Nuclear Fission

Nuclear fission is a process, by which a heavy nuclide splits into two or more pieces. There are spontaneous fission and induced fission reactions.

Some nuclei of the nuclide 256Fm undergo spontaneous fission.

256Fm100 ® 140Xe54 + 112Pd46 + 4 n. Note that the fission products are nuclides with masses range from 80 to 160.

Typical and well-known neutron-induced fission reactions are:

235U (n, 3 n) fission products with mass number ranges 80-160
239Pu (n, 3 n) fission products with mass number ranges 80-160

Since the mass excess of 235 or 238U is higher than nuclides with mass in the range of 80-160, fission reactions release a lot of energy. The annimation of neutron-induced fission is shown here:

This annimation is provided by Energy Matters - Fission theory.

Discovery of Nuclear Fission

Three groups studied neutron-induced reaction of uranium
  • I. Curie and R.F. Joliot in Paris
  • Otto Hahn (1879-1968), Lise Meitner, and Fritz Strassmann in Berlin
  • Enrico Fermi (1901-1954) in Rome
  • All groups thought the reaction to be:

    238U (n, g) 239U92 (, b) 239E93 (, b) 239E94. Their assumption of neutron capture reaction hindered their interpretation of experimental results. The three groups criticized each other for their interpretations.

    The Berlin group co-precipitated radioactive nuclides with barium. They have also precipitated radioactive materials using hydrogen sulfide. Half-life measurements indicated to them that not one but many elements were produced. They interpreted their results based on the fission reaction. The discovery was known to Frisch and Bohr, and eventually Fermi's group learned of the fission.

    Further study revealed that only 235U undergoes fission when irradiated by slow neutrons.

    Uniting Political and Nuclear Power

    Since securing the presidency of Germany in 1933, Hitler became a dictator, and many top scientists in Austria, Hungary, Italy and Germany felt uncomfortable. Scientists with ethnic backgrounds other than German felt threatened. Many European scientists had escaped the Hitler regime and come to the United States. At the time fission was discovered, Hitler invaded Poland, Hungary, Slovak and other European countries. Many scientists were concerned that Hitler would make use of fission to build bombs. Such a move is a threat to the entire world.

    Hungarian refugee scientists Leo Szilard, Eugene Wigner, and Edward Teller, who thought the time has come to unite political force with nuclear power.

    composed a letter for Einstein, and Einstein signed the letter* as the sender. They took the matter so serious that they convinced the economist Alexander Sachs to personally deliver the letter to the White House. Einstein's letters to Roosevelt contains this letter plus others.

    E-mail: cchieh@uwaterloo.ca