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Where do the initial neutrons come from?

  1. Mar 12, 2014 #1
    In atomic bomb there are two hemispheres of fission material (Uranium or Plutonium), they are joined by means of conventional chemical explosives, the necessary critical mass is reached and the atomic bomb explodes, everybody knows this:
    But is this condition enough? Indeed, the initial neutrons are needed to trigger the fission reaction, where do these neutrons come from? Definitely not from the Uranium or Plutonium………….:rolleyes:
  2. jcsd
  3. Mar 12, 2014 #2
    Yes, the Uranium or Plutonium have a low level of spontaneous fission happening all the time that may provide the neutrons needed to initiate the chain reaction. It is possible in theory, but in practice it works better to have a separate more reliable neutron source such as a Polonium-Beryllium alloy. The Polonium provides alpha particles and the Beryllium converts them into neutron radiation.
    Last edited: Mar 12, 2014
  4. Mar 12, 2014 #3


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    Actually, Plutonium produces too many neutrons from spontaneous fission for this design. The reaction would start too early and the pieces would repel each other before most of the material was used.

    Therefore, all Plutonium bombs (and all current bombs in general) implode a hollow ball and have a separate neutron source - that is faster and more reliable.
  5. Mar 12, 2014 #4
    I hate to intrude on someone else's topic and ask questions but would you mind explaining how Beryllium converts Alpha particles into Neutrons, does it somehow capture the protons?
  6. Mar 13, 2014 #5


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    See this article on Beryllium: 'Nuclear Properties'


    The part of a nuclear device which furnishes the initial neutrons to start the fission reaction is called an 'initiator'. The initiator for the first atomic bombs was made by smashing Polonium and Beryllium together when the bomb was detonated. The neutrons produced by the initiator then stuck the fissile uranium or plutonium fuel and started the chain reaction.

    When a Be-9 nucleus is struck by an energetic alpha particle produced by the decay of Polonium, the alpha particle fuses with the beryllium nucleus to produce a carbon-12 nucleus plus a neutron.

    Later designs of initiators cause a few atoms of deuterium and tritium to fuse by use of high-voltage electrical current to generate neutrons.

  7. Mar 13, 2014 #6
    Thanks guys :smile:
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