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Matter-antimatter annihilation

  1. Nov 26, 2007 #1
    Why can't we use electron-positron annihilation as an energy source? Unlike antiprotons, positrons sometimes pop out of beta decay and positron-producing isotopes with short half lives are routinely produced in cyclotrons around the world for hospital PET-scanners.

    Positrons and electrons both have a charge, so magnetic confinement should be possible.

    And electrons are 1/1836th of the mass of a proton, so compared to the equivalent mass of protons/antiprotons, we'd need about 1836 times as many positrons/electrons. Since positrons come out of beta decay, this shouldn't be that difficult.

    Reactors that rely on matter-antimatter annihilation are popular in science fiction. How feasible would it be to use these isotopes to produce positrons for positron-electron annihilation and energy production?
     
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  3. Nov 26, 2007 #2

    mathman

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    The problem is primarily a quantitative problem. For PET scanners the amount of isotope need is miniscule compared to what would be needed for a useful energy source.
     
  4. Nov 26, 2007 #3
    Would creating the isotope in a cyclotron use up more energy than positron-electron annihilation could produce? Because if the average amount of energy needed to create a single atom of the isotope is smaller than a positron-electron annihilation produces, then this could be a viable energy source, if scaled up.
     
  5. Nov 27, 2007 #4

    malawi_glenn

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    I can tell you this:
    In space satteliets (like Voyager 2) they have a solid that creates energy from alpha decaying plutonium-238. The device is called radio-thermal generator (RT), the efficancy is approx 5%. But here it is enough, because you dont need a lot of plutunoim. So compared with other energy sources available at the edge of the planet system, this device is superior.

    Using these devices on eart is useless, due to their very small efficany and that building RTG's so you can replace one nuclear - power plant is... too too large! :P

    The best we can do with energy I think, is to better our fission, the saftey, and trying to get fission of Thorium. Also research more about transmutation, converting used Uranium to create both more energy (using fuel more efficent) plus it takes down the half life time of the waste, so we dont get this very big danger about that. And the waste after transmutation, you cant build nuclear bombs of. Then of course, fusion, but fusion is approx 50y in the future, maybe 100 til we can make good and reliable enough.
     
  6. Nov 27, 2007 #5

    Astronuc

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    Basically it is not feasible to use electron-positron annihilation as an energy source.


    Yes! And then there is the inefficiency associated with the transformation of the 0.511 MeV gamma rays, which result from the electron-positron annhilation, into useful electrical or thermal energy.
     
  7. Nov 27, 2007 #6
    If you think about it, this has to be true. Fission and fusion work because you're releasing energy that's already stored by nature in either very heavy or very light nuclei. It's not really "free", but you don't care how much energy nature required to create the nuclei, you just want to release what you can for your own purposes - you have to spend some energy to do this, but you can get back more than what you put in if you're careful. In this situation, you're using man-made energy to create the isotopes, then hoping they'll provide free energy back when their decay products annihilate each other - the net effect has to be a loss of energy.
     
  8. Nov 27, 2007 #7
    Not if their decay products are to a state that is lower than what you used to create the isotopes. Basically using something as "fuel" as it were. No free energy but just using energy to prep something that could potentially cascade to a lower state, releasing some energy in the process (more than the prep).

    (in general, obviously, not really in this situation)
     
  9. May 7, 2010 #8
    The Reason Anitmatter and matter annihalation reactions cannot be used as pure energy producers as in sci-fi films, is because of the quantum instability of the anti-matter.
    Anti-atoms can only be stored in magnetically charged chambers because of its quantum instability and the fact that it has a negative electron charge, once we join the anti-protons with anti-neutrons to create an anti-atom the anti-matter produced becomes electron-neeutral and so the chamber can no longer contain the anti-matter. This then results in the anti-matter coming into contact with the chamber wall and both being annihalated. SO the reason we cannot use these anti-matter reactions as energy is because the anti-matter only has an extremely short life span because of the quantum instability, and the energy produced by reactions is not economic because of the amount of energy needed to make the reaction happen.
     
  10. May 11, 2010 #9
    On the subject....in what way could Gamma rays be used as an energy source. Obviously I know they're high frequency electromagnetic waves, but in what way could energy be retrieved from them? I can't get the idea of using them as a heat source out of my head.
     
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