Nuclear Physics

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Nuclear Physics

• Paul J. Thomas
• Department of Physics and Astronomy
• UW - Eau Claire

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Early Atomic Ideas

• Greek atomos cannot be cut.
• Early atomistic theories of Democritus and
  Leucippus.
• Dalton elements combine in constant ratios of
  mass.
• Brownian motion

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The Structure of the Atom

• Static electricity implies that atoms contain
  separate charges.
• Plum pudding model of Thomson.
• Rutherfords experiment.

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Particles Inside Atoms

• Inside the nucleus
• Protons, charge 1, mass 1.
• Neutrons, charge 0, mass 1.
• Surrounding the nucleus
• Electrons, charge -1, mass 1/2000.

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Atomic and Mass Number

• The type of element is determined by the number
  of protons. This is the atomic number (Z).
• The number of protons neutrons is the mass
  number (A).
• For light elements, there are roughly as many
  protons and neutrons. Heavier elements have more
  neutrons than protons.

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Isotopes

• Atoms of the same element, with different numbers
  of neutrons in the nucleus.
• They are chemically identical, and thus cannot be
  separated by any chemical process.

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Isotopes

• Have the same chemical behavior.
• Can be very different in nuclear behavior (e.g.
  radioactivity).
• Example 12C is stable, but 14C is radioactive,
  with a half-life of 5730 y.
• Half-life time required for half of the initial
  sample to decay.

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Half-lives

• 26Al 1.6 106 y
• 3H 12.33 y
• 14C 5,730 y
• 90Sr 28.1 y
• 235U 7.04 108 y
• 239Pu 24,000 y
• After N half-lives, ½N of original remains.

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Marie and Pierre Curie

• Deduced that the radiation emitted by uranium and
  thorium was identical and must be a property of
  the interior of the atom.
• Discovered the radioactive elements Polonium and
  Radium.
• First known victims of radiation poisoning.
• Awarded the 1903 Nobel Prize in Physics Marie
  won the 1911 Nobel Prize in Chemistry.

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Radioactive Decay

• Alpha Decay
• 212Bi ? 208Tl 4He
• 238U ? 234Th 4He
• Beta Decay
• 14C ? 14N e- ?
• 12N ? 12C e ?
• Gamma Decay
• 4He ? 4He ?

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Alpha Decay
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Beta Decay
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? decay and the neutrino
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Neutrinos

• Probably most common fundamental particle.
• Neutral.
• Small mass (recently discovered) 0.14 millionth
  of electron mass.
• Extremely nonreactive with other particles.
• Three types electron neutrino ne, muon neutrino
  nm and tau neutrino nt.

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How are Neutrinos Made?

• Nuclear reactions, particularly at very high
  energies.
• During the Big Bang.
• In the centers of stars.
• In supernovas.
• When cosmic rays hit the Earths atmosphere.
• Radioactive beta decay.

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Neutrinos are nonreactive!

• 100 billion solar neutrinos pass through each
  square inch of our bodies every second, day or
  night!
• Big bang models predict there should be 50
  billion neutrinos per electron.
• Neutrino mass has to be very small, or the
  universe would have already collapsed.

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Super-Kamiokande

• In Kamioka, Gifu-ken, Honshu, Japan.
• 12.5 million gallons of pure water in a stainless
  steel lined cavity, with 13,000 photomultiplier
  tubes.
• Data taken during 1996-98 indicates neutrinos
  undergo oscillations.
• This means that they possess mass (0.14
  millionth of an electron mass).

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Super-Kamiokande
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Super-Kamiokande
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Neutrino Detection at S-K
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The Sun in Neutrino Light
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Neutrino Oscillations

• Super-Kamiokande observed muons and electrons
  from atmospheric neutrinos.
• Well-established nuclear physics theories predict
  2? as many muon neutrinos than electron
  neutrinos.
• In fact, the numbers are equal.
• Furthermore, there are fewer muon neutrinos
  coming from the other side of the Earth.

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Neutrino Oscillations

• Conclusion the muon neutrinos are changing to
  other types of neutrinos!
• If they do this, they cannot travel at the speed
  of light.
• If they do this, they have mass - and we can
  calculate how much.
• This may explain the solar neutrino puzzle!
• This may account for 10 or more of the dark
  matter!

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NuMI The Next Test (2003)

• Neutrino beam generated at Fermilab by proton
  beam collision.
• Beam is aimed at Soudan 2 neutrino detector,
  Minnesota (730 km away).
• Goal to observe neutrino oscillations.

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Forces inside the Atom

• Typical size of a nucleus 10-15 m. Strong
  nuclear force holds protons and neutrons
  together.
• Typical size of an atom 10-10 m. Electromagnetic
  force holds protons and electrons together.

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Particle Accelerator
Fermilab Accelerator (Proton Synchrotron),
Batavia, Illinois
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Particle Accelerator
Main accelerator ring, Fermilab
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Four Forces
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Hundreds of New Particles

• The muon, who ordered that?
  I.I. Rabi
• If I wanted to remember the names of all of
  these particles, I would have been a biologist.
  
  Leon Lederman
• As the number of particles increases, all that
  increases is our ignorance. Martinus
  Veltman

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Leptons and Hadrons

• Leptons
• e, ?, ?, ?e, ??, ??,
• no internal structure
• conserved
• do not interact via the strong force
• Hadrons
• p, n hundreds of particles
• internal structure (quarks)
• baryons are conserved, mesons are not
• interact via the strong force

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Quark Properties
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Cosmic Rays

• Energetic particles from space.
• When particles interact with our atmosphere, they
  release showers of high energy particles.

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Where do Cosmic Rays come from?

• Most (lower energy) cosmic rays come from the
  Sun.
• The amount of solar cosmic rays correlates with
  low numbers of sunspots.
• This is because the Suns magnetic field
  partially shields the Earth.

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Galactic Cosmic Rays

• Other cosmic rays come from outside the solar
  system. (And perhaps the Galaxy).
• Possible origins
• Supernovas
• Pulsars

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Cloud Chamber

• First developed in 1911 by Charles Wilson.
• Uses a supercritical vapor that is triggered to
  condense when a charged particle passes through.