Radioisotope and Radiation Physics. An Introduction

245 Pages·1973·19.08 MB·English

by  M. Miladjenovic (Auth.)

RADIOISOTOPE
AND
RADIATION PHYSICS
AN INTRODUCTION
Μ. MLADJENOVIC
Translated by SONJA SU BOT IC
1973
ACADEMIC PRESS New York and London
A Subsidiary of Harcourt Brace Jovanovich, Publishers
COPYRIGHT © 1973, BY ACADEMIC PRESS, INC.
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To my wife Olga
PREFACE
The book is based on lectures given by the author to participants of
the "course on the use of the radioactive isotopes" organized by the B.
Kidric Institute of Nuclear Sciences in Belgrade. The six-week course
has been regularly repeated six times a year for the last eighteen years,
so that the author has delivered these lectures more than sixty times.
The content has been continuously evolving in the inevitable direc­
tion of giving more by offering less. The initial ambitious level of the
course, comprising the mathematical apparatus so dear to physicists,
had to be altered to reach those participants who at some time in their
lives had only a year of physics —at the beginning of their studies of
medicine, agriculture, etc.
Five hours, allotted to "interaction of radiation and matter" within
the course on the use of radioisotopes, are barely sufficient for a very
elementary introduction. For a serious student it represents only an
initial stimulus for further reading. The aim of this book is to help with
further reading. Starting from the five lectures of the basic course, which
are much expanded, material has been added to give a more complete
picture of penetration of radiation through matter. The chief aim has been
to help the reader handle the quantitative data given in specialized hand­
books.
The book conserves the approach developed during dialogues with
many generations of different students. Some readers might find the
section on classical analogies given in the introductory chapter too ele­
mentary. They can omit it to have more time for Feynman diagrams and
virtual particles. Others will not consider it time lost to read the intro­
ductory chapter, and would rather omit everything connected with
xi
xii Preface
quantum electrodynamics. Typical of the approach are the Appendixes,
summarizing each chapter, which were developed with the full encourage­
ment of the students. They are intended to help the student visualize
the important aspects of physical pictures of various processes.
The two chapters on matter and radiation represent a selection of
topics from nuclear and atomic physics, chosen to help understand the
phenomena of collisions between particles and atomic systems. This is
not a course on nuclear or atomic physics.
The three chapters on penetration of alpha, beta, and gamma radiation
do not uniformly cover the available material. The chapter on beta par­
ticle penetration contains more material on bremsstrahlung and especially
on back scattering than is normally given in books of this level. Much
work has been done in these fields, which has not yet found its way into
books for nonspecialists.
Ugo Fano's "Principles of Radiological Physics"* served as model
and inspiration for the initial pattern of the author's lectures. Fano's
presentation remains unique and after nearly two decades is still a highly
recommended text for a beginner.
An intermediate step between the course in Belgrade and this book
was the course given in the Radioisotope School of the Japanese Atomic
Energy Research Institute in Tokyo. The author is deeply grateful
to Professor Kimura and Dr. Murakami for encouraging him to write
up the lectures and for the effort made to have them published in "Text­
book of Radioisotope School," Vol. I, Tokyo, 1959.
The author is grateful to the B. Kidric Institute for support and to
Nikola Skorupan for the drawings.
The author would like to express his gratitude to his wife Olga, former
Head of the Radioisotope School in Belgrade, for stimulating interest in
lectures and the book, and for critical reading, comments, and suggestions
as the book was written.
*In "Radiation Biology," Vol. I (A. Hollaender, ed.). McGraw-Hill, New York, 1954.
CHAPTER 1
INTRODUCTION
Every experiment with radiation represents in essence the passage of
radiation through matter. Consider, for example, the simplest measure­
ment of alpha particles. Let the experimental setup consist only of a
radioactive source and a detector (Fig. 1.1). The radioactive material is
deposited in the form of a very thin layer on the source carrier. However,
even if the thickness of the layer is less than a micron, this is still larger
than the atomic radius by a factor of 104. Hence, an alpha particle, after
it has left the nucleus, may have to traverse as many as 102-103 atomic
layers before it gets out of the source. Here it undergoes collisions with
the atoms of the radioactive layer and loses a small part of its energy.
While moving between the source and the detector, the alpha particle
passes through air in which, at normal pressure, there are 2.7 x 1019
molecules per cubic centimeter. In numerous collisions with air mole­
cules, the alpha particle will again lose energy. Finally, it arrives at the
detector, where it loses the rest of its energy and captures two electrons,
to become a stationary helium atom. Thus, we see that from the instant it
left the nucleus to the termination of its "alpha phase" and its transforma­
tion into the helium atom, the alpha particle incessantly interacted with
the atoms it encountered along its path. In analyzing data from such an
experiment, the entire path should be taken into account.
ι
2 1. Introduction
Fig. 1.1. The path of an alpha particle from a source to a detector. (The thicknesses of the
carrier, the source, and the window of the Geiger-Müller counter are disproportionally
magnified, but their true numerical values are given.)
Detection of radiation is based on processes induced by radiation as it
traverses a detection medium. For example, in gaseous detectors, such as
the Geiger counter or the ionization chamber, radiation ionizes the gas.
Detection is carried out by the action of electric fields on the ions
produced.
Radiation, especially highly penetrating gamma radiation, traverses not
only experimental devices, but also reaches those who work with them.
To avoid danger to health, it is necessary to know what shielding is ade­
quate for each type of radiation.
Thus, it is seen that throughout the experiment, from the radioactive
source and the detector to the radiation shielding, we necessarily deal
with the passage of radiation through matter. The expertness of an ex­
perimenter can be judged by the thoroughness with which he analyzes
various effects produced by the passage of radiation through matter.
1.1. Accuracy and Errors
Our knowledge of the processes of the interaction of radiation with
matter has advanced together with the development of physics in general.
Classical physics could not offer even an approximate description of some
of the processes. The endeavor to interpret the processes of the inter­
action of electromagnetic radiation with matter played a particularly
important role in rejecting inadequate classical interpretations, and in
creating quantum physics. This can be seen, for example, by the fact that
four Nobel Prizes were awarded in connection with three fundamental
effects. One prize was awarded to Einstein for the interpretation of the
photoelectric effect, one to Compton for a study of the electron-photon
scattering (Compton) effect, one to Dirac for the prediction of pair pro­
duction, and one to Anderson and Neddermeyer for the discovery of
pair production.
1.1. Accuracy and Errors 3
It is useful to discuss at the very beginning the accuracy of theory and
experiment, since it will influence the approach to a consideration of the
interaction of radiation with matter.
The accuracy of theoretical predictions may depend on three factors.
These factors are the law of physics, the model of the system under con­
sideration, and the method of calculation used. Even before the creation
of quantum mechanics, Bohr had calculated the energy loss of charged
particles in their passage through matter by making use of classical
electrodynamics and of the basic postulates of quantum physics. This ap­
proach could give only an approximate picture since the, underlying
principles were inadequate. The creation of quantum electrodynamics
led to accurate basic laws, but their application can be very complicated.
Hence, use is often made of approximate models. Sometimes such models
represent a better approximation of the actual system for certain values of
characteristic parameters, and become less accurate when these values
are increased or decreased. As an example, we can mention the Born ap­
proximation, which is used in treating collisions between particles. The
approximation is more accurate for lower atomic charges and higher
energies of the particles. When a formula is obtained on the basis of a
model, the calculation of its numerical value for given parameters still re­
mains. This may sometimes be so complex that approximate methods are
also used in the computation. In this last case it is only a question of the
mathematical technique to be used, and usually the accuracy may be im­
proved by spending more time on the computation. This is no longer a
serious problem since fast electronic computers are now available.
An experiment may in principle involve a considerable number of
effects whose probabilities of occurrence may differ greatly from one
another. Irrespective of whether charged particles or photons are in
question, usually under given conditions (radiation energy, kind of
absorber, geometry, and so on) some effects will predominate and others
will be barely noticeable. For certain effects, it is necessary to adjust
experimental conditions and to use a special setup in order to make
them observable. Stronger effects might be measured more precisely,
provided that suitable devices are available.
The answer to the question of whether or not all the effects occurring in
an experiment should be taken into consideration, and an absolutely com­
plete description given, is negative. At first glance it might be expected
that exact sciences tend to completeness. The exact sciences are termed
"exact," among other things, because they take into account the in­
evitable uncertainty due to the nature of measurement and the short­
comings of the theoretical models used. Experimental error cannot be
avoided because of the statistical character of the emission of radiation.
4 1. Introduction
If, for example, the intensity of the radiation is measured twice under the
same conditions, two different values are obtained. The error can be re­
duced by increasing the number of particles to be detected, which re­
quires either a stronger source or a measurement of longer duration.
Errors in measurement of radiation intensity are seldom less than 1%,
and sometimes may be considerably larger. The uncertainties in theo­
retical calculations, considered at the beginning of this section, are of the
same order of magnitude. All these errors impose natural restrictions
upon completeness. If a larger effect can be theoretically calculated with
an error of a few percent, then an effect smaller by a factor of 100 is not
worth taking into account, since its contribution is within the limits of
error and cannot be noticed. Hence, very small effects can be disregarded.
It can be claimed that there are no experiments with radiation that do not
neglect some effects.
In what follows, we shall always take into consideration the possi­
bility of neglecting some effects. Taking 1% as the limit of accuracy, we
will disregard processes with contributions lower by one or more orders
of magnitude, that is, less than 0.1%.
1.2. Mechanical Analogy
Before proceeding to a consideration of matter and radiation, it is use­
ful to discuss the simplest mechanical analogy of the process of their
interaction. We shall then see which physical quantities play the most im­
portant role. This will allow us, in considering the properties of matter
and radiation, to confine ourselves to what is indispensable for the
treatment.
In a large number of experiments on the interaction of radiation with
matter, the following elements are present:
(1) a radiation beam obtained from a source;
(2) a material through which the radiation passes and induces a num­
ber of interactions;
(3) a radiation detector.
While the first two elements are always present, the third may be absent
or only temporarily used for control.
How could this be pictured if microparticles were replaced by spheres
(Fig. 1.2)?
A sphere of the beam can be characterized by its mass, velocity, di­
rection of motion, and kinetic energy. Let the beam be approximately
parallel and composed of spheres of the same kind, so that their masses
1.2. Mechanical Analogy 5
absorber
Fig. 1.2. An imaginary passage of a beam of spheres through an absorber made up of
bound systems of two spheres each. (In a collision, the system may receive energy from the
projectile and still remain bound, which would correspond to excitation. The system may
also receive a greater amount of energy, which would correspond to ionization or dis­
sociation.)
are the same. One of the most important characteristics of the beam is its
kinetic energy. This is the "capital" with which the whole operation is
carried out.
The material through which a beam passes is sometimes called the
absorber or the target. Suppose that it is also made up of spheres of
different masses and sizes bound into systems, just as atoms are bound
into molecules. It can be assumed that these spheres are at rest. The
energies of motion of atoms at ordinary temperatures are lower by a
factor of 107-108 than those of the particles of radioactive radiation and,
therefore, can be disregarded. Spheres from the beam, which we shall call
projectiles, undergo collisions with spheres of the absorber as they pass
through it. The "hardness" of a collision depends on whether the pro­
jectile hits the center of the target or hits the periphery, so that it only
grazes the target. A central collision is usually called a head-on collision.
In such a collision, the interaction between the projectile and the target
is the strongest.
Three effects are involved in the process of a collision.
(1) Energy transfer: The projectile transfers energy to the struck
sphere of the absorber which was at rest before the collision. Energy
transfer is the largest in a head-on collision, and decreases when the
distance of the projectile's direction from the center of the struck sphere
increases. It also depends on the relative masses of the target and the
projectile.
(2) Deflection of the projectile: The collision leads to a deflection
of the projectile. The largest deflection occurs in a head-on collision.