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Why are only some particles fissionable/fissile?

  1. Nov 26, 2008 #1
    I understand how fission happens. But what I do not understand is why only a few certain nuclei are actually fissionable/fissile.

    Can anyone help?

    Thanks,
    Jamie
     
  2. jcsd
  3. Nov 26, 2008 #2

    mgb_phys

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  4. Nov 26, 2008 #3
    If more neutrons means a more stable nucleus, then why in nuclear fission does the absorbtion of an extra neutron lead to the nucleus splitting in half?
     
  5. Nov 26, 2008 #4

    mgb_phys

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    More neutrons only means more stable upto a certian level.
    There is a most stable ratio of neutrons/protons which gets slightly larger with larger atoms.
    So a small atom with an equal number of protons/neutrons would become very unstable if you added a single neutron. As you get larger the most stable configuration is a few more neutrons than protons - but if you add more neutrons than this you go off the line in the other direction and make the nucleus less stable.

    The exact number to have best stability can be calcuated but the theory (Quantum Chromo Dynamics) is very complicated.
     
    Last edited: Nov 27, 2008
  6. Nov 26, 2008 #5
    Okay, thanks very much.
     
  7. Nov 27, 2008 #6
    adding more neutrons results in a stronger force of attractin holding the neucleus together but it takes energy to produce neutrons which are themselves unstable. allowing a sungle neutron in such a nucleus to decay would cause the nucleus to expand but it also releases the energy stored in than nutron.

    uranium decays by a different process than most. it splits in half. the protons are in 2 shells which simply separate.
     
  8. Nov 27, 2008 #7

    Vanadium 50

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    No, it only does this if the nucleus is proton rich. Adding a neutron to a nucleus that is already neutron rich makes it worse.

    Yes, if I want a beam of neutrons it takes energy and they are unstable. But what does this have to do with the question.

    I don't think so.

    Alpha decay is common, and it's simply one example of spontaneous fission.
     
  9. Nov 27, 2008 #8
    the strong force increases when you add neotrons. but if you hove too many neutrons then the energy released by allowing a neutron to decay is greater than the decrease in energy due to the strong force.
     
  10. Nov 27, 2008 #9

    Vanadium 50

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    If your theory were true, Helium 5 would decay by beta emission, like a free neutron. In fact, it decays by simply falling apart to a neutron and an alpha.
     
  11. Nov 28, 2008 #10
    yes and a nucleus of 2 alpha particles will fall apart too. theres something inherently unltable about those configurations. dont know what that has to do with what I am saying though.
     
  12. Nov 28, 2008 #11

    Vanadium 50

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    It shows your theory is wrong. You said "the strong force increases when you add neotrons [sic]" until the neutron decays (via beta decay). Here's a situation where the nucleus becomes less stable and the neutron does not beta decay inside the nucleus.
     
  13. Nov 28, 2008 #12
    a single exception doesnt necessarily prove a rule wrong. its a big complex world with lots of exceptions.
     
  14. Nov 29, 2008 #13

    Vanadium 50

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    This is science. It matters when your theory gets something wrong. Especially when mainstream theory gets it right.
     
  15. Nov 29, 2008 #14

    malawi_glenn

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    granpa, the strong force is a saturating force. You are mixing wild between concept of force and binding energy.

    Uranium exihibit same fission spectra as the other heavy nuclei...


    Unredeemed: All nuclei are fissionable, but not everyone you can gain energy by splitting them.
     
  16. Nov 29, 2008 #15
    my 'theary' as you call it IS the mainstream view.
     
  17. Nov 29, 2008 #16

    malawi_glenn

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    "uranium decays by a different process than most. it splits in half. the protons are in 2 shells which simply separate. "

    Now first define WHAT uranium isotope you are referring to and state your source (peer reviewed article or textbook)
     
  18. Nov 29, 2008 #17
    ok I'll bite. what other heavy nuclei and what fission spectrum and what does any of that have to do with what I said?
     
  19. Nov 29, 2008 #18
    I was talking to malawi about the stability of lighter nuclei. please read the whole forum before you jump on people.
     
  20. Nov 29, 2008 #19

    malawi_glenn

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    I can't see where you state that: "For light nuclei...."

    Then you brought up the discussion about Uranium, therefor I asked what is so special about it and what isotope you are referring to. Don't you know what a fission spectra is?
     
  21. Nov 29, 2008 #20
    well I would assume you mean the different masses of the resulting nuclei but that doesnt seem to follow from what you said since I cant see any relevance to anything being discussed.
     
  22. Nov 29, 2008 #21

    malawi_glenn

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    eh?

    "uranium decays by a different process than most. it splits in half. the protons are in 2 shells which simply separate. "
     
  23. Nov 29, 2008 #22
    and?
     
  24. Nov 29, 2008 #23

    malawi_glenn

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    can't you see who ridiculous that post is?

    First of all, what uranium isotope are you referring to. Second, what is so special about uranium? Third, it seems to me that you think they are split into equal halves, but infact the fission spectrum for uranium isotopes are smooth. Forth, what is "protons are in two shells which simply separate"?? Shells as in shells in nuclear structure theory or what?
     
  25. Nov 29, 2008 #24
    split into equal halves

    I never said that nor do I have any idea how you would have gotten it from any thing I said. the 2 proton shells are not equal. one is filled and the other isnt. in addition to the proton shells there are also neutron shells. each proton shell carries away a somewhat random number of neutrons. some of those neutrons then change to protons and some ore emitted

    its really quite neat if you think about it. can you imagine how strongly they must repel one another? no wonder the atom bomb is so powerful.


    edit: yes this part of what I am saying is my own thinking. but i'm not aware of any conflict with 'mainstream' science.
     
  26. Nov 29, 2008 #25
    oh wait. its been so very long since i gave any thought to any of this that i've forgotten many things.

    the mainstream view is that there are shells but not 2 shells. my thinking is this. each shell consists of 5 subshells of 1, 3, 5, 7, and 9 pairs of protons. totalling 50 protons. one shell is full. the other not quite full. this agrees with most of the magic numbers of the nucleus.


    I've also observed that the number of neutrons not part of alpha particles (and presumably therefore in a shell of their own) in the nucleus is approximately (Z/12)^2. for uranium this is approximately 50 neutrons. one complete shell.
     
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