facilities utilized film rather than

TLDs. One reason for this was the Commission position that the permanence of film

gave it an overriding advantage over TLDs. During 1967, it was decided that the permanent record of the film should not be a deciding factor and that TLDs could be

considered for use. Consequently, during

the late 1960 's many AEC contractors began

changing over from film dosimetry to TLD

No data base for routine performance existed

In addition, there obviously were no authoritative TLD performance criteria. In that

respect, the film badge criteria proposed by

Battelle were not an authoritative standard

and the Commission felt some reluctance to use such a standard without consensus approval .

In 1969, the American National

Standards Institute (ANSI) requested

the Health Physics Society to write a document that could be used as an ANSI standard.

The Society accepted the task and as a result the ANSI standard (N13 . 7-Criteria for

Film Badge Performance) was approved on July 11, 1972^0 . This standard was based

largely on the Battelle study and the

approach to judging performance criteria

acceptance are identical.

AEC Regulatory formally endorsed the

ANSI criteria for film badge performance

in 1972. This endorsement was by means of

Regulatory Guide 8.3 which also pointed out

the Commission's exceptions to certain

parts of the standard. It should be

remembered that regulatory guides are merely

recommendations and do not directly place

requirements upon licensees. The publication of this regulatory guide has not had a

279
profound effect on film dosimetry processors

or licensees. To the best of our knowledge,

no licensee or processor has ever performed

the tests specified in the Standard and

guide. In this same year, an ANSI com- mittee was formed to consider standards for

performance for TLDs. A draft of this document which has been circulated for letter

ballot indicates that the approach which

is being considered differs considerably

from the film badge performance standard.

This standard has not been approved by the

American National Standards Institute, as

of this report.

NSF continued to conduct its program

for evaluating film badge performance on

a voluntary basis. The first "Approval

Listing for Film Badge Services" was issued

by NSF on July 1, 1967 and listed 9 approved

services (7 commercial services, 1 military

and 1 AEC contractor) . In addition, there were two other participants in the program

but not desiring to have an NSF approval.

However, this small number of participants

was not considered adequate to hire full

time NSF personnel to run the program or upgrade the physical facilities and instrumentation available. As a result, NSF was

subjected to some criticism with regard to the technical adequacy of their program and

administrative errors in implementing the

early program. Some of this may have been

aggravated by the loss of services of the

individual who played the major role in

initiating the program. For whatever the reason, the NSF program has continued to operate but has not gained the amount of

official or technical recognition to enable

it to have attained the impact originally

envisioned or gained the confidence of users

or regulatory agencies. The NSF program

continues to operate, but with fewer participants than when it began.

1973 saw the introduction of a new

participant in the attempts to evaluate

personnel dosimetry performance. The

Conference of Radiation Control Program

Directors holds an annual meeting sponsored

by the Bureau of Radiological Health (FDA)

,

the Environmental Protection Agency (EPA)

,

and the Nuclear Regulatory Commission.

During these meetings, workshops on various

subjects of interest and concern to the State

Radiation Program Control Directors are held.

During the 1973 meeting, the conference held

a workshop which was to consider how a personnel dosimetry certification program could

be established to assure that dosimetry re- sults were accurate.

The major recommendations of the workshop were that a federal agency be designated to direct a continuing performance

testing program and that the executive

committee of the conference establish a task force of federal and State representatives to consider this proposal during the next year.-*--^ Such a task force was established and included representatives of three states plus individuals from FDA, EPA,

OSHA, AEC (Regulatory) and the NBS.

The workshop had endorsed the idea of

NBS involvement to provide credibility to a testing program. As had been recognized

several years earlier, the NBS stature and

unique capabilities were felt to be factors

that could provide any program developed

with the necessary prestige and technical

backup which could not be obtained from a regulatory agency. As a result of the

initial meetings of this task force it was concluded that the only practical way of

having personnel dosimetry suppliers submit

to testing was through some regulatory device. From a practical standpoint, the

only agencies which had the authority

either directly or indirectly to compel

suppliers to participate in a testing program were OSHA and AEC. OSHA had limited

interest in such a program so it was left

up to AEC to determine whether it could

either condition licenses to require use of

tested dosimeters or change the regulations

to require their use. Although AEC had no control over X-ray users, it was felt that

essentially all dosimetry suppliers would

have to submit to a testing program to remain competitive. In early 1974, the

Director of Regulatory Standards directed

his staff to develop regulatory requirements

to require all licensees to use personnel

dosimetry services evaluated as adequate by

a testing or intercalibration laboratory.

In order to assist AEC in writing a rule and

establishing criteria for the program, a work group composed of directly concerned

federal agencies was formed which included

representatives of NBS, FDA, EPA, ERDA and

various NRC offices. This work group had

periodic meetings which began to crystallize

a testing laboratory concept and identified

remaining problems. Conceptually, it was

felt that the program probably would operate

in the following manner: Part 20 of the NRC

regulations requires the use of personnel

dosimetry under certain circumstances, i.e.,

when there is a possibility of an individual

exceeding 25% of the maximal permissible

exposure or where an individual enters a high radiation area. This section could be

amended so that all licensees must use dosimetry services whose performance is tested
to meet certain minimum criteria. The

testing could then be performed by a testing

laboratory using standards set by the Commission. The accuracy of a testing laboratory's calibrations and the manner in which

the testing laboratory could be technically

evaluated by the NBS which in turn could be

supported by NRC under an interagency

agreement

When the writing of the proposed rule

began, it was felt that two things should be

accomplished: 1) establishing a testing

laboratory in the proper regulatory framework; and 2) establishing standards for

the testing of personnel dosimetry suppliers.

In order to get the program moving, it was

felt that it would be advantageous to adopt

the ANSI film badge standard rather than

going through the process of promulgating new standards. It was believed that in

that way there would be only one problem

establishment of a testing laboratory.

However, as drafting began on a proposed

rule and discussions were held with various

individuals, it became evident that the

existing ANSI criteria for film badge

performance would not be satisfactory for

the use envisioned. It was generally agreed

that the ANSI criteria are a good set of

sophisticated criteria which may be useful

for judging state of the art for performance

of film. However, it became apparent that

what was needed for the regulatory process

are simpler standards based primarily on health physics considerations and only

secondarily on the dosimetry system limitations. A second problem as noted above, is

that there are no ANSI criteria for TLBs. These may be forthcoming but will also raise

similar problems as the film criteria. An

alternative to the ANSI criteria which was considered was the National Sanitation

Foundation No. 16. Although this standard

had some advantages, its rather simplistic

statistical criteria and emphasis of system

performance over health physics consideration

directed us toward consideration of new

performance criteria. The NBS, under sponsorship of FDA, attempted to develop these new criteria. This initial attempt of

developing criteria based on health physics

needs served as a spur to establishing a committee to produce a standard which could

eventually receive ANSI approval.

During the 1975 meeting of the Health

Physics Society, the standards committee

approved the formation of a working group

to produce a standard test method and

performance criteria to be used by a testing

laboratory to determine the output of a supplier of personnel dosimetry services. This

group has been designated Working Group 15

and is being chaired by Dr. Margarete Ehrlich

of the National Bureau of Standards.

Included on the working group are two com- mercial personnel dosimetry suppliers, a military processor, an ERDA contractor, a reactor health physicist, representatives

from two potential testing laboratories, NSF

and Bat telle and the author is the NRC representative. Eventually, it is anticipated

that these efforts will form the technical

basis of a regulatory action by the NRC.

The working group anticipates that a final draft will be submitted to the parent

Health Physics Society Standards Committee

in early May 1976. It would be improper for me to discuss the details of the efforts of

the working group but a brief discussion of

the philosophy and some of the basic ideas

that are emerging would be of interest.

Nowhere in the proposed standard is it

planned to make direct reference to the type

of personnel monitoring device employed. The

performance tests and test criteria will be

the same for all devices presently employed

and contemplated for future use. Performance

criteria will be based, as much as feasible, on the recommendations o f the ICRUI^ and the

ICRPl6 for monitoring accuracies adequate in

the field of radiation protection. Only

where it is known that the recommended

tolerance limits cannot be met, at the present state of the art, will wider limits be

set. An early revision date for reconsideration of these limits will also be recom- mended .

The various categories for which a supplier could request testing is shown in

figure 1. The first three categories refer

to photons in three different dose equivalent ranges between 30 or 50 mrem and 3 rem. Further tests for accident levels of 3 to 800 rads are specified in the first two categories. The next category is for beta

particles with the testing laboratories

prepared to use a sealed strontium 90 source

or a uranium slab. For the fast neutron

tests, either calif ornium-252 whose spectrum

is close to a moderated fission spectrum

or a source of higher average energy, such

as plutonium-beryllium will be available.

Mixtures of high and low energy photons;

photons and beta particles; and fast neutrons and photons constitute the last three

categories

281
Figure 1 RADIATION CATEGORIES AMD TEST RANGES

RADIATION CATEGORY TEST RANGE

1 Photons, 200 keV< E^^^ <3 MeV

I I Photons, 30 keV < Eg^^ < 200 keV

III Photons, 16 keV< Egff <30 keV

IV Beta Particles

\/ P;^c:t Npiitrnn*^

Vl Photon Mixtures

(Categories 1,11, and III)

VII Photons and Beta Particles (Categories i, and iV)

VIII Photons and Fast Neutrons

(Categories 1, and V)

(a) Protection: 30 mrem to 10 rem

(b) Accident: 10 rad to 800 rad

(a) Protection: 30 mrem to 10 rem

(b) Accident: 10 rad to 800 rad

50 mrem to 10 rem

150 mrem to 10 rem

100 mrem to 5 rem

50 mrem to 10 rem (total)

150 mrem to 10 rem (total)

100 mrem to 10 rem (total)

Each full category test will be con- ducted over a period of several months. It was considered rather important to test a supplier over an extended period of time in

order to be able to more effectively judge a supplier's performance for the entire period

under consideration. In general, a supplier

will not know which radiation types and

categories were used for the irradiations of

particular badges. There will be an exception in the accident range where the testing

laboratory will identify the badges involved

just as the users do when an accident has

occurred

Pass-fail criteria will be based on the

weighted sum of bias and standard deviation.

These quantities will be computed from the

results of the current test and the results

of past tests of the particular supplier in

the category. Bringing in past test results

will enable one to draw conclusions regarding the long term consistency of a supplier's

performance. This is important if inferences

are to be made about his daily processes from

the results obtained from a relatively small test sample. The bias for the process will

be determined from those irradiation levels

where it is found that the performance is

independent of the radiation level. A supplier will fail a test in a given category

if the weighted sum of the bias and the

standard deviation for the current test lies

outside the specified tolerance, either for

his interpretation of total dose equivalent

or for his interpretation of the dose equivalent due to penetrating radiation alone.

Depending on the category, the tolerance

will be set at 30 to 50%.

The pass-fail criterion will also take

into account the length of a supplier's

monitoring period. This was considered

important because if a supplier offers a weekly service, for example, imprecision in

his individual interpretations will cancel

out more readily over a period of a year

than if he offered a monthly or quarterly

service. Therefore, the weight attached to the bias relative to that attached to the

standard deviation will be greater the

shorter the monitoring period for which a supplier's service is offered.

The standard will also stipulate that

all badges will be calibrated in terms of

282
dose equivalent. This could be accomplished

by computing the dose equivalent from the

measured quantities (e.g., exposure, absorbed dose, flux density) at the prescribed

depth in a tissue equivalent phantom or the personnel dosimeters can be mounted on a phantom during irradiation.

NRC is delaying proposal of a rule

requiring testing of personnel dosimetry

until the performance standard has been

approved and the technical requirements of

the performance standard can be effectively

incorporated. If an existing standard could

have been adopted in a rulemaking action,

publication for public coimnent would already

have been made. However, we believe that

the delay will have significant benefits.

Recognizing that implementation of a performance testing program is complex and

will have a great impact on the dosimetry

suppliers and licensees, the NRC has decided

to hold a public meeting on this subject

prior to publishing a proposed rule for

comment

One would hope that the past efforts

in this endeavor would provide some guidance

for this current attempt at developing a system for testing the accuracy of personnel

dosimetry services. Considering the past

obstacles which caused delay and defeat,

three points can be identified, which should

be considered in the development of any mandatory testing program:

1. It is impractical to assume that a highly competitive industry will take

a unified action to police itself in

such a manner as to satisfy regulatory

authorities without adequate regulatory incentives.

2. A regulatory authority will not be in

a position to approve or recognize the

testing laboratory results unless that

testing laboratory is either an authoritative body, an arm of the

regulatory authority or a contractor

to the regulatory authority. Providing continuing regulatory recognition

to a testing or certification organization implies that the regulatory

agency can oversee the operation of

that organization and/or the testing

organization has national recognition

and peer acceptance.

3. The criterion for judging acceptability of a supplier's process should

be based on the health physics needs

of the users rather than the

capability of individual systems

developed from prior testing. This

will provide incentive for improving

dosimetry performance independent of

the system used for evaluating that

performance.

REFERENCES

1. Staff Reports: Intercomparison of Film

Badge Interpretations. Isotopics (No. 2) 5:8-33, April 1955.

2. W. A. Longmead, N. Adams, Investigations

of the Accuracy Attained in Routine Film

Badge Dosimetry, United Kingdom Atomic

Energy Authority, AHSB(RP)R62, 1966.

3. R. 0. Corson, N. Suntharalingam and

John W. Thomas, Results of Film-Badge

Reliability Study, Radiology, 84, No. 2,

333-346, February 1965.

4. D. E. Barber, Film Badge Service

Performance, Radiological Health Data

and Reports, Vol. 7, No. 11, 623-626,

November 1966.

5. C. M. Unruh, H. V. Larson, T. M. Beetle,

and A. R. Keene, The Establishment and

Utilization of Film Dosimetry Perfor- mance Criteria, BNWL-52, Battelle Pacific Northwest Laboratory, September

1967.

6. D. E. Barber, Standards of Performance

For Film Badge Services, Public Health

Service Publication No. 999-RH-20,

September 1966.

7. L. L. Nichols, Performance Tests of

Commercial Personnel Monitoring

Services, Presented at Health Physics

Society Meeting, Buffalo, N.Y., July 16,

1975.

8. U. S. Atomic Energy Commission, Notice

Regarding Need For Establishment of

Film Dosimetry Calibration Laboratory

61F.R. 9411, September 1963.

9. U. S. Atomic Energy Commission, Press

Release No. IN-616, September 9, 1965.

10. Criteria For Film Badge Performance,

ANSI N13. 7-1972, American National

Standards Institute, July 11, 1972.

11. U. S. Atomic Energy Commission, Regulatory Guide 8.3, Film Badge Performance

Criteria, February 1973.

12. Thermoluminescent Dosimetry Standards

For Performance, Draft ANSI Standard,

HPSSC/N B/42/WG-1.

13. Workshop No. 3, Personnel Dosimetry

Evaluation and Control, 5th Annual

National Conference on Radiation

Control, DHEW Publication (FDA) 74-

8008, 342-343, October 1973.

14. U. S. Nuclear Regulatory Commission,

283
10 CFR Part 20, Standards for Protection

Protection Against Radiation.

15. Radiation Protection Instrumentation

and its Application, Section C, ICRU

Report 20, Washington, D.C., 1971.

16. General Principles of Monitoring for

Radiation Protection of Workers,

ICRP Publication 12, 1969.

284
NBS SP456 (1976)

OPTICAL INTERFEROMETRIC MEASUREMENTS OF ULTRASONIC

RADIATION AND ITS APPLICATIONS TO MEDICINE

M. E. Haran and H. F. Stewart

Bureau of Radiological Health

Food and Drug Administration

Rockville, Maryland 20852

A modified Michelson interferometer is currently being used to map

particle displacement amplitude within a beam of pulses ultrasound.

One leg of the interferometer is raster scanned across a 4 micrometer thick, opaque gold coated membrane upon which the sound is incident. The reference leg of the interferometer has been modified to provide stability. Sensitivity has been found to be at least 10 picometers which corresponds to 1.0 micro watts/cm^ at 1.5 MHz. Particle

displacement amplitude is displayed in two modes: 1) displacement

vs. position for each horizontal scan, 2) displacement vs. position

for an entire frame by Z-axis modulation of a CRO. Frame rates of

2/sec. can be achieved. This system has been used to determine

measured ultrasonic field patterns. Parameters of interest include

peak intensity, intensity distribution, ratio of peak to average intensity and calculation of beam area. Methods for specifying beam

areas will be discussed as well as the application of the system to evaluate medical diagnostic and therapy transducers. This system was developed by RCA Laboratories, Princeton, New Jersey.

INTRODUCTION

A modified Michelson interferometer is currently being used by the Bureau of Radiological Health (BRH) to map particle displacement within a beam of pulsed ultrasound.

This system, developed by RCA Laboratories,

Princeton, N.J., employs a 15 mW HeNe laser

beam to raster scan a four (4) micrometer

thick, opaque gold-coated membrane upon

which sound is normally incident. Since the

membrane is extremely thin compared to the

acoustic wavelengths of interest, it is

transparent to the sound while experiencing

the same particle displacement as the sur- rounding transmission medium. At each point

in the raster scan, the particle displacement amplitude is measured and displayed in two modes: 1) displacement vs. position for

each horizontal scan, 2) displacement vs. position for an entire frame by Z-axis modulation of an oscilloscope. The first display has been calibrated to sensitivities

corresponding to 10~^ watts/cm^ of acoustic

intensity.

Under the purview of the Radiation Control for Health and Safety Act, Public Law

90-602, the Bureau is currently investigating the output characteristics of medical

ultrasonic devices. Some of these characteristics that can be measured by this interferometric system are peak intensity, intensity distribution, ratio of spatial peak

to average intensity, and calculation of

beam area. The last parameter is discussed

in some detail since calculation of an area

from experimental data requires a definition

of beam area. This definition will influence

the methods of data collection as well as computational techniques. In this paper two methods for specifying beam area are discussed and compared. In addition, the interferometer is discussed in some detail and

examples are given of its measurement capability.

Description of System

The optical interferometer is of the

Michelson type and has been referred to as the "ultrasonovision . " Figure 1 shows the

basic optical arrangement. The beam from the NeHe laser is split into the two legs of the

interferometer. In the reference leg is

mirror Ml. In the scanning leg are galvanometer mirrors M2 and M3 which provide the

vertical and horizontal scanning, respectively. Lens L4 collimates the raster scan- ning beam to provide normal incidence to the

thin, gold coated membrane (pellicle) . The

transducer is located behind the pellicle.

Characteristics of interferometers is

the brightness curve (figure 2) relating the

relative phase difference between the legs of

the interferometer with the optical irradiance measured at the photodetector . The

optimum operating condition for the ultrasonovision occurs when the relative phase

difference is 11/2. At that point any phase

change caused by motion of the pellicle will

285
cause a maximum brightness change and the

functional relationship between them will be

nearly linear. To guarantee this operating

condition an open loop technique is employed in which the reference mirror Ml is "wiggled" along the optic axis at a frequency of

approximately 30 kHz. The amplitude of the

wiggler is such that the relative phase difference is swept through at least H radians.

At the photodetector the combined effect of

the pellicle motion at the acoustic frequency and the wiggler motion is a signal

varying in time as the acoustic pulse and

contained in an envelope having the wiggler

frequency. The peak of that envelope occurs

when the relative phase difference is 11/2. Therefore, peak detection of the photodetector signal results in the displacement

amplitude at that scan location in the

acoustic field. Another advantage of this

technique is to eliminate noise due to thermal drift and vibration.

The operation of the interferometer is

described by the following analysis • Consider an acoustic wave of angular frequency

ojg and displacement amplitude A passing

through the pellicle. When one point on the

pellicle is interrogated by the laser, the

photodiode generates a current ig having the

form

ig = 2rvJlj^Ip cos [$+ 41fA sin (Ugt) ] (1) X

where n is the quantum efficiency of the

photodiode, Ir and Ip are the irradiances

in the reference and pellicle legs of the

interferometer, respectively, $ is the re- lative phase difference between legs, and A is the optical wavelength. The d.c. terms

of this equation have been eliminated by

the use of high pass filter. If A/A <<1 and

$=1T/2 then equation (1) reduces to

ig = 8 Hn A ^iRlp sin (logt), (2)

A where the signal amplitude is directly proportional to the displacement amplitude of

the acoustic wave. This approximation is

good for acoustic intensities up to several

watts/cm^

.

The introduction of the wiggler for the

reference mirror alters equation (2) such

that

ig = 811n A_.^lRlp sin (Wgt)

sin [$+ 1[ sin (w^t) ] - (3)

2

If the wiggler displacement is greater than

A/4 and the ratio of the acoustic frequency

to wiggler frequency is sufficiently high,

then the relative phase difference will be

11/2 at the same time sin (ajst)=l. Therefore, peak detection of the wiggler envelope

will yield the displacement amplitude at the

point of interrogation.

The sensitivity of the system is determined by thermal noise and shot noise. At low light levels thermal noise predominates. For a pellicle leg irradiance of 0.1 mW and

unity signal to noise ratio, the particle

displacement is on the order of 0.03A. From

equation (3) it can be seen that lower displacement amplitudes can be detected for

higher irradiances at a fixed photodetector

signal. It has been estimated that the ultimate system sensitivity is 10"-'-'-' W/cm^

(acoustic) at 1.5 MHz. The BRH system presently can detect displacements lower than

O.IA corresponding to 10~^W/cm2 (acoustic)

at 1.5 MHz

.

The system was calibrated by illuminating the photodetector with a known intensity of light and measuring the resultant

voltage after the photodetector preamplifier. An NBS C-series laser calorimeter

was used to determine the irradiance. The

total estimated uncertainty is on the order

of +4%.

Examples

Several transducers were scanned at various distances along the propagation

axis. These scans show the ability of the

ultrasonovision to measure the radiating

patterns from both NDT and medical transducers .

Figure 3 shows the development of the

acoustic radiation from a 2 . OMHz , air-backed

ceramic transducer. The pulse width was of

the order of 9ms. and the crystal diameter

was 1.3cm. At each location in the field

two displays are shown. The first is a brightness modulated picture of the beam

cross-section. The grey scale was individually adjusted for each photograph to show

maximum detail. Therefore, no comparion

can be made between grey levels of any two photos. The second display is a single

horizontal scan through the beam center. The horizontal axis corresponds to scan position and the verticle axis corresponds

to particle displacement amplitude. The

verticle and horizontal scales are the same

286
within each scan series of a transducer for

purposes of comparison.

An immersion NDT transducer is shown in

figure 4. It is nominally rated at l.OMHz,

1.0 inch diameter, and 5.0cm focal length.

Using a Hewlett-Packard 4815A RF vector impedance meter the resonant frequency was

found to be 1.16MHz. This transducer was driven at its resonant frequency with a pulse width of 9. Ops. According to theory,

the diffraction pattern produced at the

focus of a finite aperture lens is the product of the Fourier transforms of the aperture function and the field distribution

across that aperture. For a plane distribution across a circular aperture, the focal

pattern takes the form of a Besinc function^

This function is characterized by a large

central peak (Airy disc) and several con- centric rings (Bessel rings) of lower amplitude .

Referring to figure 4, it can be seen that the distribution at the transducer face

does not occupy the full aperture nor is it

plane

Measurement of the Airy disc shows it to correspond to an aperture diameter of

0.83 inches. This is confirmed in the scan

of the transducer face.

Figures 5 through 8 are scans of medical diagnostic transducers. Each is nominally rated as having a 2.25 MHz resonant

frequency. As before, the vector impedance

meter was used to determine the resonant frequency and each transducer was driven at the

resonant frequency with a 9.0vis. pulse width

to produce each figure.

Figure 5 shows a nominal 19 mm diameter,

10 cm. I.F. (internally focussed) transducer

driven at its resonant frequency of 2.12MHz.

At the focus (10cm) the Bessel rings are not

well defined because of the lack of uniform

amplitude distribution at the transducer

face

The transducer depicted in figure 6 has

a nominal 13mm diameter radiating area, no focussing, and a 2.05 MHz resonant frequency

The uneven amplitude pattern at the transducer face is carried through in near field

in the unusual structure of the field pattern.

In figures 7 and 8 are two biopsy

transducers having resonant frequencies of

2.91 MHz and 2.52 mz , respectively. The

first shows a relatively uniform

amplitude distribution at the transducer

face and the field development is relatively even. The second transducer radiates

primarily from the upper right quadrant of

the face, and this is reflected in the development of the field. It is interesting

to note that these last two transducers

were production line rejects because they

failed leakage current tests but not for

their acoustic behavior.

Beam Area

During medical diagnostic imaging or flaw detection, the beam area (lateral dimensions for pulses) strongly determines the

effective resolution of adjacent objects.

The definition of beam area or radiating

area at the crystal face must be sufficiently broad to include the various outputs

demonstrated in the previous figures. In its specification for ultrasonic therapeutic

equipment, the American National Standards

Institute (ANSI) defines effective radiatin

area as "that [area] which is 1.11 times th area, measured at the applicator face and

using circular baffles..., which transmit

90 percent of the effective acoustic power

The difficulty with this definition is its

assumption of a radially symmetric distribution of acoustic intensity. Significant

errors can occur when applying this definition to transducers of the type shown in

figure 9. This is a commercial therapy

transducer of multiple crystal construction

the output from which is shown in this

schlieren photograph.

As an alternative, the radiating area

(or beam area depending on location) can be

defined as the area in a plane normal to the

beam axis consisting of the locus of all

points at which the intensity is greater

than a given percentage of the maximum intensity in that plane. Using this definition several transducers were scanned with

the ultrasonovision at a distance of 4 mm

from the transducer face. Each scan was digitized and area versus percent of peak

intensity calculated on a Hewlett-Packard

9820 programable calculator. The results

are plotted in figures 10 through 12. Each transducer is a therapy applicator and

was driven at its nominal resonant frequency

(1.0, 1.0, and 0.925 MHz respectively).

Figures 10 through 12 show the beam

areas calculated from the peak intensity,

calculated from the ANSI definition, and

that area specified by the manufacturer.

Figure 12 corresponds to the multiple crystal therapy applicator whose schlieren

287
photograph was shown above. The large discrepancies between measured areas and the

manufacturers specification is apparent.

SUMMARY

The optical interferometr ic system has

been shown to be an effective method to determine ultrasonic field patterns. With

a sensitivity below 0.1^, this system can detect acoustic displacement amplitudes

corresponding to intensities below 1.0

microwatts/cm2 . It has been used on therapy,

diagnostic and NDT type transducers to measure peak intensity, intensity distribution and beam area.

In the calculation of beam area, an alternative to the ANSI definition has been

suggested using the peak intensity in the

plane of measurement. This definition has

been shown to be more general and applicable

to transducers of multiple crystal design

as well as those with asymmetric radial

intensity distributions. It is felt that

this alternative definition would better

apply to the specification of transducer

characteristics

.

References

1. R.S. Mezrich, K.F. Etold, and D.H.R. Vilkomerson "System for Visualizing

and Measuring Ultrasonic Wavefrents",

RCA Review , Vol. 35, No. 4, Dec 1974,

p. 483-519.

2. G.B. Parrent, Jr., and B.J. Thompson

Physical Optics Notebook , Society of

Photo-optical Instrumentation Engineers

Redondo Beach, Calif. 1969, p. 5.

3. American National Standard Specification for Ultrasonic Therapeutic

Equipment z24. 18-1956, para. 2.3.4.

288

RELATIVE PHASE DIFFERENCE

Figure 2. Brightness Curve Showing the Relationship of

Relative Phase Difference Between Interferometer Legs

And Optical Irradiance Upon Recombination

I

290

'I
291
i
293
Figure 9. Schlieren Photograph Showing Output From A Commercial

Multiple Crystal Therapy Applicator.

294
1 2

I I

m ^ ^ a

EE

Cil

CE ^ U

RNUF-RCTUREZR 5 BPETC . < MD- I >

RN5 I

!IZI 30 3S M0 Hi

PETRCETNT F" PETHK I NTETNB I TV

Figure 10. Plot of Beam Area Calculated As A Function of Peak Intensity

At The Face of A Commercial Therapy Applicator (f=1.0MHz).

295
J

in

CE

U

q:

I I

I 121 3

B

V

B

CE ^

LJ

to M

3 ..

-MRNUFBCTURETR iPETC . < RM-H >

J=iN5 I

I IZ) IE] 121 pe:rce:nt df- peirk N

H0

N

MS

I TY

Figure 11. Plot of Beam Area Calculated As A Function of Peak Intensity

At The Face of A Conimercial Therapy Applicator (f=1.0HMz).

296
m

IT

U

(Z

[T

Ld

HS 1

M2I

323 ..

2IZ1 ..

1 E3

^^RNUF-BCTURE:R ' B BPET-C . < I_ — 3*7 >

J=1N5 I

S: IE IS 20 SS' 30

pe:rce:nt of" pe:hk NT

H0

N5 T Y

Figure 12. Plot of Beam Area Calculated As A Function of Peak

Intensity At The Face of The Commercial

Therapy Applicator of Figure 9. (f=925kHz)

297

NBS'SP456 (1976;

PHOTON MASS ENERGY-ABSORPTION COEFFICIENT AIR/MEDIUM

RATIOS FOR lONOtlETRIC DOSIMETRY 0.1 keV TO 20 MeV*

J. H. Hubbell

National Bureau of Standards

The determination of photon absorbed dose in a medium by means of an air-ionization chamber depends in part on the ratio of the mass energyabsorption coefficient y /p for air to that for the chamber wall

en material, to account for the differing atomic compositions. In this

paper, mass attenuation coefficients y/p developed at the National

Bureau of Standards by the X-Ray and Ionizing Radiation Data Center

from the latest theoretical cross section data are compared with

measurements for selected low-Z elements of dosimetric interest. From, this cross section data base, mass energy-absorption coefficients Vg^^/P are derived for air and selected media, along with air /medium ratios,

over the photon energy range 0.1 keV to 20 MeV including values at the

cesium-137 and cobalt-60 energies. Comparisons are made with earlier

published y /p values and air/medium ratios,

en (Attenuation coefficient; cross section; dosimetry; energy-absorption

coefficient; gamma-ray; photon; x-ray)

Introduction

The mass energy-absorption coefficient

/p enters into medical and industrial

en irradiation x-ray metrology in various essential ways. For example, as has recently been

discussed by Loevinger"*" and by Niatel, Loftus

2 and Oetzmann, present reference standards for

gamma-ray beam calibration in the cesium-137

(0.66 MeV) and cobalt-60 (1.17, 1.33 MeV)

energy region rely on air cavity ionization

detectors by which the exposure rate X at a point in the radiation field is determined as

m f (y/P^air

(^en/P>wall

UK. (1)

where I/m is the measured ionization_curr ent

per unit mass of air in the cavity, f is the

ratio of the average mass collision stopping

powers for air and the cavity wall material

and IlK. is the product of additional correction factors.

3 As defined in ICRU Report 19 and

schematized in Fig. 1, the mass energy-absorption coefficient y /p may be written as en ^

Work supported by the NBS Office of Standard

Reference Data.

y en mc

inc

+ --- • f + — • f (2)

in which y . . mc/p, y /p and y /p are the contril T K tributions to the mass attenuation coefficient

y/p from incoherent (bound-electron Compton)

scattering, photoeffect and pair-production

(including triplet) respectively. These

H-o/p H-^/P /^en/P

Fig. 1.

299
partial coefficients y^/p are related to the

respective atomic cross sections o. as

P,/P = a. (N^/M) (3)

where N, is Avoeadro's number and M is the

A

^ atomic weight. In Eq. (2) each factor

weights the respective partial cross section

by the average fraction of the initial

photon energy transferred to kinetic energy

of charged particles. The remaining fraction

(1 - f^) of the initial photon energy

includes secondary photons from the primary

interaction, such as Compton-scattered and

fluorescence photons, and from subsequent

radiative processes of the charged particles

such as bremsstrahlung and positron annihilation.

Previous Compilations of U /p en

The first major reviews and tabulations

of the mass energy-absorption coefficient

^ were the independent efforts of R. T. Berger

5 and J. W. Allison. The Berger tabulation

(under the name "energy transfer coefficient")

included 15 elements plus air and water over

the range 3 keV to 10 MeV and took into

account positron annihilation in flight as well as Compton scattered photons (but

neglecting electron binding effects), fluorescence, and bremsstrahlung by Compton and

pair electrons. A procedure for including

photoelectron bremsstrahlung was described

but not used by Berger. Allison's tabulation

(under the name "absorption coefficient") for seven elements plus air and water for energies

10 keV to 100 MeV was similar to Berger 's except for the inclusion of photoelectron

bremsstrahlung and omission of positron

annihilation in flight.

y /p values for all elements Z = 1 to 100

en over the range 1 keV to 100 MeV. They

included electron binding effects in their

incoherent (Compton) scattering cross sections, combined with free-electron energyabsorption fractions. The Storm and Israel

energy-absorption fractions in the range 10

to 100 MeV were taken from Allison^ and hence

do not include positron annihilation in

flight.

Addit ional tabulations of y /p include

en those given in ICRU Report 10b (NBS Handbook

85)"'""'" relying prim.arily on the Berger^ data,

for 12 elements and 8 mixtures over the range

10 keV to 10 MeV. This tabulation was revised

12 and extended down to 1 keV in ICRU Report 17 for 12 elements and 9 mixtures over the energy

range 1 to 150 keV. In deriving the latter

tabulation, the basic interaction cross sections used were combined and smoothed from

(1) NSRDS-NBS 29, (2) McMaster et al, and

14

(3) an earlier version of Storm and Israel

Present V^^/P Compilation

The mass energy-absorption coefficients

presented in table 1 for the five low-Z

elements H, C, N, 0 and Ar , and seven mixtures of these elements, extend previous

y^^/p compilations down to 0.1 keV and

incorporate new theoretical and experimental

cross-section data evaluated by the NBS X-Ray

and Ionizing P^adiation Data Center from the

j published and report literature. The air

composition was taken as .755 N, .232 0 and

.013 Ar by weight. The treatment of the

incoherent scattering, photoeffect and pair

production cross sections indicated in

Eq. (2) is described below.

Incoherent (Bound-Electron Compton) Scatterin;

As part of the continuing X-Ray and

Ionizing Radiation Data Center program at NBS

the Berger results were updated in NSRDSNBS 29^ for 18 elements plus air and water

over the range 10 keV to 10 MeV using revised

basic cross section data and f . values

4

^ interpolated from Berger. Earlier versions

of this revised set of y /p values were also

en

^ published in Radiation Dosimetry , the IAEA

g Engineering Compendium on Radiation Shielding

9 and The Physics of Radiology . Storm and

Israel"'"'^ used similar procedures to derive

The combined quantity (y . /p) • f. ^ ' inc. inc. was computed by numerical integration, over all outgoing photon angles, of the product

of (1) the differential Klein-Nishina formula

(2) the incoherent scattering function S(q,Z)

as recently tabulated by Hubbell et al,'''^ (3) the ratio T/k of the Compton electron

kinetic energy T to the incident photon

energy k and (4) the bremsstrahlung loss

correction [1 - G(T,Z)] where G(T,Z) is the

"radiation yield" tabulated by Berger and

Seltzer. "'"^ The combined radiative and \ double-Compton correction as calculated by

300
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301
Mork was included as a multiplicative

correction outside the integral sign.

Fluorescence resulting from Comp ton-created

vacancies was neglected.

Photoeffect

Photoeffect energy-absorption fractions

f , ... were calculated for the K, K

. subshell groups using the procedure

4 5 described by Berger and used by Allison

to take into account photoelectron bremsstrahlung as well as primary fluorescence,

18 modified as suggested by Carlsson to include additional fluorescence cascade

effects. Fluorescence and absorption-edge

19 energies were taken from Bearden, the

bremsstrahlung data were again taken from

Berger and Seltzer,"''^ fluorescence yield

20 21 data from Bambynek et al and from Freund

and data on L-shell vacancies resulting from

primary-event K-shell vacancies were taken

22 from Robinson and Fink.

The total photoeffect energy-absorption

cross section T

OAMANY- ASTOIN (1971. 75) HENKE (1967) DENNE (1970) BEARDEN (1966) WOERNLE (1930) MILLAR (1974)

PRESENT — VEIGELE ET AL (1971) SCOFIELD (1973), HF RENORM.

ICRU REP, 17 (1970)

-J \ I

I I I I 10

-J I

I I I I I I I I I I I I I

E , keV

f , where T is related to

T y^/p according to Eq,

lated as

T K

(3), was then calcu-

(4)

where theoretical photoeffect subshell-group

cross sections T

Veigele et al

K' 23,24L' were taken from

Fig. 2.

and extrapolated from the 1.5 to 10 MeV values

4 given by R. T. Berger (Table X) which take

into account positron annihilation in flight.

The graphical extrapolation to 20 MeV results

in uncertainties in f^ at 20 MeV of the order

of + 5% for the low-Z materials in this report

to ~ + 15% for Pb.

However, the pair production (nuclear

field) and triplet (electron field) cross sections were recalculated since new mass attenuation coefficient measurements by Henry

32 33

and Kennett, Ahrens et al, Bar-Noy and

34 Moreh and others indicate the need of

in the region 0.1 to

,25

revision of earlier NBS values 6,10 in the

1 keV and from Scofield from 1 keV to

1.5 MeV with some adjustments to fit measured

total attenuations coefficients as shown for

example in Fig. 2 for nitrogen. In Fig. 2

2 all cross sections are multiplied by E to better display differences between the selected

^ , 26-31 ^, _ ,23-25 , experimental, theoretical and

12 compiled ICRU Report 17 values. The Scofield

values were extrapolated above 1.5 MeV by an empirical formula (Eq. 2.-1) given in NSRDSNBS 29.^

Pair and Triplet Production

The pair production energy-absorption

fractions f^ in Eq. (2) were not recalculated

for the present tables, but were interpolated

threshold (1.022 MeV for K , 2.044 MeV for K ) n e to 20 MeV region.

The pair production cross section used

in the present work was combined from (a) the

near-threshold exact Coulomb-field relativ35 istic results of 0verb^, Mork and Olsen,

(b) the near-threshold HFS (Hartree-Fock3 6 Slater) screened results by Tseng and Pratt

37 and (c) the Mork-Olsen radiative correction

factor. These results join smoothly (differing by 0.2% or less at 30 MeV) with new highenergy screened results calculated using

fi Eq. (2. - 31) in NSRDS-NBS 29 in which the

empirical correction factor is replaced by an

38 updated correction factor fitted by Ahrens

to recent mass attenuation coefficient measure

ments

302
The triplet cross section for energies

less than 15 MeV, where screening is assumed

negligible, was taken directly from the new

39 rree-electron results by Haug who extends

40

the 2.044 - 10 MeV Mork results up to 2.55

GeV by an improved numerical integration

procedure. These results join smoothly

(differing by 1.5% or less at 15 MeV) with

higher-energy screened results calculated using

41

a procedure first suggested by Bethe

,

40 according to Mork, in which the VJheeler42 Lamb TF (ThoTcas -Fermi) screened triplet

result is corrected for electron-proton differences neglected by ^^Jheeler and Lam.b. In the

present calculation this correction is obtained

43

by subtracting the bare-proton Maximon

39 results from the Haug free-electron results.

The pair-production energy-absorption

cross section was then calculated as

K • f = (K + K )

• f (5) K n e K

in which K is related to y /p according to

Eq. (3).

Air/Medium Ratios

Table 2 lists the ratio (p /p) . / en air

(y /p) ,. as called for in the exposureen medium

rate determination in Eq. (1) for air vs all

TABLE 2

MASS EnFRGY A B SDRP T i C N COEFFICIENT RATIOS . AIR/MEOIUM.

fcNE hGY AIT)/ AIR/ A IK/ A IH/ A IR/ AIR/ AIR/

E V CARBON WA TER POL YSTYRLUC I TH POLYETH. 3AKELI TE AMBER

1 .CCCO+02 1 . fiSoS . <2527 1 . 9399 1 .3243 2.0 176 1 . 54 38 1.706 6

i .5CU0+02 1 . aS79 . d 198 2. 0022 1 . 3 124 2.1012 1 .5501 1 .7 360

2 .CCOO +02 1 . Ho2. 7955 1. 2875 2.1057 1 . 5282 1 . 72 1

3 . C C CC+ 02 .0924 .9124 .1001 .1451 .1078 .1164 . 115 2 ^^^.0000 + 02 .0912 . 9t;4S . 096fi .1440 .1064 .1151 .114 0 5.CCC0+02 1.077^ 11. 5636 I . 1 6 73 1 . 7004 1 .2572 1 .3597 1.3460

6 .CCCO + 02 1 . 50 73 . 9 2 £ 1 1 • 6331 1 . 2704 1 .7590 1 .3 Jl 8 1 .5346

6.CCC0+02 i .56 6* .9025 1 . 69 7^1 1 . 279

1 .H282 1 .4201 1.5748

1 .CCCC+P3 1 . 6279 . t5B54 1 . 7641 . 2903 1 .9002 1 .4502 1.6163

1 .S.CCO + 03 1.779 1 . 86 5 3 1 . 9279 1 . 32 73 2. 0767 1 . 5289 1 . 7230

2. CCOO +0 3 1 ,SIA3 . 8574 1 .9705 1.3338 2.1227 1 . 54 54 1 . 74H

3 .CCOO+03 1 . 3599 .8455 2. 0 1 56 1 . 33o5 2.1713 1 .56 11 . 7722

4.CCCO+03 2. 1134 . 940 1 2.29 57 1.5014 2.4731 1 . 76 39 2.0063

s.t;cco + 03 2 . 1 o96 . 9455 2. 3513 1 . 52 1 8 2.5329 1 .7950 2.0454

6 .CCCO+03 2 . 2 0a« . 9504 2 > 393 7 1 • 5378 2.5786 1 .8189 2. 0755

8 .CCCC+03 2.2650 .9524 2. 45 44 1 . 5 1' 5 8 2.6438 1 .8496 2. 1 1 54

I .COOO + 04 2.3206. . 96 06 2.5143 1 . 5 792 2,70 78 1 .8840 2. 1580

1.5CCC+04 2 . 40 30 . 9731 2 . 6S 0 0 2 1.6134 2.7968 1 34 0 2.2186

2.C0C0+04 2 .4394 .98 13 2 . 63 0 2 1 . 63 0 0 2.8 19 1 1 .9564 2. 2404

3 .CCOO + 04 2.3479 .9903 2. 4842 1.6029 2.6143 1.9032 2.1412

4. CCOO + 04 2. 0547 .9863 2. 1 020 1 . 4909 2 . 1 443 1 .7203 1.8658

5.(;CC0 + 04 1 .-'1 12 . 9722 1 . 6919 1 . 3374 1 .6756 1 .4891 1 . 5505

6 .C CQC + 04 1 . 4477 . 9545 1 . 3970 1. 20 16 1.3563 1 .2987 1.3106

8.CCC0+04 1 . 1796 . 9272 1 . : 1 23 1 . 04 4 5 1 .0604 1.0925 1 . 0678

1 . C CCO + 05 1 .082

. 9 1 35 1.0120 .9616 .9587 1 .01 38 . 9794

1 .5CC0+O5 1 . 0 1 87 .90 30 .9479 . 9 389 .8946 .96 1 8 .9222

2 .C CCC+05 1 . 0064 . 9C07 .9355 . 9304 .P822 .95 1 5 .9111

3 .CCC0+05 I . CO 1 0 . 8996 .9301 . 9267 .8769 .9470 .9062

4 .CCCO+05 .9998 . o994 .9289 .92 59 .3757 .9460 .905 1 5.CCC0+0S .9994 .8993 .9284 . 9255 .8752 . 9456 .9047

6.CC00+05 .9992 . 8992 .9282 .925.3 .8749 .9454 .904 4 ^ .£1 63+05 • 9991 . 8991 . 9281 .9253 .8740 .9453 .9043

8. CCCO+05 .9989 , 899 1 .9279 . 9251 .8746 .9452 .904 1

1 .ccoo+oa .9967 .8990 . 9^76 . 9250 .8743 .9450 . 9039

1 . i 7 ;- 2 + 0 e .9986 . 8990 . 92 74 . 9243 .874 1 .9448 .9037

1 .2522 + 06 .9 935 . 89 B9 . 9274 . 9248 . 8740 .9448 .9037

1 .3325 + 06 . 6989 . 92 73 . 9248 .8740 .9448 .9036

1 .5cco+oe .9966 . 8989 .9274 . 9248 .874 1 .94 49 .9037

2.CCC0+06 .9996 . 3995 . 9288 .92 58 .8757 .9459 .9051

3 .CCOO+06 1 . C043 .90 17 . 9354 .9306 .88 35 .95 1 0 .9116

4 .CCCO + 06 1 .01 06 .9048 .9444 .9370 .894 1 .9580 . 9206

S .CCCO + 06 1.0175 .9082 . 9542 . 944 1 .9059 .9655 .9304

6.CCG0+O6 1-0243 .9116 . 9640 .951 1 .9177 .9731 .9402

8. CCCO+06 1.0371 .9179 .9828 .9643 .9406 .9873 .9589

I .CCOO + 07 1 . 0488 . 9236 I .0000 . 97 64 .96 17 1 .0003 .9761

1 ,5CCCV07 1 . 0710 . 9355 1 . 0342 1 .0003 1 .0046 1 .0258 1.0106

2.CCC0+07 1 .0865 . 9437 1 . 0539 1 . 01 73 1 .0364 1 .0439 I . 0357

303
the remaining mixtures in Table 1, also vs carbon. The unusually large and small ratios

in the region 0.3 to 0.5 keV result from

displacements of the K absorption edge in C, N and 0.

Comparison of Present Results

with Previous Compilations

In tables 3 and 4 the present y /p en values for air and carbon are compared with

Table 3. Air Mass Energy-Absorp

4 Berger

5 Allison ICRU 10b

E(MeV) 1961 1961 1962

0.0001

.0003

.001

.003

.01 0.454 0.456 0.466

.03 .0145 .0144 .0147

.1 .00231 .00231 .00231

.3 .00287 .00287 .00288

1.0 .00279 .00280 ,00280

3.0 .00205 .00206 .00205

10.0 .00143 .00145 .00144

20.0 .00131

11

ion Coefficient y /p, m^/kg

NSRDS Storm „ ^ NBS 29 Israel ICRU 17 Present

1969 1970 1970 1976

4827.

4093.

358. 360.8

16.1 16.07

0.461 0.458 0.459 .4648

.0148 .0149 .0140 .01504

.00234 .00229 .00229 .002319

.00288 .00287 .002872

.00278 .00279 .002787

.00205 .00205 .002055

.00145 .00147 .001445

.00133 ,001306

Table 4, Carbon flass Energy-Absorption Coefficients y /p, m^/kg

4 Berger ICRU 10b''-"'- NSRDS

NBS 29

Storm

Israel ICRU 17-'-^ Present

E(MeV) 1961 1962 1969 1970 1970 1976

0,0001 2600.

,0003 4430,

,001 218. 209. 221.6

.003 8.90 8.92 8,85 8.641

.01 0,187 0.194 0.198 0.198 0, 194 0.2003

.03 .00595 .00592 .00596 .00637 ,00578 .006407

.1 .00214 .00213 .00216 .00212 ,00212 .002143

.3 .00287 .00288 .00288 .00287 .002869

1.0 .00279 .00279 .00279 .00279 .002791

3.0 .00204 .00204 .00204 .00204 . .002046

10.0 .00136 .00137 .00138 .00140 ,001378

20,0 .00122 ,001202

T . .-, . 4-6,10-12 values from the previous compxlations

surveyed in this report. Of particular

12 interest are the ICRU Report 17 y /p values ^ en

at 30 keV for both air and carbon which are

seen to be 5% lower than the average of the

other compilations including the present

results. The ICRU Report 17 y /p values

en appear to be systematically 5% to 10% low for

C, N and 0 over the range 20 to 50 keV,

In table 5 the present air/carbon ratio

given in table 2 is compared with this ratio

derived from air and carbon V-^^/ P data from

the previous compilations as listed in tables

3 and 4. Where Compton scattering predominates

(100 keV to 10 MeV for the low-Z materials in

this report) the different compilations yield

air/carbon ratios differing by less than 0.5%.

Below 100 keV, where the photoeffect predominates, the differences are seen to range up

to 5%. 304
Table 5. Air/Carbon Ratios (y /p) . /(y /p) , en air en carbon

Compared with Previous Compilations

, NSRDS, Storm „ Berger ICRU 10b NBS 29 Israel ICRU 17 Present

E(MeV) 1961 1962 1969 1970 1970 1976

0.0001 1.857

.0003 .092

.001 1.64 1.628

.003 1.80 1,860

.01 2.43 2.40 2.33 2.31 2,37 2.321

.03 2.44 2.48 2.48 2.34 2.42 2.348

.1 1.079 1.085 1.083 1.080 1.080 1.082

.3 1.000 1,000 1.000 1.000 1.001

1.0 1.000 1.004 .996 1.000 .999

3.0 1.005 1.005 1.005 1.005 1.0043

10.0 1.051 1.051 1.051 1.050 1.047

20.0 1.090 1,087

Acknowledgement

The author thanks M, J, Berger and

R. Loevinger for suggesting this work and for

their helpful comments.

References

1. Loevinger, R. , "Realization of the Unit of

Exposure. II, Cavity Chambers," Proc. Int. Course on Ionizing Radiation Metrology,

Varenna, Italy (1974).

2. Niatel, M.-T., Loftus, T. P., and Oetzmann,

W., Metrologia 11, 17 (1975).

3. Int. Commission on Rad. Units and Meas.

,

ICRU Rep. 19 (1971).

4. Berger, R. T. , Radiat. Res. 15, 1 (1961).

5. Allison, J. W. , Aust, J. Phys . 14, 443

(1961).

6. Hubbell, J. H. , Nat. Stand. Ref . Data Ser.

,

Nat. Bur. Stand. 29^ (1969).

7. Evans, R. D. , Radiation Dosimetry, Vol. 1,

p. 128, Attix, F. H. , and Roesch, W. C.

,

eds., (Academic Press, N. Y. , 1968).

8. Hubbell, J. H. , and Berger, M. J.,

Engineering Compendium on Radiation Shielding (IAEA), Vol. I, p. 180, Jaeger, R. G.

,

ed. in chf . (Springer-Verlag, Berlin, 1968)

9. Johns, H. E. , and Cunningham, J. R. , The

Physics of Radiology , 3rd ed., p. 735

(Thomas, Springfield, 111., 1969).

10. Storm, E, , and Israel, H. I., Nucl. Data

Tables A, 1_, 565 (1970)

11. Int. Commission on Rad. Units and Meas.

,

ICRU Rep. 10b (NBS Handbook 85, 1962).

12. Int. Commission on Rad. Units and Meas.

,

ICRU Rep. 17 (1970).

13. McMaster, W. H. , Del Grande, N. Kerr,

Mallett, J. H. , and Hubbell, J. H.

,

Lawrence Livermore Rep. UCRL-50174,

Sec. II, Rev. 1 (1969).

14. Storm, E. , and Israel, H. I., Los Alamos

Sci. Lab. Rep. LA-3753 (1967).

15. Hubbell, J, H. , Veigele, Wm. J., Briggs,

E. A., Brown, R. T. , Cromer, D, T. , and

Howerton, R. J., J. Phys. Chem. Ref. Data

j4, 471 (1975).

16. Berger, M. J., and Seltzer, S. M. , NASA

SP-3012 (1964).

17. Mork, K., Phys. Rev. A 4_, 917 (1971).

18. Carlsson, G. A., Health Phys. 2^, 653

(1971).

19. Bearden, J. A., Nat. Stand. Ref. Data Ser.,

Nat. Bur. Stand. 14_ (1967).

20. Bambynek, W. , Crasemann, B. , Fink, R. W.

,

Freund, H.-U., Mark, H. , Swift, C. D.

,

Price, R. E. , and Rao, P. V., Rev. Mod,

Phys. 44, 716 (1972).

21. Freund, H.-U., X-Ray Spectrom. _4, 90 (1975).

22. Robinson, B. L, , and Fink, R. W. , Rev. Mod.

Phys. 32., 117 (1960).

23. Veigele, Wm. J., Briggs, E. , Bates, L.,

Henry, E. M, , and Bracewell, B., Kaman

Sciences Rep. KN-71-431 R, Vol. 1, Rev. 1 (1971).

24. Henry, E. M. , Bates, C. L. , and Veigele,

Wm. J., Phys. Rev. A 6_, 2131 (1972).

25. Scofield, J. H. , Lawrence Livermore Lab.

Rep. UCRL-51326 (1973).

26. Damany-Astoin, N. , pers. comm. (Oct. 29,

1975). Also de Reilhac, L. , and DamanyAstoin, N. , J. Phys. (Paris) C4_, 32 (1971).

27. Henke, B. L. , Elgin, R. L. , Lent, R. E. , and Ledingham, R. B., Norelco Rep, 14,

112 (1967).

28. Denne, D. R. , J. Phys. D 3^, 1392 (1970).

29. Bearden, A. J., J. Appl. Phys. YL, 1681

(1966).

30. Woernle, B. , Ann. Phys. (Leipzig) 5^, 475

(1930)

31. Millar, R. H. , and Greening, J. R.

,

J. Phys. B 7_, 2332 (1974).
32. Henry, L. C. , and Kennett, T. J,, Can. J. 37. Mork, K. ; and Olsen, H. , Phys. Rev. 140 , Phys. 49_, 1167 (1971); _50, 2756 (1972). B1661 (1965); 166, 1862 (1968).

33. Ahrens, J., Borchert, H. , Czock, K. H. , 38. Ahrens, J., pers. comm. (Sep. 6, 1974).

Eppler, H. B. , Gimm, H. , Gundrum, H. , 39. Haug, E., Z. Naturforsch. 30a , 1099

Kronig, M. , Riehn , P., Sita Ram, G. , (1975).

Zieger, A., and Ziegler , B. , Nucl. Phys. 40. Mork, K. J., Phys. Rev.' 160 , 1065 (1967).

A251, 479 (1975). 41. Bethe, H. A., and Ashkin, J., Experimental

34. Bar-Noy, T. , and Moreh, R. , Nucl. Phys. Nuclear Physics , Vol. 1, p. 334, Segre,

A229 , 417 (1974). E. , ed., (Wiley, New York, 1953).

35. 0verb(!5, I., Mork, K. J., and Olsen, H. A., 42. Wheeler, J. A., and Lamb, W. E. , Phys.

Phys. Rev. A 8, 668 (1973). Rev. 55_, 858 (1939); 101, 1836 (1956).

36. Tseng, H. K. , and Pratt, R. H. , Phys. Rev. 43. Maximon, L. C., J. Res. Nat. Bur. Stand., A 6_, 2049 (1972). Sect. B 72_, 79 (1968).

306
NBS SP456 (1976)

ELECTRON DOSIMETRY WITH IONIZATION CHAMBERS

M. J. Berger and S. M. Seltzer

Pvadiation Theory Section

S. R. Domen and P. J. Lamperti

Dosimetry Section

National Bureau of Standards

Washington, D. C. 20234

This paper describes work at NBS dealing with the conversion of ionization

measurements to absorbed-dose values in high-energy electron dosimetry.

This work was done by two approaches: experimentally, through the com- parison of calorimeter and ionization-chamber measurem.ents in a graphite

phantom, for incident-beam energies from 15 to 50 MeV; theoretically, via

transport calculations of electron flux spectra and application of BraggGray cavity theory, for beam energies from 1 to 60 MeV and for various

m.edia including water, tissue, graphite and plastics. The pertinent

dose/ionization conversion factor has been determined with an estimated

accuracy of 1% as a function of the beam energy and of the depth in the

phantom. Experimental evidence has also been obtained on the influence

of lead scattering foils on the value of the conversion factor.

(Electrons; dosimetry; ionization chamber; calorimeter; dose/ionization

conversion; stopping-power ratios)

This talk is concerned with one aspect of

electron dosimetry, namely, the translation of

measured ionization data into absorbed-dose

values. Let D(med) be the absorbed dose, and

J (air) the ionization measured with an airfilled ionization chamber. In order to be

able to interpret the ionization measurement,

one must know the dose/ionization conversion

factor C(med/air) = D (med) /J (air )

According to the Bragg-Gray theory of

cavity ionization, the conversion factor is

proportional to the medium/air stopping-power

,med ratio S Thus

C(med/air) ,- I V ^med

(W/e) p S^.^, (1)

where W is the mean energy needed for the

production of an ion pair in air, e is the

charge of the electron, and p is a correction

factor''' (usually close to unity) that takes

into account the perturbation of the electron

flux by the presence of the air cavity. We

have adopted the value (W/e) = 33.7 joules/

coulomb

_med . , . - , S . IS the ratio of the average electron air ^ stopping power in the medium to that in air,

the average in both cases being taken over the

electron spectrum at the point of measurement.

In the case of irradiation by high-energy

This work was supported in part by the

National Cancer Institute.

electron beams, the average stopping-power

ratio STM?'^ is not constant but varies as much air

as thirty percent depending on the incident

beam energy T^ and the depth z in the phantom.

This is due to the combination of the energydependent polarization effect (which reduces

the stopping power in water or tissue relative

to that in a gas) and the change of the

electron spectrum with depth.

The dependence of s'''?'^, and therefore air

C(med/air), on T^ and z is especially important for the calibration of ionization

chambers. It cannot be disregarded even when

only the relative shape of the depth-dose

curve is of interest. This is illustrated

by means of the example in Fig. 1, which shows curves of D(med) ys z and of J(air) vs z, for a beam of 20-MeV electrons incident

onto a water phant om. Both curves are normalized to unit peak height. It can be seen that close to the phantom surface J (air)

is 5% higher than D(med), whereas at depths

beyond the peak it becomes from 5 to 10%

smaller. The peak of the curve of ionization

vs depth, in this example, occurs at

z = 3.34 cm, whereas the peak of the depthdose curve lies somewhat deeper, at

z = 3.80 cm.

The variability of the dose/ionization

conversion factor is inconvenient. However,

air-filled ionization chambers have so many

practical advantages that they continue to 307
1 \

\ \ \

^

1

^ \ r- were obtained; and a portable calorimeter

which can serve as a field instrument.'''^ Tests with 20-MeV and 50-MeV electrons have

shown that the two calorimeters give results

that agree to within 0.1%."'"^

The experimental arrangement is illustrated by Fig. 2 which shows the electron

beam from the MBS Linac passing through the

accelerator window and a lead scattering foil

into a graphite phantom (density 1.7 g/cm^).

Fig. 1. Curves of absorbed dose vs depth and of

ionization VS depth, calculated for a 20-MeV beam incident onto a water

phantom. The beam, is assumed to be

nondivergent and to have a circular

cross section with an area of 16.5 cm^.

SCATTERING

FOIL

MONITOR

be used widely for electron dosimetry. Considerable effort on the part of radiological

physicists has therefore gone into mapping

out the conversion factor as a function of

beam energy and depth. Most of the systematic

determinations have been made in water,

through the intercomparison of ferroussulfate dosimeter and ionization chamber

measurements.^ ^ A more limited number of

experimental determinations have been made

with calorimeter and ionization chamber

7-9

measurements in graphite. An excellent

review of the literature up to 1972 can be

found in a report of the ICRU.''"^

As part of an absorbed-dose standards

program, coordinated experimental and

theoretical work has also been done at the

National Bureau of Standards to determine

dose/ionization factors for electron

11 12 dosimetry. ' The purpose of this talk is to describe briefly some of the results

obtained.

In the experimental work at NBS, a specially designed heat-loss-compensated

calorimeter has been used as the primary

13 standard of absorbed dose. In fact two instruments have been built: a large calorimeter permanently installed at the NBS Linac

facility, with which the results in this paper

Fig. 2. Layout of the NBS experiment.

Radiation measurements at the same depth were

carried out, alternately, with the graphite

calorimeter, and with a graphite-walled

parallel-plate air-filled ionization chamber.

The collecting volume of the ionization

chamber was placed at the same effective

depth, and had the same size and shape, as the calorimeter core. The calorimeter/

ionization chamber measurements were done for

six beam energies from 15 to 50 MeV, at depths

in graphite from 0.9 to 51 g/cm^, and with

different lead scattering foils with thick- nesses from 0.144 to 1.584 g/cm^. The overall

uncertainty of the experimental values of

C (graphite/air ) is estimated to be not greater

than 1%.

The theoretical evaluation of the dose/

ionization factor was carried out in two

16 17 stages. First, the Monte Carlo method ' was used to calculate electron flux spectra

at various depths in homogeneous phantoms

irradiated with broad, parallel monoenergetic

electron beams. Then, the required average

medium/air stopping power ratios were evaluated from the electron spectra, within the

18 framework of the Spencer-Attix version of

Bragg-Gray cavity ionization theory. These

calculations were comparable to earlier work

19 by Kessaris, but included energy-loss

308
straggling instead of the continuous-slowingdown approximation and avoided various other

approximations made by Kessaris. Theoretical

stopping-power ratios were obtained for

incident beam energies from 1 to 50 MeV, in

various media of low atomic number (water,

muscle tissue, graphite, polystjTrene and

acrylic plastic). The overall uncertainty of

the "theoretical results is estimated to be 1%.

In order to treat the case of a strictly

monoenergetic incident beam, and to allow

comparison with the calculations, the experimental data were extrapolated to zero thickness of the lead scattering foil. It was

also assumed that the perturbation factor p in the NBS experiment is unity. Figure 3 compares the variation of the experimental

and theoretical values of C(graphite/air) as

a function of incident beam energy T , at

•H

n)60

u

a

CO

i)

«

u

1 1 '1 1

- \

\ r

CARBON /

T„ = 30 MeV - A

- A

_/i

'

1

!

1 r

Fig. 3. Conversion factor C (graphite/air) as

a function of beam energy. The curves are calculated and the points

are experimental.

z, g/cm

Fig. 4. Conversion factor C(graphite/air) as

a function of depth in a graphite

phantom. The curve is calculated and

the points are experimental. To chang to units of joules/coulomb, multiply

the ordinate by 38.8 = 33.7/0.869.

depth of 15 g/cm^ where it reaches a peak

value. This depth corresponds to the practical range of 30-MeV electrons in graphite,

i.e. , the greatest depth to vjhich the incident

electrons can effectively penetrate. The

value of the conversion factor at depths

greater than the practical range is determined

by the spectrum of Compton and pair electrons

that have been set in motion by penetrating

secondary bremsstrahlung

.

Comparisons between experimental and

calculated conversion coefficients in graphite

have been made for sixty-three combinations

of beam energies and depths. The experimental

values tend to be slightly lower than the

calculated values at shallow depths. Overall

there is good agreement, usually within 1%

which is well with the combined limits of

experimental and theoretical uncertainty.

three depths. The experimental results lie

slightly below the calculated results, the mean deviation for the data shown amounting

to 0.5% and the root-mean-square deviation

to 0.7%. Figure 4 shows the dependence of

C(graphite/air) on the depth in a graphite

phantom, for an incident beam energy of 30

MeV. The experimental results lie somewhat

below the calculated results for shallow

depths, and are in close agreement for intermediate and great depths. The conversion

factor increases (almost linearly) up to a

In view of this satisfactory agreement we think it possible to rely on the theoretical

approach to obtain conversion factors for

conditions not readily accessible with the

NBS experimental setup. Illustrative

calculated results are shown in the next two figures. Figures 5a and 5b give curves of

the stopping-power ratio S^^^^^ us depth,

systematically displayed for eighteen beam

energies from 1 to 60 MeV. These results can be used to evaluate C (water/air) according to

Eq. (1). Just as in the example in Fig. 4,

the curves reach a peak value at or near a depth corresponding to the practical range

309
~i—I—I—I—I—

Fig. 5a. Calculated water /air stopping power

ratios, for beam energies from 1 to 10 MeV.

Fig. 5b. Calculated water/air stopping power

ratios , for beam energies from 10 to

60 MeV,

of the incident electr

dose/ionization conver with those in muscle t graphite. The results

of the beam energy, at which may be of intere

Stopping-power ratios

been given in referenc

be given for other med

ons. Figure 6 compares

sion factors in water

issue, polystyrene, and are given as functions

a depth of 2 g/cm^

st for calibrations,

in tabular form have

e 11 for water, and will

ia in a separate report.

Various medical physics organizations

have carefully analyzed the experimental and

theoretical literature on dose/ionization

conversion factors and have recommended the

adoption of certain values of C(water/air)

for instrument calibration. Provided the

40

Eo

o 35

30

25

Fig. 6.

20 40

Tq, MeV

60

Calculated dose/ionization conversion

factors in various media at a depth

of 2 g/cm^. Assumed composition of

tissue is: 10.20% H, 12.30% C, 3.50% N, 72.90% 0, 0.08% Na, 0.02% Mg

0.20% P, 0.50% S, and 0.30% K by

weight

same assumptions are made about the values

of (W/e) and the perturbation factor p, the

percentage deviations of these recommended

values from the values calculated by us are

as follows:

Recommended by;

ICRU^O

HPA^O

Percent Deviation

Mean R.M.S.

-1.0 1.1

1.3 1.5

NACP

21 -0.0, 0.4

It appears from this comparison, and

from other more detailed comparisons in

reference 11, that the new NBS results are quite consistent with the older results in

the literature. There are only minor changes

in the value of the dose/ionization conversior

coefficients. However, the accuracy has been

significantly improved to the 1-percent level

(necessary for standards work), and conversion

factors have been made available for a wider

range of beam energies, depths and materials

than heretofore.

The discussion so far has been confined

to the idealized case of irradiation with

mono energetic beams. In practical applications (e.g. , in radiation therapy) the

electron beam must pass through various kinds

of material (scattering foil, collimator,

310
nonitor) before reaching the phantom. Even a beam which is initially quite monoenerget ic

(which is not the case for all accelerators)

will usually arrive at the phantom with a rather complex energy spectrum..

Scattering foils which are used to broaden the beam are perhaps the most important factor influencing the beam spectrum.

The passage of the beam through the foil has

the following effects: (1) the electrons lose

a certain amount of energy and their spectrum

is broadened; (2) their angular distribution

is diffused, which in turn modifies their

penetration into the phantom; (3) a significant amount of bremsstrahlung is produced in

the foil which penetrates into the phantom

and gives rise to a spectrumi of Compton and

pair electrons and their secondaries.

The influence of scattering foils is

reduced when a magnetic field is present which

removes a large fraction of the scattered lowenergy electrons from the beam. This was the case in a series of experiments m.ade by

3 6 Svensson ' and coworkers at various medical

betatrons. Svensson estimated the most

probable energy E of the electrons reaching

^ 11 the phantom. It has been sho\-m that his

experimental values for C (water/air) , for

beam energies from 10 to 30 MeV, are in good

agreement with theoretical values calculated

for monoenergetic beams of energy .

The NBS experiment provided a good deal

of information about the effect of lead scattering foils, in the absence of a magnetic

field. As an example. Fig. 7 shows the percentage change of C (graphite/air) that occurs

when a lead scattering foil is inserted into

0 10 20 30 40 50 DEPTH IN GRAPHITE, q/cm^

Fig. 7. Change of C (graphite/air) due to the

introduction of a lead scattering

foil. Results have been determined

experimentally for a 30-MeV beam of

electrons

.

a 30-Me^' beam. Up to depths equal to the

practical range of the incident electrons,

there is an increase which can be as great as several percent for the thickest foils. This

increase is approximately proportional to the

foil thickness, and is presumably due to the

lowering of the mean energy of the electrons

in their passage through the foil. At depths

beyond the practical range there is a reversal

of the effect, C(graphite/air) being reduced

slightly. This may be due to hard bremsstrahlung produced in the lead foil, but no thorough

analysis has as yet been made. Finally, the

values of C (graphite/air) at very great depths

appear to be unaffected by the presence of a foil. „References ^

1. Harder, D. , Biophysik 5_, 157 (1968); see also reference 10.

2. Almond, P. R. , J. Appl. Radiat. and

Isotopes 21_, 1 (1970) . 3. Svensson, H. , Hettinger, G., Acta Radiol.

Ther. Phys. Biol. 10, 369 (1971).

4. Wambersie, A., Dutreix, A., Prignot, M.

,

Studia Biophysika, 26_, 167 (1971).

5. Svensson, H. , Pettersson, C. , Hettinger,

G. , Acta Radiol. Ter. Phys. Biol. 10, 504

(1971) . 6. Svensson, H. , Acta Radiol. Ther. Phys.

Biol. 10, 631 (1971).

7. Bradshaw, A. A., Phys Med. Biol. 10, 355 (1965)

.

8. Pinkerton, A. P., Ann. New York Acad.

Sci. , 161, 63 (1969)

.

9. Sakata, S., Nippon Acta Radiol. 32_, 619

(1972) . 10. International Commission on Radiation

Units and Measurements (ICRU) , Report 21,

Radiation Dosimetry: Electrons with

Initial Energies Between 1 and 50 MeV,

Washington (1972).

11. Berger, M. J., Seltzer, S. M. , Domen

,

S. R. , and Lamperti, P. J., Biomedical

Dosimetry (IAEA-SM-193/ 39) , International

Atomic Energy Agency, Vienna, 589-609

(1975).

12. Domen, S. R. , and Lamperti, P, J,, in

press for publication in Medical Physics.

13. Domen, S. R. , and Lamperti, P. J.,

J. Res. NBS, 78A , 595-610 (1974).

14. Kase, K. R. , and Domen, S. R. , Nucl. Instr. Meth. 118 , 469 (1974),

15. Domen, S. R. , Mikado, T. , and Lamperti,

P. J., NBS-IR-76-1023 (1976).

(Available as PB 256 191 from the National

Technical Information Services, Springfield,

Va. 22161.

)

16. Berger, M. J., Ch. 5, Methods in Computatational Physics (Adler, B., Fernbach, S.,

Rotenberg, M. , Eds.), Academic Press,

New York (1963)

.

17. Berger, M. J., Seltzer, S. M, , Ann. New

York Acad. Sci. 161 , 8 (1969).

18. Spencer, L, V., Attix, F. H. , Radiat.

Res. 3, 239 (1955).
9. Kessaris, N. D. , Radlat. Res. 43 , 288 21. Nordic Association of Clinical Physics

(1970); also private communication. (NACP) , Acta Radiol. Ther . Phys. Biol.

0. Hospital Physicists' Association (HPA) , 11, 603 (1972).

Report No. 4, A Practical Guide to Electron Dosimetry, London (1971).

312
NBS SP456 (1976)

DOSE PROFILES IN TISSUE AND IN AIR FOR THERAPEUTIC ELECTRON BEAMS

G. Osman

Roswell Park Memorial Institute

Buffalo, NY 14263

Electron dose profiles in body tissues and in air were obtained in this work theoretically and experimentally for 3 to 30 MeV incident beam energies. A knowledge of

dose profiles in air is necessary for .any extended SSD treatments or air gap beyond

the end of collimator. Theoretical dose profiles were predicted at various depths

in tissue using the Osman 's Model for therapeutic electron beams. Experiments were

carried out with unfiltered beams in air and in tissue equivalent materials and results were compared with those predicted from our theoretical model. During the initial depth

of penetration in muscle, fat, bone, lung and air, the electron dose profiles showed

flat-top distributions within the geometrical configuration of the beam. At considerable

depths in air or tissue (comparable to the range and field size) , electron dose profiles

have shown Gaussian distributions of various half-widths. Between the two extremes,

electron dose distribution experiences a gradual transition from flat-top to the

Gaussian. Slopes of these Gaussian curves varied inversely with the incident electron

energy. The Gaussian dose distributions in tissues are generally not clinically

acceptable. The flat distributions, however, can be obtained by proper design of

scattering foils and electron cones. Tissue dose profiles as obtained from our model

could provide useful input data for programming treatment planning with high energy

electron beams, using mini-computers. It is expected that the predicted dose distribution

by our model could also account for cone design and body-inhomogeneity

.

(Electron disometry, pencil beam, therapeutic beam, dose profiles)

I. Introduction

High energy electron beams produced by

linear and circular accelerators are now in common use in radiation therapy. Electron

beams with initial kinetic energies between

3 and 30 MeV have been found to form useful

therapeutic agents. Considerable physical

and clinical experience has been gained in

the last two decades. 1"^ Electron beams

offer absorption characteristics in tissue

enabling treatments of any superficial volume and depth with good protection of underlying vital organs and surrounding tissues.

Electron depth dose curves lose their sharp

cut-off at high energies and the dose distribution is very sensitive to a change in mass density. It was also found that a linear energy-range relationship could be a reliable and convenient basis for determining the kinetic energy of the inciok<irelectron beams within the useful therapeutic energy

range. It is important to be aware of a number of discrepancies^ in field uniformity

and depth dose curves between different

machines or even for identical units depending on the scattering foils used and the

design of accelerator collimating system,

field defining cones and the transmission

monitoring devices. For this reason, depth

dose on the major axis as a function of portal size and depth below the surface is a function of the machine generating the beam. For an acceptable therapeutic machine, the

unit should deliver a uniform flat field of

variable size, the initial depth dose curves should parallel the surface in a homogeneous

medium and the x-ray contamination should be at a minimum level. We have, therefore,

assumed in our model for therapeutic electron

beams, that the clinical beam is adequately

uniform at the portal entry and central axis

depth dose data for individual fields must

be experimentally determined, \4hen the

electron isodose surfaces are not parallel

to the phantom surface under normal incidence e.g. near the edge of the field, measures should be taken to correct the

Q initially incident beam.° In the present

investigation, when a uniform therapeutic

electron beam enters a tissue or air medium

directly from the collimating device of the

clinical machine, a flat-top dose distribution was observed at shallow depths changing gradually to gaussian as the depth in

medium is increased. However, the tendency

of electron isodose surfaces to remain parallel to the entrance surface can be extended

all the way up to the end of the range by

proper design of their scattering foils and

electron cones placed directly on the skin

except for very large fields or for total

body superficial therapy. The interactions

of a narrow beam of fast electrons with air,

water or other media is well known. Theoretical and physical analyses for these

313
interactions are reviewed in literature? '1^ In these studies, the main interest was the

interactions of electron pencil beams with

matter. In an attempt to study the mode of

interactions of a broad therapeutic electron

beam with air and body tissues and to relate

the dose distribution at a particular depth

to the basic parameters of the medium (e.g.

the average density and the effective atomic

number), we present our theoretical electron

model in which we consider the broad therapeutic electron beam to be an integral of

electron pencil beam componentsl^ , 13

which the basic theories of interactions

apply. We have also studied the divergence

of electron pencil beams since they are the

input elements in our model. Seeking a work- able mathematical expression that can lead

to a simple computer program, this work was aimed to find a simple relation between the

percent depth dosages at locations away from

the central axis and those on the central

axis at the same depth in air or in tissue

and for any field configuration. The axial

percent depth dose as a function of trans- verse displacement off the central axis of

the therapeutic electron beam in planes

perpendicular to the central axis are called:

dose profiles.

II . Experimental Methods

Electron absorbed dose is measured''-'^ by

the fluence of electrons together with the

stopping power of the medium. Electron dose

in the air was measured with a calibrated

parallel plate build-up chamber connected to a Keithley electrometer. Readings at each

location refer to local peak dose values in

a polystyrene phantom. Suitable build-up

polystyrene plates were added for equilibrium dose measurements. Readings were then

corrected for tissue equivalence and energy

was calculated from absorption range measure- ments carried out separately. This method

can be considered fairly accurate since

there was no beam diffuser, flattening filters, scattering foils or direct in-beam

monitoring device. Dose distribution in

electron pencil beams in air were measured

at the equilibrium depth by a 0.2 cm-^ Baldwin-Farmer ion chamber mounted in a water phantom-automatic remote scanner. The

chamber moves in planes perpendicular to the beam for dose profile measurements. The

chamber was calibrated for Cobalt 60 beams.

Two separate methods for tissue dose measure- ments were used. The first method of measurement was made with an automatic remote

scanning water phantom of 30 x 30 x 30 cm

dimension. The second procedure utilized a solid "SCRAD" phantom made of polystyrene

blocks and slabs of 25 x 25 cm and of

sufficient depth for each particular

energy. Translite Kodak films were pres^e^^

evenly between two flat polystyrene slabs J All phantoms were exposed at right angles to the central axis of the therapeutic beam.

Electron beam dose profiles were then obtained by measuring the optical densities

on an automatic microdensitometer . Films were processed under controlled and reproducible conditions and readings were normalized to ionization measurements. Density

readings were all taken in the linear range

of the characteristic curve for Translite

films. Optical film density measurements were checked and found to be in good agreement with those obtained from small probe

ion chamber measurements in polystyrene

phantom,

COMPARISON Of MEASUREMENTS

IN POLYSTYRf/VE V^^A/TOn

Fig. (1)

0 10 2 0 3,0 cm DEPTH IN PHANTOM

Figure (1) shows a comparison of dosimetric

techniques where different dosimeters were

used to obtain depth dose curves. The

insert in this figure shows the ceJative

probe dimensions while the scanning spot of

the microdensitometer used in this experiment was of 2mm diameter. X-ray contamination in the therapeutic beam was measured

by the ionization current with the ion

chamber placed beyond the range of the electrons. For narrow beam experiments, a well

defined pencil beam of electrons was obtained

by further collimation of the uniform electron therapy beam delivered from the Roswell

Park Memorial Institute Clinic 6 and 35

linear accelerators. The pencil beam collimator consists of two parallel lead sheetabsorbers, each 2mm thick, 5cm apart with

a circular pin-hole of 1mm radius exactly

in line with the central axis of the therapeutic beam. The lead sheet-absorbers were mounted at the end of the machine collimator

314
perpendicular to the central axis of the

therapeutic beam.

Fig. (2) Energy Distribution of Electron

Beams in Planes Perpendicular to The Central axis

.

DISTANCE FROM VIRTUAL SOURCE POSITION

X At ICQ cm • At 200 cm O At 4O0cm

At 550 cm T At 200cm A At 550cm

Fig. (3)

3^

2m

5.5m NT

0 10 20 30 40 50 60 70 80 90

C.A. Cm

Distonce froin C.A.

CO

&

Diagram Showing the Geometry of

the Scattering Angles.

Y

p

1 /

Is the Incident Direction of Electron

Is the Scattered

Is the Projected Scattered Direction

of Electron

Is the Angle of Scattering in Space

Is the Projected Scattered Angle on

a Plane Containing C.A.

X ( C.A.I

iment that spatial dose distribution of

absorbed energy can be accurately determined within the 50 percent decrement

curve

.

Ill Theory of Electron Beams

A spatial distribution of the electron fluence in energy was made in air of the treatment room within the limits of the clinical

beam in order to detect any energy losFes

upon penetrating into the air medium. Fig.

(2) shows the results of measurements at 1,2

4 and 5.5 meters SSD for a clinical electron

beam of initial beam energies 6.7 and 3.6

MeV at isocenter. Energy was calculated

from MarKus relation between the energy at the surface of normally incident electrons

on the build-up chamber and the practical

range. It is concluded from this experiment that energy fluence from the central

axis remains fairly constjyt within the 50

percent decrement surface of the beam.

Beyond this region a gradual drop in energy

was observed as the transverse displacement

is increased.

This is probably due to the greater path

length and the wide range of energies result

ing from electron back scattering at further

locations. In addition to the energy loss

function, the energy of electrons that have

suffered extreme deviations differs from the

energy at same depth on the central axis,

resulting in a straggling distribution effect.

Consequently, the energy based on absorption range measurements beyond the 50 percent decrement surface has very little meaning. It is also concluded from this exper-

(i) Pencil Beams : Electron dose distribution can be assumed to be proportional to the electron number distribution i.e. the fluence, provided the incident energy is normal to the surface and

essentially constant across the dose profiles

within the useful width of the beam. In fact

the overall correction factor which converts

number of electrons into dose is a function

of the mean energy of the primary beam at the depth of measurement. The number distribution of electrons due to one single pencil

beam is based on the central limit statistical theory when applied on individual scattering events. The ddjStribution function is,

therefore, a Gaussian with the mean value

lying along the original direction of the

pencil beam. Figure (3) shows the geometry

for scattering angles where Q is the gross

scattering angle of the electrons which is a function of depth is the projected angle

of scattering and for small angles such as those encountered in elastic scattering,

<o<^> is one-half of <&^>. Dose profiles in

planes at right angles to the pencil beam

as the electrons traverse a thickness x in

a medium is given b}/^

where D represents the electron dose at a

315
particular location on the profiles.

Since the Gaussian peaks

along the original direction of the beam, the mean of the scattering

angles ~ 0

In the projection plane

the last equation will

reduce to:

Jt)(*)= J? (o; exp (-otViGT^) ( 0

if is normalized such

that the area under the curve is unit};, then

where (T is the standard

deviation of the projected

scattering angles, defined

as the square root of the

average of the squares

of the angular deviations

from the meap i.e.j^

Fig. (4) THEORETICAL 607% DECREMENT LINES OF A PENCIL BEAM OF ELECTRONS

<r . R M S ANGLES OF DEVIATION AT VARIOUS S S D 1^ OUiv

<t, NET ANGLES OF DEFLECTION AT TREATMENT PLANE 5 6m S S D. BSA BETHE a ASHKIN

DISTANCE OFF

C.A IN METERS

HORIZONTAL C A

5.0

The root-mean-square projected transverse

displacement "d" in the dose profiles can

be calculated from the following equation19

Theoretical derivations of the divergence of

electron pencil beams have been the subject

of a large amount of literature based on multiple scattering. The theories most relevant to the prgsent work are^ those of

AshkiUj^and Bethe''^ and of Rossi-'"'^ from which

3.0 4 0 SSD IN METERS

e=Electronic charge

= "t.H ^ IC

'° euA 1n=Planck's Const, divided by 277

Figure (4) shows the divergence of two elec tron pencil beams in air obtained theoretically. Because the decrement curves are symmetrical around the initial direction of

the pencil beam, only one half for each diwhere: ^ ^^«3

N=Number of scattering centers /cm

C=Speed of light in Vacuum

Pjj=Average momentum of electrons at cm in medium

Vx=Average velocity of electrons at cm in medium

E =Effective atomic number of material

transversed by the incident electrons.

137

N

P.

) ] —r- (5;

depth X.

depth X

vergence

the 60.7

obtained

cones is given. In this figure,

percent decrement curves were

= ^-"t/ a, where

2/14-

aj^, ... are contents of electrons belonging to the fractional

elements

Z^, Z2, 1'. of the medium.

m=Rest mass of the electron

based on elastic multiple scattering theories of Bethe and Ashkin and Rossi

for 3.6 and 6.7 MeV Initial beam energies. The upper half refers to the divergence

of a 3.6 MeV electron pencil beam while

the lower half refers to the 6.7 MeV pencil

beam. Th^ projected R.M.S. angular deviation (ai^y''^-C and the corresponding projected transverse displacement "d" were

calculated for each energy at different

depths X from equations 3, 4 and 5. After

defining the points of a particular trans- verse displacement "d" and the corresponding

316
C.A.

tangential directions , the

decremental curves were dra^ra for each energy. The net deflection angle <^ can also be obtained for any depth in tissue

or any treatment plane as shown

in this figure.

(ii) ' Therapeutic Beams

Figure (5) shews the basis of our electron Model. The dose

distribution in a therapeutic

beam at a depth x in tissue or in air medium can be considered

to be the resultant of a number

of dose contributions of elec-1

trons coming from different locations within the collimated

broad beam. When a clinical

machine gives a uniform dose at the end of the collimator, it is

reasonable to consider the therapeutic beam as being made up of

an infinite number of identical

and initially parallel pencil

beam components each of minute width.

Each pencil beam, emerging at different locations within the

width of the beam, will diverge into a cone

on entering the uniform medium such as those

shown in Fig. (4). Considering a point at

a location p at a depth x in medium and

distance d off the central axis of the therapeutic beam, or at a deviation cp> from the

beam central axis, the resultant dose B^[4')

will be the summation of different dose

components (the overlapping gaussians) contributed from each pencil beam. The resultant dose profiles in air or in tissue can, therefore, be predicted from the scattering

characteristics of individual electron pencil beams. Therefore the resultant dose Dp(.4'/' can be expressed as an integral between ^ and as the limits as shown in Fig. (5).

DIAGRAM ILLUSTRATING THE MATHEMATICAL MODEL

FOR A THERAPEUTIC ELECTRON BEAM

(x.d)

where X is hay-f the initial beam width at end of collimator. The parameter <S in this

basic equation can be substituted from the

well known expressions of R.M.S. angular

deviation for electron pencil beams. The

above mentioned integral is the basis for

the resultant dose profiles which can be

applied for air, tissue or any other medium.

The body inhomogeneity can be easily accounted for, from the basic par" neters such as

average density and atomic number. The

projected R.M.S. angular deviation 'U" is

expressed in terms of electron energy (e.g.

momentum, velocity) , depth of penetration

X and the average density of the medium.

The integral in equation (6) cannot be integrated in terms of a closed form but can be ntanerically integrated by applying

"Simpson's rule''^!.

(iii) Method of Calculation

Our main objective is to obtain theoretically derived values of the electron dose

distribution across the therapeutic beam in

tissues which can predict accurate experimental results. The resultant dose profiles were calculated from eq (6) with the

central axis dose normalized to 100 percent.

The computer print out values are the theoretically predicted dose distribution at successive planes i.e. dose profiles.

Theoretical dose profiles leading to electron dose distribution were obtained in air,

subcutaneous fat, lung, muscle and bone

for initial electron beam energies ranging

from 3 to 30 MeV taking into account the

energy loss in the media concerned. The

central axis dose, however, was taken for

all media from experimental depth dose

curves at the particular energy and SSD

concerned. Dose profiles on planes perpendicular to the central axis were calculated at successive increments and for

317
adequate lateral displacements

In muscle, dose profiles were

obtained to a depth of 15cm with

successive increments of 1cm

depth and up to 8cm lateral displacement on either side for a portal field of 10 x 10cm with

increments of 2mm between successive locations. In air, for

a therapeutic electron beam with

initial configuration of 20cm x 20cm at the end of the collimator of the linear accelerator,

dose profiles for all points

between central axis and 100cm

displacements on either side were calculated at successive

increments of 2cm between two locations. For relatively

large distances, i.e. from 3.5

to 5.6m they were also calculated between the beam central

axis and 300cm lateral displacement at successive increments of

5cm between each two locatior.s.

(iv) Formulae Used in the Calculations

The spatial dose distribution of the absorbed energy of electrons was calculated

from eq (6) on the basis of our theoretical

Model. The parameter '7 in this basic equation was taken from the pencil beam expressions for electrons as the basic elements

.

The R.M.S. angular deviation <.-V^>^ = ^

is expressed in terms of depth of penetration X, average density f of the medium,

effective atomic number Z of the medium,

number of scattering centers/cm-^ "N" and

the electron energy at a depth in the medium

(average momentum and average velncity

Vj^) . For any tissue the following relation

was applied in our calculations:

j'Rp=0.530 E - 0.106

_f Rp=ln g/cm2 and E is the kinetic energy

of the incident electrons in MeV.

In muscle we have used the following equa—

tion:23 Rp=0.521 E - 0.376, knowing the

initial energy and density of medium, the

practical range Rp could be calculated. The

effective total energy Ex of the electrons ^/j. at depth x in the medium was calculated from

- 0.511 = E C I—x/Rp J

Fig. (6). Divergence of Electron Pencil Beam in Air at 1, 2 and 3 Meters from Pin Hole. Theoretical Gaussians are dotted.

MeV + 0.511. The number of scattering

centers "N" in different body tissues or in

air can be calculated from simple relations.

In our calculations we have used the following methods

In Tissue : Number of atoms/cm-* — —r

where No is the number of electrons/g. The

following table^^ gives f , No and Z for

different tissues which were used in our calculations

Tissue Dens it; No ? g/cm

Muscle 1.00

Fat 0.91

Bone 1.85

Lung 0.51

In Air:

7.42

5.92

13.8

7.53

3.36x10^3

3.48x10^^

3.00x10

,23

and V„ can be obtained from the effective

electron energy E^^ at depth x in the medium

from the end of collimator or pinhole by the

following relations-'-^

Number of atoms/cm-' of air "N" = ^

22,

3.8x10''

is the initial

the end of collimator=Kinetic energy E in

E^ is the initial electron total energy at

where f — fo ^"^^f

J' is the density of air (at prevailing tem- perature T and pressure P) in g/cm"^.

^0 is the density of air at "normal T and P"

= 1.2929 X 10"^ g/cm^.

318
=Temperature and pressure correction

factor. Since the prevailing conditions in our treatment room are P=745 mm Hg and T=

2 95 °K

Fig. (8)

- 3 = 111',- A ic y/Lw of a'ty

IV Results ! Experimental and

Theoretical

(i) Pencil Beams and Gaussian Fitting : Equation (1) can be written:

Did)

where d distribution i.e. when d = dx D(d) = i D(o)

on (1) can be written: .

d is the half width of the gaussian

and has the value

:

Fig. (6) shows the experimental results for an electron pencil beam diverging in air.

Dose distributions at 1,2 and 3 meters from

pin-hole were recorded at the equilibrium

position in water phantom. Experimental

curves were fitted with theoretical gaussian

distributions as obtained from eq (7) . We

can compute 100 D (c<)/D(o) for different

(d/d-^ ) with d I. and D(o) taken from experimental data, '^n all these experiments, the

chart recorded lines represent experimental

data while the dotted curves represent eq

(7) with dt.= 8,24 and 38cm respectively.

It is, therefore, concluded that applying

the multiple scattering elastic theories

is justifiable and the divergence of the

electron beam should be accurately known

from the basic parameters of the medium

such asfd-Z. From these equations, it is

also possible to account for body inhomogeneity as ^ and Z varies with depth.

Fig. (7). Experimental Electron Dose Profiles for 3.6 MeV Therapeutic Beam in Air

at IQOcm SSD.

ANGULAR DOSE DISTRIBUTION OF THERAPEUTIC ELECTRON BEAMS

IN VERTICAL TREATMENT PLANE ATJ^Jcm SSO

ANGLE AT ISOCENTER FROM HORIZONTAL

(ii) Therapeutic Beams :

The present results refer to a broad therapeutic electron beam as used in radiation

therapy. No electron cone or beam diffuser were used in the following experiments in air

or in tissue,

(a) Dose Profiles in Air

The Roswell Park Memorial Institute linear

accelerator delivered the electron therapeutic beams across the length of the treatment room with a horizontal central axis as used

in total body superficial treatments. The

main interactions of the travelling electrons

is with air moleculsB and^^n^^nalysis of dose

distribution was reported ' . The transmission electron ion chamber was removed and

placed outside the accelerator head, intercepting the edge of the beam. This ion chamber has no effect on the beam and served

as the monitor unit indicator of the linear

accelerator. Figure (7) shows the experimental dose distribution for a 21 x 21cm

electron beam of 3.6 MeV initial energy at 100cm SSD or at about 40cm from the end of

the machine collimator. The position of

the transmission electron ion chamber can be observed in the chart recorded diagram

of the dose profile Fig. (7). Figure (8)

shows the angular dose distribution for 3.0

and 6.0 MeV incident electron beam energy

at 560cm SSD for same field as obtained from

experimental results together with their

gaussian fits. From Figs. 7 and 8 it is clear that electron dose distributions vary

from an essentially uniform field to gaussian

distributions. The agreement with gaussian

curves indicates that electrons are mainly

suffering multiple elastic scattering. Figs.

9 and 10 show the dose profiles in air for

319
3.6 and 6.7 MeV incident electron energy as obtained from our theoretical Model.

Fig.(Q) % DOSE PROFILES IN AIR FOR THERAPEUTIC ELECTRON BEAM . AT SHORT SSD

Theory Bosod On ICRU a Rossi Thao.y Based On BETHE 8 flSHKIN

.110 cm SSO

.lOOcm SSD

. 90cmSSD

. aOemSSO

Theory 8ose<) On iCRUSftoi £.67 M«V

Theory Boied On BETHE 8 aSHKlN

E . 67 MiV

y

. Troni«ef»e Oi»pioca.Ti«n'

1 Depth inAir

'-'3 (JC)

'/o DOSE PROFILES IN AIR FOR THERAPEUTIC ELECTRON BEAM

AT EXTENDED SSD

Theory SaiadOn ICRU a Rosst 3 6 MeV

y Tfonivette Oiiplacemen*

Theory Boied On BETHE 8 ASHKIN

3.6 Uev

100

• DItpIocemin

(O 100 200 cm

C A

*i W- HALF BEAM WIDTH

Theory Based On ICRU Q Rom

010 100 200 CI C.A.

•41 l» HALF BEAM WIDTH

Theory Btned On BETHE 8 ASHKIN

M M- HALF BEAM WIDTH

K- HALF BEAM WIDTH

Fig. (11) Off-Axis Electron Dose Distribution of a Therapeutic Beam at Different Treatment Distances.

EXPERIMENTAL THEORETICAL (BETHE a ASHKIN) THEORETICAL (ROSSI) X BOUNDARY OF THE INITIAL BEAM

PERCENTAGE OF CENTRAL AXIS "CA" DOSE V& TRANSVERSE DISTANCE "d"

Fig. (11) shows a comparison of electron

dose profiles in air of a therapeutic beam

at different treatment distances for 3.6

and 6.7 MeV initial beam energies. Two pencil beam theories were applied as the

basic element in our Model for obtaining

the electron dose distributions in the profiles,

at short, intermediate and

long SSD as shown in this

figure

.

X=5I0 Cm SSD

e 12 iF'o 20 40 60 80 To^O 40 80 120 160 200 240

CA. „ ^ " DISTANCE OFF CENTRAL AXIS d IN Cm

320
(b) Dose Profiles in Tissue : Fig. (12)

Fig. (12) shows an example of

the chart recorded electron

dose profiles across 30 MeV

beam at various depths in

water phantom for a 10 x 10cm.

The gradual transition from

flat-top uniform distribution

to gaussian is observed as the

depth in phantom is increased

Dose profiles in muscle were

predicted from our theoretical

model using the physical properties of the tissue concerned as explained in Section

III. Figure 13 shows these

profiles at various depths

and for 6.7, 10, 20 and 30

MeV normally incident electrons of the therapeutic beam

and for a 10 x 10cm portfield. Fig. 14 gives a comparison of dose profiles

for 30 MeV incident electron beam as obtained from

experimental results and

from our theoretical model

for electron beams.

Experimental Dose Profiles across 30 MeV Electron

Beam at Various Depth In Phantom for a 10 x 10cm

Field Without Electron Cone or Benm Diffuser.

y = Transverse Displacement

X = Scm % DOSE

FIGI3 % DOSE PROFILES IN MUSCLE

FOR THERAPEUTIC ELECTRON BEAM

Ec- 10 MeV

E,. 67 MeV y . TRANSVERSE DISPLACEMENT 7' TRANSVERSE DISPLACEMENT _ _ MUSCLE

1 • DEPTH m MUSCLE

Fig|^fZ^ COMPARISON OF THEORETICAL a EXPERIMENTAL

DOSE PROFILES FOR 30 MeV ELECTRON SEAM FOR lOxlOcm FIELD

012345678012345678

Cm Cm

100

50

01 234567801 2345678

Cm Cm

321
FIG. 15 % DOSE PROFILES IN LUNG

FOR THERAPEUTIC ELECTRON BEAM

FIG. 17 % DOSE PROFILES IN FAT FOR THERAPEUTIC ELECTRON BEAM

Ec = 20MeV

y = TRANSVERSE DISPLACEMENT

FIG. 16 % DOSE PROFILES IN BONE

FOR THERAPEUTIC ELECTRON BEAM

Ec=IOMev Ec = 20Mcv

y : TRANSVERSE DISPLACEMENT y = TRANSVERSE DISPLACEMENT

« --DEPTH IN BONE > = DEPTH IN eONE

% DOSE PROFILE IN BONE

Ec= 30 Mev

y

= TRANSVERSE DISPLACEMENT

X = DEPTH IN BONE

Figi:r'rs(15,16, and 17) are another examples

of electron dose profiles in different

tissues e.g. lung, bone and subcutaneous fat

as predicted from our theoretical model. In both experimental and theoretical results,

the electron depth dose in tissue

changes from uniform flat-top distribution

to gaussian as the depth in tissue is

increased. Tissue viose profiles as obtained from our electron model are in very

good agreement with experimental data.

Based on this evidence, a computer program

has been derived as explained in Section III

in application of our model and on the

knowledge of the physical parameters of the

concerned tissue.

(c) X-Ray Contamination In Tissue

X-ray contamination was measured in polystyrene phantom for port fields of 10 x 10cm and for normally incident electron

beams of 4, 10, 15, 20, 25 and 30 MeV

initial kinetic energies. Readings were

then corrected for tissue equivalence.

In fact this experiment was part of electron

depth dose curves after the beams have

reached their practical range as shown m

Fig. (18). VfiR14TJ0N OF ELECTRON OOSE WITH DEPTH IN TISSUE FOR TYPICAL ELECTRON BEAMS ANO FOR PORTAL FIELDS OF 10 i 10 cm'

DEPTH INTISSUE ( cm )

322
Beyond the electron range each curve levels

off reaching a fairly constant value. The

X-ray contamination dose was found to in- crease with increase of the energy of the

incident beam, rising from about '/: percent

at 4 MeV to about 10 percent for the 30

MeV beam. In Fig. (18) the kinetic energy

of each beam as calculated from the practical range were 4, 10.5, 15.5, 20.3, 25.1

and 32 MeV respectively.

(d) Electron Cones and Scattering Foils :

Flatness of electron beams used in radiation

treatments has been improved by introducing

one or more scattering foils inserted in

the beam . These^Joils may be flat or shaped or combination . Obviously this

will increase the X-ray contamination and

therefore they should be avoided for ^^ole

body superficial electron treatments

When using scattering foils they should be

as thin as possible so that the contamination is minimized. However, in some clinical accelerators, the uniformity of the

electron beam and the shape of isodose

curves are not adequate for treatments ex- cept when using heavy scattering foils.

Electron cones have also been d^isigned so that lead blocks of adequate thickness intercept the edges of the electron beams. These

electron cones are usually made of Incite

and meant to make immediate contact with the

skin. The lead blocks are built towards the

end of the electron cone and acts as a diaphragm. The diaphragm size is usually

smaller than the field size and an experimental relation between field size versus diaphragm size could be easily established.

The scattered electrons from the wall material contributes to the skin dose and may

also vary the shape of depth dose and isodose curves. Both electron cones and

scattering foils are made to extend the

electron beam flatness over the full range

of electron energies and field sizes.

Fig. (19). Isodose Distribution of 14 x 14cm

Field for 10 MeV.

Figure (19) shows a typical electron isodose

distribution for a 10 MeV initial beam

energy and for a port field 14 x 14cm using

electron cone and a thin scatterer of 0.02"

Pb + .012" Al. These isodose curves were

obtained using the 0.2cm Farmer probe

mounted on an automatic scanner in water

phantom.

(e) Air-Gaps :

In some clinical situations it is necessary

to treat a patient at distances greater

than the length of the electron cone leaving an air gap between the end of the electron cone or collimator and the skin.

These air gaps could be small when electron

cone fails to make good contact with the

skin, or intermediate e.g. in the perineum

or in the head and neck region, or could be

large in the treatments of extensive or whole body superficial skin lesions. The

air-gap has also strong influence on the

electron dose profiles particularly at shallow depths.

Figs. (20

and 21) .

Typical

electron

Dose

Profiles

with 1 and

2cm AirGaps .

323
Fig. (22).

(i) Small Gaps : Experimentation at Roswell

with 1 to 5cm air gaps showed an arise or

inflections in each profiles range near the

edges of the beam. However, these inflections are eliminated at about 2cm depth from

the skin surface for 10 and 15 MeV electron

beam energies of the depth dose curves. Figures (20,21 and 22) show examples of

these results for 15 MeV electron beams

and for air gaps of 1, 2 and 3cm. Electron

dose uniformity at the surface of phantom

and at 1cm depth is improved as the air-gap

increases from 1 to 5cm. However, dose

profiles have optimal uniformity at 2cm depth

for the range of energy under investigation.

(ii) Intermediate Gaps : In this case con- sideration should be given to the change in

beam flatness. As mentioned in Section II,

a gradual transition from flat-top distribution to gaussian depending on distance.

With such intermediate gaps of about 20cm

probably the gaussian distributions will

not yet be formed, but there is a definite

increase in the beam half width while the

sharp and rapid fall-off in the electron

dose profiles are gradually diminished. In this case the profile shape will be some- where between the uniform or flat-top distribution and the gaussian depending on distance and energy.

(iii) Large Gaps : Significantly large air

gaps about several times the initial width

of the beam, the distribution curves of

electron dose profiles are essentially

gaussians, such as those obtained in total

skin irradiations for the treatments of ex- tensive skin lymphomas. In fact the relative dimensions of field size to SSD is very

small, consequently the therapy beam in

this casp- could be considered as a pencil

beam where electron cones or scattering

foils are avoided. The uniform dose distribution can best be obtained by a dual

electron beam . In this case the angles

of the central axis of the beam above and

below the horizontal are a function of beam

energ^^

V . Discussion and Conclusion :

A study has been made on electron dose distribution in air and in tissue for therapeutic beams of 3 to 30 MeV incident beam

energies. Electron dose profiles were ex- perimentally determined and were compared

to those predicted from theoretical considerations. An electron model for clinical

beams has been presented and is based on considering the therapeutic beam to be made

up of an infinite number of pencil beam

components. The divergence of electron pencil beams has been studied in the present

work and dose profiles were found to be in

excellent agreement with theoretical gaussian curves over a wide range of transverse

displacement except for the tail ends of the

distributions where the experimental curves indicate broader dose profiles than the

gaussians. It was also found in the present

work that the root -mean-square angular deviations of an electron pencil beam is in

fact the 60.7 percent decrement curve. Therefore the average divergence of an electron beam could be easily visualized. It is also concluded that the scattering is

mainly elastic obeying a multiple small angle

scattering formula. The non-gaussian parts

of the curves may be explained by a major

change in the mode of electron interactions.

This is probably due to the transition from

small angle multiple scattering to ±arge

angle single scattering acts. Instead of

interacting with the nuclear coulomb field

of the atomic nuclei without loss of energy,

there will be electron-electron collisions

with the atoms of the stopping medium

accompanied by energy loss. This, in fact,

is a minor process of about 14 percent after

X-ray contamination was removed, as indicated by the experimental curves when com- pared with the gaussian fits. Therefore,

the spatial dose distributions are dominated

by a multiple scattering process. Scattering of the incident electrons with the

electrons of the absorbing medium results in

large angle single scattering of >J? I miili radian accompanied by energy loss due to collision. Elastic scattering of the incident electrons by the atomic nuclei is a

324
more probable interaction with | wii/i radian. It is the large number of such

scattering acts at successive collisions

which produces the multiple scattering

events. Moreover, in all tissues or air media, slopes of the gaussian distributions

varied inversely with the initial normally

incident electron energy. As the energy of

the beam is increased, narrower gaussian

distributions are expected particularly at considerable depths in the medium. Tissue

dose profiles as obtained from our electron

model are in very good agreement with experimental data. Electrons scattered by the cone and diaphragm as well as air gaps between patient and collimator may contribute

substantially to the surface absorbed dose.

Changes in variables, standard conditions or formulae used could be easily accounted for

in the presented program. It was not our

aim to investigate the validity of the existing theories on electron pencil beams.

This aim can probably be achieved by using

a more realistic theoretical model taking

into account the quantum mechanical manybody effect (i.e. the various interactions

among electrons in the therapeutic beam)

,

the scattering within collimator and symmetry monitor etc. However, our simple model

has sufficiently served the purpose and our theoretical predictions were consistent with

our experimental results. The validity of our mathematical model is based on the

accuracy of the available theories of multiple scattering expressions for narrow

electron beams. With the proper choice of

the pencil-beam theory which is used as the

basic element in our model, an agreement

with the experimental results of about 5 percent is achieved. Our theoretically

predicted results can be used for routine

programming of treatment planning with high

energy electron beams using mini-computers.

Further, it is also possible to account for

I body inhomogeneity , not from the stopping

j power formula, but from the basic parameters

I of the media concerned

.

! ACKNOWLEDGEMENT

(

I The authors acknowledge considerable assistance from Dr. H. Yu in particular in the

computer and theoretical aspects of this

investigation. The authors also are grateful to Dr. T. Dougherty for his comments on

the text and to Mr . H. Kowal for helping in

all measurements on the Clinac 35.

REFERENCES

1. H.A. Bethe, Handbuch der Physik,

Julius Springer, Berlin, 24 (1933).

2. H.W. Lewis, Phys . Rev. 78, 526-9

(1930)

.

3. J.S. Laughlin and J.S. Beatty, Phys.

Rev. 83, 692 (1951).

4. L.V. Spencer and U. Fano , Phys. Rev.

93, 6 (1954).

5. I. Berger and Seltzer, NASA-SP-3012

Suppl. to circular No. 577 (1958).

6. J. Ovadia et al. Radiology 72, 99 (1959).

7. I.A.E.A. Atlas Vol. 1, Electron Beams,

p. 131-155 (1965).

8. R. Loevinger, C.J. Karzmark and

M. Weissbluth, Radiology 77, 906-927

(1961).

9. J. Ashkin and H. A. Bethe, Experimental

Nuclear Physics Vol. 1 Part II,

John Wiley and Sons, Inc., 166 (1953).

10. B.B. Rossi, High Energy Particles,

Englewood Cliffs, N.Y. (1964).

11. N.D. Kessoris, Phys. Rev. 145,

164-173 (1966).

12. G. Osman, Proceedings of AAPM, Phys. in

Med. and Biol. 18, No. 4 (1973).

13. B.C. Lillicap, W. Patricia and

J.W. Boag, Phys. in Med. and Biol.

20, 30 (1975).

14. I.C.R.U. Report 21 (1972)

15. J. Dutreix and A. Dutreix, Annals of

the N.Y. Academy of Sciences 161,

33-43 (1969).

16. AAPM Protocol "SCRAD", Phys. in Med.

and Biol. 11, 4 505-520 (1966).

17. G. Osman, Proceedings of AAPM, Med. Phys.

Vol, 1, No. 2, p. 97 (1974). Also

Med. Phys. Vol. 2, No. 3, p. 155 (1975).

325
i

18. A. Hald, Statistical Theory with

Engineering Applications, John Wiley

and Sons Inc. 119-188 (1952).

19. J.D. Jackson, Classical Electrodynamics, John Wiley and Sons Inc. 456-463 (1967).

20. W.V. Mayneord, Acta of the International Union Against Cancer,

2:271 (1937).

21. A.D. Booth, Numerical Methods,

Academic Press Inc., 31-34 (1957).

22. L. Katz and A.S. Penfold, Rev. Mod.

Phys. 24, 28 (1952).

23. B. Markus, Strahlentherapie 116,

280 (1961).

24. D. Harder, Symposium on High Energy

Electrons, Montreux 1964, Springer

Verlag, Berlin and Heidelberg. Also:

W. Pohlit, Annals of the N.Y. Academy

of Sciences 161, 195 (1969).

25. H.E. Johns and J.R. Cunningham, The

Physics of Radiology, Thomas Charles

(1974).

26. M. Sempert and R. Wideroe, Elektronenstrahlen. Springer Verlag, Berlin, 182

(1958)

27. A. Dahler, Proceedings of the International Symposium on High Energy

Electrons, Montreux, p. 98, Springer

Verlag, Berlin (1964).

28. C. Lindskoug, and K.A. Johansson, Acta. Radiol. Ther. Phys. Biol. 10, 21 (1971),

29. H. Svensson, and G. Hettinger, Acta. Radiol. Ther. Phys. Biol. 6, 404 (1967).

30. B. Lindskoug, and A. Dahler, Collimating

System for Electron Beams, Acta. Radiologica, New Series Vol. 10

454-463 (1971).

31. J. A. Rawlinson, The Scientific Program

of R.S.N. A. , 186 (1975)

32. G. Osman, Proceedings of the Third

International Conference on Medical

Physics, Sweden (1972).

326
NBS SP456 (1976)

CALORIMETRIC DOSE MEASUREMENTS IN FAST NEUTRON AND COBALT-60

GAMMA-RAY FIELDS*

J.C. McDonald and J.S. Laughlin

Memorial Sloan-Kettering Cancer Center

New York, N.Y., 10021

L.J. Goodman

Radiological Research Accelerator Facility

Brookhaven National Laboratory

Upton, N.Y., 11973

A portable calorimeter, constructed of A-150 tissue equivalent

(T.E.) plastic, was employed to measure dose in a cyclotron60 produced fast neutron field and in a Co gamma-ray field.

The precision of these measurements ranged from ±1-2% depending

upon dose rate and signal to noise ratio. The absolute accuracy

of measurements in the neutron field is estimated to be ±5% and

60 those in the Co field to be ±2%. A method for determining

the magnitude of endothermic radiochemical effects which occur

in T.E. plastic will be described along with other sources of

uncertainty in calorimetric dosimetry.

(Tissue equivalent; calorimeter; fast neutron; dosimetry)

Introduction

Cyclotron-produced fast neutron fields

are presently employed for radiobiological

research and clinical investigations of

their potential advantages for radiotherapy. In order to properly evaluate the

results of these studies, it is necessary

to develop a high degree of precision and

accuracy in dosimetric measurements. At our laboratory calorimeters have been designed,

constructed and employed as primary dosimetric standards for measurements in electron and photon beams^"^. This work has

recently been expanded to include measureg ments in fast neutron fields . The advantages of calorimetric dosimetry are that

dose is determined directly by measurement

of the temperature rise in an isolated test

volume, calibration is carried out by electrical heating of the test volume thus obviating a radiation calibrating source and

calorimeter response does not require a knowledge of stopping power ratios or W values. There are also some disadvantages,

such as relatively poor sensitivity and the

physical size of some calorimeter systems.

The instrument described in this report is compact and has been designed to be portable, in addition, measurements have been

carried out at dose rates as low as 0.5

mGy-s'^(3 rad-min"^), and a precision of

±2% has been achieved for dose rates of 1.9

mGy.s"^(ll rad.min"^).

The material selected for use in con- struction of this calorimetric dosimeter is

Q "in A-150 T.E. plastic ' since it is almost

universally employed for ionization chambers and proportional counters now used in fast neutron and pion dosimetry. Although

the composition of the T.E. plastic is not identical to the formulation adopted by the

ICRU to characterize tissue^\ the kerma

correction which is employed to convert

dose in T.E. plastic to dose in tissue is small (^5%) for most photon and neutron

energy spectra of current interest in radiobiology and radiotherapy. Unfortunately,

nearly all irradiated plastics are charac2 12-15

terized by a thermal defect ' " which is the quantity of absorbed energy that does

not result in the production of heat.

When fast neutrons and pions interact

with tissue and tissue equivalent plastic,

an array of secondary charged particles is

produced which includes protons, alpha

*This research is supported in part by ERDA Contracts E(l 1 -1 )-3522 , E(ll-l)-3243 and by NCI Grants CA-08748, CA-12536.

327
particles and nuclei of carbon and oxygen.

The numbers and energy spectra of these

secondary charged particles are determined

by the initial energy spectrum of the primary particles. The thermal defect in T.E. plastic has been measured using 1.7 MeV protons^^, however, there have been no such

measurements to date for higher energy protons, alpha particles or heavy ions for

which the thermal defect may be quite different. A method for extending the thermal

1

defect measurements of Fleming and Glass

will be described in this report, since

this method will help to reduce the uncer- tainty associated with the determination of

dose in fast neutron fields using a T.E. plastic calorimeter.

Calorimeter Elements

Core

The central absorbing element, or core,

is cylindrical in shape; 2 cm in diameter

and 0.2 cm thick. This design was chosen

to insure reasonably good spatial resolution in fields whose intensity decreases

rapidly with depth. The core is thermally

isolated from its surroundings by hollow

nylon suspension points which provide a minimal thermal conduction path. The opposing faces of the core have been coated with

a thin layer of conducting silver dag providing electrical contacts for calibration

heating of the core, and also lowering radiative heat loss from the core to its sur- roundings by producing a lower emissivity

surface compared to uncoated T.E. plastic.

A thermistor has been bonded into the core

with an electrically insulating epoxy which

has relatively good thermal conductivity.

The cross-sectional view of the calorimeter,

shown in Figure 1, indicates some of the

details of construction.

EUCTFBCAL

CAUBRATION LEADS

.THERMISTOR EMBEDDED IN THERMALLY

CONDUCTIVE EPOXY

Since it is necessary to determine

the quantity of energy deposited per unit

mass in a defined medium, a precise amount

of energy must be introduced into the core. The electrically conductive faces are used to pass current through the T.E. plastic which

results in a heating pattern similar to that produced by the radiation field. The

resistivity of A-150 T.E. plastic has been

measured as a function of temperature and

was found to be 48.21 ohm-cm at 300C. The

electrical calibration circuit and the

method used to measure the current flowing

through the core will be described below.

The composition of the core assembly

is shown in the following table. The percentage of material other than T.E. plastic

has been kept to a minimum in order to maintain a nearly homogeneous medium in which dose is measured.

Component Weight Percentage

Tissue equivalent plastic .7365 98.29 Conducting paint .0055 .73 Conducting epoxy .0027 .36 Thermal epoxy for thermistor .0022 .29

Thermistor head .0020 .27

Electrical leads on core .0004 .05

.7493 gm 99.99%

Figure 1. Cross-section of the calorimeter.

TABLE I, Calorimeter Core Composition.

Jacket

Surrounding the core is a "pill-box"

shaped adiabatic jacket whose inside dimensions are approximately 0.05 cm larger than

the core in all directions. The gaps be- tween all calorimeter elements, which are necessary for thermal isolation, have been

kept to a minimum practical size in order

to maintain homogeneity throughout the

instrument. The jacket is electrically

heated in the same manner as the core, by means of conducting silver dag contacts.

During electrical calibration the jacket

and core heating rates are adjusted so that

the initial temperature rise in both ele- ments is nearly equal. This quasi-adiabatic operation, initially described by

Laughlin^ provides a method of reducing

heat loss from the core to its surroundings, and evaluating the magnitude of this

heat loss during irradiation and cali- '

328
brat ion.

Thermal Buffer and Control Shields

The three bodies surrounding the core

and jacket serve as static thermal buffer

shields whose thermal capacity tends to damp out temperature oscillations before

they reach the core and jacket. These

elements also rise in temperature during

irradiation which helps to reduce the heat

loss of the core and jacket. Radiative

losses between elements have been minimized

by evaporating gold coatings on the faces

of these buffer shields to reduce their

emissivity. The control shield, indicated

in Figure 1, serves as a constant temperature enclosure which is regulated to within tO.OOloc of a set point a few degrees

above room temperature. The inner components are housed within a vacuum enclosure

also constructed of T.E. plastic. The

pressure within this enclosure is estimated

to be <.013Pa(10~^ torr) which is necessary

to reduce heat losses from convection and

air conduction.

Signal Detection and Calibration Systems

A d-c Wheatstone bridge is employed to measure the resistance of the core thermistor. This circuitry must be quite precise and capable of resolving small resistance changes since the bridge imbalance

that occurs during calibration or irradiation may be due to a change in resistance

of only a few ohms in approximately 30,000

ohms. The precision of the bridge system

is estimated to be ±0.05%. The specific

heat of A-150 T.E. plastic is approximately

1 .8J-g"''oc~^ so that the temperature rise

which occurs upon absorption of 0.17mGy

-6

(1 rad) is approximately 7.5x10" OC in a one gram sample. The temperature increase

and resulting relative resistance change,

aR/R, when a known amount of energy is

deposited in the core by electrical heating

serves as the calibration. The instruments

used for measurement and calibration are

shown in Figure 2.

The calorimeter in its styrofoam

thermal enclosure appears at the far right,

and the Wheatstone bridge is just to the

left of the calorimeter. The bridge

enclosure is also insulated with styrofoam

in order to minimize the effect of room

temperature changes on resistance values.

The bridge potential of 12.15 volts is supplied by three mercury batteries, and

the variable resistance arm is remote controlled by three stepping motors so that

re-balancing can be achieved without

entering the irradiation area. The temperature regulator for the calorimeter control

shield and a microvoltmeter , which serves

as a null detector, are in place above the

bridge. The remote controller for the

bridge and the calibration circuit appear

at the far left and the strip chart re- corder, which provides a graphic record of

the core temperature versus time by indicating the bridge imbalance potential, is

in the left foreground.

Figure 2. Tissue equivalent plastic calorimeter, along with the measurement, calibration and control systems.

A Leeds and Northrup N.B.S. type

standard resistor of 1000 ohms is connected

in series with the calorimeter core (3.41

ohms) and the potential across both resistive elements is measured with a digital

voltmeter calibrated using an Eppley standard cell. The electrical power dissipated

in the core during calibration is known to

an accuracy estimated to be ±0.25%, and the

precision of electrical calibration mea- surements ranges from ±0.5-1% depending on the heating time and the magnitude of the

relative resistance change.

Dose Measurements in Radiation Fields

Calorimeter

A mixed fast neutron and gamma-ray

field was generated at the Sloan-Kettering

Institute cyclotron by using a beam of deu- terons to bombard a thick Be target. The

neutron spectrum produced by the

9 10

Be(d,n) B interaction has a mean energy

1 ft of approximately 3.5 MeV (for - 7.5 MeV deuterons). The percentage of gamma-ray

dose present at the position of the calorimeter core has been computed to be approxi329
mately 6% from paired ionization chamber

measurements in a water phantom"*^. The cyclotron area is an electrically noisy environment which is also subject to somewhat

larger temperature variations than other

laboratory areas. In spite of these difficulties, a series of measurements was , carried out at a dose rate of 1.9 mGy-s'

(11 rad-min"^) with a precision of ±2%

(standard deviation about the mean). An example of one such irradiation run is shown below in Figure 3.

OFF

R= 29, 205.40 ohm

0 ' ' ' ' 5

'

'

' ' io min

Figure 3. Core temperature vs. time

recording for neutron irradiation.

A series of measurements was also

carried out in a Co gamma-ray field at Memorial Hospital. The calorimeter was set

up in the evening and allowed to equilibrate

overnight. The thermal drift rate on the

following day was approximately 6x10

°C-min ^ for the duration of exposures

which took place over a period of two hours.

The dose rate at the calorimeter core was 10.22 mGy-s'^ (60.09 rad-min"'') and the precision of this series of measurements was

±1.2%. The absolute accuracy of the calorimetric measurements is estimated to be

approximately ±2% in ^°Co field and ±5% in

the cyclotron-produced fast neutron field.

The uncertainty in the thermal defect is a contributing factor affecting the accuracy

of these measurements, and a method of re- ducing this uncertainty will be described

below.

Ionization Chamber

An ionization chamber which is cylindrical in shape, has been constructed of

T.E. plastic. The collecting volume is guarded and has a diameter of 2 cm and a

thickness of 0.2 cm. T.E. gas flows

through the chamber at a rate of approxi3-1 1 mately 0.08 cm .s (5cc-min ) at a pressure of 2kPa (15 torr) above atmospheric

pressure. Collection efficiency at 400 V in ^'^Co gamma-ray and fast neutron fields

at dose rates of about 10 mGy-s"^

(60 rad-min~^) has been measured to be

99.85% and 99.3% respectively (assuming

columnar recombination only). The charge

measured when using positive and negative

collecting potentials agrees to within

±0.6%.

An irradiation phantom which places

both the calorimeter core and the collecting volume of the ionization chamber at the

same depth in T.E. plastic has been con- structed, and is shown below in Figure 4. The dimensions of the phantom approximate

those of the calorimeter and inserts have

been machined to accept other dosimeters.

Figure 4. T.E. ionization chamber in place

within a T.E. plastic irradiation phantom.

Measurement of the Thermal Defect

In A-150 T.E. Plastic

One of the sources of uncertainty in

calorimetric dosimetry which employs T.E. plastic is the thermal defect. The amount

of energy which is consumed by endothermic

radiochemical reactions when A-150 T.E. plastic is irradiated with 7 MV x-rays has

been determined by Bewley^^ to be (4±2)%.

1

Fleming and Glass employed a differential

calorimeter in which absorbers of T.E. plastic and aluminum were irradiated with

1.7 MeV protons. This method was quite

330
precise and yielded a value for the thermal

defect extrapolated to zero dose of

(4.2±0.2)%. As accumulated dose increased

radiation damage eventually caused lowering

in this value to 3.67% for doses above

6 8

10 Gy(10 rad). Over the useful range of a calorimetric dosimeter, the thermal defect

would be relatively constant, however, this

accumulated dose dependence is a source of

uncertainty which will be investigated

further.

The thermal defect value measured by

Fleming and Glass for T.E. plastic could be

employed for neutrons of relatively low

energy since the dose is predominately deposited by recoil protons. However, for

higher energy cyclotron-produced fast neutron beams an increasingly larger fraction

of dose is deposited by alpha particles and

heavy recoil nuclei. Radiochemical yields

for these densely ionizing charged particles are usually quite different than those

obtained with sparsely ionizing radiation.

In order to properly evaluate the thermal

defect for fast neutron irradiated A-150

T.E. plastic, the technique of Fleming and

Glass will be extended by using different

energy protons and other charged particles.

This project will be a collaborative

effort of the Sloan-Kettering Biophysics

Laboratory and The Radiological Research

Accelerator Facility (RARAF). Thermal

defect measurements will be performed on a number of T.E. plastic samples from three

laboratories in the United States and one in Europe. An attempt will be made to evaluate

samples molded at the extremes of temperature and pressure which will demonstrate

whether the thermal defect is dependent on sample processing technique. Hydrogen, helium, carbon and possibly oxygen ions of

various energies will be accelerated at the RARAF 4 MV Van de Graaff. The calorimeter

constructed by Fleming and Glass will be

employed, however, certain critical internal components have been rebuilt or re- placed. A block diagram of the system is shown in Figure 5.

This calorimeter system has been des1

on bed previously and a brief summary of

its operation will now be given. The

charged particle beam from the accelerator

is swept across the absorber face by electrostatic deflection plates, in the manner

of a video raster, in order to produce a uniform irradiation pattern. The calorimeter is adiabatically controlled by a thermopile which detects the temperature

difference between the absorber and the

surrounding adiabatic jacket. The thermopile signal is amplified and used to control

a thermal regulator which provides current

for a heater surrounding the adiabatic

jacket.

ADIABATIC JACKET CONTROLLER

VAN de GRAAFF^

BEAM

DEFLECTION PLATES

CALORIMETER

THERMOPILE OUTPUT

VOLTAGE

SWEEP

CONTROL

CIRCUIT

ELECTRICAL

CALIBRATION

CIRCUIT

Figure 5. Block diagram of the experimental

system.

Since the absorber and adiabatic jacket are constrained to rise in temperature

simultaneously, the measuring thermistor is placed in the jacket in order to minimize

power dissipation in the absorber. Electrical calibration is achieved through the

use of a resistance element between the T.E. plastic and aluminum faces.

PLASTIC THERMAL

INSULATOR

ALUMINUM

ISOTHERMAL

JACKET

-0 THERMAL BEAM RADIATION APERTURE SHIELD

MULTIPIN'^ ELECTRICAL CONNECTORS

Figure 6. Cross-sectional view of thermal

defect calorimeter.

The cross-sectional diagram in Figure

6 shows the various elements of the calorimeter. The aluminum isothermal jacket is maintained at a set temperature by a thermal regulator, and its interior is at vacuum. Within this element is the adiabatic jacket, which is thermally and electrically insulated from the outer aluminum

331
wall. The adiabatic jacket is maintained

at a negative potential in order to suppress

the emission of secondary electrons from

the absorber and assure a reliable measurement of the beam current. The measuring

thermistor and thermopile are mounted with

a high thermal conductivity epoxy and are electrically guarded from the negative high

potential to prevent leakage currents.

The absorber can be rotated so that

either face will completely stop the

charged particle beam. One face is aluminum 0.018 cm thick and 1.4 cm in diameter

and the other is 0.025 cm thick T.E. plastic of the same diameter. The ratio of

power measured using both sides of the

absorber yields the thermal defect. Details

of the absorber assembly are shown below

in Figure 7.

ABSORBER

CURRENT LEflO\ CROSS SECTION

JACKET WflLL"

Figure 7. Details of the thermal defect

calorimeter absorber assembly.

Summary

A series of measurements has been

carried out in ^'^Co and fast neutron fields

using a portable tissue equivalent calorimetric dosimeter. These measurements have

a precision of from ±1 to 2% depending

somewhat on dose rate and signal to noise

ratio. The absolute accuracy at present is estimated to be approximately ±5°^. Much of

this uncertainty is associated with the

thermal defect, and measurements are in progress which will help reduce this uncertainty. Comparisons will then be completed

for calorimetric dose and ionometric

specific charge in cyclotron-produced fast

neutron fields.

References

1. J.S. Laughlin, AEC Progress Report for

Contract AT(30-1 ) -1 451 (1954)

2. P. Mi Ivy, S. Genna, N. Barr and J.S. Laughlin, Proc. 2nd Int. Conf.

Peaceful Uses of Atomic Energy , Geneva (1958), p. 142.

3. J. Gei ssel soder , K. Koepke and J.S. Laughlin, Rad. Res. 20_, 423 (1963).

4. J.S. Laughlin, Proc. XI Int. Cong, of

Radiol., Rome (1955), p. 1772.

5. J.S. Laughlin and S. Genna, Radiation

Dosimetry , ed. F.H. Attix, W.C. Roesch

and E. Tochilin (Academic Press, New

York, 1966) , Vol . 2, pp. 389-439.

6. A. P. Pinkerton, Ann. N.Y. Acad, of

Science 161, 63 (1969).

7. J.C. McDonald, A. P. Pinkerton, H. Weiss

and E.R. Epp, Rad. Res. 49, 495 (1972).

8. J.C. McDonald, J.S. Laughlin and R.E. Freeman, Med. Phys. 3_ No. 2 (in press

1976).

9. J.J. Spokas, Presented at Rad. Res. Soc. Meeting Miami Beach, May 11-15,

1975.

10. J.B. Smathers, V.A. Otte, A.R. Smith,

P.R. Almond, F.H. Attix, J.J. Spokas,

W.M. Quam and L.J. Goodman, Presented

at R.S.N. A. - A. A. P.M. meeting

Chicago, Nov. 30-Dec. 5, 1975.

11. I.C.R.U. Report 108 (1964), p. 4.

12. D.M. Fleming and W.A. Glass, Rad. Res.

37_, 316 (1969).

13. D.K. Bewley, E.C. McCullough, B.C.Page

and S. Sakata, Phys. Med. Biol. 1_7, 95 (1972).

14. M. Sabel , Th. Schmidt and H. Pauly,

Health Physics 23, 744 (1972).

15. G. von Weimer, Atomkernergie, 20_, 327

1972

332
F.T. Kuchm'r, L.S. Skaggs, A.J. Elwyn.

F.P. Mooring and N.A. Frigerio, in

Cyclotrons 1972 , Proc. of the 6th Int. Cycl. Conf. (American Institute of

Physics, New York, 1972).

17. T.R. Canada, J.C. McDonald,

B.R. Freed, T.Y.T. Kuo and

J.S. Laughlin, Rad. Res. 50,

205 (1974).

333

NBS SP456 (1976)

SCATTERED RADIATION FROM A NEUTRON COLLIMATOR

F.H. Attix, L.S. August, and P. Shapiro

Naval Research Laboratory

Washington, D.C. 20375

Fast-neutron beams are being employed in radiotherapy trials and associated radiobiology studies at numerous centers in the U.S., Europe,

and Japan. Since collimated beams of various sizes and shapes are employed, it is desirable to know the composition of the scattered

radiation component contributed by the collimator. A simple method is

shovm for deducing the field composition in terms of a 3-component

model, from measurements made with three ionization chambers (tissueequivalent, graphite, and magnesium). The dose contributed by the

scattered radiation in the present example was found to be predominantly due to fast neutrons indistinguishable from those in the primary

spectrum (from 35-MeV d"^ on Be) . This method may prove useful for

measurements in phantoms as well.

(Neutrons; scattering; collimation; albedo; dosimetry; ion chambers; n-Y dosimetry)

Introduction

It is well known that the presence of a neutron-beam collimator near a source tends

to increase the absorbed dose (tissue)^ in

the beam, compared to what it would be at

the same point with the collimator removed.

This is, of course, due to the contribution

of scattered and secondary radiation by the

collimator. It is the purpose of this note

to describe quantitatively a specific example

of this effect as a function of collimator

size, and to show that about 90% of the observed dose increase can be ascribed to fast neutrons which have undergone little energy

loss in being scattered by the collimator.

Collimators 40 60 ' 80

DISTANCE FROM TARGET.cm

Figure 1 is a schematic diagram (either

plan view or elevation) of the collimator

design originally employed at the NRL Cyclotron to provide a neutron beam for the

MANTA^ program. Each collimator is made

of Benelex 401'* having a thickness of 66 cm, with aperture walls sloped so that their

extension toward the source is tangential to the circular beryllium target 2.5 cm in

diameter. The extension of the collimator

walls away from the source intersects the

125-cm SSD plane to give the nominal beam

size for that collimator. The 35-MeV

deuteron beam which generates the neutrons

in the Be target is focused in a central

spot less than 1 cm in diameter, which

*This work was partially supported by the

National Cancer Institute under Interagency

Agreement No. 1Y01C040001-01

.

Figure 1. Schematic diagram of the Benelexcollimator geometry. The 35-MeV deuteron

beam is incident on the thick beryllium

target from the left. Ion chamber measurements were made on the beam axis in free

space 125 cm from the target. A monitor

chamber (not shown) was located 10 cm from

the target. Note the exaggerated scale

perpendicular to the beam axis.

yields a beam size at 125 cm SSD somewhat

larger (at ~80% of max.) than the nominal

designation, as approximated by the dotted

rays shown emanating from the target , center

in Fig. 1. Typical isodose curves have been

reported in an earlier paper. ^ The present

study was made with three such collimators

having square apertures which pass neutron

beams of 5 x 5, 10 x 10, and 15 x 15 cm in

nominal size. A flat polystyrene-walled
monitor chamber, centered on the beam axis

»10 cm from the target, provided exposure

control for the present measurements. The

effect of neutrons backscattered from the

collimators into the monitor chamber was

found to be negligible, inasmuch as the ratio

of monitor reading to deuteron charge on the

Be target was independent of collimator size.

100

I 98

y 92

a: 90

TISSUE DOSE VS INNER SURFACE- + AREA OF SQUARE BENELEX COLLIMATORS / (ON AXIS AT 125cm SSD IN FREE SPACE, /

FOR 35 MeV D"^ ON Be NEUTRONS) /

/ / I5xl5cm // / /

//t

/ lOxlOcm CORRECTED FOR ANGULAR / DISTRIBUTION OF NEUTRON FLUX DENSITY //

//^ /o /'/ 5x5cm

~~RAW DATA

//(Q/A)^g = 90l FOR PRIMARY RADIATION (EXTRAPOLATED)

1

1 1 1 500 1000 1500 2000

INNER SURFACE AREA OF COLLIMATORS, cm^ 2500

Figure 2. Graph of the TE-plastic chamber

reading (for a constant charge collected in

the monitor chamber) vs^ the inner surface

area of the collimators. The lower points

(circles) are raw data; the upper points

(crosses) have been corrected for the

angular distribution of the neutron flux

density according to Serber stripping

theory, and therefore should apply to an isotropic neutron source.

Measurements

The lower points in Fig. 2 represent the

ionization readings obtained with a TE-plastic

chamber^ located on the beam axis at 125 cm from the target, for a constant monitorchamber reading. The abscissa of the graph

was arbitrarily chosen to be the inner surface area of the collimators, since that

surface probably contributes most of the

scattered neutrons reaching the chamber.

Although there was no a. priori reason for

believing that the chamber reading due to the

scattered radiation component would be proportional to such a simple parameter, the raw

data points in Fig. 2 are seen to be approximately collinear, but with a slight downward

curvature

Correction for Angular Dependence

of Neutron Flux Density

August et al.'' have shown that the

present neutron beam is generated primarily

by the deuteron stripping reaction described

by Serber,^ and as a result the neutron flux

density is strongly peaked at 0° (the

direction of the deuteron beam axis) and

decreases rapidly with increasing angle.

This forward peaking persists even for thick

targets.-'''' Because of the angular dependence, relatively fewer neutrons can strike

the collimator wall and be scattered than

would occur for an equivalent isotropic

source, and the disparity increases with

collimator aperture size.

A correction to the data in Fig. 2 was made for this angular dependence, to raise

the points to where they would be if the source were isotropic. Initially we obtained

a first approximation of the TE-chamber

response due to primary radiation only, by

extrapolating the raw data points to the zero-area axis in Fig. 2. This quantity was

then subtracted from each of the raw data

points to obtain the approximate scatteredTable I. Correction for Angular Dependence of Flux Density in the

Neutron Beam from 35-MeV d"*" on Be Nominal

collimator

size

Area

of

walls

Raw

TE chamber

reading

Representative angle

Scatter

correction

factor

Corrected

TE chamber

reading

15 X 15 cm 2080 cm2 100.0 4 75°

43°

1 159 101 6 10 X 10 cm 1498 cm^ 97.7 3

' 1 088 98 4

5 X 5 cm 941 cm^ 95.1 2 15° 1 038 95 3- 0^ 0^ 0 1 000 90 1

336
radiation contribution to the TE-chamber

reading for each collimator. The appropriate

angular corrections were then applied to these scatter components, which were then re- combined with the primary radiation component

and then replotted. Iteration of this procedure rapidly converged on the upper points

and solid curve shown in Fig. 2, which are strictly linear and extrapolate with a slope

of 5.5 X 10"'^ cm ^ to a relative TE chamber

reading of 90.1 for the primary radiation

component

Table I contains the numerical data for

this correction. The angle 6 taken to be

representative of each collimator was assumed

to be that lying between the beam axis and a ray drawn from the target center to a point P

in the perpendicular midplane of the collimator 53 cm away (see Fig. 1). Imagining the

square aperture cross-section which lies in

that plane, point P was taken to be located

on the collimator wall, one-fourth of the way

from one comer of that square to the next

adjacent comer. Letting the length of one side of that square be d cm, 6 is given by

53 cos (arc tan 0.5)

The fact that a straight line results

from these considerations tends to support

the idea that the tissue dose component on

axis due to scattered radiation is closely

proportional to the collimator wall area for

an isotropic neutron field, at least within

the range of square Benelex collimators

studied here. This could be further verified

with 14-MeV neutrons from the D-T reaction,

for which the neutron field is nearly isotropic. It seems reasonable, moreover, that

the increasing scatter trend with collimator

size must eventually reverse itself for very

large collimator apertures, in order for the

dose to retum smoothly to its uncollimated

level again when the aperture has been enlarged to the point of eliminating the collimator entirely. That interesting transition

would be a suitable subject for further experimentation as well as Monte Carlo calculations .

The last column in Table I shows that the

TE chamber reading contribution due to scattered radiation from these collimators

ranges from~6 to 13% of that delivered by

primary radiation alone. It is important,

both for radiobiological and radiotherapeutic

applications, to know the composition of such

a sizeable scattered radiation component. A

satisfactory method has been devised for obtaining this information in terms of a simple

three-component model.

Three-Chamber, Three-Component Method

Let us assume the total radiation field

at the point of interest to be made up of

only three components:

a.. Fast neutrons having a spectrum

practically the same as the primary neutron

spectrum (see Refs. 3 and 5)

_b. A slower monoenerget ic neutron component, E^.l MeV, low enough in energy to provide a negligible response in nonhydrogenous chambers, but high enough to give

normal response per tissue rad in a TE plastic chamber

£. Y rays from whatever source.

We make measurements of this mixed field

for each of the collimators with three

ionization chambers:

1. The TE-plastic chamber with TE-gas flow,

2. A graphite chamber (EG&G Model IC-17G)

containing static air, and

3. A magnesium chamber (EG&G Model IC-17M)

with argon gas flow.

For equal arbitrary monitor readings the

relative ionization charge readings Q divided

by the y-ray sensitivity A (coulombs/tissuerad) are listed in Table II for each collimator.

Also shown in parentheses are the ratios

for each chamber of (Q/A) relative to that

for the 5 x 5 cm collimator. It is evident

that the three chambers show quite similar

variations of Q/A vs collimator size. This can only occur if the tissue dose component

due to scattered radiation consists mostly of

fast neutrons, rather than the slower neutron

component of y radiation in this 3-component

model, as can be seen from the following

argument

Suppose the scatter contribution to the

tissue dose, as measured approximately by the

TE chamber, were due solely to the slower

neutron component (b above) . Since the

response of both the G-air and Mg-Ar chambers

is negligible for such neutrons, as recently

verified by Kuchnir et al.,^ those chambers

would show little or no change in output vs collimator aperture size, which is contrary

to the observed findings in Table II. Thus

the slower-neutron component must be

relatively small.

337
Table II. Relative Response (Q/A) for Equal Monitor-Chamber Readings

Nominal

collimator TE-TE G-air Mg-Ar

size

15 X 15 cm 100.0 (1.052) 32.76 (1.053) 16.80 (1.059)

10 X 10 cm 97.7 (1.027) 31.92 (1.026) 16.29 (1.027)

5 X 5 cm 95.1 (1.000) 31.10 (1.000) 15.86 (1.000)

Suppose, on the other hand, that the

scatter component were due entirely to y rays. The TE chamber, for which the ratio of

neutron-to-7 sensitivity per tissue rad

(B/A) is ~1 independent of neutron energy,

sees a dose increase of ~5% in going from the

smallest to the largest collimator, on top of

a primary beam due almost entirely to fast

neutrons. The response (Q/A in Table II)

of the G-air chamber to these neutrons is evidently only about one-third as great, so that the y^i^sy contribution to this chamber's

response would appear relatively three times

as large, or ~15%. Likewise the relative

response of the Mg-Ar chamber to the same

Y ray contribution would appear to be

~6x5% = 30%, since this chamber is only about

l/6th as responsive as the TE chamber to the

fast neutrons. The data in Table II give no indication of marked differences in variation

among the three chambers; therefore no substantial part of the scatter dose com- ponent can be due to y rays.

By process of elimination we are therefore draitm to the conclusion that the

scattered radiation from the collimators must consist mostly of fast neutrons (a^ above)

whose energies have been altered very little

by a glancing elastic collision with the

collimator wall.

The data in Table II can be incorporated

into three simultaneous linear equations to obtain a quantitative solution for the three

dose components due to a^, b^, and £ in the

simple model proposed above. Using the

notation established earlier^' we can write for each collimator:

TE-TE

TE Y \A

TE WtE

G-air \f] = D + ^ + (0) D

Mg-Ar {^\ = D + (0) D

where D is the y i^^y dose (tissue)

D^^ is the fast-neutron dose (tissue) from

component a above

D is the slower-neutron dose (tissue)

from component b^ above

and (B/A)^ a; (B/A)„ 0 for slower neutron

G Mg

component b^.

We assume the values (B/A) TE 0.991 and

(B/A) = 0.319 for our primary neutron

G spectrum, in agreement with earlier studies

of this neutron beam.-^^ This leaves us with

three equations in four unknowns, since we have not yet established a reliable value

for (B/A) with argon gas flow for the NRL

Mg

primary neutron spectrum. ^ Instead, we will treat (B/A)_, as a fourth unknown in

Mg

addition to D , and D . It is reasonY fn sn able to suppose that D^^ increases approximately in proportion to the scatter component

of the TE chamber readings (raw data in

Fig. 2) so that, referring to Table I,

^°sn)5x5cm

= n (95.1 - 90.1) = 5.0 n

^°sn^lOxlOcm

^°sn^l5xl5cm

n (97.7 ~ 90.1) = 7.6 n

n (100 - 90.1) = 9.9 n

where n is a proportionality constant which

is a single unknown to be solved for in

place of the three different values of D^^

for the three collimators. In this way we

can solve the data in Table II for each

pair of collimators, for which there will be

6 equations in 6 unknowns. This allows

three different solutions for the three pairs

of collimators employed.

The values obtained for (B/A)j_jg in the

three solutions were as follows:

338
Table III. Components of Dose (Tissue) in Benelex-Collimated

Neutron Beam from 35-MeV d"*" on Be

Nominal

collimator

size fn

15 X 15 cm 109 4 0.44 1 13 111 0

10 X 10 cm 107 1 0.37 0 95 108 4

5 X 5 cm 104 4 0.28 0 92 105 6

0^ 99 2^ 0^ 0 8^ 100 Oa,b

Extrapolated

Normalized to 100 rads, which equal 1 gray (Gy)

Collimator Pair

5 X 5 cm. 10 X 10 cm 0.1583

5 x 5 cm. 15 X 15 cm - .1584

10 X 10 cm. 15 X 15 cm .1587

Avg. .1585

Although D was constrained by

sn assumption to vary in proportion to the TE chamber reading, the D contribution is so

sn small (0.3 - 0.4%) that the substitution of

any other reasonable assumption about its

dependence upon collimator size would not change the data in Table III significantly.

The spread of these values is only about

± 0.1%, indicating good internal consistency

among the data in Table II. However the 2.2%

discrepancy from the earlier value for

(B/A)^. (see Ref. 13) must await later

Mg

resolution. In the present context 0.1585

is clearly the more appropriate value.

Table III contains a resume of the

average values obtained in these calculations

for , D , D and their sum D„. Also

fn sn 7 T shown are the corresponding data extrapolated

to the case of zero aperture area, using the same iterative technique already described

in relation to Fig. 2. All the figures were

then normalized to a value of 100 rad for

with zero aperture scatter. The value of

0.8 rad for D in that case was obtained as

Y the difference between the extrapolated

values of D_ and , and it agrees with the

J. rn figure for "aperture y-rays" given in Ref.

10 for a 2-cm diameter collimator. These

Y-rays evidently originate in the target,

and the collimators in the present study are seen to add only about 0.1 rad more y^^^Y

dose per collimator size, so that the y-vay

dose percentage only varies between 0.9 and

1.0% of D over the range of collimator

sizes employed here.

Conclusions

The most important finding in Table III

is that, of the 5.6 to 11% dose increase

caused by the collimators, over 9/lOths is

due to the fast neutron component . Thus

rn these collimators act to intensify the

neutron beam in the desired direction while

degrading the quality and purity of the beam

only to a negligible degree.

This has the practical consequence that

one should not expect detectable differences

in the RBE or OER of any biological system

exposed in free space to the collimated vs uncollimated neutron field, assuming

independence of dose rate. Likewise in the use of various-sized Benelex collimators in

neutron radiotherapy, the quality and purity

of the incident beam may be assumed to remain unchanged, although the characteristics of the mixed field in the body would be

'

expected to depend upon beam dimensions.

Other types of collimators may behave

similarly to those studied here, but that

remains to be demonstrated elsewhere.

In this connection a recent time-offlight study by Hertel-^^ at Texas A&M with a

339
49-MeV D on Be neutron beam showed no significant degradation in the energy spectrum

(within the range 7-53 MeV) on axis for a

7 X 7-cm square pyramidal collimator vs a straight-sided circular one 3.8 cm in

diameter. Both were made of iron filings

and water-extended polyester.

It should be pointed out that, having

once established a consistent set of

values for (B/A)_, (B/A)„, and (B/A)^ for TE G Mg

the primary neutron spectrum, it should then

be possible to solve for , D , and Dy

fn sn

at any measurement location, including points

within a phantom. The conventional twodetector, two-component (n,Y) method fails

when applied within a phantom unless one knows the neutron spectrum at each point and

the neutron energy dependence of the non- hydrogenous detectors ^ In other words,

one must know the effective (B/A) of the

non-hydrogenous detector for the neutrons

present at the point of interest. In the

present model we do this by allowing the

parameter D^^ to grow and take on the

appropriate value as a weighting factor so that the effective B/A for the graphite or magnesium chamber becomes

(B/A) eff

(B/A)-D^ + (0)-D

tn sn

D_ + D fn sn

Although we do not use (B/A) eff

explicitly in the method, this viewpoint

helps clarify the role of the D^^ component.

Finally, one can employ the present

results to obtain an estimate of the effect

of the collimator on the neutron output

measurements given by August et al.-''^ They

reported that the neutron tissue dose rate^ on the central axis at 125 cm from the d"*" on Be neutron source, with a 5 x 5 cm beam

defined by the same collimator as one of

those used in the present experiment, was

given by

Y = a(E^) rad pA'^min"!

where a = 1.24 x lO"** b = 2.99

and Ej^ = the deuteron beam energy (MeV).

This a. value takes into account a 3%

downward correction for y radiation in the

beam on the basis of the best estimate then

available. In the present Table III it will

be seen that 0.9% should be used in place of

3%, and the value of a is increased thereby

to 1.27 x

10"'+ for the 5 X 5 cm collimator.

Since the value of a_ should be proportional

to (D^ - D ) in Table III for the other

T Y collimators, one obtains = 1.33 x lO"'* for

the 15 X 15 cm collimator, 1.30 x 10"^ for

the 10 X 10 cm, and 1.20 x 10~^ for the

uncollimated source.

A very slight collimator effect on the

value of Id in the above equation, due to the

energy dependence of the neutron-flux

angular distribution, can be neglected in

comparison with the change in a^. Moreover

August et al.^ have shown from Serber

stripping theory considerations that b^ should approximate 3.

If the corresponding neutron dose rates

are desired for the bare source unencumbered

by the neutron attenuation of 3.5 mm Al

backing the target and 9.5 mm polystyrene in

the monitoring ion chamber^ ^, the foregoing

values of should all be increased by the

factor 1.145.

Shapiro et al.^^ made time-of-f light

studies of this beam with no collimator, and

with the monitor chamber removed but the Al

target-backing plate in place. For this case

the value of a = 1.20 x 10~^ should be increased by the factor 1.105 to remove the

attenuating effect of the 9.5 mm of polystyrene, so that the above output equation

gives 54.9 rad/min at 125 cm for 10 uA of

deuteron current on the target. The time-offlight results indicated that the corres- ponding neutron flux density at the same place was 8.4 x 10^ n/cm^- min, from which

one can obtain a value 6.5 x 10~^ rad n~-^cm^ for the average energy-transfer coefficient

of this neutron beam in muscle tissue. This

agrees fortuitously well with the value given

by Bach and Caswell^ ^ for E = 14 MeV, which

n

is approximately the average as well as modal energy of the neutron spectrum

obtained in the time-of-f light measurements

(see Ref . 3 or 5)

340
References

1. Throughout this paper we will employ the terms dose and absorbed dose (tissue) in

place of kerma (tissue) by invoking the

equality which results from the approximation

of charged particle equilibrium at the point

of interest.

2. The Middle Atlantic Neutron Therapy

Association (MANTA) , is a consortium of

medical centers engaged in a clinical trial

of the effectiveness of fast neutrons in cancer radiotherapy under the sponsorship

of the National Cancer Institute (see Ref.

3). The Principal Investigator is Charles

C. Rogers, M.D.

3. R.B. Theus, R.O. Bondelid, F.H. Attix,

L.S. August, P. Shapiro, R.E. Surratt, and

C.C. Rogers, Cancer 34, 17 (1974).

4. Benelex 401 is a composition product of

density p~1.42 g/cm^ made of pressed wood

chips and lignin by the Masonite Corporation,

29 North Wacker Drive, Chicago, Illinois.

5. F.H. Attix, L.S. August, P. Shapiro, and

R.B. Theus, The physics and dosimetry of

fast neutrons for radiotherapy. NRL Memorandum Report 3123, 1975.

6. Model IC-17, manufactured by EG&G, Inc.,

130 Robin Hill Road, Goleta, California.

This is a spherical chamber of nominal value

1 cm^, having walls of type A-150 tissueequivalent plastic 5 mm in thickness. TE

gas was flowed through the chamber continuously.

7. L.S. August, F.H. Attix, G.H. Herling,

P. Shapiro, and R.B. Theus, 1976, Stripping

theory analysis of thick-target neutron

production for D + Be. In press.

8. R. Serber, Phys. Rev. 72^, 1008 (1947).

9. F.T. Kuchnir, C.J. Vyborny, and L.S. Skaggs, A precise method for measuring the

neutron response of a "neutron-insensitive"

dosimeter. IAEA-SM-193/51 , 1975.

10. F.H. Attix, R.B. Theus, and C.C. Rogers,

Measurement of dose components in an n-y

field, Proceedings of the 2nd Symposium on Neutron Dosimetry in Biology and Medicine,

Neuherberg. Edited by G. Burger and H.G. Ebert, EUR 5273 d-e-f, p. 329, 1975.

Commission of the European Communities,

Luxembourg.

11. F.H. Attix, R.B. Theus, P. Shapiro, R.E. Surratt, A.E. Nash, and S.G. Gorbics, Phys.

Med. Biol. 18, 497 (1973).

12. Note that (B/A) = 0.318 for an airfilled graphite chamber, as given in Ref. 10,

has been remeasured recently at 0.319 by the same authors using an improved technique to be reported later.

13. A preliminary determination of

(B/A),, = 0.162 was obtained, but this is

Mg

evidently too large since it gives some slightly negative D and D values in ° ° sn Y solving the above equations using the data in

Table II.

14. N.E. Hertel, Fast Neutron Spectra

Produced by a 49-MeV Deuteron-Beryllium

Reaction and its Modification by Selected

Absorbers. Master's Thesis, Texas A&M

University, (1975).

15. G.H. Herling, Monte Carlo Calculations

of the Neutron and Photon Spectra for the INDI Project, NRL Report No. 7886 (1975).

16. F.H. Attix, R.B. Theus, S.G. Gorbics,

and C.C. Rogers, Report on NRL Measurements

as a Participant in the INDI, NRL Memorandum

Report No. 3051 (1975)

17. L.S, August, R.B. Theus, F.H. Attix,

R.O. Bondelid, P. Shapiro, and R.E. Surratt,

Phys. Med. Biol. 18, 641 (1973).

18. P. Shapiro, R.B. Theus , R.E. Surratt,

L.S. August, F.H. Attix, R.O. Bondelid,

and C.C. Rogers, (Abstract), Transactions

of the American Nuclear Society, 19, 52 (1974). Also unpublished data of Shapiro.

19. R.L. Bach and R.S. Caswell, Radiation

Research 35, 1 (1968) and 60, 363 (1974).

341

NBS SP456 (1976)

DOSE CONTROL IN RADIATION THERAPY

K.R. Kase, Ph.D., B.E. Bjarngard, Ph.D., B.J. Maddox, M.S., G.K. Svensson, Ph.D.

Joint Center for Radiation Therapy and Department of

Radiation Therapy, Harvard Medical School

Boston, Massachusetts

Accurate and precise dose delivery is necessary in radiation therapy because of the

steep tissue response-dose curve. This paper discusses the precision of dose measuring

devices, therapy machines, patient dose delivery at the Joint Center for Radiation

Therapy. Ion Chambers used for therapy machine calibrations show a precision of 0.7

to 1.3% while a thermoluminescent dosimetry (TLD) system shows a precision of 1.5%.

Therapy machines are calibrated on a weekly schedule and the variation in dose delivery

between calibrations varies from 1 to 2%. Daily constancy checks on the linear

accelerators using TLD show a variation of about 2% from day to day. Patient dose

measurements using TLD at orthovoltage energies appear to have a precision of about

5% except for very small treatment fields where it is about 12%. Patient dosimetry

at megavoltage X-ray energies shows a precision of 3.5% while at megavoltage electron

energies it is about 5%.

(Therapy, Dosimetry, Precision, Ion Chamber, TLD, Patient)

Need for Dose Control

The need for accurate and precise dose

delivery in radiation therapy is primarily

a consequence of the steep tissue responsedose curve, which implies that a small

change of dose can make a large difference

in tumor control probability^ » ^ or in

severity of complications. It must be

possible to deliver repeatedly and re- producibly high doses to the treatment

volume and at the same time spare adjacent

normal tissue or critical organs. To plan

a treatment for optimum ratio of tumor dose

to healthy tissue dose a large amount of

empirical data are required. These include

the relative distribution of dose for

various field sizes and depths, the therapy

machine output expressed in rad and a quantitative description of the influence

of different beam modifying devices such

as wedges and compensators on the dose

distributions. Measurements for this

data base are made under good experimental

conditions and should ideally be versatile

enough to allow corrections for almost all

variations of field configuration and

patient geometry.

There are also a number of corrections

one would like to apply to the machine

output to correct for tissue heterogeneity

and nonuniform field configuration.

Because exact information on individual

tissue composition is lacking many of these

corrections can not be done with any great

accuracy. Consequently one would expect

that the dose delivered to a point in a patient is known with even less accuracy.

However, the capability of precisely

repeatable dose delivery is a necessity

for patient treatment.

To be certain that therapy machines

deliver a precisely repeatable dose to the

treatment volume when operated in accordance

with the treatment plan, periodic measurements of the radiation fields must be made.

Absolute accuracy of these measurements is

assured by proper calibration of dose

measuring devices and nationwide intercomparisons. For accelerators and X-ray

machines, whose characteristics depend on

the reliability of electrical systems,

a program of frequent constancy control

is required which can supplement the more

time-consuming calibrations. However, we must also be convinced of the precision

of these dose meters and therapy machines,

i.e. for a given programmed dose the

variability in dose delivered and the

variability in dose measured must be

carefully assessed and determined to be

within the limits that can be tolerated.

This paper discusses the dose measurement systems routinely in used at the

Joint Center for Radiation Therapy (JCRT)

and studies undertaken to determine the

precision of these measurement systems

and patient dose delivery. The measure

of precision used is the coefficient of

variation^ a xlOOj of a series of measurements.

Dose Measurement Systems

Ionization Chambers

At the present time the standards for

dose measurements at the JCRT are Victoreen

R-meters model 621 for megavoltage machines

and model 131 for orthovoltage machines.

343
Every two years one set of these chambers

is calibrated at the NBS, and will be

referred to as the local standard

instruments. Another set was calibrated

at the JCRT against the local standard.

This set is used in routine calibrations

and will be referred to as field

instruments

.

Periodic constancy checks are made on these four chambers to insure that they

hold their calibration. These checks are done by exposing the chambers in a fixed

geometry to a 137Cs source for a fixed

time. Results of these checks corrected

for 137cs decay are plotted in Fig. 1. These constancy checks show a coefficient

of variation of 0.7-0.8% for the local

standard instruments and 1.3% for the

field instruments over a 22 month period.

During this period the reader for the

field instruments became defective and was replaced. This together with the

harsher environment experienced by the

field instruments accounts for their

greater variability relative to the local

standard instruments.

Thermoluminescent Dosimeters

Precision of the TLB system was tested

by exposing 21 dosimeters to 13''Cs in a series of 8 identical exposures over a period of 3 weeks. The coefficient of

variation in this series was calculated

for each dosimeter and plotted in Fig. 2. The average coefficient of variation for

the 21 dosimeters in this study was 1.5%.

Other Measurement Devices

In addition to R-meters and TLD, film

has been used for field size and uniformity

and relative depth dose measurements. Film

also plays an important role in routine

monitoring of the relative distribution of

exit dose in mantle field treatments. The

information obtained is subsequently used

in the calculation of the patient midline

dose. Fricke dosimeters have been used

for absolute dose measurements in X-ray and

electron beams but no detailed study has

been done on the precision of our Fricke

system. Several other ionization chambers

have been used from time to time, but

only for relative dose measurements.

X-Ray Treatments on 4 and 8 MV Linear Accelerators

The other dose measurement system in

routine use at the JCRT is a TLD system

using Li2B407;Mn Teflon disks. These dose

meters were calibrated against the local

standard instruments in the beam of a

4 MV Linac and at orthovoltage energies

used in therapy. TLD disks have been

used for routine daily checks on therapy

machine calibration, for depth dose

measurements in X-ray and electron beams,

and for some patient dosimetry.

The method we use for dose

interpretation involves a double readout

cycle which determines the sensitivity of

each dosimeter each time it is used. This

improves precision by eliminating the

effects of dosimeter history and day-to-day

variations in operator technique and reader

performance. Following exposure to the

unknown dose the dosimeter is annealed

at 80°C for 5 min and then its light output (Li) is measured. The dosimeter is

then irradiated to a standard dose in a 137cs irradiator, annealed at 800C for

5 min and its light output (L2)

determined. The unknown dose is then

calculated from:

D= (L1/L2) C

where C is the calibration constant for

the radiation producing Ll.

Weekly Calibrations

Therapy machines are calibrated at approximately weekly intervals using the

field instrument. The R-meter is exposed

in a polystyrene phantom under standardized

conditions (Fig. 3). If this measurement

indicates that the dose delivered to Dmax

is more than 1% different from the dose

indicated by the monitor chamber on the

machine, appropriate adjustments are made

on the machine. Machine output is then

remeasured and if 3 consecutive measurements

show dose delivery within 1% of the

programmed dose, the machine is considered

calibrated

.

In general the therapy machines hold

their calibration very well. A study of

calibration measurements over a period of

12 months indicated that the mean difference

between R-meter and machine monitor readings

prior to machine adjustment was very close

to zero on all machines (Fig. 4). The

coefficient of variation for four machines

ranged from 1.2% to 2.1%, The coefficient

of variation of an R-meter measurement is

1.3%, thus the maximum coefficient of

variation in the dose delivered by a therapy

machine is about 1.7%, since some imprecision will arise in the calibration

setup.
Morning Checks

Each morning prior to starting

treatment the calibration of each therapy

LINAC is checked by exposing 2 TLD at Dmax to a predetermined fixed dose (300

rad on the machine monitor) in a standardized geometry (Fig. 3) . The TLD measurement must fall within + 5% of the

programmed dose. If it does not, a second

measurement is taken and corrective action

would be initiated if the second measurement is also more than 5% different from

the programmed dose.

A six month period in 1975 was broken into intervals corresponding with

the intervals between machine calibration

adjustments. The percent difference

between the morning check measurement and

the machine monitor dose was averaged

over this interval to get an indication

of the precision of our dose control base

on these daily TLD checks. The results

are shown in Fig. 5 and indicate that the

relationship between the TLD measurement

and the machine monitor followed the same pattern on all machines. The coefficient

of variation ranged from 2.1 to 2.4% for

the four machines. Since the maximum

coefficient of variation of the dose

delivered by the machine is about 1.7%

from above, the minimum coefficient of

variation of a TLD measurement is in the

range 1.2 to 1.7% which compares well with

that found in the laboratory study reported

above.

An interesting observation is that the

periods in which differences between the

TLD and machine monitor exceed 4% are generally associated with the replacement

of a machine component such as an accelerator wave guide, magnetron or pulse

transformer. Because our morning check

measurements are made at a depth of 1 cm

on the 4 MV and 2 cm on the 8 MV LINAC

while the machine calibration measurement

is made at 5 cm depth, a large difference

between the dose measured by the TLD and

the machine monitor would indicate a shift

in the depth dose characteristics of the

beam. This might indicate that a temporary

but significant energy shift occurs as a result of these component replacements.

Patient Dosimetry

Routine dose measurements are done on patients treated with a mantle field. TLD measurements are made during the early

portion of the treatment on the surface of

the patient in polystyrene build-up blocks.

Measurements made at various points in the

field (Fig. 6) are used to verify that the

prescribed dose is being delivered and to measure dose uniformity throughout the

treatment field. Undertreated areas as determined by the TLD are boosted.

Doses delivered to 52 patients

treated with mantle fields since Jan. 1975

have been measured. The distribution of

dose measurements is shown in Fig. 7. Results of these measurements show that 47 of these patients received midline doses

within + 5% of the prescribed dose. The

coefficient of variation in the measurement

of the ratio of Measured Dose/Prescribed

Dose for the 52 patients is 3.5%, which is an indication of the precision of dose

delivery for mantle fields.

X-Ray Treatments at Orthovoltage

Energies

Orthovoltage X-ray treatments are done

at the JCRT using one of two beam qualities

140 kVp, 3.5 mm Al HVL or 300 kVp, 1.5 mm

Cu HVL. Tissue areas treated vary from

1.5 cm equivalent square to 14.0 cm equivalent square fields. Patient doses

are verified by exposing a pair of TLD dosimeters in the center of the treatment

field on the skin surface during treatment. Treatment fields were defined using a lead

mask at the surface of the patient and

overlapping the mask with the machine

collimators

Over a period of time covering 116

treatment measurements it was observed that

although there is a large amount of scatter

in the data, measurements of dose delivered were generally slightly lower than the

dose expected at the skin surface based on the prescribed dose (Table 1) . Also the

measurement precision appeared to be worse

for the very small field sizes. TABLE 1

No. of \ Coeff.

Equiv. Square(cm) HVL Meas. Dc Var.

< 2.0 3 5 mm Al 14 0 91 11 6%

2 1-3 0 3. 5 mm Al 27 0 98 5 9%

3 1-4 0 3. 5 mm Al 22 0. 98 6. 8%

*T>f^ is the mean of the ratio of measured to

13^ calculated skin dose.

345
A 1 R 4

1-D .

oJ

t; J inm A 1 T 9 1 Z U

c

H . i— J . U i J mm LU 1 9 u

Q Q oJ

3 . 1-0 .1

cJ mm o U y /o

D . i- / . U 1

rD mm J u

Q 7 7 1

7.1-8.0 1 5 mm Cu 5 0 95 3 0%

8.1-10.0 1 5 mm Cu 8 0 94 8 3%

10.1-12.0 1 5 mm Cu 5 0 93 4 6%

A series of experiments were undertaken to determine what factors other

than TLD calibration might cause a difference between measured and calculated

skin dose. A 30x30 cm polystyrene box

filled with water was used as a phantom.

Pairs of TLD were placed at the water

surface in the center of the phantom for

the measurements. Field size was defined

in 3 ways: (1) the field size was set with the machine collimator; (2) field

size was defined with lead strips at the

water surface and the machine collimators

were set to overlap the lead by 1 cm on all sides; (3) field size was again

defined with lead strips but the machine

collimators were set at 10x10 cm. The

majority of patient treatments are done

under condition 3. Another measurement

was done setting field size with the

machine collimators and exposing pairs of TLD with no water in the phantom. This was done to study the effect of field size

without the influence of backscatter.

Field flatness was measured under

conditions 1 and 3 above using TLD microrods (1 mm x 6 mm) spaced at 0.5 cm intervals across the field.

Results of the first set of measurements are given in Table 2. The dose was

first measured at the center of a 10 cm

x 10 cm field. The dose expected for

smaller field sizes was then calculated

using published backscatter factors

in the following way:

Dp = DlOxlOf L where

BiOxlO'

Dp = dose expected for field size F. ^10x10 ~ dose measured in 10 cm x 10 cm

field.

Bp = backscatter factor for field size F. BjOxlO ~ backscatter factor for 10 cm x

10 cm field.

Results shown in Table 2 are the ratio of

the measured dose (D14) to the expected dose

(Dp) for various field sizes under the

three conditions.

It is apparent that dose delivered is

within a few percent of the expected dose

when field size is defined by a lead mask

and the machine collimators are opened to 10 X 10 cm. However, if the field size is defined by the machine collimators, doses

delivered

TABLE 2

Dm/Df

Field Defined

Lead

Field Collimators Lead Mask & Mask

Size HVL Only Collimator Only

2x2 3.5 mm 0.80

Al

2x3

3x3

4x4

5x5

2x2 1 . 5 mm

Cu

2x3

3x3

4x4

5x5

0.89

0.92

0.98

0.99

0.79

0.87

0.93

0.95

0.96

0.89

0.94

0.93

0.98

1.06

0.93

0.95

1.00

0.99

1.01

0.99

1.03

1.00

1.01

1.01

0.97

1.00

1.01

1.00

1.05

to the small fields are significantly less

than expected. The measurement in air (no

water in phantom) showed that the dose

measured decreased with field size and

that this non-uniformity accounts for the

discrepancy between dose measured and dose

expected when the machine collimators are used to define the field size. The

precision of the dosimetry in these experiments was quite good (less than 3% coefficient of variation)

The field flatness measurement con- firmed that the dose is very non-uniform

for small fields when the field is defined

by the machine collimators. This occurs because there is no scattering into the

beam from outside its edges. Field

346
uniformity is much improved when a lead

mask is used to define the field size.

However the dose still falls off rapidly at the edges of the field. Since the

dosimeters used are about 1 cm diameter,

placement is critical in small fields which

probably accounts for the large variations

in dose measurement for patients treated

with very small fields.

Electron Treatments at 4, 7 and 11 MeV

Measurements to characterize the

radiation field when treating patients with

intermediate energy electron beams,

characterization of the electron depth dose

for the treatment configuration is most

important. At the JCRT the radiation field

is defined at the patient using a lead

mask cut to the desired field size and shape

and placed in contact with the patient.

An adjustable aluminum collimator defines

the electron be'am impinging on the lead

mask. This collimator is set so that the

electron beam is 1 to 2 cm larger than the

cutout in the lead mask at the surface of

the patient. The collimator jaws on the

therapy machine are set to 30 x 30 cm. In

this configuration we have been able to achieve reasonably consistent depth dose

curves and field uniformity^.

Prior to beginning treatment on a patient, the lead mask for that patient is

used to define the electron field for a depth dose measurement in a polystyrene

phantom at the treatment energy.

Thermoluminescent dosimeters are used for

this measurement which provides the information required to calculate the machine

meter dose needed to deliver the prescribed

treatment dose.

Patient Dosimetry

Pairs of TLD are placed in the

center of the radiation field on the

patient during each treatment. These

dosimeters are used both to verify the

dose delivered during a particular treat- ment and to measure the cumulative dose

during the treatment period. Based on

the delivered dose as measured by the

TLD, the treatment plan may be modified to deliver higher or lower doses for the

remainder of the treatment. To date 24 patients have been treated in this manner. Nineteen have received cumulative doses

within 5% of the prescribed dose.

Day-to-Day variations in dose

delivered could be caused by variations

in the TID , machine fluctuations or patient

setup. TLD precision was discussed above

where it is sho^m that the coefficient

of variation in a series of exposures is

about 1.5%. Electron beam stability was checked in a series of measurements in

which a Baldwin-Farmer ionization chamber

and TLD were exposed together to electron

beams of 4, 7 and 11 MeV. These measurements showed that the coefficient of

variation in the comparison between TLD,

Baldwin-Farmer chamber and the monitor

chamber on the machine was about 2%. In

a more extensive series of comparisons

between the Baldwin-Farmer chamber and

machine monitor chamber the coefficient of

variation of the ratio, B-F dose/monitor

dose, was 2.0% for 4 MeV, 2.6% for 7 MeV and

0.9% for 11 MeV electrons.

The ratio of monitor dose (MD) to dose

delivered during patient treatment as measured by TLD should remain constant

throughout the treatment period for any

given patient. Variations in this ratio

from patient to patient are caused by

adjustments in monitor chamber sensitivity,

SSD, field size, etc., but are taken into

account in the treatment plan. The

coefficient of variations in the ratio

MD/TLD over the treatment sessions for a single patient is a measure of the precision of dose delivery for electron treatments. Table 3 presents the mean of the

ratio MD/TLD and the coefficient of

variation in the ratio for patients treated

to date.

TABLE 3 Coefficient

of No. of VariaEnergy Patient Sessions MD/TLD tion

4 a 9 0 968 7 4%

4 b 13 0 384 3 8%

4 c 8 0 462 4 7%

4 d 5 0 450 9 8%

7 e 7 1 258 5 8%

7 f 10 0 855 6 1%

7 g 13 0 281 7 1%

7 h 10 0 324 5 7%

7 i 5 0 283 4 2%

7 3 6 0 792 2 4%

7 k 8 0 870 1 9%

7 1 4 0. 997 3 3%

7 m 4 0. 907 4 5%

7 n 9 1 048 8 7%

7 o 3 1 101 3 2%

347
11 p 3 0. 677 0 8%

11 q 9 0 681 6 6% 11 r 15 0 577 6 6% 11 s 10 0 479 3. 8% 11 t 5 0 566 4 7% 11 u 15 0 477 5 2% 11 V 18 0 743 3 3%

11 w 10 0 794 2 9% 11 X 4 0 823 4 4%

As seen in Table 3 the precision varies

widely from one patient to another from

less than 1% to almost 10%, while the

average value is about 5%. An analysis

of the 203 sessions involving 24 patients

shows that in 85% of the sessions the dose

measured was within + 10% of the prescribed

dose and in 61% of the sessions the

measured dose was within + 5% of the

prescribed dose.

Based on these data the uncertainty

in dose delivery averages about 8.5% at the

95% confidence level. However, because of

as yet unexplained non-statistical

fluctuations in dose delivered, we must

continue to measure patient dose during

each treatment session.

Conclusions

Patient Dose Measurement and

Treatment Verification

- Orthovoltage 3% to 12%

- Mantle Fields 3.5%

- Electrons 5.1%

Based on these results our policy is

to investigate any measurement which

exceeds these values and if deemed

necessary repeat the measurement, adjust

the machine calibration, or revise the

patient treatment plans.

As a result of this study we have

decided that continued daily verification

of therapy machine calibration is

essential. However, we will soon change

from using TLD at one point for this

purpose to a system similar to that

recently published by Jones and Schumacherusing diode detectors at 3 positions

in the beam. This will allow an immediate

check on dose output, beam symmetry and

depth dose without waiting 2-3 hours for

the TLD to be read out.

References

Dose control in radiation therapy

depends on the precision of many instruments and techniques. Using the

coefficient of variation as a measure of

precision we have attempted to determine

the individual precision of some of these

factors and relate that to the precision

of patient dose delivery. Table 4 summarizes our findings and gives the

precision with which we can calibrate

therapy machines, verify their calibration, measure patient dose and verify treatment

plans

.

TABLE 4

1. M.L. Mendelsohn, Front. Radiation

Ther. One. 6, 512 (1972) (ed. J.M. Vaeth)

2. W.D. Bloomer & S. Hellman, New Eng.

J. Med. 293, 80 (1975).

3. British J. Radiol. Supplement No. 11

(1972)

.

4. B.E. Bjarngard, R.W. Piontek and

G.K. Svensson, Med. Phys . 3^, (1976)

(in press)

.

5. D. Jones and D. Schumacher, Am. J. Roentgenol. 123, 198 (1975).

Instrument or Technique Precision

R-meter and Therapy Machine

Calibration 1.3%

TLD Controlled Experiment 1.5%

LINAC Dose Delivery 1.7%

Verification of LINAC

Calibration 2.2%

348
36-

34-

32^

30-

32-

30-

28-

621

* Local Standords o Field Instruments

131

UONTHS 4 6 8 tO

1974

6 !

1975

2 4

1976

Fig. 1. Constancy measurements for

Victoreen R-meters over 22 month

period corrected for source decay.

3.0

I "

1 20

I

1.0

8 0.5

15

J I \ L

Fig. 2.

2 4 6 8 10 t2 14 16 18 20 22 TLD DOSIMETER NUMBER

Coefficient of variation for 8 identical exposures of 21 TLD over

a 3 week period.

Fig. 4. % Difference between machine monitor

and R-meter dose prior to machine

adjustment for 4 MV and 8 MV LINAC's.

Fig. 3. Setup for LINAC calibrations and

morning checks

349
PBBH4

PBBH 8

JUN JUL AUG SEP OCT NOV DEC 1975

Fig. 5. % Difference between morning dose

check and machine monitor dose

averaged over the period between

calibration adjustments for 4 MV

and 8 MV LINAC's

Fig. 6. Points of dose measurement for

mantle fields.

i

0.90 0.95 1.00 1.05 MEASURED DOSE/PRESCRIBED DOSE

Fig. 7. Distribution of mantle patient

dose measurements.

350
NBS SP456 (1976)

DOSIMETRY PROCEDURES AT THE NORTHWEST MEDICAL PHYSICS CENTER

Douglas Jones, Douglas Schumacher & John Washington

Northwest Medical Physics Center

University of Washington, RC-08

Seattle, Washington 98195

The Northwest Medical Physics Center provides physics support

to 28 radiotherapy institutions in the Northwest from a central location. The methods employed in on-site physical measurements, organization of data and long-term quality assurance

are described.

(Calibration; megavoltage radiotherapy; quality assurance)

Introduction

The Northwest Medical Physics Center

developed from funds from the Washington/

Alaska Regional Medical Program in the

years 1969 through 1971. The Center sup- plies radiotherapy physics support to 28 radiotherapy departments in Washington,

Oregon and Alaska at which over 7,000 cancer patients are treated annually. The

growth in participation in the Center is illustrated in Table I, and the radiation

therapy equipment currently in use at the

participating institutions is shown in Table II. TABLE I Participation in the Center

Y Number of Number of New -^-^ Participants Cancer Patients

1970 16 3,182

1971 22 4,103

1972 23 5,505

1973 27 6,500

1974 29 7,778

1975 28 7,400*

*Data for 1975 is estimated.

TABLE II Radiation Therapy Equipment

January, 1976

Machine Type Number

Superficial X-Ray 9 Orthovoltage X-Ray 17 Cs-137 2 Co-60 21

4 MV Linear Accelerator 10

8 MV Linear Accelerator 1

The Center provides a broad range of

physics support services to radiotherapists

which have been described^. The purpose of

this report is to describe the methods

employed to ensure accurate dose delivery

at these institutions with an emphasis on megavoltage photon therapy which is the

modality employed in over 94% of the treat- ments delivered in our region.

Equipment Available & Its Calibration

Routine Field Instruments

The Farmer dosimeter, Model 2502/3,

with the associated 0.6 cc chamber is the

basic field instrument. In our experience,

the new graphite wall chamber has proved

too fragile for routine field work, and the

nylon wall chamber is preferred for this

purpose. The precision of readings with

this instrument is defined by a standard

deviation of less than 0.2% of the mean

value. The nonlinearity of our particular

instrument over its range is less than 0.5%

and a calibration curve has been constructed

to account for this deviation. Other chambers which are used with this system in specialized applications include a 0.03 cc parallel plate chamber with a 2.3 mg.cm"^

window and a 30 cc chamber for low level

measurements. A Keithley Model 602 Electrometer has been modified to accommodate

the Farmer chambers and is used when dose

rate measurements are required. The local

standard to which these field ionization

chambers are referred is a 0.6 cc Farmer

graphite chamber which is calibrated an- nually at a Regional Calibration Laboratory.

Depth dose and off axis dose measure- ments are routinely made using a 50 x 50 x 38 cm water phantom to which is attached a linear scanner having variable speed, remote

operated drive. The position of the chamber is sensed electronically allowing auto- matic plotting of beam profiles. Prior to 1975, a silicon diode was used as the detector for off axis dose measurements but

studies showed that this device overresponds

at the periphery of large fields from 4 MV x-ray machines and its use was discontinued.

Where indicated, film is employed for
measurements, and Kodak RP/V is preferred

for its slow speed. Our experience is that

unit density is obtained from a dose around

50 rads. The dose/density relationship of

the film is measured on each occasion.

Plastic and tissue-equivalent rubber

phantoms are occasionally used for check

measurements or depth dose checks at beam

orientations other than vertically down.

The reference material is water and when

plastics are used, the depth is scaled on the basis of linear attenuation coefficients rather than simple density. For example, a scaling factor for lucite for

Co-60 gamma rays of 1.15 is employed and

good agreement with depth dose measurements

in water has been obtained on this basis.

The routine armamentarium for field

work is completed by a survey meter. The

Eberline Rad-Owl has proved satisfactory

for our purposes. The facility to integrate exposure is a particular advantage.

This instrument is calibrated by a standardized Cs-137 source.

Laboratory Apparatus

Several film densitometers are available for quantifying the film images pro- duced in the field. The one most commonly

used is a Joyce-Loebl Scanning Microdensitometer. The automatic and high resolution

features of this instrument are particularly

advantageous. A Tel edyne- Isotopes Model

7300B Thermoluminescent Dosimetry Reader is available for the evaluation of LiF-Teflon

and Li aBijOy :Mn-Tef Ion dosimeters which are

the thermoluminescent dosimeters usually

employed at the Center. A chemical dosimetry system built around a Beckman DoubleBeam Spectrophotometer is available for

Fricke dosimetry. A considerable assortment of test equipment and general laboratory apparatus together with a well-stocked

workshop complete the set of tools required

by a physicist supporting radiotherapy departments in the 1970' s. The data processing capabilities of the

Center should be mentioned in this general

category. These range from pocket calculators through desk top calculators, typified

by the Hewlett-Packard Model 9810, to a CDC 6400 Computer which are all used for

data manipulation and reduction.

Measurements at the Institutions

Initial Acceptance Testing

In 1976, we anticipate responsibility

for the acceptance testing of one 10 MV, two

6 MV and six 4 MV radiotherapy machines.

With a load of this magnitude, a systematic

approach is necessary. Table III is a guide

employed by us structured on the AAPM recom- mendations^.

TABLE III The Guide used by the Center in Acceptance Testing of Megavoltage

Photon Radiotherapy Machines

Safety Considerations

1. Leakage Radiation, Beam Off

2. Leakage Radiation, Beam On

3. Exposure in the Environs

4. Door Interlock

5. Couch Interlock

6. Beam Direction Interlocks

7. Emergency Off Switches

8. Signs and Labels

9. Radiation On Indicators

10. Backup Dosimetry

11. Operating Procedures

Mechanical Aspects

1. Dimensions

2. Isocenter Location and Stability

3. Optical Distance Indicator

4. Collimator Symmetry in Rotation

5. Light Field Size

6. Radiation Field Size

7. Coordination of Light and Radiation

Fields

8. Radiation Field Symmetry

9. Isocenter Indicators

Dosimetry Data

1. Shutter Time

2. Quoted Rmm

3. Dose Rate as a Function of Field Size

4. (TAR)o Off Axis

5. Off Axis Dose in Water (Flatness)

6. TAR or TMR Measurement

7. Surface Dose

8. Interface Dose Between Couch

9. Output as a Function of Gantry Angle

10. Stability in Rotation

11. Output as a Function of Distance

12. Timer and Integrator Linearity

External Inhomogeneities

1. Wedges 4. Block Transmission

2. Compensators 5. Couch

3. Bolus 6. Trays

Machine Log

1. Beam Monitor Readings

2. Frequent Check Dosimeter Setup

3. Review Measurements and Checks to be

made by Local Staff

352
The guide also defines the form of the re- port, which will be discussed later. The

tasks enumerated in Table III will be rec- ognized by those experienced in the field.

Our approach and methods are elaborated in the subsequent sections and where appropriate, examples are given to illustrate

specific tasks.

Safety Considerations

The objects of this section of the re- view are to ensure that the machine complies

with local regulations and that hazards to patients and operators are minimized. Head

leakage measurements in the beam off condition are only relevant to isotopic machines

when measurements are made in accordance

with the methods described in NCRP Report

3 No. 33 . There are two considerations in

measurements of the head leakage in the

beam on condition. The first is if the

housing is designed to minimize total body

exposure to the patient and the second is if high leakage areas will result in high

exposures in the environs of the room. In the case of electrical machines, a search

is made for high leakage areas by wrapping

the head with film. If Kodak RP/V film is used for this purpose, the machine should

be set to deliver a dose on the order of

7,000 rads at one meter. Following the

demarcation of these areas, the magnitude

of the leakage is determined using an ionization chamber. An interesting characteristic of the Varian Clinac 4 operating with

the uranium flattening filter is that measurements with the jaws closed indicate

leakage in excess of 0.1% of the primary

but this zone of high leakage is obscured

by the upper jaws so that for clinically

useful beams, the leakage is within specification.

The checking and evaluation of interlocks has to be done with reference to the

day-to-day operation of the machine. It is necessary that the local technical staff be

on hand during these tests so that they

fully comprehend the function of all the

interlocks and that the purpose of ongoing

checks is realized. Tests are conducted to ensure the efficient operation of backup

dosimetry systems also.

Mechanical Aspects

There is a class of measurements to be

made on a therapy machine which relate to

its construction and accurate design. Generally, a test which does not require the

use of an ionization chamber falls into this

category. The dimensions of the machine

should be noted with particular reference

to points of measurement such as the isocenter and wedge location. The stability

of the isocenter during gantry, collimator

and couch rotation is assessed using slit

beams on film. The accuracy of the optical distance indicator is assessed over its range. The rigidity of the collimator

system is checked by film exposures at various gantry angles. The size of the light

and radiation fields and their coordination

are checked over the range of field sizes

available using film. We have found it

convenient to use a marked polystyrene

plate with radiopaque markers for this purpose. The field symmetry is assessed by

dose measurements at symmetrical locations

in the beam.

Dosimetry Data

Besides the items mentioned in Table

III, measurements are made to provide the

radiotherapist with clinical data for special treatment regimes such as moving strip

therapy and whole body irradiation. But

in general, the purpose of this section is to develop a comprehensive set of data to allow computer or hand calculations of dose

distributions

The output of the machine as a function of field size is assessed and a separation made of collimator and phantom

scatter. The variation in energy across

the field of low energy linacs is an important consideration in dose calculations.

4 Recently Bruels described a routine to accommodate this effect in Clarkson-type

irregular field calculations which is cur- rently under review at the Center. Initial

measurements were made using a narrow column of water, but in order to expedite

this measurement, a semi-empirical relationship was developed to convert attenuation measurements in lead to those in water. For beams of nominal energy of 4 MV the equation has the following form:

HVL Water = 7.11(HVLLead) + 4.91 cm. Typical values for the change in energy are shown in Table IV. The dose off axis is assessed by scanning in a water tank. A range of field sizes are normally measured

and the data tabulated in the form of off

center ratios to enable computer calculation of patient dose distributions. For depth dose determination, the standard

practice at the Center is to measure tissue

air ratios for beams up to 4 MV and tissue

maximum ratios for higher energies. A computer program, Tarman, has been developed

and is used to generate percentage depth

353
dose tables and tables of the dose along

the central axis in isocentric treatments.

We are currently extending this program to smooth and extrapolate tissue air ratio

data to produce scatter air ratio tables

for large radii. Surface dose measurements

are made under various conditions and serve

to illustrate reduction in skin sparing

under such conditions as increasing field

size and inclusion of plastic block trays

in the beam. Where the couch is fitted

with a cutout section, an assessment is made of the loss of skin sparing introduced

by the patient support structure. The op- eration of the machine in the rotation

therapy mode is evaluated in order to en- sure uniform output as a function of gantry

angle. The output is correlated to inverse

square law to establish the location of the

virtual source which is used in depth dose

computations. Generally, all measurements

are made with the machine running at the

dose rate to be employed clinically. How- ever, the deviation of the output due to nonl i neari ti es in the integrator circuitry

is determined over the range of operation

of the unit.

Machi nes

Clinac 4 Pb Filter

CI i nac 4

U Filter

TABLE IV HVL, Water, cm Distance Off Axis at 100 cm

0 1 10 15. 20

12.9 12.5 12.0 11.5 11.1

13.5 13.4 13.1 12.7 12.4

SsTilter ^^'^ ^^'^ ^^'^ ^^-^

SS^FiUer°° 12.1 12.1 12.1 11.9 11.3

LMR-4

Pb Filter 12.5 12.3 11.8 11.3 10.9

External Inhomogeneities

This term refers to measurements of

the effect of the introduction of any absorbers in the beam. The standard algorithm employed at the Center for dose com- putations is that developed at Memorial

Hospital^ with some local modifications^.

The computation of the dose under wedges is based upon the assumption of exponential

absorption on a ray through the wedge.

This technique does not adequately account

for phantom scatter and other effects, and

thus it is necessary to fit wedge dimensions to experimental data. As a routine,

wedged beams are scanned at several depths

in a water phantom and the wedge dimensions

are modified to yield a best fit to the

empirical data.

The evaluation of compensation, bolus

and blocking techniques employed would be

the subject of a report in themselves.

The general approach we make is to ensure

that sufficient information is collected to enable the calculation of dose distributions. In this regard, mention should be

made of recent work at the Center using the

concept of negative fields'' to calculate

the dose distribution under narrow blocks.

The attenuation of the support structures of the couch are determined and this

information is used in dose calculations

where the couch interposes the beam. The

transmission factors of blocking trays are determined and if the variation with field

size is less than + 1%, a single factor is quoted

Machine Log

The foregoing has provided an outline

of the initial measurements on a megavoltage radiotherapy machine. It must be

stressed that while we do use this guide,

it is construed as an outline and that

individual variations in the installation

will require innovation on the part of the

responsible physicist.

Following this extensive series of

tasks, we see the essential task to be one

of ensuring the fidelity of the original

data and providing adequate documentation

to this effect. For electrical machines,

we consider the particularly suspect param- eters to be the dose delivered per unit

monitor setting and, to a lesser extent,

beam energy and symmetry for bent beam or multimode machines.

We have developed a device called the

g beam monitor which consists of two diode

detectors located at 5 and 17 cm depth in

a phantom with electronic circuitry at- tached to the phantom. Our design considerations in developing this device were

that it be simple and convenient to use

and that long-term drift of the readings

of the integrated current from the diodes

be less than 3% per year. Seven instruments were produced which have been in use

on the machines in our region over the last

three years. A commercial version of this

g unit is now available . A standard procedure is in use which consists of two exposures of the device under well defined

conditions at the beginning and end of the

treatment day. At the time of a calibration

354
using ionization chambers, the physicist

will establish the readings obtained by the

beam monitor under these standard conditions. At the end of each month, a form

developed by the Center is completed in

which the average readings are shown as well as the deviations from the exact

values. A copy of this is returned to the

Center for review. When an isotopic source

is available at an institution, this is used for a weekly check on the constancy of

the beam monitor. This system has proved

very useful in diagnosing problems with the

machine and has enabled the analysis of the

long-term stability of the machines in our

region''''^. A typical example of a completed

analysis form is contained in an appendix

to this paper.

Another parameter which requires regular review is the coordination of the light

and radiation fields. For this purpose, we

have developed a frequent check dosimeter which consists of a piece of DuPont

Radiation Imaging film contained in a specially marked cassette. The cassette has two rectangles scribed on its surface with

corners defined by radiopaque markers.

These devices are exposed each month on the

therapy machines in the region and returned

to the Center where the coordination of the

image of the radiation field produced on the film to the scribed rectangle is determined. The device also contains two pre- calibrated Li2Bi*07:Mn-Teflon dosimeters

which are used to check the output of the

machine.

Check lists have been designed for use

by the local staff which define the param- eters to be checked and their frequency.

For example, a check list for an isocentric

4 MV machine requires a daily check on the

accuracy of the optical distance indicator,

a weekly check on the accuracy of sidelights and a monthly check on all safety

interlocks. A copy of this form is re- turned to the Center for review. The form

employed is shown in the appendix.

Information Flow & Documentation

Methods of documentation are extremely

important in this field and the methods em- ployed at the Center today are the result

of considerable effort and time. While

simple in concept, they are worthy of description here.

A central repository for all data per- taining to a therapy machine is maintained.

This takes the form of a ring binder which

is kept close to the therapy machine. An exact replica is maintained at the Center

to ensure the application of uniform data

355

in dose calculations. The format of these

binders follows that described in Table

III. We have not found it possible to formalize the information further than a narrative style report structured on the

topics shown in the table. Except that as already discussed, the presentation of

depth dose is formalized by the output format of the program Tarman and as in most

repetitious tasks, a uniform pattern tends

to be enforced in all sections. The

Machine Log Section also contains forms

for recording downtime on the unit and

narrative reports by the maintenance engineer and the technicians on the machine

operation. In summary, this approach pro- duces a data base for use by the radiation

oncologist, physicist, dosimetrist, technician and maintenance engineer and by

this means, ensures uniform data and clear

communication of information. The structure is logical and in our experience, it

has proved easy to access information on the machine.

There exists a manual of operations

describing all the services offered by the

Center. With reference to the object of

this paper, the manual contains a section

dealing with the standards employed by the

Center and this general statement makes it

unnecessary to repeat in reports such items

as the value of the displacement factor em- ployed or the method used to determine

shutter time.

The Center conducts annual workshops

for participants to review the operation

of the services provided by the Center and

while these tend to emphasize the treatment

planning services offered by the Center,

there is discussion of other dosimetry

procedures.

Future Trends

Electron Beam Therapy

There is currently only one center

employing this modality but this is ex- pected to expand in the coming years. The

Center is developing the tools and techniques necessary to support this modality

in the same rigorous fashion currently em- ployed for megavoltage photon beam.

Patient Information Systems

While this topic is only obliquely

related to the general subject of this

paper, the Center is developing a system

to automate the therapy record and thus

obviate arithmetic errors that do occur in

manual updates of treatment records. A development from this project will be a
unified data base on radiation therapy in

the Northwest.

Conci usion

The remote nature of the Center and

its scale of operation have required us to formalize, in many aspects, the physics

support of radiotherapy. It is a matter of

semantics to decide whether a physicist is remote if he or she is in the next office,

building, town or state from the therapy

facility so that some of the techniques we

have described should be universally applicable.

Acknowl edgment

We wish to express our appreciation to the Board of Directors of the Northwest

Medical Physics Center for their constant

and enthusiastic support of the activities

of the Center.

References

1. Jones, D.: Five years activity of a re- gional physics program. Presented at the American College of Radiology Workshop on Current Trends in Radiotherapy,

San Diego, December, 1974.

8. Jones, D. and Schumacher, D.: An instrument for the rapid check of output, energy and field symmetry of

supervoltage therapy equipment. Am . J. Roentgenol . 123:198, 1975.

9. Monitrex-100. Lawrence Soft Ray Corp.

Mountain View, California.

10. Jones, D., Rubach, A. and Schumacher,

D.: Variations in linear accelerator

radiotherapy machine output. Medical

Physics 2:160, 1975. (AbstractT"^

11. Jones, D.: A system for the frequent

check by mail of the operation of

radiotherapy machines. AAPM Quarterly

Bulletin 7:100, 1973. (Abstract)

Appendix

The following forms are employed at the Center for the analysis of beam monitor

readings and for checks made by the local

staff.

2. Task Group 10, P. Wootton, Chairman,

AAPM: Code of practice for x-ray

therapy linear accelerators. Medical

Physics 2:110, 1975.

3. Medical x-ray and gamma ray protection

for energies up to 10 MeV: Equipment

design and use. NCRP Report No. 33,

1968.

4. Bruels, M.C. and Hanson, W.F.: Off axis

large field corrections for linacs.

Medical Physics 2:150, 1975. (Abstract)

5. DeClemente, A., et al: The new Memorial

Hospital external beam treatment planning program. Memorial Hosp., New York,

New York, 1971.

6. Jones, D. and Washington, J.: The quantitative description of a radiation

therapy plan. Radiology 115 :451, 1975.

7. Jones, D., Gifford, J. and Washington,

J.: On the use of fields of negative

weight to simulate blocks in radiotherapy calculations. Int'1. J. Rad . One. , Biol . & Phys . In press.

356
CHECK LIST 4 MV ISOCENTRIC UNIT

Machine I .D.

Month and Year:

DAILY CHECK LIST

Optical Distance Indicator - Check accuracy at usual treatment distance and initial

if within + 3 mm.

iDay !i

i

! Int 1

3 L 5 5 7 8 9 1 0 1 1 1 2 1. 1 5

16 '17

1

!

1

1 8 1 9 2 0 2 1 22 2 3 2 h 2 5 2 6 2 7 2 8 2 9 3 0 3 1

WEEKLY CHECK LIST

Sidel ight Accuracy - Set up 4.5 mm diameter pinhead at isocenter. If sidelights

impinge on sphere, initial and date.

Initial Initial Initial Initial

Date Date Date Date

MONTHLY CHECK LIST

Door Interlock - Check function and initial if o.k. Initial

Date

Emergency Off Switch - Check function and initial if o.k. Initial

Date

Source On Indicator Lights - Check that all lights function Initial_

and initial if o.k. Date

Frequent Check Dosimeter - Expose NMPC frequent check dosimeter as per instructions

on the first Wednesday of each month.

PLEASE MAKE COPIES OF THIS FORM FOR YOUR USE. RETURN ONE COMPLETED COPY TO THE NORTHWEST MEDICAL PHYSICS CENTER EACH MONTH AND RETAIN ORIGINAL IN BLUE BINDER.

357
ANALYSIS OF BEAM MONITOR READINGS

Month of

First Readings

A.M. P.M.

Upper Lower Upper Lower

Number of Readings

; s 1 t

Average Reading

Second Readings

A.M. P.M.

Upper Lower Upper Lower

Number of Readings

; Average Reading %03>

DISTRIBUTION OF SECOND READINGS

Upper

Number of Readings

Inside Bracket Lower

Number of Readings

Inside Bracket

Exact

A.M. P.M. A.M. P.M.

+ 1% Bracket ^// ToAO> 0 /rVTo /5i?

+ 2% Bracket H /5-5"to /VI 5"

+ 3% Bracket SL/S'To^O^ 1 0 /57to ;V7 fi-

+ 4% Bracket ^/7-To '^of r 3 I5T To /V4» t's

+ 5% Bracket n /^To /vy n

ISOTOPIC SOURCE CHECK ON BEAM MONITOR

Upper Lower

Average reading of the diode per rad *?5^ ^^Q3

PLEASE MAKE COPIES OF THIS FORM FOR YOUR USE AND RETURN ONE

COMPLETED COPY TO THE NORTHWEST MEDICAL PHYSICS CENTER EACH MONTH

358
MBS SP456 (1976)

INTERCOMPARISON OF PRESCRIPTION DOSE DELIVERY IN THE NEW

ENGLAND REGION

J.S. Blinick* and J. Spira,

Boston University Medical Center

Boston, Massachusetts 02118

and

D. Call and P. Plazeski,

Boston City Hospital

Boston, Massachusetts 02118

An inter comparison of radiation dosimetry among 16 institutions within the New England area was performed using a hand

carried water phantom-ionization chamber system. Participants were asked to deliver 200 rads using their own beam

data and computations to the chamber for each of eight

field size and depth combinations. Average measured doses

ranged from 200 to 204 rads . For all combinations , the

standard deviation of the measured values was 37o. (Dosimetry; radiation; measurements; intercomparisons

supervoltage ; therapy)

Introduction

Intercomparisons of radiation

dosimetry are an important means of

assuring accurate dose delivery to

radiation therapy patients.

An informal intercomparison

among several hospitals in Boston

was conducted in 1973 using a handcarried water phantom-ionization

chamber system."*" The results of this

pilot study indicated close agreement between the hospitals. However,

some discrepancies were found that

exceeded 57o, and these were subsequently investigated and corrected

by the institutions themselves,

On the basis of these results

it was felt that an expanded intercomparison would be of use to radiation physicists in the area. The

New England Radiological Physics

Organization (NERPO) agreed to

sponsor the survey, and a total of 21

supervoltage units (14 Co units

4 linear accelerators, and 3 betatrons) were measured at 16 different

institutions in Maine, Massachusetts,

New Hampshire, and Vermont.

Procedure

Equipment

All measurements were made with

a water phantom and ionization

chamber system. The phantom measured

30 X 30 X 30 cm. and, except for the

front face, was made from lucite.

The front face was made from polystyrene and was inscribed with lines

outlining different field sizes. An

adjustable lucite sleeve was used to

hold a 0.6 cc Farmer ionization

chamber. The chamber and its electrometer were calibrated at Co

before, during, and after the study

with a Shonka-type transfer standard

2 chamber which in turn was calibrated

by the National Bureau of Standards.

Water for the phantom was obtained

at each institution, and was adjusted

whenever possible to the same temperature as the ambient air, The

ambient pressure was measured with a portable aneroid barometer which was frequently compared to a standard

mercury-in-glass barometer.

Measurements

The equipment was hand carried

to each institution by one member

of a three man measurement team.

The phantom was filled with water

and placed on the treatment table.

It was then centered and adjusted to

the usual source to surface distance

(SSD) used by the institution by means of the devices normally

employed by that institution. Once

the setup was completed, and the

359
water and chamber temperature stabilized, the participants were asked

to deliver 200 rads to the chamber

for each of eight different field

size and depth combinations. A

minimum of two readings were taken

for each combination. If these

readings did not agree to within the

reading error of the instrument , a sufficient number of additional

readings were taken to obtain a meaningful average. The water temperature and ambient pressure were measured before and after the series

of ionization measurements.

Reports

Reports indicating the measured

dose values for each field size and

depth combination and the percentage

difference of these values from the

nominal prescribed dose of 200 rads were sent to the physicist affiliated

with each institution shortly after

the measurements at that institution

were completed. In addition, at the

conclusion of the survey, a summary

of the results was also sent to each

physicist to allow him to compare his

results with the average values.

A novel feature of this study

was the presence at each trial of an impartial observer chosen from among

the members of the NERPO radiation

therapy physics committee. These

"inspectors" provided an extra check

of the measurement setup, and helped

prevent any bias on the part of the

members of the measurement team from

affecting the results. No committee

member served as an inspector at his own institution.

Calculations

The ionization chamber readings

were converted to absorbed dose by means of the standard equation'^

D = RNC^ C, tp A (1)

where D is the absorbed dose in rads

R is the chamber reading, N is the

chamber calibration factor for 60Co

C^p is the temperature-pressure cor- rection factor, and C,^ is the overall correction factor.

Values for C^ were obtained

from a tabulation in Report 14 of

the ICRU.^ A value of 0.87,

obtained by extrapolation of these

tables, was used for the 42 MeV and

45 MeV betatrons.

Separate correction factors were calculated for the temperatures

and pressures measured before and

after the ionization measurements.

The average of these factors was then used in Eq . (1).

Results

Frequency distributions sum- marizing the results of the survey

are shown in Fig. 1. It can be seen

that for most of the field size and

depth combinations, the majority of

the measured dose values fall within

the range of 196-204 rads (±2% of

the nominal prescribed dose) . How- ever , some values were obtained well

outside this range. The lowest

reading was 184 rads , and the highest 216 rads, both of which represent an 87o deviation from the prescribed dose . Table I shows the

average measured dose value and

standard deviation for each field

size and depth combination. The

average measured dose values should

not be given too much significance.

It may be noted that a reduction of

17o for the chamber calibration

factor or C would bring the average

A measured values even closer to the

nominal prescribed dose.

A more relevant parameter for

the purpose of intercomparison is

the standard deviation. For all

field size and depth combinations,

the value for the standard deviation

was about 2>%. These results are comparable to those obtained by the

Radiological Physics Center in their

study. The data indicate that the

uniformity of basic dosimetry within

the region is quite good considering

the diversity of machines and institutions surveyed.

360
FIGURE I. FREQUENCY OF MEASURED DOSE VALUES

5 X 5 @ 5 CM.

8 -

4 -

8

0

5 X 5 lOCM.

178 187 196 205 214

186 195 204213 222

MEASURED DOSE VALUES

(RADS)

6 - 10 X 10 <a 5CM.

178 187 196 205 214

186 f95 204 213 222

MEASURED DOSE VALUES

(RADS)

12 - 15 X 15 @ lOCM.

4

•-

. 178 187 196 205 214

186 195 204 213 222

MEASURED DOSE VALUES

(RADS)

12 - 20 X 20 (a lOCM.

1?8 187 196 205 2)4

• l86 (95 204213 222

MEASURED DOSE VALUES

( RADS)

0

178 187 196 205 214

186 195 204 213 222

MEASURED DOSE VALUES

(RADS)

10 X 10 @ (OCM.

>

o 12

z:UJZD 8 OUJ

cc 4

0

178 187 196 205 214

186 195 204 213 222

MEASURED DOSE VALUES

( RADS) _ 15 X (5 © 15CM,

I -

178 187 1^6 205 214

186 195 204 213 222

MEASURED DOSE VALUES

( RADS)

20 X 20 @ 15CM.

361

214

186 (95 204 213 222

MEASURED DOSE VALUES

(RADS)
Table II shows a comp

the results for ^'-'Co units

institutions (those having more full-time physicists)

institutions (those having

time or consultant physics

Only the data for a 10 x 10

at 10 cm. depth is presente

results were essentially th

all combinations. It can b

this study that 1, the aver measured dose is closer to

nominal value, and 2, the s

arisen of

at large

one or and small

only partservices) . cm, field

d, but the

e same for

e seen in

age

the

tandard

deviation is less for the large institutions .

It should be emphasized that

irradiating a water phantom is not

the same as irradiating a patient.

Factors such as technologist error,

setup reproducibility, and proper

chart maintenance have not been con- sidered in this study. Nevertheless,

basic dosimetry, as evaluated in this

study, is fundamental to the accurate

treatment of radiation therapy

patients

.

Table I Measured Doses - Average and Standard Deviation

ield Size Depth Average Value Std. Dev: cm X cm) (cm) (rads) (7o)

5 X 5 5 202 2.8

5 X 5 10 203 2.8

10 X 10 5 202 2.5

10 X 10 10 204 2.9

15 X 15 10 202 3.0

15 X 15 15 202 3.2

20 X 20 10 202 3 . 0

20 X 20 15 200 3.5

Table II

Measured Doses - Average and Standard Deviation

60co 10 X 10 cm. Field at 10 cm. Depth

Large Institutions (8)

Small Institutions (6)

Summary

Measurements using a handcarried water phantom-ionization

chamber system were made on 21 supervoltage units at 16 institutions.

Eight different field size and depth

combinations were examined. For all

combinations, the average measured

dose was within TL of the nominal

prescribed dose. Standard deviations

of the measured doses were about 37o,

indicating good uniformity of dose

delivery at the institutions studied.

Average Value

(rads)

204

210

Std. Deviation

(%)

1.6

2.5

Acknowledgments

The authors would like to thank

the members of the NERPO therapy

physics committee who voluntarily

provided their time and expertise to

act as inspectors. The cooperation

and hospitality of the participating

institutions are also greatly

appreciated. We would like to thank

Dr. Edward S. Sternick for his

assistance in the statistical

evaluation of the data.

362
References

"Present address: Maine Medical

Center, Portland, Maine 04102

4. H. E. Johns and J.R. Cunningham,

The Physics of Radiology , (3rd

ed . Charles C. Thomas, Springfield

Illinois, 1969) , p. 288.

1. J. Spira and D. Cail, unpublished

2. Obtained from Physical Sciences

Laboratory, Illinois Benedictine

College, Lisle, Illinois 60532

3 . Members of the committee at the

time of this survey were

5. "Radiation Dosimetry: X-Rays and

Gamma Rays with Maximum Photon

Energies Between 0.6 and 50 MeV"

ICRU Report 14 (1969)

6. W. Grant, III, Medical Physics 1,

41 (1974)

J. Spira, Chm. BostonMA

F. Bagne

Hanover , N.H.

A. Boyer

BostonMA

A. Pinkerton

Manchester, N.H

D. Reinhard

BostonMA

G. Svensson

Boston

MA

K. Wright

BostonMA

363

UBS SP456 (1976)

ELECTRON-THERAPY DOSIMETRY

M, Ehrlich and P. J. Lamperti

National Bureau of Standards

Washington, D.C. 20234

A service for checking the dosimetry of electron beams in the energy

range from 5 to 50 MeV was established in 1957, mainly for use by

radiation-therapy departments. Since then, 34 radiation-therapy

departments and several other groups have participated, a relatively

large number of them only intermittently or for a total of only a few

tests. There has been little improvement of overall performance over

the years, less than one-half of the total number of irradiated dosimeters involved in any one test showing agreement to within 5 percent

between the dose reported by the participants and the NBS dose interpretation, and up to one-quarter of the dosimeters showing disagreements of more than 10 percent. Only in a few exceptional cases was there an improvement in individual performance with continuing participation. It is concluded that means must be sought, in collaboration

with the therapy community, to secure a more favorable impact of this

service on electron-therapy dosimetry.

(Electrons; dosimetry; service; radiation therapy; performance; impact;

improvement)

Description of Service

Since 1967, the National Bureau of Standards (NBS) has been providing a service for

testing the status of dosimetry for electron

beams in the energy range from about 5 to

50 MeV. The service, which is directed

mainly toward radiation-therapy applications

in the United States, was established upon

the request of the American Association of

Physicists in Medicine (AAPM), with the initial financial assistance of the Bureau of

Radiological Health (BRH). Ferrous-sulfate

(Fricke) dosimeters, prepared and checked at

NBS by means of cobalt-60 pre-irradiation,

are shipped to the participating institutions

with the request to irradiate them in a polystyrene phantom, using the dosimeter depth

and the phantom and field sizes specified by

the AAPM.-^

Figure 1. Dosimetry Assembly

Figure 1 shows the dosimeter assembly,

consisting of Fricke solution in a stoppered

spectrophotometer cell, cradled in a polystyrene block. Each participant receives

two control dosimeters and at least two dosimeters to each of which he is asked to deliver between 50 and 80 Gy (5000 and 8000

rad). After irradiation, the dosimeters are returned to NBS for evaluation of their re- sponse in terms of absorbed dose in water.

Finally, the participants are informed of

their own test results and those of all other

participants, whose identity, however, is not

divulged. They then are free to discuss

their results and possible problems with NBS. Technical details on dosimeter preparation

and on the pre-irradiation checks have been

2 publ ished.

Inherent Uncertainties

Irradiation of each dosimeter batch

prior to shipment permits (1) a statistical

study of the random variations from dosimeter to dosimeter in the response to a given

irradiation level, as measured spectrophotometrically by the change in optical density;

and (2) the detection and elimination from

further use of dosimeters with responses far

removed from the mean for the particular

batch. Furthermore, it permits the evaluation of the absorbed doses administered by

the participants from higher (more precise)

optical densities. Both before and after

shipment, the relative standard deviation

from the mean response, which is a measure

365
of the random uncertainty of the method, is

about 1.2 percent for the currently used

dosimeters

The overall uncertainty of the method

was determined by irradiating batches of

dosimeters at two dose levels, shipping them

cross-country and back as if they had been

irradiated by participants, and evaluating

their response in terms of absorbed dose in

the usual manner. The difference between

actual dose delivered to the dosimeters and

dose evaluation from dosimeter response,

which is taken to be a measure of the total

uncertainty of the method, was found to be

about 4 percent at the 99-percent confidence

level. Not considered are uncertainties

(1) due to the assumption of a constant efficiency (a G value of 15.5/100 eV, as recom- mended by the AAPM'^) of the Fricke-dosimetry

system over the electron-energy range from

5 to 50 MeV, and (2) due to inhomogenei ties

in absorbed dose over the 1-cm depth of the

dosimeter's sensitive volume. Thus, the

service does not provide a beam calibration,

but simply a check on the uniformity of

electron dosimetry throughout the United

States at a given electron energy. However,

since the participants are informed of the

details of the NBS evaluation procedure,

they are in a position to derive information

about their beam calibration from the

supplied test results.

The Participating Therapy Departments

During its first year of operation, the

service was subsidized by BRH and thus could

be offered quarterly and at a fraction of

actual cost. After this period, the frequency was reduced to twice yearly, and the

charges were increased to cover the actual cost of the service. During the past nine

years, a total of about 40 groups have participated. Of these, 34 are therapy users of electron beams, two use their beams for

biological research and development, one is

a food processor. On a few occasions, some non-medical groups desiring cobalt-60 gamma

irradiations have been accepted. Some of

the groups originally participating have

dropped out while others joined later. How- ever, ten of the original groups are still

participating.

Table 1 gives a survey of the machines

used by the current participants who employ

their beams for therapy. There are 18 users of betatrons in the energy range from

5 to 45 MeV and 9 users of linear accelerators, in the energy range from 6 to 50 MeV.

Table 1. MACHINES USED BY CURRENT PARTICIPANTS

Type and

Maximum

Energy (MeV)

Manufacturer Number Number

< 10 MeV

Used

loin Energ

-30 MeV

y Range

>30 MeV

BETATRON, 11 Applied Rad. (Siemens) 1 1 1

18 Si emens 2 2 2 22 ATC (Allis-Chalmers) 1 1 1 25 ATC (Allis-Chalmers) 11 8 11 42 Siemens 1 1 1 45 Brown-Boveri 2 2 2 2

LINAC, 6 Varian 2 2

7 Varian 2 2

10 Applied Rad. (Siemens) 1 1 1

18 Varian 2 2 2 33 Non-commercial 1 1 1 1

. 50 HV Engineering-Varian 1 1

366
Performance

Figure 2 gives a review of the performance of all therapy participants in each of

the nine years of operation of the service,

indicating the deviations between NBS dose

interpretation and the participants' reported dose, as well as the energies of the

electron beams employed. It is evident that

some of the current users of 6-MeV beams do

not take into account that the average

absorbed dose in the sensitive volume of

the dosimeters is of the order of 10 percent

smaller than the dose at the peak of the

6-MeV depth-dose curve. During the past

year a statement of caution regarding this

effect was mailed to the participants invol ved.

The results shown in Fig. 2 are reviewed

numerically in Table 2, in which the number

of participants also is broken down according to whether or not they had participated

before. On the whole, the performance is seen to have improved only very little over

the years, up to one-half of the total number of dosimeters having interpretations

that differ from the reported dose by more

than 5 percent, with differences of more

than 10 percent occurring in up to one- fourth of all dosimeters.

Examples of Individual Performance

Further insight is gained through a detailed study of the performance of individual groups. In Table 3, the performance

of Group I is seen to have remained essen- tially unchanged throughout the years,

agreement to within 5 percent occurring

about 70 percent of the time. Group II is better, with about 80 percent of the dose

interpretations agreeing to within 5 percent.

Group III is seen to have improved, showing

no disagreement larger than 5 percent over

the last five years. The performance of

Group IV remains poor, less than one-half

of its reported doses being within 5 percent

of the NBS dose interpretation, and as many

differing by more than 10 percent. Group V shows some improvement.

Groups VI and VII are included here

because there is evidence that participation

Table 2. PERFORMANCE OF ALL PARTICIPANTS

Calendar Number of Number of Dosimeters with Difference A* of

Year

Parti :ipants < 5% 57o < |a < 10% 1 A 1 > 10%

Old New absol ute relative absol ute rel ati ve

{%)

absol ute relative

(%)

1967 0 14 33 66 14 28 3

1968 10 2 39 48 24 30 18 22

1969 10 5 25 45 16 29 14 25

1970 12 3 34 56 12 20 15 25

1971 17 0 30 52 13 22 15 26

1972 13 2 35 74 5 11 7 15

1973 17 2 38 63 9 15 13 22

1974 12 2 38 78 6 12 5 ,0

1975 17 3 38 51 24 32 12 16

*The symbol A stands for the difference between the absorbed dose reported and the NBS

absorbed-dose assignment.

367
Figure 2. PERFORMANCE OF ALL PARTICIPANTS IN EACH OF 9 YEARS OF SERVICE

25r—

50

! 24 20

15

10

1967

35

3330

25

28 24

20 18

18

15

15 !± IT

10 6

F4l

1.968 20

Ii4l |24l

1969

M.

Z2

33120 go|

_2I 201 18

18

20 —

o

o

UJ

CD3

15

10

19 70 42

33

3o|l5 25

20

25 20 18

15. il 10 L.. 7

1971

lis!

33

2520

18

42

15 20

36 14

30

II I0_ 9

18

18 7

15 Ts

6

1972

3933

30

33 25

3C25 24

20 20

18

18

ie"

15

15

T2

10 13 8 10

20

15

10

_ 1973

I

30

25

36 20 30

25

18

2422

15 20

18 13

15

12.

10

10 6 T

30 25

15

i\o o\o o\o V / o\o o\o o\o o\r ^ ^

\

/

.0 vO V° <^ * c

" V" V' <-> V

1974 ^33

30 —

25

35

20

X.-V

o\o nP4

1975

fel fsl [el [is T

30

25

) n? tP '^P vP <3

<D VO .O

<0 sP

368
Table 3. FOLLOW-UP ON PERFORMANCE OF INDIVIDUAL GROUPS

aTOUp

Number of Dosimeters in a Calendar Year Percentage of

Total Number

No. Per Group

A*

1 n "7 lyb/ 1 nc o lyby 1 n "7 n ly /u ly /1 n"70 ly /^i1 n "7 o ly / J ly /4 1975

1 A 1 < 5% 2 4 1 3 2 3 3 4 3 66

I 5%<|a! < 10% 0 1 0 0 1 0 1 0 1 11 |a|> 10% 0 1 4 0 2 1 0 0 1 24

T T

1 1

1 A 1 < 5% 2 5 2 4 4 4 1 , 4 4 81

5% < 1 A 1 < 10% 0 1 2 0 0 0 3 0 0 _16

|a|> 10% 0 0 0 1 0 0 0 0 0 3

1 A 1 < 5% 3 2 2 2 2 3 4 2 78

III 5% < 1 A

1 < 10% 1 3 2 0 0 0 0 0 0 22

|a|> 10% 0 0 0 0 0 0 0 0 0

|a| < 5% 1 1 5 0 2 2 2 0 42

IV 5%< |a| ^ 10% 0 2 1 0 0 0 1 1 16

|a|> 10% 3 0 2 0 2 1 3 42

1 A 1 5 5% 1 0 0 1 0 2 4 2 3 38

V 5%< |a1 < 10% 1 2 1 2 3 2 0 2 0 38

|a|> 10% 0 2 3 1 1 0 0 0 1 24

1 A 1 < 5% 2 0 0 1 2 2 1 1 2 50

VI 5%< |a| < 10% 2 3 1 1 0 0 1 1 0 41

|a|> 10% 0 1 1 0 0 0 0 0 0 9

1 A 1 ^ 5% 2 3 4 1 63

VII 5% < 1 A

1 < 10%

|a|> 10%

1

1

10

0

0

2

1

25

13

1 A 1 < 5% 4 0 0 29 VIII 5%< |a| s 10%

|a|> 10%

2

3

0

3

1

1

2150

*The symbol A stands for the difference between the absorbed dose reported and the NBS

absorbed-dose assignment. _ .._

assisted them in finding and removing certain difficulties in their operation of

which they had not been aware prior to their

participation in the service. For instance,

Group VI discovered a faulty integrator,

while the monitor chamber of Group VII was

found to heat up and thus show an increase

in sensitivity during irradiation. Unfortunately, Group VII seems to be unable to afford regular participation, and after a

3-year interruption now performs as poorly

as it did in 1967. Group VIII is an ex- ample of a participant who felt that his

system was not made to perform over the long

exposure times required for the test irradiations. Therefore, after three years of

poor performance, he dropped out, declaring

that he would consider resuming participation when a more sensitive dosimetry system

becomes available.

369
Conci us ions

After nine years of operation of the NBS

electron-dosimetry uniformity service, it seems that the impact of the service on the

therapy community does not match the effort

expended. Participation is relatively poor

and irregular, and improvement in performance is an exception instead of the rule.

Therefore it was decided to initiate an active dialogue with the therapy community,

in an effort to explore (1) means for a more

personal interchange with the participants

and for increasing participation; and (2)

the priority to be given to a change-over

to a more sensitive dosimetry system, and

inclusion of high-energy bremsstrahl ung beams. It is hoped that this presentation

will assist in stimulating such a dialogue.

References

1- The Subcommittee on Radiation Dosimetry

(SCRAD) of the AAPM, Phys. Med. Biol. 11,

505 (1966).

2- M. Ehrlich, P. J. Lamperti, Phys. Med.

Biol. 14, 305 (1969).

370
NBS SP456 (1976)

THYROID MONITORING AND MINIMIZING 1-125 UPTAKE

M. M. Gabel, K. W. Price and G. R. Holeman

Health Physics Division

Yale University

1136 Kline Biology Tower

219 Prospect Street

New Haven, Connecticut 06520

A method is described for the quantitative determination of 1-125

uptake by the thyroid gland of all users of 1-125. Causes for up- take are discussed and the need for monitoring is illustrated.

(Thvroid monitoring; 1-125 uptake; bioassay)

Introduction

The rapidly increasing use of 1-125

labeled compounds in biomedical research has

presented several radiation protection problems. In order to keep exposures to 1-125

to "as low as practicable," Yale has included in its Cancer Center a radioisotope

facility, intended as a safe, supervised,

multi-use research laboratory. One of the

first uses of the facility was for iodination experiments. The control program for

1-125 usage has evolved with time and has

become rather complex. The following information summarizes some of our experiences

with 1-125, its control and methods of dose

assessment

Materials and Methods

1-125 in the Thyroid

To measure the amount of radioactive

material which has accumulated in the thyroid, a detector can be placed over the neck

in the region of the thyroid. Equipment

which has been used for thyroid monitoring

is as follows: 31/32" x 1/32" Nal crystal,

high voltage power supply, preamplifier,

amplifier, 100 channel analyzer and printer.

The count rate obtained from the detector is related to the amount of activity in

the thyroid by calibration. The method of

calibration includes the use of an anthropomorphic phantom filled with water, representing the thyroid of an average man. The

thyroid cavity of the phantom will hold a calibrated 1-125 source.

The standardized 1-125 source in the

TcUige of five microcuries is counted for

four minutes. A four-minute background count

is also made by removing the source from the

phantom. In order to maximize counting sta- tistics, the optimum channel width must be

obtained. This may be found by adding the

counts in successively increasing channels on either side of the peak channel (see

Table I). The ratio of the counts in ± X channels inclusive of the peak channel, and

the corresponding background counts may be

plotted against the number of channels on either side of the peak (see Figure I). The

number of channels on either side of the peak

channel, yielding the highest value, determines the optimum channel width.

Once the channel width has been established, system efficiency may be measured.

This is the ratio of the gamma rays which

interact in the crystal to the total gamma

rays emitted by the source or

2 _ cpm in optimum channel width

actual activity in dpm

A minimum detectable level of activity

present in a four-minute count at a 95% con- fidence level may now be found using the following equation:

where : t = sample count time ,

tg = cpm, Bkg

o = standard deviation Bkg

D Y = X gross counts needed

o K = number of standard deviations

for X% accuracy; K=2 for 95%

Y = number of counts to be accumu- lated in time t . G The Minimum Detectable Activity may

then be defined as
Table I

2 Optimizing Channel V/idth, S /B

CHANNELS sxioS B S2/BX109

±2 1.44 9 2.3

^ 4 2.44 10 5.95

t 6 3.24 12 8.74

18 3.83 16 9.14

tio 4.22 20 8.9

+12 4.48 24 8.36

the equipment described above. Iodine 125

concentration in the thyroid was measured by

placing the probe in the region of the thyroid, between the laryngeal prominence and

the manubrium. A standard value for background was obtained by counting the thyroids

of 15 individuals not exposed to 1-125. The

results of these measurements yielded a con- sistent value for these control individuals.

The background counts per minute are then

subtracted from the gross counts per minute

of the individual being counted.

Questions may arise as to the reproducibility of positioning the probe in suc- cessive counts. This was measured experimentally by altering the normal counting

geometry. The source was placed at various

distances about the detector probe. The

count at each distance was taken as a percentage of the total counts at the normal

counting geometry. The results are shown

graphically in Figure II. It can be seen

that the best results are obtained when the

probe is flush against the object being

counted. A concentric deviation of more

than 1/2" from the center of the probe, in

the flush position, will result in a greater

MDA =

where: MDA = minimum detectable activity

in ^-minute count at a 95%

confidence level

Y = number of counts to be accu- mulated in time t

tg = sample count time

tg = Bkg cpm

E = system efficiency, 1.2%

C = conversion constant,

4.5 X 10"^

^

The system is checked weekly with the calibrated 1-125 source for accuracy and stability. To date, the system has remained

consistent.

Sixty-eight individuals working with

1-125 were given thyroid counts utilizing
Figure I 1-125 PEAK AT^ 636 KEV/CHANNEL

FOR 4 MINUTE COUNT

2 4 6 8 10 12 INCLUSIVE ± CHANNELS FROM PEAK CENTER

than 15% error.

1-125 in the Urine

In addition to the analysis of individual thyroids using the thyroid monitor, a urine sampling program was initiated for the

determination of 1-125 whole body content.

The objectives of the urine sampling program

were (a) to try to correlate urine sample

activities with actual measurements of

thyroid 1-125 content, and (b) to try to determine if a urine sampling procedure is

sufficient for routine screening for 1-125

uptakes. Therefore, in instances where a thyroid count indicated activity in the

organ, a urine sample was taken immediately

afterwards. Urine samples were analyzed

utilizing a liquid scintillation spectrometer

and urine sample activity in MCi/liter was determined.

For an in- house program for urine analysis , one wishes to reduce the time involved

per sample and at the same time have an ac- ceptable sensitivity for detecting the isotope in question. Instagel was chosen as a counting medium for all urine samples. The

amount of urine added to the Instagel was determined by pipetting several different

volumes of samples into 10 ml of gel. This

procedure was repeated for 20 different

urine samples. The resultant cocktail mixture was inspected visually for gelling

quality. If too little urine was present in

the sample, a gelling did not occur. When

an excess of urine was added to the gel , a precipitate formed. It was determined that

the best mixture was 3 ml of urine and 10 ml

of gel. This proportion always provided an acceptable sample that gelled and was not severely quenched. Although this mixture

does not always yield an optimum quenching

condition, the amount of time saved is

Figure II — Normal Position

1/2" Concentric

I" Concentric

1/4" 1/2" 3/4" DISTANCE FROM PROBE

1"

significant and the loss of counting efficiency is not unacceptable.

Urine samples were counted in a Tri-Carb

Liquid Scintillation Spectrometer. Counting

efficiencies were determined by adding a known amount of calibrated 1-125 standard to various urine-gel samples. An efficiency

curve was eventually determined and is

plotted in Figure III. In Figure III the

counting efficiency is plotted versus the

automatic external standard ratio (AES

ratio). It may be seen that the counting

efficiencies range from 28% to 39.4% depending on the degree of quench of the sample.

In order that a quench curve, such as shovm

here, may be used, a system reliability

check must be made with each batch of sam- ples analyzed. As values of 1-125 counting

efficiency were determined, a set of six

sealed quenched standards and a blank system

standard were counted also. As a result,

AES ratios of the sealed standards and blank

standard are related to system performance.

With each batch of samples analyzed, the

quench curve shown in Figure III is applied

if the sealed standards efficiency curves

do not deviate from the expected values.

When one begins to analyze urine sam- ples of occupationally exposed individuals

at very low counting rates, the question of

what sample background to use becomes a significant factor in determining the net

activity. We have chosen to use as a sample

background counting rate that activity which

is an average of non-occupationally exposed

individuals. A total of 15 urine samples

373
Figure III COUNTING EFFICIENCY VS. AES RATIO FOR 1-125

Ui URINE, 3 ML URINE, lOML GEL

.40 r

.32

30

.26

.24 AES RATIO

were collected from the Yale Health Services

Center and analyzed. It was determined that

these individuals were not involved in

radioactive isotope work at the University.

Each sample was counted for a 50-minute

period, and all counts were averaged and a standard deviation computed. It was found

that the background counting rate was

11.52 ± 1.07 cpm in the 1-125 channel of the

liquid scintillation spectrometer. This

background was applied to all urine samples

if no background was available for the individual in question. In determining the net sample activity, a gross count rate of at least three sigma (11.62 + 3x 1.07) or 14.83

cpm was required. Any sample analyzed that

yielded a count rate less than 14. 83 cpm was considered to be not different from the background level. This minimum detectable level

corresponds to a net sample activity of

(1.72 ± 0.37) X 10"^ MCi/1 and (1.22 ± 0.46)

_ 3 X 10 /xCi/l of urine for the worst and best

quenched samples, respectively. These mini- mum detectable levels are based on a 50-

minute counting time per sample.

Once the urine activity at a specific

point in time is determined, an approximation to the original amount of 1-125 uptake

may be calculated. The fractional retention

equation for stable iodine in the human body

is given by (ICRP, 1967):

R(t) = 0.7 exp

+ 0.3 exp

(3)

where T. , and T^„ are the biological half

bl bz

lives of iodine in the whole body and thyroid, respectively. The excretion function

is of interest when analyzing urine sample

data, and may be obtained by taking the

differential of R(t).l In addition, dR(t)/dt

is multiplied by exp (-X^t) to account for

radioactive decay. The equation for the

fraction excreted per day via all routes is given by:

Y(t)

Y(t)

-exp (-X t) r

or

dR(t)

dt

(4)

(

-exp (-X t) r -.7(.693)

Tbl

(.3)(.693)

Tb2

exp (5)

where is the radioactive decay constant

for 1-125, The values of T, , and T, ^ are

bl d2

assumed to be 0.353 days and 96.37 days,

respectively. These values represent aver- ages of the data.-^ The initial amount of

1-125 uptake may now be computed by:

X(t) =

where X(t)

Y(t)

Y(t) U F o u (6)

urine concentration in //Ci/

liter at time t

fractional excretion equation,

fraction lost per day

F = fraction of activity lost that

is present in the urine (F^ = 1 in present analysis)

= initial mCI uptake of 1-125

G = quantity of urine excreted per

day in liters/day.

The approximate value for G may be calculated

from the expressions^

G = 0.02 W adult males,

G = 0.017W adult females.

where W is the mass of the individual in

kilograms. The ii finally given by:kilograms. The initial uptake is then

374
u = o

G X(t)

T T 485 r bl

).593t

T.bl

^ T ,208 r b2

b2

).693t

(7)

Now that U is determined, the amount of

o 1-125 in the thyroid at any time T after an acute uptake may be computed by the use of

the retention equation for the thyroid alone.

This is

1

U(T) = 0.3 U e o

,1T T

t b2

).593T

MCi (8)

Therefore, by determining the urine

sample activity at time t days after an acute uptake of 1-125, the amount in the thyroid at

T days after the uptake may be calculated.

This value may then be compared with the

measured quantity present at time T. To date

a few comparisons of thyroid burdens as determined from urine analysis data and thyroid count data were made. The results of

these comparisons will be given later in the

present paper.

Another objective of the urine analysis

was to determine how effective urine sampling would be in routine monitoring procedures

for 1-125 uptake. In calculating the sensitivity of urine analysis, the minimum detectable activity was assumed in the sample under

worst and best quenching conditions (1.72 x

10""^ MCi/1 and 1.22 x lO"^ /j-Ci/1, respectively). This activity was converted to 1-125

uptake Uq by the use of equation (5). It was assumed that the minimum detectable ac- tivities were measured at various times t after the uptake. In addition to the calculation of Uq from urine data, the initial

uptake was calculated from a thyroid count T days after the exposure. The thyroid ac- tivity U(T) may then be related to Uq by the

use of equation (6). The minimum detectable

activity in the thyroid was used at various

times T for the variable U(T). The minimum

detectable thyroid activity is approximately

0.0002 /xCi ± 0.00007 ^xCi at the two-sigma

level of confidence. The results of the calculations are plotted in Figure IV, which is a plot of the minimum uptake detectable of

1-125 based on urine analysis and thyroid

count data. It is very evident from the plot

that the thyroid counting technique is much

more sensitive than is urine sampling. However, the urine sample should not be ruled

out completely. One must be certain to

obtain a urine sample within the first few

days of a suspected uptake to determine if

an uptake was recent or occurred some time

Figure IV Plot of Amount of 1-125

Uptake vs. Time of Bioassay Where Minimum

Detectable Level was Measured

THYROID COUNT 12 3 4 9 6

t, DAYS AFTER INTAKE

in the past. Our data to date, as summa- rized in Table II, have allowed us to determine which laboratory procedures are con- tributing to thyroid burdens. By using the

thyroid count and the urine sample in con- junction, the time exposure may be determined with acceptable accuracy.

Table II Summary of the Factors Affecting Uptake"

N.D. N.D. - .005 ^iCi . 005 /xCi No. of Users 26 19 23

Form

Free 16% 22% 31%

. Bound 22% 6% 3%

Qty. . <500 MCi 27% 9% 6%

, >500 fxCi 11% 19% 28%

' <1 year 23% 19% 19%

Exp. 1-5 years 13% 6% 10%

>5 years 1% 3% 4%

Septum

'On [Off24%

15%

15%

13%

4%

29%

Hood

'Yes

1 No

10%

28%

22%

6% 25%9%

"Expressed as % of total

375
Summary of Results

1-125 in the Thyroid

these offsetting factors, it is likely that

uptake would increase with decreases in

prior experience.

Of the 68 users who were counted, 38%, or 25, had no detectable uptake, while 28%

and 34-% had uptakes which were greater than

non-detectable but less than 0.005 ;^Ci and

0.005 /j-Ci or greater, respectively. Sixtynine percent of the individuals were using

the free form of 1-125, and the remaining

31% a bound form of 1-125. In terms of

quantities used, 34% of those counted were utilizing 500 or less ^Ci in their laboratory procedures and the remaining 59% were

using more than 500 jaCi. The vast majority

of individuals, 61%, had one year or less

experience. Twenty-nine percent had from

one to five years experience and only eight

percent had more than five years experience.

Fifty-seven percent of those counted reported

that they removed the vial septum prior to working with the material and 43% remarked

that the vial septum had not been removed.

Finally, the hood was utilized by 57% of the

individuals counted.

Characteristics of Investigators with

Non-Detectable Uptake

As mentioned, 25 of the 68 individuals

counted had non-detectable 1-12 5 uptake. Of

this group, 58%, or 15, were using the bound

form of 1-125 while 11, or 42%, employed the

free form. This pattern departs from the

pattern established for the group as a whole

where 69% and 31% were using the free and

bound forms, respectively, and points out that the widespread use of the less volatile

bound form may in part have contributed to

this group's non-detectable uptake.

Also, those with non-detectable uptakes

utilized smaller quantities , generally asso- ciated with the bound form, than the total

group. Seventy percent of those with non- detectable uptakes were experimenting with

less than 500 jxCi and 30% with 500 or more

/xCi. This compares with 42% using less than

500 ixCi and 58% using 500 or more iJuCi for the

group as a whole. Obviously, quantity used

should be positively related to uptake.

Prior experience should correlate highly

with uptake, but in the present context, 62%

of those with no detectable uptake had one year or less experience. Thirty-five percent

had from one to five years prior experience

and the remaining 3% had more than five years

experience. Uptake may have been minimized

in this largely inexperienced group due to the widespread use of the bound form of 1-125

and the small quantity. In the absence of

Sixty-two percent of those counted with

non-detectable uptakes reported working with

the vial septum on, while 32% reported it

had been removed. This departs from, the

total sample where 57% of the 58 counted

reported removing the septum.

The overall pattern seems to indicate

that those with a non-detectable uptake used

the less volatile bound form, less than 500

MCi and did not remove the vial septum.

There is no doubt that these factors are crucial in minimizing uptake, but there may

also be a chronic uptake from repeated ex- perimentation with these small quantities

and it is unlikely that such a largely inexperienced group (94% of those working in

this group had less than six months experience) could have yet become subject to this

effect. Future counting of these individuals

may reveal detectable uptakes as this influence takes effect.

Characteristics of Individuals with a Greater than Non- Detect able but Less Than 0.005 /xCi Uptake

Nineteen of the 68 individuals counted

had uptakes which were greater than non- detectable but less than 0.005 (xCi. Of this

group, 79% were experimenting with the free

form of 1-125 and 21% with the bound form.

This differs from the group with a non- detectable uptake where 58% used the bound

and 42% the free form and suggests that up- take is correlated with the form utilized.

Sixty-nine percent of the individuals

with greater than non-detectable to 0.005

ju.Ci uptake utilized 500 /iCi or more and 31%

used less than 500 /u,Ci. Again, those with

non-detectable uptakes exhibited a different

pattern. In their case, 70% experimented

with 500 fxCi or less.

Those with a greater than non- detectable

to 0.005 ;u,Ci uptake had less prior experience than those with non-detectable uptakes.

In the former case, 58%, 21% and 11% had

one year or less, one to five years and more than five years prior experience, respectively. In the latter case, 62% had one year or less prior experience. Limited

prior experience coupled with the use of the more hazardous free form and 500 or more /x,Ci seems in the present context to have con- tributed to a higher uptake.

Use of the septum had inconclusive
results. Fifty- three percent of those counted reported not removing the septum and 47%

removed it. For the group as a whole, 57%

removed the septum while 43% did not remove

it, and for those with non-detectable uptakes

62% did not remove the septum and 38% did.

Seventy-nine percent of those with a greater than non-detectable to 0.005 fxCi up- take used the hood while 21% did not use the

hood. A far smaller group of those with non- detectable uptake used the hood where only

27% reported its use.

The combined evidence from this group

and those with non-detectable uptakes indicates that primary factors in determining

uptake are form, quantity used and prior ex- perience. To this point, however, the influence of the septum and the hood on uptake

is uncertain.

Characteristics of Individuals with a Greater than 0.005 ^Ci Uptake

Twenty-three of the 58 individuals

counted had a greater than 0.005 p.Ci uptake.

Far more individuals in this group used the

free form of 1-125 than did in any other

group. Here 91% used the free form where

only 79% did so in the greater than non- detectable to 0.005 ^^Ci uptake group. Nine

percent of the current group used the bound

form.

Five hundred or more p.Ci were used by

83% of the individuals with a greater than

0.005 txCi uptake and 17% used 500 or less

fiCi. Only the group with greater than non- detectable to 0.005 ^lCi uptake came close

where 69% reported using 500 or more jxCi.

Progressively, it can be seen that as uptake has increased, so had the percentage

of individuals using the free form and the

quantity utilized. Therefore, there appears

to be a strong correlation between uptake

and these variables.

Experience has not increased with the

level of uptake. On the other hand, in

this group uptake and experience appear to

be negatively correlated. For those with

greater than 0.005 ^Ci uptake, 57% reported

one year or less experience. Thirty percent

reported one to five years experience and

13% more than five years experience. Sixtytwo percent of those with no detectable up'- take reported one or less years experience

and 68% of those with a greater than nondetectable to 0.005 piCi uptake reported one year or less experience.

The septum was reported removed by 87%

of the individuals with greater than 0.005

fiCi uptake and not removed by 13%. Forty- seven percent of those with greater than

non-detectable to 0.005 ^Ci uptake reported

the septum off and 38% of those with non- detectable uptakes reported it off. The

increasing percentage of those removing the

septum seems to indicate that uptake is in

part influenced by the absence of the

septum.

Seventy-four percent of those with

greater than 0.005 /j.Ci uptake reported that

the hood was used, while 25% reported that

it was not used. The majority of the group

with a greater than non-detectable to 0.005

p-Ci uptake also reported that the hood was

used. In this group, 79% reported that the

hood was used. Widespread use of the hood

in combination with a detectable uptake

would indicate that inexperience coupled

with unfamiliarity with experimental procedures has resulted in the ineffective

utilization of the hood and the resultant

pattern.

Current Yale University guidelines suggest that only users of one millicurie or more of 1-125 and performing iodinations

need be counted for thyroid uptake. To have

adhered to this guideline would have meant

that the number of people counted would have

been reduced from 58 to 35 or by 32%. More

important, however, of the total 68 individuals counted, 40 or 59% had detectable up- takes. Of this group, 12 or 30% were ex- perimenting with less than one millicurie

and were not iodinating. Obviously, to have

adhered in this case to the guideline would

have meant that a sizeable group with detectable uptakes would have been overlooked.

1-125 in the Urine

Urine samples were obtained from twelve

individuals immediately after a thyroid

count indicated a thyroid uptake of more

than .001 /^Ci. This cutoff level was based

on the minimum detectable activity in the

urine of 1.72 x lO"'^ nCi/1 and 1.22 x lO"^

/LtCi/l for worst and best quenched samples,

respectively. Table III gives the results

of both the thyroid counts and the urine

analysis. Of the 12 urine samples analyzed,

four showed a detectable 1-125 concentration.

Utilizing the urine sample data along

with equation (7), the initial uptake, U^, was calculated. Once U^ is determined, the

retention function given by equation (3) is

377
Table III Table IV Urine Concentrations at Time t Vs. Thyroid Burdens at Time t

Amount in Urine

t T Thyroid ^tCi Activity /^Ci/1

1.125d Id 0. 141 3. 52x10

2 wks. 2 wks. 0. 0155 N.D.

U3d 4 3d 0. 0055 N.D.

.75d .75d 0. 0024 2. 76x10

3d 3d 0 005 N.D.

X DQ 0 007

15d 15d 0 003 N.D.

88d 88d 0 005 N.D.

1. 5d 1. 5d 0 0038 N.D.

8d 8d 0 0105 N.D.

1.125d 1.125d 0 007 2. 89x10

Id Id 0 0095 2. 23x10

-3

applied in order to determine the thyroid ac- tivity at the time of the thyroid count. A comparison of calculated and measured thyroid

burdens is given in Table IV. The thyroid

count data was assumed to yield the actual

amount present. The percent error given in

the last column of Table IV reflects the

ability of the urine sample data to approximate the measured thyroid burden. The

authors feel the results are better than what

one would expect, the worst error being 34,4%

too low. One must remember that in calculating a thyroid burden from urine data, errors

result from estimating total daily urinary'

excretion, assumptions in biological half

lives of stable iodine in the body and thyroid, and the actual time the uptake occurred.

It is extremely difficult to correlate urine

sample activity and thyroid burden. This is

easily seen by referring to Figure V. Plotted in Figure V is the excretion function

for 1-125 given in terms of the fraction of

the initial uptake excreted per day, including radiological decay. The amount excreted

from the thyroid is insignificant as compared

to the initial whole body excretion. The

body eliminates the isotope rather quickly,

and for typical thyroid burdens of 0.01 to

Calculated and Measured Thyroid Burdens

A B Measured Thyroid Activity

Thyroid Calculated

No. Activity

MCi

From Urine

Analysis , /xCi A-B —r— X 100%A

1 0.141 0.12 14. 9%

2 0. 007 0.0059 15.7%

3 0.0024 0.0031 29.2%

4- 0.0095 0.0053 34.4%

0.001 txCiy no detectable activity is present

in the urine. We have found that in order

to make comparisons between urine activity

and thyroid burden, a sample must be taken

within three days of the uptake. After

three days the urine activity is below the

minimum detectable level of our system.

Figure V EXCRETION OF 1-125 VS. TIME

TIME AFTER UPTAKE, DAYS

378
It should also be pointed out that a thyroid count always has a better sensitivity than does a urine sample. Plotted

in Figure IV is the minimum uptake Uq of

1-125 that may be determined from urine

sample data and thyroid count data. In con- structing the plot , it was assumed that the

minimum detectable level was measured in the

urine or thyroid t days after an uptake.

The initial uptake was then calculated and

is the minimum detectable. Urine sample

sensitivity increases with time while thyroid count sensitivity remains approximately

constant.

References

1. International Commission on Radiological

Protection (ICRP), Publication 10 , 1957

(Oxford: Pergamon Press).

2. International Commission on Radiological

Protection (ICRP), Publication 23 , 1975

(Oxford: Pergamon Press )

379

NBS 5P456 (1976)

ATTENUATION PHANTOMS FOR PATIENT EXPOSURE MEASUREMENT DURING

RADIOGRAPHIC EXAMINATIONS UTILIZING PHOTOTIMING TECHNIQUES

Pei-Jan Paul Lin

Northwestern University Medical School

Chicago, Illinois 60611

Aluminum phantoms have been obtained for patient simulation

in radiographic procedures utilizing phototiming techniques.

Thickness of these phantoms have been determined by (a)

monitoring radiographic parameters of phototimed examinations

and (b) attenuation measurements of x-rays in aluminum. Four

of the eight selected radiographic procedures, namely (1) A-P

cervical spine, (2) Lateral lumbar spine, (3) A-P abdomen,

and (4) A-P skull examinations, have been subject to patient

exposure limitation in the State of Illinois since January 1,

1975. Use of these aluminum phantoms in compliance testing

and estimation of patient exposure are discussed.

(Phantoms; attenuation; exposure; phototiming;

radiographic examination; x-ray tube potential)

Introduction

To measure patient exposure from

medical radiographic examinations

utilizing phototiming techniques, it

is necessary to recreate the clinical

situation with an appropriate phantom in place of a patient. The

thickness of the phantom employed

should correspond to the body part of

interest to simulate x-ray beam

attenuation. The x-ray exposure is

then terminated at a preset radiation

level striking the film-cassette

system.

The purpose of this study is to assess phantom thicknesses for x-ray

attenuation of various routine radiographic examinations. Patient ex- posure can then be estimated with the use of the phantoms when the phototiming technique is employed for the

examinations. Eight radiographic

examinations are chosen for this

study. These eight examinations are

(1) A-P cervical spine, (2) Lateral

lumbar spine, (3) A-P abdomen, (4)

A-P skull, (5) A-P thoracic spine,

(6) A-P lumbar spine, (7) high

voltage P-A chest, and (8) P-A chest.

The examinations (1) through (4) have

been subject to patient exposure

limitation in the State of Illinois

since January 1, 1975^. These eight

radiographic examinations are most

often performed, therefore are chosen

for this investigation.

Experimental Methods

Monitoring Clinical Factors

Accurate assessment of the

clinical situation is necessary to

facilitate proper measurement from

which the attenuation phantoms can be

derived. Monitoring and recording

several physical quantities during

actual radiographic examinations en- sures such an accurate assessment.

These physical quantities are the

radiographic technique factors em- ployed for the examinations, including x-ray tube potential (kVp) , tube

current (mA) , phototimer density se- lection, source - to - film distance, and

the thickness of the body part to be

examined. The exposure time or the

value of milliampere-second (mAS)

must be measured and recorded.

In this study, a Machlett Dynalizer 11^ system was employed during

actual radiographic examinations to

monitor values of kVp , mA, mAS and

exposure time. The experimental

arrangement is illustrated in Figure

1.Attenuation Phantoms

With sufficient data obtained

from the arrangement shown in Figure

1, the patient is then replaced by an appropriate phantom. The thickness

of the phantom is varied while all

other factors are held constant. Thus

the exposure time or the mAS value

which provides adequate radiographic

film density for a given examination

is dependent upon the thickness of

the phantom only. By placing phantoms of different thickness in

place of the patient, the mAS values

381
High Voltage Supply Film Cassette Figure 1. Schematic

illustration for the

Experimental Arrangement .

Patient

X-ray Tube

High Voltage Divider

Digital Display Unit

CRT Display

are measured. The phantom thickness

which provides the same exposure

time or the mAS value as the average

patient is a realistic phantom for

this examination. The phantoms,

with their thickness determined for

various examinations, can then be

utilized for patient simulation in x- ray beam attenuation to activate the

automatic termination of exposure.

Primarily, because of convenience and reasons that will be discussed in a later section, aluminum

is chosen for simulation of patient

attenuation. The phantom consists of

a number of aluminum (type 1100)

sheets of dimensions 1 2"xl 2"xl/ 1 6"

.

The total thickness of the phantom

can be adjusted in increments of

1/16".

Although it is designed for

dosimetry verification in radiation

therapy, the Rando Phantom^ is also

utilized for the purpose of com- parison with the aluminum phantom.

With soft tissue equivalent material

incorporating a human skelton, the

Rando Phantom is a potentially useful

phantom for phototiming attenuation

measurements

.

Results and Discussion

Clinical Factors

The thickness of the body parts

and the x-ray tube potentials em- ployed for the radiographic examinations investigated are shown in

Table I. The values in the second

and the fifth columns under the

headings "Technique Chart" are those

obtained from the technique charts

posted in the x-ray rooms. The

second column indicates the average

thickness of the body parts and the

fifth column identifies the x-ray

tube potential dial settings. Notice

that the thickness for each examination shown in columns of "Technique

Chart", "Rando Phantom", and

"Patient" are very close to each

other. The Rando Phantom is, therefore, indeed a "standard" man phantom with respect to its physical

size.

Figures 2 through 8 depict the

variation of mAS values as a function

of the patient thickness for the

examinations investigated. The x-ray

tube potentials are held constant at

the values indicated in Table I . The

mAS values measured with the Rando

Phantom are also included in the

figures. The Rando Phantom appears

to be denser than an average patient

for A-P skull, A-P thoracic spine,

and Lateral lumbar spine examinations.

Consequently, for these examinations,

the Rando Phantom requires higher mAS

values to obtain proper radiographic

film density. However, the Rando

Phantom is, radiographically speaking,

a "standard" man for the rest of the

examinations included in this study.

The mAS values which will provide adequate film density for an average patient may be obtained from

Figures 2 through 8. " These mAS

values facilitate the determination

of the phantom thickness as will be

shown in the next section.

Attenuation Phantoms

382
Table I. Radiographic examinations investigated.

Examination Thickness of Body Part (cm) X-ray Tube Potential (kVp)

Technique Chart Rando Phantom Patient { # Technique Chart' Manufacture ^ Measured"*

(1) A-P Cervical Spine 12-13 13.0 12.8 (32) 65 60 75

(2) A-P Lumbar Spine 20-21 20.5 20.1 (41) 75 70 85

(3) Lat. Lumbar Spine 25-27 25.5 25.9 (39) 85 77 95

(4) A-P Thoracic Spine 21-22 22,0 20.9 (45) 75 73 85

(5) A-? Abdomen 20-21 20.5 21.2 (43) 75 80 80'

(6) A-P Skull 19-20 19.0 19.0 1.23) 70 70 80

(7) P-A Chest? 21-22 21 .5 21.1 (SO) 115 120 120

(8) P-A Chest^ 21.5 70 70

The nui^ber in the parenthesis indicates the total number of patients.

^ The grid employed for these examinations has 12:1 grid ratio.

' Manufacturer suggested X-ray tube potential ''or use with a grid having 8:1 grid ratio.

.X-ray tube potential measured at the transformer terminals in series with the X-ray tube.

5 High voltage chest radiography. Grid ratio is 12:1.

° Chest radiography. No grid is employed.

' Large focal spot is employed for this examination.

Figure 2. mAS vs patient

thickness, for A-P

cervical spine examination .

Thickness of Patient (cm)

383
1 1 1 1 1 — LAT. LUMBAR SPINE

1 1 1 1

RANDO PHANTOM

1

1

1 1 1 1 1 1 1 1 1

Figure 4. mAS vs patient,

thickness, for Lateral

lumbar spine examination.

22 23 24 25 26 27 28 29 Thickness of Patient (cm)

30 31 32

Figure 5. mAS vs patient

thickness, for A-P thoracic

spine examination.

16 17 18 19 20 21 22 23 24 25 26

Thickness of Patient (cm)

110

1 1 1 1 1 1

1 1 1

100 - A-P ABDOMEN • _

-o

90 RANDO PHANTOM o

u 80

O)

l/l

1 CD

s_ 70

OJQ-E

(O 60

50 E

40 1 \

1 1 1 1 1 1

1

Figure 6. mAS vs patient

thickness, for A-P abdomen

examination

.

16 17 18 19 20 21 22 23 24 25 26

Thickness of Patient (cm)

384
E 50

01

S-Q.

40 —

30 —

Ra'nDO PfiANTOM 6^

^ A-P SKULL

Figure 7. mAS vs patient

thickness, for A-P skull

examination.

1 7 18 1 9 20 21 22 Thickness of Patient (cm)

s-Q.E

8 —

7 -

Figure 8. mAS vs patient

thickness, for high

voltage P-A chest ex- amination .

16 1 7 18 19 20 21 22 23 24 25 26 27 Thickness of Patient (cm)

The attenuation measurements

with the phantom, based on the

results obtained in the previous

section, are then performed to determine the thicknesses of aluminum

which provide the same mAS values as the average patient. The thicknesses

of aluminum determined by the attenuation measurements are to be utilized

as attenuation phantoms for the ex- aminations included in this study.

The thickness of the phantoms are specified in Table II. These alumi- num phantoms are only valid at the x- ray tube potentials (measured) of

respective examinations.

The attenuation properties of

the aluminum phantoms and the Rando

Phantom with respect to the variation

of x-ray tube potential are depicted

in Figures 9 through 16. The aluminum phantom tends to be "too thin"

and "too thick" at x-ray tube potentials "higher" or "lower", re- spectively, than at the x-ray tube

potential that is employed for the

examinations. This can be explained

by noting that the attenuation of x- rays in the photon energy range of

diagnostic radiology (average energy

less than 75 keV) is approximately

proportional to the cube of the

atomic number and the thickness of

the attenuation media"*' ^. For the purpose of comparison,

attenuation measurements with copper

and water were also performed to determine the thickness required to

simulate an A-P abdomen examination.

The thickness obtained were 2 . 5 mm

for copper and 22 cm for water.

These two phantoms provide the same mAS value as the 2" aluminum phantom

or the average patient for A-P abdomen examination at 80 kVp . Figure

17 depicts the attenuation properties

of the 2.5 mm copper phantom and the

22 cm water phantom with respect to

thevariation of x-ray tube potential.

Comparison of Figures 13, and 17

clearly indicates that (1) the water

phantom and the Rando Phantom behave

385
-High volto5tf L*iest radiography Figure 11. mAS vs kVp, for

Lateral lumbar spine ejcamination.

s-Q.

10

• RANOn PHANTOM

1 .312" ALUMINUM

A-P CERVICAL SPINE _J \ \

i 60 65 70 75 80 85 90 95 X-ray Tube Potential (kVp)

Figure 9. mAS vs kVp , for A-P

cervical spine examination.

<C 150 E

"O

100 o 90 o

0) 80

l/l

1 70

CD 60

r

^ 50 E

to 40

30

i I • RANDO PHANTOM

2.125" ALUMINUM

55 70 75 80 85 90 95 100 X-ray Tube Potential (kVp)

T3o

u

OJ

S10090

80

7(1

^0

_ 1 1

1

1 1 L

— • RANDO PHANTOM ~

l.«75' ALUMINUM

A-P THORACIC SPINE

1 1 1

1

! 1 = 6S 70 7-j 30 85 90 95 100 X-ray Tube Potential (kVp)

Figure 12. mAS vs kVp , for A-P

thoracic spine examination.

<D

S-O)CLs

ft3

200

150

100 —

90-

80

70

60

50

40

3060

RANDO PHANTOM

2.0" ALUMINUM

X-ray Tube Potential (kVp)

95 100

Figure 10. mAS vs kVp , for A-P Figure 13. mAS vs kVp , for A-P

lumbar spine examination. . abdomen examination.

386
60 65 70 75 80 85 90 95 X-ray Tube Potential (kVp)

Figure 14. mAS vs kVp , for A-P

skull examination.

X-ray Tube Potential (kVp)

Figure 15. mAS vs kVp , for high

voltage P-A chest examination.

55 60 65 70 75 80 85 X-ray Tube Potential (kVp)

Figure 16. mAS vs kVp , for P-A

chest examination. (Routine chest

radiography)

X-ray Tube Potential (kVp)

Figure 17. mAS vs kVp , for A-P

abdomen examination.

in a very similar manner, (2) the

copper phantom has a steeper curvature, and (3) the aluminum phantom

has an intermediate characteristic.

The water phantom is probably the

best choice for patient simulation,

however, transportation and handling

of this phantom in x-ray rooms is

often inconvenient. Due to its

relatively high atomic number (Z=29),

the copper phantom does not attenuate x-rays in a manner similar to

that of muscle (average Z=7.4), or water. Therefore, copper may not be an appropriate choice for attenuation phantom when the variation of

x-ray tube potential is anticipated.

Aluminum (Z=13) , on the other hand,

has an attenuation property which is

not significantly different from

that of muscle or water. In addition, its reasonable thickness for

handling is most suitable in field

applications

.

Use of the Phantoms and Summary

For patient exposure measurement or estimation of, for example,

the abdomen examination, the 2"

aluminum phantom is placed on the

examination table. The radiation

field is aligned with the phantom

and the phototimer-film cassette

system. The geometry and the parameters of radiographic techniques

are arranged identically with those

of the clinical situation for an average patient. An ionization

chamber is positioned 21 cm (the

average thickness) above the center

of the phantom. The exposure

obtained is an index of the estimated

387
Table III. Thickness of aluminum phantom for various kVp values.

A-P Cervical

Spine

kVp

inch

65 - 70

1 1/4

71 - 79

1 5/16

80 - 85

1 3/8

A-P Abdomen kVp

i nch

70 - 75

1 15/16

76 - 84

2

85 - 90

2 1/16

A-P Lumbar

Spi ne

kVp

inch

75 - 80

2 1/16

81 - 89

2 1/8

90 - 95

2 3/16

A-P Skull kVp

inch

70 - 75

1 11/16

76 - 84

1 3/4

85 - 90

1 13/16

Lat. Lumbar

Spine

kVp

i nch

85 - 90

2 5/8

91 - 99

2 3/4

100-105

2 7/8

P-A Chest kVp

i nch

110-115

7/8

116-124

1

125-130

1 1/8

A-P Thoracic

Spi ne

kVp

i nch

75 - 80

1 13/16

81 - 89

1 7/8

90 - 95

1 15/16

P-A Chest kVp

i nch

60 - 65

11/16

66 - 74 3/475 - 80

13/16

exposure that may be received by an average patient at the surface of the

primary beam entrance without a backscatter correction.

The measurement protocol for

patient exposure limitation implemented by the State of Illinois calls

for positioning of the ionization

chamber at the tabletop^. The ex- posure obtained above, therefore,

should be corrected by the inversesquare law for compliance testing.

There was no statement made

regarding the x-ray tube potential by

the State of Illinois when the abdomen phantom was specified (2.1 mm

of copper). However, it is clearly

demonstrated in Figures 9 through 17

that an appropriate compensation for

correcting the exposure values due to

the variation of x-ray tube potential

is necessary. The investigator finds

a correction chart derived from

attenuation measurements with various

thickness of aluminum phantoms, shoim

in Table III, to be useful. This

chart is currently employed at our institution for patient exposure

estimation of radiographic examinations utilizing phototiming techniques .

References

1. Rules and regulations for protection against radiation,

adopted April 18, 1961, amended

May 1, 1974. Illinois Department

of Public Health.

2. Dynalizer II x-ray equipment

calibration and verification

system, available from the

Machlett Laboratories, Inc.,

Stamford, Connecticut.

3. The Rando Phantom, available from

Alderson Research Laboratories,

Inc., Stamford, Connecticut,

4. William R. Hendee, Medical

Radiation Physics (Year Book

Publishers, Inc., Chicago,

Illinois)

5. Harold E. Johns and John R. Cunningham, The Physics of

Radiology (Charles C. Thomas

Publisher, 1969, Springfield,

I 1 1 inois

)

Acknowledgments

The author would like to express

his appreciation to L. Lanzl for his

useful suggestions, to P. Johnson for

her reviewing of the manuscript, and

to the technologists involved, es- pecially M. Klein, for their technical assistance in obtaining parts

of the data for this study.

388
NBS SP456 (1976)

EFFECTS OF MEASURING APPARATUS ON X-RAY ATTENUATION MEASUREMENTS

Tomraie J. Morgan, Ph.D., Libby B. Smith, M.S.,

and Zia R. Hashini, Ph.D. Medical Physics Branch

Division of Electronic Products

Bureau of Radiological Health

Food arid Drug Administration

Rockville, Maryland 20852

Measurements of x-ray exposure are affected by the photon energy dependence of instrument response, and measurement geometry.

The effects of these factors on measured x-ray attenuation curves and

estimates of first half-value layer in aluminum under certain test conditions are described, and estimates of the magnitudes of the effects

are given.

A method for correcting half-value layer estimates obtained under less

than ideal, but well-controlled, conditions is discussed.

(Half-value layer; geometry effects; chamber energy response; photon

attenuation)

Introduction

An investigation was conducted to determine the effects of measurement geometry and

radiation detector energy dependence on x-ray beam half-value layer determinations

within the diagnostic energy range. Two

discrete x-ray beam qualities were selected

for this investigation. The two x-ray beam

qualities selected were approximately 1 mm

Al half-value layer (HVL) and 4 mm Al HVL,

with nominal 48 kVp and 90 kVp tube potentials, respectively. It was felt that these

two beam qualities were representative of

the lower and medium beam qualities encountered in diagnostic radiology. In addition,

it was suspected that the greatest error in

half-value measurements might occur at the

lower beam quality.

This investigation encompassed three

geometries commonly employed in x-ray attenuation measurements, and three instruments

having significantly different energy re- sponse functions. The response of one instrument had a rather significant but

not necessarily atypical photon energy

dependence. An attempt was made to quantify

the magnitude of the effect of the individual parameters on the determination of

the two half-value layers.

Test Geometries. Three geometries were selected for evaluation in this investigation. These geometries are commonly employed in attenuation measurements in the diagnostic energy range. Geometry A can be

described as follows: the distance from

the source to the center of the sensitive

volume of the ionization chamber was approximately 75 cm; the source to aperture dis- tance was approximately 30 cm; the field was defined by the beam-limiting device

of the x-ray source assembly, i.e., the

collimator of the x-ray system itself; the

x-ray field cross-sectional area at the

plane of the center of the sensitive volume

of the ionization chamber was approximately

2 220 cm (12 cm X 18 cm); the attenuators were perpendicular to the central ray of

the beam placed approximately midway be- tween the x-ray source and the center of

the sensitive volume of the ionization chamber. It may be noted that this geometric

configuration corresponds to that routinely

employed in the Bureau of Radiological

Health (BRH) routine compliance testing

of radiographic x-ray systems, utilizing

the BRH test stand and associated hardware.

Experimental Design and Equipment

Description

Two x-ray beam qualities were selected

for this investigation. The peak tube

potentials were 48 kVp and 90 kVp.

389
30 cm J

75 era

35 cm

/ 1 f

/ V

/ \

I \

I \

x-ray tube target

Col L ima tor

Beam-defining assembly

Aluminum attenuators

BRH test stand

Ionization chamber

Narrow beam geometry (geometry C), was used to determine the reference half-value

layers throughout this investigation.

Narrow beam geometry is described as follows: the distance from the x-ray source

to the center of the sensitive volume of

the ionization chamber was approximately

100 cm; the beam-defining assembly was midway between the x-ray source and the ionization chamber, perpendicular to and centered

on the central ray of the x-ray beam; the

x-ray field was large enough to expose

uniformly only the entire sensitive volume

of the ionization chamber; and the attenuators were placed on the source side of

and in direct contact with the beam-defining

assembly

Geometry A

Geometry B can be described as follows:

the distance from the x-ray source to the

center of the sensitive volume of the ionization chamber was approximately 75 cm; the source to aperture distance was approximately 38 cm; the beam-defining assembly

was a 15 cm X 15 cm square lead/Lucite sandwich with a 5 cm X 5 cm centered aperture;

the X-ray field cross-sectional area at the plane of the center of the sensitive

volume of the ionization chamber was approx2 imately 100 cm (10 cm x 10 cm). The con- figuration described as geometry B can be

achieved by using the BRH test stand and

associated hardware, and corresponds closely

to the test configuration used in gathering

HVL data for NEXT (Nationwide Evaluation

of X-Ray Trends .

)

x-ray tube target

Collimator

Aluminum attenuators

Beam defining assembly

BRH test stand

Ionization chamber

100 cm

X-ray tube target

Collimator

Aluminum attenuators

Beam-defining assembly

Ionization chamber

Geometry B

Geometry C

The BRH test stand used in geometries

A and B is a rectangular Lucite frame which

provides fixed geometry for placement of

the ionization chamber, the beam-defining

assembly, and the aluminum attenuators.

Instrumentation . Three ionization

chambers and their respective readout sys- tems were selected for this investigation. A Victoreen 555 Radocon II with a 555-1 DAS chamber was used in making the reference

half-value layer determinations. The energy

dependence of this ionization chamber is depicted in figure 1. This chamber is a propionate cylinder with a metal wire as the central anode.

390
I.I -

1.0-

0.9 -

Half-Value Layer {mm Al)

Figure 1. Energy Dependence of

a Victoreen 555-1 DAS Chamber.

The second instrument selected for evaluation in this investigation, an MDH 1015A

X-Ray Monitor with a 10x5-6 chamber, involved the use of an ionization chamber com- posed of two concentric polystyrene cylinders. The energy response of this chamber is depicted in figure 2.

1.0 -

0.9 - 12T 1 r

3 4 5 Half Value Layer (mm Al)

Figure 2. Energy Dependence of an MDH 10x5-6 Chamber.

The 666-10 ionization chamber selected

for the use with the third instrument em- ployed in this investigation, a Victoreen

666 Survey Meter, can be described as a right cylinder, constructed of phenolic,

of parallel plate design with a round disk

anode. The characteristic energy response

of this chamber model is depicted in figure

3.

1.0 2.0 3.0 4.0 5.0 Half-Value Layer (mm Al)

Figure 3. Energy Dependence of

a Victoreen 666-10 Chamber.

Throughout the course of this investigation the peak tube potential, the product

of the tube current and exposure time,

and the line voltage regulation of the

x-ray machine were monitored continually

and found to be reproducibly stable. The

peak tube potential was monitored with

a Philips model 8122 100 voltage divider

equipped with a digital readout system.

Calibration of this voltage divider was traceable to the National Bureau of Standards. The tube current/exposure time

product measurements were made with a Hewlett-Packard model 4288 clip-on probe

milliammeter , modified to provide the

exposure time/ tube current product. The

line voltage regulation was observed with

a specially designed BRH line voltage

regulation monitoring system.

Data and Results

Extended attenuation curves were developed with the use of the Victoreen 666

Survey Meter in all three geometries:

A, B, and C. The MDH 1015 X-Ray Monitor

was also used in the three geometries.

The Victoreen 555 Radocon II was used only

in geometry C. The relationships among

instruments and geometries are depicted

in Table 1.

From the extended attenuation curves

the first half-value layers (observed,

uncorrected) were determined. The results

of these uncorrected data are presented

in Tables 2 and 3. It is obvious from

391
Ins trument

A Geometry

B C

Victoreen 666 with 666-10 chamber X X X

MDH 1015A with 10x5-6 chamber X X X

Victoreen 555 with 555-1 DAS chamber X

Table 1. Instrument/Geometry Combinations Employed.

Ins trument

A Geometry

B C

Victoreen 666 with 666-10 chamber 0 98 1 00 1 03

MDH 1015A with 10x5-6 chamber 0 87 0 84 0 87

Victoreen 555 with 555-1 DAS chamber 0.88

Table 2. Observed First Half-Value Layers for X-Ray Beam at 48 kVp.

Instrument

A Geometry

B C

Victoreen 666 with 666-10 chamber 5 02 4.62 4 70

MDH 1015A with 10x5-6 chamber 3 85 3.80 3 76

Victoreen 555 with 555-1 DAS chamber 3 74

Table 3. Observed Half-Value Layers for X-Ray Beam at 90 kVp.

392
Tables 2 and 3 that" significant differences

occur in the determination of the half-value

layer with different geometries and instruments. The implication is that these variations can be attributed to the different

chamber energy responses and geometries.

An attempt was made to correct the observed half-value layers determined in geometries A and B for the energy responses

of the Victoreen 666-10 and MDH 10x5-6

chambers, such that the observed half-value

layers would be equal to the reference

half-value layer. The iterative technique

of correcting half-value layers for the ionization chamber energy dependence, as suggested by Morrison^, was applied to the

data. This technique attempts to regress

the observed attenuation curve to the true attenuation curve, independent of the energy

dependence of the ionization chamber employed. The number erf iterations required to achieve this goal is a function of the

severity of the energy dependence of the

particular ionization chamber employed.

Figure 4 shows the uncorrected attenuation data (Geometry C) as well as the re- sults of the first iteration applied to this data for a beam having a reference

half-value layer of 3.75 mm Al . Inherent

in the iterative technique is an implied

knowledge of the energy response of the

particular ionization chamber employed as well as the selection of the appropriate

correction factor for the individual points

on the attenuation curve. Since half-value

layer measurements are relative measurements

and not absolute exposure measurements,

it is frequently concluded that it is unnecessary to use any calibration data for the

ionization chamber employed in such m.easure- ments. It is obvious from figure 4 that

this is not the case at all, and that it

is necessary to apply the appropriate cor- rections for the particular beam quality

being determined.

c

0) u

u

PL,

I

1 1 1 1

1 2 3 4 5 6 Absorber Added (mm Al

)

Figure 4. Attenuation Curves for a Beam of 3.75 mm Al Reference Half-Value

Layer. 1. Observed attenuation curve

for Victoreen 666-10 chamber in

geometry A. 2. Above attenuation

curve, corrected by one iteration.

3. Reference attenuation curve for

Victoreen 555-1 DAS chamber.

It is imperative to recognize that in

order to use the iterative technique effectively on attenuation data, it is necessary

to know the energy response of the ionization chamber employed over a range of energies significantly exceeding that corresponding to the particular observed half-value

layer; e.g., for an observed half-value

layer of 3.74 mm Al , it is necessary to know the energy response of the ionization

chamber to approximately 8 ram Al half-value

layer if the iterative technique is to be

effectively applied to the attenuation data.

It is also worth noting that the application

of the iterative technique requires that

the attenuation data be extended so that

the initial radiation exposure rate is reduced to 25% or less, and, if possible,

to 12%.

Instrument response is not solely dependent on the energy dependence of the

ionization chamber, and certain other parameters can significantly affect the instrument response in an attenuation measurement.

Some of these parameters are instrument

precision, sensitivity, and scale linearity

for analog readout systems. In addition,

all radiation measurements have an associ393
ated inherent uncertainty dependent on the

reproducibility of the detector readout

system, the accuracy of the calibration

of that particular system, the exposure

rate, the number of observations, and the

magnitude of the particular exposure being

measured. It is appropriate to re-emphasize

the importance of selecting the appropriate

energy response data for the particular

ionization chamber employed as well as selecting from the energy response data

the appropriate correction factor for the

particular attenuation data being corrected.

Conclusions

From Table 2, which gives the results

for the lower beam quality as measured with

the three geometries and three instruments,

it can be seen that at this lower half-value

layer (0.88 mm Al reference value) the magnitude of the percentage error is significant. The apparent geometry and instrument

effects result in a measured half-value

layer which can be as much as 15% greater

than the reference value. The first application of the iterative technique gave a good approximation of the lower reference

half-value layer. Application of the iterative technique seems to be valid, within

reasonable limits, regardless of the severity of the energy response of the particular

ionization chamber employed for measurement

of the lower half-value layer.

Table 3 shows the results of the several instrument/geometry combinations employed

in measuring the quality of the beam with

a reference half-value layer of 3.75 mm

Al . It can be seen that the magnitude of

the effects of the several instrument/geometry combinations is more severe at higher

energies than at lower energies. For the

instrument/geometry combinations evaluated

here, the worst-case condition reflected

a half-value layer approximately 34% larger

than the reference value. The first application of the iterative technique to the

data in Table 3 for the Victoreen 666-10

chamber did not correct the observed halfvalue layer to the reference value. There

is strong indication that it is not the

iterative technique that is at fault, but

the lack of knowledge of the energy response

of the particular chambers employed for

beam qualities greater than 5 mm Al halfvalue layer.

Another interesting point is the effect

of geometry on the determination of the

half-value layer for a particular beam.

Within the framework of this investigation,

the effects of geometry provided a source

of variation not greater than 4%. It is not reasonable to believe that the 4% variation can be attributed solely to geometry,

since there are other variations that could

contribute to this value. From this, the

conclusion can be made that it is much more

important to review the energy response

of the ionization chamber being employed

in half-value layer measurements than it is to put severe restrictions on the geometry for making the measurement. If the

data are to be used for comparative evaluations of the same particular piece of

x-ray equipment, then a fixed reproducible

geometry for making the attenuating measurement is very important; however, in the event that one is simply making a single

estimate of the half-value layer, it is much much more important to have precise

knowledge of the energy response of the

ionization chamber employed. It is also

worthwhile to note that the effects of the

different source to chamber distances employed here and the effects of the x-ray field

size were essentially insignificant in the case of the chamber with minor energy dependence (i.e., the MDH 10x5-6 chamber).

The implication is that a geometry

which is simple to establish and reproduce

is an acceptable configuration in making

routine beam quality evaluations of diagnostic x-ray equipment. Geometry effects

are minimal in such determinations of halfvalue layer when such measurements are made

with an instrument having a minor energy

dependence . 1 ~ . Margaret T. Morrison and G. W. Reed,

Brit. J. Radiol., 25:270, 1952.

394
fjBS SP456 (1976)

kVp MEASUREMENT FOR DENTAL X-RAY UNITS

M. S. Moroz and J. A. Vukan

Pennwalt Corp., S. S. White Div.

Holmdel, New Jersey 07733

The kVp measurement of a sealed x-ray

system using filtered radiation from

a scatterer is discussed and compared

to direct voltage measurement techniques.

(kVp; dental; x-ray)

Introduction

Federal legislation has made necessary

the accurate measurement of all parameters

associated with x-ray production. Among

these parameters is the peak potential impressed on the x-ray tube by the high voltage generator.

An accurate measurement of this parameter is important for two (2) reasons:

1. It is an indication of the penetrating

power of an x-ray beam.

2. The radiation incident upon the patient

is dependent on the kVp.

The measurement of this parameter in a dental x-ray unit offers a particular challenge to the investigator. The diagnostic

source assembly contains the x-ray tube

housing, beam limiting device and the high

voltage generator. These are immersed in

oil or gas and sealed in a metal enclosure

with expansion compensators. The high voltage area of the diagnostic source assembly

is therefore inaccessible without breaking

the seal on the metal enclosure.

Five (5) methods of measuring this pa- rameter for dental x-ray systems are discussed below:

1. Direct high voltage measurement in an open oil filled tank using a high resistance

voltage divider and an oscilliscope.

2. Direct high voltage measurement in an open oil filled tank using a peak reading kV

meter of the charging capacitor type.

3. Direct pulse height analysis using a 1024 channel multichannel analyzer, GeLi

detector and heavy filtration of the x-ray

beam.

4. Pulse height analysis of a scatter spectrum from an element whose "K" line is at or

near the peak x-ray potential indicated,

using a 1024 channel multichannel analyzer

and a sodium iodide scintillation detector.

5. Method #4 modified by a filter with an atomic number one higher than that of the

scatterer placed between the scatter source

and the detector.

Method #1

The open tank/high voltage divider

measurement

The apparatus used for this measurement is depicted in Fig. 1.

Schematic of Open Tank/High voltage Divider Apparatus

The entire assembly was put into an electrically insulated tank, evacuated to 200 microns and held there for 24 hours.

Hot transformer oil (140°F) was allowed to cover the assembly and the unit was held at 200 microns for another 24 hours. The

transformer (T]^) , the x-ray tube (C, A) and

transformer (T2) constitute the electrical

components of a dental diagnostic source assembly.

395
To calibrate this apparatus, a pure

sine wave was impressed on the primary of

T^; the primary of the filament transformer

(T2) was disconnected so that no distortion

of the sine wave was introduced into the

secondary. A Hipotronics AC/DC kilovoltmeter, Model 110, was put in parallel to the

divider. The meter used has an N.B.S. traceable calibration. The oscilliscope was calibrated for a full scale reading of the

highest peak potential to be measured by adjusting potentiometers and P^. This produces the proper peak reading corresponding

to the rms reading on the Hipotronics meter. A pure sine wave was needed at the output of

transformer T^ to ensure that the conversion

from rms kV to peak kV is without harmonic

error

.

The Hipotronics meter was then removed,

the primary of transformer was re-con- nected and the entire assembly was connected

to the proper x-ray control. Readings were

then taken at the maximum rated peak x-ray

potential of the x-ray system and compared

to its specifications. The kVp was varied

when it was possible, or the mA was varied

and the indicated value of kVp was compared

to the actual.

Fig. 2 is a picture of the waveform

seen on the oscilliscope face; note that due

to the half wave loading of the high voltage

transformer by the x-ray tube, the waveform

departs from that of a true sine wave.

Waveform at 90 kV indicated, 10 mA

indicated obtained by the voltage

divider method.

The advantages of the method are listed

below:

1. The actual secondary waveform is seen

and can be studied if the oscilliscope is a storage type oscilliscope.

2. The mA can be displayed with the secondary voltage waveform to determine the effects that changing the mA has on the kVp.

3. The primary voltage can be studied along

side the secondary voltage and the effects

that waveform changes in the primary have on the secondary waveform can be determined.

4. The kVp reading is instantaneous.

These reasons are all good research

tools and this method allows one to use

them. However, what effects are ignored by

this method or what new effects are introduced by this method?

A list of the major aberrations of this

system are in order:

1. This system ignores the possible capacitance currents that a rapidly varying high

voltage field generates when surrounded by a grounded metal enclosure.

2. The introduction of a resistive load un- balances the two (2) coils in the transformer.

3. The resistive load also tends to lower

the kVp reading since the voltage regulation

in dental diagnostic source assemblies is low.

4. The resistors used (100 meg, ^7o) have a high voltage coefficient.

5. The width of the oscilliscope trace also

lends itself to inaccuracy.

The error is estimated at a full-scale

inaccuracy of 2-57o by this method, depending

on the type oscilliscope used and the individual researcher. However, this is a valuable research tool in determining high voltage wave shapes and factors effecting them.

Method #2

Direct high voltage measurement in an open oil tank using a charging

capacitor type kVp meter

The meter used in this method was an ITT Jennings Model 12102 high voltage meter

with a calibration traceable to N.B.S.

395
This meter has an oscilliscope output

and digital readout s^ith an option of reading the positive half cycle, the negative

half cycle or rms kilovoltage.

The diagnostic source assembly was evacuated in an open tank and filled with

oil by a method similar to that described in

Method *1.

N.B., in certain cases, when the x-ray

tube and high voltage generator are sealed

by bolting the outer tank together, the diagnostic source assembly may be put into a larger tank of oil and the bolts removed and

the diagnostic source assembly opened in the

oil to avoid the necessity of re-evacuation.

This procedure is somewhat difficult and

many dental diagnostic source assemblies are

of a welded or soldered construction and

this procedure is not applicable.

After the diagnostic source assembly

was put into oil, the high voltage input

lead from the metering circuit was connected

to either the anode or cathode. The diagnostic source assembly was connected to the

control designed for it. The control was

then set for the maximum rated kVp and mA. An exposure was made of at least five (5)

seconds duration and a reading was taken.

The exposure was of long duration because

the rise time of the meter was quite long.

Charge time is a very important factor in

the use of peak reading devices of this nature.

If there is more than one technique

factor available, the machine should be

tested at every combination of preset techniques and in the case of continuously variable techniques it should be tested in a series of combinations at predetermined

steps, i.e. variations of 10 kV if the kVp

is continuously variable at each of the preset mA' s.

The positive aspects of this type of

measurement are easily recognizable.

1. The machine has a digital readout and

readability is good.

2. One has the option of viewing the waveform on an oscilliscope independent of the

readings

.

3. One has the option of reading the magnitude of the positive peak, negative peak or the rms kV.

However, this type of measurement has a few subtleties not as readily apparent as

the positive aspects:

1. The long rise time forces the researcher into making a long exposure. In dental

diagnostic source assemblies, this is un- desirable due to duty cycle considerations

(most dental x-ray control panels do not have a time setting greater than five (5)

seconds)

.

2. Due to the long rise time of this unit,

the instantaneous peak voltage cannot be

measured. Some dental machines may produce

a kVp that varies in magnitude on alternate

conducting half cycles, ref. Fig. 2. Others

may have a low kVp on the initial impulses

of x-ray which then stabilizes to a higher

value of kVp. Finally, the kVp may be a high value which stabilizes to a lower value

of kVp. None of these cases can be detected

with this type of instrumentation due to the

long rise time of the metering circuit. The

kVp value obtained by this method may at times be erroneous.

Fig. 3 is an example of an x-ray unit

whose kVp does not stabilize in the first

few x-ray impulses. The kVp is very low

initially and stabilizes at a higher value.

Positive half cycle of the kVp of a dental x-ray machine

3. The nature of the circuitry itself tends

to allow the calibration to change over relatively short periods, 1-3 months, necessitating frequent re-calibration.

The biggest problem associated, both,

with this method (#2) and the previous

method (#1) is that in order to test the

diagnostic source assembly, the high voltage generator and the x-ray tube must be

removed from their housing. The tube housing, expansion devices or both may be damaged by the process. A significant amount

of the testing of dental machines involves

machines that are already installed.

397
Methods #1 and #2 have a serious drawback

for this application.

The accuracy of Method #2 is dependent

on the type of system being tested. For

instance, a unit in which the kVp varies on alternate peaks, Method #2 yielded a value

4-6 kV lower than the true value in the

range 80-100 kVp; another example is the

unit whose kVp drops down to a point of

stabilization, the initial high kVp is never

seen by the observer only the long term stabilized kVp. The error here was measured

to be 8-12 kV in the 50-100 kVp range. However, if the unit being tested has the same peak kV for the first pulse of x-ray as for

the 300th pulse of x-ray and for all those

in between or if the kVp stabilizes at a point higher than the kVp of the first x-ray

pulse and remains stable, then measurement

of the peak kV would be within the machine's

accuracy of t 17^.

Method #3

Pulse height analysis of a heavily

filtered x-ray beam using a GeLi detector

prior to the test using two (2) radioactive

isotopes which have gamma emission lines

bracketing the indicated kVp. This was re- peated after the location of the spectrum

end was found. The spectrum end was found

by drawing a line along the noise level at the end of the spectrum and drawing a line

on the downward slope of the last peak;

where these two (2) lines intersected was considered to be the end of spectrum. The

kVp was then calculated by the formula:

kVp Pi - (Pi - P2)(Cf

- C„ )

Where

The energy of the calibration peak

nearest the end of the spectrum.

The energy of the other calibration

peak.

The channel number of calibration

peak #1.

This method, depicted in Fig. 4, was tried briefly. This apparatus was an x-ray

source, narrowly collimated by a heavy lead

collimator. The x-ray beam was then heavily

X-ray

Source

Filter Combination

of Lead, Copper,

Aluminum and Tin

1024 Channel

Multichannel

Analyzer

Fig. k Block diagram of the apparatus for

direct pulse height analysis

of a heavily filtered

x-ray beam

filtered by a filter comprised of aluminum,

copper, lead and tin. The GeLi detector was brought in as close to the source as physically possible. The system was calibrated

C_ = The channel number of calibration

^2 peak #2.

Cg = The channel number of the end of spec- trum.

+/- is determined by:

+ if C is lower than C Pi

- if C is higher than C Pi

The calibration points, both at the start and the end of a test, were each used

to calculate a separate kVp and the spread

between the two (2) values was used as an indication of error. The kVp was found to be repeatable by this method to + 3 kV in

the 50-100 kVp range. Also noted during the course of experimentation was that equally

good results in this application could be

obtained by using a sodium iodide detector.

The sodium iodide detector had the added

advantages of not needing cryogenic temperatures and being less bulky to handle (use

of the GeLi detector was discontinued in

favor of a sodium iodide detector)

The above method was reasonably repetitive but relies too much upon the judgement

of the individual experimenter. It has the

advantages of:

1. Not disturbing the electrical potentials

398

J
being measured.

2. The diagnostic source assembly need not be disassembled to measure the kVp,

3. The highest kVp pulse is measured each time and not the steady state kVp.

4. Results are not dependent on the secondary wave shape.

There are a few problems with this sys- tem when applied to dental x-ray units These are;

1. The inherent storage characteristics de- mand extremely long cumulative exposure times.

2. Long term drift of the system adds an additional x 1 kv error to the absolute

measurement

.

3. Due to the pulsed half sine wave, the electron flow in the conducting half cycle

see the peak potential only' intermittently

not continuously as in the case of a fully

essence, filtered

what

d.c. potential across the tube m

this means is that the true value of the kVp may be buried somewhere in the noise of this experiment and that even though the values are relatively repeatable they are in fact too low to be the actual value This is indicated by the results ob- tained in Method #5. The values obtained in Of H^hod T " '--^

A typical spectrum generated by this method is shown in Fig. 5. It can be easily

seen that the noise level at the end of the spectrum is barely differentiated from the signal, an inherent problem in this method

Method #4

kVp measurement of an x-ray beam by analysis of the scatter spectrum

of a known pure element

The apparatus (seen in Fig. 6) is rel- atively simple; the x-ray beam was highly collimated by a lead collimator, it then passed through a thick lead aperture which allows a beam to exit which is slightly

Known Absorber Whose Atomic Number is one higher than the scatterer (used

in Method #5)

Typical spectrum from a dental diagnostic

source assembly derived from the analysis

of a highly filtered x-ray beam

Aperture

Col 1 itnator

X-ray Source

102A Channel Multichannel Analyzer

Fia. Block diagram of the apparatus used m Methods #4 and #5. The additional filter is not used in Method #4.

smaller than the cross section of the scat- terer that the beam sees. The scatter

spectrum was. then detected by a sodium iodide detector which was connected to the 1024 channel multichannel analyzer The scatterer was an ultra-pure foil and has a K line near the indicated kVp being meas- ured.

For this experiment, the following scatterers were chosen:

1. 50 kVp - Terbium has a of 50.391 keV.

2. 70 kVp - Platinum has a of 75.736 keV.

3. 90 kVp - Lead has a K^^^ of 88 keV.

Each line has a width so a reasonable variation from the nominal kVp can be deter399
mined

,

The apparatus was calibrated by radioactive isotopes before and after an analytical run was made to collect the scatter spectrum. The isotopes were chosen so that their

gamma emission lines bracketed the nominal

kVp.

Typical scatter spectrum from

platinum generated by a 68 kVp

x-ray field

Due to the strength of the "L" and "M" lines of the scatterer, the higher energy

"K" lines were obscured.

Fig. 7 is a picture of the spectrum

given off by this scatterer (platinum in

this case) in the region of 70 kVp. The

unit was subsequently measured at 68 t 0.9 kVp, however, in the best judgement

of the experimenter the kVp was determined

to be 73 t .5 kVp with Method #4. The 68 kVp

figure was verified by both Method #5 and

Method #1.

A method was sought to enhance the "K" line radiation or reduce the "L" line radiation. The solution became obvious and

Method #5 was tried. Method #4 was discarded due to the fact that the information

desired was buried in the noise level.

Method #5

kVp determination of a dental diagnostic

source assembly by analysis of the

scatter spectrum of an ultra-pure foil

which is filtered by another pure foil

whose atomic number is one greater

than that of the scatterer.

The apparatus for this method is depicted in Fig. 6. The x-ray beam exited

from the collimator and passed through the

aperture which allowed a stream of photons

through which had dimensions equal to the

projected dimensions of the scattering foil.

The scattering foil was angled at 45° to the

central ray of the system. The axis of the

scintillation detector passed through the

center of the scatterer and was orthogonal

to the central ray. If desired, the axis of

the detector may be orthogonal to the sur- face of the scatterer to gain in spectrum

intensity, but the surface of the absorber

should be perpendicular to the axis of the

scintillation probe. The scatterer /absorber

pairs chosen for this experiment were:

1. 50 kVp - Terbium/Dysprosium.

2. 70 kVp - Platinum/Gold.

3. 90 kVp - Lead/Bismuth.

The 70 kVp materials were chosen be- cause these materials are commonly found in

the pure form. The 90 kVp materials were chosen because they were the closest

non-radioactive substances that could be

found

Fig. 8 is a spectrum obtained by ana- lyzing the filtered scatter spectrum from a diagnostic source assembly. This is the same diagnostic source assembly that produced the spectrum of Fig. 7. The difference lies in the fact that the spectrum of

Fig. 7 was further filtered by a gold foil

filter to obtain Fig. 8. It can be seen that less experimenter judgement is needed

to determine the kVp than in the previous

two (2) methods.

Same spectrum as Fig. 7 except the

scatter spectrum was further filtered

by a gold foil

400
The positive aspects of this type of

measurement are;

1. Less room for experimenter errors in

judgement

.

2. This method does not disturb the electrical system of the x-ray machine.

3. The diagnostic source assembly does not have to be disassembled to gain access to the high voltage areas.

4. The measurement of kVp is independent of

secondary waveform.

The disadvantages of this method are:

1. The equipment is not portable for field

use

.

2. The length of time needed for a complete

analytical run ranges from 1-6 hours depending upon the x-ray system's duty cycle.

This method is impractical as a quality control method in a high volume production situation .

Conclusions

A comparison of the results obtained by

the above methods are expressed in Table #1.

It can be seen that Methods V-'l and #5 agree

Method 70 kVp 90 kVp

#1 68 kVp 91 kVp

#2 69 kVp 88.6 kVp

#3 67 kVp 89.8 kVp

#4 73.5 kVp indeterminate

#5 68 kVp 91.5 kVp

Table -- ,--1 A set of typical values for the

inter-comparison of methods discussed

in this paper

rather closely with each other. The differences are mainly attributed to the loading effects of the high voltage divider and

the line width of the oscilliscope trace. Other discrepancies are attributed to the

effects of a low signal to noise ratio and errors in operator judgement because of the

high noise levels.

Research will continue to obtain a portable and/or fast response kV measurement system that can measure the kVp accu- rately in the field and on the production

line.

Bibliography

1. Clark, G. L., ed. , "The Encyclopedia of

X-rays and Gainma Rays," Reinhold Publishing

Corp. , New York (1963)

2. Green, M. , Cosslett,V. E., "The Efficiency of Production of Characteristic X-Radiation in Thick Targets of a Pure Element,"

Proc. Phys. Soc. LXXVIII, 6 (1961)

3. Richards, A. G., et al, "Samarium Filters for Dental Radiography," Oral Surgery

(May 1970)

4. Richards, A. G., private communication,

(1974)

5. Rothenberg, L. N. , Williams, D. J.,

"Compliance Test Methods for Diagnostic Medical X-ray Equipment," Vol. I, Vol. II (June 1973)

6. U. S. Department of Health, Education

and Welfare, "Draft of Proposed Test Methods

for Determining Compliance with the diagnostic X-ray Equipment Performance Standard

(42CFR78. 213-1, 78.213-2, 78.213-3)"

(March 1972)

401

ms SP456 (1976)

NATIONAL IONIZING RADIATION STANDARDS

James E. Leiss

National Bureau of Standards

Washington, D. C. 20234

Ionizing radiation in its many forms is used in an increasing number

of ways to help solve important problems of our technological society.

As these applications become more mature, the needs for reliable radiation measurement continue to increase, both to achieve the maximum

benefit from the radiation use and also to assure that these applications are carried out safely. To meet these measurement needs, the

National Bureau of Standards provides a wide variety of measurement

standards and services. New standards are also being developed to meet

a variety of new needs.

It is recognized that these radiation measurement standards by themselves are of limited usefulness unless they can be related to measurements made at the user level. Various mechanisms exist for this, including measurement calibrations, measurement traceability testing, and

measurement assurance programs. The National Bureau of Standards carries

out an increasing number of activities of this type. In some cases new

or augmented institutional arrangements within the national measurement

system for ionizing radiation appear necessary.

(ionizing radiation, standards, measurement assurance, radiation users)

Introduction

The history of x-rays and of ionizing

radiation in general is a remarkable example

of rapid technology transfer from the basic

physics discovery to practical applications.

Almost from the day of their discovery in 1896 the value of x-rays for visualizing the

interior of the human body was recognized

and applied in the medical profession. Within about five years of their discovery x-rays were used to assist doctors in the setting

of broken bones, locate tumors, relieve the

suffering of cancer, and "cure" a variety of

human ailments. Many of these applications,

of course, were not really successful and

some of them are only in very recent years

reaching the point of significant success.

The harmful effects of ionizing radiation were also recognized very early in the

history of x-rays, mostly because of the severe skin burns received by some of the

early radiation workers. Thus ionizing radiation is a classic example of new scientific

discovery which has led to very important

benefits for mankind, while at the same time

carrying significant risk to man if not used

carefully and intelligently. The problem of

balancing these benefits and risks continues

today in the national debate regarding nu- clear power plants and in the debate over x-ray screening of women for breast cancer.

Our conference title, "Measurements for

the Safe Use of Radiation," is thus very

timely since it relates directly to one aspect of this important benefit-risk problem.

Reliable and accurate measurement relates directly to achieving benefits from the use of

ionizing radiation, and also is essential if

meaningful measurements are to be made to protect the public from undue harmful effects.

As uses and the effects of ionizing radiation

become better understood, this need for ac- curate and reliable radiation measurement

will become increasingly important.

The use of ionizing radiation within our society is widespread and applications are growing. One categorization of these applications which we find useful in thinking of our programs at NBS is given in Table I where

I have tried to identify radiation applications by their relationship to important

national interests.

TABLE I. Ionizing Radiation Application Areas

• Health and Public Safety

• Energy and Environment

• Industrial Productivity

• National Defense

• Basic Measurement Science

Our conference this week is devoted primarily to the first of these areas, Health

and Public Safety . NBS, howeyer, must at- tempt to provide appropriate measurement

standards and measurement assistance to all of these applications. What I hope to do in this paper is to give you at least a partial

look at some of our activities related to all 403
of these radiation applications. I will at- tempt to do this by taking you on a brief

tour of some of our facilities and some of

the activities carried out at these facilities. Some of the other speakers from our laboratory have spoken on some of these

same activities; however, my objective is to give you an overview, particularly of some of our ionizing radiation measurement standards

and of measurement techniques which are being

developed. I will also try to briefly relate

these activities to why we are doing the

work.

Dr. Caswell in his paper has discussed

"The Standards Laboratory and the Radiation

Measurement System." This is primarily a look at the national radiation measurement

system and how we at NBS appear to relate to this system. Without too much overlap with

his presentation, I also will discuss some

of the activities at NBS related to radiation measurement calibrations and measurement assistance to the radiation user com- munity as well as some of the problems which

we perceive with the present operation of

this national measurement system.

NBS Radiation Laboratories and Activities

Although ionizing radiation is used ex- tensively in many different activities within the National Bureau of Standards, the re- sponsibility for ionizing radiation stan- dards and measurement science is located

within the Center for Radiation Research

(CRR). Most of the activities of the Center

are located in the Radiation Physics Building. An aerial view of this laboratory is

shown in Fig. 1. The large stack is for ex- haust air from our linear accelerator complex, and the horizontal pipes going out from the building (up to 200 meters) are evacuated flight tubes for neutron time-offlight measurements. A number of different

radiation sources are located within this

building, and are required for the many different types of radiation standards and ser- vices which we provide, A list of some of

these radiation sources is given in Table H.

Some of our activities using these sources

will be discussed below.

TABLE II. CRR Radiation Facilities

• 0.5-, 1.5-, 4-, and 1 50-MeV high intensity

accel erators

.

• 250-MeV electron storage ring (for synchrotron light).

• 3-MeV Van de Graaff (used as a neutron

source)

.

• 2-MeV and 600-keV flash x-ray machines.

• Five constant potential x-ray machines

(50-250 kV).

• Various gamma, beta, and alpha radiation

sources.

• Isotope separator.

NBS also has a 10 megawatt research

reactor which is operated by the Reactor Ra- diation Division in the Institute for Materials Research. The Center for Radiation Re- search is an active user of this facility in part of our neutron measurement standards

programs. A view of the NBS reactor is shown

in Fig. 2.

One of our users of the reactor is in

the area of reactor dosimetry to support the

growing nuclear power industry. Fig. 3 is an artist's view of an intermediate energy stan- dard neutron field (ISNF) which has been developed within the graphite thermal column of

the reactor. A spherical cavity has been

created within the thermal column with small

fission plates on the surface. This creates

a nearly pure fission neutron spectrum within

the cavity. An inner spherical shell of B-10

moderates this neutron spectrum to a spectrum

similar to that of a breeder reactor. Instru- ments or activation foils being calibrated

can be located as shown within this inner re- gion (inside the B-10 shell).

The instrument shown at the center of

Fig. 3 is shown in more detail in Fig. 4. It is a double fission chamber developed at NBS for reactor dosimetry and reaction rate measurements. This instrument can operate

in a wide variety of environments, including

those within an operating reactor. Duplicates

of this instrument are now used in many reactor research centers throughout the world.

A second use which we make of the reactor is for the production of low-energy mono- energetic filtered neutron beams. These

beams and their use to calibrate neutron personnel monitors were discussed by Dr. Schwartz

in his paper, "Filtered Neutron Beams at the

NBS Reactor." We are now working actively

with a number of ERDA sponsored laboratories

to develop improved personnel monitoring and

calibration techniques using these beams.

404
The largest of our radiation facilities

within the Radiation Physics Building is the

150 MeV linear electron accelerator shown in

Fig. 5. This facility produces high intensity electron beams from 10-150 MeV which can

be directed into a nunber of different experimental areas. Some of the magnet assemblies

used to direct the beam to different areas

are shown in Fig. 6. This was one of the

first fully achromatic beam switching systems ever developed and has contributed greatly

to the ease with which such accelerators can

be applied to a diverse research program. I wish to discuss three of our applications of

this accelerator, primary beam monitoring

standards, calorimetric dosimetry standards

for electron beam cancer therapy, and mea- surement of standard reference neutron cross sections.

Before the advent of modern linear ac- celerators most high energy electron accelerators were of relatively low current and used

primarily for production of bremsstrahl ung

beams. Beam measurements v;ere done mostly

with ionization chambers. The NBS P-2 ionization chamber, shown in Fig. 7, was developed as a primary beam monitoring standard

for such accelerators. This NBS P-2 chamber

has been duplicated by many national standards laboratories throughout the world, and

is still the primary standard in many countries. Faraday cup monitors are used with

modern high intensity accelerators (external

electron beams), to collect beam charge.

Fig. 8 shows the NBS primary electron beam

standard. This is a Faraday cup collector

which will operate reliably up to 100 kW of

average beam power. We have also developed

non-intercepting monitors for measurement of

these high energy electron beams using ferrite transformers. Fig. 9 shows that we

know the performance of our primary standard to about 500 ppm. Only the most exacting of physics research activities need

such measurement accuracy and we now only

maintain this standard.

High energy electron beams are used ex- tensively in cancer therapy. As discussed

by several speakers during this conference,

reliable radiation dosimetry is essential for

successful cancer radiation therapy. Fig. 10

shows an experimental arrangement at the NBS linac for development of calorimetric absorbed dose standards. The large wheel shown

in the figure contains a variety of carbon

absorbers used to obtain absorbed dose mea- surements at different depths in the carbon

medium. This apparatus contains both a calorimeter which measures absorbed dose directly and an ionization chamber of nearly identical geometry. The objective in the use of

this apparatus is to measure the relative

response of the calorimeter and of the ionization chamber, and to compare this experimental ratio with theoretical calculations.

Fig. 11 shows one such comparison for 25 MeV electrons. Agreement between experiment and

theory is within one percent. We have also

developed a portable calorimeter which is suitable for field use. This instrument,

shown in Fig. 12, has been successfully used

in the field. Recently it was carried to the

Saclay laboratory and compared with the

French national standard using Co-60 radiation, with agreement to within a few tenths

of a percent.

The safe and economical development of

nuclear power plants, especially the breeder

reactor, requires a vast amount of accurate

nuclear data. Most measurements of such

nuclear data are actually measurements of the

ratio of an unknown cross section to some reference cross section. Thus the data files

which are used in reactor design can only be

as good as the reference cross sections.

There is great need to improve and verify the

accuracy of these reference cross sections.

At the present time the major use of our lin- ear accelerator is as a pulsed neutron source

Using time-of-f 1 ight (see Fig. 1) to measure

neutron energy, we have an active effort un- derway to accurately measure a set of stan- dard reference neutron cross sections, especially in the energy region of greatest inter

est to breeder reactors and fusion reactors. The energy dependence of some of these stan- dard reference neutron cross sections is shown in Fig. 13. For neutron energies below one MeV the accuracies we have achieved

at present are about 3 percent. Our target

goal is an accuracy of 1 percent.

A primary standard of neutron source strength which has been maintained at NBS for

a number of years is the Ra-Be {y ,n) neutron

source shown in Fig. 14. Both this source

and unknown 'sources which are sent to NBS for calibration are measured in a manganous

Sulfate bath. The arrangement is shown

schematically in Fig. 15. Most of the neu- trons are captured by Mn-55 to form Mn-56.

The radioactive decays from Mn-56 are detected with scintillation counters. One special source at NBS which has been calibrated in this manner is a specially encapsulated

Cf-252 source shown in Fig. 16. The postage

stamp gives a size comparison and Fig. 17 shows an x-ray of the source. The small

dark spot in the center is the Cf-252 material, very nearly a point neutron source. One

example of the use of this CF-252 source is shown in Fig. 18, in which two of the NBS double fission chambers shown in Fig. 4 are placed symmetrically around the CF-252.

Through the chain of measurements indicated
here and in the previous discussion, we have

established a direct measurement chain from

reaction rates and dosimetry within nuclear

power reactors to the primary neutron source

standards.

Within the areas of national defense,

industrial radiation processing, and con-- trolled thermonuclear research there is con- tinuing need to make radiation measurements

under conditions of extremely high radiation

intensity, and often in low pulse repetition

rate situations. Fig. 19 shows one of the

radiation sources used in developing stan- dards for such radiation applications. It

is a 2 MeV, 5000 ampere pulsed accelerator

with beam pulse length of a few nanoseconds.

Either x-rays or the direct electron beam

from the accelerator can be used. One ex- ample of how this accelerator is used is indicated on the left side of Fig. 20. When

electrons strike a target material, the breinsstrahlung production goes primarily in the

forward direction. However, the production

of K x-rays is nearly isotropic. Thus by

looking in the backward direction, beams of

nearly monoenergetic K x-rays are available

and can be used for instrument calibrations.

Using such sources we have provided calibrations for weapons testing programs for several years, and are now providing such calibrations to laboratories in the controlled

thermonuclear research program. We also use

this radiation source to investigate the rate dependence of radiochromic dye dosimeters

used in industrial processing. Fig. 21 shows the change in optical density of solutions of these dye dosimeters for different

absorbed dose levels. The dye dosimeters

developed at NBS are remarkably dose rate independent.

A last dosimetry technique which has

been developed using these high current

pulsed accelerators is indicated in Fig. 22. This figure shows the interference fringe

pattern of laser light produced when one beam burst from the accelerator passes

through a liquid cell in one arm of a two arm interferometer. The fringes are a re- sult of the radiation heating of the liquid

cell. This technique has the great advantage that the absorbed dose measurement is absolute, knowing the heat capacity of the

liquid and the change of the optical index

of refraction with temperature. It has the

further advantage that the entire depth dose

distribution is obtained with one radiation

burst, which is very important in some cases.

In more conventional radiation calibration activities, x-ray and Co-60 calibration

ranges are utilized to calibrate ionization

chamber instruments for medical radiation

monitoring applications. Fig. 23 shows a view of an x-ray calibration range, which is now almost completely automated. Fig. 24 shows a Co-60 calibration range. For x-ray

calibrations a free-air chamber is the primary reference standard. For Co-60 the ref- erence standards are graphite cavity ionization chambers. A number of these graphite

cavity chambers are shown in Fig. 25.

An active area of standards development

within CRR is that of radioactivity standards.

These standards are needed for nuclear medicine, scientific research, instrument manufacturers, and environmental monitoring. Most

of the radioactivity standards which we produce are distributed by NBS through the Standard Reference Materials program. We also

have substantial involvement in measurement

assurance testing programs relating to radioactivity measurements. Some examples of these

programs are discussed later in this report.

Fig. 26 shows a photograph of some of

the radioactivity standards which we develop.

Included are liquid and gaseous sources at different activity levels, point sources, and sources which have been developed for specific

applications. An example of this latter case

is a series of mixed radionuclide sources which simulate the type of radiation measurement problem encountered in monitoring the

radioactive effluent from nuclear power plants.

These sources are now in use by most operating

nuclear power plants in the United States for

calibration and testing of their instruments

and spectrum analysis programs.

Some examples of the apparatus used in the development of these radioactivity stan- dards is given in the next few figures. Fig. 27 is a view of the high resolution isotope

separator operated by the Radioactivity Standards Section. One example of the use of this

facility in standards development is indicated

in Fig. 28. Krypton gas is passed through the

isotope separator and imbedded in pure aluminum foil as shown in this figure. The Kr-84

component of this is cut-out and irradiated

in the NBS reactor. Through the neutron cap- ture process in Kr-84, Kr-85 is formed, re- sulting in an isotopically pure source of

Kr-85. Such sources are valuable in environmental monitoring of the Kr-85 release from

nuclear power plants. A second piece of ap- paratus useful in development of environmental standards is shown in Fig. 29. This is

a two-crystal high-efficiency, low background scintillation spectrometer. The ar- rangement of this spectrometer is shown in

Fig. 30. One great advantage of this spectrometer is that its response is nearly in406
dependent of sample size and shape. For com- plex spectra the energy resolution of this

spectrometer is not adequate and high-resolution germanium spectrometers must be utilized.

Fig. 31 illustrates such an example, the

gamma ray spectrum from Columbia River sediment. This material has been prepared as Standard Reference Material and the activity

of many of the nuclides present has been

certified.

For the more conventional radioactivity

standards more conventional apparatus is used. Fig. 32 shows some of the gas handling

apparatus used in the preparation of gaseous

radioactivity standards. Fig. 33 shows portions of a beta-gamma coincidence apparatus

used in checking the activity of standard

sources. This apparatus is highly automated

so that many standard sources can be measured

economi cal ly

Measurement Dissemination Activities

In the above discussion I have tried to give an overview of many needs for NBS stan- dards v/hich result from the diverse applications of ionizing radiation in our society.

I would like now to discuss the difficult

problem of getting the output of our work to the radiation users. For many of our activities the need is to provide standards for a specialized type of measurement to a relatively small community of users. Getting our services to these groups is not too difficult.

Where we experience our greatest problems is with large diverse user groups.

The various measurement dissemination

mechanisms which we now use are listed in Table III.

TABLE III. NBS Standards Dissemination Mechanisms

• Cal ibrations

• Standard Reference Materials

• Publications

• Measurement Traceability Testing

• Measurement Assurance Programs

The first of these, calibrations, is usu- ally only applicable to a general situation

where there is a common calibration need of

a relatively large users group. There is also

the problem that calibrations of an instrument

for a user really provides little assurance

that the user is making measurement adequate

to his needs. The various ionizing radiation

calibrations formally offered by NBS are listed in Table IV. In addition to these,

many calibrations are provided on an individTABLE IV. NBS Calibration Services in Ionizing Radiation

• Neutrons

-- Neutron source strength

-- Neutron instrument (thermal neutrons)

-- Neutron irradiation of foils

• Radioactivity

-- Gamma-ray emitters

-- Alpha particle emitters

-- Beta-ray emitters

• Radiological equipment

-- X and gamma ray equipment

-- Gamma ray sources

-- High energy electron beam, dosimetry — Thin film dosimeters (high dose)

• Many special calibrations upon request

Standard Reference Materials offered by NBS in the ionizing radiation area are radioactivity standards, and essentially all of our radioactivity standards are available in this manner. These also have a difficulty in that

purchase of SRM's provide no assurance that

sufficiently accurate measurements are actu- ally being made in the field.

The measurement dissemination mechanisms

which appear most valuable and are increasingly being employed by NBS are those of Measurement Traceability Testing and Measurement As- surance. In essence these could be called

Measurement Quality Control programs. Table

V lists a number of traceability and measurement assurance programs which have been con- ducted recently. I wish to discuss three of

these as examples.

TABLE V. NBS Measurement Assurance and

Traceability Testing Programs

Radiopharmaceutical Users (CAP)

Radiopharmaceutical Producers (AIF)

Radioactivity Reference Standards (ANSI) Dose Calibrators (CAP)

Environmental Radioactivity (Users)

Measurement Traceability Testing (NRC, EPA,

FDA)

Nuclear Fuel and Reactor Dosimetry (ILRR)

Radiation Hardness Testing (ASTM)

Personnel Monitors (Pilot Test)

Brachytherapy Source Dosimetry (Pilot Test)

Electron Therapy Dosimetry
The first example of measurement assur- ance testing is in the area of environmental

radioactivity monitoring. In this program

mixed-radionucl ide standards are offered to users by NBS. These are initially distributed to purchasers without the calibration

certificate. After the purchaser measures

the content of this standard and reports his

observations to NBS, the calibration certificate is sent to him. This type of test is extremely valuable. It allows a normal calibration standard to serve the dual purpose

of standard and mechanism for measurement

assurance testing. The results of one such test are indicated in Fig. 34.

A second example of a measurement assur- ance test is a program conducted with the

College of American Pathologists (CAP). In this case, samples of 1-125 were distributed

by NBS as unknowns to the participants. The

participants measured the sources with their

dose calibrators, identified the dose calibrator model, and reported their findings to NBS. The results of this test are shown in Fig. 35. Two significant facts are apparent

from this test. First, in no single case did

a participant's measurement come within the

accuracy limits recommended by the U.S. Pharmacopeia. Second, there is clearly a bias

of almost a factor of two in the readings of

dose calibrators from one particular manufacturer. The problem was found to be an error essentially in dial calibration, and was reported to the manufacturer.

The third example is in the area of mea- surement assurance programs for electron

therapy dosimetry. Fig. 36 shows the results

of measurement assurance testing by NBS for a 9-year period. Chemical dosimeters are distributed to participants by NBS. The participant irradiates the dosimeter in a phantom

according to a prescribed protocol, sends the

irradiated dosimeter back to NBS with his measure of the absorbed dose given to the dosimeter. NBS in turn sends back to the participants the actual dose indicated by our reading of the dosimeter, the distribution of performance by all participants, and suggestion

that corrective action be taken if appropriate.

The results of this measurement assurance program indicate that a substantial fraction of

participants lie outside of the accuracy limits normally considered desirable for radiation therapy, and furthermore the situation

has not appreciably improved during the nine

years that the program has been in operation.

Clearly many of the participants do not use

the information available to them from this

measurement assurance program.

Problems of the National Measurement

System for Ionizing Radiation

The examples given above of the results

from NBS measurement assurance testing programs clearly indicate the great value of

such programs in evaluating radiation measure

ment performance at the user level, and the

value that such programs can have in identifying problems. These programs unfortunately

clearly reveal that many users are not making

measurements at an accuracy which is considered generally desirable. It is also clear,

and best illustrated in Fig. 36, that users often do not take corrective measures even when a measurement assurance test clearly

reveals the need for corrective action.

In attempting to understand this situation we have concluded that there are a number of specific problems which exist within

the national measurement system for ionizing

radiation. These are the following:

• Lack of user recognition of need.

• Need to document accuracy goals and procedures . • Lack of adequate national measurement dissemination system.

• Lack of follow-up and corrective action.

• Some standards do not exist.

• Inadequate resources at all levels.

We believe that the challenge to the

national measurement system is clear. It is to achieve adequate and assured radiation

measurement accuracy at the user level. I believe that the evidence presented in this

paper and in other papers of this conference

clearly indicate that in some areas this goal

is not being achieved at the present time. A number of corrective actions which we believe

would help this situation are the following:

• Develop concensus documents of accuracy

needs

• Organize national measurement assurance pro grams with follow-up procedures, including

substantially complete user participation.

• Establish educational programs where

appropriate.

• Develop missing standards for the cases where they do not exist.

• Recognize adequate measurement accuracy

as a legitimate business cost.

Above all else, it is necessary for us

in NBS and the user community to recognize

that we each have a role to play in making

the national measurement system work effectively, and for each of us to effectively

carry out our appropriate role.

408
Fig. 1 Aerial view of NBS Radiation Physics

building. The long horizontal pipes (up to 200 meters) are evacuated neutron flight

paths for neutron standard cross section

measurements.

Fig. 2 View of NBS 10-megawatt reactor.

Fig. 3 Artist's view of Intermediate-Energy

Standard Neutron Field (ISNF). This calculable reference field matches the typical

spectrum within a breeder reactor.

Fig. 4 NBS double fission chamber. This

standard instrument is now in use in reactor

research laboratories throughout the world.

Fig. 5 View of NBS 150-MeV linear electron

accelerator.

Fig. 6 Beam handling system which deflects

the NBS linac beam into different experimental areas.

409
Fig. 9 Indication of accuracy of NBS Faraday cup. Independent calibration is provided

by an absolute non-intercepting current trans- former monitor. The correction indicated is largely due to backscattered electrons from

the Faraday cup.

Fig. 7 NBS P-2 ionization chamber, a primary standard instrument for measurement of

^ov^ intensity high-energy bremsstrahl ung beams.

Fig. 8 NBS primary standard Faraday cup for

measurement of high-energy electron beams.

Fig. 10 Apparatus for development of absorbed dose standards for electron therapy.

Apparatus is located in one of the electron

linac measurement rooms.

410
Fig. 14 NBS primary standard neutron source

(NBS-1 ). This Ra-Be (y.n) source is the U.S. primary standard.

MANGANOUS SULFATE BATH SOURCE CALIBRATION Fig, n Ratio of absorbed dose as calorimetrically measured to specific ionization

observed in an ionization chamber. The

theoretical curve was calculated by Berger

and Seltzer.

i[iim

Feb. 15 Schematic illustration of the physical processes which must be considered in the manganous sulfate bath technique for neutron source calibration. Although capable of

absolute source calibration, it is usually

used to obtain the ratio of the strength of

an unknown source to a standard source.

Fig. 12 Portable calorimeter for absorbed

dose measurements in field applications.

NEUTRON ENERGY, MlV

Fig. 13 Energy dependence of several standard reference neutron cross sections. Most

neutron data used in nuclear power development is measured relative to such reference

cross sections.

411
F1g. 16 View of NBS specially encapsulated

Cf-252 neutron source. The postage stamp

gives an indication of the actual size of

this source. The strength of this source

has been measured relative to NBS-1 using

the manganous sulfate bath technique.

Fig. 17 X-ray of NBS Cf-252 neutron source. The dark spot in the center is the actual

neutron emitting material, and closely ap- proximates a point source.

Fig. 18 Arrangement of NBS double fission

chambers with Cf-252 neutron source for

determination of absolute fission chamber

response to Cf-252 neutrons.

Fig. 19 View of 2-MeV flash x-ray machine.

This accelerator is capable of 5000 ampere

current with few nanosecond pulses. The ap- paratus on the left is used for calibration

of detector response to low energy K x-rays.

412
PRODUCTION OF

HONOCHROIIATIC PHOTONS

BY nxcraoM EzorATiON P06ITBON ANNIHIUTiaN OFKXUTS

y

INFU6I1T

ss. C -02kmV 2

Al - L2k»V - ae k>v

Fig. 20 Arrangements used by NBS to produce

nonochromatic photon beams. At the left, K x-ray beams largely free of bremsstrahl ung are produced by viev/ing targets at large

angles. At the right, positrons produced

in the NBS linac are annihilated in flight to produce nearly monochromatic beams in the

forv;ard direction.

Fig. 22 Interference fringes produced in laser light by radiation dose heating of

liquid in one arm of a double arm interferometer. The electron beam enters from the

top. Knowing the change of optical path

length with temperature and the heat capacity of the liquid, absolute dosimetry is achieved by counting interference fringes.

K/LORADS 0 5 (0 15 25 50 75 100

Fig. 21 Response of radiochromic dye dosimeters to different radiation doses. These

dosimeters have been shown to be nearly dose

rate independent over a wide range of dose

rates

413
Fig. 24 View of Co-60 calibration range. A radiation survey instrument is being mounted

for calibration.

Fig. 26 Examples of several radioactivity

standards offered by NBS as Standard Refer- ence Materials.

Fig. 27 View of NBS isotope separator used

in preparation of some radioactivity stan- dards .

Fig. 25 Graphite cavity ionization chambers

for Co-60 radiation. The national standard

is the average response of several of these

instruments.

80 82 83 84 86

Fig. 28 lon-tmplanted krypton gas in aluminum foil. The Kr-84 portion under neutron

irradiation undergoes neutron capture and

becomes Kr-85, an important gaseous effluent

from nuclear power plants.

414
Fig. 29 View of two-crystal spectrometer

for development of environmental radioactivity standards.

Fig. 30 Illustration of detector, sample

and shield arrangement of two-crystal spec- trometer.

-o^l

Fig. 32 Gas handling and counter system

used in preparation of gaseous radioactivity

standards. Included in this arrangement are counters of several different lengths to allow correction for counter end effects.

Fig. 33 Automatic" sample changer and betagamma coincidence counting apparatus for

measurement of "point-source" radioactivity

standards.

Fig. 31 Gamma ray spectrum of Columbia

River sediment obtained with a high resolution gernamium detector. This material is now offered by NBS as a Standard Reference

Material

.

415
1 r—! r-71 f 1 ^

B 10 2 x/Nas

Fig. 34 Results of measurement assurance

test of user ability to measure mixedradionucl ide solutions distributed by NBS.

PEHFOKWiNCE OF ALL P4P T ICI PANT S IN EACH OF 9 YEARS Of SERVli

Fig. 36 Results of measurement assurance

testing of dosimetry for electron beam therapy over a nine-year period. The lack of

improvement at the user level during this

period is striking.

IODINE-125, OCT.. 1972, DOSE CALIBRATOR RESULTS NBS VALUE: 7 B.IO /j.Ci ± 1.3g% 1200 EST. OCT. 14

i*-

N M

1

1 1

U

iccep

S PH

rABLE ARM&RANG

OPOE

E

1 Al —

C

2 S 299 0

L

E

A

30 50 70 90 110 MICROCURIES

130 150 170 190

Fig. 35 Results of NBS-CAP measurement as- surance test of dose-calibrators in field

environment to measure 1-125 samples distributed by NBS. Dose calibrators of type N exhibit a clear bias (later confirmed).

416
>I3S SF455 (1976)

liOI^-IOIJIZII^'C- ELECTROI-IAGITETIC RADIATIOII STAIIDAEDIZATIOW

R. C. Baird and A. A. Sanders

National Bureau of Standards

Boulder, Colorado 80302

Calorime usrs have "been developed "by iJBS as accurate (± 1%) standards

for calibra-cing laser power and energy measuring devices. A pilot

measurement assurance program (MAP) is underway, with 12 industrial

organizations participating. The anticipated accuracy is ± 3%- A

formal I-LAP service will be announced ahout July 1976. Accurate

standards for rf and microwave fields are not as well developed hecause of the inherently greater difficulty in measuring these fields

caused by diffraction, interference, reactive fields, modulation, etc.

iraS has developed accurate measurement and calibration techniques which

will be fully implemented about August 1976 when a new anechoic

( reflection-free ) chanber will be completed. ?JBS is also developing

stable transfer standards suitable for measurement assurance programs

,

and a pilot ViAP with -oncertainties of about ± 0.5 dB (± 12%) is anticipated during FY 1977.

(Non-ionizing E2-! radiation hazards; laser calorimeters; RF and micro- wave measurements; calibrations; standards; measurement assurance programs)

Requirements to compare and correlate

measurements made by different laboratories

,

to settle disputes arising in the market

place, to determine biological effects of

radiation, and to assure the safety of

workers and the public exposed to non- ionizing radiation have produced an increasing demand by a number of laboratories

and organizations for measurement traceability to national standards . This paper

reviews the present status of standards for

laser, microwave, and rf frequencies, giving

ranges of applicability and associated

meas-orement uncertainties. A brief review

of how one can establish traceability to these national standards through a measurement assurance program (LlAP) is included.

In the case of microwave and rf measurements, some discussion of the difficult

measurement problems that have impeded the

development of adequate standards is also

presented.

Laser Standards

Description of IJBS Standards

The KBS standards for power and energy

measurements are the C, K, and Q series

calorimeters.^"^ Their essential features

include a constant temperature jacket sur- rounding a cavity which absorbs an incident

laser beam. The temperature rise due to the

absorbed optical radiation is measured and

compared to the quantity of electrical

energy causing the same temperature rise.

The calorimeter is thus a comparative device

which compares -the energy of the optical

radiation from a laser with an equivalent

quantity of electrical energy.

The approximate ranges and accuracies

of these calorimeters are shown in Figures

1, 2, and 3. Each figure shows the laser

power (or energy) range of the calorimeter.

For example. Figure 1 demonstrates that the

C-series calorimeter is useful, for the

nominal accuracy shown, for energies in the

range of 0.1 joule to 22 joules. Experience

has demonstrated that 300 seconds is the

maximum laser (or electrical) power injection time compatible with reasonable precision. Thus, since 0.1 joule is the minimum

amoiint of energy that can be accurately

measured, this sets a lower power limit of

0.3 milliwatts. The 300 second injection

time also determines the left sla.nted boundary. The response of the calorimeter becomes non-linear for energies above the

upper energy boundary, and the boundary on the right is the maximum power that can be

produced by the electrical heaters used in

the calibration. Laser powers in excess of

this boiindary may also damage the calorimeter

Figure k depicts the optical arrangement of a measTirement system formed from

these calorimeters (NBS maintains two or more of each type of calorimeter). A laser

beam is incident upon the beam splitter

which both reflects and transmits portions

of the beam. The action of the beam splitter

causes the incident beam to be dispersed

into two or more beams; one of these dispersed beams is incident on each calorimeter. Since the power in the transmitted

beam is usually larger than that in the

reflected beam, we normally refer to them
ELECTRICAL

CALIBRATION LIMIT

0.3 X 10 10 X 10"^ 0.1 2.0

POWER RANGE ( WATTS

)

FIGURE 1. The enclosed, region defines the energy -power range for the C calorimeter. The nominal uncertainty is one percent.

3.0

1.5

0.3

300 SECOND

'LIMIT LINE

10 100

POWER RANGE ( WATTS

)

LIMIT ON HEATER

WIRE LOAD

500 1000

FIGURE 2. The enclosed region defines the energy-power range for the K

calorimeter. The nominal uncertainty is four percent.

10

FIGURE 3,

300 SEC

LIMIT LINE

MAX. ENERGY FOR SINGLE PULSE MEASUREMEI T

_L

1 X 10 .20 POWER RANGE (WATTS

)

1 X 10

The' enclosed region defines the energy -power range for the

calorimeter. The nominal uncertainty is two percent.

418
NBS LASER MEASUREMENT SYSTEM

3 5m

laSER

CALORIMETER lOW lEVEl BEAM)

Figure k. Diagram of NBS calorimeter

system used for calibrating laser power and

energy m.eters

respectively as the high and low level beams

The actual ratio of power in the two beams

is referred to as the beam splitter ratio

and can be measured. Once this ratio is

known, an unknown power or energy meter can be substituted for one of the calorimeters

The response of the remaining calorimeter

and knowledge of the beam splitter ratio,

permits one to determine the amount of

energy incident on the \inknown instrument

and hence its calibration constant. This

beam splitter meas\irement technique helps

prevent imprecision in the measurement

caused by instabilities of laser radiated

power

The calorimeters are strictly energy

measurement devices and are therefore insensitive, within reasonable limits, to the

temporal characteristics of the laser.

However, they can be utilized in certain

circumstances to calibrate power meters.

Consider, for example, the case where the

power in the laser radiation is nominally

constant {O/J) . By means of a timed shutter,

the actual time that the laser radiation is

incident upon the calorimeter can be determined. A measurement of the total energy

and knowledge of this time interval allows

one to determine the average power incident

on the calorimeter. Since the beam splitter

ratio is known, the power incident on the

unknown power meter is also known. Thus

calibration of CW power meters can be ac- complished. Other power measurements are possible to the extent that the temporal

characteristics of the laser are known, but

the detailed measurement of laser power is

still an important area of research.

The inaccuracy of these calorimeter

measurement systems is comprised of systematic uncertainties and imprecision. Systematic errors are composed principally of

the uncertainties in the electrical energy

to which the optical radiation is compared,

uncertainties in the percentage of radiation

absorbed by the calorimeter, and any inequivalence in the response of the calorimeter to electrical versus optical energy.

These uncertainties have been carefully

studied and bounded. Errors due to imprecision are determined through a careful and

thorough internal quality assurance program.

Laser Measurement Dissemination Program

Considerable effort is being expended

to disseminate measurement services and

traceability to the national standards to other laboratories. In some cases copies of

the calorimeter have been built for other

government laboratories, e.g., Newark Air

Force Station, the Bureau of Radiological

Health, and Fort Belvoir. By means of

measurement assurance programs (MAPS) with

NBS, these laboratories can establish es- sentially the same measurement capability as roS. The Air Force laser calibration laboratory at Newark Air Force Station is cur- rently duplicating most of the laser power

and energy measurement systems maintained by

NBS. They will then be able to extend

calibration and traceability to other DoD

laboratories and DoD contractors.

However, duplicating and maintaining

these calorimeter measurement systems is an expensive endeavor and not practical for

most laboratories. For these laboratories

we are offering some limited calibration

services and are establishing measurement

assurance programs. We feel that reliable

measurement assurance programs have the

greatest potential for providing measurement

traceability in an efficient and cost effective manner.

Laser Measurement Assurance Programs

The measurement assurance program (MAP)

is one of the primary vehicles used by NBS

to provide measurement services to other

laboratories. Within this concept, measurements are viewed as the product of a com- plete measurement system. Hence, the total

meas\irement is evaluated including the

measurement environment, operator, data

reduction, laboratory, standards, and anything else that may influence the measurement.

419
In practice a laser MAP might work as follows. A participant in the laser MAP will establish his own measurement system

and req^uired. level of accuracy. He would

then establish an internal program for

documenting the precision of his system (NBS

guidance and suggestions will be available

for each of these tasks). This might, for

example, consist of intercomparing two or three different power or energy meters and

maintaining records and control charts.

Instead of having devices calibrated, the

participant would occasionally be sent a transfer standard by NBS. (A transfer

standard is simply a stable optical detector

or power meter that has been extensively

evaluated with respect to its stability,

accuracy, and suitability for use as a standard in intercomparisons . ) The calibration constant of this transfer standard

would not be known to the participant in

advance. He would calibrate the transfer

standard by his measurement system and

forward the results and data to NBS. NBS

would analyze the data and forward a formal

report of the precision and accuracy to the

participant. Any systematic error would

have been determined and appropriate corrections could be made. Transfer standards

would be forwarded on some periodic basis

(or more often if required). The level of

accuracy is determined primarily by the user. The results of the inter comparison

and the participant's own documentation of

his measurement system furnish detailed information on the system measurement accuracy

and provide traceability to the national

standards maintained by NBS. The user may now use his measurement system to extend

calibration to other devices or establish

internal MAP's of his own.

NBS now has underway a pilot laser MAP involving twelve laboratories. This pilot

program is restricted to the measurement of

approximately 1 milliwatt of HeNe (632.8 nm)

laser radiation. A formalized laser MAP for

this frequency and power level will be

available approximately July 1976. The cost of the service to a participant will be

approximately $1500.00 per year.

An extensive evaluation of potential

transfer standards is being conducted by NBS

for these MAP's. The total uncertainties of

the transfer standards for the 1 mW MAP is

about 3-5^. By means of this MAP a participant should be able to dociiment measurement

accuracies of 5-10^ at a nominal cost. Transfer standards for other wavelengths,

energies , and powers are currently being

evaluated and additional laser MAP's will be

instituted as needs are established.

RF and Microwave Standards

Electromagnetic hazards (EMH) and

electromagnetic interference (EMI) are problem areas of increasing importance and concern due to expanding use of electromagnetic (em) radiation for communications and

industrial purposes, and the growing use of

electronic equipment that may be affected by

EM radiation. These areas require accurate

measurements of EM fields for adequate

monitoring and control of undesired radiation, and for determining biological effects

and the susceptibility of sensitive electronic devices to EM fields. It is worth

pointing out that very real (though indirect)

hazards can result from the effects of EM

fields on certain electric and electronic

devices —for example, the unplanned detonation of electro-explosive devices and the

failure of automatic braking systems on vehicles. The development of adequate

measurement instruments and techniques

,

standards, and calibration methods has been

painfully slow in both the EMH and EMI areas

due to several severe measurement problems.

Although the following discussion of these

problems is oriented toward EMH, it is also

generally applicable to EMI.

Measurement Problems at RF and Microwave

Frequencies

With lasers, one is usually dealing

with a well defined beam of reasonably small

cross section. The total energy or power in

the beam can be measured by the techniques

previously described, and the power density

(W/m^) can be inferred from the size of the

beam. In contrast, rf or microwave energy

is usually distributed throughout large

volumes of space so the best one can do is to measure the pertinent field parameters

in a localized region of the field. Perhaps

as a carryover from the practice in laser

and ionizing radiation safety, workers in rf

and microwave radiation safety also began to characterize field levels in terms of power

density. However, it is now recognized that

power density is not always a good parameter

to use because: (l) it is not measured

directly, but is derived from measurements

of electric and/or magnetic field strength;

(2) it does not always provide a good indication of the hazard and (3) it cannot be

accurately measured or calculated in many

hazardous situations. Since hazards are usually most closely related to the electric

field E (vectors are indicated by underbars ) , we feel that measurements of |E_p or the electric energy density U„ are most

E

appropriate. (There is some evidence that
iHp or U-j may te important at the lower

frequencies.'*"^) Measurements of E would be

equally good, but it is much easier to measure |E^p. Probes based on resistive

heating, and composite probes which indicate

the s\im of the squares of three orthogonal

components of are now available for measiiring |e_

choosing

directly. The rationale for

will become more apparent from

the following discussion.

Under far-field plane-wave conditions

any one of a number of parameters is a valid

index of the field. These parameters include electric field strength |e | , magnetic

field strength |h_| , time average power

density |S_| , electric energy density U

E > magnetic energy density U„, and the total

11 energy density U. If any one of these

parameters is known, all of the others can be obtained from relatively simple rigorous

relationships . Under these conditions power

density is a suitable parameter. Unfortunately, both Elffi and EMI tend to occur in

near-field situations where the EM field

configurations may be very complicated,

where the simple plane-wave relationships

among the various field parameters no longer

hold, and where conventional field intensity

meters are totally inadequate for measiirements. The situation is further complicated

by the wide frequency range (> 10^°) involved. Wavelengths extending from thousands

of kilometers to millimeters are common, and

different measurement approaches are re- quired for different portions of the spectrum. The principal complexities and

problem areas that need to be coped with are the following. (For a more detailed discussion see reference 7-)

Diffraction and Interference . Since

the wavelengths associated with rf and

microwave radiation are not, in general,

short compared to the dimensions of the

objects or subjects involved, diffraction

and interference effects can cause significant spatial variations in field intensity. These variations show up as standing

waves, hot spots, dead spots, etc., with the

distance between maxima (minima) being the

order of a wavelength. Interference occurs when multiple or extended sources are involved, and where there is scattering from

the ground, walls, or other objects. This

condition is the rule rather than the exception for most hazardous situations, and is a good example of a case where power density

is not an appropriate measiire of the field.

If we consider two waves of equal amplitude

and frequency, travelling in opposite directions, the power density is exactly zero. However, the fields (and therefore the

hazard) can be large where the resulting

standing wave is a maximum. The foregoing

implies that , at least for wavelengths

larger than the diameter of the human eye

,

the field intensity at the maxima of

standing wave patterns should be used as the

hazard indicator. Thus, for frequencies up

to about 10 GHz, a good hazard meter should

employ a field sensor that is smaller than

the shortest wavelength of interest.

Polarization . In the near field and in

the presence of multipath interference, the

polarization can be arbitrary, is probably

not known in advance, and may change radically from point to point. It is not practicable to determine power density in a general field of this type since one would

need to measure the equivalent of the amplitudes and relative phases of all six E and H field components —a very difficult measurement task. Further, if directional probes

or probes that respond to a single component

of E^ or H_ are used, the measurement procedures for finding the maximum response are tedious and time consuming, and one is never really sure that the probe has been

oriented for maximum response.

Reactive Mear-field Components . Within

distances of approximately one wavelength

from some sources and antennas , strong

reactive fields can exist which contribute

to the hazard but not to the time-average

energy flow (power density). Such fields

are more common at the lower frequencies,

but can be important up to about 1 GHz.

Interactions With the Field Being

Measured . Several types of interactions

occur which can cause serious errors in EM field measurements. For instance, a measuring instrument or object close to a source, particularly in a reactive zone, can interact with the source and alter the

amount of energy emitted. Even at large

distances where the source is not affected,

a dielectric or conductive body can strongly

affect the field in the immediate vicinity

and perturb the measurement , Thus , the

metallic transmission lines used with early

model hazard meters have been observed to produce variations in field intensity of 10 to 20 dB. A good probe should, therefore,

be designed to produce very little field

perturbation. The close proximity of an operator can also cause significant errors

,

even with the best of probes

.

421
Complicated Modulation of the Field . Fields may vary drastically with time,

depending on the type of modulation present,

and this can affect the potential hazard.

For example, with pulsed fields some effects

may he functions of peak field strength

instead of the average; some effects may

even depend on the pulse shape and repetition rate. Meters with fast response times,

capable of measuring both peak and average

fields are needed but are difficult to design and produce.

Summary . In the complicated fields

associated with EMH and EMI, measurements

may vary with time, position, frequency,

polarization, modulation, probe orientation,

etc., and the simple plane-wave relationships between parameters are not valid. A

good general purpose instrument capable of

accurately measuring such fields should,

therefore, possess most of the following

characteristics which we believe to be

essential or highly desirable. (l) The

instrument should measure E or U^, and

ii possibly H

2 or U„, since measurements of

11 and H are often too difficult and time

consuming. (2) The probe should be small

enough to resolve significant spatial variations in the field. (3) For most purposes

the probe response should be isotropic,

i.e., independent of the orientation of the

probe in the field. {h) An ideal instrument

should also be capable of measuring both

average and peak field levels. Useful

probes that satisfy the first three conditions are now available,^ but there is still

a need for probes with faster response times

for peak measurements.

Free-space Standard Field Method . The

objective of this method is to establish a known plane-wave calibrating field in free

space by means of the basic experimental

arrangement shown in Figure 5 . The power

density |s_| at a point on the transmitting

axis at a distance d from the transmitting

antenna is given by

E =

i+TTd^ (1)

where is the net power delivered to the

transmitting antenna, and G is the effective gain of the transmitting antenna. The

gain is normally determined in advance, and

and d are measured as part of the regular

calibration procedure. The rms electric

field strength |e| can be obtained from the

relation |e| = sfEp, where e is the permittivity of the medium.

REFLECTED POWER METER

COAX

TO WG

ADAPTOR

UNDER TEST

INCIDENT POWER

METER

FIGURE 5. The basic experimental arrangement for the free-space standard field

method of calibration.

Standards and Calibrations

Existing calibration methods are based

on the premise that a known field intensity

can be established through measurement,

calculation, or a combination of both. The

device to be calibrated is placed in the

"standard" field and the meter indication is compared with the known field value . There

are three basic methods of producing the

"standard" calibrating field: (l) the freespace standard field method, (2) guided wave methods and, (3) the standard probe or "transfer standard" method. The choice of

technique will depend on such things as the

type and size of probe, frequency range,

available facilities and equipment , and the

accuracy requirements. The basic concepts

of each method are presented below; see reference 9 fo^ more details.

The principal sources of error in this

method are multipath interference and uncertainties in the gain determination. Multipath effects are often overlooked, but every

calibrating facility will have some scat- tering associated with it which may cause

the field in the calibrating region to be

significantly different from that predicted

by equation 1. Even high quality anechoic

(reflectionless ) chambers are not perfect

and should be carefully evaluated if the

best accuracy is desired. Reference 10

contains a useful discussion of this problem.

It is relatively easy to obtain accurate

(± 2%) gain values for large distances;^

in fact, the far-field gain of some standard

horns can be calculated to about ± T percent. However, large distances require

greater transmitter power and the multipath

situation is worse. On the other hand,

there are some fundamental difficulties

422

I
associated with accurate gain determinations

at short distances.^ These problems are gradually "being overcome and it seems possible to reduce the total maximum uncer- tainty of this technique to less than

± 0.5 dB (10^) for relatively close

distances and moderate transmitter powers.

Guided-Wave Methods . Several types of

guided-wave structures have been considered

for generating known fields for calibration

purposes. These include waveguides, parallel plane lines, parallel wire lines, and

TEM cells. Only waveguides and TEM cells

will be considered here because they are fully shielded systems that do not radiate

into the surroiinding environment and are, therefore, much preferred. The main advantage of a guided-wave system is that con- siderably less power and space are required.

One disadvantage is that the maximum trans- verse dimensions of the structure must be

less than X/2 at the highest frequency in

order to avoid higher order modes which

result in more complicated field distributions. Hence, the method is only useful for

frequencies below 1 or 2 GHz , since the

device being calibrated must be small

compared to the guide dimensions.

the calibration. The probe to be calibrated

is inserted into the waveguide through a hole in either the top or side wall and

positioned in the center of the guide where

the field is most uniform. The access holes

should be as small as possible. Equations

for calculating the field distribution from

P (the net power delivered to the section)

nand the guide dimensions can be found in any

book on waveguide theory. It is difficult

to estimate the total uncertainty of this

method because the field intensity will be

modified by the size and nature of the probe

being calibrated. Woods describes a system which operates from hOO to 6OO MHz with an estimated uncertainty in the field

intensity of ± 12%, and Asian slaims an uncertainty of approximately ± 5^ in WRl+30 guide (l.T-2.6 GHz).

The basic TEM Cell is illustrated in

Figure T . It consists of a section of twoconductor transmission line operating in the

transverse electromagnetic mode (TEM), hence

the name. The main body of the cell consists of a rectangular outer conductor and a flat center conductor located midway between

the top and bottom walls. The dimensions of

Figure 6 shows how a section of rec- tangular waveguide can be used for calibrations. A reflectionless load is con- nected to the output end to prevent standing

waves which would cause serious errors in

Calibration Plane

Probe

Propagated Wave

FIGURE 6. A section of rectangular waveguide modified for use as part of a probe

calibration system.

CROSS SECTION

I^L/2 -4 L = *^-L/2

Coaxial

Input

Coaxial

Output

SIDE VIEW

FIGURE T. A type of TEM transmission cell

used at the NBS for calibrating hazard

meters and for measuring susceptibility of

electronic equipment to EM fields. These

cells can also be used as exposure chambers

for studying biological effects.

423
the main section and the tapered ends of the

cell are usually chosen to provide a 50-ohin

impedance along the entire length of the

cell. Hence, when the cell is carefully

made, terminated in a reflectionless load,

and connected to a 50-ohin coaxial cable, the

input VSWR is usually less than 1.05 and

always less than 1.1 for all frequencies

helow the cutoff limit. The principle

advantages of the cell (compared to a waveguide) are: (1) the field intensity in the

center of the cell midway between the top

wall and center conductor can be made uniform over a much larger region, and (2) the wave impedance of the TEM field is nominally

377 ohms, the same as a plane wave in free

space. These features are the principal

reasons for using TEM cells for

calibrations . ^ ^

The cells can be made in various sizes

to suit particular needs and cover certain

frequency ranges. However, since the width

(W in Fig. 7) must be less than X/2 to avoid multimoding, the upper useful frequency of the devices is probably about

500 MHz. As in a waveguide, the introduction of a probe into the test region will

change the field distribution in the vicinity of the probe, but if the probe is

much smaller than the transverse cell dimensions, this error will be small. Fully

evaluated cells should be capable of about

the same accuracy as waveguides (5^-10^),

and are expected to play an important role

in EMH and EMI measurements.

Transfer Standard Method . This method

may be the simplest and best method of

calibrating hazard meters for general field

use. The idea is to have a stable and

reliable probe that has been accurately

calibrated (by one of the previous techniques) and use it as a "transfer standard."

That is, the standard probe is used to measure the field intensity at a particular

point in space (or in a guided system) produced by an arbitrary transmitter. The

probe to be calibrated is then placed in the same location in the field and the meter

reading compared with the known value of the

field. The only requirements on the transmitter are that it generate a field which

has the desired magnitude, is constant in

time, and is sufficiently uniform over the

calibrating region. Accuracies of about

± 0.5 dB (12^) can probably be achieved with

this method. The biggest advantages of this

approach are convenience, reliability, and

simplicity. The ideal probe for use as a transfer standard should be stable, rugged,

and not easily biirned out; it should have a

large dynamic range, cover a broad frequency

range, and possess an isotropic response

pattern. Suitable probes are becoming

available.^ Organizations that cannot

justify the construction and maintenance of

a calibration facility could have a transfer

standard calibrated by NBS or some other

reputable laboratory and make measurements

that would be satisfactory for most purposes.

Practical Measurement Accuracy . It is important to understand that one cannot

expect to achieve the full calibration

accuracy when using meters for practical

measurement applications. Some of the reasons are as follows. (a) Hazard meters

are usually calibrated in nominally planewave fields. Such fields are seldom encoiintered in practice, and the sensor may

not respond in the same way to non-planar

fields. (b) In most calibration methods,

only the sensor (probe) is exposed to the

field, while in practice the complete

system, including the indicating unit and

connecting cable, is immersed in the field,

(c) With hand-held meters, the presence of

the operator can significantly affect the

reading. The additional uncertainity caused

by these factors is difficult to assess and

will vary with the type of meter. However,

if good measurement procedures are followed,

accuracies of ± 1 or 2 dB (10-50^) can probably be achieved under typical measurement conditions.

Measurement Assurance Programs at HBS

The Electromagnetics Division of NBS

(Boulder) will be in a position to offer

map's in rf and microwave measurements in

about the fall of 1976. During recent years

KBS has concentrated on developing the

required calibration systems and techniques

and instruments for measiiring EM fields

since adequate instrxmientations and measirrement techniques did not exist. By the

simaner of 1976 we will have an anechoic

(reflectionless) chamber, adequate for free

space calibrations down to about 500 MHz. We plan to employ a series of TEM cells to extend the calibration capabilities down to very low frequencies, so we should be in a position to calibrate instruments and conduct map's in any frequency range of interest.

There is still a need for probes with

better characteristics for use as transfer

standards, but progress in this direction is being made, both commercially and at KBS. The NBS effort is based on the 3-axis dipolediode probe described by Bowman.^ We believe it is possible to make such a probe

424
with, an improved temperature coefficient and

with sufficient stability, sensitivity,

dynamic range, and ruggedness to qualify as an excellent transfer standard. The units

employ very small sensors and high resistance lines that do not unduly pert'ur'b the

fields being measured. In any event,

existing probes coiild "be used for interim

f^IAP's and the overall accuracy could "be improved later on as better transfer

standards become available.

References

1. E.D. West and K.L. Churney, "Theory of

isoperibol calorimetry for laser power

and energy measurements," J. of Applied

Physics, i+l, 2705-2712, May 1970.

2. E.D. West, W.E. Case, A.L. Rasmussen,

and L.B. Schmidt, "A reference calorimeter for laser energy measurements,"

J. of Research of the National Bureau

of Standards-A. Physics and Chemistry,

76A , No. 1, 13-26, January-February,

1972.

3. D.L. Franzen, "An absolute reference

calorimeter for measuring high power

laser pulses," manuscript in preparation. National Bureau of Standards,

Boulder, Colorado 80302.

k. C.C. Johnson, C.H. Durney, and H. Massoudi, "Long wavelength electromagnetic power absorption in prolate

spheroidal models of man and animals ,

IEEE Trans, on Microwave Theory and

Techniques, I4TT-23 , 739-7^7, September

1975.

5. CP. Gandhi, "Frequency and orientation

effects on whole animal absorption of

electromagnetic waves," IEEE Trans, on Biomedical Engineering, BI'lE-22 , 536-

5^2, November 1975-

6. A.W. Guy, M.D. Webb, and C.C. Sorensen,

"Determination of power absorption in man exposed to high frequency electromagnetic fields by thermographic measurements on scale models," submitted

for publication in IEEE. Trans, on Biomedical Engineering, April 1975-

7. P.F. Wacker and R.R. Bowman, "Quantifying hazardous electromagnetic fields

Scientific basis and practical con- siderations," IEEE Trans, on Microwave

Theory and Techniques, MTT-I9 , 178-

187, February 1971.

8. R.R. Bowman, "Some recent developments

in the characterization and measurement

of hazardous electromagnetic fields,"

Proceedings of International Symposium

on Biologic Effects and Health Hazards

of Microwave Radiation, 217-227, Warsaw,

October 1973. (Copies are available

from the author at the National Bureau

of Standards, Boulder, Colorado 80302.)

9. R.C. Baird, "Methods of calibrating

microwave hazard meters," Proceedings

of International Symposium on Biologic

Effects and Health Hazards of Microwave

Radiation, 228-235, Warsaw, October

1973. (Copies are available from the

author at the National Bureau of Standards, Boulder, Colorado 80302.)

10. R.R. Bowman, "Field strength above

1 GHz : Measurement procedures for

standard antennas," Proc. IEEE, 55., 981-990, June 1967.

11. A.C. Newell, R.C. Baird, and P.F. Wacker, "Accurate measurement of

antenna gain and polarization at reduced distances by an extrapolation

technique," IEEE Trans, on Antennas and

Propagation, AP-21 , U18-U31, July 1973.

12. E.V. Jull, "Errors in the predicted

gain of pyramidal horns," IEEE Trans,

on Antennas and Propagation , AP-21 , 25-31, January 1973.

13. M.L. Crawford, "Generation of standard

EM fields using TEM transmission cells,"

IEEE Trans, on Electromagnetic Compatability, EMC-I6 , 189-I95 , November

197i+.

ih. D. Woods, "Standard intensity electromagnetic field installation for calibration of radiation hazards monitors

from kOO MHz to hO GHz," Non-ionizing

Radiation, 1, 9-17, June I969

15. E. Asian, "Simplify leakage probe

calibration," Microwaves, lk_, 52-57,

December 1975-

425

PANEL DISCUSSION

E. Dale Trout, Oregon State Urn" versity--Moderator

G. S. Parker, Massachusetts Department of Public Health

J. R. Cameron, University of Wisconsin

W. A. Mills, U.S. Environmental Protection Agency

R. S. Caswell, National Bureau of Standards

Dr. Trout: The Panel consists of the four Chairmen of the Invited Paper Sessions--

each comes from a different area of interest. Each represents a consumer area for measurement. There is Gerald Parker who represents

the State's Regulatory Program, John Cameron from Medical Physics,

Bill Mills from a Federal Regulatory Group and Randy Caswell from the source of the standards which we use. To be sure that we didn't waste

a lot of time I, with Jim's assistance, put some questions to these

fellows which we asked them to consider during the course of the

Symposi um.

1. Did the Symposium contribute to the stated objective?

That is, did it draw attention to the rapidly increasing

need for high quality measurements of ionizing and

non-ionizing radiation throughout the country?

2. In the session you chaired what were the major contributions

to the objective of the Symposium?

3. Is the current measurement system working in your area of

interest and if not, how can it be improved?

4. Should there be further symposia sponsored by NBS and other

sponsoring organizations? If so, how often? What is your

recommendation for a title and an objective for the next symposi um?

We will first of all give each of the four Chairmen five minutes in

which to speak to the four propositions we put in front of them; then

ten minutes for them to kick these things back and forth among

themselves. Now to take them in order of their sessions .. .Gerry, tell

us about it.

Mr. Parker: Thanks Dale. After seeing Jim Leiss' slides I'm not sure there's much

that the panelists can add on what's occurred here. I will try to amplify on a few of these items and answer some of the questions that

Dale has raised with us. As far as the high quality radioactivity

standards that we've heard about, I think that Laurie Taylor perhaps

summed it up when he said we'll have to have a little sense of humor

about this. Why do we need 0.5% when the last speaker told us the

things that he would use in the field would be about +_ 50% and we

heard one of the state program directors saying the same thing.

Perhaps, we are over-emphasizing the need for that degree of accuracy.

Now in the first session we heard from the Federal agencies, and the Bureau of Radiological Health talked about P.L. 90-602. One of the

items of compliance in the x-ray performance standard promulgated

under P.L. 90-602 was positive beam limitation. Some investigators

indicate that perhaps we could reduce exposures by 90% if we could

limit the beam to the image receptor.

427
We heard about therapy calibration centers and availability of precise

standards. Yet we heard that 6% of the practitioners that were doing

therapy were delivering doses more than 15% off from the calculated

dose. Larry Lanzl showed some curves with very steep slopes of

dose vs. treatment effectiveness. If the delivered dose is more than

15% off from the desired dose, then the clinical implications are rather frightening; i.e. the therapy has no effect whatsoever. So I wonder, why have we been spending hundreds of thousands of dollars on therapy calibration centers when perhaps what we should be doing is concentrating on the user--the people that are going to be using these

therapy units. The standards are available but apparently they are

not being used correctly.

I was rather shocked about laboratories. Fifty percent of the environmental radioactivity laboratories in the country can't even do a gross

beta within three standard deviations. Twenty-three percent. can't

even do a Cesium-137 within three standard deviations. The standards

are there, where are the procedures? Where is the training? What

are we going to do about these users?

As far as measuring systems are concerned, what are the things that we need? We could use some low energy dosimeters for use around

mammographic units. Those of you who've tried to do some measurements

of mammographic units find it very difficult to find a dosimeter that's

down in that energy range with the exception of TLD chips. I think

NBS should take a real close look at this problem. This is an important

problem because of the possibility of radiation induced tumors. I believe the states need the mobile calibration lab that Randy spoke

about. However, I think NBS should understand the states aren't

going to have the money to pay for this kind of service, and NBS

better be able to support it themselves if they want to put a mobile

lab out there, and provide this service to the states.

I don't think most people realize that the state surveyors don't go around with three quarter ton trucks when we go out to survey an installation! We have to carry our instrumentation, and I think you

ought to concentrate on getting instruments small enough so that they

are really portable and can give us reproducible results. We don't

need 0.5% accuracy. We'll settle for 5, 10, 15 and as the last

speaker said even 25%. What do I propose? I propose the next time

NBS wants to have a meeting they bring in more people living out in the real world. Find out what's going on out there in the states where the people have to visit all these installations, have to do all these measurements and have to make the decisions whether a medical

office will stay open, whether a reactor will continue to operate.

How can we make these decisions when the laboratories cannot produce

accurate results and when the health and radiation physicists at therapy installations are not able to deliver the desired dose?

Thank you.

The next Chairman represents the area of medical physics and John, I want you to clean ud your stories because remember, this is being

taped and it has to come out rated G. John Cameron.

I'll try one of my general stories that I have left over from an earlier meeting concerning the person who asked a Frenchman what the

temperature was and the Frenchman answered that it was 38 degrees.

The fellow says is that in Fahrenheit or Celsius? He said no, no, it's

right here in Paris.

428
The answer to the first question: Did this Symposium contribute to the stated objective? My answer would be, in general, yes. The answer to the second question, what were the major contributions: I would like, in an effort to reduce time, to read a few pages I have

written here for that.

Dr. Trout: Up to 5,000 words.

Dr. Cameron: Bob Shalek reviewed the experiences of the Radiological Physics Center

which he has directed. His measurements at 175 radiation therapy

installations indicated that about 20% had errors larger than 5% in

delivering the prescribed tumor dose and 6% had errors exceeding 15%.

These measurements were made at centers which are large enough to participate in a national treatment trial. Essentially no data exists

on the much larger number of smaller centers doing radiotherapy, many

of whom do not have full time physicists to help them. The accuracy

of calibration of ionization chambers by NBS and the two Regional

Calibration Laboratories (RCL's) sponsored by the AAPM was good.

However, one commercial calibration laboratory produced calibrations

that sometimes exceeded errors of 5%. It would appear that the

country would benefit from more RCL's but subsidization will probably

be necessary if they are to be started. In general, quality assurance

in radiotherapy appears satisfactory, at least in the larger centers. Measurements should be made at the large number of small centers

doing radiotherapy to determine how well they are doing.

Mel Siedband reviewed the need for more quality assurance in diagnostic

radiology and described very simple test tools to aid in these

measurements. Of the three major areas of radiology: therapy, nuclear

medicine and diagnosis, it appears that the largest area, diagnostic

radiology, is most in need of improvement in quality assurance. Relatively few medical physicists work in diagnostic radiology, and at NBS very few calibrations involve instruments for diagnostic radiology.

The situation is improving but there is a long way to go. The advent

of computerized tomography is bringing more physicists into diagnostic

radiology and once they are there they will undoubtedly have a beneficial effect on the general level of quality assurance. Most

states have an inspection program for diagnostic x-ray equipment.

Unfortunately, many x-ray users confuse this inspection with quality assurance and feel that everyone is o.k. because the state inspector

checks their equipment every three years.

Buck Rhodes discussed the status of quality assurance in nuclear

medicine. As the newest major field of radiology, this area is in pretty good shape, at least compared to diagnostic radiology. There

is a need for specification of guidelines or standards in the biodistribution of radionuclides. There is also a need for more education

of nuclear medicine technologists in the area of quality assurance.

The next speaker, Larry Lanzl , discussed state and federal radiation

regulations related to medical facilities. He described in some

detail the radiation control program in Illinois that limits the

exposure to typical patients from some standard x-ray projections.

The limits were determined largely on the basis of data obtained from

the N.E.X.T. Program. In the discussion period, some questions were

raised about the advisability of such limits. However, it seems unlikely that the rules will be rigorously enforced and I view the

rules as a step in the right direction. I think more states should

follow the Illinois lead in this area. The regulation implies that

measurements will be made, otherwise the rules will be ignored. In general, when you start to measure radiation the exposure(s) tend to decrease as the users become more aware of the overexposures.

429
The last speaker in the session, John Shaver, discussed the N.E.X.T.

Program. Nearly all states now participate in this program. I think

this is the best program yet developed for obtaining realistic data on patient exposures from diagnostic x-rays. N.E.X.T. data show large

variations of exposure and exposure area products from one facility to another. However, the actual situation is probably somewhat worse

since it is likely that the x-ray operator is somewhat more careful

during an inspection than at other times. We have done an analysis of

the N.E.X.T. data at Wisconsin to see how the radiation to the standard

patient depends on the type of facility and the training of the

operator. This is a different analysis than BRH has done. The results

will be available, hopefully, this year when BRH reproduces our report.

In summary, I think quality assurance in daily radiotherapy practice is in fair shape. Quality assurance in nuclear medicine appears o.k. but

good field data is needed, and quality assurance in diagnostic

radiology needs all the help it can get. In answer to the fourth

question, are further symposia needed or should they be sponsored by

NBS, I think meetings are needed in the specialized areas rather than

the general areas. General meetings like this are useful but not too often, maybe every three or four years. Diagnostic radiology could use many more meetings. Thank you.

Dr. Trout: Next we have a Chairman from a Federal regulatory group and if you

don't believe that the Chairman of one of these sessions has a bad

time you weren't here yesterday afternoon when John Matuszek was working

Bill over.

Dr. Mills: I thought I'd have a few slides to show to rebut everything

John Matuszek said, but that didn't seem appropriate, so let me speak

to the points that were raised. It seems to me that in looking at the

program across the board, you've had a breadth of representation but

perhaps not to the depth the states and the users, like Gerry Parker,

would suggest. I would tend to agree with Gerry. I think you've been

told how standards are designed and how they can be used. Now what

happens after that, it seems to me, is really up to you. If you don't

go back and use the information then the Symposium hasn't accomplished

much. Certainly there's a need for these standards and certainly

there's a need to push it to the greatest extent possible. As far as the session I chaired, I'll run through briefly the summary that I got

from the speakers.

Jim McLaughlin, I thought, described well the ability to go out and

make measurements around nuclear power plants at very low levels. For

the most part, we're talking about gamma measurements and perhaps beta

measurements. The levels of interest are at the least, a few millirem

above background, and at the most, several times background. This is the area of environmental concern. It isn't with doses of 100 rads.

It isn't doses of one rad even in a short period of time. It's of

the order of background and I think Jim described some systems that can

be used in this regard.

Matuszek impressed you with the ability of the laboratories to come

down hard on regulatory agencies and show how absurd some of the

regulations are. I'd get angry with that except for the fact that

John's objective is the same as my own. That is, protection. I can argue with him about differences in many things but as long as the

objective is protection of the public or protection of the worker, then

I think these things can be worked out.

1 National Evaluation of X-Ray Trends

430
Art Jarvis' job was to describe how the quality assurance program in EPA has worked. Art expressed some concern that it would go down the

drain. Everything, I think, will be done to assure its survival because

it seems to me that the quality assurance program is mandatory at environmental levels. Most of us who are concerned with the environment

just can't make decisions without some reliability in the measurements.

Dick Griffith discussed some of the personnel dosimetry problems,

emphasizing that he thought that the low L.E.T. radiations were in good shape. However, for the neutron dosimetry, high L.E.T. , there

is certainly a lack of confidence. I don't know how much this means

from the standpoint of the numbers of people in the exposed population

i.e. how many receive neutron doses of magnitude, but I can visualize

that this is a problem.

Bernie Weiss gave encouragement to the fact that we're beginning to make some inroad into standards for the performance of personnel

dosimeters. I'd only add to this that if he can do this for essentially

an occupational situation, there is some hope that we can apply these

same kinds of performance standards in the general environment.

So, in general, I got from this session that we can make measurements

at low levels in the environment and we must be as precise as we can be

in those we make. If you sit in the public arena like some of us do,

it doesn't do much good to rely to a large extent on models. If you get

a public inquiry and you respond that the model shows such and such it

doesn't satisfy many people. But they are satisfied if you have gone

out and made a measurement. They have more confidence in that. I think

that does more to sell the need for measurement capability, for example,

in nuclear power than anything else. If you can say you've made a measurement you can go a long way in satisfying many of the concerned.

Unfortunately, while our physics, our instrumentation, our electronics,

and our radiochemistry have gotten very precise, keep in mind that the

biology that goes with it is not that precise. We can't do anything

about that, we have to live with it. This is not to say, however, that

we shouldn't keep our sights on the preciseness of the measurements.

As far as the future of the Symposium. I would emphasize from an environmental standpoint that we have two major problems at the present

time that I see in the ionizing field. We have to be able to measure Plutonium and we have to be able to do it in a precise way. There are many areas in which plutonium contamination is being discussed and

action will have to be taken. We've also got the problem of emanations

from tailing piles, from phosphate mining in which a-daughters from

radon is an environmental problem that requires improvement in our ability to measure. The last point is for the non-ionizing field. I would plead for more environmental measurements, hopefully by the time

we can scope that problem the biology will be sufficient that we can say there is a non-problem. But at this stage in the game we just

can't make that statement. So I would say that in three to five years,

if we could come in with a program designed to look at the radiochemical

procedures, in particular in a-measurements , this would make many of us

in the environmental arena very happy.

While Randy's collecting his thoughts I want to tell you what was one

of my most enjoyable moments. I enjoyed thoroughly Harold Stewart's

presentation when he got up here and developed some standards with

nothing but some basic high school and college physics. He didn't have

a transistor on the screen, and I didn't see a computer anywhere.

431
Now from the NBS, that provides the product that goes on the shelf for

the consumer to use, here's Randy Caswell of the Bureau.

Dr. Caswell: First, I'd like to say something concerning measurements which were mentioned in connection with safeguards. NBS is trying to establish

a program of measurements and standards of special nuclear material

which is not part of the environmental problem.

Now as we come back to the question of the meeting, one of the participants in the meeting remarked to me yesterday that this was a different

meeting than he expected to find. I guess he was expecting more

technical measurements and what he found was a meeting that included

some of that but also included a lot of questions on the nature of

measurement policy. That got me thinking about who are the people who

came to the meeting. As I indicated in my talk the first day, one of the

ways of looking at the system is to divide people up into whether they are direct users of radiation or manufacturers, or standards and

calibration lab people, or regulators, or some of the other interested

groups such as ANSI, NCRP, and standards setting organizations. So I took the list of participants and went through it and counted. I arbitrarily decided which you were, according to the institution you

came from, and probably made some mistakes. If you were from a University or Hospital, I thought you were a direct user. If you were

from BRH I thought you were a regulator, and so on. I came out with

some percentages of the audience, and these are: 53% of the audience

are direct users of radiation, 18% represent manufacturers of instruments or of sources or radiopharmaceutical manufacturers, 10% are from

standards labs, 15% are from regulatory agencies of state or federal

government or local, and 3% are from other categories. The person who

made this original suggestion to me said, you looked at the wrong set

of numbers--what you really should have looked up was the fraction of

the papers that were in the various categories. So I said, I'll do

that too, that's even easier because there are less papers. It turns

out that 39% of the papers were from users, 7% from manufacturers, which

is surprisingly low to me, 28% from standards labs, 24% from regulators

and 2% from other organizations. I think these numbers in a way are

not too surprising in the sense that part of the goal of the meeting

was a communication from standards people and regulators to users.

It's not surprising to me anyway that these two groups are well

represented in the program. I thought you might be interested in that.

The one other point I want to key on, which impressed me in the meeting,

is the importance of feedback in various ways in the measurement system.

I think, certainly one of the purposes of the meeting was to have

constructive feedback from one group to another and I think this was largely achieved. I heard lots of feedback during the meeting. This

also reminded me, looking at it from an NBS standpoint, that the way

you get standards out to the user and help the user in getting

demonstrable accuracy occurs in four steps. First, develop a standard.

Second, issue a standard or calibration. Third is to carry out measurement assurance to see whether the measurements are really correct,

or whether there is some problem. Fourth, the very important feedback

or correction step. I think 15 or 20 years ago at NBS we did largely

the first two. We developed the standards and we issued them or we made calibrations available. He who wanted to come and get a calibration could do so and he who chose not to--nobody bothered him. He didn't have to do so. The third step, which is very active now, is the one of measurement assurance and there were lots of results of measurement assurance tests and round robins reported at the meeting. But I think the final step, and the one where the problem is now, is in the

feedback or correction step. I think, for example. Bob Shalek's paper

432
showed beautifully what the feedback or correction steps could do to improve measurements in a system. Some mechanism is needed to find out what's wrong, to correct it and improve the system. I think the areas where we are in trouble is where we have not, for some reason or another,

been able to carry out that fourth step. Therefore, I think that's

what we should perhaps be focusing on.

I'll just comment on one of the other questions--remarks on the

measurement system, and on my area of interest--! sort of have two hats.

As a member of the office of our Center, I'm interested in the broad

system and I think here the pattern is clearly specified. Some areas

seem to be in good shape and other areas seem to need much more work.

My other hat is that of a specialist in neutron dosimetry. There we

have sort of a strange situation in that the laboratories doing therapy

in the United States are very closely tied to each other on an arbitrary scale, but there is no standard produced by the National Bureau of Standards, and there is no wel 1 -determined absolute scale.

I think before we can feel that this area is in really satisfactory

shape we do need absolute standards.

Dr. Trout: Up to this point the Panel has been speaking to the audience. For the next ten minutes I'm going to ask them to speak to one another. Bill,

say what you think to the other panelists, ask them questions and the rest of you hold your answer until I get to you. What would you like

to say to the three other panelists?

Dr. Mills: Dr. Cameron's discussion on the medical--let me be the first to endorse

what he said about the Illinois situation. We have a great deal of

interest in the dose ranges used in the medical area and it seems to me

that that's an area in which there can be a very great reduction in the

exposures, the unnecessary parts of the exposure, and a lot of man-rems

(if you want to use man-rems) can be saved in this area. So I don't

know as I really have a question addressed to that point, but I certainly go along with the fact that if we want to make a contribution

in terms of reduction of exposure, and if it depends on good standards

in the medical field, I certainly think this is a very viable area to

go into. I guess I really don't have too much complaint with what the

other panel members have said because Matuszek isn't on this program.

I would like the chance to defend the drinking water standards, but

he's not here so it would just be wasted effort, I think. I certainly

think state labs will have to share a large burden of the drinking

water act, not just for radiation but for the other pollutants as well,

and are going to have to have federal funds to do it. There's no question. They just can't do that job by themselves.

Dr. Trout: It looks as though we have a universally approved message here to 0MB.

John, what can you say? Don't join the mutual admiration society with

Bill there. Give us a little conflict.

Dr. Cameron: I was just going to comment to Gerry's question about the need for a low

energy chamber for measuring radiation in the mammography region. The

Memorial Sloan Kettering group has developed a very nice chamber which,

I think, they are now putting on the market. They have found somebody

to produce it and Garrett Holt would be the person to contact about

that. It has accuracy, flatness of energy response relative to air of

about 1% in the energy region from a half value layer of .3 mm to about 1.5 mm of aluminum. I think to a large extent it's not really

necessary. I would like to argue that measuring mammography exposures

to 1% or even 5% is a bit of overkill. I think one of the last

speakers, Jim Leiss, made the point that there's no point in measuring

something more accurately than you need it. We measured quite a few

433
mammogrdphi c exposures in connection with our Midwest Center for

Radiological Physics. We found one center that over a period of about

a month had tripled this exposure. It had gone from about IR to 3R. This is the kind of thing we're looking for. We aren't looking for the

1% or 5% changes. TLD is quite adequate in this case, even if you get

only 10 or 20% accuracy. I am often impressed by high accuracy, but I think in the field we don't really need it.

Mr. Parker: John, I couldn't agree with you more. We don't really need that

precision. I think that was my point that I made in my talk and it's true. I spoke to Garrett about his dosimeters while he was here. We haven't seen how they will operate in the field. The proof will be if

50 states are able to get reproducible results from these dosimeters.

When you're getting 47 rads per mammographic film you really don't need

5%, 10% or 20% accuracy. I'd like to go back to two of the papers in your session John. Dr. Shalek's and Dr. Rhodes'. I'll mention Buck's

first because that was a minor point. He spoke about brain scanning

using 30 mCi , and when questioned by the audience he dropped it to 20

and I'd like to know why you, as someone from a large physics organization, didn't say "hey, what the hell are you using 20 for, why don't

you get down to 10?"

Dr. Cameron: Partly because I left the nuclear medicine field when they were still

using mercury and giving the kidneys about 200 rads. I plead

ignorance in this particular case.

Mr. Parker: With the Shalek thing I just don't see your optimism that this shows

good results. Here we have sophisticated centers and we get that kind

of data. Can you imagine what's going on in the smaller therapy

installations?

Dr. Cameron: One of the errors he pointed out was caught at Wisconsin. He found out

we had a ^°Co source that decayed with a half life of 3.5 years

instead of 5.3. Nature got the numbers reversedl Apparently the source consisted of ^°Co of various specific activities, with the hotter ^°Co

in front. The source was apparently not properly made and the colder

s^Co gradually moved forward giving more self absorption to the hotter

^°Co. Even relatively good places can make mistakes!

Dr. Caswell I hate to be agreeable with a fellow member of the Panel but I do agree

with the person who said that probably what is needed next are special

technical meetings in certain areas and that some period like three

years is probably an appropriate period' for repetition of a general

type of meeting. Secondly, one of our problems is how to make the

regional calibration laboratory system work or be viable. I believe we have three now and I think they're a fine system because I think people

in medical fields like to come to a medical-type institution for their

calibration. But unless we can find some means of making these

economically viable and create more successful ones, the approximately

10,000 institutions in the field are not going to be reached with

calibration. That's a problem that I would encourage the audience to think about and try to help. We're interested in that too, and we should work together to try to solve it. Just one other thing I wanted

to add since I have the microphone. This measurement system study that

I talked about is one of a number and they will be coming out in about

three months as NBS reports on such subjects as electromagnetics,

medical ultrasonics, acoustics, radiometry and photometry, spectrophotometry, far ultraviolet radiometry, optics, lasers, ionizing

radiation and so on. If you're in such a position where you would like

to see what somebody who tried to study part of a system found out; in about three months it will be easy to get a hold of such information

by writing to NBS.

434
"MEASUREMENTS FOR THE SAFE USE OF RADIATION"

Registrants

Roger Aamodt

National Institutes of Health

Building 10, Rm. B3B25

9000 Rockville Pike

Bethesda, Maryland 20014

Siro Abe

National Institute of Radiological

Sciences

9-1 , Anagawa-4

Chiba-Shi JAPAN 280

Robert Adams

Case Western Reserve University

2065 Adelbert Road

Cleveland, Ohio 44106

John Alexander

Oak Ridge National Laboratory

Health Physics Division

Box X

Oak Ridge, Tennessee 37830

Michael Altebrando

Grumman Aerospace Corporation

Bethpage, New York 11714

Charles Armentrout

Department of Physical Sciences

and Engineering

University of Maine

96 Falmouth Street

Portland, Maine 04103

John Arras

SAF Department

Armed Forces Radiobiology

Research Institute

NNMC

Bethesda, Maryland 20014

Frank Attix

Naval Research Laboratory

Code 6603A

Washington, D.C. 20375

Miguel Awschalom

Fermi National Accelerator Lab

P.O. Box 500

Batavia, Illinois 60510

Norman Baily

University of California, San Diego

School of Medicine

Department of Radiology M-OlO

La Jolla, California 92093

Ramon Baird

Electromagnetics Division

National Bureau of Standards

Boulder, Colorado 80302

Daniel Balkunow

E. R. Squibb and Sons, Inc. Georges Road

New Brunswick, New Jersey 08903

Stephen Baiter

Philips Medical Systems, Inc. 710 Bridgeport Avenue

Shelton, Connecticut 06484

Paul Barrans RADEF Instrumentation Test Facility

U.S. Army Communications Command

Building 22, Paulding Street

Washington Navy Yard

Washington, D.C. 20374

Harry Bartlett

E. R. Squibb and Sons, Inc. Georges Road

New Brunswick, New Jersey 08903

Arden E. Bicker

Reynolds Electrical and Engineering

P. 0. Box 14400

Las Vegas, Nevada 89114

Ralph Bish

CAPINTEC Instruments Inc. 540 Alpha Drive

Pittsburgh, Pennsylvania 15238

435
Nick Blaskovich

U.S. Department of HEW-NIOSH

944 Chestnut Ridge Road

Morgantown, West Virginia 26505

Joseph Blinick

Maine Medical Center

22 Bramhall Street

Portland, Maine 04102

Joseph Brown

Nuclear Regulatory Commission

5903 Greentree Road

Bethesda, Maryland 20034

Henry Buchanan

Yankee Atomic Electric Company

20 Turnpike Road

Westborough, Massachusetts 01581

Leroy Booth

Naval Research Lab

Washington, D.C. 20375

Arthur Boyer

Massachusetts General Hospital

Boston, Massachusetts 02114

Svresh Brahmavar

Medical Center of Western Massachusetts

759 Chestnut Street

Springfield, Massachusetts 01107

Robert Britain

Bureau of Radiological Health

Division of Compliance

Rockville, Maryland 20852

William Britt

The Machlett Laboratories, Inc. 1063 Hope Street

Stamford, Connecticut 06907

George Brodie

RADEF Instrumentation Test Facility, USACC

Building 22, Paulding Street

Washington Navy Yard

Washington, D.C. 20374

David Brown

Bureau of Radiological Health

12720 Twinbrook Parkway, //204 Rockville, Maryland 20851

Frank Brown

Eastman Kodak Company

Health, Safety and Human Factors

Laboratory

B-320, Kodak Park

Rochester, New York 14650

Gail de Planque Burke

U.S. ERDA, HASL

376 Hudson Street

New York, New York 10014

David Burton

Litton Bionetics

5516 Nicholson Lane Kensington, Maryland 20795

Robert Butenhoff

Division of Biomedical and

Environmental Research

U.S. ERDA

Washington, D.C. 20545

John Cameron

University of Wisconsin

Sterling Hall

Madison, Wisconsin 53706

Victoria Castro

American Oncologic Hospital

Central and Shelmire Avenues

Philadelphia, Pennsylvania 19111

Randall Caswell

Center for Radiation Research

National Bureau of Standards

Washington, D.C. 20234

Gul Chughtai

Radiation Therapy

Georgetown University Medical Center

Washington, D.C. 20007

Robert Clark

Searle Analytic Inc. 2000 Nuclear Drive

Des Plaines, Illinois 60018

436
D. Collins

U.S. ERDA Savannah River

P.O. Box A

Aiken, South Carolina 29801

Herbert Deich

MDH Industries, Inc. 426 Duarte Road

Monrovia, California 91016

John Connelly

NUS

4 Research Place

Rockville, Maryland 20850

Grace Conrad

Arlington County Virginia

Department of Inspection Services

Court House

Arlington, Virginia 22201

John Cooper

Nuclear Science Center

Auburn University

Auburn, Alabama 36830

Toimny Corbin

Lawrence Radiation Lab

End of Hearst Avenue

Berkeley, California 94720

Karin Corey

136 High Street

Leonia, New Jersey 07605

Bert Coursey

Center for Radiation Research

National Bureau of Standards

Washington, D.C. 20234

Sue Crain

Monsanto Research Corporation

Mound Laboratory

Miamisburg, Ohio 45342

Roland Davis

Los Alamos Scientific Laboratory

P.O. Box 1663

Los Alamos, New Mexico 87544

Neal De Gaston

Orange County Medical Center

University of California, Irvine

101 The City Drive

Orange, California 92668

Andrzej Demidecki

American Association of Physicists

in Medicine

Suite 307

6900 Wisconsin Avenue

Chevy Chase, Maryland 20015

James Deye

George Washington University

Medical Center

Radiology Department

901 23rd Street, N.W. Washington, D.C. 20037

Charles Dick

National Bureau of Standards

Washington, D.C. 202 34

Howard Dickson

ORNL

P.O. Box X

Oak Ridge, Tennessee 37830

J. William Dirksen

University of Nebraska Medical Center

42nd and Dewey Avenue

Omaha, Nebraska 68105

Ponce Dixon

U.S. Army Edgewood Arsenal, APG

2127 Round Hill Road

Fallston, Maryland 21047

Elwyn Dolecek

Rutgers University Radiation Safety

Department

Doolittle Building, Rm. 3572

New Brunswick, New Jersey 08903

Steve Domen

National Bureau of Standards

Washington, D.C. 20234

437
Richard Donovan

RAB Department

Armed Forces Radiobiology Research Inst. NNMC

Bethesda, Maryland 20014

Keith Eckerman

Nuclear Regulatory Commission

Washington, D.C. 20555

Elmer Eisenhower

National Bureau of Standards

Washington, D.C. 20234

Jack Elliott

Lawrence Livermore Laboratory

P.O. Box 808

Livermore, California 94550

David Espe j o-Villalobos

Hospital San Jose De Monterrey

Ave. Morones Prieto Pte N 3000

Monterrey, N.L. Mexico

Frederick Faw

Arnot-Ogden Memorial Hospital

Grove and Roe Avenues

Elmira, New York 14901

Sherman Fivozinsky

National Bureau of Standards

Washington, D.C. 20234

Ralph Flint

Battelle Columbus Labs

505 King Avenue

Columbus, Ohio 43201

Gary P. Foster

Division of Labor and Industry

203 E. Baltimore Street

Baltimore, Maryland 21202

Judith Foulke

U.S. Nuclear Regulatory Commission

Office of Nuclear Material Safety

and Safeguards

Washington, D.C. 20555

Douglas Fuhrman

Teledyne Isotopes

50 Van Buren Avenue

Westwood, New Jersey 07675

Gary Fullerton

University of Minnesota

Ther. Radiology Department

Box 494 Mayo Building

Minneapolis, Minnesota 55455

Marianne Gabel

Yale University

219 Prospect Street

New Haven, Connecticut 06520

David George

National Naval Medical Center

Bethesda, Maryland 20014

T. F. Gesell

University of Texas School of

Public Health

P.O. Box 20186

Houston, Texas 77025

Wallace Giffen

Mis-Hospital Shared Engineering

Services, Inc. Box 1441

Springfield, Ohio 45501

David Gilliam

National Bureau of Standards

Washington, D.C. 20234

Richard Gilmore

School of Medicine

Marshall University

Huntington, West Virginia 25701

Leon Goodman

Columbia University

Brookhaven National Lab

Building 902B

Upton, New York 11973

438
Richard Granke

1311 Downs Drive

Silver Spring, Maryland 20904

Richard Griffith

Lawrence Livermore Laboratory

Trailer 21, Room 103, L-520

P.O. Box 808

Livermore, California 94550

Michael Grissom

EXP Department

Armed Forces Radiobiology Research Institut

NNMC

Bethesda, Maryland 20014

James Grolton

Georgetown University Medical Center

Radiation Control Office

3800 Reservoir Road, N.W. Washington, D.C. 20007

Walter Gundaker

FDA-Bureau of Radiological Health

5600 Fishers Lane (HFX-470)

Rockville, Maryland 20852

Charles Hall

E. I. duPont

Photo Products Department

Chestnut Run Wilmington, Delaware 19898

Mahmoud Hammoudah

Howard University College of Medicine

Radiotherapy Department

Washington, D.C. 20060

James Hancock

Traps, Inc. and DePaul Hospital

1130 Hanover Avenue

Norfolk, Virginia 23508

Gerald Hanson

Pan American Health Organization

525 23rd Street, N.W. Washington, D.C. 20037

Rupert Henry

General Electric Company

Building 273, Rm. 299

Schenectady, New York 12345

John Hickey

U.S. Nuclear Regulatory Commission

Nicholson Lane Building

Washington, D.C. 20555

Carolyn Holman

Sawyer and Joiner, M.D., P. A. 85 W. Sturtevant

Orlando, Florida 32806

J. Garrett Holt

Memorial Hospital

1275 York Avenue

New York, New York 10021

Yingchieh Hsu

Nuclear Medicine

Veterans Administration Hospital

University Drive C Pittsburgh, Pennsylvania 15240

Jimmy Humphreys

National Bureau of Standards

Washington, D.C. 20234

Robin Hutchinson

National Bureau of Standards

Washington, D.C. 202 34

Thomas Jackson

Dosimeter Corporation

6106 Interstate Circle

Cincinnati, Ohio 45242

Ahren Jacobson

University of Louisville

Radiation Center

500 South Floyd Street

Louisville, Kentucky 40201

Arthur Jarvis

Office of Quality Assurance-Radiation

P.O. Box 15027

Las Vegas, Nevada 89114

439
Ronald Johnson

National Bureau of Standards

Washington, D.C. 20234

Tommy Johnson

Naval Research Lab

Code 6070

Washington, D.C. 20375

Donald Jones ERDA

1253 Norton

Idaho Falls, Idaho 83401

Robert Jucius

General Electric

Medical Systems Division

P.O. Box 414

Milwaukee, Wisconsin 53201

Kenneth Kase

Harvard University Medical School

50 Binney Street

Boston, Massachusetts 02115

Albert Kasenter

Henry Ford Hospital

Department of Radiology

2799 W. Grand Boulevard

Detroit, Michigan 48202

David Katzman

Teledyne Isotopes

50 Van Buren Avenue

Westwood, New Jersey 07675

Eric Kearsley

RAB Department

Armed Forces Radiobiology Research Inst. NNMC

Bethesda, Maryland 20014

Ronald Keyser

ORTEC

100 Midland Road

Oak Ridge, Tennessee 37830

Allan Kolber

NYU Institute of Environmental Medicine

138 Village Circle West Paramus, New Jersey 07652

Dean Kunihiro

RAB Department

Armed Forces Radiobiology Research Inst. NNMC

Bethesda, Maryland 20014

Paul Lamperti

National Bureau of Standards

Washington, D.C. 20234

Robert S. Landauer

R. S. Landauer, Jr. and Company

Science Road

Glenwood, Illinois 60425

Lawrence Lanzl

University of Chicago

950 East 59th Street

Chicago, Illinois 60637

Harold Larson

Battelle Northwest Laboratories

P.O. Box 999

Richland, Washington 99352

Jay Lee

U.S. Nuclear Regulatory Commission

ETSB/SSEA/NRR

Washington, D.C. 20555

Paul Leggett

Bureau of Radiological Health

Division of Compliance

Rockville, Maryland 20852

Vernon Leininger

Department of Radiology

University of Connecticut

Farmington, Connecticut 06032

.0
J

James Leiss

Center for Radiation Research

National Bureau of Standards

Washington, D.C. 20234

Joseph McDonald

Sloan-Kettering Institute

12 75 York Avenue

New York, New York 10021

Irving Lerch

Radiation Oncology

New York University Medical Center

566 First Avenue

New York, New York 10016

Norman McElroy

George Washington University

Medical Center

2300 K Street, N.W. Washington, D.C. 20037

Stuart Levin

Pennsylvania Department of

Environmental Resources

1204 Kossman Building

100 Forbes Avenue

Pittsburgh, Pennsylvania 15222

Gerald Levine

Brookhaven National Lab

Building 535

Upton, New York 11973

Pei-Jan Lin

Northwestern University Medical School

303 East Chicago Avenue

Chicago, Illinois 60611

Otha Linton

American College of Radiology

6900 Wisconsin Avenue

Chevy Chase, Maryland 20015

Joseph Lochamy

Duke Power Company

422 S. Church Street

P.O. Box 2178

Charlotte, North Carolina 28242

Robert Loevinger

National Bureau of Standards

Washington, D.C. 20234

John Lombardi

U.S. Army Ordnance Center and School

1315 Charleston Drive

Edgewood, Maryland 21040

Stephen A. McGuire

Nuclear Regulatory Commission

Washington, D.C. 20555

James McLaughlin

U.S. ERDA

Health and Safety Laboratory

376 Hudson Street

New York, New York 10014

Elaine Mackowiak

Temple University

School of Pharmacy

3307 N. Broad Street

Philadelphia, Pennsylvania 19140

Henry Magee

Radiation Health Division (Code 105.05)

Pearl Harbor, Hawaii 96610

Adam Malik

Union Carbide Corporation-Linde Division

175 E. Park Drive

P.O. Box 44 Tonawanda, New York 14150

Marjorie Malmberg

NUS Corporation

Rockville, Maryland 20850

John Matuszek

New York State Department of Health

Radiological Sciences Laboratory

30 Russell Road

Albany, New York 12206
Karl Mendenhall

RA.B Department

Armed Forces Radiobiology Research Inst. NNMC

Bethesda, Maryland 20014

William Mills

Criteria and Standards Division (AW-460)

U.S. Environmental Protection Agency

401 M. Street, S.W. Washington, D.C. 20460

Thomas Mitchell

Georgetown University Medical Center

Radiation Control Office

3800 Reservoir Road, N.W. Washington, D.C. 20007

R. W. Modrell

Goodyear Tire and Rubber Company

1144 East Market Street

Akron, Ohio 44 316

Robert Mogle

Virginia Polytechnic Institute

and State University

Robeson Hall, Rm. 13

Blacksburg, Virginia 24061

Tommie Morgan

Bureau of Radiological Health

5600 Fishers Lane Rockville Maryland 20852

Michael Moroz

Pennwalt Corporation

S. S. White Division

P.O. Box 100, South Street

Holmdel, New Jersey 07733

Robert Morton

Nuclear Medicine Branch

National Naval Medical Center

Bethesda, Maryland 20014

C. E. Moss

NIOSH

PAEB/DBBS-4th Floor

R. A. Taft Lab

4676 Columbia Parkway

Cincinnati, Ohio 45226

William Moss

Los Alamos Scientific Laboratory

MS 486

P.O. Box 1663

Los Alamos, New Mexico 87545

Joseph Motz

National Bureau of Standards

Washington, D.C. 20234

Victor Mdnoz

Centre Regional de Calibracion

y Dosimetria S.S.A.

Dr. Balmis No. 148

Mexico D.F. Mexico

John Munro

Technical Operations, Inc. 40 South Avenue

Burlington, Massachusetts 01803

Ravi Nair

Ohio State University Hospital

410 W. 10th Avenue

Columbus, Ohio 43221

Christopher Nelson

USEPA/Office of Radiation Programs

(AW-461)

401 M Street, S.W. Washington, D.C. 20460

W. Roger Ney

National Council on Radiation

Protection and Measurements

7910 Woodmont Avenue, Suite 1016

Washington, D.C. 20014

Jagannadha Rao Nibhanupudy

Howard University Hospital

Radiotherapy Department

2041 Georgia Avenue, N.W. Washington, D.C. 20060

Lowell Nichols

Battelle Northwest

P.O. Box 999

Richland, Washington 99352

kh2
Edward Nickoloff

The Johns Hopkins University

School of Hygiene and Public Health

Rm. 1102

615 N. Wolfe Street

Baltimore, Maryland 21205

Dale Nix

Tennessee Valley Authority

204 Greenfield Road

Florence, Alabama 35660

James Noyce

National Bureau of Standards

Washington, D.C. 20234

Thomas Ohlhaber

FDA, Bureau of Radiological Health

12720 Twinbrook' Parkway

Rockville, Maryland 20852

W. F. Ohnesorge

Oak Ridge National Laboratory

P.O. Box X

Oak Ridge, Tennessee 37830

G. Osman

Department of Radiation Medicine

Roswell Park Memorial Institute

Buffalo, New York 14263

William Ott

National Bureau of Standards

Washington, D.C. 20234

John Paciga

New Brunswick Electric Power Commission

527 King Street

Fredericton, New Brunswick Canada

Bhudatt Paliwal

University of Wisconsin-Madison

Radiology Department

475 N. Charter Street

Madison, Wisconsin 53706

Peter Paras

Bureau of Radiological Health

1901 Chapman Avenue

Rockville, Maryland 20852

Gerald Parker

Massachusetts Department of Public Health

80 Boylston Street

Rm. 835

Boston, Massachusetts 02116

Marshall Parrott

Radiation Control Service

State Health Division

P.O. Box 231

Portland, Oregon 97207

Sidney Parry

USNRC-Division of Organization and

Personnel

Training and Development Branch

Washington, D.C. 20555

Donald Parsons

Roswell Park Memorial Institute

666 Elm Street

Buffalo, New York 14263

Indu Patel

Howard University

College of Allied Health

Washington, D.C. 20060

Gerald Peters

Naval Surface Weapons Center

White Oak Laboratory, Code WR41

Building 2-013

Silver Spring, Maryland 20910

Paul Pfalzner

Ontario Cancer Treatment and

Research Foundation

1053 Carling Avenue

Ottawa, Canada KlY 4E9

Leigh Phillips

Brookhaven National Laboratory

Building 535

Upton, New York 11973

Robert Placious

National Bureau of Standards

Washington, D.C. 20234

A43
Robert Poe ERDA Division of Operational Safety

Washington, D.C. 20545

John Potter

Nuclear Regulatory Commission

Washington, D.C. 20555

William Powers

Naval Research Laboratory

Washington, D.C. 20375

S. Guru Prasad

Evans Hospital

2650 Ridge Avenue

Evanston, Illinois 60201

Mark Presser

Victoreen Instrument Company

10101 Woodland Avenue

Cleveland, Ohio 44104

Frederick Prevo

Bureau of Radiological Health

12720 Twinbrook Parkway

Rockville, Maryland 20852

John Pruitt

National Bureau of Standards

Washington, D.C. 20234

William Quam

EG&G

130 Robin Hill Road

Goleta, California 93017

Mohammed Quddus

Mechanical and Aerospace Engineering Dept.

University of Delaware

Newark, Delaware 19711

Philip Quinn

FDA/WEAC

109 Holton Street

Winchester, Massachusetts 01890

Gustavo Quintanilla-E.

ITESM Physics Department

Sucursal De Correos "J" Monterrey, N.L. Mexico

Buck Rhodes

University of Kansas Medical Center

Rainbow Boulevard at 39th Street

Kansas City, Kansas 66103

Gordon Riel

Naval Surface Weapons Center

Wiiite Oak

Silver Spring, Maryland 20910

Francis Roddy

Bechtel Power Corporation

15740 Shady Grove Road

Gaithersburg , Maryland 20760

Jerry Rosen

University of Pittsburgh

Graduate School of Public Health

Pittsburgh, Pennsylvania 15213

Paul Rosenbaum

Nuclear Radiation Consultants

P.O. Box 647

New Haven, Connecticut 06503

David Ross

SINE

7 Dunwoody Park-Suite 104

Atlanta, Georgia 30341

Harvey Rudolph

Bureau of Radiological Health

12720 Twinbrook Parkway

Rockville, Maryland 20852

Warren Rumble

National Institutes of Health

9000 Rockville Pike

Bethesda, Maryland 20014

Roger Schneider

Division of Electronic Products

Bureau of Radiological Health

5600 Fishers Lane

Rockville, Maryland 20852
Robert Schwartz

National Bureau of Standards

Washington, D.C. 20234

Richard Sha

Radiation Management Corporation

Science Center Building #2, Suite 400

3508 Market Street

Philadelphia, Pennsylvania 19104

Robert Shalek

Physics Department

M.D. Anderson Hospital

Houston, Texas 77025

John Shaver

University of California Medical Center

350 Parnessus Avenue, Suite 307

San Francisco, California 94117

Richard Smale

Los Alamos Scientific Laboratory

H-1, MS403

P.O. Box 1663 Los Alamos, New Mexico 87545

Robert Smith

Westinghouse Hanford

Box 1970

Richland, Washington 99352

George Snyder

U.S. Navy

19066 Montgomery Village Avenue

Gaithersburg , Maryland 20760

Julian Sparrow

National Bureau of Standards

Washington, D.C. 20234

Bruce Shields

Union Carbide Corporation

Tarrytown Technical Center

Spine Building

Tarrytown, New York 10591

Melvin Siedband

Medical Physics-Department of Radiology

University of Wisconsin Medical School

Madison, Wisconsin 53706

Robert Simpson

Bureau of Radiological Health

5600 Fishers Lane

Rockville, Maryland 20852

Lester Slaback

SAF Department

Armed Forces Radiobiology Research Inst. NNMC

Bethesda, Maryland 20014

Robert Sleever

White Sands Missile Range

Nucleonics Calibration

P.O. Box 182

White Sands Missile Range, New Mexico

E. W. Spiers

Toronto General Hospital

College Street

Toronto, Ontario M5G1L7

Jacob Spira

Boston University School of Medicine

80 East Concord Street

Boston, Massachusetts 02118

Larry Stephenson

Texas Department of Health Resources

1100 W. 49th

Austin, Texas 78756

Eugene Stickley

Medical College of Virginia

Richmond, Virginia 23298

Robert Strauss

Picker Corporation

595 Miner Road

Cleveland, Ohio 44143

Karl Stuermer

Victoreen Instrument Company

10101 Woodland Avenue

Cleveland, Ohio 44104

445
Monte Sula

United Nuclear Industries, Inc. Box 490

Richland, Washington 99352

Joseph Thiel

Texas Department of Health Resources

1100 West 49th Street

Austin, Texas 78756

N. Suntharalingam

Department of Radiation Therapy

Thomas Jefferson University Hospital

Philadelphia, Pennsylvania 19107

Arata Suzuki

CAPINTEC Instruments Inc. 540 Alpha Drive

Pittsburgh, Pennsylvania 15238

Michael Svoren

LSU School of Medicine

P.O. Box 3932

Shreveport , Louisiana 71130

John Taschner

Bureau of Radiological Health

18917 Alpenglow Lane

Brookeville, Maryland 20729

Daniel Thompson

Sandia Laboratories

Albuquerque, New Mexico 87115

Winston Thompson

Howard University

College of Allied Health

Washington, D.C. 20059

Hans Tovedal

AB Atomenergi

Studsvik, Fack

S-61101 Nykoping, Sweden

E. Dale Trout

Oregon State University

X-Ray Science and Engineering Lab

Corvallis, Oregon 97331

Lauriston Taylor

National Council on Radiation Protection

and Measurements

7910 Woodmont Avenue, Suite 1016

Washington, D.C. 20014

Warren Taylor

Sandia Laboratories-Division 9531

P.O. Box 5800

Albuquerque, New Mexico 87115

Jos'e Telich

Hospital Espaftol Av. Ejercito Nacional 613

Mexico 5, D.F. Mexico

Jerry Tetenes

Albert Einstein College of Medicine

1825 East Eastchester Road

Bronx, New York 10461

George Tucker

Sandia Laboratories

Albuquerque, New Mexico 87115

Arnold VanArt

Litton Medical Systems

515 E. Touhy Avenue

Des Plaines, Illinois 60018

Wesley Van Pelt

Hoffmann-La Roche Inc. 340 Kingsland Street

Nutley, New Jersey 07110

David Verrelli

Research Program Coordinating Office

Armed Forces Radiobiology Research In NNMC

Bethesda, Maryland 20014

446
G. Walford

ORTEC Inc. 100 Midland Road

Oak Ridge, Tennessee 37830

William Walker

Malcolm Grow USAF Medical Center

Box 3525

Andrews Air Force Base Washington, D.C. 20331

L. Walsh

Ontario Hydro

Pickering G. S

. -Operations

Pickering, Ontario Canada L1V2R5

John Wang

Federal Communications Commission

2025 M Street, N.W. , Rm. 7202

Washington, D.C. 20554

Joseph Wang

Naval Surface Weapons Center

I'Jhite Oak Laboratory

Silver Spring, Maryland 20910

John Washington

Northwest Medical Physics Center

RC-08, University Hospital

Seattle, Washington 98195

Goldie Watkins

New York State Department of Health

Empire State Plaza

Albany, New York 12237

Arthur Weis

CAPINTEC Inc. 136 Summit Avenue

Montvale, New Jersey 07645

Bernard Weiss

Radiological and Environmental Protection

Branch

Office of Inspection and Enforcement

Nuclear Regulatory Commission

Washington, D.C. 20555

M. Weiss

Bell Laboratories Inc. 600 Mountain Avenue

Murray Hill, New Jersey 07974

Bryan Westerman

Department of Nuclear Medicine

Northwestern Memorial Hospital

Fairbanks and Superior

Chicago, Illinois 60611

Robert Wheeler

R. S. Landauer, Jr. and Company

Science Road

Glenwood, Illinois 60425

Carl-0 Widell

AB Atomenergi

Studsvik

S-61101 Nykoping, Sweden

John Widman

Veterans Administration Hospital

Nuclear Medicine Service

Allen Park, Michigan 48101

Richard Wilder

Toledo Ohio Board of Health

635 N. Erie Street

Toledo, Ohio 43624

G. Wilkening

Bell Laboratories

600 Mountain Avenue

Murray Hill, New Jersey 07974

Edward Williams

RADEF Instrumentation Test Facility

U.S. Army Communication Command

6626 Reynard Drive

Springfield, Virginia 22152

C. Wright

E. I. DuPont De Nemours and Company

Savannah River Plant

en, South Carolina 29801

A47
James Wyckoff

National Bureau of Standards

Washington, D.C. 20234

George Zacharopoulos

Montefiore Hospital and Medical Center

111 East 210 Street

Bronx, New York 10467

Al Zlrkes

ICN Dosimetry Service

26201 Miles Road

Cleveland, Ohio 44128
AUTHOR INDEX

Athey, T.W. 203

Attix, F.H. 335

August, L.S. 335

Baird, R.C. 417

Barrans, P.V. 59

Bassen, H.I. 99

Berger, M.J. 307

Blinick, J.S. 359

Bjarngard, B.E. 343

Brodie, G.H. 59

Burke, G.D. 187

Cail, D. 359

Cameron, J.R. 427

Caswel 1 , R.S. 13, 427

Toaklpv \_/ \_/ t-i r\ 1 w y 9i]Will*M 215

Collg, R. 71 Cooper, J. A. 159

rntirc:p\/ V-< U 1 O C R M Jr 9 1 1 > 173

Cundiff, J.H. m

Dick, C.E. 41 Dickson, H.W. 225

Domen, S.R. 307

Easterly, D.G. 263

Ehrlich, M. 365

Fox, W.F. 225

Gabel, M.M. 371 Gagnon, W.F. 111 Gallagher, J.H. 131 Gesell, T.F. 187

Goldman, D.T. 3 Goodman, L.J. 327

Grant, W. , III 111 Griffith, R.V. 267

Hankin, N.N. 203

Hanson, W.F. 111 Haran, M.E. 91, 285

Hashmi, Z.R. 389

Haywood, F.F. 225

Herman, B.A. 91 Herman, W.A. 99

Hladik, W.B. 131 Holeman, G.R. 371 Hubbell, J.H. 299

Hull, A. P. 153

Humphreys, J.C. 47

Hutchinson, J.M R. 77

UaCODbUil, H. 65

James, R. 215

Janes, D.E. , Jr. 203

Jarvis, A.N. 263

Jones, D. 351

Kase, K.R. 343

Kennedy, P. in

Keyser, R.M. 159

Lallv J S 223

Lamperti , P.J. 307, 365

Lanzl, L.H. 139

Laughlin, J.S. 327

Lei ss , J.E. 403

Levine, G.S. 153

Lin, P.P. 381 Lochamy, J.C. 169

Loevinger, R. 37

McDonald, J.C. 327

McGarry, E.D. 83

riuLduyii Mil) u . u . coo

MarlHnY R .1 O "t J Matuszek, J.M. 249

Miller, T.L. 53 Mills, W.A. 427

Mnhan nond W n , K

C I D Morgan, T.J. 389

Moroz, M.S. 395

Nelson, J.C. 203

Ohlhaber, T.R. 53 Osman, G. 313

Ott, W.R. 107

Parker, G.S. 427

r a 1 1 u U U 5 1 1 . Inl . 31 Parsons, D.F. 223

Peterson, R.W. 215

Phillips, V.A. 223

Placious, R.C. 47

Plazeski , P. 359

Prevo, F.J. 53

Price, K.W. 371 Pruitt, J.S. 37

Rhodes, B. 131

449
Sanders, A. A. 417

Schneider, R.H. 25 Schroder, I.G. 83

Schumacher, D. 351 Schwartz, R.B. 83

Seidel , C.W. 77 Seltzer, S.M. 307

Shalek, R.J. 1 1

Shapiro, P. 335

Shaver, J.W. 147

Siedband, M.P. 119

Sorrows, H.E. 1 Smith, L.B. 389

Sparrow, J.H. 41 Spiegel , V

87

Spira, J. 359

Stewart, H.F. 91 , 285

Stovall , M. 111 Svensson, G.K. 343

Swicord, M.L. 99

Tay 1 or , L . S5 Tell, R.A. 203

Thiel , J.F. 199

Trout, E.D. 427

Vukan, J. A. 395

Walford, G.V. 159

Washington, J. 351 Weiss, B.H. 277

Widen, C. 181 Williams, E.F. 59
NBS-lUA (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS SP-456

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

MeasTorements for the Safe Use of Radiation

Proceedings of an NBS 75th ANNIVERSARY SymposiTom

held at the National Bureau of Standards

Gaithersbur^, Maryland, March 1-4, 1976

5. Publication Date November 1976

6. Performing Organization Code

7. AUTHOR(S)

Sherman P, Fivozinsky, Editor

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

National Bureau of Standards

American Association of Physicists in Medicine

Conference of Radiation Control Program Directors

Health Physics Society

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 76-60CX)60

16. ABSTRACT (A 200-word or less factual summary oC most sigpificant information. If document includes a significant

bibliography or literature survey, mention it here.)

These proceedings are the compilation of papers presented at the

Symposium, "Measurements for the Safe Use of Radiation", held in

Gai thersburg, Maryland, March 1-4, 1976. The symposium addressed the

present status and future requirements for measurements of ionizing and

non-ionizing radiation. Emphasis was placed on the needs for

standardization and measurement assurance activities in medical,

environmental, and personnel protection applications.

17. KEY Vl'GRDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Calibrations; environment; measurements; medical; radiation; standards

18. AVAILABILITY Unlimited

1

' For Official Distribution. Do Not Release to NTIS

ly ' Order From Sup. of Doc, U.S. Government Printing Office

Wa.shinpron. D.C. 20402. SD Cat. No. C13ulO:A.%

1

I Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

21. NO. OF PAGES

437

20. SECURITY CLASS

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NBS TECHNICAL PUBLICATIONS

PERIODICALS JOURNAL OF RESEARCH reports National Bureau

of Standards research and development in physics,

mathematics, and chemistry. It is published in two

sections, available separately:

• Physics and Chemistry (Section A)

Papers of interest primarily to scientists working in these fields. This section covers a broad range of physical and chemical research, with major emphasis on

standards of physical measurement, fundamental constants, and properties of matter. Issued six times a year.

Ajinual subscription: Domestic, $17.00; Foreign, $21.25.

I • Mathematical Sciences (Section B)

' Studies and compilations designed mainly for the mathematician and theoretical physicist. Topics in mathematI ical statistics, theory of experiment design, numerical

i analysis, theoretical physics and chemistry, logical de- sign and programming of computers and computer sys-

! tems. Short numerical tables. Issued quarterly. Annual

subscription: Domestic, $9.00; Foreign, $11.25.

j DIMENSIONS/NBS (formerly Technical News Bulle-

' tin) —This monthly magazine is published to inform

scientists, engineers, businessmen, industry, teachers,

students, and consumers of the latest advances in

' science and technology, with primary emphasis on the work at NBS. The magazine highlights and reviews

i such issues as energy research, fire protection, building

technology, metric conversion, pollution abatement,

health and safety, and consumer product performance.

I In addition, it reports the results of Bureau programs

' in measurement standards and techniques, properties of matter and materials, engineering standards and serv-

,' ices, instrumentation, and automatic data processing.

Annual subscription: Domestic, $9.45; Foreign, $11.85.

NONPERIODICALS

f Monographs—Major contributions to the technical liter- ature on various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and

industrial practice (including safety codes) developed

in cooperation with interested industries, professional

organizations, and regulatory bodies. Special Publications—Include proceedings of conferences sponsored by NBS, NBS annual reports, and other

special publications appropriate to this grouping such

' as wall charts, pocket cards, and bibliographies.

I Applied Mathematics Series—Mathematical tables, man-

' uals, and studies of special interest to physicists, engineers, chemists, biologists, mathematicians, computer programmers, and others engaged in scientific and technical work.

I National Standard Reference Data Series—Provides

' quantitative data on the physical and chemical properties of materials, compiled from the world's literature and critically evaluated. Developed under a world-wide

i program coordinated by NBS. Program under authority

of National Standard Data Act (Public Law 90-396).

NOTE: At present the principal publication outlet for these data is the Journal of Physical and Chemical

Reference Data (JPCRD) published quarterly for NBS

by the American Chemical Society (ACS) and the American Institute of Physics (AIP). Subscriptions, reprints,

and supplements available from ACS, 1155 Sixteenth

St. N.W., Wash. D. C. 20056. Building Science Series—Disseminates technical infor- mation developed at the Bureau on building materials,

components, systems, and whole structures. The series presents research results, test methods, and performance criteria related to the structural and environmental

functions and the durability and safety characteristics

of building elements and systems.

Technical Notes—Studies or reports which are complete

in themselves but restrictive in their treatment of a

subject. Analogous to monographs hut not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under proce- dures published by the Department of Commerce in Part

10, Title 15, of the Code of Federal Regulations. The

purpose of the standards is to establish nationally rec- ognized requirements for products, and to provide all concerned interests with a basis for common understanding of the characteristics of the products. NBS

administers this program as a supplement to the activities of the private sector standardizing organizations.

Consumer Information Series—Practical information,

based on NBS research and experience, covering areas

of interest to the consumer. Easily understandable language and illustrations provide useful background knowledge for shopping in today's technological marketplace.

Order above NBS publications from: Superintendent

of Documents, Government Printing Office, Washington,

D.C. 20U02. Order following NBS publications—NBSIR's and FIPS

from the National Technical Information Services,

Springfield, Va. 22161. Federal Information Processing Standards Publications

(FIPS PUBS)—Publications in this series collectively constitute the Federal Information Processing Standards Register. Register serves as the official source of information in the Federal Government regarding stand- ards issued by NBS pursuant to the Federal Property

and Administrative Services Act of 1949 as amended,

Public Law 89-306 (79 Stat. 1127), and as implemented

by Executive Order 11717 (38 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal

Regulations).

NBS Interagency Reports (NBSIR)—A special series of interim or final reports on work performed by NBS for outside sponsors (both government and non-government). In general, initial distribution is handled by the sponsor; public distribution is by the National Technical Information Services (Springfield, Va. 22161) in paper copy or microfiche form.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey

bibliographies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service. A

literature survey issued biweekly. Annual subscription: Domestic, $20.00; Foreign, $25.00.

Liquified Natural Gas. A literature survey issued quarterly. Annual subscription: $20.00.

Superconducting Devices and Materials. A literature survey issued quarterly. Annual subscription: $20.00.

Send subscription orders and remittances for the pre- ceding bibliographic services to National Bureau of Standards, Cryogenic Data Center (275.02) Boulder,

Colorado 80302.
U.S. DEPARTMENT OF COMMERCE

IMational Bureau of Standards

Washington. D.C- 20234

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