Nuclear Forces and Nuclear Energy

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The basic structure of nuclei and some of the ways

they change were discussed in the previous chapter.

What holds the particles in a nucleus together? The pro-

tons have positive electric charge; therefore, all the

electric forces within the nucleus are repulsive.

Neutrons are uncharged and do not participate in any

electrical interaction. Protons and neutrons both pos-

sess mass, but the gravitational force is too weak to

overcome the electrical force. If these were the only

important forces, every nucleus would fly apart, and all

atoms would be reduced to hydrogen with only one pro-ton per nucleus.

An attractive force must be holding the particles

together inside atomic nuclei. This force is called the

strong nuclear force and is due to an interaction called

the strong interaction. Another interaction, the weak

interaction, also acts within atomic nuclei. This inter-

action governs beta decay and other processes.

The Strong Interaction

The strong interaction produces a force that can

hold neutrons and protons together in an atomic nucle-

us. It is the strongest force discovered in natureabout100 times stronger than the electrical force under com-

parable conditions. Thus, the protons in a nucleus can

be held together through the strong interaction even

though they are repelled by the electrical interaction.

However, the strong interaction is not a long-range one.

The strong interaction only affects nuclear particles that

are very close to each other, within about 1015 m.

Notice that both the electromagnetic and gravita-

tional interactions are long-range interactions. Their

strengths decrease with separation, but there is no dis-

tance so great that there is not some gravitational attrac-

tion between objects with mass or some electromagnet-ic interaction between charged objects. In contrast, the

nuclear force is very strong when the nuclear particles

are essentially in contact with one another but is zero at

greater distances. Thus, two protons repel each other

through the electrical interaction if they are some dis-

tance apart, but they attract each other through the

strong interaction if they are close enough together (Fig.

25.1). This difference between the behavior of the

interactions has some interesting consequences.

Figure 25.1. (a) Two protons repel each other at long

range by the electromagnetic interaction. (b) Two pro-

tons attract each other by the strong interaction if they

are close enough. (c) A neutron is attracted to a nearby

proton or (d) to a nearby neutron.

Another interesting property of the strong interac-

tion is that it only acts between certain kinds of particles.

Any two nucleons attract each other if they are close

enough. Protons attract neutrons as well as other pro-

tons, and neutrons attract each other. The strength of the

attractive force seems to be independent of the kind ofnuclear particle involved (Fig. 25.1). On the other hand,

electrons, photons, and neutrinos do not participate in

the strong interaction. For example, there is no strong

interaction between an electron and a proton.

The features of the strong interaction can be sum-

marized as follows:

1. The resulting force is the strongest force in

nature, approximately 100 times stronger than

the electrical force under comparable circum-

stances.

2. The interaction acts only over a short range,

about 1015 m.

3. The interaction occurs only between particular

kinds of particles.

Nuclear Energy

The structure of atomic nuclei is important because

25. Nuclear Forces and Nuclear Energy


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there is considerable potential energy associated with

the strong interaction that can sometimes be converted

to other more useful forms of energy. As a result,

nuclear energy has become an important source of ener-

gy in modern technology. In warfare, nuclear energy

has become a terrifying destructive agent.

The situation with nuclear energy is similar to those

discussed earlier involving the gravitational and electri-

cal interactions. A falling object and the earth lose grav-itational potential energy, which is converted to kinetic

energy, as they approach each other. Upon collision the

kinetic energy is transformed to thermal energy. The

electrically charged particles in hydrogen and oxygen

molecules lose electrical potential energy as they collide

with each other and rearrange themselves into water

molecules. The potential energy in this case transforms

first into kinetic energy of the fast-moving product mol-

ecules and then to thermal energy as the products col-

lide with molecules in the surrounding material.

Atomic electrons lose electrical potential energy as they

make transitions to lower-energy states, often releasing

the lost energy as photons.

In a similar way, protons and neutrons can lose

nuclear potential energy as they come closer together.

The energy appears as kinetic energy of the products or

as ionizing radiation and is often absorbed and thermal-

ized, raising the temperature of the associated matter.

Gamma decay, one of the radioactive processes dis-

cussed in the previous chapter, illustrates these ideas

(Fig. 25.2). When alpha or beta decay occurs, the

nucleus is often left in an excited state. The nuclear par-

ticles can rearrange themselves, losing nuclear potential

energy, by emitting the excess energy in the form of a

high-energy photon (the gamma ray).A similar process is responsible for the energy

released in all the radioactive decay processes. There is

a reduction in nuclear potential energy in each case; the

energy transforms to kinetic energy of the emerging

particles or to electromagnetic energy.

Energy transformations involving the strong inter-

action show one feature that is not seen with the other

interactionsthe energy changes involve significant

fractions of the total mass-energy present. You proba-

bly remember Einsteins prediction that the total energy

associated with matter is related to its mass by the equa-

tion E = mc2. Energy changes caused by the electrical

and gravitational interactions are so small that they

involved only a small fraction of this total mass-ener-

gyso small, in fact, that the resulting mass changes

are immeasurable.

The large energy changes associated with the

strong interaction, on the other hand, are often as largeas a few tenths of 1 percent of the total mass-energy

originally present. When this energy is lost, either by

radiation or any other energy transfer process, the mass

of the resulting particles is measurably less than before.

The mass reduction is strictly in harmony with

Einsteins prediction and provides one of the striking

confirmations of his theoretical work.

These comparatively large energy changes associ-

ated with the strong interaction are responsible for the

high mass-efficiency of nuclear energy. Thus, a large

nuclear weapon can release as much energy as 20 mil-

lion tons (i.e., 20 megatons) of conventional high explo-

sive (TNT).

Nuclear Fusion

One process by which nuclear potential energy can

be released is nuclear fusion, a process by which small

atomic nuclei join together (fuse) to form larger nuclei.

Nuclear fusion releases the energy that maintains the

high internal temperature of the sun and is also the basic

mechanism used in the hydrogen bomb.

An important fusion reaction (Fig. 25.3), one that

has been suggested as a domestic energy source, involves

two isotopes of hydrogen, deuterium (

2

1H) and tritium(31H). Under appropriate conditions these combine to

create an alpha particle (42He) and release a neutron:

21H +

31H 42He + 10n .

Loss of nuclear potential energy in this reaction is

significant. This lost nuclear potential energy trans-

forms into kinetic energy. The products (the alpha par-

ticle and neutron) leave the reaction at high speeds and

Figure 25.2. How is energy conserved when radioactive gamma decay occurs?


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collide with other pieces of matter, atoms, and other

nuclei. As the energy is distributed and becomes more

random, the temperature of the surrounding matter

increases. The amount of energy released is about 90

million kilowatt-hours per kilogram of fuel. The entire

energy needs of the United States in 1970 could have

been supplied with only about 19 tons of material.

The primary materials used in this reaction are eas-

ily obtained. Deuterium (also called heavy hydrogen)occurs naturally and is plentiful on the earths surface.

About 0.015% of all the worlds hydrogen, including

that of seawater, is deuterium. Tritium does not exist

naturally in the earth because its radioactive half-life is

only about 12 years. However, it can be formed by

bombarding 6Li, a plentiful isotope, with neutrons. The

reaction is

63Li +

10n 42He + 31H .

It is even possible to use the neutrons released by fusion

to initiate this reaction, thus replenishing the fusion fuelfrom the products of the fusion itself.

The main technical difficulty in developing a con-

trolled fusion energy source is that the reacting nuclei

initially repel each other because of the electrical inter-

action. Imagine two protons some distance from each

other. If they are to interact via the nuclear force, they

must get close enough so that the nuclear attraction is

greater than the electrical repulsion. This means that

they must approach each other at high speed, since the

electrical repulsion becomes large at such short dis-

tances. If their initial collision speed is not high

enough, they will simply stop and then move apart with-

out ever getting close enough. The technical problemwhich has not yet been completely solvedis to devise

a controlled process by which enough of these charged

particles can approach each other fast enough so that the

nuclear reaction can take place.

The most promising approach is to raise the tem-

perature of a plasma to a high enough level so that the

normal collisions between particles in a gas are suffi-

ciently violent. The required temperature is between 30

and 100 million degrees Celsius. The plasma can be

contained by magnetic fields instead of a material con-

tainer, but no one yet has perfected a way to contain and

control such a high-temperature plasma for long enough

periods of time and at sufficient plasma density so that

more useful energy is released than is consumed by the

confinement device itself. However, great strides have

been made over recent decades and current research

efforts are close to success.

Nuclear fusion has the potential of providing theworlds energy for millions of years if the problem of

containment can be solved. The fuels are plentiful and

the hazards of radioactive by-products are much lower

than for fission. We know that the process works at the

high temperatures in the center of the sun, and we have

caused the process to occur on the earth. However, on

earth we have triggered the reaction by using a fission-

type nuclear explosion to create the required high tem-

peratures. The challenge of controlling nuclear fusion

is probably the most urgent and promising engineering

and scientific problem of our age.

Nuclear Fission

We have noted that nuclear fission occurs sponta-

neously in the case of a few natural radioactive nuclei

and that considerable energy could be released by this

process. But radioactive fission could never be a prac-

tical source of commercially significant amounts of

energy because the materials that undergo spontaneous

fission are rare and their half-lives are all long. It is not

possible to collect enough of the materials in one place

so that large amounts of energy would be released in

comparatively short times.

Fission became an important energy source withthe discovery, in the late 1930s, that certain nuclei could

be induced to fission by slow neutrons. When a slow

neutron strikes such a nucleus, it is absorbed. The

nucleus immediately becomes unstable and breaks into

two major fragments, releasing energy in the process.

Some of the released energy is in the form of kinetic

energy of the new nuclei. This quickly becomes ther-

malized as these collide with other atoms in the materi-

al, and so the net result is an increase in thermal energy

Figure 25.3. An important fusion reaction involving two isotopes of hydrogen. How is mass-energy conserved in this

process?


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and temperature.

In addition to the two major fragments, a few neu-

trons are released in the fission process. These carry

away some of the released energy. More important,

these neutrons have the capability of initiating addition-

al fissions as they, in turn, are absorbed by other fis-

sionable nuclei. These free neutrons then give the pos-

sibility of initiating a chain reaction (Fig. 25.4); for

example, one nucleus fissions spontaneously releasing

two neutrons. Each of these is absorbed by other nuclei,

causing them to fission, releasing additional neutrons.

These cause even more fissions and the release of more

neutrons. The process quickly builds until the tempera-

ture of the material is so high that it explodes. This, in

fact, is the mechanism of the so-called atom bomba

nuclear bomb. All that is needed to produce such an

explosion is to arrange a large enough collection of fis-

sionable nuclei close enough so that the neutrons

released by each fission are absorbed by other fission-

able nuclei.

Keeping the nuclear chain reaction under control in

a nuclear reactor allows energy to be released at a slow-er rate, so that the resulting thermal energy can be used

to produce electricity or some other useful form of ener-

gy. The way to control the reaction is to absorb some of

the free neutrons before they cause additional fissions.

A nuclear reactor is equipped with control rods (Fig.

25.5), usually made of cadmium metal, that are

designed to harmlessly absorb all but one of the free

neutrons. Each fission then causes one additional fis-

sion, and the reaction is controlled. If the reactor

becomes too hot, the control rods are inserted farther

into the fissionable fuel. A larger fraction of neutrons is

absorbed, and the reaction slows down. Retracting the

rods causes the reaction rate to increase because a

smaller fraction of neutrons is then absorbed.

Figure 25.5. The basic elements of a nuclear reactor.

Fission has two major weaknesses as a source of

energy: the by-products are always radioactive, and the

fuel is not plentiful. The radioactivity of fission by-

products is due to the fact that fissionable nuclei are neu-

tron richthey contain too many neutrons for their

number of protons (about 1.6 neutrons per proton).

Lighter nuclei, which are the products of fission, are sta-

ble only with roughly equal numbers of protons and neu-

trons (about 1.2 neutrons per proton). Since the fission

Figure 25.4. The fission chain reaction by which nuclear energy can be released.


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fragments have too many neutrons, they immediately

move toward stability by changing some of their neu-

trons to protons by beta decay or lowering their energy

by gamma decay. All the products of fission are radioac-

tive, some with half-lives of a fraction of a second and

some with half-lives of many thousands of years.

These radioactive by-products are the principal

environmental concern associated with nuclear reactors.

If an accident occurs, how can we be sure that theseradioactive materials do not enter the environment and

become hazardous to health? If no accident occurs,

how can we be sure that such materials are safely stored

for the thousands of years needed for them to decay to

the point where they are no longer hazardous?

Fissionable fuel is in short supply because only one

naturally occurring isotope, uranium-235, is appropriate

for the chain reaction. Uranium itself is a rare element,

and only 0.7 percent of naturally occurring uranium is

composed of the useful isotope. This percentage is not

high enough for the chain reaction to occur in natural

uranium. Uranium reactors use enriched uranium in

which the fraction of uranium-235 has been increased to

the required percentage.

It is also possible to make additional nuclear fuel in

the reactor itself. Most of the uranium in nature, urani-

um-238, is not fissionable itself, but it can absorb neu-

trons released by other fissions in the reactor. When

uranium-238 absorbs a neutron, it becomes uranium-

239 which is unstable. Two beta decays, one after the

other, transmute the uranium-239 into plutonium-239.

The resulting plutonium-239 can be recovered from the

fuel rods of uranium reactors and used as the fissionable

material in other reactors. This process is the basis of

the so-called breeder reactor in which more fuel, inthe form of plutonium-239, is produced than is used in

the operation of the reactor itself. Such a reactor, prop-

erly controlled, could provide a significant source of

energy for decades to come.

At first it seems paradoxical that fission and fusion

can both release nuclear potential energy when they

seem to be opposite kinds of processes. In both cases,

however, nuclear potential energy is lost because

nuclear particles are closer after the reaction than

before. This relationship is easy to visualize for fusion,

but somewhat more difficult for fission.

To understand the energy relationships in fission,

you must remember two things. First, the strong inter-action is a short-range interaction, so short that nuclear

particles on opposite sides of a large nucleus like urani-

um do not interact with each other in this way. Each

nucleon is affected only by those particles that are near.

And second, all the protons in a uranium nucleus have a

positive electric charge, and the electrical interaction is

a long-range interaction. Thus, all the protons in a large

nucleus repel each other.

These two factors combine in such a way that the

protons and neutrons in a large nucleus are not as close

together as they would be if only the strong interaction

were operating. The nucleus is comparatively

spongy, the protons being pushed apart from each

other by electrical forces and the neutrons following

because of their attachment to the protons through the

strong interaction. These very heavy nuclei are only

stable in the first place because of the accompanying

large number of neutrons. If there are too many pro-tons, the very heavy nuclei could not be held together at

all, even by the strong forces associated with the strong

interaction. This is the principal reason why no ele-

ments heavier than uranium occur in nature, and all ele-

ments heavier than bismuth are radioactive.

After fission takes place, the overall nuclear poten-

tial energy in the product nuclei is reduced relative to

that in the original large nucleus. The strong interaction

exerts its influence more forcefully within the two

smaller fragments so that the nuclear particles come

closer together, thus lowering nuclear potential energy.

Both electrical (from the electrical repulsion between

the two positively charged nuclear fragments as they

move apart) and nuclear potential energy are major

sources of energy converted to kinetic energy and radi-

ation whenever fission occurs.

The Weak Interaction

The weak interaction is a second interaction that

operates at close distance inside atomic nuclei. The

weak interaction governs beta decay, electron capture,

and all interactions in which neutrinos are involved.

Neutrinos do not participate in any of the other interac-

tions (except gravitational), a fact that accounts for thesmall probability they have of interacting with matter.

The weak interaction completes the list of the four

known fundamental interactions. Listed in order of

decreasing strength the four forces are as follows: the

strong interaction, the electromagnetic interaction, the

weak interaction, and the gravitational interaction. If all

four interactions were acting at the same time in a par-

ticular situation, their relative strengths would be as fol-

lows: gravity, 1; weak interaction, 1025; electromagnet-

ic interaction, 1036; and strong interaction, 1038.

Gravity, the weakest, would be important only in situa-

tions in which the other three were not operating. The

weak interaction would govern behavior only if theelectromagnetic and strong interactions did not.

The Structure of Nucleons

In all experiments performed thus far, the electron

always behaves like an idealized point. We say that it is

an elementary particle because it apparently has no

structure. Nucleons, on the other hand, are not elemen-

tary particles. Experiments similar to the Rutherford


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experiment, which revealed the structure of the atom

(particularly the existence of the nucleus), reveal that

the proton and neutron have structure. The evidence is

quite convincing that there are lumps in a proton; we

call these lumps quarks.

The present state of knowledge is that the quarks,

electrons and neutrinos are elementary particles.

Experiments have revealed three kinds of electrons (the

garden-variety electron, the muon, and the tau) whichdiffer markedly in their masses but are otherwise the

same. For each of these there is an associated, but dif-

ferent, kind of neutrino. Each kind of electron and each

kind of neutrino has a distinct antiparticle. Hence our

number of elementary particles has ballooned to 12.

There have been six kinds of quarks observed. The

six are given the quaint labels of up, down,

strange, charm, bottom, and top. Each quark

is thought to have a fraction of the charge on an electron

(plus or minus 1/3 or 2/3, depending on the quark) and

is also thought to come in three varieties called co-

lors. Each quark has an antiparticle so there are 36

kinds of quarks if the antiparticles are counted. Each is

thought to be elementary. In addition there are a whole

host of additional particles, some elementary, some not.

The strength of the quark model is that it predicts

the existence and properties of just these particles with-

out predicting the existence of some which do not exist.

The whole scheme taken together is quite complicated,

but very impressive in its internal consistency and pre-

dictive power. Still, the model represents the present

frontier of understanding and is the subject of ongoing

refinement and experimental tests.

On the submicroscopic level, particles interact

(exchange energy) by passing other particles back andforth. Imagine tennis players who interact by hitting a

ball back and forth. The different fundamental forces

(strong, electromagnetic, and weak) each have different

balls that are generically referred to as bosons. You

can further visualize what is happening by looking at a

spacetime diagram for two repelling electrons (Fig.

25.6). (Refer to Chapter 9 where spacetime diagrams

were introduced.) The two electrons are shown

exchanging a photon (represented by the wavy line) and

reversing their direction of motion in space. The verti-

cal axis is the direction of increasing time.

The example shows that charged particles

exchange photons in the electromagnetic interaction. In

the strong interaction quarks exchange gluons. In the

weak interaction the exchanged particles are so-calledvector bosons. Again we have added new particles

(usually with corresponding antiparticles) to the grow-

ing zoo of elementary particles.

Particles, such as protons, are thought to be made

up of combinations of the varieties of quarks. Protons

and neutrons are combinations of three quarks. The

strong interaction of two protons with one another is

really a result of gluon exchanges between the con-

stituent quarks (Fig. 25.7). Vector bosons are

exchanged in decay processes such as beta decay that

Figure 25.6. Spacetime diagram for the electromagnet-

ic interaction of two electrons by exchange of a photon.

Figure 25.7. Spacetime diagram for the strong interac-

tion of two quarks by exchange of a gluon.

Figure 25.8. Neutron decay results from the emission of

a vector boson by a quark. Vector bosons are exchanged

in weak interactions.


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are governed by the weak interaction (Fig. 25.8).

Spacetime diagrams are not merely conceptual pic-

tures. They are also a visual recipe for a calculation of

the rate at which the process occurs. Each line and point

of intersection can be made to correspond to a specific

piece of a mathematical representation of the probabili-

ty of the process in the picture. Just as hammers and

saws are the essentials tools of a carpenter, the spacetime

diagrams are tools of the elementary particle physicist.

Summary

The strong interaction holds the particles inside

atomic nuclei together. The resulting forces are the

strongest in nature, but act only at short range and only

between certain kinds of particles. Potential energy, in

comparatively large amounts, is associated with these

strong forces and can sometimes be changed to other

forms.

Fission and fusion are two processes by which

nuclear energy can be released. Fusion combines small

nuclei into larger ones. The raw materials are abundant

or easily produced, and there are few radioactive by-

products. The materials, however, must be kept at high

temperature during the reaction, and no one yet has

learned how to accomplish this to release commercially

useful amounts of energy.

Commercial fission reactors depend on a chain

reaction in which each fission releases neutrons that

induce additional fissions. Fissionable materials are not

plentiful and some by-products of the fission process

are intensely radioactive.

It is becoming increasingly clear that nucleons

(protons and neutrons) are made of more basic particlescalled quarks. Quarks, electrons, and neutrinos are

thought to be without structure and size and are called

elementary particles. At the submicroscopic level,

interactions are the result of the exchange of particles.

The strong interaction exchanges gluons. The electro-

magnetic interaction exchanges photons. The weak

interaction exchanges vector bosons.

Historical Perspectives

The neutron (discovered in 1932) is a marvelous

thing. It is an electrically neutral, strongly interacting

particle that can penetrate the nucleus uninhibited byelectrical repulsion. Beginning in 1934, Enrico Fermi

(1901-1954) and others began to study the transmuta-

tion of elements by neutron bombardmentbut failed

to notice that some of the bombarded nuclei were fis-

sioning. As the world moved closer to World War II,

Otto Hahn, Lise Meitner, and Fritz Strassman continued

neutron bombardment experiments in Berlin.

In 1938, Meitner lost her post in Hitlers Berlin

because she was Jewish and so moved to Stockholm,

where she and her nephew, Otto Frisch, first recognized

the evidence for fission in data sent to her by Hahn and

Strassman. She communicated the evidence to Niels

Bohr. Bohr carried the information to a theoretical

physics conference in Washington, D.C. on January 29,

1939, where Fermi wondered out loud if neutrons were

being produced in the fission process in sufficient quan-

tities to sustain a chain reaction. The meeting was im-

mediately thrown into an uproar as physicists rushed tophones and called their laboratories to initiate experi-

ments to search for the neutrons. By March 3, 1939,

Leo Szilard and Walter Zinn had detected the neutrons

in sufficient quantities to make a chain reaction feasi-

bleand create a bomb of enormous explosive power.

By April of 1939 (a month later), German scientists

had already held the first meeting concerning the build-

ing of an atomic bomb. By September of 1939, nine

nuclear physicists in Germany had drawn up a detailed

research program. Soon negotiations were begun to

acquire all the uranium and radium produced by the

Joachimsthal mines in Czechoslovakia. Later, a 3500-

ton supply of uranium was captured in Belgium.

In the United States, scientists reacted with fear to

the German initiatives. On March 16, 1939, Fermi tried

unsuccessfully to get the Navy to begin a research pro-

gram of its own. Other foreign-born scientists persuad-

ed Albert Einstein (who as a Jew had left Germany for

the United States in 1933) to use his influence with

President Franklin Roosevelt. Einstein wrote a letter to

Roosevelt on August 2, 1939, in which he warned:

In the course of the last four months it has been

made probable . . . that it may become possible

to set up a nuclear chain reaction in a largemass of uranium, by which a vast amount of

power and large quantities of new radium-like

elements would be generated. Now it appears

almost certain that this could be achieved in the

immediate future.

This new phenomenon would also lead to the

construction of bombs and it is conceivable

though much less certainthat extremely pow-

erful bombs of a new type may thus be con-

structed. A single bomb of this type, carried by

boat and exploded in a port, might very well

destroy the whole port together with some ofthe surrounding territory. However, such a

bomb might very well prove to be too heavy for

transportation by air.

In February of 1940, Roosevelt made $6000 avail-

able to start research. Two billion dollars would follow.

It soon became clear that only two isotopes, 235U

and 239Pu, were suitable. About 100 pounds of each

were needed, yet not a millionth of a pound of either


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were available, nor was there any knowledge of quanti-

ty production processes. During the next several years

large facilities had to be built at Oak Ridge, Tennessee,

and at Hanford, Washington, to provide concentrated

supplies of these materials. Meanwhile, the theorists

were given the task of designing a bomb assuming that

the isotopes would eventually be available.

In December 1942, the first controlled, self-sus-

tained chain reaction took place in the squash courtbeneath the University of Chicago football stadium.

The director of the secret project, Arthur Compton,

informed Harvard President James B. Conant in a cryp-

tic letter that the reactor had begun operation ahead of

schedule, that it was smaller than calculated, and that

Chicago was still intact:

The Italian navigator has just landed in the

New Worldthe earth was smaller than esti-

mated and he arrived several days earlier than

he had expected . . . (The natives) were indeed

(friendly). Everyone landed safe and happy

(from A. H. Compton, Atomic Quest: A

Personal Narrative, New York, Oxford

University Press, 1956).

In 1943, a secret laboratory to design the bomb was

created from scratch on a mesa at Los Alamos, New

Mexico. It was headed by J. Robert Oppenheimer. In

fact, two bombs of different design were created:

Little Boy from 235U and Fat Man from 239Pu. On

July 16, 1945, a plutonium bomb was tested at a target

area called Trinity, 120 miles southeast of Albuquerque.

The explosion came with a tremendous flash, followed

by a sudden blast of heat and then by a roar of sound. Aball of fire rose rapidly, followed by a mushroom cloud

extending to 40,000 feet. The test tower was vaporized

and the surrounding desert surface fused to glass.

On August 6, 1945, Little Boy was dropped on

Hiroshima, instantly and completely devastating four

square miles of the heart of the city, killing 66,000 and

injuring 69,000. The world had entered the terrifying

age of nuclear war. Three days later Fat Man was

dropped on Nagasaki, killing 39,000 persons and injur-

ing 25,000.

On August 14, 1945, Japan surrendered. Germany,

which had forced many of its best scientists to flee

because of its racial policies, was never able to marshalenough resources to capitalize on its early lead in the

race for the atomic bomb.

The conduct and scale of science (particularly

physics) in American society was profoundly changed

by the building of the bomb. Prior to the Second World

War, physicists worked alone or in small laboratories

with meager funding. But the large national laborato-

ries at Oak Ridge, Hanford, Los Alamos, and others that

built the bomb continued to receive support from the

federal government following the war. They currently

employ thousands of scientists, engineers, and techni-

cians. Like Alexanders support of the Library and

Museum of Alexandria, the federal government became

the patron of science.

The laboratories continued developing and testing

nuclear weapons, but they also attacked the problems of

fission and fusion as sources of energy for a rapidly

expanding industrial society. The laboratories also builtever-larger elementary particle accelerators in an

attempt to resolve the age-old question about the funda-

mental structure of matter. Other government-support-

ed scientists and engineers undertook the first steps into

space exploration, while others tackled the fundamental

question of life at the molecular level.

These admittedly expensive and often esoteric

activities continue to this day and are justified on the

basis that research and development are investments in

the well-being and future of society.

STUDY GUIDE

Chapter 25: Nuclear Forces and Nuclear Energy

A. FUNDAMENTAL PRINCIPLES

1. The Electromagnetic Interaction: See Chapter 4.

2. The Strong Interaction: See Chapter 2.

3. The Wave-Particle Duality of Matter and

Electromagnetic Radiation: See Chapters 14 and

16.

4. The Conservation of Mass-Energy: See Chapter

9.

5. The Conservation of Electric Charge: See

Chapter 7.

6. The Conservation of Mass Number: See Chapter

24.

B. MODELS, IDEAS, QUESTIONS, OR APPLICA-

TIONS

1. What are the properties of the strong interaction?

2. What is the source of energy in nuclear reactions?

3. What is nuclear fusion?

4. What is nuclear fission?

5. What are the problems associated with obtaining

and using nuclear energy?

6. What are the advantages of obtaining and using

nuclear energy?

7. What are quarks?8. How are the strong, electromagnetic, and weak

interactions explained at the most fundamental

level? What are gluons and vector bosons?

C. GLOSSARY

1. Chain Reaction: A process in which fission is

triggered by the absorption of a neutron by a nucle-

us and releases enough free neutrons as fission by-

products to trigger subsequent fissions in other


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nuclei.

2. Control Rod: Structures within the core of a

nuclear reactor whose purpose is to absorb neutrons

without fissioning and thus shut down or control a

chain reaction. Cadmium is a suitable material for

control rods.

3. Elementary Particle: A class of subatomic parti-

cles (including electrons, quarks, and neutrinos)

that are best modeled as a mathematical point with-out any measurable size and that are, therefore,

thought to be the most fundamental constituents of

matter.

4. Fuel Rod: Structures of fissionable material which

form the core of a device (nuclear reactor) to con-

trol a fission chain reaction for the practical pur-

pose of generating usable energy. Suitable materi-

als for fuel rods are 235U and 239Pu.

5. Gluons: The elementary particles exchanged

between quarks to convey energy from one to

another to create the strong interaction. Gluons

play the role for the strong interactions that photonsplay for the electromagnetic interaction.

6. Moderator: Material surrounding or within the

core of a nuclear reactor whose purpose is to slow

neutrons down by collisions so that they more read-

ily cause fission when absorbed by certain fission-

able materials. Water and graphite are suitable

moderator materials.

7. Nuclear Fission: See Chapter 24.

8. Nuclear Fusion: A nuclear process in which

nuclei of lower mass number collide and combine

to form nuclei of higher mass number.

9. Quarks: The elementary particles of which nucle-

ons are made. Each nucleon consists of threequarks.

10. Vector Bosons: The elementary particles that con-

vey energy from one particle to another to create

the weak interaction. Vector bosons play the role

for the weak interaction that photons play for the

electromagnetic interaction.

D. FOCUS QUESTIONS

1. Consider nuclear fusion:

a. Name and state in your own words the three

fundamental conservation principles that govern

adjustments in a nucleus associated with the release

of nuclear energy. What is the source of the energyreleased?

b. Describe the process of nuclear fusion in terms

of these principles. What do you begin with? What

do you end with?

c. What are the difficulties and the advantages of

fusion as a source of useful energy?

2. Consider nuclear fission:

a. Name and state in your own words the three

fundamental conservation principles that govern

adjustments in a nucleus associated with the release

of nuclear energy. What is the source of the energy

released?

b. Describe the process of nuclear fission in

terms of these principles. What do you begin with?

What do you end with?

c. What are the difficulties and the advantages of

fission as a source of useful energy?

3. Consider nuclear fission:a. What are the two fundamental forces that are

directly involved in nuclear fission? Describe each

of the two forces in terms of the kinds of particles

that experience the force, the ranges of the forces

(long or short), and the relative strengths of the two

forces under comparable conditions.

b. Describe the process of nuclear fission in

terms of the interplay of these two forces. What do

you begin with? What do you end with?

c. Why are nuclei of very large atomic numbers

suitable for fission, but nuclei with small atomic

numbers are not?

4. Consider nuclear fusion:

a. What are the two fundamental forces that are

directly involved in nuclear fusion? Describe each

of the two forces in terms of the kinds of particles

that experience the force, the ranges of the forces

(long or short), and the relative strengths of the two

forces under comparable conditions.

b. Describe the process of nuclear fusion in terms

of the interplay of these two forces. What do you

begin with? What do you end with?

c. Why are nuclei of very small atomic numbers

suitable for fusion, but nuclei with large atomic

numbers are not?

E. EXERCISES

25.1. Outline the important characteristics of the

strong interaction.

25.2. Explain why two protons repel each other

when they are separated, but attract each other when

they are close together.

25.3. Explain why a nucleus has less mass after

emitting a gamma ray than before. Show how mass-

energy is conserved in this case.

25.4. When a nucleus decays by beta decay, it

emits a high energy electron. What is the source of

energy from which the electrons mass and kinetic ener-

gy come?

25.5. Why do the nuclei of atoms not break apart

very easily? Atoms themselves can be broken up in a

simple gas discharge tube, yet nuclei maintain their

structure without change in even the most violent chem-


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ical explosions. Why are nuclei so much more stable

than atoms?

25.6. Show that mass-energy is conserved in a

fusion reaction by accounting for all the mass-energy

before and after the reaction.

25.7. Show that mass-energy is conserved in a fis-

sion reaction by accounting for all the mass-energybefore and after the reaction.

25.8. Why do the products of nuclear fission and

fusion have less mass than the original nuclear parti-

cles?

25.9. Describe the process of nuclear fusion.

25.10. Describe the process of nuclear fission.

25.11. How can nuclear fusion and fission both be

used to release energy when they are, at least in one

sense, opposite processes?

25.12. Describe the chain reaction that can occur

for nuclear fission.

25.13. Describe the function and operation of the

control rods in a nuclear power plant.

25.14. Why is high temperature required for

nuclear fusion to occur?

25.15. What would be the advantages of nuclear

fusion over nuclear fission as a domestic energy source?

25.16. Why do we not already have nuclear fusion

reactors producing energy for domestic use?

25.17. Why do we notice a change in mass for fis-

sion and fusion, but not in energy changes associated

with chemical or gravitational forces?

25.18. Nuclear potential energy is

(a) the source of energy in nuclear fusion.

(b) the source of energy in nuclear fission.

(c) converted to kinetic energy in nuclear fission.

(d) converted to kinetic energy in nuclear fusion.(e) all of the above.

25.19. Which of the following is not true?

(a) There are working fission power reactors.

(b) There are working fusion power reactors.

(c) There are working fusion bombs.

(d) There are working fission bombs.

(e) Fusion reactions power the sun.

Nuclear Forces and Nuclear Energy

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