Nuclear Physics

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Nuclear Physics Radioactivity

What holds a nucleus together? What drives
radioactive decay? What sets the timescale for
radioactive decay? What happens during
radioactive decay?
a particles in a cloud chamber
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The Four Forces of Nature
Force Strength Range Occurrence Strong
nuclear 1 ltlt1/r2 (finite, v. short) inter-nucleon
Electromagnetic 10-2 1/r2 (infinite, but
shielded nucleus, atom Weak nuclear 10-13 ltlt1/r2
(finite, v. short) B-decay, neutrinos Gr
avity 10-39 1/r2 (infinite) everywhere
Four Tenets of Nuclear Physics
1) mass-energy equivalence (Emc2) 2)
wave-particle duality (particles are waves, and
waves are particles) 3) conservation of energy,
mass, momentum 4) symmetry
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The Discovery of Radioactivity

• Henri Becquerel (1852-1908) by accident
• Pierre and Marie Curie continue investigation

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Radioisotopes

• Certain isotopes that have unstable nuclei
• Unstable nuclei lose energy by emitting radiation
  during radioactive decay
• Radioactive decay spontaneous, requires no
  energy input

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Four types of radioactive decay
1) alpha (a) decay - 4He nucleus (2p 2n)
ejected 2) beta (?) decay - change of nucleus
charge, conserves mass 3) gamma (g) decay -
photon emission, no change in A or Z 4)
spontaneous fission - for Z92 and above,
generates two smaller nuclei
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a decay
- involves strong and coloumbic forces - alpha
particle and daughter nucleus have equal and
opposite momentums (i.e. daughter experiences
recoil)
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? decay - three types
1) ?- decay
- converts one neutron into a proton and
electron - no change in mass number, but
different element - release of anti-neutrino (no
charge, no mass)
2) ? decay
- converts one proton into a neutron and
electron - no change in mass number, but
different element - release of neutrino
3) Electron capture
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g decay
- conversion of strong to coulombic E - no change
of A or Z (element) - release of photon - usually
occurs in conjunction with other decay
Spontaneous fission
- heavy nuclides split into two daughters and
neutrons - U most common (fission-track dating)
Fission tracks from 238U fission in old zircon
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Nuclear Stability

• Instability leads to decay
• Band of stability (p. 763)
• Too many nucleons alpha decay
• Too many protons beta decay (electron capture)
• Too many neutrons beta decay (emission of
  electron to make a proton)
• Too much energy gamma decay

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Which nuclei are radioactive?

• All elements with atomic numbers greater than 83
  (bismuth)
• These usuaslly undergo alpha decay
• Some lighter isotopes as well.