Radiological Safety Aspects of the Operation of Electron Linear Accelerators
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TECHNICAL REPORTS SERIES No. 188 Radiological Safety Aspects of the Operation of Electron Linear Accelerators i11 INTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 1979 jy RADIOLOGICAL SAFETY ASPECTS OF THE OPERATION OF ELECTRON LINEAR ACCELERATORS The following States are Members of the International Atomic Energy Agency: AFGHANISTAN HOLY SEE PHILIPPINES ALBANIA HUNGARY POLAND ALGERIA ICELAND PORTUGAL ARGENTINA INDIA QATAR AUSTRALIA INDONESIA ROMANIA AUSTRIA IRAN SAUDI ARABIA BANGLADESH IRAQ SENEGAL BELGIUM IRELAND SIERRA LEONE BOLIVIA ISRAEL SINGAPORE BRAZIL ITALY SOUTH AFRICA BULGARIA IVORY COAST SPAIN BURMA JAMAICA SRI LANKA BYELORUSSIAN SOVIET JAPAN SUDAN SOCIALIST REPUBLIC JORDAN SWEDEN CANADA KENYA SWITZERLAND CHILE KOREA, REPUBLIC OF SYRIAN ARAB REPUBLIC COLOMBIA KUWAIT THAILAND COSTA RICA LEBANON TUNISIA CUBA LIBERIA TURKEY CYPRUS LIBYAN ARAB JAMAHIRIYA UGANDA CZECHOSLOVAKIA LIECHTENSTEIN UKRAINIAN SOVIET SOCIALIST DEMOCRATIC KAMPUCHEA LUXEMBOURG REPUBLIC DEMOCRATIC PEOPLE'S MADAGASCAR UNION OF SOVIET SOCIALIST REPUBLIC OF KOREA MALAYSIA REPUBLICS DENMARK MALI UNITED ARAB EMIRATES DOMINICAN REPUBLIC MAURITIUS UNITED KINGDOM OF GREAT ECUADOR MEXICO BRITAIN AND NORTHERN EGYPT MONACO IRELAND EL SALVADOR MONGOLIA UNITED REPUBLIC OF ETHIOPIA MOROCCO CAMEROON FINLAND NETHERLANDS UNITED REPUBLIC OF FRANCE NEW ZEALAND TANZANIA GABON NICARAGUA UNITED STATES OF AMERICA GERMAN DEMOCRATIC REPUBLIC NIGER URUGUAY GERMANY, FEDERAL REPUBLIC OF NIGERIA VENEZUELA GHANA NORWAY VIET NAM GREECE PAKISTAN YUGOSLAVIA GUATEMALA PANAMA ZAIRE HAITI PARAGUAY ZAMBIA PERU The Agency's Statute was approved on 23 October 1956 by the Conference on the Statute of the IAEA held at United Nations Headquarters, New York; it entered into force on 29 July 1957. The Headquarters of the Agency are situated in Vienna. Its principal objective is "to accelerate and enlarge the contribution of atomic energy to peace, health and prosperity throughout the world". © IAEA, 1979 Permission to reproduce or translate the information contained in this publication may be obtained by writing to the International Atomic Energy Agency, KSrntner Ring 11, P.O. Box 590, A-1011 Vienna, Austria. Printed by the IAEA in Austria February 1979 TECHNICAL REPORTS SERIES No. 188 RADIOLOGICAL SAFETY ASPECTS OF THE OPERATION OF ELECTRON LINEAR ACCELERATORS A manual written by William P. SWANSON Stanford Linear Accelerator Center Stanford University United States of America Work supported in part by the United States Department of Energy INTERNATIONAL ATOMIC ENERGY AGENCY VIENNA, 1979 RADIOLOGICAL SAFETY ASPECTS OF THE OPERATION OF ELECTRON LINEAR ACCELERATORS IAEA, VIENNA, 1979 STI/DOC/lO/188 ISBN 92-0-125179-3 FOREWORD Electron linear accelerators are being used throughout the world in increasing numbers in a variety of important applications. Foremost among these is their role in the treatment of cancer with both photon and electron radiations in the energy range 4—40 MeV. To a greater extent linear accelera- tors are replacing 60Co sources and betatrons in medical applications. Com- mercial uses include non-destructive testing by radiography, food preservation, product sterilization and radiation processing of materials such as plastics and adhesives. Scientific applications include investigations in radiation biology, radiation chemistry, nuclear and elementary-particle physics and radiation research. This manual is conceived as a source book providing authoritative guidance in radiation protection from an important category of radiation sources. It thus supplements other manuals of the Agency related to the planning and implementation of radiation protection programmes. The author, W.P. Swanson of Stanford Linear Accelerator Center, USA, was engaged as a consultant by the Agency to compile and write the manual, and the Agency wishes to express its gratitude to him. A draft was sent to a number of experts in various countries. The Agency gratefully acknowledges the helpful comments, which have been taken into account in the final text, from J. Rassow, K. Tesch (Federal Republic of Germany), M. Ladu (Italy), T. Nakamura (Japan), G.R. Higson (United Kingdom), F.H. Attix, R.C. McCall and C.S. Nunan (United States of America). The Agency's officer responsible for this project was F.N. Flakus of the Radiological Safety Section, Division of Nuclear Safety and Environ- mental Protection. Comments from readers for possible inclusion in a later edition of the manual would be welcome; they should be addressed to the Director, Division of Nuclear Safety and Environmental Protection, International Atomic Energy Agency, Karntner Ring 11, P.O. Box 590, A-1011 Vienna, Austria. CONTENTS INTRODUCTION 1 Purpose and scope of the Manual 2 Terminology and units 5 Acknowledgements 5 1. USES AND CHARACTERISTICS OF ELECTRON LINEAR ACCELERATORS 7 1.1. Fields of application 7 1.2. Types of electron linear accelerator installations 8 1.3. Parameters of electron linear accelerators 11 2. RADIATIONS AT ELECTRON LINEAR ACCELERATOR INSTALLATIONS 27 2.1. Radiations anticipated and their quality factors 27 2.1.1. Types of radiations and their sources 27 2.1.2. Quality factors 29 2.2. Photon differential track length and estimation of yields .... 34 2.3. Electron beams 43 2.4. Photons 50 2.4.1. External bremsstrahlung 50 2.4.2. Scattered photons 57 2.5. Neutrons 61 2.5.1. Neutron quality factor and dose equivalent 61 2.5.2. Production mechanisms 65 2.5.3. Neutron yields from electron beams 79 2.5.3.1. The giant-resonance region (E0 < 35 MeV) . 82 2.5.3.2. Neutron production for 34<E0< 150MeV. 88 2.5.3.3. Neutron production for E0> 150 MeV .... 91 2.6. Radioactivity induced in components 97 2.6.1. Installations with E0 <35 MeV 101 2.6.2. Activity induced by high-energy beams 101 2.7. Activity induced in air and water 126 2.7.1. Airborne radioactivity 126 2.7.1.1. Air activation 126 2.7.1.2. Dust 131 2.7.2. Activity induced in water 136 2.8. Muons 142 2.9. Charged-particle secondary beams 147 2.10. Production of toxic gases 149 2.10.1. Concentration buildup and removal 151 2.10.2. Production rates 152 2.11. X-rays generated by microwave systems 156 2.11.1. Oscillators and amplifiers 156 2.11.2. Microwave cavities 158 3. RADIATION SHIELDING 160 3.1. Types of areas and shielding criteria 160 3.1.1. Accelerator scheduling and workload factor W 160 3.1.2. Primary and secondary barriers and the orientation (use) factor U 162 3.1.3. Occupancy factor T 164 3.2. Shielding materials 165 3.3. Physical considerations 168 3.4. Shielding against photons 175 3.4.1. Primary barriers (E0 < 100 MeV) 175 3.4.2. Secondary barriers (E0< 100 MeV) 177 3.4.3. Accelerators operating above 100 MeV 186 3.5. Shielding against neutrons 189 3.5.1. Energies below the photopion threshold (E0 < 140 MeV) 190 3.5.2. High-energy installations (E0> 140 MeV) 197 3.6. Labyrinths, doors, voids and penetrations 199 4. TYPICAL INSTALLATIONS 205 4.1. Medical installations 205 4.2. Industrial radiographic installations 207 4.3. Research and special-purpose installations 214 5. RADIATION MONITORING AND INTERPRETATION OF MEASUREMENTS 226 5.1. Characteristics and choice of monitoring equipment 226 5.2. Duty factor effects on radiation measurements 236 5.2.1. Dead-time effects in pulse counters 237 5.2.2. Recombination in ionization chambers 238 5.3. Neutron monitoring techniques 242 5.3.1. Fluence measurements 243 5.3.2. Spectral measurements 252 5.4. Radiation surveys 255 5.5. Instrument calibration and maintenance 260 6. REQUIREMENTS FOR AN EFFECTIVE SAFETY PROGRAMME.. 263 6.1. Safety organization 263 6.2. Safety programme 263 6.3. Radiation safety 265 6.4. Accelerator safety 268 6.5. Linear accelerators used in radiation therapy 270 6.5.1. General radiation safety 271 6.5.2. Reliability of dosimetry 272 6.5.3. Control of dose distribution 273 6.5.4. Safe delivery of the prescribed treatment 274 6.5.5. Provisions to facilitate accurate patient positioning .... 276 6.5.6. Control of unwanted dose to patient 277 6.5.7. Protection against other risks 279 6.6. Safety at industrial and research installations 282 7. GENERAL BIBLIOGRAPHY 286 APPENDIXES 293 Appendix A. Physical and numerical constants 294 Appendix B. Radiation parameters of materials 297 Appendix C. Rules of thumb 318 Appendix D. Addresses of organizations 322 INTRODUCTION More than a decade of experience has been gained since the publication of a manual devoted exclusively to radiological protection at high-energy electron accelerators. Because of the rapidly increasing use of electron linear accelerators, it was felt that it would be useful to prepare such a handbook that would encompass the large body of methods and data which have since been developed. Since the publication of NBS Handbook No.97, significant developments related to radiation protection at electron linear accelerators have occurred along the following lines: (a) Electron linear accelerators for medical and radiographic purposes operating in the range 4 — 40 MeV are now widely accepted. The growing number of such machines operating above 10 MeV poses additional problems of undesirable neutron radiations and concomitant component activation. (b) There has been a trend toward standardization of radiation-protection practices, and development of national and international radiation-protection guidelines for medical accelerators. (c) The introduction of high-energy, high-power electron machines has brought new types of problems and magnified old ones. The higher energy has necessitated provisions for high-energy neutron dosimetry and shielding, muon dosimetry and shielding, and treatment of the neutron skyshine problem. The higher power has aggravated such problems as radioactive air and water and the possibility of burn-through of shielding by raw electron beams. (d) Operating flexibility such as multibeam capability has placed new demands on personnel protection systems. (e) Refinement of measurements of photonuclear reactions has made more reliable predictions of neutron production and component activation possible. These developments include improved consistency among cross-section measure- ments with monochromatic photons in the giant-resonance region, as well as new data on less frequent types of reactions at all energies.