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Worlds Within Worlds: 

jry of Nuclear r _ 

Ma ss and Energy * The Neutron 
The Structure of the Nucleus 

by Isaac Asimov 

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

Mass and Energy; The Neutron; The Structure of the Nucleus 

Author: Isaac Asimov 

Release Date: August 30, 2015 [EBook #49820] 

Language: English 


Produced by Stephen Hutcheson, Dave Morgan and the Online 
Distributed Proofreading Team at 

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 


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 

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 

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 

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. 

Photograph of night sky 


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 


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 


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 

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. 


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 

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? 

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 ah 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 = me 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 V 28 of an ounce) would keep a 100-watt light bulb burning for 
35,000 years. 


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 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 

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. 

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 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 

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 x 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 x 2, or 16, while the 6 beta 
particles involve an increase in atomic number of 6 x 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 

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 

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 

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-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 

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. 

Man-made transmutation. 

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 

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, Frederic Joliot-Curie (1900-1958) and Irene 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. 

Frederic and Irene 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 

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. 

Artificial radioactivity. 

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”. 

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. 

The bombardment of lithium-7 with protons was the first nuclear reaction caused by man-made particles. 

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. 

Ernest O. Lawrence holds a model of the first cyclotron in 1930, a year after its conception. 


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 Vi. If the particle spun in one direction it 
was +Yi, if in the other it was -Vi. 

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 +Vi and two of -Vi. Suppose also that of the 2 electrons, one had a spin of 
+Vi and one of -Vi. 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 +Yi or all -Vi. 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. 

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 
+Vi or -Vi, 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 +Vi and the 4 electrons 
were all -Vi in their spins, the nuclear spin would be 7 /2 - 4 A = 3 h. 

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 

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 

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. 

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. 

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 Vi 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 

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.) 

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 

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 x 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 

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 l h 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. 

Hideki Yukawa 

Victor F. Fless 

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. 

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 

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 

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 

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 


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 Copyright © by Abelard-Shuman, Ltd., New York. Reprinted by 

front permission from Inside the Atom, Isaac Asimov, 1966. 


Photo Credits 

Cover Thorne Films 

Page The “Horsehead” Nebula in Orion, Hale Observatories. 



front cover 

Author’s Jay K. Klein 

Contents Lick Observatory 

Dr. Erwin W. Mueller, The Pennsylvania State University 
Yerkes Observatory 

From Discovery of the Elements, Mary E. Weeks, Chemical 




96 & 97 

Education Publishing Company, 1968. 

The Central Press Photos, Ltd., and Sir John Cockcroft 
Ernest Orlando Lawrence Livermore Laboratory 
Samuel A. Goudsmit 
Nobel Institute 

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 


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 

• 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 

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. 


United States 

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

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