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I Wall Fission Spectrum Fast Reactor Spectrum (FERMI) Graphite Moderated Spectrum Light-Water Moderated Spectrum IETRI (Spectra Normalized to Equal Flux Greater than 0.0674 MeV) / \ / \ (0.06741 г 3 Lethargy, u TECHNICAL REPORTS SERIES No 107 Neutron Fluence Measurements INTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 1970 NEUTRON FLUENCE MEASUREMENTS The following States are Members of the International Atomic Energy Agency: AFGHANISTAN GREECE NORWAY ALBANIA GUATEMALA PAKISTAN ALGERIA HAITI PANAMA ARGENTINA HOLY SEE PARAGUAY AUSTRALIA HUNGARY PERU AUSTRIA ICELAND PHILIPPINES BELGIUM INDIA POLAND BOLIVIA INDONESIA PORTUGAL BRAZIL IRAN ROMANIA BULGARIA IRAQ SAUDI ARABIA BURMA IRELAND SENEGAL BYELORUSSIAN SOVIET ISRAEL SIERRA LEONE SOCIALIST REPUBLIC ITALY SINGAPORE CAMBODIA IVORY COAST SOUTH AFRICA CAMEROON JAMAICA SPAIN CANADA JAPAN SUDAN CEYLON JORDAN SWEDEN CHILE KENYA SWITZERLAND CHINA KOREA, REPUBLIC OF SYRIAN ARAB REPUBLIC COLOMBIA KUWAIT THAILAND CONGO, DEMOCRATIC LEBANON TUNISIA REPUBLIC OF LIBERIA TURKEY COSTA RICA LIBYAN ARAB REPUBLIC UGANDA CUBA . LIECHTENSTEIN UKRAINIAN SOVIET SOCIALIST CYPRUS LUXEMBOURG REPUBLIC CZECHOSLOVAK SOCIALIST MADAGASCAR UNION OF SOVIET SOCIALIST REPUBLIC MALAYSIA REPUBLICS DENMARK MALI UNITED ARAB REPUBLIC DOMINICAN REPUBLIC MEXICO UNITED KINGDOM OF GREAT ECUADOR MONACO BRITAIN AND NORTHERN EL SALVADOR MOROCCO IRELAND ETHIOPIA NETHERLANDS UNITED STATES OF AMERICA FINLAND NEW ZEALAND URUGUAY FRANCE NICARAGUA VENEZUELA GABON NIGER VIET-NAM GERMANY, FEDERAL REPUBLIC OF NIGERIA YUGOSLAVIA GHANA ZAMBIA 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. 1970 Permission to reproduce or translate the information contained in this publication may be obtained by writing to the International Atomic Energy Agency. Karntner Ring 11, P.O. Box 590, A-1011 Vienna, Austria. Printed by the IAEA in Austria May 1970 TECHNICAL REPORTS SERIES No. 107 NEUTRON FLUENCE MEASUREMENTS INTERNATIONAL ATOMIC ENERGY AGENCY VIENNA, 1970 NEUTRON FLUENCE MEASUREMENTS IAEA, VIENNA, 1970 STI/DOC/10/107 FOREWORD For research reactor work dealing with such subjects as radiation ef- fects on solids and such disciplines as radiochemistry and radiobiology, the radiation dose or neutron fluence is an essential parameter in evaluat- ing results. Unfortunately it is very difficult to determine. Even when the measurements have been accurate, it is difficult to compare results obtained in different experiments because present methods do not always reflect the dependence of spectra or of different types of radiation on the induced processes. After considering the recommendations of three IAEA Panels, on Чп-pile dosimetry' held in July 1964, on 'Neutron fluence-measurements' in October 1965, and on Чп-pile dosimetry' in November 1966, the Agency established a Working Group on Reactor Radiation Measurements. This group consisted of eleven experts from ten different Member States and two staff members of the Agency. On the measurement of energy absorbed by materials from neutrons and gamma-rays, there are various reports and reviews scattered through- out the literature. The group, however, considered that the time was ripe for all relevant information to be evaluated and gathered together in the form of a practical guide, with the aim of promoting consistency in the measurement and reporting of reactor radiation. The group arranged for the material to be divided into two manuals, which are expected to be use- ful both for experienced workers and for beginners. The present manual was edited by Dr. J. Moteff, of the General Electric Company, Cincinnati, Ohio, USA, and the companion volume, on 'Determination of absorbed dose in reactors', will be published by the Agency shortly. The authors who contributed to the present manual are: Chapter 1 : Introduction J. Moteff Chapter 2: Neutron spectra S.B. Wright (Atomic Energy Research Establishment, UKAEA, Harwell, United Kingdom) Chapter 3: Thermal neutrons Y. Droulers (Commissariat à l'Energie atomique, Centre d'études nucléaires de Grenoble, France) Chapter 4: Intermediate neutrons W. L. Zijp (Reactor Centrum Nederland, Petten NH, Netherlands) Chapter 5: Fast neutrons R.E. Dahl and H. H. Yoshikawa (Pacific Northwest Laboratories, Battelle Memorial Institute, Richland, Wash., USA) Index: A. Keddar (Division of Nuclear Power and Reactors, IAEA, Vienna) The original contributions have been changed somewhat during edit- ing in order to make the manual consistent. The editor and contributing authors wish to express their appreciation to Dr. S. Sanatani (IAEA), who was the Scientific Secretary of the Working Group during the early stages and to Dr. A.Keddar (IAEA), the present Scientific Secretary, for their help in assembling the manual. Special thanks are also due to Dr. W. KOhler of the Agency for his comments and help during the initial planning of the book and during the final editorial work. CONTENTS CHAPTER I. INTRODUCTION 1 CHAPTER II. NEUTRON SPECTRA 7 II. 1. Introduction 7 II. 1.1. Maxwellian spectrum II. 1. 2. Fission neutron spectrum II. 1.3. Intermediate neutron spectrum II. 1.4. Reactor spectrum II. 1. 5. Physical processes encountered in irradiation experiments И. 1. 6. Division of the neutron spectrum for the monitoring of irradiation experiments II. 2. Thermal neutron region 19 II. 2.1. Conventional thermal flux densities II. 2. 2. True thermal flux spectrum II. 2.3. Theoretical spectra II. 2.4. Variation of the spectrum across a reactor lattice II. 3. Fast neutron region 26 II. 3.1. Calculation of fast neutron spectra II. 3. 2. Variations in the fast neutron spectra in a heterogeneous reactor II. 3. 3. Comparison of experimental and theoretical spectra II. 3.4. Comparison of theoretical spectra II. 3. 5. The effect of the neutron spectrum on experimental measurements References to Chapter II 43 CHAPTER III. THERMAL NEUTRONS 45 III. 1. Theory of detector response 45 III. 1.1. General method III. 1.2. Westcott' s notation III. 1.3. Formalism of Horowitz and Tretiakoff III. 2. Measurement of thermal neutron flux density and fluence .... 52 III. 2.1. Relation between flux density, fluence and detector activity III. 2.2. Measurement of low flux densities III. 2.3. Measurement of high fluences 1П. 3. Measurement of thermal neutron spectra 65 III. 3.1. Principle of the method III. 3. 2. Example of the experimental procedure III. 4. Other measuring methods 69 III. 4. 1. Measurement of flux density III. 4. 2. Fluence measurements References to Chapter III 75 CHAPTER IV. INTERMEDIATE NEUTRONS 77 IV. 1. Introduction 77 IV.1.1. General IV. 1. 2. Response of a resonance detector IV. 1.3. Fluence and spectrum measurements IV. 1.4. Types of resonances IV. 2. Spectrum characteristics 81 IV.2.1. The 1/E spectrum IV. 2. 2. Other spectrum representations IV. 3. Detectors 88 IV. 3.1. Resonance integral cross-section IV.3.2. Detector response IV. 3.3. Self-shielding effect IV.4. Fluence measurements Ill IV.4.1. Experimental details IV. 4. 2. Data reduction and treatment IV. 4.3. Data reporting IV. 5. Spectrum measurements 130 IV. 5.1. Experimental details IV.6. Other methods 132 IV. 6.1. Recent developments with resonance detectors IV. 6. 2. Other fluence measurement methods IV. 6. 3. Other spectrum measurement methods IV. 7. Concluding remarks 135 References to Chapter IV 136 CHAPTER V. FAST NEUTRONS 141 V.l. Introduction 141 V.2. Neutron spectrum 142 V.3. Detector response ; 145 V.4. Fluence measurements 147 V. 4.1. Experimental details V.4.2. Data reduction V.4.3. Summary of fluence measurements V.5. Spectral determination from monitor activations 176 V.6. Other methods of fluence measurements 178 References to Chapter V 179 INDEX 183 CHAPTER I INTRODUCTION For reactor research work in the fields of radiation chemistry and radiation damage to solids, the neutron energy distribution and the neutron fluence 1 are essential parameters for the evaluation of the experimental results. The accurate determination of these factors in research reactors, and particularly in materials test reactors is extremely difficult. In many reactors there is pronounced variation of the power level and, therefore, of the flux density with time. Moreover, the various experimental assem- blies, control-rod positions a:nd local burn-up will tend to perturb the flux density throughout the reactor core and reflector regions. These pertur- bations make it almost impossible to estimate doses and fluences from measurements made on cold, clean reactor cores. For this reason it is desirable to measure the^ radiation environment in exactly the same core geometry as would be used for each experiment. At the present time it seems feasible to recommend specific methods, at least for neutron fluence measurements. However, even if unified ex- perimental techniques could be suggested, there still exist serious problems in the presentation of experimental results. An example may be found in the field of graphite irradiations. Radiation damage has, on different occasions, been expressed as a function of thermal neutron fluence, fission neutron fluence, or fast neutron fluence. Without more detailed information on the reactor neutron environment, an intercomparison of results cannot even be attempted in any
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