Radioisotope Instruments: International Series of Monographs in Nuclear Energy

By J. F. Cameron and C. G. Clayton

International Series of Monographs in Nuclear Energy, Volume 107: Radioisotope Instruments, Part 1 focuses on the design and applications of instruments based on the radiation released by radioactive substances. The book first offers information on the physical basis of radioisotope instruments; technical and economic advantages of radioisotope instruments; and radiation hazard. The manuscript then discusses commercial radioisotope instruments, including radiation sources and detectors, computing and control units, and measuring heads. The text describes the applications of radioisotope instruments in the industries, including mining and quarrying; agriculture, forestry, and fishing; manufacturing industries; transport and communications; and civil engineering constructions. The manuscript also focuses on legislation and codes of practice on the use of sealed radioisotope sources and control of radiation hazard. The book is a dependable reference for readers interested in radioisotope instruments.

• Language: English
• Publisher: Elsevier Science
• Release date: Oct 22, 2013
• ISBN: 9781483159331

Book preview

International Series of Monographs in Neclear Energy, Volume 107

Radioisotope Instruments

Volume 1, Part 1

J.F. Cameron

C.G. Clayton

Table of Contents

Cover image

Title page

Copyright

Foreword

Preface

Acknowledgments to Volume 1

Chapter 1: Introduction

Publisher Summary

1.1 THE PHYSICAL BASIS OF RADIOISOTOPE INSTRUMENTS

1.2 TECHNICAL ADVANTAGES OF RADIOISOTOPE INSTRUMENTS

1.3 ECONOMIC ADVANTAGES OF RADIOISOTOPE INSTRUMENTS

1.4 SOME CHARACTERISTICS OF RADIOISOTOPE INSTRUMENTS

1.5 RADIATION HAZARD

Chapter 2: Commercial Radioisotope Instruments

Publisher Summary

2.1 INTRODUCTION

2.2 RADIATION SOURCES AND DETECTORS

2.3 MEASURING HEADS

2.4 INDICATING UNITS

2.5 COMPUTING AND CONTROL UNITS

Chapter 3: Applications of Radioisotope Instruments in Industry

Publisher Summary

3.1 INTRODUCTION

3.2 ADVANTAGES OF RADIOISOTOPE INSTRUMENTS

3.3 SUMMARY OF THE USES OF RADIOISOTOPE INSTRUMENTS

3.4 AGRICULTURE, FORESTRY AND FISHING

3.5 MINING AND QUARRYING

3.6 THE MANUFACTURING INDUSTRIES

3.7 CIVIL ENGINEERING CONSTRUCTIONS

3.8 ELECTRICITY, GAS, WATER AND SANITARY SERVICES

3.9 RESEARCH, SERVICES, TRANSPORT AND COMMUNICATIONS

Chapter 4: Health and Safety, Legal Requirements and Insurance

Publisher Summary

4.1 INTRODUCTION

4.2 RADIATION HAZARD AND ITS CONTROL

4.3 LEGISLATION AND CODES OF PRACTICE GOVERNING THE USE OF SEALED RADIOISOTOPE SOURCES

4.4 INSURANCE OF SEALED RADIOISOTOPE SOURCES AND INSTALLATIONS

Copyright

Pergamon Press Ltd., Headington Hill Hall, Oxford

Pergamon Press Inc., Maxwell House, Fairview Park, Elmsford,

New York 10523

Pergamon of Canada Ltd., 207 Queen’s Quay West, Toronto 1

Pergamon Press (Aust.) Pty. Ltd., 19a Boundary Street,

Rushcutters Bay, N.S.W. 2011, Australia

Vieweg & Sohn GmbH, Burgplatz 1, Braunschweig

Copyright © 1971 C. G. Clayton, J. F. Cameron

All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of Pergamon Press Ltd.

First edition 1971

Library of Congress Catalog Card No. 77-140495

Printed in Great Britain by A. Wheaton & Co., Exeter

08 015802 1

Foreword

This is the first comprehensive work dealing solely with the application and design of radioisotope instruments and as such is a very welcome addition to the library of information on the peaceful uses of atomic energy. But it is more than this. These instruments, which were first introduced into industry in the United Kingdom by my former collaborators, have now spread to almost all areas of economic activity so that nowadays there is hardly one mass-produced commodity which does not benefit at some stage in its production from the application of these instruments. This book provides, therefore, first and foremost, an insight into a new and advancing technology which can stand in its own right amongst the major technologies of the world.

The idea of publishing this book in two volumes is excellent and Volume 1, which deals mainly with the applications of the instruments, should appeal especially to those amongst us who must keep abreast of the outlines of expanding technologies but who are not so concerned with their most intimate details. Volume 2 is written for the specialist; the scientist and the engineer responsible for the design, installation and maintenance of radioisotope instruments. However, it contains information which should be valuable to all who are concerned in any way with radiation interactions and their application.

Although several thousand papers have already been published on almost every aspect of these instruments, this book nevertheless contains much original material and a great deal of information never before published in the Western press. For this latter reason alone it is to be highly recommended.

The authors of this book are both well known to me personally and I have had close professional contact with them for many years. I believe that they are amongst only a few who have both the wide experience and knowledge required to assemble the information contained in this extremely valuable work.

Although I have been in close contact with the field since its inception, I found a high amount of useful new information. I have no hesitation to recommend this book to all classes of industrialists including management, control engineers, scientists and everyone interested in the practical application of radiation techniques, analysis, product control and non-destructive testing.

HENRY SELIGMAN,     Vienna

Preface

Our intention in writing this book has been to give an account of the design and application of instruments based on the radiations emitted by radioactive substances. Such instruments, which include a radioactive source in combination with a radiation detector, are referred to as radioisotope instruments. They are used to measure a variety of physical properties of materials in the solid, liquid and gaseous states and they are now designed to operate in the laboratory, in the field and in industrial plant where they form an essential part of many types of process control system.

The simplest radioisotope instruments, and the ones which have been most fully developed, are used to measure thickness and density and are based on the attenuation and scattering of radiation. However, as a result of a more detailed study of radiation interaction phenomena during the past few years a number of other types of instrument have been developed. Typical of these are instruments for analysis and coating thickness measurement using X-ray fluorescence spectrometry, instruments for the determination of hydrogen–carbon ratios and the sulphur and tetraethyl lead concentration of petroleum products using preferential absorption techniques, instruments for the analysis of gases in gas chromatography using ionization phenomena and instruments for following the water–oil interface in oil wells based on measurements of the rate of thermal neutron capture.

It has become clear that an empirical approach to design which was adequate for the early generations of the simpler type of instruments is no longer sufficient and that a sound understanding of the basic principles of radiation interactions has lead to noteworthy improvements in techniques and to the development of a range of new and successful devices.

For several years the authors have had the opportunity of discussing the design and application of radioisotope instruments to problems posed by physicists, chemists and engineers associated with many different areas of industrial activity and of gaining some insight into many problems to which radioisotope instruments could be applied. It was as a result of such discussions that the need for a detailed description of the various applications and for a formal exposition of basic principles became apparent and this book was conceived. It is aimed at bringing together into one work the more relevant ideas now scattered in a vast amount of scientific and technical literature.

The book has been written in two volumes. Volume 1 is devoted to the use of radioisotope instruments whilst in Volume 2 an account is given of the underlying physical principles so as to enable the design, construction and use to be more fully understood.

Although the two volumes are complementary, Volume 1 has been written so that it can be read and referred to independently of Volume 2. As a result it should appeal to those who wish to acquaint themselves with the applications of radioisotope instruments but who feel no need to become familiar with the finer points of instrument design. It has been this attempt to satisfy the demands of different classes of reader which has led to our decision to write this book in two volumes. Although, to appreciate the applications of radioisotope instruments, little understanding of their design features is necessary, some acquaintance with the principal characteristics of these instruments is required. We have, therefore, in the first chapter of Volume 1, given a very brief account of the main features of these instruments with no more detail than is necessary to appreciate the contents of the following chapters. Those interested in the design of a particular type of instrument can refer to Volume 2.

The second chapter is intended to illustrate the various types of commercial instruments which have been designed for different applications. In the selection of examples, emphasis has been given to proven instruments in routine use and care has been taken to show how each type of instrument has been constructed to cope with the often extremely arduous environmental conditions in which it must operate.

Chapter 3 is concerned with the technical and economic advantages and the application of these instruments in all branches of economic activity. It has clearly been impossible to describe in detail all the applications reported in the literature. Emphasis has therefore been placed on those applications which the authors consider to be most important: other applications are mentioned briefly and references are given at the end of each section.

In some of the original papers referred to it was not always clear whether applications which have been developed are still in use or have been abandoned, and wherever there is reason for doubt we have tried to imply this in the text. In some papers, even in those referring to well-established applications such as the measurement of paper thickness by β-transmission techniques, not all the information was available to satisfy the different interests of all readers. Many papers refer to new developments which have as yet been only briefly reported but which could become important in the future. To enable readers to form their own opinions and to enquire further into many of the applications quoted, more than usually liberal lists of references have been included.

In the final chapter in Volume 1, the implications of regulations relating to radiation hazard and their effect on the application of radioisotope instruments are discussed. What is often regarded as a radiation hazard resulting from the application of radioactive isotopes in industry and the regulations designed to ensure their safe use are frequently quoted as inhibiting the more widespread acceptance of radioisotope instruments. In fact it is now strikingly evident that the incorporation of radioactive material need not constitute a major restriction on their use for virtually any application. All instruments can be designed so that the dose rates at accessible points are less than the agreed maximum permissible levels and the regulations relating to the granting of licences for the use of radioactive sources, the registration of premises and the insurance of plant are things which do not constitute a difficult barrier. Many of these requirements are in fact common to other types of instruments.

In Volume 2, particular attention has been given to those properties of radioisotopes which are relevant to a proper appreciation of the operation and application of these instruments. Radiation interaction phenomena and the performance of radiation detectors, an understanding of which underlies the design of all radioisotope instruments, have been dealt with in some detail. A chapter has also been included outlining the principles of statistics relevant to radioisotope instruments. The importance of having a clear understanding of this subject cannot be overemphasized, since the accuracy of radioisotope instruments is partly dependent on statistical fluctuations and, as with other instruments, the ultimate accuracy of measurement can only be adequately expressed by statistical methods. The broad principles of the electronic equipment, in which the signal from the detector is translated into a form which can be observed visually or used for automatic process control, has been described briefly. The last chapter in Volume 2 is devoted entirely to a description of the design of radioisotope instruments such as thickness and density gauges, level gauges, analytical instruments (based on techniques such as X-ray fluorescence and X-ray and β-particle scatter), borehole logging instruments (using a wide variety of different radiation interactions) and a group of miscellaneous instruments such as gas-flow meters and gas chromatographic detectors based on ionization phenomena in gases. By adopting a semi-empirical mathematical approach to design, feasibility studies can be carried out with sufficient accuracy for most applications and the agreement between this simplified theory and actual measurements is illustrated for a number of typical instruments in common use.

In purely scientific works it is good practice to ensure that the same system of units is used consistently throughout the text. In writing on technological practice, however, as we are doing in much of this book, we feel that such strict adherence to uniformity would more likely lead to confusion than it would to clarity. There seems little point, for instance, in quoting the thickness of a coating of tin on steel solely in g/cm² when it is accepted practice in the United Kingdom to measure this thickness in microinches or oz/ft² or even in units of lb/basis box. When generally unfamiliar units are first encountered, therefore, we have tried to give the cgs equivalent. We trust the reader will accept our reasons for the frequent use of mixed units.

Acknowledgments to Volume 1

The writing of this volume has been made possible only through the generous help of many colleagues and friends throughout the world. We are extremely grateful to all of them for the valuable information which has been made freely available to us.

The co-operation of S. J. Wright, now of Nuclear Enterprises Ltd., who for many years has given us the benefit of his experience, is especially acknowledged.

For valuable discussions relating to various parts of the text, we should particularly like to thank; A. G. M. Batten (Alliance Assurance Co., Ltd.), E. Bell (formerly of S.R.N.E., Ltd.), P. Boyle (Boyle Nucleonic Gauging Systems, Ltd.), L. E. Taylor and W. E. Thompson (Ekco Electronics, Ltd.), S. Margolinas (formerly of Nucleometre, France), K. Ljunggren (Isotope Techniques Laboratory, Sweden), S. Bosch (Frieseke-Hoepfner, GmbH, West Germany), A. Tröst (Bertholdt Laboratories, West Germany), C. O. Badgett (Industrial Nucleonic Corps., U.S.A.), R. L. Caldwell (Socony-Mobil Oil Corp., U.S.A.), C. A. Ziegler (Panametrics Inc., U.S.A.), S. H. U. Bowie and M. J. Gallagher (Institute of Geological Sciences), P. Martinelli (C.E.A., Saclay, France), B. Dziunikowski, J. A. Czubek, T. Florkowski and A. Zuber (Institute of Nuclear Techniques, Krakow, Poland), A. G. Darnley (Geological Survey of Canada), W. E. Mott (Division of Isotopes Development, U.S.A.E.C.), D. F. White (U.K.A.E.A., Harwell) and the late G. Appleton (U.K.A.E.A., Risley).

Inevitably we have drawn on much information from our former colleagues at Wantage Research Laboratory, especially P. F. Berry (now at Texas Nuclear Corp., U.S.A.), I. S. Boyce, T. W. Packer and J. R. Rhodes (now at Columbia Scientific Research Institute, U.S.A.). Mr. O. Armstrong, librarian at Wantage, has checked many of the references.

To Dr. J. L. Putman we are especially indebted as he has laboured through the entire manuscript and suggested innumerable improvements in content and style.

Our secretaries Poldi Baumgartner, Jill Mitchell and Johanna Spielauer have worked through several nearly illegible drafts and in so doing have shown almost infinite patience with us. We owe them our most sincere thanks.

There is no need to thank our wives. They are more relieved even than we that this book is finished.

J.F. CAMERON,     Edinburgh

C.G. CLAYTON,     Harwell

1

Introduction

Publisher Summary

This chapter discusses the physical basis of radioisotope instruments. A radioisotope instrument consists of a source of radiation, usually a sealed radioisotope preparation, a radiation detector, and an electronic unit to convert the output from the detector into a signal that is capable of operating a visual display or actuating an automatic control system. Radioisotope instruments are used to measure a variety of physical properties of materials such as, thickness and density, viscosity, coating thickness, and elemental composition by a number of methods that are based on the interaction of the radiations from radioactive isotopes. To understand the physical basis of radioisotope instruments is to understand the nature of primary and secondary radiation interactions with materials in the solid, liquid, and gaseous phases. When radiation passes through matter, it is absorbed and scattered to a degree that depends upon the nature of the material and the type and energy of the incident radiation.

1.1 THE PHYSICAL BASIS OF RADIOISOTOPE INSTRUMENTS

A radioisotope instrument consists of a source of radiation, usually a sealed radioisotope preparation, a radiation detector and an electronic unit to convert the output from the detector into a signal capable of operating a visual display or actuating an automatic control system. Radioisotope instruments are used to measure a variety of physical properties of materials such as thickness and density, viscosity, coating thickness and elemental composition by a number of methods based on the interaction of the radiations from radioactive isotopes with the materials under examination. The techniques employed and some typical applications are given in Table 1.1.

TABLE 1.1

TECHNIQUES EMPLOYED IN RADIOISOTOPE INSTRUMENTS AND SOME TYPICAL APPLICATIONS

To understand the physical basis of radioisotope instruments is to understand the nature of primary and secondary radiation interactions with materials in the solid, liquid and gaseous phases.

In this book, therefore, we will be concerned with α- and β-particles, X- and γ-radiations, bremsstrahlung and neutrons emitted during the disintegration of radioactive nuclides and during the interaction of these radiations with other atoms and molecules. We will attempt to understand the mechanisms of the different interactions and how these can be used to measure the properties of materials. The design concepts, advantages and limitations of these instruments should then become apparent.

When radiation passes through matter it is absorbed and scattered to a degree which depends upon the nature of the material and the type and energy of the incident radiation. A beam of radiation traversing a material is thus attenuated as a result of these processes, the attenuation generally increasing with thickness and density, and in some cases with atomic number, but being independent of the physical state of the material. These facts form the basis of a simple radioisotope instrument. If a given material is placed between a radioactive source emitting β-particles or γ-radiation and a detector, a measurement of the percentage radiation transmitted enables the thickness of the material to be determined if the density is constant. As a corollary, the density may be determined if the thickness is constant. An instrument based on the above principle, where the absorber is placed between source and detector, is known as a transmission gauge and the arrangement, which is shown diagrammatically in Fig. 1.1, is generally referred to as having transmission geometry. A simple extension of the transmission gauge principle enables the levels of liquids and solids in closed containers to be determined.

FIG. 1.1 Schematic diagram of a simple transmission gauge.

In some cases thickness and density may be measured with the source and detector on the same side of the material. An instrument based on this principle is known as a backscatter gauge and the arrangement is said to have backscatter geometry. An elementary form of backscatter gauge is shown in Fig. 1.2.

FIG. 1.2 Schematic diagram of a simple backscatter gauge.

In addition to this classification according to geometrical configuration, a further classification of instruments is possible according to the type of radiation used; thus we have, for example, a β-transmission gauge (i.e. an instrument based on the transmission of beta particles) or a γ-backscatter gauge (an instrument based on the backscatter of gamma radiation). Unfortunately this simple classification system cannot be applied to all types of measurement and this is especially true when the interaction results in the emission of a different type of radiation from that incident: in these cases the total interaction is usually written down.

The intensity of β-particles backscattered from any substance increases with thickness and atomic number. Hence, if a thin layer of one material is built up on a thick base material, the thickness of the layer may be determined provided the atomic numbers of the two materials are sufficiently different.

Somewhat similar considerations apply to the backscatter of γ-radiation which also depends on the composition of the scattering medium. Use is made of this fact to measure the varying concentration of one component in an otherwise homogeneous matrix. The measurement of coal ash is an outstanding example of a successful application of this type. Coal sensing probes used to determine the thickness of residual layers of coal on the floor and roof of a coal face during automatic coal-cutting also operate by measuring the change in backscattered intensity between coal (mainly carbon and hydrogen) and underlying or overlying deposits of shale.

The absorption of high energy γ-radiation (above about 0·5 MeV) is only weakly dependent on atomic number and depends mainly on thickness and density. In contrast the absorption of low energy electromagnetic radiation (below 100 keV) is critically dependent on atomic number, as well as on thickness and density. A number of techniques now widely used in analysis are based on this strong dependence of the attenuation of low-energy radiation on the atomic number of the absorber. They are called preferential absorption techniques and are used to determine the concentration of materials of high atomic number in a low atomic number matrix. The analysis of sulphur, cobalt and lead in petroleum products are the most important examples at present.

During the processes of absorption and scattering, the incident radiation excites and ionizes atoms of the material through which it passes. Excitation and ionization are transient phenomena which ultimately give way to fluorescence radiation with energies characteristic of the excited and ionised atoms. Characteristic X-ray fluorescence radiation resulting from de-ionization is also used in analysis and to determine the thickness of a thin coating of one material on a base material of different composition. X-ray fluorescence analysis has a wide and currently increasing application in geological prospecting, mining, ore-refining, chemical engineering and in the manufacturing industries.

Except in certain insulators and semiconductors, ionization produced as a result of radiation interactions in solids has a short lifetime. In gases, however, because of the relatively long mean free paths between the atoms and molecules, the lifetime of an ion is greatly increased. When an electric field is applied to an ionized gas, the ions are separated and may be collected at electrodes: they then constitute an electric current. These facts are used in anemometry, the measurement of gas flow rates and in radiation dosimetry.

The rate at which ions form and recombine varies from one gas to another. Provided that the temperature and pressure are constant the magnitude of the ionization current is a measure of gas composition. Impurities in gas streams, such as occur at the exit of gas chromatographic columns, are analysed in this way.

The pattern of neutron interactions with matter is generally somewhat different from the pattern of interactions of other types of radiations. High-energy neutrons (several keV to several MeV) interact with matter by inelastic collisions with nuclei—as a result of which their energy is reduced—and by capture to form radioactive nuclides. Several capture reactions for high energy neutrons exist, mainly (n,p), (n,α) and (n,2n) reactions. Low-energy neutrons interact mainly by the (n,γ) reaction. The cross-sections for the (n,γ) reaction for thermal neutrons are usually higher than are high-energy neutron capture cross sections. The variation of capture cross-section for both high- and low-energy neutrons may fluctuate rapidly and violently with atomic number and it is in this respect that neutron interaction probabilities differ most from interaction probabilities for other types of radiation.

Radioisotope instruments using both isotopic neutron sources and 14 MeV neutron generators are mainly confined to the field of analysis. The most important applications are in borehole logging, especially in oil-field exploration, and in the continuous analysis of oxygen in steel during steel-making.

Apart from an understanding of radiation interactions, several other important topics are included within the general concept of the physical basis of radioisotope instruments. An understanding of factors governing the choice of detector for different applications is probably most important but other considerations such as source design and geometrical effects of shape and size of source and detector are also important.

1.2 TECHNICAL ADVANTAGES OF RADIOISOTOPE INSTRUMENTS

One of the most important advantages of a radioisotope instrument is that contact with the material being examined is not required. Consequently these instruments are used on production lines travelling at high speeds, in systems operating at extreme temperatures, or on soft materials or materials where the surface finish is important. In the food industry, non-contact measurements ensure sanitary conditions throughout the manufacturing processes.

The penetrating nature of high-energy gamma radiation enables measurements to be made through the walls of sealed containers. Densities or levels of solids, liquids or slurries in pipes and tanks, for instance, can be determined using an externally mounted source and detector. Rejection of empty or underfilled packets from a production line is another example of this application. Scanning techniques to measure the degree of homogeneity across the width of a continuously produced sheet can be carried out more easily with a non-contact type of instrument.

If the mean atomic number of a material is constant, the attenuation of a beam of β-particles or X- or γ-radiation is dependent solely on the product of the thickness and density of the material. The attenuation is thus a direct function of the mass per unit area of the material. Since many sheet materials are manufactured and sold on the basis of weight per unit area, a direct and accurate measurement of this quantity has an important technical advantage. In the production of plastic sheet and certain types of paper, however, the composition of the material may vary from one production batch to another and this variation in atomic number may affect the attenuation of the radiation as well as the variation in mass per unit area. This difficulty can be overcome by several techniques, the simplest of which is to use a set of clip-on scales calibrated for material composition, or to include an electronic circuit, operated by manual control, which can be adjusted to give exact compensation for variations in the mean atomic number of the material.

An important feature of radioisotope instrument design and application is that in principle there is no limit to the size of the area of material which can be examined, although in practice limitations to the largest area may be imposed by the cost of the system and by considerations of radiation hazard, for example. As far as the radioactive source is concerned the main limitation to its physical size is generally the size of available reactor space, although this can generally be overcome by fabricating a large source from a number of smaller ones. The ionization chamber, which is probably the most used radiation detector in applications of this type, can also be manufactured with a very large sensitive area. A radioisotope instrument can thus be constructed to be sensitive to both gross or local variations and this is an important feature in many types of process control.

For many applications, radioisotope instruments are an attractive alternative to X-ray machines. They have the advantage of being compact and relatively inexpensive: the radiation energy and intensity are stable (apart from radioactive decay which is usually small and can be allowed for accurately) and electrical power supplies are not required. These particular characteristics have resulted in many important applications on production lines where only a small space is available for the installation, and reliability is of extreme importance. The small size of radioisotope instruments has also allowed many applications to be carried out at great depths in boreholes; initially, mainly for oil-field exploration but more recently in a variety of different types of geological ore prospecting. In recent years compact portable instruments, mainly for analysis and plating thickness measurement, have been developed using low energy β- and bremsstrahlung sources. The low radiation intensities from these sources have permitted routine manual application of instruments with only very simple safety devices. Because of the low cost and flexibility of radioisotope analytical instruments they are now being used in large numbers in process control systems in circumstances where previously a price restriction would have allowed only one alternative instrument to be used.

1.3 ECONOMIC ADVANTAGES OF RADIOISOTOPE INSTRUMENTS

The economic advantages to be gained from the use of radioisotope instruments vary greatly according to the application and the financial circumstances of different industries and for these reasons it is not possible to give a general formula for the guidance of the potential user. Particular examples will, however, serve to give some indication of their value in the different areas of application. In general the value of radioisotope instruments, as with many other types of instrument, is calculated in terms of the operating time during which the savings resultingfrom the installation are equal to the initial cost of the instrument. This amortization period for large radioisotope instrument installations is usually between 6 months and 2 years but in particular applications it can be a great deal less than this.

Instruments used to control the manufacture of sheet material enable closer tolerances to be achieved. When materialmanufactured to a given mass per unit area is sold by area, accurate measurement of mass per unit area allows the mean value to be maintained close to the specification and thereby reduces the total amount of material which need be used. When sheet materials are sold by weight there is no direct saving of raw material by using radioisotope instruments buta more uniform material can be produced and this often has important advantages during subsequent processing.

By rapidly recognizing material which is outside the specified tolerance excessive waste can be avoided. This is particularly true in industries like the paper industry where the same machine is often used to produce material to a wide range of different specifications and accurate control is required, especially when the specification is being changed.

The use of level gauges enables processes to operate continuously with the assurance that the rate at which the product is moving through the system is consistent with the highest efficiency. Instruments which operate outside closed containers can be maintained easily without interfering with the manufacturing process. Empty packet detectors protect the customer from being sold under-weight and unfilled packages—and thereby protect the reputation of the manufacturer.

The use of analytical instruments in large numbers on process control systems in the petroleum, chemical engineering and basic metal industries is enabling automatic process control to be carried out with resultant increase in production efficiency. In some cases the use of analytical radioisotope instruments allows new processes to be carried out which would otherwise be impossible. Coal blending based on the measurement of coal ash, and coal sensing during automatic mining operations are typical examples.

Borehole logging instruments in use in the oil and mineral ore prospecting industries are enabling important measurements to be made which would otherwise be difficult, expensive and time consuming, if not impossible. The economic advantages from applications of this type are difficult to assess but there is no doubt they are considerable.

1.4 SOME CHARACTERISTICS OF RADIOISOTOPE INSTRUMENTS

Perhaps the most unique feature of a radioisotope instrument, and the one that distinguishes it from other types of instrument, is that the measurement is derived from a radiation intensity subject to statistical fluctuations, due to the random nature of the emission of radiation from radioactive isotopes. Every measurement is thus associated with an intrinsic error which in nearly all cases is accountable in terms of Poisson statistics. One of the main problems in radioisotope instrument design is to recognize the magnitude of this error and to ensure that it is small in comparison with the variations in the property being examined. This is not always possible in practice and the desired accuracy of many measurements is often limited by statistical considerations.

An appreciation of the influence of statistics in radioisotope instrument design can be obtained from an elementary consideration of the problem of thickness measurement using a system such as that shown in Fig. 1.1. The radiation from the source, for this purpose assumed to be γ-radiation, is emitted discontinuously and isotropically: backwards into the source as well as forwards into the detector. Without the absorber, the detector receives only a percentage of the emitted radiation determined by the solid angle subtended by its sensitive area. This gives us our first limitation to the total available intensity in the direction of the detector since, unlike many other radiation sources, nothing can be done to focus the radiation in the required direction. Some of the primary radiation will, however, be scattered from the source holder, but since this will in general be radiated isotropically, only the same proportion of the total scattered radiation as of the primary radiation will arrive at the detector. Although focusing is not possible, a narrow beam of radiation can be produced by using a collimator which screens the source in all but one or more preferred directions. The greater the degree of collimation, i.e. the narrower the beam of radiation, the greater the intensity of the source required to produce the same total response at the detector.

With an absorber present between the source and detector of Fig. 1.1 the radiation intensity at the detector is reduced. The greater the thickness and density of the absorber, the greater the attenuation of the radiation in the direction of the detector.

Suppose that the number of photons detected per second is n and that the measurement is required in a time t sec. The number of photons actually recorded in t sec will be nt ± √(nt) where √(nt) is the standard deviation of the measurement according to Poisson statistics and is a measure of the uncertainty in the true value of nt. The relative uncertainty (coefficient of variation) is given by √(nt)/(nt) = 1/√(nt).

A radioisotope instrument is used to measure some quality X of a material in terms of the output I of a radiation detector. The instrument sensitivity, or relative sensitivity, S, is defined as the ratio of the fractional change δI/I in detector output which results from a given fractional change δX/X in the quality being measured, i.e.

If, in a measurement, the only source of error is the statistical variation in the number of recorded events, the coefficient of variation in the value of the quality measured

To reduce this to as small a value as possible then S, n or t, or all three of these variables should be increased to as high a value as possible. In many cases, however, the time available for measurement is short. This is particularly true on high-speed production lines of sheet material where only a few milli-seconds may be available for the measurement.

We can now see how measurement time, collimation, detector size and absorber thickness may affect the error in the measurement. The shorter the measurement time, the greater the degree of collimation, the thicker the absorber and the smaller the detector, the greater the source activity required to maintain a constant error. If a large source activity is required then this itself may impose a limit on accuracy, since the higher the activity of the source the greater its cost. An increase in source activity may also necessitate an increase in its physical size, after the highest specific activity has been achieved and this also may prove to be a restriction if an intense, highly collimated beam is required.

In the following chapters, the precision or reproducibility of a measurement is defined in terms of the ability to repeat measurements of the same quantity. Precision is expressed quantitatively in terms of the standard deviation from the average value obtained by repeated measurements. In practice it is determined by statistical variations in the rate of emission of radiation, instrumental instabilities and variations in measuring conditions.

The accuracy of a measurement is an expression of the degree of correctness with which an actual measurement yields the true value of the quantity being measured. It is expressed quantitatively in terms of the deviation from the true value of the mean of repeated measurements. The accuracy of a measurement depends on the precision and also the accuracy of calibration. If the calibration is exact, then in the limit, accuracy and precision are equal. When measuring a quantity such as thickness it is relatively easy to obtain a good calibration. In analysing many types of samples, on the other hand, the true value is often difficult to obtain by conventional methods and care may have to be taken in quoting the results.

In general, therefore, a result is quoted along with the calculated error in the result and the confidence limits to which the error is known. Confidence limits of both one standard deviation, lσ (68% of results lying within the quoted error), and two standard deviations, 2σ (95% of results lying within the quoted error), are used. Occasionally results are quoted with a higher degree of confidence (up to 5σ).

In analytical instruments, when commenting on the smallest quantity or concentration which can be measured, the termlimit of detection is preferred and used wherever possible. This is defined as the concentration at which the measuredvalue is equal to some multiple of the standard deviation of the measurement.

Care has been taken to adhere to the above definitions. However, in many papers which were referred to, these terms were used indiscriminately, with a variety of meanings and without definition. Although it was not always clear to what degree of confidence published results were given, we have, in fact, often considered it more important to quote available data (to unknown confidence limits) than to omit it altogether.

In practice, the accuracy of radioisotope instruments used to measure the thickness of materials is generally within ± 1 %, except for very light-weight materials when it is about ±2%. Coating thickness is usually measured

Reviews for Radioisotope Instruments

[Wajid Hussain · Rating: 4 out of 5 stars]

Very good explanation coupled with illustrations about the theory, principals and applications of the Radioisotope Instruments in industrial production world.