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Slow neutrons

  1. Feb 11, 2010 #1
    Here's a question.
    Why does slow neutron is most likely to cause a nuclear fission in Uranium 235, compare to a fast neutron?
  2. jcsd
  3. Feb 11, 2010 #2


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    In simple terms because a fast neutron doesn't spend enough time near the nucleus to have an effect
  4. Feb 11, 2010 #3
    Hmm...How about the reason for slow neutrons not being able to be absorb by uranium 238?
  5. Feb 11, 2010 #4


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    Here is the plot of cross-section of (n, gamma) reaction for U-238 as a function of neutron energy.


    Note that the absorption cross-section is much higher for thermal (slow) neutrons than fast.

    U-238 does capture neutrons and becomes U-239, which decays to Np-239, which decays to Pu-239.

    Fission cross-section for U-238 as a function of neutron energy.

    U-238 is a much more stable nuclide than U-235.

    (n,gamma) for U-235

    (n, fission) for U-235
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