Cameron / Clayton / Silverleaf Radioisotope Instruments

International Series of Monographs in Nuclear Energy
1. Auflage 2013
ISBN: 978-1-4831-5933-1
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 320 Seiten

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.

Autoren/Hrsg.

Cameron, J. F.

Clayton, C. G.

Silverleaf, D. J.

Leseproben

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 a- and ß-particles, X- and ?-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 ß-particles or ?-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 ß-transmission gauge (i.e. an instrument based on the transmission of beta particles) or a ?-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 ?-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 ?-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,a) and (n,2n) reactions. Low-energy neutrons interact mainly by the (n,?) reaction. The cross-sections for the (n,?) 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...