2. Processes of Nuclear
Energy
Fission
A nucleus of large mass number splits
into two smaller nuclei
Fusion
Two light nuclei fuse to form a
heavier nucleus
Large amounts of energy are
released in either case
3. Nuclear Fission
A heavy nucleus splits into two
smaller nuclei
The total mass of the products is less
than the original mass of the heavy
nucleus
First observed in 1939 by Otto Hahn
and Fritz Strassman following basic
studies by Fermi
Lisa Meitner and Otto Frisch soon
explained what had happened
4. Fission Equation
Fission of 235U by a slow (low energy)
neutron
236U* is an intermediate, short-lived state
Lasts about 10-12 s
X and Y are called fission fragments
Many combinations of X and Y satisfy the
requirements of conservation of energy and
charge
5. More About Fission of 235U
About 90 different daughter nuclei
can be formed
Several neutrons are also
produced in each fission event
Example:
The fission fragments and the
neutrons have a great deal of KE
following the event
6. Sequence of Events in
Fission
The 235U nucleus captures a thermal (slow-
moving) neutron
This capture results in the formation of
236U*, and the excess energy of this
nucleus causes it to undergo violent
oscillations
The 236U* nucleus becomes highly
elongated, and the force of repulsion
between the protons tends to increase the
distortion
The nucleus splits into two fragments,
emitting several neutrons in the process
8. Energy in a Fission
Process
Binding energy for heavy nuclei is about
7.2 MeV per nucleon
Binding energy for intermediate nuclei
is about 8.2 MeV per nucleon
Therefore, the fission fragments have
less mass than the nucleons in the
original nuclei
This decrease in mass per nucleon
appears as released energy in the
fission event
9. Energy, cont
An estimate of the energy released
Assume a total of 240 nucleons
Releases about 1 MeV per nucleon
8.2 MeV – 7.2 MeV
Total energy released is about 240 Mev
This is very large compared to the
amount of energy released in chemical
processes
10. Chain Reaction
Neutrons are emitted when 235U
undergoes fission
These neutrons are then available to
trigger fission in other nuclei
This process is called a chain reaction
If uncontrolled, a violent explosion can
occur
The principle behind the nuclear bomb,
where 1 kg of U can release energy equal to
about 20 000 tons of TNT
12. Nuclear Reactor
A nuclear reactor is a system designed
to maintain a self-sustained chain
reaction
The reproduction constant, K, is defined
as the average number of neutrons
from each fission event that will cause
another fission event
The maximum value of K from uranium
fission is 2.5
In practice, K is less than this
A self-sustained reaction has K = 1
13. K Values
When K = 1, the reactor is said to be
critical
The chain reaction is self-sustaining
When K < 1, the reactor is said to be
subcritical
The reaction dies out
When K > 1, the reactor is said to be
supercritical
A run-away chain reaction occurs
14. Basic Reactor Design
Fuel elements
consist of
enriched uranium
The moderator
material helps to
slow down the
neutrons
The control rods
absorb neutrons
15. Reactor Design Considerations
– Neutron Leakage
Loss (or “leakage”) of neutrons from
the core
These are not available to cause fission
events
The fraction lost is a function of the
ratio of surface area to volume
Small reactors have larger percentages lost
If too many neutrons are lost, the reactor
will not be able to operate
16. Reactor Design Considerations
– Neutron Energies
Slow neutrons are more likely to cause
fission events
Most neutrons released in the fission
process have energies of about 2 MeV
In order to sustain the chain reaction, the
neutrons must be slowed down
A moderator surrounds the fuel
Collisions with the atoms of the moderator
slow the neutrons down as some kinetic
energy is transferred
Most modern reactors use heavy water as
the moderator
17. Reactor Design Considerations
– Neutron Capture
Neutrons may be captured by
nuclei that do not undergo fission
Most commonly, neutrons are
captured by 238U
The possibility of 238U capture is lower
with slow neutrons
The moderator helps minimize the
capture of neutrons by 238U
18. Reactor Design Considerations
– Power Level Control
A method of control is needed to adjust
the value of K to near 1
If K >1, the heat produced in the runaway
reaction can melt the reactor
Control rods are inserted into the core to
control the power level
Control rods are made of materials that
are very efficient at absorbing neutrons
Cadmium is an example
By adjusting the number and position of
the control rods, various power levels can
be maintained
20. Pressurized Water Reactor
– Operation Notes
This type of reactor is commonly used
in electric power plants in the US
Fission events in the reactor core supply
heat to the water contained in the
primary system
The primary system is a closed system
This water is maintained at a high
pressure to keep it from boiling
The hot water is pumped through a
heat exchanger
21. Pressurized Water Reactor
– Operation Notes, cont
The heat is transferred to the water
contained in a secondary system
This water is converted into steam
The steam is used to drive a turbine-
generator to create electric power
The water in the secondary system is
isolated from the water in the primary
system
This prevents contamination of the
secondary water and steam by the
radioactive nuclei in the core
22. Reactor Safety –
Containment
Radiation exposure, and its potential
health risks, are controlled by three
levels of containment
Reactor vessel
Contains the fuel and radioactive fission
products
Reactor building
Acts as a second containment structure
should the reactor vessel rupture
Location
Reactor facilities are in remote locations
23. Reactor Safety – Loss of
Water
If the water flow was interrupted, the
nuclear reaction could stop immediately
However, there could be enough residual
heat to build up and melt the fuel elements
The molten core could also melt through the
containment vessel and into the ground
Called the China Syndrome
If the molten core struck ground water, a steam
explosion could spread the radioactive material to
areas surrounding the power plant
Reactors are built with emergency cooling
systems that automatically flood the core if
coolant is lost
24. Reactor Safety –
Radioactive Materials
Disposal of waste material
Waste material contains long-lived, highly radioactive
isotopes
Must be stored over long periods in ways that protect
the environment
Present solution is sealing the waste in waterproof
containers and burying them in deep salt mines
Transportation of fuel and wastes
Accidents during transportation could expose the
public to harmful levels of radiation
Department of Energy requires crash tests and
manufacturers must demonstrate that their
containers will not rupture during high speed
collisions
25. Nuclear Fusion
Nuclear fusion occurs when two
light nuclei combine to form a
heavier nucleus
The mass of the final nucleus is
less than the masses of the
original nuclei
This loss of mass is accompanied by a
release of energy
26. Fusion in the Sun
All stars generate energy through fusion
The Sun, along with about 90% of other
stars, fuses hydrogen
Some stars fuse heavier elements
Two conditions must be met before
fusion can occur in a star
The temperature must be high enough
The density of the nuclei must be high
enough to ensure a high rate of collisions
27. Proton-Proton Cycle
The proton-proton cycle
is a series of three
nuclear reactions
believed to operate in
the Sun
Energy liberated is
primarily in the form of
gamma rays, positrons
and neutrinos
2
1H is deuterium, and
may be written as 2
1D
28. Fusion Reactors
Energy releasing fusion reactions are
called thermonuclear fusion reactions
A great deal of effort is being directed
at developing a sustained and
controllable thermonuclear reaction
A thermonuclear reactor that can
deliver a net power output over a
reasonable time interval is not yet a
reality
29. Advantages of a Fusion
Reactor
Inexpensive fuel source
Water is the ultimate fuel source
If deuterium is used as fuel, 0.06 g of
it can be extracted from 1 gal of
water for about 4 cents
Comparatively few radioactive by-
products are formed
30. Considerations for a
Fusion Reactor
The proton-proton cycle is not feasible
for a fusion reactor
The high temperature and density required
are not suitable for a fusion reactor
The most promising reactions involve
deuterium (D) and tritium (T)
31. Considerations for a
Fusion Reactor, cont
Deuterium is available in almost
unlimited quantities in water and is
inexpensive to extract
Tritium is radioactive and must be
produced artificially
The Coulomb repulsion between
two charged nuclei must be
overcome before they can fuse
32. Requirements for Successful
Thermonuclear Reactor
High temperature 108 K
Needed to give nuclei enough energy to
overcome Coulomb forces
At these temperatures, the atoms are
ionized, forming a plasma
Plasma ion density, n
The number of ions present
Plasma confinement time,
The time the interacting ions are
maintained at a temperature equal to or
greater than that required for the reaction
to proceed successfully
33. Lawson’s Criteria
Lawson’s criteria states that a net
power output in a fusion reactor is
possible under the following conditions
n 1014 s/cm3 for deuterium-tritium
n 1016 s/cm3 for deuterium-deuterium
The plasma confinement time is still a
problem
34. Magnetic Confinement
One magnetic
confinement device is
called a tokamak
Two magnetic fields
confine the plasma inside
the doughnut
A strong magnetic field is
produced in the windings
A weak magnetic field is
produced in the toroid
The field lines are helical,
spiral around the
plasma, and prevent it
from touching the wall of
the vacuum chamber
35. Some Fusion Reactors
TFTR
Tokamak Fusion Test Reactor
Princeton
Central ion temperature of 510 million
degrees C
The n values were close to Lawson criteria
JET
Tokamak at Abington, England
6 x 1017 DT fusions per second were
achieved
36. Current Research in Fusion
Reactors
NSTX – National Spherical Torus
Experiment
Produces a spherical plasma with a hole in the
center
Is able to confine the plasma with a high pressure
ITER – International Thermonuclear
Experimental Reactor
An international collaboration involving four major
fusion programs is working on building this reactor
It will address remaining technological and scientific
issues concerning the feasibility of fusion power
37. Other Methods of Creating
Fusion Events
Inertial laser confinement
Fuel is put into the form of a small pellet
It is collapsed by ultrahigh power lasers
Inertial electrostatic confinement
Positively charged particles are rapidly
attracted toward an negatively charged grid
Some of the positive particles collide and
fuse
38. Elementary Particles
Atoms
From the Greek for “indivisible”
Were once thought to the elementary
particles
Atom constituents
Proton, neutron, and electron
Were viewed as elementary because
they are very stable
39. Discovery of New Particles
New particles
Beginning in 1937, many new particles were
discovered in experiments involving high-
energy collisions
Characteristically unstable with short
lifetimes
Over 300 have been cataloged
A pattern was needed to understand all
these new particles
40. Quarks
Physicists recognize that most particles
are made up of quarks
Exceptions include photons, electrons and a
few others
The quark model has reduced the array
of particles to a manageable few
The quark model has successfully
predicted new quark combinations that
were subsequently found in many
experiments
41. Fundamental Forces
All particles in nature are subject
to four fundamental forces
Strong force
Electromagnetic force
Weak force
Gravitational force
42. Strong Force
Is responsible for the tight binding of
the quarks to form neutrons and
protons
Also responsible for the nuclear force
binding the neutrons and the protons
together in the nucleus
Strongest of all the fundamental forces
Very short-ranged
Less than 10-15 m
43. Electromagnetic Force
Is responsible for the binding of
atoms and molecules
About 10-2 times the strength of
the strong force
A long-range force that decreases
in strength as the inverse square
of the separation between
interacting particles
44. Weak Force
Is responsible for instability in certain
nuclei
Is responsible for beta decay
A short-ranged force
Its strength is about 10-6 times that of
the strong force
Scientists now believe the weak and
electromagnetic forces are two
manifestations of a single force, the
electroweak force
45. Gravitational Force
A familiar force that holds the planets,
stars and galaxies together
Its effect on elementary particles is
negligible
A long-range force
It is about 10-43 times the strength of
the strong force
Weakest of the four fundamental forces
46. Explanation of Forces
Forces between particles are often
described in terms of the actions of
field particles or quanta
For electromagnetic force, the photon
is the field particle
The electromagnetic force is
mediated, or carried, by photons
47. Forces and Mediating
Particles (also see table 30.1)
Interaction (force)
Mediating Field
Particle
Strong Gluon
Electromagnetic Photon
Weak W and Z0
Gravitational Gravitons
48. Paul Adrien Maurice Dirac
1902 – 1984
Instrumental in
understanding
antimatter
Aided in the
unification of
quantum mechanics
and relativity
Contributions to
quantum physics and
cosmology
Nobel Prize in 1933
49. Antiparticles
For every particle, there is an antiparticle
From Dirac’s version of quantum mechanics that
incorporated special relativity
An antiparticle has the same mass as the
particle, but the opposite charge
The positron (electron’s antiparticle) was
discovered by Anderson in 1932
Since then, it has been observed in numerous
experiments
Practically every known elementary particle
has a distinct antiparticle
Exceptions – the photon and the neutral pi particles
are their own antiparticles
50. Hideki Yukawa
1907 – 1981
Predicted the
existence of
mesons
Nobel Prize in
1949
51. Mesons
Developed from a theory to explain the
strong nuclear force
Background notes
Two atoms can form a covalent bond by the
exchange of electrons
In electromagnetic interactions, charged
particles interact by exchanging a photon
A new particle was proposed to explain
the strong nuclear force
It was called a meson
52. Mesons, cont
The proposed particle would have a
mass about 200 times that of the
electron
Efforts to establish the existence of the
particle were done by studying cosmic
rays in the 1930’s
Actually discovered multiple particles
Pi meson (called pion)
Muon
Plays no role in the strong interaction
53. Pion
There are three varieties of pions
+ and -
Mass of 139.6 MeV/c2
0
Mass of 135.0 MeV/c2
Pions are very unstable
- decays into a muon and an
antineutrino with a lifetime of about
2.6 x10-8 s
54. Richard Feynmann
1918 – 1988
Contributions include
Work on the Manhattan
Project
Invention of diagrams to
represent particle
interactions
Theory of weak interactions
Reformation of quantum
mechanics
Superfluid helium
Challenger investigation
Shared Nobel Prize in
1965
55. Feynman Diagrams
A graphical representation of the
interaction between two particles
Feynman diagrams are named for
Richard Feynman who developed
them
56. Feynman Diagram – Two
Electrons
The photon is the field
particle that mediates
the interaction
The photon transfers
energy and momentum
from one electron to the
other
The photon is called a
virtual photon
It can never be detected
directly because it is
absorbed by the second
electron very shortly after
being emitted by the first
electron
57. The Virtual Photon
The existence of the virtual photon
would be expected to violate the
law of conservation of energy
But, due to the uncertainty principle
and its very short lifetime, the
photon’s excess energy is less than
the uncertainty in its energy
The virtual photon can exist for short
time intervals, such that E t
58. Feynman Diagram –
Proton and Neutron
The exchange is via the
nuclear force
The existence of the pion
is allowed in spite of
conservation of energy if
this energy is
surrendered in a short
enough time
Analysis predicts the rest
energy of the pion to be
130 MeV / c2
This is in close
agreement with
experimental results
59. Classification of Particles
Two broad categories
Classified by interactions
Hadrons – interact through strong
force
Leptons – interact through weak force
60. Hadrons
Interact through the strong force
Two subclasses
Mesons
Decay finally into electrons, positrons, neutrinos
and photons
Integer spins
Baryons
Masses equal to or greater than a proton
Noninteger spin values
Decay into end products that include a proton
(except for the proton)
Composed of quarks
61. Leptons
Interact through weak force
All have spin of 1/2
Leptons appear truly elementary
No substructure
Point-like particles
Scientists currently believe only six
leptons exist, along with their
antiparticles
Electron and electron neutrino
Muon and its neutrino
Tau and its neutrino
62. Conservation Laws
A number of conservation laws are
important in the study of
elementary particles
Two new ones are
Conservation of Baryon Number
Conservation of Lepton Number
63. Conservation of Baryon
Number
Whenever a baryon is created in a
reaction or a decay, an antibaryon is
also created
B is the Baryon Number
B = +1 for baryons
B = -1 for antibaryons
B = 0 for all other particles
The sum of the baryon numbers before
a reaction or a decay must equal the
sum of baryon numbers after the
process
64. Conservation of Lepton
Number
There are three conservation laws,
one for each variety of lepton
Law of Conservation of Electron-
Lepton Number states that the
sum of electron-lepton numbers
before a reaction or a decay must
equal the sum of the electron-
lepton number after the process
65. Conservation of Lepton
Number, cont
Assigning electron-lepton numbers
Le = 1 for the electron and the electron neutrino
Le = -1 for the positron and the electron
antineutrino
Le = 0 for all other particles
Similarly, when a process involves muons,
muon-lepton number must be conserved
and when a process involves tau particles,
tau-lepton numbers must be conserved
Muon- and tau-lepton numbers are assigned
similarly to electron-lepton numbers
66. Strange Particles
Some particles discovered in the 1950’s
were found to exhibit unusual properties in
their production and decay and were given
the name strange particles
Peculiar features include
Always produced in pairs
Although produced by the strong interaction,
they do not decay into particles that interact via
the strong interaction, but instead into particles
that interact via weak interactions
They decay much more slowly than particles
decaying via strong interactions
67. Strangeness
To explain these unusual properties, a new
law, conservation of strangeness, was
introduced
Also needed a new quantum number, S
The Law of Conservation of Strangeness states
that the sum of strangeness numbers before a
reaction or a decay must equal the sum of the
strangeness numbers after the process
Strong and electromagnetic interactions
obey the law of conservation of
strangeness, but the weak interactions do
not
68. Bubble Chamber
Example
The dashed lines
represent neutral
particles
At the bottom,
- + p 0 + K0
Then 0 - + p
and
K0 + µ- + µ
69. Murray Gell-Mann
1929 –
Worked on
theoretical studies
of subatomic
particles
Nobel Prize in
1969
70. The Eightfold Way
Many classification schemes have been
proposed to group particles into families
These schemes are based on spin, baryon
number, strangeness, etc.
The eightfold way is a symmetric pattern
proposed by Gell-Mann and Ne’eman
There are many symmetrical patterns that
can be developed
The patterns of the eightfold way have
much in common with the periodic table
Including predicting missing particles
71. An Eightfold Way for
Baryons
A hexagonal pattern
for the eight spin 1/2
baryons
Strangeness vs.
charge is plotted on
a sloping coordinate
system
Six of the baryons
form a hexagon with
the other two
particles at its center
72. An Eightfold Way for
Mesons
The mesons with spins of 0
can be plotted
Strangeness vs. charge on
a sloping coordinate
system is plotted
A hexagonal pattern
emerges
The particles and their
antiparticles are on
opposite sides on the
perimeter of the hexagon
The remaining three
mesons are at the center
73. Quarks
Hadrons are complex particles with size
and structure
Hadrons decay into other hadrons
There are many different hadrons
Quarks are proposed as the elementary
particles that constitute the hadrons
Originally proposed independently by Gell-
Mann and Zweig
74. Original Quark Model
Three types
u – up
d – down
s – originally sideways, now strange
Associated with each quark is an
antiquark
The antiquark has opposite charge,
baryon number and strangeness
75. Original Quark Model, cont
Quarks have fractional electrical
charges
+1/3 e and –2/3 e
All ordinary matter consists of just
u and d quarks
76. Original Quark Model –
Rules
All the hadrons at the time of the
original proposal were explained
by three rules
Mesons consist of one quark and one
antiquark
This gives them a baryon number of 0
Baryons consist of three quarks
Antibaryons consist of three
antiquarks
77. Additions to the Original
Quark Model – Charm
Another quark was needed to account
for some discrepancies between
predictions of the model and
experimental results
Charm would be conserved in strong
and electromagnetic interactions, but
not in weak interactions
In 1974, a new meson, the J/ was
discovered that was shown to be a
charm quark and charm antiquark pair
78. More Additions – Top and
Bottom
Discovery led to the need for a more
elaborate quark model
This need led to the proposal of two
new quarks
t – top (or truth)
b – bottom (or beauty)
Added quantum numbers of topness
and bottomness
Verification
b quark was found in a Y meson in 1977
t quark was found in 1995 at Fermilab
79. Numbers of Particles
At the present, physicists believe
the “building blocks” of matter are
complete
Six quarks with their antiparticles
Six leptons with their antiparticles
See table 30.5
80. Color
Isolated quarks
Physicist now believe that quarks are
permanently confined inside ordinary
particles
No isolated quarks have been observed
experimentally
The explanation is a force called the color
force
Color force increases with increasing distance
This prevents the quarks from becoming isolated
particles
81. Colored Quarks
Color “charge” occurs in red, blue,
or green
Antiquarks have colors of antired,
antiblue, or antigreen
Color obeys the Exclusion Principle
A combination of quarks of each
color produces white (or colorless)
Baryons and mesons are always
colorless
82. Quark Structure of a
Meson
A green quark is
attracted to an
antigreen quark
The quark –
antiquark pair
forms a meson
The resulting
meson is colorless
83. Quark Structure of a
Baryon
Quarks of
different colors
attract each other
The quark triplet
forms a baryon
The baryon is
colorless
84. Quantum
Chromodynamics (QCD)
QCD gave a new theory of how quarks
interact with each other by means of
color charge
The strong force between quarks is
often called the color force
The strong force between quarks is
carried by gluons
Gluons are massless particles
There are 8 gluons, all with color charge
When a quark emits or absorbs a gluon,
its color changes
85. More About Color Charge
Like colors repel and opposite colors
attract
Different colors also attract, but not as strongly
as a color and its anticolor
The color force between color-neutral
hadrons is negligible at large separations
The strong color force between the constituent
quarks does not exactly cancel at small
separations
This residual strong force is the nuclear force
that binds the protons and neutrons to form
nuclei
86. QCD Explanation of a
Neutron-Proton Interaction
Each quark within the
proton and neutron is
continually emitting and
absorbing virtual gluons
Also creating and
annihilating virtual
quark-antiquark pairs
When close enough,
these virtual gluons and
quarks can be
exchanged, producing
the strong force
87. Weak Interaction
The weak interaction is an extremely
short-ranged force
This short range implies the mediating
particles are very massive
The weak interaction is responsible for
the decay of c, s, b, and t quarks into u
and d quarks
Also responsible for the decay of and
leptons into electrons
88. Weak Interaction, cont
The weak interaction is very important
because it governs the stability of the
basic particles of matter
The weak interaction is not symmetrical
Not symmetrical under mirror reflection
Not symmetrical under charge exchange
89. Electroweak Theory
The electroweak theory unifies
electromagnetic and weak
interactions
The theory postulates that the
weak and electromagnetic
interactions have the strength at
very high particle energies
Viewed as two different
manifestations of a single interaction
90. The Standard Model
A combination of the electroweak
theory and QCD form the standard
model
Essential ingredients of the standard
model
The strong force, mediated by gluons, holds the
quarks together to form composite particles
Leptons participate only in electromagnetic and
weak interactions
The electromagnetic force is mediated by
photons
The weak force is mediated by W and Z bosons
92. Mediator Masses
Why does the photon have no mass
while the W and Z bosons do have
mass?
Not answered by the Standard Model
The difference in behavior between low
and high energies is called symmetry
breaking
The Higgs boson has been proposed to
account for the masses
Large colliders are necessary to achieve the
energy needed to find the Higgs boson
93. Grand Unification Theory
(GUT)
Builds on the success of the
electroweak theory
Attempted to combine electroweak
and strong interactions
One version considers leptons and
quarks as members of the same
family
They are able to change into each other
by exchanging an appropriate particle
94. The Big Bang
This theory of cosmology states that
during the first few minutes after the
creation of the universe all four
interactions were unified
All matter was contained in a quark soup
As time increased and temperature
decreased, the forces broke apart
Starting as a radiation dominated
universe, as the universe cooled it
changed to a matter dominated
universe
96. George Gamow
1904 – 1968
Among the first to
look at the first half
hour of the universe
Predicted:
Abundances of
hydrogen and helium
Radiation should still
be present and have
an apparent
temperature of about
5 K
97. Cosmic Background
Radiation (CBR)
CBR represents the
cosmic “glow” left over
from the Big Bang
The radiation had
equal strengths in all
directions
The curve fits a
blackbody at ~3K
There are small
irregularities that
allowed for the
formation of galaxies
and other objects
98. Connection Between Particle
Physics and Cosmology
Observations of events that occur
when two particles collide in an
accelerator are essential to
understanding the early moments
of cosmic history
There are many common goals
between the two fields
99. Some Questions
Why so little antimatter in the Universe?
Do neutrinos have mass?
How do they contribute to the dark mass in the
universe?
Explanation of why the expansion of the
universe is accelerating?
Is there a kind of antigravity force acting
between widely separated galaxies?
Is it possible to unify electroweak and
strong forces?
Why do quark and leptons form similar but
distinct families?
100. More Questions
Are muons the same as electrons, except
for their mass?
Why are some particles charged and others
neutral?
Why do quarks carry fractional charge?
What determines the masses of
fundamental particles?
Do leptons and quarks have a
substructure?
