Worlds Within Worlds: The Story of Nuclear Energy, Volume 2 (of 3) - 3

Total number of words is 2324
Total number of unique words is 781
41.4 of words are in the 2000 most common words
56.6 of words are in the 5000 most common words
65.6 of words are in the 8000 most common words
Each bar represents the percentage of words per 1000 most common words.
experiencing the attractions and repulsions. Under some conditions,
these “exchange particles” moving back and forth very rapidly between 2
bodies might force those bodies apart; under other conditions they might
pull those bodies together.
In the case of the electromagnetic interaction, the exchange particles
seemed to be “photons”, wave packets that made up gamma rays, X rays, or
even ordinary light (all of which are examples of “electromagnetic
radiation”). The gravitational interaction would be the result of
exchange particles called “gravitons”. (In 1969, there were reports that
gravitons had actually been detected.)
Both the photon and the graviton have zero mass and there is a
connection between that and the fact that electromagnetic interaction
and gravitational interaction decline only slowly with distance. For a
nuclear interaction, which declines very rapidly with distance, the
exchange particle (if any) would have to have mass.
In 1935 the Japanese physicist Hideki Yukawa (1907- ) worked out in
considerable detail the theory of such exchange particles in order to
decide what kind of properties the one involved in the nuclear
interaction would have. He decided it ought to have a mass about 250
times that of an electron, which would make it about ¹/₇ as massive as a
proton. Since this mass is intermediate between that of an electron and
proton, such particles eventually came to be called “mesons” from a
Greek word meaning “intermediate”.
Once Yukawa published his theory, the search was on for the hypothetical
mesons. Ideally, if they existed within the nucleus, shooting back and
forth between protons and neutrons, there ought to be some way of
knocking them out of the nucleus and studying them in isolation.
Unfortunately, the bombarding particles at the disposal of physicists in
the 1930s possessed far too little energy to knock mesons out of nuclei,
assuming they were there in the first place.
There was one way out. In 1911 the Austrian physicist Victor Francis
Hess (1883-1964) had discovered that earth was bombarded from every side
by “cosmic rays”. These consisted of speeding atomic nuclei (“cosmic
particles”) of enormous energies—in some cases, billions of times as
intense as any energies available through particles produced by mankind.
If a cosmic particle of sufficient energy struck an atomic nucleus in
the atmosphere, it might knock mesons out of it.
In 1936 the American physicists Carl David Anderson (1905- ) and Seth
Henry Neddermeyer (1907- ), studying the results of cosmic-particle
bombardment of matter, detected the existence of particles of
intermediate mass. This particle turned out to be lighter than Yukawa
had predicted; it was only about 207 times as massive as an electron.
Much worse, it lacked other properties that Yukawa had predicted. It did
not interact with the nucleus in the manner expected.
[Illustration: _Hideki Yukawa_]
[Illustration: _Victor F. Hess_]
[Illustration: _C. D. Anderson_]
In 1947, however, the English physicist Cecil Frank Powell (1903-1969)
and his co-workers, also studying cosmic-particle bombardment, located
another intermediate-sized body, which had the right mass and all the
other appropriate properties to fit Yukawa’s theories.
Anderson’s particle was called a “mu-meson”, soon abbreviated to “muon”.
Powell’s particle was called a “pi-meson”, soon abbreviated to “pion”.
With the discovery of the pion, Yukawa’s theory was nailed down and any
lingering doubt as to the validity of the proton-neutron theory
vanished.
[Illustration: _C. F. Powell_]
(Actually, it turns out that there are two forces. The one with the pion
as exchange particle is the “strong nuclear interaction”. Another,
involved in beta particle emission, for instance, is a “weak
interaction”, much weaker than the electromagnetic but stronger than the
gravitational.)
The working out of the details of the strong nuclear interaction
explains further the vast energies to be found resulting from nuclear
reactions. Ordinary chemical reactions, with the electron shifts that
accompany them, involve the electromagnetic interaction only. Nuclear
energy, with the shifts of the particles inside the nucleus, involves
the much stronger nuclear interaction.

Neutron Bombardment
As soon as neutrons were discovered, it seemed to physicists that they
had another possible bombarding particle of extraordinary properties.
Since the neutron lacked any electric charge, it could not be repelled
by either electrons on the outside of the atoms or by the nuclei at the
center. The neutron was completely indifferent to the electromagnetic
attraction and it just moved along in a straight line. If it happened to
be headed toward a nucleus it would strike it no matter how heavy a
charge that nucleus might have and very often it would, as a result,
induce a nuclear reaction where a proton would not have been able to.
[Illustration: _J. Robert Oppenheimer_]
To be sure, it seemed just at first that there was a disadvantage to the
neutron’s lack of charge. It could not be accelerated directly by any
device since that always depended on electromagnetic interaction to
which the neutron was impervious.
There was one way of getting around this and this was explained in 1935
by the American physicist J. Robert Oppenheimer (1904-1967) and by his
student Melba Phillips.
Use is made here of the nucleus of the hydrogen-2 (deuterium) nucleus.
That nucleus, often called a “deuteron”, is made up of 1 proton plus 1
neutron and has a mass number of 2 and an atomic number of 1. Since it
has a unit positive charge, it can be accelerated just as an isolated
proton can be.
Suppose, then, that a deuteron is accelerated to a high energy and is
aimed right at a positively charged nucleus. That nucleus repels the
deuteron, and it particularly repels the proton part. The nuclear
interaction that holds together a single proton and a single neutron is
comparatively weak as nuclear interactions go, and the repulsion of the
nucleus that the deuteron is approaching may force the proton out of the
deuteron altogether. The proton veers off, but the neutron, unaffected,
keeps right on going and, with all the energy it had gained as part of
the deuteron acceleration, smashes into the nucleus.
Within a few months of their discovery, energetic neutrons were being
used to bring about nuclear reactions.
Actually, though, physicists didn’t have to worry about making neutrons
energetic. This was a hangover from their work with positively charged
particles such as protons and alpha particles. These charged particles
had to be energetic to overcome the repulsion of the nucleus and to
smash into it with enough force to break it up.
Neutrons, however, didn’t have to overcome any repulsion. No matter how
little energy they had, if they were correctly aimed (and some always
were, through sheer chance) they would approach and strike the nucleus.
In fact, the more slowly they travelled, the longer they would stay in
the vicinity of a nucleus and the more likely they were to be captured
by some nearby nucleus through the attraction of the nuclear
interaction. The influence of the nucleus in capturing the neutron was
greater the slower the neutron, so that it was almost as though the
nucleus were larger and easier to hit for a slow neutron than a fast
one. Eventually, physicists began to speak of “nuclear cross sections”
and to say that particular nuclei had a cross section of such and such a
size for this bombarding particle or that.
The effectiveness of slow neutrons was discovered in 1934 by the
Italian-American physicist Enrico Fermi (1901-1954).
Of course, there was the difficulty that neutrons couldn’t be slowed
down once they were formed, and as formed they generally had too much
energy (according to the new way of looking at things). At least they
couldn’t be slowed down by electromagnetic methods—but there were other
ways.
A neutron didn’t always enter a nucleus that it encountered. Sometimes,
if it struck the nucleus a hard, glancing blow, it bounced off. If the
nucleus struck by the neutron is many times as massive as the neutron,
the neutron bounced off with all its speed practically intact. On the
other hand, if the neutron hits a nucleus not very much more massive
than itself, the nucleus rebounds and absorbs some of the energy, so
that the neutron bounces away with less energy than it had. If the
neutron rebounds from a number of comparatively light nuclei, it
eventually loses virtually all its energy and finally moves about quite
slowly, possessing no more energy than the atoms that surround it.
(You can encounter this situation in ordinary life in the case of
billiard balls. A billiard ball, colliding with a cannon ball, will just
bounce, moving just as rapidly afterward as before, though in a
different direction. If a billiard ball strikes another billiard ball,
it will set the target ball moving and bounce off itself with less
speed.)
The energy of the molecules in the atmosphere depends on temperature.
Neutrons that match that energy and have the ordinary quantity to be
expected at room temperature are called “thermal” (from a Greek word
meaning “heat”) neutrons. The comparatively light nuclei against which
the neutrons bounce and slow down are “moderators” because they moderate
the neutron’s energy.
Fermi and his co-workers were the first to moderate neutrons, produce
thermal neutrons, and use them, in 1935, to bombard nuclei. He quickly
noted how large nuclear cross sections became when thermal neutrons were
the bombarding particles.
It might seem that hope could now rise in connection with the practical
use of energy derived from nuclear reactions. Neutrons could bring about
nuclear reactions, even when they themselves possessed very little
energy, so output might conceivably be more than input for each neutron
that struck. Furthermore because of the large cross sections involved,
thermal neutrons missed far less frequently than high-energy charged
particles did.
But there was a catch. Before neutrons could be used, however low-energy
and however sure to hit, they had to be produced; and in order to
produce neutrons they had to be knocked out of nuclei by bombardment
with high-energy protons or some other such method. The energy formed by
the neutrons was at first never more than the tiniest fraction of the
energies that went into forming the neutrons in the first place.
It was as though you could indeed light a candle with a single match,
but you still had to look through 300,000 useless pieces of wood before
you found a match. The candle would still be impractical.
Even with the existence of neutron bombardment, involving low energy and
high cross section, Rutherford could, with justice, feel right down to
the time of his death that nuclear energy would never be made available
for practical use.
And yet, among the experiments that Fermi was trying in 1934 was that of
sending his neutrons crashing into uranium atoms. Rutherford had no way
of telling (and neither had Fermi) that this, finally, was the route to
the unimaginable.


FOOTNOTES

[1]The attempt to work out the structure of the nucleus resulted in a
_false_, but useful, theory that persisted throughout the 1920s. The
great advances in nuclear science in this decade were made in the
light of this false theory and, for the sake of historical accuracy,
they are so presented here. The theory now believed correct will be
presented shortly, and you will see how matters can be changed from
the earlier concept to the later one.

Quotation Credit
Inside front cover Copyright © by Abelard-Shuman, Ltd., New York.
Reprinted by permission from _Inside the Atom_,
Isaac Asimov, 1966.

Photo Credits
Cover Thorne Films
Page facing inside The “Horsehead” Nebula in Orion, Hale
front cover Observatories.
Author’s Photo Jay K. Klein
Contents pages Lick Observatory
68 Dr. Erwin W. Mueller, The Pennsylvania State
University
70 Yerkes Observatory
86 From _Discovery of the Elements_, Mary E. Weeks,
Chemical Education Publishing Company, 1968.
89 The Central Press Photos, Ltd., and Sir John
Cockcroft
91 Ernest Orlando Lawrence Livermore Laboratory
93 Samuel A. Goudsmit
96 & 97 Nobel Institute
99 Copyright © 1965 by Barbara Lovett Cline,
reprinted from her volume _The Questioners:
Physicists and the Quantum Theory_ by permission
of Thomas Y. Crowell, Inc., New York.
105 & 106 Nobel Institute
107 Alan W. Richards
★ U.S. GOVERNMENT PRINTING OFFICE: 1975——640—285/14


A word about ERDA....

The mission of the U. S. Energy Research & Development Administration
(ERDA) is to develop all energy sources, to make the Nation basically
self-sufficient in energy, and to protect public health and welfare and
the environment. ERDA programs are divided into six major categories:
· CONSERVATION OF ENERGY—More efficient use of existing energy sources,
development of alternate fuels and engines for automobiles to reduce
dependence on petroleum, and elimination of wasteful habits of energy
consumption.
· FOSSIL ENERGY—Expansion of coal production and the development of
technologies for converting coal to synthetic gas and liquid fuels,
improvement of oil drilling methods and of techniques for converting
shale deposits to usable oil.
· SOLAR, GEOTHERMAL, AND ADVANCED ENERGY SYSTEMS—Research on solar
energy to heat, cool, and eventually electrify buildings, on conversion
of underground heat sources to gas and electricity, and on fusion
reactors for the generation of electricity.
· ENVIRONMENT AND SAFETY—Investigation of health, safety, and
environmental effects of the development of energy technologies, and
research on management of wastes from energy production.
· NUCLEAR ENERGY—Expanding medical, industrial and research applications
and upgrading reactor technologies for the generation of electricity,
particularly using the breeder concept.
· NATIONAL SECURITY—Production and administration of nuclear materials
serving both civilian and military needs.
ERDA programs are carried out by contract and cooperation with industry,
university communities, and other government agencies. For more
information, write to USERDA—Technical Information Center, P. O. Box 62,
Oak Ridge, Tennessee 37830.
[Illustration: ENERGY RESEARCH & DEVELOPMENT ADMINISTRATION USA]
United States
Energy Research and Development Administration
Office of Public Affairs
Washington, D.C. 20545


Transcriber’s Notes

--Retained publication information from the printed edition: this eBook
is public-domain in the country of publication.
--In the text version only, underlined or italicized text is delimited
by _underscores_.
--Where possible, UTF superscript and subscript numbers are used; some
e-reader fonts may not support these characters.
--In the text versions only, other superscript text is preceded by caret
and delimited by ^{brackets}.
--In the text versions only, other subscripted text is preceded by
underscore and delimited by _{brackets}.
You have read 1 text from English literature.