Radioisotope Instruments

307 Pages·1971·10.249 MB·English

by  J. F. Cameron, C. G. Clayton and D. J. Silverleaf (Auth.)

Radioisotope
Instruments
Part 1
J. F. Cameron and C. G. Clayton
P E R G A M ON P R E SS
OXFORD . NEW YORK
TORONTO · SYDNEY · BRAUNSCHWEIG
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 infor-
mation 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 collabor-
ators, 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 infor-
mation 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
xiii
Preface
OUR intention in writing this book has been to give an account of the design and applica-
tion 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 note-
worthy 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 radio-
isotope 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
XV
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 instru-
ments 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 applica-
tion of these instruments in all branches of economic activity. It has clearly been impos-
sible 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
ß-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 maxi-
mum 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 detec-
tors, 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 prin-
ciples of statistics relevant to radioisotope instruments. The importance of having a clear
xvi
understanding of this subject cannot be overemphasized, since the accuracy of radioiso-
tope 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 thick-
ness and density gauges, level gauges, analytical instruments (based on techniques such
as X-ray fluorescence and X-ray and ^-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 ioniza-
tion 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/cm 2 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 encoun-
tered, therefore, we have tried to give the cgs equivalent. We trust the reader will accept
our reasons for the frequent use of mixed units.
xvii
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. Trost (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. Martineiii (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
xix
1
Introduction
1.1. THE PHYSICAL BASIS OF RADIOISOTOPE INSTRUMENTS
A radioisotope instrument consists of a source of radiation, usually a sealed radio-
isotope 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 radia-
tions from radioactive isotopes with the materials under examination. The techniques
employed and some typical applications are given in Table 1.1.
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 a- and ß-particles, X- and y-radia-
tions, 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 radia-
tion. 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 ß-particles or y-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 deter-
mined.
1
TABLE 1.1. TECHNIQUES EMPLOYED IN RADIOISOTOPE INSTRUMENTS AND SOME TYPICAL APPLICATIONS
Property measured Technique Typical applications
Thickness, or mass a-transmission Very light-weight materials, e.g. cigarette paper
per unit area ^-transmission Light-weight materials, e.g. paper, plastic, metals
Transn. of low-energy Sheet metals
X-rays and brems-
strahlung
Transn. of high-energy Trimming hot-steel blooms; hot rolling;
y-radn. materials on conveyor belts
ß-backscatter Paper
y-backscatter Walls of pipes, tanks, process vessels
Coating thickness ^-transmission Coated textiles and papers, e.g. leathercloth,
(differential method) abrasive papers and cloths
ß-backscatter Zn on steel ; Ba S0-coating on photographic
2 4
paper; rubber on calenders ; coal-dust deposits
y-backscatter Thickness of coal on rock to control coal-
cutting machines
X-ray fluorescence Sn and Zn on Fe, precious metals on Cu
Density a-transmission Gases
^-transmission Cigarettes; fluids and slurries in pipes and
tanks ; gases and gas-fluidized solids ;
gas-liquid emulsions ; steam-water ratios, etc.
y-transmission Fluids and slurries in pipes and tanks
Bulk density y-transmission Soil ; borehole cores
y-backscatter Soil-measurements, on the surface and in
boreholes ; rocks and ores-measurements in
boreholes
Composition X-ray fluorescence Ores, alloys, silver content of photographic films
(Selective methods) X-ray absorption-edge Slurries, solutions
n-activation Alloys, ores, slurries, solutions, e.g.
Mg, Al, Si, F in ores, oxygen in steel
Ores—in boreholes and in the laboratory,
(n,y capt.)
nitrogen in bulk cereals
(n,y i„el.) Ores—in boreholes and in the laboratory
(y,n) Be in ores
(α,η) Be in ores
Mössbauer effect Sn and Fe content of ores
(Non-selective methods) ^-transmission C/H ratios of hydrocarbons
y-transmission C/H ratios of hydrocarbons
Transn. of low-energy S, Pb, Co in hydrocarbons, ash in coal
X-rays and brems-
strahlung
jS-backscatter Ores, alloys, ash in coal
Selective (y—y) Heavy metallic ores in boreholes
(n,n ) Ί H content of rocks—in boreholes (to find oil,
th
(n,y) \ gas and water and to calculate porosity) ;
((nn,,nn )) JJ moisture content of soil, coke, sinter mix,
eeppii
detergents, etc. ; content of B, Cd, and other
elements with large thermal neutron absorption
cross-sections
Neutron-die-away Position of oil-water and oil-gas interfaces
in boreholes
Ionization Analysis of gas from chromatographic columns ;
detection of smoke in fire detectors ; measure-
ment of methane concentrations in mines
2 Radioisotope Instruments
TABLE 1.1 (cont.)
Property measured Technique Typical applications
Level ^-transmission Liquids (not in widespread use)
y-transmission Liquids, powders, slurries, ores in containers
y-backscatter Liquids in tanks (not in widespread use)
Position or proximity Alignment of y-ray Correct positioning of lifts, skips, hoppers
beam and detector
Changes in source- To avoid collision of overhead cranes
detector distance
Alignment Alignment of collimated Control of coke oven pushing operations
y-ray beam and detector
Pressure Ionization by a- and Vacuum gauges
(from gas density) j3-particles
Forward scattering of Altimeters
0-particles
Temperature ^-transmission Furnaces
(from gas density)
Flow rate Changes in source- Gases and liquids
detector distance or
cyclic movement of
source (e.g. on a
rotameter)
Ionization: Continuous Gases
and by pulse-timing
Torque Relative movement of Engine shafts
source, detector and
absorber
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 ele-
mentary form of backscatter gauge is shown in Fig. 1.2.
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 ^-transmission gauge (i.e. an instrument based on the transmission
of beta particles) or a y-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 ß-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 y-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
§1.1] Introduction 3