In this appendix we list a number of radionuclides which have been referred to in the text in connection with the X-radiation emitted during decay. Many decay schemes are now understood to a high degree of complexity, and we have attempted to present the data in a simplified manner, so as to stress the emitted X- and γ-radiation, and including only those γ-rays which are relatively readily observable by γ-ray and X-ray spectroscopy. The data are based for the most part on the compilation by Lederer et al. (1966).

In table A3.1 are listed 14 radionuclides which decay wholly by electron capture and which yield little or no γ-radiation. If the decay goes directly to the ground state of the daughter nuclide, the only γ-radiation emitted will be the very weak internal Bremsstrahlung associated with the electron capture decay. If the decay proceeds to an excited state, some γ-radiation will be emitted, to an extent depending upon the fraction of electron capture events proceeding to that level, and upon the degree of internal conversion. The latter process becomes important for low transition energies and high multipolarities (large changes of angular momentum during the γ-transition). When internal conversion occurs, the conversion electrons are able to produce external Bremsstrahlung in the source material.

The Q-values for the electron capture transitions are given to enable the free-recoil energy of the nucleus to be calculated (Er = Ev2/2Mc2), and to facilitate the calculation of the ratio of K: L capture (7.14).

Ionization cross-sections (experimental) for protons and α-particles

The observation that characteristic X-radiation can be produced by α-particle bombardment seems to have been first made and reported by Rutherford, Chadwick and others during the years immediately following the discovery of characteristic radiation. Since that time, the subject has been re-examined at intervals. In 1928, Bothe and Franz reported a series of measurements in which various elements were irradiated with α-particles from radioactive decay. Cork (1941) carried out a series of measurements of characteristic radiation produced by deuteron bombardment, using a maximum deuteron energy of 10 MeV. He observed that K X-rays were detectable up to an atomic number of 38. Above this value, no K X-rays were observable, but between atomic numbers of 52 and 78 the L X-rays could be detected. Evidently the L electrons can be ejected from the L shell in circumstances where the K electrons are too strongly bound to be ejected by deuterons of this energy.

For detailed measurements we must wait until the 1950s, when the increasing availability of small accelerators opened up the possibility of experimental work on an absolute basis over a wide range of particle energies. From investigations for example by Messelt (1958) and by Khan and Potter (1964) it became clear that the X-ray yield increased rapidly with proton energy.

Data from five tables of experimental values of mass attenuation coefficients are included in the information presented here. The data of Cooke and Stewardson (1964) extend from 730 eV to 1.74 keV and are given in their entirety (table A2.1). Hughes and Woodhouse (1966) reported data in the energy region 1.3–22 keV and made a comparison between their data and theoretical and experimental data reported elsewhere in the literature. A few additional values at low photon energies were included in Hughes et al. (1968). The whole of their data is included here (table A2.2).

Hopkins (1959) has reported data in the energy range 6–40 keV for aluminium and copper, and in the range 6–14 keV for four other elements. This is presented in table A2.3. To fill out the information in the range 10–25 keV we have drawn on the earlier data of Laubert (1941) (table A2.4).

A detailed set of experimental data, with estimated errors, was published by McCrary et al. (1967) extending from 24 to 131 keV, and is reproduced in table A2.5.

The full range of data given in this appendix thus extends from 730 eV to 131 keV. A great deal of additional experimental data, both within and outside this energy range, exists in the literature. A survey of measurements extending from 10 eV to 100 GeV has been given by Hubbell (1971), covering the period 1909 to June 1971, including a complete bibliography.

Absorption and scattering cross-sections

The interaction between electromagnetic radiation and matter represents one of the most varied classes of phenomena in the whole of experimental physics. Even within the range of energies normally associated with X-rays (itself covering several orders of magnitude of the electromagnetic spectrum) many different processes occur, all of which possess their own individual characteristics.

The nature of the matter with which the radiation interacts offers almost as wide a range of phenomena as does the nature of the radiation. This is true even within the relatively restricted domain of X-ray physics. For example, the subject of X-ray crystallography is essentially a study of the interactions between ordered matter and a radiation field, and any discussion of the absorption and scattering processes in crystals must have as its basis the collective behaviour of a large number of atoms bound by chemical bonds or other interatomic forces into a recognisable structure.

However, in the present work we are concerned mainly with situations in which the overall behaviour of an absorber or scatterer can be deduced by regarding it as a collection of individual atoms each absorbing or scattering independently of its surroundings. In such cases we can assert that interactions between X- or γ-ray photons and matter are single, identifiable, processes, each associated with an individual atom, and can therefore be characterised by a cross-section.

Introduction

The experimental study of X-rays involves consideration of the methods of producing X-rays over a wide range of photon energies, methods of detecting the X-radiation and analysing it in terms of intensity, photon energy and polarization, and the effect of materials placed between source and detector. We have already seen that X-radiation is produced whenever a beam of charged particles encounters any target material, solid, liquid, or gaseous, and that it is also emitted by various processes during radioactive decay. The charged particles used in X-ray generators are normally electrons, but protons or alpha particles are readily available in accelerated beams, and cause X-rays to be produced when slowed down in a target. Accelerating voltages, for electrons or other particles, may range from a few hundred volts to many MV, and the photon energies of interest extend from this upper limit down almost to the ultra-violet region of the electromagnetic spectrum. X-ray detectors depend upon the ionizing qualities of the radiation (or, very occasionally, upon nuclear excitation), and include photographic emulsions, gas-filled devices (such as the ionization chamber, and Geiger and proportional counters), the scintillation counter, and the solid-state detector. Some of these detectors have an intrinsic ability to distinguish between photons of different energy, but often this energy resolution will need to be improved upon by the use of diffraction gratings, or by utilizing Bragg reflection from single crystals.

X-ray microscopy and microanalysis

The development of X-radiography for medical purposes was one of the early successes in the application of X-rays to practical problems, and the attractions of applying radiography on the microscopic scale, for study of small biological specimens or sections, are obvious. In recent years, methods of viewing small objects by means of a magnifying system using X-rays have been developed and are in use in many biological and metallurgical laboratories.

The first exploratory studies appear to have been made by Sievert (1936). In this work an aperture a few micrometres in diameter was placed in front of an X-ray tube, enabling magnified images to be produced by shadow projection. A system such as this would suffer from the rather small intensity of X-radiation which would be available through a small aperture used in this way, but photographs with a resolution of 5–10 μm were obtained.

However, developments in electron optics were necessary before projection X-ray microscopy, at anything approaching optical resolution, became a practical proposition. Von Ardenne (1939) proposed the use of an electron lens for demagnifying an electron source, thereby enabling X-rays to be generated in a region with a diameter of the order of a few micrometres only. An X-ray tube using this principle was constructed by Cosslett and Nixon (1952), and the method of projection X-ray microscopy has undergone continuous development from that time onwards, and has become an investigational method of considerable importance in biology and metallurgy.

When planning a historically-orientated introduction to a book on X-rays it becomes clear at an early stage that there is no shortage of material, so well documented are the facts relating to the discovery and early investigation of X-radiation. Many of the facts are well-known, but the very richness of the subject presents interesting opportunities for discussion, not only because of the large amount of material published during the twelve months or so following the discovery (it has been stated that almost 1000 communications were published during that one year), but also because of the great increase in X-ray studies in recent years, which naturally stimulates an interest in the origins and early days of the subject.

The discovery of X-rays, which took place on 8 November 1895 in Würzburg, Bavaria, constituted an event the importance of which was immediately obvious to the discoverer, and which became apparent to the world at large within a very few weeks of Röntgen's announcement. The subject of X-rays has, of course, always enjoyed the twin privilege of being important as a branch of physics in its own right and of having applications in medical practice the importance of which is known to all. Therein lies a clue to the great speed with which the early researches were put in hand.

Röntgen reported his discovery on 28 December, and dispatched some X-ray photographs and a few copies of his paper to friends on New Year's Day, 1896.

The main purpose of this book is to give a concise account of the production and properties of X-rays. This may at first sight seem a rather restricted aim, but, as the title of the book indicates, its coverage extends beyond the ‘conventional’ aspects of the subject to include other branches of physics in which X-rays play an important rôle. I have brought together several of the areas of physics in which X-rays are encountered, because it has become very evident in recent years that much of the great body of X-ray knowledge acquired several decades ago is highly relevant to other fields such as the recent developments in radioactivity, plasma physics and astrophysics.

In a book of moderate size it is not possible to give a comprehensive treatment, in depth, of the whole of X-ray physics. But I have included a reasonably full account of the continuous X-ray spectrum at low and medium energies, and have also described the production of characteristic X-rays (by electron bombardment) in sufficient detail to enable the research workers to make useful predictions about what is happening or is likely to happen in a wide variety of circumstances when electrons impinge on matter.

X-ray research necessarily involves the use of radiation detectors, and an account of some of these techniques is given.

In the second edition, new material on characteristic X-ray production has been added in Chapter 3. During the intervening period the study of X-ray production by proton bombardment has been actively pursued, and this has merited the inclusion of a new chapter (Chapter 6). Chapter 8 has been extended and up-dated.

The S.I. units are now in very widespread use, and this has justified the conversion of the whole book into this system of units. Occasionally, in topics where the old system of units is particularly entrenched, older units have been retained.

Although many aspects of X-ray physics have expanded very considerably since the first edition of this book appeared, I have attempted to keep the book within reasonable proportions, and hope that not too much of the work of recent years has been omitted.

I am indebted to Mrs Erica Gaize, Mrs Pauline Goddard and Mrs Eileen Shinn for re-typing the whole work, and to many friends and colleagues over the years for discussion of X-ray processes. I am especially indebted to Dr R. Sokhi, Dr R.G. Harris, and Dr M. Church for reading Chapters 6, 7, and parts of Chapter 8 respectively, and for their valuable comments. My thanks are due also to Longman Group UK Limited, publishers of the first edition of this work, for permission to reproduce here a substantial number of line diagrams which appeared originally in that edition.

The Origins of the Theory

1. As soon as man began to think of abstract problems at all, it was only natural that speculations as to the nature and ultimate structure of the material world should figure largely in his writings and philosophies.

Among the earliest speculations which have survived are those of Thales of Miletus (about 640–547 B. C.), many of whose ideas may well have been derived from still earlier legends of Egyptian origin. He conjectured that the whole material universe consisted only of water and of substances derived from water by physical transformation. Earth was produced by the condensation of water, and air by its rarefaction, while air when heated became fire. About 500 B. C. Heraclitus advanced the alternative view that earth, air, fire and water were not transformable one into the other, but constituted four distinct unalterable “elements”, and that all material substances were composed of these four elements mixed in varying proportions—a sort of dim anticipation of modern chemical theory. At a somewhat later date, Leucippus and Democritus maintained that matter consisted of minute hard particles moving as separate units in empty space, and that there were as many kinds of particles as there are different substances.

Unhappily nothing now remains of the writings of either Democritus or Leucippus; their opinions are known to us only through second-hand accounts. From these we learn that they imagined their particles to be eternal and invisible, and so small that their size could not be diminished; hence the name ἄτομος—indivisible.