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How does fission work?

  1. Apr 11, 2009 #1
    I've been really interested in fission lately and have been trying to find out as much about it as I can. A lot of explanations I've read explain that the nucleus spits apart from the force of a neutron hitting it. I've also read that sometimes a nucleus will decay by spontaneous fission if it is unstable enough. This makes me wonder if it's the force of the neutron that makes it split.

    It makes sense to me for fissionable isotopes where the neutron has to have a certain energy to split it, but for fissile isotopes I get the impression that something like U-235 for instance would absorb the neutron and become U-236 for an instant then spontaneously fission. An http://en.wikipedia.org/wiki/Neutron_cross_section" [Broken] had this written in the section "Actinide decay:" U-235+n=U-236*; U-236*(fission 81%) which looks like it shows the U-235 turning into U-236 for a moment before it fissions.

    I have no idea which is correct since I've seen it explained two different ways (or at least I think I have.) Can someone help clear this up for me?

    Another thing I'm curious about is how much energy a neutron has to have to be absorbed by a nucleus. Does it vary for different elements and isotopes of them?

    And one more: I read a bit on nuclear isomers where the particles in the nucleus have more energy than is normal. If fission happens when a nucleus absorbs a neutron that makes it unstable and then splits, does this explain that? (The * in the example above means that the nucleus is energized.) And if that's true, then does that mean that U-236 made from neutron absorption of U-235 would fission but U-236 made from beta decay of Pa-236 would not?

    Thanks for any help clearing this up for me.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Apr 11, 2009 #2


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    When U-235 absorbs a neutron, it forms an excited U-236 nucleus, which either fissions or decays by gamma-emission. Recycled U from spent fuel has an increased amount of U-236 in it.

    It's not the force of the neutron colliding with the nucleus that causes fission, but the simple fact of absorption. Increasing the energy of the colliding neutron will change the out come of the fissions with distribution of fission product elements moving a bit above A=100 and below A=130. Either two or three neutrons are released.

    Here is a nice overview.
    Last edited by a moderator: Apr 24, 2017
  4. Apr 13, 2009 #3


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    It's not so much the kinetic energy of the neutron - but the fact that it is falling into a nuclear potential

    Imagine you had an old well - the type people used to haul water up from in a bucket. Except this well
    is dry - it's just a very deep hole in the ground lined with stones.

    Now suppose you have a rock sitting on the side of the well - and you lightly push it off into the well.
    You didn't put much energy into the rock. However, when the rock falls into the well, it is falling into
    a gravitational potential well and the force of gravity gives it a lot of energy when it falls. When it hits
    the bottom of the well, it ricochets around the bottom of the well bouncing off the stone walls and making
    a loud racket that you hear.

    Obviously, there's energy in the sound given off as the rock bounces around. Where did that energy
    come from? Did it come from the little push you gave the rock? No - it came from the fact that the
    rock fell into this gravity well.

    When a neutron is absorbed by a nucleus, it is also falling into a well - it's just a nuclear potential well
    and not a gravity potential well. THAT is where most of the energy comes from that breaks the nucleus

    Dr. Gregory Greenman
  5. Apr 13, 2009 #4
    Thanks! I understand that better now.
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