Positron questions

[McC]

I've been doing a bunch of reading on matter/antimatter annihilation recently, and I'm trying to wrap my head around how you might use it as an energy source. As I understand it, the 'best' form of annihilation is electron/positron -- proton/antiproton has a bunch of energy, but also spits out a bunch of 'junk.' With electron/positron, you get a pair of predictable-energy gamma rays. My questions focus specifically on this type of annihilation.

1) What is the best positron source and how difficult are they to create? I know that the theorized Hawking radiation might give rise to them, as does B+ radiation, which itself is a more rare variety of Beta decay, but are these the only sources?

2) When you obtain a positron, one might suppose you can then contain it in a magnetic field. Is this correct? What would the limits be on such a containment? I imagine that the positrons would form a kind of high-charge 'soup' if pooled together for fuel storage. Is this an accurate conception, or totally irrational? I can think of no case where we in reality have a 'pool' of electrons. So, in what way would one store fuel for electron/positron reactions? And would simply maintaining the storage environment be prohibitively energy-expensive?

3) Supposing you had a way to do all of the preceding, and a way to reliably transport the electrons and positrons to a meeting point, would their mutual charge attraction (e+ and e-) result in complete annihilation? Essentially, would they seek one another out as a result of their charges, or does the possibility exist that they might not reliably react?

4) Once the annihilation takes place, how would one best utilize the resultant gamma rays to provide energy? Is a boiling-water turbine really the best we can do? Since we're dealing with extremely energetic photons, would something like a photovoltaic cell make any sense? If so, would you really get more electricity from that cell than if you were to simply utilize your stored cache of electrons directly?

Sorry if some of these questions seem inane or obvious, but any help is muchly appreciated.

Thanks!

[Winston Blake]

1) What is the best positron source and how difficult are they to create? I know that the theorized Hawking radiation might give rise to them, as does B+ radiation, which itself is a more rare variety of Beta decay, but are these the only sources?

Smashing nuclei in a collider will produce lots of positrons (and lots of other junk too). You'd get a bonus if you transmuted the beam or target into a beta-+ emitter.

There's also pair production, which might be feasible in a scifi-verse with gamma ray lasers.

2) When you obtain a positron, one might suppose you can then contain it in a magnetic field. Is this correct? What would the limits be on such a containment?

Penning trap. I think more complex versions can hold particles indefinitely. I don't think it can get dense enough for practical fuel.

I imagine that the positrons would form a kind of high-charge 'soup' if pooled together for fuel storage. Is this an accurate conception, or totally irrational? I can think of no case where we in reality have a 'pool' of electrons. So, in what way would one store fuel for electron/positron reactions? And would simply maintaining the storage environment be prohibitively energy-expensive?

I'd say more like a cloud than a soup. If you're using a CRT screen, there's a fast moving cloud of electrons continuously flying towards your face right now.

IIRC there's something out there about stabilising positronium using E and B fields. That would probably give better energy density.

3) Supposing you had a way to do all of the preceding, and a way to reliably transport the electrons and positrons to a meeting point, would their mutual charge attraction (e+ and e-) result in complete annihilation? Essentially, would they seek one another out as a result of their charges, or does the possibility exist that they might not reliably react?

Depends how fast they're going. If it's hot enough you could lose a good portion to collisions with the walls. In fact, the energy released by the annihilation might put a limit on how fast you can feed in fuel. The gamma rays released can knock other electrons/positrons around via the Compton effect, heating up the reacting cloud.

4) Once the annihilation takes place, how would one best utilize the resultant gamma rays to provide energy? Is a boiling-water turbine really the best we can do? Since we're dealing with extremely energetic photons, would something like a photovoltaic cell make any sense? If so, would you really get more electricity from that cell than if you were to simply utilize your stored cache of electrons directly?

I don't really know, but I expect steam turbines are the easiest way to do it (or a variation like a nuclear thermal rocket for propulsion). Even if such PV cells existed, they would necessarily be continuously destroyed by the radiation; it might be possible to use the photoelectric effect in some other way, which could be neat for a scifi-verse.

You're probably better off with antimatter-initiated fusion.