Nuclear Science Minor Program

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Nuclear Science Minor Program

• 14 upper division units from the following
• CHEM 482 Directed Study in Advanced Topics of
  Chemistry
• NUSC 341 Introduction to Radiochemistry
• NUSC 342 Introduction to Nuclear Science
• NUSC 344 Nucleosynthesis and Distribution of the
  Elements
• NUSC 346 Radiochemistry Laboratory
• NUSC 444 Special Topics in Nuclear Science
• NUSC 485 Particle Physics
• PHYS 385 Quantum Physics

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What weve discussed last time

• History of radioactivity
• Interactions and Force Carriers
• Standard Model and Subatomic Particles
• Structure of Matter
• Nucleus
• Chart of Nuclides

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Forces in Matter and the Subatomic Particles

• Chapter 1

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Natural Decay Chains
http//hyperphysics.phy-astr.gsu.edu/
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(4n 0)
6 a particles 4 ß- particles
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(4n 2)
8 a particles 6 ß particles
http//hyperphysics.phy-astr.gsu.edu/
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(4n 3)
7 a particles 4 ß particles
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lt 4.7 109 y
The members of this series are not presently
found in nature because the half-life of the
longest lived isotope in the series is short
compared to the age of the earth.
7 a particles 4 ß- particles
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(No Transcript)
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Types of Radioactive Decay

• Chapter 2

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Radioactive Decay

• Statistical process
• Spontaneous emission of particle or
  electromagnetic radiation from the atom
• Unaffected by temperature, pressure, physical
  state, etc
• Exoergic process
• Conserves total energy, linear and angular
  momentum, charge, mass number, lepton number,
  etc.

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Units of Energy

• Mass and energy are interchangeable
• E mc2
• where energy usually expressed in MeV
• 1 eV 1.602 x 10-19 J 1.60219 x 10-12 erg
• 1 MeV 1.602 x 10-13 J 1.60219 x 10-6 erg
• 1 u 931.5 MeV/c2

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Decay Modes

• Alpha decay
• Beta decay
• Gamma decay
• Spontaneous fission
• Delayed neutron and proton emission
• Two-proton decay
• Composite particle emission
• Double beta decay
• Prompt proton decay (new)

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Molecular Rotations and Vibrations (Bjerrum 1912)
moments of inertia bond and force length
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Vibrations
http//wwwnsg.nuclear.lu.se
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Rotation
oblate rotor
prolate rotor
http//wwwnsg.nuclear.lu.se
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Reflection Asymmetric Shape
octupole
http//wwwnsg.nuclear.lu.se
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Gamma-Ray Radiation and Nuclei
?
?
?
Excitation energy, keV
Angular momentum, h
?-ray energy, keV
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Alpha Decay

• 210Po ? 4He 206Pb ?
• t1/2 (210Po) 138.4 d Ea 5.304 MeV
• Typically for Agt150 Z gt 83 (144Nd, 147Sm)
• Geiger-Nuttall rule

216Rn 8.05 MeV, 45µs 144Nd 1.83 MeV, 2.1 x 1015
y
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Conservation of Energy for Alpha Decay

• Etrans Ea Erecoil
• E ½ mv2
• 2mE m2v2 (mv)2
• p mv p2 m2v2 (mv)2 2mE
• pa precoil
• 2m aE a 2mrecoilErecoil
• Erecoil (m a/mrecoil)E a

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Alpha Spectrum
Parent
a1(20)
a2(40)
?1
a3(40)
?3
?2
5.0
5.5
6.0
6.5
7.0
7.5
Daughter
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What we have learned last time

• Natural decay chains
• Excited molecules and nuclei
• Alpha decay

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Alpha Decay
238U ? 234Th 4He2 238U ? 234Th 4He
Parent
a1(20)
a2(40)
Counts
?1
a3(40)
?3
?2
Daughter
Ea (MeV)
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Beta Decay
Unlike alpha decay, which occurs primarily among
nuclei in specific areas The periodic table,
beta decay is possible for certain isotopes of
all elements
b-
change a neutron to a proton
(negatron decay)
b- is an electron
b
change a proton to a neutron
ß is an anti-electron or positron
EC
electron capture, change a proton to a neutron
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Negatron (ß-) Decay
Neutron rich nuclei Large N/Z ratio t1/2
ß1
ß2
?
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Beta decay Energy spectrum

• Emax
• Antineutrino in ß-
• No charge
• No magnetic moment
• Near zero rest mass
• Spin ½
• Conservation of lepton number

Etrans Enegatron Eantineutrino Erecoil
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Antineutrino discovery
1953 by F. Reines and C.L. Cowan Jr.
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Positron (ß) Decay
change a proton to a neutron
b
ß is an anti-electron, or positron

• Proton rich nuclei
• Similar spectrum as in negatron decay
• Change a proton to a neutron ? positive
  electron is emitted by the nucleus
• and an orbital electron originally present in the
  parent atom is lost
• to form a neutral daughter atom.
• equivalent to the creation of a
  positron-electron pair from the available
  transition energy
• 2 x 0.511 MeV 1.02 MeV necessary to create 2
  electrons
• ß decay is possible only when the energy of the
  transition is greater than 1.02 MeV

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The fate of the positron

• Conversion to pure energy by positron
  annihilation
• After the positron slows down to energies
  comparable to that of surroundings
• Formation of 1, 3, or 0 annihilation photons,
  depending on the spin orientation of the
  electron-positron pair
• If the spins are parallel ? triple state
• If the spins are anti-parallel ? a single state
• Positronium atom ? light isotope of
  hydrogen, with the positron substituting for the
  nuclear proton

• Ortho positronium paralell spins
• 10-7 s
• Para positronium anti-parallel spins
• 10-10 s

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Electron Capture (EC ore)
EC
electron capture, change a proton to a neutron
x-rays or Auger electrons inner
bremsstrahlung
excited nucleus
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Gamma Decay

• Pure ? decay
• Internal conversion (IC)
• Pair production (PP)

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Pure Gamma-Ray Emission
234mPa
?
99.8 ß1, t1/2 1.17 m
234gPa
0.2 ß2, t1/2 6.70 h
234U
92U
91Pa
2 keV lt E lt 7 MeV monoenergetic
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Internal Conversion

• The excited nucleus transfers the energy to an
  orbital electron, which is then ejected from the
  atom (monoenergetic).
• EIC electron Etrans BEatomic electron
• IC and gamma decay are competing processes
• Internal conversion coefficient (a)
• a Fraction of decays occurring by gamma
  emission/Fraction of decays occurring by IC

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Pair Production

• E gt 1.02 MeV
• 16mO ? 16O
• Etrans 6.05 MeV
• t1/2 7 x 10-11 s

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Spontaneous Fission Decay
Induced Fission Reaction
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Oklo, Gabon A natural fission reactor

• 235U natural abundance is well known 0.00720
  0.00001
• Uranium deposit where self-sustained nuclear
  chain reactions have occurred.
• 235U abundance 0.00717, about 3 standard
  deviations below the accepted value.
• The only process which can lead to reduction of U
  is fission by low-energy neutrons.
• 2 x 109 y, 235U (3) reactor moderated by
  groundwater.
• Fission product isotope signatures Nd, Ru

Geological Situation in Gabon leading to natural
nuclear fission reactors 1. Nuclear reactor
zones2. Sandstone3. Ore layer4. Granite
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Fossil Reactor 15, located in Oklo, Gabon.
Uranium oxide remains are visible as the
yellowish rock.
Source NASA
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Oklo

• Estimations
• 5 tonnes of 235U were fissioned.
• Total energy released 2 x 1030 MeV or 108 MWh. A
  contemporary power reactor can operate at 103 MW.
• Average power 0.01 MW, operating for 106 y.
• Important feature
• the fission products are still in place in the
  reactor zone and have migrated very little.
  Despite climate changes, no substantial movement
  of the fission products has taken place over the
  past 2 x 109 y.

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Confirmation Nd signature

• natural neodymium contains 27 142Nd
• the Nd at Oklo contained less than 6 but
  contained more 143Nd
• the isotopic composition matched that produced
  by the fissioning of 235U.

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Delayed-Neutron Emission

• Following beta decay of fission products such as
  140Ba and 94Kr
• 87Br ? 87Kr ? 86Kr n ß-
• neutron rich

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Delayed-Proton Emission

• Production of precursor 54Fe(p,2n)53Co
• Decay by proton emission 53Co ? 52Fe p

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Double-Beta Decay

• 130Te, 82Se stable to ordinary beta decay, but
  unstable toward 2-beta decay
• Simultaneous 2 beta emission

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Two-Proton Decay

• 22Al (1960), 54Zn (2005)
• 45Fe (2003, 2007)
• 48Ni

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End of Chapter 2

• Questions?