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Why do different nuclei need different neutron speed for neutron capture?

  1. Jun 20, 2012 #1
    Why do different nuclei need different neutron speed to achieve fission??

    My understanding is that U-235 requires a thermal neutron in order to be likely to cause a fission reaction, however Pu-239 requires the additional energy of a fast neutron in order to cause the nucleus to undergo fission rather than just absorbing the neutron and becoming Pu-240.

    I also think that a U-235 nucleus is unlikely to absorb a fast neutron. (is this true?)

    Is it true that the strong nuclear force in the vicinity of the nucleons is responsible for the "capture" of a passing neutron? If so why should U-235 and Pu-239 be so different in terms of the likelihood of them capturing fast neutrons?

    Also, if a U-235 nucleus did absorb a fast neutron, would it be likely to split?

    Thanks for any help.
     
    Last edited: Jun 20, 2012
  2. jcsd
  3. Jun 20, 2012 #2

    mfb

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    Did you find values for the cross-section of fast neutrons at U-235 and Pu-239? Maybe they are not so different, and the cross-section for thermal neutrons at U-235 is just larger.

    This is true.
     
  4. Jun 20, 2012 #3

    Bill_K

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    Thermal neutron fission is observed for some even-odd nuclei such as U235 and Pu239. The compound nucleus formed is even-even, which has greater binding energy, and therefore there's plenty of excess energy available to cause instability and result in fission.

    Fast neutron fission is observed in some even-even nuclei like U238. In this case the compound nucleus is even-odd and has less binding energy. Meaning at least a certain amount of excitation energy must be supplied by the incoming neutron for fission to occur.

    Your statement that Pu239 requires fast neutrons is incorrect.
     
  5. Jun 20, 2012 #4
    Is fast neutron fission possible or just as likely as thermal neutron fission in the even-odd nuclei like U235 and Pu239?
     
  6. Jun 20, 2012 #5
    From what I can see for thermal neutrons the cross-sections for fission for the two nuclei are:

    U235 - 346 barns
    Pu239 - 695 barns

    And for fast neutrons the the cross-sections are

    U235 - 1.8 barns
    Pu239 - 1.7 barns

    So could I conclude from these that both nuclei are much more likely to encounter, and therefore undergo fission as a result of a thermal neutron than a fast one?

    Why should the speed of the neutron effect the likelihood of a collision? Is this related to the nuclear force?
     
  7. Jun 20, 2012 #6

    mathman

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    Yes - fast fission is possible. That's how bombs work. Thermal fission is for reactors.
     
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