How could we produce electricity thanks to antimatter?

Question

Imagine we had an easy way to produce antimatter, we could use it for interstellar travel, deadly weapons or more; but I've thought: How could we use it to produce electricity?

As Antimatter annihilates itself with normal antimatter producing a huge amount of energy, I thought we could harvest its potential to make power and fortunately replace petrol, oil, and polluting substances that now are mostly used to power the world. Therefore, how could we harvest antimatter efficiently to produce power?

When antimatter and normal matter annihilates themselves, they produce a large amount of gamma rays, which would be absorbed by special materials (which ones?) that would convert them to heat (which can easily make electricity). But as antimatter annihilation makes a lot of gamma rays, we couldn't place the absorbing materials near the antimatter annihilation source, because the gamma rays would be so intense that they would destroy the absorbing materials. This would mean that we should put the absorbing materials far enough for them to absorb gamma rays without getting themselves destroyed. Unfortunately, we would then only harvest a fraction of the annihilation great potential. What are the structures or solutions that could enable this "project" to work?

Maybe this concept is completely absurd and wrong, do you have another concept? I've thought that we could combine the destroying power of antimatter with nuclear fission and nuclear fusion to ease the fission of (for example) uranium 238 which would start the fusion of a D-T pellet (deuterium-tritium) and produce heat that is converted into electricity easily. But how could we sustain the fusion? Actually, this concept mainly is to use antimatter to help the start of a nuclear fusion and hopefully to sustain it.

I just don't know and I would very pleased if you answered the most of the questions I asked as possible, giving your theories and concepts even if they have nothing to do with the ones I've said. Thank you!

Answer 1

How efficiently you can convert matter-antimatter annihilation to useful energy depends on the specific particles involved. The simplest and most efficient is electron-positron annihilation which produces 0.511 MeV gamma rays. Proton-antiproton and neutron-antineutron interactions are more complex and produce mostly particles.

Gamma rays in the energy range of 0.1 to 10.0 MeV are absorbed mostly by Compton scattering. I've included a link below showing the absorption of 0.309 MeV gamma rays from the decay of cobalt 60. It shows that 1.12 cm of lead will absorb half of 0.309 MeV gamma rays. It turns out that the absorption per unit mass is about constant, so it would take a layer about 0.67 cm thick of tungsten for the same absorption, since it is 70% denser than lead.

So the short answer to your question is: you use electrons and antielectrons (positrons) as your fuel, and make a heat engine by absorbing the gamma rays in a few inches (~ 10 cm) of tungsten. Compared to lead, tungsten has a much higher melting point (3422 C), is harder, stronger, and easier to machine. The simplest heat engine for producing electricity would be a thermopile (an array of thermocouples). This is what the current RTGs (radioisotope thermal generators) use (such as those on the Cassini (Saturn) and New Horizons (Pluto) missions. Although not very efficient, there are no moving parts or fluids so, assuming you have enough fuel, it should work for a century or two. If you want to use the antimatter reactions for propulsion, use the electric generator to power an ion engine. Although the thrust from an ion engine is many powers of ten less than that of a modern chemical rocket, it can provide thrust for a really, really long time.

Although a simple net search will find many papers on gamma ray absorption, most require math worthiness; the one below does not. 'Just look for the Dsub(1/2) which is the distance/thickness required to absorb one half of the gamma rays for the particular material. Twice this thickness will absorb three quarters, three times the thickness will absorb seven eighths, etc.

Absorption of Gamma Rays from Cobalt 60: https://www.nikhef.nl/~h73/kn1c/praktikum/phywe/LEP/Experim/5_2_41.pdf