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

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