# What is the percentage of useful energy that we get from antimatter ?

1. Aug 10, 2013

### Hurricane93

This is a theoretical question since we haven't made enough antimatter to try it in reality of course. But I am asking about the physics part in this.

Also, by "useful energy" I mean the energy we are able to use either as a heating source for something like a nuclear reactor, or as energy for an explosion like nuclear explosions.

If I am not mistaken, a large part of the energy we get from the annihilation is in the form of neutrinos, which we for some reason can't consider them useful energy. So now, if we subtract the energy of the neutrinos, it is safe to consider the rest as useful energy as I explained ?

Please try to be as simple as possible because I don't speak English very well.

2. Aug 10, 2013

### HallsofIvy

I'm not sure what you are asking here. Currently, there is NO method of getting ANY "useful energy" from matter-anti-matter interaction.

3. Aug 10, 2013

### mathman

I believe you are mistaken about the output of annihilation. It consists of photon pairs. For example, electron-positron ends up as two 511 kev photons.

4. Aug 10, 2013

### Staff: Mentor

@mathman: Hadron annihilations produce more pions than photons, and charged pion decays lead to neutrinos.

snorkack and I discussed that in this thread. We could not find a number, but neutrinos will get a significant fraction of the total energy.

5. Aug 10, 2013

### Hurricane93

So as the link you provided states, is it safe to consider at minimum that 50% of the energy goes to the explosion ?
Also, does matter and antimatter annihilation produce harmful radiation ?
If a large amount of gamma rays is released, then we should consider this bomb a source of radiation, shouldn't we ?

6. Aug 10, 2013

### Staff: Mentor

I don't see where that number comes from.
Sure.
Sure.

7. Aug 10, 2013

### Hurricane93

One last thing. It is known that gamma radiation require less shielding than neutron radiation, so in case of a "hypothetical" Antimatter reactor, would that mean that it would need less massive shielding to reduce the radiation to non-harmful levels ?

8. Aug 10, 2013

### Crazymechanic

By the way , energy doesn't go into explosion like people go into a house , the explosion or the artistic effects of it like mushroom cloud , a blast wave and a lot of heat are just different side effects of the primary nuclear reaction going on at the heart of the bomb or whatever , alot of heat is produced which rapidly expands creating a shock wave , all kinds of radiation is produced.

you could use antimatter for energy production , there is only a slight problem , we don't have any and as it turns out , atleast with our current understanding and methods it takes a lot of energy to make it so in the end of the day you wasted energy and got back the same or even less + you have to contain it somewhere as normally it would try to annihilate with matter.

9. Aug 11, 2013

### Staff: Mentor

Ah. Hmm, sounds reasonable. This applies to bombs only, as it is not reasonable to keep all muons within some container.

That depends on the energy. If the neutron energy is not too high (below 1 GeV), they are easier to shield. To contain the gamma rays, something like 1-3m of iron is a good start.

10. Aug 13, 2013

### Hurricane93

What about the kinetic energy of the pions ?
should we consider them as lost energy too ?

11. Aug 13, 2013

### Drakkith

Staff Emeritus
If electrically charged, then they probably interact readily with normal matter like any charged particle and can transfer their kinetic energy. But if they aren't charged, then it's likely they don't interact before they decay, as their lifetimes are so short and without electric charge they would pass through matter in a similar manner to neutrons.

12. Aug 13, 2013

### Hurricane93

Charged pions have a mean lifetime in the range of a nanosecond, so does this mean they will transfer their kinetic energy during this relatively short lifetime ?

13. Aug 13, 2013

### Staff: Mentor

The interactions between charged pions and electrons are similar to those between muons and electrons - they lose some energy, but not enough to get stopped within the bomb.
Nuclear interactions are different.

14. Aug 13, 2013

### Hurricane93

Could you please explain a little bit more ?
Like just before decaying, would the charged pion have lost all of its kinetic energy ?

15. Aug 13, 2013

### Drakkith

Staff Emeritus
If they hit something, then yes, they will transfer at least part of their kinetic energy.
Of course, I'm assuming that if a pion hits a proton with enough force then there are other interactions possible other than a simple transfer of kinetic energy. I don't know how much energy the pions from an antimatter reaction have.

16. Aug 13, 2013

### Hurricane93

Should we consider the air as "something" ?
Also, what will happen if it decays before it loses its kinetic energy ? where will this energy go ?

Last edited: Aug 13, 2013
17. Aug 13, 2013

### Drakkith

Staff Emeritus
I would.

The decay products retain this energy.

18. Aug 13, 2013

### Hurricane93

I don't really know how fast charged pion move, but assuming it moves at the speed of light, with its 70 nanosecond mean lifetime, it will travel 21 meters in its lifetime. So it will surely go through the casing of the bomb for example. So if this material is say 5 cm thick of iron or lead, would this material get a significant part of the kinetic energy of the pion ?

Sorry if I have been asking too many question, but I think this is the last one. Thanks in advance.

19. Aug 13, 2013

### Drakkith

Staff Emeritus
A pion is a particle that has mass. As such it will never go the speed of light. Such a thing is impossible.

20. Aug 14, 2013

### Staff: Mentor

That is very unlikely.

At least not via the electromagnetic interaction. Some pions will do inelastic collisions with nuclei, in that case they can deposit most of their energy within 5cm of iron or lead.

That is true, but most pions from baryon annihilations are high-energetic (average gamma factor of 2-3 if I remember correctly), for their mean flight distance the speed of light is a reasonable approximation.

@Hurricane93: Don't forget time dilatation.