X-Rays and Their Applications

259 Pages·1975·14.512 MB·English

by  J. G. Brown (auth.)

X-RAYS AND
THEIR APPLICATIONS
X-RAYS
AND THEIR
APPLICATIONS
J.G. BROWN
Royal College of Advanced Technology, Salford
A PLENUM/ROSETTA EDITION
Library of Congress Cataloging in Publication Data
Brown, James Graham.
X-rays and their applications.
"A Plenum/Rosetta edition."
Includes bibliographies and index.
1. X-rays. I. Title. [DNLM: 1. Radiation QC481 B878x)
QC481.B881975 539.7'222 75-34146
ISBN-13: 978-1-4613-4400-1 e-ISBN-13: 978-1-4613-4398-1
DOl: 10.1007/978-1-4613-4398-1
First paperback printing 1975
© J. G. BROWN, 1966
A Plenum/Rosetta Edition
Published by Plenum Publishing Corporation
227 West 17th Street, New York, N.Y. 10011
United Kingdom edition published by Plenum Press, London
A Division of Plenum Publishing Company, Ltd.
Davis House (4th Floor), 8 Scrubs Lane, Harlesden, London, NW10 6SE, England
All rights reserved
No part of this book may be reproduced, stored in a retrieval system or transmitted,
in any form or by any means, electronic, mechanical, photocopying, microfllming,
recording, or otherwise, without written permission from the Publisher
CONTENTS
Preface 7
Chapter 1 Introduction 9
2 Generation of X-rays 13
3 Measurement and Detection of X-rays 40
4 Absorption and Scattering of X-rays 53
5 Elementary Crystallography 79
6 Diffraction of X-rays 95
7 X-ray Spectroscopy 122
8 Refraction and Reflection of X-rays 147
9 Health Hazards and Safety Precautions 157
10 Radiography 165
11 X-ray Crystallography I: The Methods of Ob
serving the X-ray Diffraction Patterns of
Crystals 182
12 X-ray Crystallography II: Applications of
X-ray Diffraction by Crystals 203
13 Spectroscopic and Other Applications of X-rays 237
Appendix 1 Selected Problems 245
2 Answers to Problems 250
Index 251
PREFACE
This book is intended to provide a treatment of the production, properties
and applications of X-rays suitable for undergraduate courses in physics.
It is hoped that parts of it, at least, will be useful to students on other
courses in physics, materials science, metallurgy, chemistry, engineering,
etc. at various levels. It is also hoped that parts of it will serve as an
introduction to the subject of X-ray crystallography, and to this end the
treatment of X-ray diffraction has been designed to show the relation
between the simple approach and the more sophisticated treatments.
During many years of teaching this subject to Degree, Diploma in
Technology and Higher National Certificate students, I have been unable
to find a single book which attempts to cover the whole of this field.
This lack of a treatment of X-rays and their applications in one volume
has prompted me to attempt to fill the gap and this present volume is
the result.
Obviously in writing such a book I have referred to many existing
books and I acknowledge my indebtedness to the authors of all the books
which I have used. I believe that all these books are included in the re
ferences at the ends of the chapters but if I have omitted any, then my
apologies are offered to the authors concerned.
My thanks are also due to Dr. B. Brown for his help and guidance
throughout the preparation of the manuscript and to my colleagues at
the Royal College of Advanced Technology, Salford (the proposed Univer
sity of Salford) for many helpful discussions on the subject matter of the
book and its presentation. The photographs reproduced as Plates 11.1 to
11.6 were taken at the Royal College of Advanced Technology, Salford,
and are reproduced by permission of the Principal.
Salford, 1966 J.G.B.
1
INTRODUCTION
1.1. THE DISCOVERY OF X-RAYS
X-rays were discovered in 1895 by Rontgen 1 during the course of an
examination of the fluorescence produced in the walls of a discharge
tube when an electrical discharge occurred in the residual gas in the
tube. It was known at that time that the fluorescence occurred when
the pressure in the tube was low, so that no visible discharge took place
and that it was under these conditions that cathode rays were most
easily observed.2 In order to observe the fluorescence more easily,
Rontgen fitted the tube with a closely fitting sheath of thin black
cardboard and the room was darkened. He also had a paper screen
coated with barium platino-cyanide. He observed that when the
potential difference produced by an induction coil was applied to the
discharge tube the barium platino-cyanide fluoresced brilliantly
whether the treated or untreated surfaces of the screen faced the tube.
Furthermore, the screen lit up at appreciable distances from the
tube-up to 2 m. Rontgen was able to convince himself that the
agency which caused the fluorescence had its origin in that part of the
discharge tube at which the walls were struck by the cathode rays.
He also realised the importance of his discovery and began to
study the properties of these new rays which he named 'X-rays'.
In the original paper in which the discovery was announced, he
recorded his observations of some of the properties of X-rays, as
follows:
1. All substances are penetrated by X-rays to an appreciable
extent. Thus wood is very transparent even several centimetres
thick. A IS-mm thickness of aluminium weakens the fluorescence
considerably. Lead glass appears to be quite opaque but other
types of glass are much more transparent.
(Rontgen also observed that the bones of the hand absorb
X-rays more readily than the surrounding flesh. As a result of
this observation, X-rays were used in a hospital in Vienna as an
aid to surgery within three months of their discovery.)
9
10 X.rays and their Applieations
2. Many substances fluoresce under irradiation with X-rays, e.g.
calcium compounds, uranium glass, rock salt, etc.
3. Photographic plates and films are sensitive to X-rays.
4. X-rays are not deflected by magnetic fields.
5. X-rays discharge electrified bodies-of either sign.
6. X-rays travel in straight lines.
7. X-rays are generated whenever cathode rays strike a solid body.
Heavy elements are more effective as producers of X-rays than
light ones. .
Although, at the time, Rontgen was unable to reflect or to refract
X-rays, it is now known that both reflection and refraction of X-rays
can be observed under special conditions. (See Chapter 8).
It is remarkable that the paper in which the original discovery of
X-rays was recorded should also contain evidence for most of the
basic properties of the radiation.
1.2. THE NATURE OF X-RAYS
It is natural that many of the early experiments on X-rays should
have been designed to find out something about the nature of this
new radiation. The first evidence was provided by Rontgen himself
who showed that X-rays are not deflected by magnetic fields. It must
therefore be concluded that X-rays are not charged particles as are
cathode rays and the a-and fJ-rays emitted by radioactive substances.
Since at this time uncharged particle radiation was not known, it
was natural to assume that X-rays were some kind of wave motion.
Attempts were made to observe, therefore, the well-known wave
phenomena of interference and diffraction. The early attempts were
unsuccessful3 but in 1899 Haga and Wind4 obtained some more
positive evidence. They passed a beam of X-rays through a narrow
V-shaped slit a few thousandths of a millimetre wide, and allowed the
transmitted beam to fall on to a photographic plate. The image of the
slit was found to be slightly broadened. Haga and Wind attributed
this effect to diffraction and estimated the wavelength to be of the
order of 10-8 cm. Some later work by Walter and Pohl5 suggested
that diffraction did not perhaps occur. However, Sommerfeld6
reconsidered Walter and Pohl's results and showed that they con
firmed the work of Haga and Wind.
In the meantime a new approach had been tried. It was assumed
that Haga and Wind's evidence was sound, and that X-rays were,
therefore, waves, and that they were, in fact, electromagnetic waves.
The electromagnetic theory was, of course, well developed by this
Introduction 11
time, so that its application to the case of X-rays was a logical
step.
The application of this theory to the production of X-rays visualises
the X-rays as being produced as electromagnetic waves by the
decelerating cathode-ray particle. It follows that the X-rays may be
expected to be plane polarised with the electric vector parallel to the
direction of the cathode-ray beam. The application of the electro
magnetic theory to the scattering of X-rays provides a method of
testing this experimentally. According to this theory the scattered
X-rays are produced by the oscillations of the electrons in the scatter
ing material and these oscillations are produced by the periodic
electric intensity variation in the incident electromagnetic wave.
Since this electric intensity is necessarily perpendicular to the direc
tion of propagation of the (transverse) electromagnetic waves and
since the electric intensity of the scattered X-rays must be perpendi
cular to the direction of propagation of these scattered X-rays as
well as being parallel to the line of oscillation of the electrons, it
follows that X-rays scattered through exactly one right angle must be
plane polarised. This is because the line of oscillation of the electrons
responsible for the scattered X-rays must be perpendicular to the
directions of both the incident and scattered beams and, if these are
mutually perpendicular, the line of oscillation can lie in only one
possible direction. Thus the scattered X-rays are plane polarised
with the electric intensity perpendicular to the plane containing the
incident and scattered beams. It also follows from this theory that
if the incident X-rays are plane polarised there should be zero scat
tered intensity in a direction parallel to the electric intensity of the
incident plane polarised beam.
These predictions were tested experimentally by Barkla 7 who found
that the X-rays scattered through 90° were about 70 %p lane polarised.
He pointed out, however, that certain errors existed in the experi
ment which would lead to incomplete polarisation. Subsequently
Compton and Hagenow8 carried out experiments in which these
errors were eliminated or allowed for, and these showed that the
polarisation of the scattered X-rays was complete to within I or 2%.
The scattering of X-rays is discussed more fully in Chapter 4.
Since the predictions of the electromagnetic theory on the scatter
ing of X-rays were found to be correct, this led to the conclusion that
X-rays must be electromagnetic waves. If this is so then interference
and diffraction effects should be observable. The experiments of
Haga and Wind and of Walter and Pohl indicated that the wave
lenl!th ofX-ravs is of the order of 10-8 em which is too small for the
12 X-rays and their AppHeatieDi
use of slits to demonstrate these effects, and, therefore, some other
technique is required.
It was realised by von Laue that the average distance between the
atoms in a solid, which he calculated from the known number of
molecules per unit volume, is of the same order of magnitude as
the wavelength of X-rays as indicated by Raga and Wind and by
Walter and Pohl. Furthermore, if crystals are built up by the regular
repetition in three dimensions of some unit, which is presumably of
atomic or molecular size, then a crystal may form a kind of three
dimensional diffraction grating whose spacing should be of the same
order of magnitude as the wavelength of X-rays. An experiment to
test this idea was carried out by Friedrich and Knipping who passed
a beam of X-rays through a crystal of zinc blende.9 This experiment
showed that diffraction by the crystal did in fact occur. It also indi
cated the range of wavelengths present in the X-ray beam and of
course provided confirmation of von Laue's assumptions with regard
to crystals and X-rays.
This experiment, therefore, can be regarded as providing conclu
sive evidence that X-rays are waves and in view of Barkla's experi
ments they must be electromagnetic waves. The experiment of
Friedrich, Knipping and von Laue also marks the beginning of the
science of X-ray crystallography which is treated more fully in
Chapters 11 and 12.
REFERENCES
1. RONTGEN, Sitzber, Wurzburger Physick. Med. Ges. (1895). Translated-The
Electrician, 36, 415 and 850 (1896); Nature, 53,274 (1896). Reprinted-Ann.
Physik., 64, 1 (1898).
2. YARWOOD, Atomic Physics, University Tutorial Press (1958).
3. GoUY, Compt. Rend., 122, 1197 (1896).
4. HAGA & WIND, Ann. Physik., 68, 884 (1899).
5. WALTER & POHL, Ann. Physik., 29, 331 (1909).
6. SOMMEFELD, Ann. Physik., 38, 473 (1912).
7. BARKLA, Proc. Roy. Soc., A, 77, 247 (1906).
8. CoMPTON & HAGENOW, Rev. Sci. Instr., 8, 487 (1924).
9. FRIEDRICH, KNIPPING & VON LAUE, Bayer. A cad. Wiss., 303 (1912).
2
GENERATION OF X-RAYS
2.1. INTRODUCTION
In Rontgen's investigations of the properties of X-rays he found that
X-rays are generated whenever cathode rays strike a solid body and
are stopped by it. Thus, in order to generate X-rays, three main
components are required, namely, a source of cathode rays or elec
trons, a means of accelerating them and a solid target to stop them.
In most modern X-ray tubes the source of electrons is a heated fila
ment and the necessary acceleration is produced by a large potential
difference applied between the target and the electron gun. Thus an
X-ray generator consists of an X-ray tube together with equipment
to provide the necessary electrical supplies.
It is the purpose of this chapter to consider these matters. The
X-ray tubes and the electrical supplies will be considered separately
first and then typical complete installations will be described.
2.2. X-RAY TUBES
It is possible to classify X-ray tubes in a number of ways:
(a) in terms of the type of electron source:
(i) the cold cathode tube-also known as the ion tube or the gas
tube
(ii) the hot filament tube-also known as the electron tube or
the hard tube.
In the first of these tubes the electrons are provided by a bombard
ment of the plate cathode by positive ions which are produced in the
gas which is deliberately left in the tube. On the other hand in the
second type of tube the electrons are produced by thermionic emis
sion in a heated filament.
(b) in terms of the method of maintaining the necessary vacuum:
(i) the continuously evacuated tube or the demountable tube
(ii) the sealed-off tube.
In the first of these two types the tube is continuously evacuated by
13