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

[Hurricane93]

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 :)

 

[Bandersnatch]

Well, if you think about it, you can't really "lose" antimatter. It annihilates with any sort of normal matter, including air.

 

[Hurricane93]

Well, if you think about it, you can't really "lose" antimatter. It annihilates with any sort of normal matter, including air.

How ? Can antihydrogen react with something like plastic or steel ?

 

[phinds]

How ? Can antihydrogen react with something like plastic or steel ?

Which part of "It annihilates with any sort of normal matter, including air." did you not understand?

 

[Nugatory]

How ? Can antihydrogen react with something like plastic or steel ?

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.

 

[HallsofIvy]

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.

 

[Hurricane93]

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 ?

 

[Nugatory]

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 ?

Yes, although a two-megaton nuclear bomb would be a less technically demanding way of doing the same amount of damage.

 

[Bandersnatch]

Yes, although a two-megaton nuclear bomb would be a less technically demanding way of doing the same amount of damage.

Huh, my back of an envelope says 40 megatons.
Or is there large neutrino content taking away that energy?

 

[Hurricane93]

Yes, although a two-megaton nuclear bomb would be a less technically demanding way of doing the same amount of damage.

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.

 

[Nugatory]

Huh, my back of an envelope says 40 megatons.
Or is there large neutrino content taking away that energy?

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.