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I Fission caused by positrons - in a fission fragment rocket?

  1. Jan 30, 2017 #1

    Al_

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    I was fascinated to learn about dusty plasma fission fragment rockets.
    https://en.wikipedia.org/wiki/Fission-fragment_rocket
    There's talk that they might enable high delta-V exploration like a manned Jupiter mission! However, they need to be surrounded very many tons of neutron moderator material so the neutrons can cause a chain reaction.
    If you could save the mass of the neutron moderator, the thrust to mass ratio goes up, and the voyage times come down, as well as the cost of construction. Something said on another thread made me wonder if antimatter (positrons) could be fired at Thorium dusty plasma with similar effects? I found a report that large numbers of positrons were made by bombarding gold with electrons - https://www.llnl.gov/news/billions-particles-anti-matter-created-laboratory
    Could this work from a Particle Physics viewpoint?
     
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  3. Jan 30, 2017 #2

    mfb

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    What effect do you expect from the positrons? "Did you try X" is useless without an idea how X could be interesting.

    Positron production needs a huge amount of energy, by the way.
     
  4. Jan 30, 2017 #3

    Al_

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    I was wondering if Thorium fission could be caused by positrons.
     
  5. Jan 30, 2017 #4

    ZapperZ

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    You are not answering the question. Your response is more of a blind shot into a barrel hoping to hit something.

    You need to come up with the physics that might support what you are claiming. That is how we do science, and that is how it is done here, not simply make up blind guesses hoping that one would stick.

    Zz.
     
  6. Jan 30, 2017 #5

    mfb

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    Or electrons, muons, taus, grains of sand, apples, hitchhiker books...?
    That approach doesn't work.
     
  7. Jan 30, 2017 #6

    Vanadium 50

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    Very small rocks...cider..grape gravy...cherries...churches...lead
     
  8. Jan 30, 2017 #7

    Al_

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    I'm puzzled at your responses. How would you like the question to be framed?
     
  9. Jan 30, 2017 #8

    mfb

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    The whole type of question does not lead to interesting results.

    If you have a reason to expect that X can influence Y, feel free to ask if and how it does. But randomly combining two things in physics does not make a useful question.
     
  10. Jan 30, 2017 #9

    Al_

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    I see!
    The reason to think that was a post by ProfChuck : https://www.physicsforums.com/threa...may-enable-trips-to-mars.889815/#post-5604600
    where he says: " I am currently consulting with a company that is researching a propulsion system based on positron annihilation. There are several promising approaches including a positron powered VASIMR engine and another that involves direct conversion of anti matter reactions to thrust."
    Positronium might become more available thanks to : https://www.llnl.gov/news/billions-particles-anti-matter-created-laboratory

    But maybe I read his post wrong, he linked to : http://thefutureofthings.com/3031-new-antimatter-engine-design/

    If you combine two design ideas - stored Positronium and a rocket with a magnetic nozzle, then is it possible to build a rocket that has both high thrust and a high specific impulse?
    I think it depends if the annihilation results in charged fragments.
    Would Positronium annihilation in dust particles result in a substantial % of charged dust fragments with velocities of a fraction of c ?
     
  11. Jan 30, 2017 #10

    ZapperZ

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    You do know that matter-antimatter annihilation has nothing to do with what you asked in this thread, don't you?

    Zz.
     
  12. Jan 30, 2017 #11

    Al_

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    OK, more thinking required.
     
  13. Jan 30, 2017 #12

    mfb

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    No. And you are doing the same mistake again. You randomly combine two unrelated concepts.

    Concerning positron production: Here is the research paper. Scientists used a laser that spreads over multiple rooms to produce 100 billion positrons, annihilating them will release 0.016 J. The laser can probably produce a few shots per second. And even if it could produce more: They need about 60 GeV laser energy per positron/electron pair produced. That is an efficiency of 0.0016%. I didn't find numbers for the laser efficiency, but those pulsed lasers are probably below 1% efficient. Your overall efficiency of electricity to energy released in positron annihilation is now 0.000016%. And where is the point? We still have no connection between positrons and fission.

    Do you see the problem? Science is not about randomly mixing concepts. It is about coming up with a plausible mechanism how X can cause Y, and then calculating if that looks reasonable quantitatively.
     
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