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                         Worlds Within Worlds:
                      The Story of Nuclear Energy
                                Volume 2
      Mass and Energy · The Neutron · The Structure of the Nucleus


                            by Isaac Asimov


          U. S. Energy Research and Development Administration
                        Office of Public Affairs
                         Washington, D.C. 20545

           Library of Congress Catalog Card Number: 75-189477
                                  1972

_Nothing in the history of mankind has opened our eyes to the
possibilities of science as has the development of atomic power. In the
last 200 years, people have seen the coming of the steam engine, the
steamboat, the railroad locomotive, the automobile, the airplane, radio,
motion pictures, television, the machine age in general. Yet none of it
seemed quite so fantastic, quite so unbelievable, as what man has done
since 1939 with the atom ... there seem to be almost no limits to what
may lie ahead: inexhaustible energy, new worlds, ever-widening knowledge
of the physical universe._
                                                            Isaac Asimov

                [Illustration: Photograph of night sky]




The U. S. Energy Research and Development Administration publishes a
series of booklets for the general public.

Please write to the following address for a title list or for
information on a specific subject:

  USERDA—Technical Information Center
  P. O. Box 62
  Oak Ridge, Tennessee 37830

                      [Illustration: Isaac Asimov]




ISAAC ASIMOV received his academic degrees from Columbia University and
is Associate Professor of Biochemistry at the Boston University School
of Medicine. He is a prolific author who has written over 150 books in
the past 20 years, including about 20 science fiction works, and books
for children. His many excellent science books for the public cover
subjects in mathematics, physics, astronomy, chemistry, and biology,
such as _The Genetic Code_, _Inside the Atom_, _Building Blocks of the
Universe_, _Understanding Physics_, _The New Intelligent Man’s Guide to
Science_, and _Asimov’s Biographical Encyclopedia of Science and
Technology_.

In 1965 Dr. Asimov received the James T. Grady Award of the American
Chemical Society for his major contribution in reporting science
progress to the public.

                [Illustration: Photograph of night sky]

CONTENTS


                                VOLUME 1
  Introduction                                                          5
  Atomic Weights                                                        6
  Electricity                                                          11
      Units of Electricity                                             11
      Cathode Rays                                                     13
      Radioactivity                                                    17
      The Structure of the Atom                                        25
      Atomic Numbers                                                   30
      Isotopes                                                         35
  Energy                                                               47
      The Law of Conservation of Energy                                47
      Chemical Energy                                                  50
      Electrons and Energy                                             54
      The Energy of the Sun                                            55
      The Energy of Radioactivity                                      57


                                 VOLUME 2
  Mass and Energy                                                      69
  The Structure of the Nucleus                                         75
      The Proton                                                       75
      The Proton-Electron Theory                                       76
      Protons in Nuclei                                                80
      Nuclear Bombardment                                              82
      Particle Accelerators                                            86
  The Neutron                                                          92
      Nuclear Spin                                                     92
      Discovery of the Neutron                                         95
      The Proton-Neutron Theory                                        98
      The Nuclear Interaction                                         101
      Neutron Bombardment                                             107


                                 VOLUME 3
  Nuclear Fission                                                     117
      New Elements                                                    117
      The Discovery of Fission                                        122
      The Nuclear Chain Reaction                                      127
      The Nuclear Bomb                                                131
      Nuclear Reactors                                                141
  Nuclear Fusion                                                      147
      The Energy of the Sun                                           147
      Thermonuclear Bombs                                             149
      Controlled Fusion                                               151
  Beyond Fusion                                                       159
      Antimatter                                                      159
      The Unknown                                                     164
  Reading List                                                        166

[Illustration: _A field-ion microscope view of atoms in a crystal. Each
tiny white dot is a single atom, and each ring system is a crystal facet
or plane. The picture is magnified 1,500,000 times._]




                            MASS AND ENERGY


In 1900 it began to dawn on physicists that there was a vast store of
energy within the atom; a store no one earlier had imagined existed. The
sheer size of the energy store in the atom—millions of times that known
to exist in the form of chemical energy—seemed unbelievable at first.
Yet that size quickly came to make sense as a result of a line of
research that seemed, at the beginning, to have nothing to do with
energy.

Suppose a ball were thrown forward at a velocity of 20 kilometers per
hour by a man on top of a flatcar that is moving forward at 20
kilometers an hour. To someone watching from the roadside the ball would
appear to be travelling at 40 kilometers an hour. The velocity of the
thrower is added to the velocity of the ball.

If the ball were thrown forward at 20 kilometers an hour by a man on top
of a flatcar that is moving backward at 20 kilometers an hour, then the
ball (to someone watching from the roadside) would seem to be not moving
at all after it left the hand of the thrower. It would just drop to the
ground.

There seemed no reason in the 19th century to suppose that light didn’t
behave in the same fashion. It was known to travel at the enormous speed
of just a trifle under 300,000 kilometers per second, while earth moved
in its orbit about the sun at a speed of about 30 kilometers per second.
Surely if a beam of light beginning at some earth-bound source shone in
the direction of earth’s travel, it ought to move at a speed of 300,030
kilometers per second. If it shone in the opposite direction, against
earth’s motion, it ought to move at a speed of 299,970 kilometers per
second.

Could such a small difference in an enormous speed be detected?

[Illustration: _Albert A. Michelson_]

The German-American physicist Albert Abraham Michelson (1852-1931) had
invented a delicate instrument, the interferometer, that could compare
the velocities of different beams of light with great precision. In 1887
he and a co-worker, the American chemist Edward Williams Morley
(1838-1923), tried to measure the comparative speeds of light, using
beams headed in different directions. Some of this work was performed at
the U. S. Naval Academy and some at the Case Institute.

The results of the Michelson-Morley experiment were unexpected. It
showed no difference in the measured speed of light. No matter what the
direction of the beam—whether it went in the direction of the earth’s
movement, or against it, or at any angle to it—the speed of light always
appeared to be exactly the same.

To explain this, the German-Swiss-American scientist Albert Einstein
(1879-1955) advanced his “special theory of relativity” in 1905.
According to Einstein’s view, speeds could not merely be added. A ball
thrown forward at 20 kilometers an hour by a man moving at 20 kilometers
an hour in the same direction would not seem to be going 40 kilometers
an hour to an observer at the roadside. It would seem to be going very
slightly less than 40 kilometers an hour; so slightly less that the
difference couldn’t be measured.

However, as speeds grew higher and higher, the discrepancy in the
addition grew greater and greater (according to a formula Einstein
derived) until, at velocities of tens of thousands of kilometers per
hour, that discrepancy could be easily measured. At the speed of light,
which Einstein showed was a limiting velocity that an observer would
never reach, the discrepancy became so great that the speed of the light
source, however great, added or subtracted zero to or from the speed of
light.

Accompanying this were all sorts of other effects. It could be shown by
Einstein’s reasoning that no object possessing mass could move faster
than the speed of light. What’s more, as an object moved faster and
faster, its length in the direction of motion (as measured by a
stationary observer) grew shorter and shorter, while its mass grew
greater and greater. At 260,000 kilometers per second, its length in the
direction of movement was only half what it was at rest, and its mass
was twice what it was. As the speed of light was approached, its length
would approach zero in the direction of motion, while its mass would
approach the infinite.

Could this really be so? Ordinary objects never moved so fast as to make
their lengths and masses show any measurable change. What about
subatomic particles, however, which moved at tens of thousands of
kilometers per second? The German physicist Alfred Heinrich Bucherer
(1863-1927) reported in 1908 that speeding electrons did gain in mass
just the amount predicted by Einstein’s theory. The increased mass with
energy has been confirmed with great precision in recent years.
Einstein’s special theory of relativity has met many experimental tests
exactly ever since and it is generally accepted by physicists today.

Einstein’s theory gave rise to something else as well. Einstein deduced
that mass was a form of energy. He worked out a relationship (the
“mass-energy equivalence”) that is expressed as follows:

                              _E_ = _mc_²

where _E_ represents energy, _m_ is mass, and _c_ is the speed of light.

If mass is measured in grams and the speed of light is measured in
centimeters per second, then the equation will yield the energy in a
unit called “ergs”. It turns out that 1 gram of mass is equal to
900,000,000,000,000,000,000 (900 billion billion) ergs of energy. The
erg is a very small unit of energy, but 900 billion billion of them
mount up.

The energy equivalent of 1 gram of mass (and remember that a gram, in
ordinary units, is only ¹/₂₈ of an ounce) would keep a 100-watt light
bulb burning for 35,000 years.

 [Illustration: ENERGY CREATED compared to MATTER (OR MASS) DESTROYED]

It is this vast difference between the tiny quantity of mass and the
huge amount of energy to which it is equivalent that obscured the
relationship over the years. When a chemical reaction liberates energy,
the mass of the materials undergoing the reaction decreases slightly—but
_very_ slightly.

Suppose, for instance, a gallon of gasoline is burned. The gallon of
gasoline has a mass of 2800 grams and combines with about 10,000 grams
of oxygen to form carbon dioxide and water, yielding 1.35 million
billion ergs. That’s a lot of energy and it will drive an automobile for
some 25 to 30 kilometers. But by Einstein’s equation all that energy is
equivalent to only a little over a millionth of a gram. You start with
12,800 grams of reacting materials and you end with 12,800 grams minus a
millionth of a gram or so that was given off as energy.

No instrument known to the chemists of the 19th century could have
detected so tiny a loss of mass in such a large total. No wonder, then,
that from Lavoisier on, scientists thought that the law of conservation
of mass held exactly.

Radioactive changes gave off much more energy per atom than chemical
changes did, and the percentage loss in mass was correspondingly
greater. The loss of mass in radioactive changes was found to match the
production of energy in just the way Einstein predicted.

It was no longer quite accurate to talk about the conservation of mass
after 1905 (even though mass was just about conserved in ordinary
chemical reactions so that the law could continue to be used by chemists
without trouble). Instead, it is more proper to speak of the
conservation of energy, and to remember that mass was one form of energy
and a very concentrated form.

The mass-energy equivalence fully explained why the atom should contain
so great a store of energy. Indeed, the surprise was that radioactive
changes gave off as little energy as they did. When a uranium atom broke
down through a series of steps to a lead atom, it produced a million
times as much energy as that same atom would release if it were involved
in even the most violent of chemical changes. Nevertheless, that
enormous energy change in the radioactive breakdown represented only
about one-half of 1% of the total energy to which the mass of the
uranium atom was equivalent.

Once Rutherford worked out the nuclear theory of the atom, it became
clear from the mass-energy equivalence that the source of the energy of
radioactivity was likely to be in the atomic nucleus where almost all
the mass of the atom was to be found.

The attention of physicists therefore turned to the nucleus.




                      THE STRUCTURE OF THE NUCLEUS


The Proton

As early as 1886 Eugen Goldstein, who was working with cathode rays,
also studied rays that moved in the opposite direction. Since the
cathode rays (electrons) were negatively charged, rays moving in the
opposite direction would have to be positively charged. In 1907 J. J.
Thomson called them “positive rays”.

Once Rutherford worked out the nuclear structure of the atom, it seemed
clear that the positive rays were atomic nuclei from which a number of
electrons had been knocked away. These nuclei came in different sizes.

Were the nuclei single particles—a different one for every isotope of
every element? Or were they all built up out of numbers of still smaller
particles of a very limited number of varieties? Might it be that the
nuclei owed their positive electrical charge to the fact that they
contained particles just like the electron, but ones that carried a
positive charge rather than a negative one?

All attempts to discover this “positive electron” in the nuclei failed,
however. The smallest nucleus found was that produced by knocking the
single electron off a hydrogen atom in one way or another. This hydrogen
nucleus had a single positive charge, one that was exactly equal in size
to the negative charge on the electron. The hydrogen nucleus, however,
was much more massive than an electron. The hydrogen nucleus with its
single positive charge was approximately 1837 times as massive as the
electron with its single negative charge.

Was it possible to knock the positive charge loose from the mass of the
hydrogen nucleus? Nothing physicists did could manage to do that. In
1914 Rutherford decided the attempt should be given up. He suggested
that the hydrogen nucleus, for all its high mass, should be considered
the unit of positive electrical charge, just as the electron was the
unit of negative electrical charge. He called the hydrogen nucleus a
“proton” from the Greek word for “first” because it was the nucleus of
the first element.

[Illustration: _One proton balances 1837 electrons._]

Why the proton should be so much more massive than the electron is still
one of the unanswered mysteries of physics.


The Proton-Electron Theory

What about the nuclei of elements other than hydrogen?

All the other elements had nuclei more massive than that of hydrogen and
the natural first guess was that these were made up of some appropriate
number of protons closely packed together. The helium nucleus, which had
a mass four times as great as that of hydrogen, might be made up of 4
protons; the oxygen nucleus with a mass number of 16 might be made up of
16 protons and so on.

This guess, however, ran into immediate difficulties. A helium nucleus
might have a mass number of 4 but it had an electric charge of +2. If it
were made up of 4 protons, it ought to have an electric charge of +4. In
the same way, an oxygen nucleus made up of 16 protons ought to have a
charge of +16, but in actual fact it had one of +8.

Could it be that something was cancelling part of the positive electric
charge? The only thing that could do so would be a negative electric
charge[1] and these were to be found only on electrons as far as anyone
knew in 1914. It seemed reasonable, then, to suppose that a nucleus
would contain about half as many electrons in addition to the protons.
The electrons were so light, they wouldn’t affect the mass much, and
they would succeed in cancelling some of the positive charge.

Thus, according to this early theory, _now known to be incorrect_, the
helium nucleus contained not only 4 protons, but 2 electrons in
addition. The helium nucleus would then have a mass number of 4 and an
electric charge (atomic number) of 4 - 2, or 2. This was in accordance
with observation.

This “proton-electron theory” of nuclear structure accounted for
isotopes very nicely. While oxygen-16 had a nucleus made up of 16
protons and 8 electrons, oxygen-17 had one of 17 protons and 9
electrons, and oxygen-18 had one of 18 protons and 10 electrons. The
mass numbers were 16, 17, and 18, respectively, but the atomic number
was 16 - 8, 17 - 9, and 18 - 10, or 8 in each case.

Again, uranium-238 has a nucleus built up, according to this theory, of
238 protons and 146 electrons, while uranium-235 has one built up of 235
protons and 143 electrons. In these cases the atomic number is,
respectively, 238 - 146 and 235 - 143, or 92 in each case. The nucleus
of the 2 isotopes is, however, of different structure and it is not
surprising therefore that the radioactive properties of the
two—properties that involve the nucleus—should be different and that the
half-life of uranium-238 should be six times as long as that of
uranium-235.

The presence of electrons in the nucleus not only explained the
existence of isotopes, but seemed justified by two further
considerations.

First, it is well known that similar charges repel each other and that
the repulsion is stronger the closer together the similar charges are
forced. Dozens of positively charged particles squeezed into the tiny
volume of an atomic nucleus couldn’t possibly remain together for more
than a tiny fraction of a second. Electrical repulsion would send them
flying apart at once.

On the other hand, opposite charges attract, and a proton and an
electron would attract each other as strongly as 2 protons (or 2
electrons) would repel each other. It was thought possible that the
presence of electrons in a collection of protons might somehow limit the
repulsive force and stabilize the nucleus.

Second, there are radioactive decays in which beta particles are sent
flying out of the atom. From the energy involved they could come only
out of the nucleus. Since beta particles are electrons and since they
come from the nucleus, it seemed to follow that there must be electrons
within the nucleus to begin with.

The proton-electron theory of nuclear structure also seemed to account
neatly for many of the facts of radioactivity.

Why radioactivity at all, for instance? The more complex a nucleus is,
the more protons must be squeezed together and the harder, it would
seem, it must be to keep them together. _More and more electrons seemed
to be required._ Finally, when the total number of protons was 84 or
more, no amount of electrons seemed sufficient to stabilize the nucleus.

The manner of breakup fits the theory, too. Suppose a nucleus gives off
an alpha particle. The alpha particle is a helium nucleus made up, by
this theory, of 4 protons and 2 electrons. If a nucleus loses an alpha
particle, its mass number should decline by 4 and its atomic number by 4
- 2, or 2. And, indeed, when uranium-238 (atomic number 92) gives off an
alpha particle, it becomes thorium-234 (atomic number 90).

Suppose a beta particle is emitted. A beta particle is an electron and
if a nucleus loses an electron, its mass number is almost unchanged. (An
electron is so light that in comparison with the nucleus, we can ignore
its mass.) On the other hand, a unit negative charge is gone. One of the
protons in the nucleus, which had previously been masked by an electron,
is now unmasked. Its positive charge is added to the rest and the atomic
number goes up by one. Thus, thorium-234 (atomic number 90) gives up a
beta particle and becomes protactinium-234 (atomic number 91).

If a gamma ray is given off, that gamma ray has no charge and the
equivalent of very little mass. That means that neither the mass number
nor the atomic number of the nucleus is changed, although its energy
content is altered.

Even more elaborate changes can be taken into account. In the long run,
uranium-238, having gone through many changes, becomes lead-206. Those
changes include the emission of 8 alpha particles and 6 beta particles.
The 8 alpha particles involve a loss of 8 × 4, or 32 in mass number,
while the 6 beta particles contribute nothing in this respect. And,
indeed, the mass number of uranium-238 declines by 32 in reaching
lead-206. On the other hand the 8 alpha particles involve a decrease in
atomic number of 8 × 2, or 16, while the 6 beta particles involve an
increase in atomic number of 6 × 1, or 6. The total change is a decrease
of 16 - 6, or 10. And indeed, uranium (atomic number 92) changes to lead
(atomic number 82).

It is useful to go into such detail concerning the proton-electron
theory of nuclear structure and to describe how attractive it seemed.
The theory appeared solid and unshakable and, indeed, physicists used it
with considerable satisfaction for 15 years.

—And yet, as we shall see, it was wrong; and that should point a moral.
Even the best seeming of theories may be wrong in some details and
require an overhaul.


Protons in Nuclei

Let us, nevertheless, go on to describe some of the progress made in the
1920s in terms of the proton-electron theory that was then accepted.

Since a nucleus is made up of a whole number of protons, its mass ought
to be a whole number if the mass of a single proton is considered 1.
(The presence of electrons would add some mass but in order to simplify
matters, let us ignore that.)

When isotopes were first discovered this indeed seemed to be so.
However, Aston and his mass spectrometer kept measuring the mass of
different nuclei more and more closely during the 1920s and found that
they differed very slightly from whole numbers. Yet a fixed number of
protons turned out to have different masses if they were first
considered separately and then as part of a nucleus.

Using modern standards, the mass of a proton is 1.007825. Twelve
separate protons would have a total mass of twelve times that, or
12.0939. On the other hand, if the 12 protons are packed together into a
carbon-12 nucleus, the mass is 12 so that the mass of the individual
protons is 1.000000 apiece. What happens to this difference of 0.007825
between the proton in isolation and the proton as part of a carbon-12
nucleus?

According to Einstein’s special theory of relativity, the missing mass
would have to appear in the form of energy. If 12 hydrogen nuclei
(protons) plus 6 electrons are packed together to form a carbon nucleus,
a considerable quantity of energy would have to be given off.

In general, Aston found that as one went on to more and more complicated
nuclei, a larger fraction of the mass would have to appear as energy
(though not in a perfectly regular way) until it reached a maximum in
the neighborhood of iron.

Iron-56, the most common of the iron isotopes, has a mass number of
55.9349. Each of its 56 protons, therefore, has a mass of 0.9988.

For nuclei more complicated than those of iron, the protons in the
nucleus begin to grow more massive again. Uranium-238 nuclei, for
instance, have a mass of 238.0506, so that each of the 238 protons they
contain has a mass of 1.0002.

By 1927 Aston had made it clear that it is the middle elements in the
neighborhood of iron that are most closely and economically packed. If a
very massive nucleus is broken up into somewhat lighter nuclei, the
proton packing would be tighter and some mass would be converted into
energy. Similarly, if very light nuclei were joined together into
somewhat more massive nuclei, some mass would be converted into energy.

This demonstration that energy was released in any shift away from
either extreme of the list of atoms according to atomic number fits the
case of radioactivity, where very massive nuclei break down to somewhat
less massive ones.

Consider that uranium-238 gives up 8 alpha particles and 6 beta
particles to become lead-206. The uranium-238 nucleus has a mass of
238.0506; each alpha particle has one of 4.0026 for a total of 32.0208;
each beta particle has a mass of 0.00154 for a total of 0.00924; and the
lead-206 nucleus has one of 205.9745.

This means that the uranium-238 nucleus (mass: 238.0506) changes into 8
alpha particles, 6 beta particles, and a lead-206 nucleus (total mass:
238.0045). The starting mass is 0.0461 greater than the final mass and
it is this missing mass that has been converted into energy and is
responsible for the gamma rays and for the velocity with which alpha
particles and beta particles are discharged.


Nuclear Bombardment

Once scientists realized that there was energy which became available
when one kind of nucleus was changed into another, an important question
arose as to whether such a change could be brought about and regulated
by man and whether this might not be made the source of useful power of
a kind and amount undreamed of earlier.

Chemical energy was easy to initiate and control, since that involved
the shifts of electrons on the outskirts of the atoms. Raising the
temperature of a system, for instance, caused atoms to move more quickly
and smash against each other harder, and that in itself was sufficient
to force electrons to shift and to initiate a chemical reaction that
would not take place at lower temperatures.

To shift the protons within the nucleus (“nuclear reactions”) and make
nuclear energy available was a harder problem by far. The particles
involved were much more massive than electrons and correspondingly
harder to move. What’s more, they were buried deep within the atom. No
temperatures available to the physicists of the 1920s could force atoms
to smash together hard enough to reach and shake the nucleus.

In fact, the only objects that were known to reach the nucleus were
speeding subatomic particles. As early as 1906, for instance, Rutherford
had used the speeding alpha particles given off by a radioactive
substance to bombard matter and to show that sometimes these alpha
particles were deflected by atomic nuclei. It was, in fact, by such an
experiment that he first demonstrated the existence of such nuclei.

Rutherford had continued his experiments with bombardment. An alpha
particle striking a nucleus would knock it free of the atom to which it
belonged and send it shooting forward (like one billiard ball hitting
another). The nucleus that shot ahead would strike a film of chemical
that scintillated (sparkled) under the impact. In a rough way, one could
tell the kind of nucleus that struck from the nature of the sparkling.

In 1919 Rutherford bombarded nitrogen gas with alpha particles and found
that he obtained the kind of sparkling he associated with the
bombardment of hydrogen gas. When he bombarded hydrogen, the alpha
particles struck hydrogen nuclei (protons) and shot them forward. To get
hydrogen-sparkling out of the bombardment of nitrogen, Rutherford felt,
he must have knocked protons out of the nitrogen nuclei. Indeed, as was
later found, he had converted nitrogen nuclei into oxygen nuclei.

This was the first time in history that the atomic nucleus was altered
by deliberate human act.

Rutherford continued his experiments and by 1924 had shown that alpha
particles could be used to knock protons out of the nuclei of almost all
elements up to potassium (atomic number 19).

There were, however, limitations to the use of natural alpha particles
as the bombarding agent.

First, the alpha particles used in bombardment were positively charged
and so were the atomic nuclei. This meant that the alpha particles and
the atomic nuclei repelled each other and much of the energy of the
alpha particles was used in overcoming the repulsion. For more and more
massive nuclei, the positive charge grew higher and the repulsion
stronger until for elements beyond potassium, no collision could be
forced, even with the most energetic naturally occurring alpha
particles.

[Illustration: _Man-made transmutation._]

  Nitrogen-14 (7N,7P) + Helium-4 (2N,2P) (Alpha particle)
  → Oxygen-17 (9N,8P) + Hydrogen-1 (1P)
  Neutron=N, Proton=P

Second, the alpha particles that are sprayed toward the target cannot be
aimed directly at the nuclei. An alpha particle strikes a nucleus only
if, by chance, they come together. The nuclei that serve as their
targets are so unimaginably small that most of the bombarding particles
are sure to miss. In Rutherford’s first bombardment of nitrogen, it was
calculated that only 1 alpha particle out of 300,000 managed to strike a
nitrogen nucleus.

The result of these considerations is clear. There is energy to be
gained out of nuclear reactions, but there is also energy that must be
expended to cause these nuclear reactions. In the case of nuclear
bombardment by subatomic particles (the only way, apparently, in which
nuclear reactions can be brought about), the energy expended seems to be
many times the energy to be extracted. This is because so many subatomic
particles use up their energy in ionizing atoms, knocking electrons
away, and never initiate nuclear reactions at all.

It was as though the only way you could light a candle would be to
strike 300,000 matches, one after the other. If that were so, candles
would be impractical.

In fact, the most dramatic result of alpha particle bombardment had
nothing to do with energy production, but rather the reverse. New nuclei
were produced that had _more_ energy than the starting nuclei, so that
energy was absorbed by the nuclear reaction rather than given off.

This came about first in 1934, when a French husband-and-wife team of
physicists, Frédéric Joliot-Curie (1900-1958) and Irène Joliot-Curie
(1897-1956) were bombarding aluminum-27 (atomic number 13) with alpha
particles. The result was to combine part of the alpha particle with the
aluminum-27 nucleus to form a new nucleus with an atomic number two
units higher—15—and a mass number three units higher—30.

The element with atomic number 15 is phosphorus so that phosphorus-30
was formed. The only isotope of phosphorus that occurs in nature,
however, is phosphorus-31. Phosphorus-30 was the first man-made
nucleus—the first to be manufactured by nuclear reactions in the
laboratory.

[Illustration: _Frédéric and Irène Joliot-Curie_]

The reason phosphorus-30 did not occur in nature was that its energy
content was too high to allow it to be stable. Its energy content
drained away through the emission of particles that allowed the nucleus
to change over into a stable one, silicon-30 (atomic number 14). This
was an example of “artificial radioactivity”.

Since 1934, over a thousand kinds of nuclei that do not occur in nature
have been formed in the laboratory through various kinds of
bombardment-induced nuclear reactions. Every single one of them proved
to be radioactive.


Particle Accelerators

Was there nothing that could be done to make nuclear bombardment more
efficient and increase the chance of obtaining useful energy out of
nuclear reactions?

In 1928 the Russian-American physicist George Gamow (1904-1968)
suggested that protons might be used as bombarding agents in place of
alpha particles. Protons were only one-fourth as massive as alpha
particles and the collision might be correspondingly less effective; on
the other hand, protons had only half the positive charge of alpha
particles and would not be as strongly repelled by the nuclei. Then,
too, protons were much more easily available than alpha particles. To
get a supply of protons one only had to ionize the very common hydrogen
atoms, i.e., get rid of the single electron of the hydrogen atom, and a
single proton is left.

[Illustration: _Artificial radioactivity._]

  Aluminum-27 (14N,13P) + Helium-4 (2N,2P) (Alpha particle)
  → (16N,15P)
  → N + Phosphorus-30 (Radioactive) (15N,15P)
  → Positron + Silicon-30
  Neutron=N, Proton=P

Of course, protons obtained by the ionization of hydrogen atoms have
very little energy, but could energy be imparted to them? Protons carry
a positive charge and a force can therefore be exerted upon them by an
electric or magnetic field. In a device that makes use of such fields,
protons can be accelerated (made to go faster and faster), and thus gain
more and more energy. In the end, if enough energy is gained, the proton
could do more damage than the alpha particle, despite the former’s
smaller mass. Combine that with the smaller repulsion involved and the
greater ease of obtaining protons—and the weight of convenience and
usefulness would swing far in the direction of the proton.

Physicists began to try to design “particle accelerators” and the first
practical device of this sort was produced in 1929 by the two British
physicists John Douglas Cockcroft (1897-1967) and Ernest Thomas Sinton
Walton (1903-    ). Their device, called an “electrostatic accelerator”,
produced protons that were sufficiently energetic to initiate nuclear
reactions. In 1931 they used their accelerated protons to disrupt the
nucleus of lithium-7. It was the first nuclear reaction to be brought
about by man-made bombarding particles.

Other types of particle accelerators were also being developed at this
time. The most famous was the one built in 1930 by the American
physicist Ernest Orlando Lawrence (1901-1958). In this device a magnet
was used to make the protons move in gradually expanding circles,
gaining energy with each lap until they finally moved out beyond the
influence of the magnet and then hurtled out of the instrument in a
straight line at maximum energy. This instrument was called a
“cyclotron”.

[Illustration: _Inventors of one of the first accelerators, Ernest T. S.
Walton, left, and John D. Cockcroft, right, with Lord Ernest Rutherford
at Cambridge University in the early 1930s._]

[Illustration: _The bombardment of lithium-7 with protons was the first
nuclear reaction caused by man-made particles._]

  Lithium-7 (4N,3P) + Hydrogen-1 (Proton)
  → Helium-4 (2N,2P) (Alpha particle) + Helium-4 (2N,2P) (Alpha
          particle)
  Neutron=N, Proton=P

The cyclotron was rapidly improved, using larger magnets and
increasingly sophisticated design. There are now, at this time of
writing, “proton synchrotrons” (descendants of that first cyclotron)
that produce particles with over a million times the energy of those
produced by Lawrence’s first cyclotron. Of course, the first cyclotron
was only a quarter of a meter wide, while the largest today has a
diameter of some 2000 meters.

As particle accelerators grew larger, more efficient, and more powerful,
they became ever more useful in studying the structure of the nucleus
and the nature of the subatomic particles themselves. They did not
serve, however, to bring the dream of useful nuclear energy any closer.
Though they brought about the liberation of vastly more nuclear energy
than Rutherford’s initial bombardments could, they also consumed a great
deal more energy in the process.

It is not surprising that Rutherford, the pioneer in nuclear
bombardment, was pessimistic. To the end of his days (he died in 1937)
he maintained that it would be forever impossible to tap the energy of
the nucleus for use by man. Hopes that “nuclear power” might some day
run the world’s industries were, in his view, an idle dream.

[Illustration: _Ernest O. Lawrence holds a model of the first cyclotron
in 1930, a year after its conception._]




                              THE NEUTRON


Nuclear Spin

What Rutherford did not (and could not) take into account were the
consequences of a completely new type of nuclear bombardment involving a
type of particle unknown in the 1920s (though Rutherford speculated
about the possibility of its existence).

The beginnings of the new path came about through the reluctant
realization that there was a flaw in the apparently firmly grounded
proton-electron picture of nuclear structure.

The flaw involved the “nuclear spin”. In 1924 the Austrian physicist
Wolfgang Pauli (1900-1958) worked out a theory that treated protons and
electrons as though they were spinning on their axes. This spin could be
in either direction (or, as we would say in earthly terms, from
west-to-east, or from east-to-west). Quantum theory has shown that a
natural unit exists for what is called the angular momentum of this
spin. Measured in terms of this natural unit of spin, the proton and the
electron have spin ½. If the particle spun in one direction it was +½,
if in the other it was -½.

When subatomic particles came together to form an atomic nucleus, each
kept its original spin, and the nuclear spin was then equal to the total
angular momentum of the individual particles that made it up.

For instance, suppose the helium nucleus is made up of 4 protons and 2
electrons, as was thought in the 1920s. Of the 4 protons, suppose that
two had a spin of +½ and two of -½. Suppose also that of the 2
electrons, one had a spin of +½ and one of -½. All the spins would
cancel each other. The total angular momentum would be zero.

Of course, it is also possible that all 6 particles were spinning in the
same direction; all +½ or all -½. In that case the nuclear spin would be
3, either in one direction or the other. If 5 particles were spinning in
one direction and 1 in the other, then the total spin would be 2, in one
direction or the other.

[Illustration: _Wolfgang Pauli lecturing in Copenhagen in April 1929._]

In short if you have an even number of particles in a nucleus, each with
a spin of +½ or -½, then the total spin is either zero or a whole
number, no matter what combination of positive and negative spins you
choose. (The total spin is always written as a positive number.)

On the other hand, suppose you have lithium-7, which was thought to be
made up of 7 protons and 4 electrons. If the 7 protons were all +½ and
the 4 electrons were all -½ in their spins, the nuclear spin would be
⁷/₂ - ⁴/₂ = ³/₂.

If you have an odd number of particles in the nucleus, you will find
that any combination of positive and negative spins will _never_ give
you either zero or a whole number as a sum. The sum will always include
a fraction.

Consequently, if one measures the spin of a particular atomic nucleus
one can tell at once whether that nucleus contains an even number of
particles or an odd number.

This quickly raised a problem. The nuclear spin of the common isotope,
nitrogen-14, was measured accurately over and over again and turned out
to be 1. There seemed no doubt about that and it could therefore be
concluded that there were an even number of particles in the nitrogen-14
nucleus.

And yet, by the proton-electron theory of nuclear structure, the
nitrogen-14 nucleus, with a mass number of 14 and an atomic number of 7,
had to be made up of 14 protons and 7 electrons for a total of 21
particles altogether—an odd number.

The nuclear spin of nitrogen-14 indicated “even number” and the
proton-electron theory indicated “odd number”. One or the other had to
be wrong, but which? The nuclear spin was a matter of actual
measurement, which could be repeated over and over and on which all
agreed. The proton-electron theory was only a theory. It was therefore
the latter that was questioned.

What was to be done?

Suppose it is wrong to count protons and electrons inside the nucleus as
separate particles. Was it possible that an electron and a proton,
forced into the close confinement of the atomic nucleus might, by the
force of mutual attraction, become so intimately connected as to count
as a single particle. One of the first to suggest this, as far back as
1920, was Rutherford.

Such a proton-electron combination would be electrically neutral and in
1921 the American chemist William Draper Harkins (1873-1951) used the
term “neutron” as a name for it.

If we look at the nitrogen-14 nucleus in this way then it is made up,
not of 14 protons and 7 electrons, but of 7 protons and 7
proton-electron combinations. Instead of a total of 21 particles, there
would be a total of 14; instead of an odd number, there would be an even
number. The structure would now account for the nuclear spin.

But could such a revised theory of nuclear structure be made to seem
plausible? The proton-electron theory seemed to make sense because both
protons and electrons were known to exist separately and could be
detected. If an intimate proton-electron combination could also exist,
ought it not exist (or be made to exist) outside the nucleus and ought
it not be detected as an isolated particle?


Discovery of the Neutron

Throughout the 1920s scientists searched for the neutron but without
success.

One of the troubles was that the particle was electrically neutral.
Subatomic particles could be detected in a variety of ways, but every
single way (right down to the present time) makes use of their electric
charge. The electric charge of a speeding subatomic particle either
repels electrons or attracts them. In either case, electrons are knocked
off atoms that are encountered by the speeding subatomic particle.

The atoms with electrons knocked off are now positively charged ions.
Droplets of water vapor can form about these ions, or a bubble of gas
can form, or a spark of light can be seen. The droplets, the bubbles,
and the light can all be detected one way or another and the path of the
subatomic particle could be followed by the trail of ions it left
behind. Gamma rays, though they carry no charge, are a wave form capable
of ionizing atoms.

All the particles and rays that can leave a detectable track of ions
behind are called “ionizing radiation” and these are easy to detect.

The hypothetical proton-electron combination, however, which was neither
a wave form nor a charged particle was not expected to be able to ionize
atoms. It would wander among the atoms without either attracting or
repelling electrons and would therefore leave the atomic structure
intact. Its pathway could not be followed. In short, then, the neutron
was, so to speak, invisible, and the search for it seemed a lost cause.
And until it was found, the proton-electron theory of nuclear structure,
whatever its obvious deficiencies with respect to nuclear spin, remained
the only one to work with.

Then came 1930. The German physicist Walther Wilhelm Georg Bothe
(1891-1957) and a co-worker, H. Becker, were bombarding the light metal,
beryllium, with alpha particles. Ordinarily, they might expect protons
to be knocked out of it, but in this case no protons appeared. They
detected some sort of radiation because something was creating certain
effects while the alpha particles were bombarding the beryllium but not
after the bombardment ceased.

[Illustration: _Walther W. G. Bothe_]

To try to determine something about the properties of this radiation,
Bothe and Becker tried putting objects in the way of the radiation. They
found the radiation to be remarkably penetrating. It even passed through
several centimeters of lead. The only form of radiation that was known
at that time to come out of bombarded matter with the capacity of
penetrating a thick layer of lead was gamma rays. Bothe and Becker,
therefore, decided they had produced gamma rays and reported this.

In 1932 the Joliot-Curies repeated the Bothe-Becker work and got the
same results. However, among the objects they placed in the path of the
new radiation, they included paraffin, which is made up of the light
atoms of carbon and hydrogen. To their surprise, protons were knocked
out of the paraffin.

Gamma rays had never been observed to do this, but the Joliot-Curies
could not think what else the radiation might be. They simply reported
that they had discovered gamma rays to be capable of a new kind of
action.

[Illustration: _James Chadwick_]

Not so the English physicist James Chadwick (1891-    ). In that same
year he maintained that a gamma ray, which possessed no mass, simply
lacked the momentum to hurl a proton out of its place in the atom. Even
an electron was too light to do so. (It would be like trying to knock a
baseball off the ground and into the air by hitting it with a ping-pong
ball.)

Any radiation capable of knocking a proton out of an atom had to consist
of particles that were themselves pretty massive. And if one argued like
that, then it seemed that the radiation first observed by Bothe and
Becker had to be the long-sought-for proton-electron combination.
Chadwick used Harkins’ term, neutron, for it and made it official. He
gets the credit for the discovery of the neutron.

Chadwick managed to work out the mass of the neutron from his
experiments and by 1934 it was quite clear that the neutron was more
massive than the proton. The best modern data have the mass of the
proton set at 1.007825, and that of the neutron just a trifle greater at
1.008665.

The fact that the neutron was just about as massive as the proton was to
be expected if the neutron were a proton-electron combination. It was
also not surprising that the isolated neutron eventually breaks up,
giving up an electron and becoming a proton. Out of any large number of
neutrons, half have turned into protons in about 12 minutes.

Nevertheless, although in some ways we can explain the neutron by
speaking of it as though it were a proton-electron combination, it
really is not. A neutron has a spin of ½ while a proton-electron
combination would have a spin of either 0 or 1. The neutron, therefore,
must be treated as a single uncharged particle.


The Proton-Neutron Theory

As soon as the neutron was discovered, the German physicist Werner Karl
Heisenberg (1901-    ) revived the notion that the nucleus must be made
up of protons and neutrons, rather than protons and electrons. It was
very easy to switch from the latter theory to the former, if one simply
remembered to pair the electrons thought to be in the nucleus with
protons and give the name neutrons to these combinations.

Thus, the helium-4 nucleus, rather than being made up of 4 protons and 2
electrons, was made up of 2 protons and 2 proton-electron combinations;
or 2 protons and 2 neutrons. In the same way the oxygen-16 nucleus
instead of being made up of 16 protons and 8 electrons, would be made up
of 8 protons and 8 neutrons.

The proton-neutron theory would account for mass numbers and atomic
numbers perfectly well. If a nucleus was made up of _x_ protons and _y_
neutrons, then the atomic number was equal to _x_ and the mass number to
_x_ + _y_. (It is now possible to define the mass number of a nucleus in
modern terms. It is the number of protons plus neutrons in the nucleus.)

[Illustration: _Werner Heisenberg_]

The proton-neutron theory of nuclear structure could account for
isotopes perfectly well, too. Consider the 3 oxygen isotopes, oxygen-16,
oxygen-17, and oxygen-18. The first would have a nucleus made up of 8
protons and 8 neutrons; the second, one of 8 protons and 9 neutrons; and
the third, one of 8 protons and 10 neutrons. In each case the atomic
number is 8. The mass numbers however would be 16, 17, and 18,
respectively.

In the same way uranium-238 would have a nucleus built of 92 protons and
146 neutrons, while uranium-235 would have one of 92 protons and 143
neutrons.

By the new theory, can we suppose that it is neutrons rather than
electrons that somehow hold the protons together against their mutual
repulsion, and that more and more neutrons are required to do this as
the nucleus grows more massive? At first the number of neutrons required
is roughly equal to the number of protons. The helium-4 nucleus contains
2 protons and 2 neutrons, the carbon-12 nucleus contains 6 protons and 6
neutrons, the oxygen-16 nucleus contains 8 protons and 8 neutrons, and
so on.

For more complicated nuclei, additional neutrons are needed. In
vanadium-51, the nucleus contains 23 protons and 28 neutrons, five more
than an equal amount. In bismuth-209, it is 83 protons and 126 neutrons,
43 more than an equal amount. For still more massive nuclei containing a
larger number of protons, no amount of neutrons is sufficient to keep
the assembly stable. The more massive nuclei are all radioactive.

The manner of radioactive breakdown fits the theory, too. Suppose a
nucleus gives off an alpha particle. The alpha particle is a helium
nucleus made up of 2 protons and 2 neutrons. If a nucleus loses an alpha
particle, its mass number should decline by 4 and its atomic number by
2, and that is what happens.

Suppose a nucleus gives off a beta particle. For a moment, that might
seem puzzling. If the nucleus contains only protons and neutrons and no
electrons, where does the beta particle come from? Suppose we consider
the neutrons as proton-electron combinations. Within many nuclei, the
neutrons are quite stable and do not break up as they do in isolation.
In the case of certain nuclei, however, they do break up.

Thus the thorium-234 nucleus is made up of 90 protons and 144 neutrons.
One of these neutrons might be viewed as breaking up to liberate an
electron and leaving behind an unbound proton. If a beta particle leaves
then, the number of neutrons decreases by one and the number of protons
increases by one. The thorium-234 nucleus (90 protons, 144 neutrons)
becomes a protactinium-234 nucleus (91 protons, 143 neutrons).

In short, the proton-neutron theory of nuclear structure could explain
all the observed facts just as well as the proton-electron theory, and
could explain the nuclear spins, which the proton-electron theory could
not. What’s more, the isolated neutron had been discovered.

The proton-neutron theory was therefore accepted and remains accepted to
this day.


The Nuclear Interaction

In one place, and only one, did the proton-neutron theory seem a little
weaker than the proton-electron theory. The electrons in the nucleus
were thought to act as a kind of glue holding together the protons.

But the electrons were gone. There were no negative charges at all
inside the nucleus, only the positive charges of the proton, plus the
uncharged neutron. As many as 83 positive charges were to be found (in
the bismuth-209 nucleus) squeezed together and yet not breaking apart.

In the absence of electrons, what kept the protons clinging together?

Was it possible that the electrical repulsion between 2 protons is
replaced by an attraction if those protons were pushed together closely
enough? Can there be both an attraction _and_ a repulsion, with the
former the more important at very short range? If this were so, that
hypothetical attraction would have to have two properties. First, it
would have to be extremely strong—strong enough to overcome the
repulsion of two positive charges at very close quarters. Secondly, it
would have to be short-range, for no attractive force between protons of
any kind was ever detected outside the nucleus.

In addition, this short-range attraction would have to involve the
neutron. The hydrogen-1 nucleus was made up of a single proton, but all
nuclei containing more than 1 proton had to contain neutrons also to be
stable, and only certain numbers of neutrons.

Until the discovery of the neutron, only two kinds of forces, or
“interactions”, were known in the universe. These were the
“gravitational interaction” and the “electromagnetic interaction”. The
electromagnetic interaction was much the stronger of the two—trillions
and trillions and trillions of times as strong as the gravitational
attraction.

The electromagnetic attraction, however, includes both attraction
(between opposite electric charges or between opposite magnetic poles)
and repulsion (between like electric charges or magnetic poles). In
ordinary bodies, the attractions and repulsions usually cancel each
other entirely or nearly entirely, leaving very little of one or the
other to be detected as surplus. The gravitational interaction, however,
includes only attraction and this increases with mass. By the time you
have gigantic masses such as the earth or the sun, the gravitational
interaction between them and other bodies is also gigantic.

Both the gravitational and electromagnetic interactions are long-range.
The intensity of each interaction declines with distance but only as the
square of the distance. If the distance between earth and sun were
doubled, the gravitational interaction would still be one-fourth what it
is now. If the distance were increased ten times, the interaction would
still be 1/(10 × 10) or 1/100 what it is now. It is for this reason that
gravitational and electromagnetic interactions can make themselves felt
over millions of miles of space.

But now, with the acceptance of the proton-neutron theory of nuclear
structure, physicists began to suspect the existence of a third
interaction—a “nuclear interaction”—much stronger than the
electromagnetic interaction, perhaps 130 times as strong. Furthermore,
the nuclear interaction had to decline very rapidly with distance much
more rapidly than the electromagnetic interaction did.

In that case, protons in virtual contact, as within the nucleus, would
attract each other, but if the distance between them was increased
sufficiently to place one outside the nucleus, the nuclear interaction
would decrease in intensity to less than the electromagnetic repulsion.
The proton would now be repelled by the positive charge of the nucleus
and would go flying away. That is why atomic nuclei have to be so small;
it is only when they are so tiny that the nuclear interaction can hold
them together.

In 1932 Heisenberg tried to work out how these interactions might come
into being. He suggested that attractions and repulsions were the result
of particles being constantly and rapidly exchanged by the bodies
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




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