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Neutron causing Nuclear Fission how?

  1. Jul 12, 2008 #1
    If anyone can help me with this question I'd be greatly appreciative....

    Here's what I think I understand:
    Nuclear fission is the breaking of an atomic core of an atom. Like with U-235.
    It absorbs a slow moving neutron which yields the unstable U-236. Because of the instability the atomic core fissions. Yielding Kr and some other element (can't remember which one) along with 2 neutrons (or more). These 2 neutrons hit other U-235 atoms, amplifying the nuclear fission with each generation of more neutrons.

    So, where does the stray neutron come from?
    I kind of understand radioactive decay.... but I haven't read any mention of a neutron coming out.
    Radioactive decay releases alpha particles, beta particles and gamma rays.
    Alpha particles are Helium atoms without the electrons, so they are positive and would be repelled from the atomic core of U-236. Also, they have alot of mass, travel slowly and have short range.
    Beta particles are electrons, they move faster, have a longer range but would get repelled from the electron cloud (I think), and wouldn't be able to make an atomic core unstable.
    Gamma rays aren't particles but are waves and have no mass. So they wouldn't be able to make an atomic core unstable.

    What am I missing!?!?!?!?!?!?
    Everywhere I read the article just assumes this neutron floating around approaching a some heavy elements atomic core. But where does this neutron come from? Is it a 4 type of radioactive decay? Along with alpha, beta and gamma.... a neutron can get shot out?

    Thanks for any help.
  2. jcsd
  3. Jul 12, 2008 #2


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    Neutrons can come from sponateous fission, from cosmic rays (high energy muons/protons hitting atmospheric nuclei and producing neutron amongs other particles)
    And yeah, some nuclei undergoes neutron emission, but those are very very few (if there is any natural occuring nuclei which does so)
  4. Jul 12, 2008 #3


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    Neutrons may also come from (p,n) reactions, but that would be very inefficient,

    from (d+t => alpha + n, or d+d => t + n, and others) fusion reactions,

    from photoneutron reactions, with d or Be-9, and

    from alpha + Be9 spallation reactions. The source of alpha particles could be Ra, Po, or Pu, or other alpha emitters.

    Nuclear reactors use a photoneutron source for startup, and continue to use the sources until they have accumulated fuel with sufficiently high burnup, which then has sufficient amount of spontaneous fissions from transuranic radionuclides which can be used instead of the startup source.

    See page 108 of this document - http://hss.energy.gov/NuclearSafety/techstds/standard/hdbk1019/h1019v1.pdf [Broken]
    Last edited by a moderator: May 3, 2017
  5. Jul 13, 2008 #4
    Thanks for the answers.

    Making a bit more sense now.
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