Nuclear Physics

1
Nuclear Physics

• Chapter 29

2
Chapter 29 Objectives

• Students will understand the significance of the
  mass number and charge of nuclei
• Students will understand the nature of the strong
  nuclear force
• Students will understand nuclear fission

3
Chapter 29 Objectives

• Students will understand the significance of
  half-life radioactive decay
• Students will understand the relationship between
  mass and energy

4
Modern Physics

• Virtual Concept Map

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Properties of Nuclei

• Atomic (Z)
• Neutron (N)
• Mass ( A )
• AZX

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Properties of Nuclei

• Nuclei of all atoms of a particular element must
  contain the same number protons.
• Nucleons
• Isotopes

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Properties of Nuclei

• Charge and Mass
• Electron me 9.11 X 10-31 kg
• Proton mp 1.6726 X 10-27 kg
• Neutron mn 1.6750 X 10-27 kg
• Unified Mass Unit
• 1 u 1.660540 X 10-27 kg

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Properties of Nuclei

• Size of Nuclei
• Average radius r0A1/3
• r0 1.2 X 10-15 m
• All nuclei have nearly the same density

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Properties of Nuclei

• Nuclear Stability
• Nuclear Force ( Strong Force )
• Coulomb Force ( Electro-magnetic )
• When Z 83 the repulsive forces between protons
  cannot be compensated by the addition of more
  neutrons

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Binding Energy

• The total energy of the nucleus is less than the
  combined energy of the separated nucleons.
• Binding Energy
• Eb (mp mn ) melement) c2
• The total energy required to break up a nucleus
  into its constituent protons and neutrons can be
  calculated from E Dmc2, called nuclear binding
  energy

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Binding Energy
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Radioactivity

• Radioactivity

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Radioactivity

• Decay Constant and Half-Life
• Decay rate lN
• N N0e-lT
• l ln 2
• T1/2

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Radioactive Decay
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Radioactive Decay

• Methods of Radioactive Decay
• Periodic Table of Elements

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Decay Processes

• Decay Rules
• 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

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Decay Processes

• Alpha Decay
• Parent nucleus
• Daughter nucleus
• Spontaneous decay

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Decay Processes

• Beta Decay
• The daughter nucleus has the same of nucleons
  as the parent nucleus, but the atomic is
  changed by 1
• A neutron is transformed into an electron,
  proton, and neutrino.

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Positron Decay

• Something inside the nucleus of an atom breaks
  down, which causes a proton to become a neutron.
• It emits a positron and a neutrino which go
  zooming off into space.
• The atomic number goes DOWN by one and mass
  number remains unchanged.

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Electron Capture

• An electron from the closest energy level falls
  into the nucleus, which causes a proton to become
  a neutron.
• A neutrino is emitted from the nucleus.
• Another electron falls into the empty energy
  level and so on causing a cascade of electrons
  falling. One free electron, moving about in
  space, falls into the outermost empty level.
  (Incidently, this cascade of electrons falling
  creates a characteristic cascade of lines, mostly
  (I think) in the X-ray portion of the spectrum.
  This is the fingerprint of electron capture.)
• The atomic number goes DOWN by one and mass
  number remains unchanged.

1)
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Electron Capture
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Decay Processes

• Gamma Decay
• Nucleus undergoes decay to achieve a lower energy
  state

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Natural Radioactivity

• Radioactive Nuclei
• Unstable nuclei found in nature (Natural)
• Nuclei produced in the laboratory (Artificial)
• Decay series

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Works Cited

• http//dbhs.wvusd.k12.ca.us/webdocs/Radioactivity/
  Writing-Alpha-Beta.html
• http//library.thinkquest.org/17940/texts/radioact
  ivity/radioactivity.html
• http//pearl1.lanl.gov/periodic/default.htm
• http//hyperphysics.phy-astr.gsu.edu/hbase/nuclear
  /halfli2.html

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Chapter 30

• Nuclear Energy
• and
• Elementary Particles

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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

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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

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Fission Equation

• Fission of 235U by a slow (low energy) neutron
• 236U is an intermediate, short-lived state
• X and Y are called fission fragments
• Many combinations of X and Y satisfy the
  requirements of conservation of energy and charge

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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

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Sequence of Events in Fission Diagram
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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

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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

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In the first atomic bomb, the energy released was
equivalent to about 30 kilotons of TNT, where a
ton of TNT releases an energy of 4.0 109 J. The
amount of mass converted into energy in this
event is nearest to (a) 1 ?g, (b) 1 mg, (c)
1 g, (d) 1 kg, (e) 20 kilotons
QUICK QUIZ 30.1
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(c). The total energy released was E (30 103
ton)(4.0 109 J/ton) 1.2 1014 J. The mass
equivalent of this quantity of energy is
QUICK QUIZ 30.1 ANSWER
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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 g
  of U can release energy equal to about 20000 tons
  of TNT

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Chain Reaction Diagram