Atomic Physics

1
Chapter 28

• Atomic Physics

2
Importance of Hydrogen Atom

• Hydrogen is the simplest atom
• The quantum numbers used to characterize the
  allowed states of hydrogen can also be used to
  describe (approximately) the allowed states of
  more complex atoms
• This enables us to understand the periodic table

3
More Reasons the Hydrogen Atom is so Important

• The hydrogen atom is an ideal system for
  performing precise comparisons of theory with
  experiment
• Also for improving our understanding of atomic
  structure
• Much of what we know about the hydrogen atom can
  be extended to other single-electron ions
• For example, He and Li2

4
Sir Joseph John Thomson

• J. J. Thomson
• 1856 - 1940
• Discovered the electron
• Did extensive work with cathode ray deflections
• 1906 Nobel Prize for discovery of electron

5
Early Models of the Atom

• J.J. Thomsons model of the atom
• A volume of positive charge
• Electrons embedded throughout the volume
• A change from Newtons model of the atom as a
  tiny, hard, indestructible sphere

6
Early Models of the Atom, 2

• Rutherford, 1911
• Planetary model
• Based on results of thin foil experiments
• Positive charge is concentrated in the center of
  the atom, called the nucleus
• Electrons orbit the nucleus like planets orbit
  the sun

7
Scattering Experiments

• 1911 Rutherford, Geiger and Marsden performed
  scattering experiments
• Established the point mass nature of the nucleus
• Nuclear force was a new type of force

8
Scattering Experiments

• The source was a naturally radioactive material
  that produced alpha particles
• Most of the alpha particles passed though the
  foil
• A few deflected from their original paths
• Some even reversed their direction of travel

9
Difficulties with the Rutherford Model

• Atoms emit certain discrete characteristic
  frequencies of electromagnetic radiation
• The Rutherford model is unable to explain this
  phenomena
• Rutherfords electrons are undergoing a
  centripetal acceleration and so should radiate
  electromagnetic waves of the same frequency
• The radius should steadily decrease as this
  radiation is given off
• The electron should eventually spiral into the
  nucleus, but it doesnt

10
Emission Spectra

• A gas at low pressure has a voltage applied to it
• A gas emits light characteristic of the gas
• When the emitted light is analyzed with a
  spectrometer, a series of discrete bright lines
  is observed
• Each line has a different wavelength and color
• This series of lines is called an emission
  spectrum

11
Examples of Emission Spectra
12
Emission Spectrum of Hydrogen Equation

• The wavelengths of hydrogens spectral lines can
  be found from
• RH is the Rydberg constant
• RH 1.097 373 2 x 107 m-1
• n is an integer, n 1, 2, 3,
• The spectral lines correspond to different values
  of n

13
Spectral Lines of Hydrogen

• The Balmer Series has lines whose wavelengths are
  given by the preceding equation
• Examples of spectral lines
• n 3, ? 656.3 nm
• n 4, ? 486.1 nm

14
Absorption Spectra

• An element can also absorb light at specific
  wavelengths
• An absorption spectrum can be obtained by passing
  a continuous radiation spectrum through a vapor
  of the gas
• The absorption spectrum consists of a series of
  dark lines superimposed on the otherwise
  continuous spectrum
• The dark lines of the absorption spectrum
  coincide with the bright lines of the emission
  spectrum

15
Applications of Absorption Spectrum

• The continuous spectrum emitted by the Sun passes
  through the cooler gases of the Suns atmosphere
• The various absorption lines can be used to
  identify elements in the solar atmosphere
• Led to the discovery of helium

16
Absorption Spectrum of Hydrogen
Emission Spectra
Absorption Spectra
17
Chapter 29

• Nuclear Physics

18
Milestones in the Development of Nuclear Physics

• 1896 the birth of nuclear physics
• Becquerel discovered radioactivity in uranium
  compounds
• Rutherford showed the radiation had three types
• Alpha (He nucleus)
• Beta (electrons)
• Gamma (high-energy photons)

19
More Milestones

• 1911 Rutherford, Geiger and Marsden performed
  scattering experiments
• Established the point mass nature of the nucleus
• Nuclear force was a new type of force
• 1919 Rutherford and coworkers first observed
  nuclear reactions in which naturally occurring
  alpha particles bombarded nitrogen nuclei to
  produce oxygen

20
Milestones, final

• 1932 Cockcroft and Walton first used artificially
  accelerated protons to produce nuclear reactions
• 1932 Chadwick discovered the neutron
• 1933 the Curies discovered artificial
  radioactivity
• 1938 Hahn and Strassman discovered nuclear
  fission
• 1942 Fermi and collaborators achieved the first
  controlled nuclear fission reactor

21
Ernest Rutherford

• 1871 1937
• Discovery of atoms being broken apart
• Studied radioactivity
• Nobel prize in 1908

22
Some Properties of Nuclei

• All nuclei are composed of protons and neutrons
• Exception is ordinary hydrogen with just a proton
• The atomic number, Z, equals the number of
  protons in the nucleus
• The neutron number, N, is the number of neutrons
  in the nucleus
• The mass (nucleonIB) number, A, is the number of
  nucleons in the nucleus
• A Z N
• Nucleon is a generic term used to refer to either
  a proton or a neutron
• The mass number is not the same as the mass

23
Symbolism

• Symbol
• X is the chemical symbol of the element
• Example
• Mass number is 27
• Atomic number is 13
• Contains 13 protons
• Contains 14 (27 13) neutrons
• The Z may be omitted since the element can be
  used to determine Z

24
Symbolism (2)

• A nuclide is the name given to a particular
  species of atom(one whose nucleus contains a
  specified number of protons and a specified
  number of neutrons.
• Some nuclides are the same element they have
  the same chemical properties and have the same
  number of protons

25
More Properties

• The nuclei of all atoms of a particular element
  must contain the same number of protons
• They may contain varying numbers of neutrons
• Isotopes of an element have the same Z but
  differing N and A values
• Example

26
Charge

• The proton has a single positive charge, e
• The electron has a single negative charge, -e
• The neutron has no charge
• Makes it difficult to detect
• e 1.602 177 33 x 10-19 C

27
Mass

• It is convenient to use unified mass units, u, to
  express masses
• 1 u 1.660 559 x 10-27 kg
• Based on definition that the mass of one atom of
  C-12 is exactly 12 u
• Mass can also be expressed in MeV/c2
• From ER m c2 (Einsteins equation)
• 1 u 931.494 MeV/c2

28
Summary of Masses
29
The Size of the Nucleus

• First investigated by Rutherford in scattering
  experiments
• He found an expression for how close an alpha
  particle moving toward the nucleus can come
  before being turned around by the Coulomb force
• The KE of the particle must be completely
  converted to PE

30
Size of Nucleus, Current

• Since the time of Rutherford, many other
  experiments have concluded
• Most nuclei are approximately spherical

31
Density of Nuclei

• The volume of the nucleus (assumed to be
  spherical) is directly proportional to the total
  number of nucleons
• This suggests that all nuclei have nearly the
  same density
• Nucleons combine to form a nucleus as though they
  were tightly packed spheres

32
Maria Goeppert-Mayer

• 1906 1972
• Best known for her development of shell model of
  the nucleus
• Shared Nobel Prize in 1963

33
Nuclear Stability

• There are very large repulsive electrostatic
  forces between protons
• These forces should cause the nucleus to fly
  apart
• The nuclei are stable because of the presence of
  another, short-range force, called the strong
  nuclear force
• This is an attractive force that acts between all
  nuclear particles
• The nuclear attractive force is stronger than the
  Coulomb repulsive force at the short ranges
  within the nucleus

34
Nuclear Stability, cont

• Light nuclei are most stable if N Z
• Heavy nuclei are most stable when N gt Z
• As the number of protons increase, the Coulomb
  force increases and so more nucleons are needed
  to keep the nucleus stable
• No nuclei are stable when Z gt 83

35
Binding Energy

• The total energy of the bound system (the
  nucleus) is less than the combined energy of the
  separated nucleons
• This difference in energy is called the binding
  energy of the nucleus
• It can be thought of as the amount of energy you
  need to add to the nucleus to break it apart into
  separated protons and neutrons

36
Binding Energy per Nucleon
37
Binding Energy Notes

• Except for light nuclei, the binding energy is
  about 8 MeV per nucleon
• The curve peaks in the vicinity of A 60
• Nuclei with mass numbers greater than or less
  than 60 are not as strongly bound as those near
  the middle of the periodic table
• The curve is slowly varying at A gt 40
• This suggests that the nuclear force saturates
• A particular nucleon can interact with only a
  limited number of other nucleons

38
Mass Defect

• Because a bound system is at a lower energy level
  than its unbound constituents, its mass must be
  less than the total mass of its unbound
  constituents. For systems with low binding
  energies, this "lost" mass after binding may be
  fractionally small. For systems with high binding
  energies, however, the missing mass may be an
  easily measurable fraction.
• Since all forms of energy have mass, the question
  of where the missing mass of the binding energy
  goes is of interest. The answer is that this mass
  is lost from a system which is not closed. It
  transforms to heat, light, higher energy states
  of the nucleus/atom or other forms of energy, but
  these types of energy also have mass, and it is
  necessary that they be removed from the system
  before its mass may decrease. The "mass defect"
  from binding energy is therefore removed mass
  that corresponds with removed energy, according
  to Einstein's equation Emc2.
• Mass in kg, E in Joules

39
Marie Curie

• 1867 1934
• Discovered new radioactive elements
• Shared Nobel Prize in physics in 1903
• Nobel Prize in Chemistry in 1911

40
Radioactivity

• Radioactivity is the spontaneous emission of
  radiation
• Experiments suggested that radioactivity was the
  result of the decay, or disintegration, of
  unstable nuclei

41
Radioactivity Types

• Three types of radiation can be emitted
• Alpha particles
• The particles are 4He nuclei
• Beta particles
• The particles are either electrons or positrons
• A positron is the antiparticle of the electron
• It is similar to the electron except its charge
  is e
• Gamma rays
• The rays are high energy photons

42
Distinguishing Types of Radiation

• A radioactive beam is directed into a region with
  a magnetic field
• The gamma particles carry no charge and they are
  not deflected
• The alpha particles are deflected upward
• The beta particles are deflected downward
• A positron would be deflected upward

43
Penetrating Ability of Particles

• Alpha particles
• Barely penetrate a piece of paper
• Beta particles
• Can penetrate a few mm of aluminum
• Gamma rays
• Can penetrate several cm of lead

44
The Decay Constant

• The number of particles that decay in a given
  time is proportional to the total number of
  particles in a radioactive sample
• ? is called the decay constant and determines
  the rate at which the material will decay
• The decay rate or activity, R, of a sample is
  defined as the number of decays per second
• Radioactive decay is a random and spontaneous
  process and the rate of decay decreases
  exponentially with time

45
Decay Curve

• The decay curve follows the equation
• The half-life is also a useful parameter
• The half-life is defined as the time it takes for
  half of any given number of radioactive nuclei to
  decay

46
Units

• The unit of activity, R, is the Curie, Ci
• 1 Ci 3.7 x 1010 decays/second
• The SI unit of activity is the Becquerel, Bq
• 1 Bq 1 decay / second
• Therefore, 1 Ci 3.7 x 1010 Bq
• The most commonly used units of activity are the
  mCi and the µCi

47
Alpha Decay

• When a nucleus emits an alpha particle it loses
  two protons and two neutrons
• N decreases by 2
• Z decreases by 2
• A decreases by 4
• Symbolically
• X is called the parent nucleus
• Y is called the daughter nucleus

48
Alpha Decay Example

• Decay of 226 Ra
• Half life for this decay is 1600 years
• Excess mass is converted into kinetic energy
• Momentum of the two particles is equal and
  opposite

49
Decay General Rules

• When one element changes into another element,
  the process is called spontaneous decay or
  transmutation
• The sum of the mass numbers, A, must be the same
  on both sides of the equation
• The sum of the atomic numbers, Z, must be the
  same on both sides of the equation
• Conservation of mass-energy and conservation of
  momentum must hold

50
Beta Decay

• During beta decay, the daughter nucleus has the
  same number of nucleons as the parent, but the
  atomic number is changed by one
• Symbolically

51
Beta Decay, cont

• The emission of the electron is from the nucleus
• The nucleus contains protons and neutrons
• The process occurs when a neutron is transformed
  into a proton and an electron
• Energy must be conserved

52
Beta Decay Electron Energy

• The energy released in the decay process should
  almost all go to kinetic energy of the electron
  (KEmax)
• Experiments showed that few electrons had this
  amount of kinetic energy

53
Neutrino

• To account for this missing energy, in 1930
  Pauli proposed the existence of another particle
• Enrico Fermi later named this particle the
  neutrino
• Properties of the neutrino
• Zero electrical charge
• Mass much smaller than the electron, probably not
  zero
• Spin of ½
• Very weak interaction with matter

54
Beta Decay Completed

• Symbolically
• ? is the symbol for the neutrino
• is the symbol for the antineutrino
• To summarize, in beta decay, the following pairs
  of particles are emitted
• An electron and an antineutrino
• A positron and a neutrino

55
Gamma Decay

• Gamma rays are given off when an excited nucleus
  falls to a lower energy state
• Similar to the process of electron jumps to
  lower energy states and giving off photons
• The photons are called gamma rays, very high
  energy relative to light
• The excited nuclear states result from jumps
  made by a proton or neutron
• The excited nuclear states may be the result of
  violent collision or more likely of an alpha or
  beta emission

56
Gamma Decay Example

• Example of a decay sequence
• The first decay is a beta emission
• The second step is a gamma emission
• The C indicates the Carbon nucleus is in an
  excited state
• Gamma emission doesnt change either A or Z

57
Ionization Properties

• Every decay emits one of the 3 types of radiation
• All three are ionizing
• As they go through a substance, collisions occur
  which strip electrons form the atoms.
• Atoms that have lost or gained electrons are
  called ions.
• This ionization allows radiation to be detected
• Ionization can be dangerous in living tissue
  altering DNA causing mutations

58
Effects of ionization

• Direct action occurs when alpha particles, beta
  particles or x-rays create ions which physically
  break one or both of the sugar phosphate
  backbones or break the base pairs of the DNA.
• Two types of direct effects
• Base substitutions (ATGC)
• Frameshift mutations
• Insertions or deletions (addition or loss of one
  or more nucleotides)

59
Effects of ionization (2)

• Ionizing radiation can also impair or damage
  cells indirectly by creating free radicals. Free
  radicals are molecules that are highly reactive
  due to the presence of unpaired electrons (ions),
  which result when water molecules are split. Free
  radicals may form compounds, such as hydroxyl
  radical, hydrogen peroxide which could initiate
  harmful chemical reactions within the cells. As a
  result of these chemical changes, cells may
  undergo a variety of structural changes which
  lead to altered function or cell death.

60
Enrico Fermi

• 1901 1954
• Produced transuranic elements
• Other contributions
• Theory of beta decay
• Free-electron theory of metals
• Worlds first fission reactor (1942)
• Nobel Prize in 1938

61
Uses of Radioactivity

• Carbon Dating
• Beta decay of 14C is used to date organic samples
• The ratio of 14C to 12C is used
• Smoke detectors
• Ionization type smoke detectors use a radioactive
  source to ionize the air in a chamber
• A voltage and current are maintained
• When smoke enters the chamber, the current is
  decreased and the alarm sounds

62
More Uses of Radioactivity

• Radon pollution
• Radon is an inert, gaseous element associated
  with the decay of radium
• It is present in uranium mines and in certain
  types of rocks, bricks, etc that may be used in
  home building
• May also come from the ground itself

63
Natural Radioactivity

• Classification of nuclei
• Unstable nuclei found in nature
• Give rise to natural radioactivity
• Nuclei produced in the laboratory through nuclear
  reactions
• Exhibit artificial radioactivity
• Three series of natural radioactivity exist
• Uranium
• Actinium
• Thorium
• See table 29.2

64
Decay Series of 232Th

• Series starts with 232Th
• Processes through a series of alpha and beta
  decays
• Ends with a stable isotope of lead, 208Pb

65
Nuclear Reactions

• Structure of nuclei can be changed by bombarding
  them with energetic particles
• The changes are called nuclear reactions
• As with nuclear decays, the atomic numbers and
  mass numbers must balance on both sides of the
  equation

66
Nuclear Reactions Example (induced
transmutation)

• Alpha particle colliding with nitrogen
• Balancing the equation allows for the
  identification of X
• So the reaction is

67
Q Values

• Energy must also be conserved in nuclear
  reactions
• The energy required to balance a nuclear reaction
  is called the Q value of the reaction
• An exothermic reaction
• There is a mass loss in the reaction
• There is a release of energy
• Q is positive
• An endothermic reaction
• There is a gain of mass in the reaction
• Energy is needed, in the form of kinetic energy
  of the incoming particles
• Q is negative

68
Radiation Damage in Matter

• Radiation absorbed by matter can cause damage
• The degree and type of damage depend on many
  factors
• Type and energy of the radiation
• Properties of the absorbing matter
• Radiation damage in biological organisms is
  primarily due to ionization effects in cells
• Ionization disrupts the normal functioning of the
  cell

69
Types of Damage

• Somatic damage is radiation damage to any cells
  except reproductive ones
• Can lead to cancer at high radiation levels
• Can seriously alter the characteristics of
  specific organisms
• Genetic damage affects only reproductive cells
• Can lead to defective offspring

70
Radiation Levels

• Natural sources rocks and soil, cosmic rays
• Background radiation
• About 0.13 rem/yr
• Upper limit suggested by US government
• 0.50 rem/yr
• Excludes background and medical exposures
• Occupational
• 5 rem/yr for whole-body radiation
• Certain body parts can withstand higher levels
• Ingestion or inhalation is most dangerous

71
Applications of Radiation

• Sterilization
• Radiation has been used to sterilize medical
  equipment
• Used to destroy bacteria, worms and insects in
  food
• Bone, cartilage, and skin used in graphs is often
  irradiated before grafting to reduce the chances
  of infection

72
Applications of Radiation, cont

• Tracing
• Radioactive particles can be used to trace
  chemicals participating in various reactions
• Example, 131I to test thyroid action
• CAT scans
• Computed Axial Tomography
• Produces pictures with greater clarity and detail
  than traditional x-rays

73
Applications of Radiation, final

• MRI
• Magnetic Resonance Imaging
• When a nucleus having a magnetic moment is placed
  in an external magnetic field, its moment
  processes about the magnetic field with a
  frequency that is proportional to the field
• Transitions between energy states can be detected
  electronically

74
Chapter 30

• Nuclear Energy
• and
• Elementary Particles

75
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

76
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

77
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

78
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

79
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

80
Sequence of Events in Fission Diagram
81
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

82
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

83
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

84
Chain Reaction Diagram
85
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

86
K Values

• When K 1, the reactor is said to be critical
• The chain reaction is self-sustaining
• When K lt 1, the reactor is said to be subcritical
• The reaction dies out
• When K gt 1, the reactor is said to be
  supercritical
• A run-away chain reaction occurs

87
Basic Reactor Design

• Fuel elements consist of enriched uranium
• The moderator material helps to slow down the
  neutrons
• The control rods absorb neutrons

88
Reactor Design Considerations Power Level
Control

• A method of control is needed to adjust the value
  of K to near 1
• If K gt1, 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

89
Pressurized Water Reactor Diagram
90
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

91
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

92
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

93
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

94
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

95
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

96
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

97
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

98
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

99
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

100
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)

101
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

102
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

103
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

104
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