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Homework Help: GRE Question - Fission

  1. Feb 17, 2010 #1
    1. The problem statement, all variables and given/known data
    The Uranium (A = 238) nucleus has a binding energy of about 7.6 MeV per nucleon. If the nucleus were to fission into two equal fragments, each would have a kinetic energy of just over 100 MeV. From this, it can be concluded that

    A) Uranium (A = 238) cannot fission spontaneously
    B) Uranium (A = 238) has a large neutron excess
    C) nuclei near A = 120 have masses greater than half that of Uranium (A = 238)
    D) nuclei near A = 120 must be bound by about 6.7 MeV/nucleon
    E) nuclei near A = 120 must be bound by about 8.5 MeV/nucleon

    2. Relevant equations
    conservation of mass-energy:
    Binding Energy + rest mass of uranium = rest masses of two equal fragments + 100 MeV for each fragment + Binding energy of each fragment

    3. The attempt at a solution
    So I know that the equation I should write is (using approximate values for the mass numbers):

    240*-7.6 + Mc^2 = 2mc^2 + 200 MeV + 240*X

    X -> the binding energy of the fragments.
    Mc^2 -> rest mass of Uranium
    2mc^2 -> rest masses of the two equal fragments created

    There is a solution to this problem online, however they neglected to include the rest mass terms on both sides of the equation. I'm a little confused as to why. Is that just because this is an approximate calculation and the rest masses will more or less cancel out on each side?

    Any clarification there would be great. Thanks.
  2. jcsd
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