Chapter 30
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Transcript of Chapter 30
Chapter 30
Nuclear Energy and
Elementary Particles
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
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
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
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
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
Sequence of Events in Fission – Diagram
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
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
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
Chain Reaction – Diagram
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
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
Basic Reactor Design Fuel elements
consist of enriched uranium
The moderator material helps to slow down the neutrons
The control rods absorb neutrons
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
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
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
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
Pressurized Water Reactor – Diagram
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
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
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
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
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
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
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
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
21H is deuterium, and
may be written as 21D
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
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
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)
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
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
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
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
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
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
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
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
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
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
Fundamental Forces All particles in nature are subject
to four fundamental forces Strong force Electromagnetic force Weak force Gravitational force
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
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
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
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
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
Forces and Mediating Particles (also see table 30.1)
Interaction (force)Mediating Field Particle
Strong Gluon
Electromagnetic Photon
Weak W and Z0
Gravitational Gravitons
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
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
Hideki Yukawa 1907 – 1981 Predicted the
existence of mesons
Nobel Prize in 1949
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
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
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
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
Feynman Diagrams A graphical representation of the
interaction between two particles Feynman diagrams are named for
Richard Feynman who developed them
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
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
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
Classification of Particles Two broad categories Classified by interactions
Hadrons – interact through strong force
Leptons – interact through weak force
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
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
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
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
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
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
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
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
Bubble ChamberExample The dashed lines
represent neutral particles
At the bottom, - + p 0 + K0
Then 0 - + p and
K0 + µ- + µ
Murray Gell-Mann 1929 – Worked on
theoretical studies of subatomic particles
Nobel Prize in 1969
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
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
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
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
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
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
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
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
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
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
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
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
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
Quark Structure of a Baryon
Quarks of different colors attract each other
The quark triplet forms a baryon
The baryon is colorless
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
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
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
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
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
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
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
The Standard Model – Chart
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
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
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
A Brief History of the Universe
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
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
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
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?
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?