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Guidelines for Radioelement Mapping Using Gamma Ray Spectrometry Data

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Guidelines for Radioelement Mapping Using Gamma Ray Spectrometry Data IAEA-TECDOC-1363 Guidelines for radioelement mapping using gamma ray spectrometry data July 2003 The originating Section of this publication in the IAEA was: Nuclear Fuel Cycle and Materials Section International Atomic Energy Agency Wagramer Strasse 5 P.O. Box 100 A-1400 Vienna, Austria GUIDELINES FOR RADIOELEMENT MAPPING USING GAMMA RAY SPECTROMETRY DATA IAEA, VIENNA, 2003 IAEA-TECDOC-1363 ISBN 92–0–108303–3 ISSN 1011–4289 © IAEA, 2003 Printed by the IAEA in Austria July 2003 FOREWORD Gamma rays are the most penetrating radiation from natural and man-made sources, and gamma ray spectrometry is a powerful tool for the monitoring and assessment of the radiation environment. Gamma ray surveys are carried out from aircraft, field vehicles, on foot, in boreholes, on the sea bottom and in laboratories. Ground and airborne gamma ray measurements cover large areas of the earth’s surface, and many national and regional radiometric maps have been compiled and published. Standardized maps of terrestrial radiation and radioelement concentrations can be compared and regionally unified, showing general regional trends in radionuclide distribution and making the radiological assessment of the environment possible. Radiometric surveys and maps are applicable in several fields of science. They retain their geological and geophysical information for mineral prospecting, geochemical mapping and structural geology, and enable the comparison of geological features over large regions. Although the gamma ray method was originally developed for geoscience, it has also been successfully applied in emergency situations for mapping the contamination from nuclear fallout and for the location of lost radioactive sources. The use of modern computer data processing has enabled the introduction of new interpretation methods and the achievement of greater reliability in solving geological and environmental problems. The purpose and scope of this report is to introduce the theoretical background of gamma ray spectrometry in its application to the radiation environment, and to emphasize and illustrate new procedures in this field based on current knowledge. The report introduces the principles of radioactivity, contemporary radiation units, and the fundamentals of gamma ray spectrometry and its application to airborne, ground, car-borne, borehole and laboratory measurements. Examples of the use of gamma ray spectrometry in environmental studies and geological mapping illustrate the conditions, requirements and procedures for data acquisition, processing and reporting using this method. For many years the International Atomic Energy Agency (IAEA) has been involved in the treatment of exploration data and their multiple applications related to mineral exploration, agricultural, and environmental assessment. Recognizing that techniques and methods are rapidly progressing in this field, the IAEA invited a group of specialists to write a comprehensive report on radioelement mapping using gamma ray spectrometry. The authors of this publication are known and recognized specialists from four countries involved in mapping and data treatment using spectrometry. They all have extensive experience in the application and use of gamma ray spectrometry for radioelement mapping. This TECDOC is one of a series of IAEA publications covering all aspects of the uranium mining industry, from exploration to exploitation, decommissioning, and the application of techniques in other non-uranium resources areas. The IAEA officers responsible for this publication were J.-P. Nicolet and G. Erdi-Krausz of the Division of Nuclear Fuel Cycle and Waste Technology. EDITORIAL NOTE The use of particular designations of countries or territories does not imply any judgement by the publisher, the IAEA, as to the legal status of such countries or territories, of their authorities and institutions or of the delimitation of their boundaries. The mention of names of specific companies or products (whether or not indicated as registered) does not imply any intention to infringe proprietary rights, nor should it be construed as an endorsement or recommendation on the part of the IAEA. CONTENTS 1. INTRODUCTION ............................................................................................................ 1 1.1. Purpose of the report........................................................................................... 1 1.2. Historical review................................................................................................. 1 1.3. Gamma ray methods........................................................................................... 2 1.4. Scope of the report.............................................................................................. 2 2. PRINCIPLES OF RADIOACTIVITY ............................................................................. 4 2.1. Basic radioactivity .............................................................................................. 4 2.2. Types of radioactive decay................................................................................. 4 2.3. Statistical nature of radioactive decay................................................................ 5 2.4. Natural sources of radiation................................................................................ 6 2.5. Disequilibrium.................................................................................................... 7 2.6. Interaction of gamma rays with matter............................................................... 7 2.7. Detectors and instruments................................................................................ 12 2.8. Quantities and units .......................................................................................... 14 2.8.1. SI derived quantities and units and non-SI units accepted for use with SI................................................................................................... 15 2.8.2. Ground radioelement concentration units and conversion constants................................................................................................ 16 2.8.3. Conventional ground reporting units .................................................... 17 2.9. Natural and man-made absorbed doses ............................................................ 18 3. FUNDAMENTALS OF GAMMA RAY SPECTROMETRY....................................... 20 3.1. Sources of gamma radiation ............................................................................. 20 3.2. Properties of gamma ray spectra ...................................................................... 20 3.2.1. The detector response ........................................................................... 21 3.2.2. Source-detector geometry..................................................................... 21 3.3. Measurement of gamma radiation .................................................................... 25 3.4. Physical models................................................................................................ 25 3.5. Environmental effects....................................................................................... 29 4. GROUND RADIOMETRIC METHODS ...................................................................... 31 4.1. Portable gamma ray spectrometry .................................................................... 31 4.1.1. Instrumentation ..................................................................................... 31 4.1.2. Field measurement................................................................................ 31 4.1.3. Instrument calibration........................................................................... 34 4.1.4. Data processing procedure.................................................................... 38 4.1.5. Errors .................................................................................................... 39 4.2. Car-borne gamma ray spectrometry ................................................................. 40 4.2.1. Instrumentation ..................................................................................... 40 4.2.2. Field measurement................................................................................ 40 4.2.3. Instrument calibration........................................................................... 41 4.2.4. Data processing procedure.................................................................... 41 4.3. Borehole gamma ray spectrometry................................................................... 42 4.3.1. Instrumentation ..................................................................................... 42 4.3.2. Field measurement................................................................................ 43 4.3.3. Instrument calibration........................................................................... 43 4.3.4. Data processing procedure.................................................................... 43 4.4. Laboratory gamma ray spectrometry................................................................ 45 4.4.1. Rock samples and laboratory equipment .............................................. 45 4.4.2. Laboratory determination of natural radionuclides .............................. 46 4.4.3. Instrument calibration........................................................................... 48 4.4.4. Data processing procedure...................................................................
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