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What happens when a large amount of antimatter gets in contact with matter ?

  1. Dec 14, 2012 #1
    Hi everyone, I'm new to the forums and I don't have the knowledge you guys have about Physics, so my questions will be very simple.

    Yesterday I was reading a topic about using the antimatter as a weapon, and someone said it is impractical for many reasons, and he stated some, but only one got my attention. He said, if you have a large amount of antimatter contained safely in a container, when this antimatter meets the matter, it won't annihilate completely as the first particles that are annihilated will prevent the rest of the antimatter particles from touching their "matter" particles and thus, stopping the reaction. So is that true ?

    So my question here is, how fast is the reaction between matter and antimatter ?
    Because if it is not fast enough, then using large amount of antimatter won't be useful, because only the first particles and antiparticles to touch each others will annihilate and the rest of antimatter will be lost.

    I'm sorry if my question seem too ignorant or unclear. I don't speak English very well so I did my best to describe what I wanted. Thanks :)
     
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  3. Dec 14, 2012 #2

    Bandersnatch

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    Well, if you think about it, you can't really "lose" antimatter. It annihilates with any sort of normal matter, including air.
     
  4. Dec 14, 2012 #3
    How ? Can antihydrogen react with something like plastic or steel ?
     
  5. Dec 14, 2012 #4

    phinds

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    Which part of "It annihilates with any sort of normal matter, including air." did you not understand?
     
  6. Dec 14, 2012 #5

    Nugatory

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    Yes. Plastic and steel, like all matter, are composed of protons, neutrons, and electrons.

    Anti-hydrogen, like all anti-matter, is composed of anti-protons, anti-neutrons, and anti-electrons (well, anti-hydrogen is a bit short on anti-neutrons, for the same reason that normal hydrogen is a bit short on neutrons, but that doesn't change the result).

    Thus, no matter what you choose for your matter and anti-matter, you get the same set of happily (and violently) interacting and annihilating particles and anti-particles.
     
  7. Dec 14, 2012 #6

    HallsofIvy

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    It doesn't matter what "atoms" they are assembled into. The anti-electrons (positrons) and anti-protons (which don't appear to have a special name) will be annihilated by the regular electrons and protons in the air.
     
  8. Dec 14, 2012 #7
    So how fast is that reaction ?
    Is it fast enough to annihilate say 1 kg of antimatter exposed to air to be effective as a weapon ?
     
  9. Dec 14, 2012 #8

    Nugatory

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    Yes, although a two-megaton nuclear bomb would be a less technically demanding way of doing the same amount of damage.
     
  10. Dec 14, 2012 #9

    Bandersnatch

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    Huh, my back of an envelope says 40 megatons.
    Or is there large neutrino content taking away that energy?
     
  11. Dec 14, 2012 #10
    one kilogram of antimatter annihilating with another kilogram of matter would produce :
    2 * (3*10^8)^2 Joules which is 1.8*10^17 joules. A megaton of TNT has the equivalent energy of 4.184*10^15 joules. So 1 kg of antimatter with 1 kg of matter produces about 43 megatons TNT equivalent of energy, not two.

    And although neutrino take away energy, it ever exceed 50%-60% of the energy released (usually, it is much less than that) depending on the antimatter used. I might be wrong though.
     
  12. Dec 14, 2012 #11

    Nugatory

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    You're right.
    I lost a factor of two by forgetting that a kilogram of normal matter is being annihilated along with the anti-matter and I lost a factor of ten by forgetting the coefficient in front of 108 in the speed of light... c2 is 1017 not 1016... I thought it sounded too low.
     
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