Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Conditions for thermal neutron induced fission

  1. Aug 17, 2007 #1
    Does anyone know what the necessary conditions are for a nucleus to undergo fission with a thermal neutron? I have found something for the chain reactions, but not very helpful. I want to find out the conditions for ONE nucleus to undergo fission with a thermal neutron.
     
  2. jcsd
  3. Aug 17, 2007 #2

    malawi_glenn

    User Avatar
    Science Advisor
    Homework Helper

    Well on a very fundamental basis, it has to do with the activation energy and excitation energy. And that is related to if the nuclei you want to fission is an Even-even, or odd-odd or even-odd. But when we talk about the resonances, it gets a more complicated.

    I belive that the chapter about fission in Krane's book "introductory nuclear physics" have a good explantion.
     
  4. Aug 17, 2007 #3
    A particular fission reaction has a neutron energy-dependent cross section, regardless of whether or not the neutron is "thermal". Are you asking for links to these energy-dependent cross sections?
     
  5. Aug 18, 2007 #4
    No, actually. I want to know just about the thermal neutrons and the conditions that a nucleus must follow, in order to fission by it.
     
  6. Aug 19, 2007 #5

    Astronuc

    User Avatar

    Staff: Mentor

    Nuclides that are readily fissionable, of fissile, by thermal neutrons are:
    U-233, U-235, Pu-239, and Pu-241, as malawi_glenn indicated.

    The condition for fission is that the fissile nucleus absorbs the neutron, just the same condition which is necessary for a neutron of any energy.

    Even then, there is stil a finite probability that a gamma-ray will be emitted and the nucleus will simply increase in mass by ~ 1amu.

    With the absorption of a thermal neutron, there is ~ 84% chance of fission and 16% chance of gamma-decay and U235 becomes U236.
     
  7. Aug 20, 2007 #6
    Actually, what I meant by the question, was if the nucleus has a low activation energy, be heavy (A > 210) and has a specific ratio N/Z. It seems that only even N nuclei can fission by thermal neutron. Is that correct?
     
  8. Aug 20, 2007 #7

    malawi_glenn

    User Avatar
    Science Advisor
    Homework Helper


    And the reason for that is the pairing-term in the semi emperical mass formula.
     
  9. Aug 20, 2007 #8

    Astronuc

    User Avatar

    Staff: Mentor

    Well, the fissile nuclei have an odd mass number intially, but they form an even numbered nucleus up absorption of a neutron, e.g. U-233 + n -> U-234*, U-235 + n -> U-236*, Pu-239 + n -> Pu-240*, where the * indicates an excited state of the nucleus. The excited nucleus can fission or it can release a gamma-ray, in which case it forms a more stable nucleus. The point is that fissile nuclei are more likely to form an excited nucleus which does fission, as opposed to simply releasing a gamma ray (gamma decay).

    As malawi_glenn indicated, there is a forumula for nuclear stability that more or less indicates which nuclei are prone to fission (instability).

    One can also look at the binding energy per nucleon for the isotopes of U and Pu.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Conditions for thermal neutron induced fission
Loading...