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Lecture 3

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Author: Grant Mathews

The discovery of the neutron

  1.  By 1932, the nucleus was thought to consist of protons and electrons which were emitted in β-decay. Chadwick’s new experiment revealed a third particle, the neutron.

  2. His experiment: a strong polonium source emitted α particles which bombarded Be; radiation was emitted which – based on energy and momentum transfer arguments, could only be neutral particles with similar mass as protons: neutrons.

Properties of the Nucleus

  1. Nucleus with Z protons (p) and N neutrons (n) has a total mass number: A=Z+N   
  2. According to Einstein’s formula each nucleus with certain mass (m) stores energy:  E=mc2
    3 - bindingenergy.jpg
  3. Nuclear Binding Energy - energy per mass number (B/A)
    1. Nuclear Reactions and Energy Release:  Frederic Joliot and Irene Curie at Paris had observed the first nuclear reaction. Enrico Fermi showed the existence of neutron induced reactions which produce artificial radioactivity.
    2. Nuclear reactions can produce energy (Q):   (Q > 0  exothermic) or need energy (Q): (Q < 0 endothermic)

The discovery of fission, 1938

  1. Hahn and Strassmann identified a broad variety of elements they thought to be near Uranium, (e.g. Z=88 Radium).
  2. Hahn and Strassmann repeated the experiment numerous times and were never able to isolate the ‘radium’ from barium. They reported their results as follows: "As chemists, we must actually say the new particles do not behave like radium but, in fact, like barium; as nuclear physicists, we cannot make this conclusion, which is in conflict with all experience in nuclear physics." Hahn, the chemist, was reluctant to go against the ideas of nuclear physicists, despite clear chemical evidence of barium.
  3. Close correspondence with collaborator Lise Meitner who had emigrated to Stockholm in 1938  to Manne Siegbahn. Now whenever mass disappears, energy is created, according to Einstein's formula E = mc2, and... the mass was just equivalent to 200 MeV; it all fitted!  Meitner was convinced that the product actually was Barium rather than a homologue.  The nightmare of contradictory evidence all fit the explanation that Uranium had fissioned (broke into pieces) rather than form a heavier element.  Frisch calculated the energy needed, and Meitner calculated the energy available.  The other fragment must be Krypton to conserve atomic number.

Niels Bohr

  1. Brought news of fission to the US at the fifth Washington Conference on Theoretical Physics. 
  2. Within a few month Bohr and Wheeler predicted the possibility of a chain reaction by fission of U-235 with similar energy output as U-238.  The problem was to generate U-235, which is a very rare Uranium isotope.  Because of this, Germany quickly stopped all Uranium exports.

Fission fragments

  1. When U-235 undergoes fission, the average of the fragment mass is about 118 amu, but very few fragments near that average are found. It is much more probable to break up into unequal fragments, and the most probable fragment masses are around mass 95 and 137 amu. Most of the fission fragments are radioactive, intense short lived and long-lived radioactive elements are released into the environment.

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