In the far future, antimatter is used for energy storage: Huge space stations with powerful solar arrays orbit the sun a bit inside Mercury's orbit, using the energy to create small amounts of anti-hydrogen (or other antimatter). This is then packaged in special containment devices and shipped to places in need of energy, to be used in reactors or even as fuel.
But what do these reactors look like? Matter-Antimatter Annihilation releases energy in the form of mostly Gamma rays, neutrinos, (fast) electrons and positrons. This is frigging hard to turn into useful energy!
If you happen to know which directions the charged particles will fly, you can build something like a particle accelerator in reverse to utilize their kinetic energy: an electron flies through conductive rings and induces a slight charge, the difference in charge along the flight path can be used to generate power. Same for positron but each particle species needs a dedicated generator! So you need to know which particle will fly which way.
Gamma rays are far harder since they tend to penetrate lots of matter without interacting. The only way I can think of to harvest Gamma-rays would be to have really thick lead (or other heavy metal) shielding that will be heated by the radiation, then harvest the heat.
Half value thickness of lead depends on energy of the Gamma rays, German wikipedia gives ~ 4mm for 0.5MeV (Energy of gammas released by electron-positron annihilation).
For simplicity's sake we might as well forget the electron harvesting described above, let the electrons/positron smash into the lead shield (the positron will annihilate with bound electrons releasing more Gamma radiation) to generate heat
Neutrinos ... Neutrinos mostly just pass through matter. I'd say the we can forget about harvesting the energy released as neutrinos.
So our reactor will likely consist of a small annihilation chamber embedded in a huge vessel full of molten lead, the heat generated will drive a steam engine. The energy content of the neutrinos will be wasted.
From this, several questions arise:
- Is the idea above even feasible - while the half value thickness for 0.5 MeV is not too bad, other annihilation reactions might release harder gamma radiation requiring more shielding
- How much energy is wasted via neutrinos?
This all ties into my ultimate question: Is a reactor as described above feasible? Are there more elegant solutions on the horizon? How will an actual matter-antimatter reactor work and what will it look like?
(AM containment is outside of the scope of this question (Which is admittedly a major handwave))