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Fissionable Elements - Substitute teacher looking for answer

  1. Sep 22, 2008 #1
    Got a question about fissionable (as opposed to fissile) elements.

    I vaguely remember the minimum A number for (theoretically) fissionable elements is somewhere in the vicinity of 153 to 156.

    But I don't recall the exact figure, neither google nor ask.com have been of help and I'd like to be able to go back to class this afternoon with the answer.

    Anyone lend a hand?
  2. jcsd
  3. Sep 22, 2008 #2


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    It's more like 232, as in Th-232, which like U-238, is fissionable with fast neutrons. However Pa-231 apparently has a very small probability of spontaneous fission, but it itself is a decay/spallation product.

    Ref: http://www.nndc.bnl.gov/chart/ - put cursor over element (Z,N), click on it, and then click Zoom 1.

    A=153-156 is in the range of Gd (primarily 155, 157), Dy (161, 163) which are good neutron absorbers.

    Lighter elements experience spallation in which a high energy neutron may knock out some nucleons ((n,p), (n,2n), (n,3n)), or an alpha particle.
  4. Sep 22, 2008 #3
    Thanks for the reminder on spallation, and you're right, Gd and Dy have pretty decent cross sections.

    But the thing is this; I distinctly remember being surprised that nuclei with a mass 80 amu or so below what I was used to seeing could fission and further surprised that the reaction was either endothermic or close to it.

    Any thoughts as to how we could further check to verify the probabilities of induced fission with masses this low? I'd hate to think it was only my memory that was decaying.
  5. Sep 23, 2008 #4


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    One can search here for available reactions

    There do not appear any fission reactions with A=80.

    There is a comment that nuclides heavier than iron can release energy by fission, but I believe that in most cases, absorption of neutrons yields gamma emission, or other nucleons, or spallation - but not fission. Nature tends to take the easiest path to emit excess energy.
  6. Sep 23, 2008 #5


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    Are you thinking of the Iron minimum, in stars you can form heavier elements by fusion only upto Iron. After that it requires energy to join heavier elements and so presumably energy would be released if they split.
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