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

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


    User Avatar
    2017 Award

    Staff: Mentor

    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


    User Avatar
    Science Advisor

    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


    User Avatar
    Science Advisor

    Yes - fast fission is possible. That's how bombs work. Thermal fission is for reactors.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook