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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))

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    $\begingroup$ I'm gonna tentatively suggest that you may want to move this over to the physics page. $\endgroup$
    – TCAT117
    Commented Sep 3, 2018 at 8:29
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    $\begingroup$ Isn't water only a couple orders of magnitude worse than lead for absorbing gamma rays, with the major advantage that it heats up and powers the steam engine directly without all that conducting-through-solid-metal-without-melting-it stuff to worry about? $\endgroup$
    – abarnert
    Commented Sep 3, 2018 at 8:43
  • $\begingroup$ Also, you're talking about using anti-hydrogen, not just a bunch of positrons. Isn't that going to mean lots of the energy goes into producing pions and other heavier particles that can be magnetically deflected, and then it's just a matter of capturing the energy from those pions decaying to muons? Yeah, there's still neutrino loss at every step, but it seems like the rest of the energy is a lot easier to control than just a sphere of gamma rays. $\endgroup$
    – abarnert
    Commented Sep 3, 2018 at 8:48
  • $\begingroup$ good point. pions will combine and decay, releasing energy in the form described above. But at what timescales? What does it mean for reactor design? I don't know! $\endgroup$
    – mart
    Commented Sep 3, 2018 at 8:50
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    $\begingroup$ Hello Mart. I'd like to point out that anyone who could give a detailed answer to this question wouldn't post it here, they'd post it at the Patent Office and in peer-reviewed scientific journals. Knowing this, could you clarify your expectations for an answer? Are you looking for generalizations based on current knowledge? Are you asking for plausible extrapolations based on non-existing knowledge? $\endgroup$
    – JBH
    Commented Sep 3, 2018 at 15:30

2 Answers 2

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No differently from a conventional nuclear reactor

How are the fission fragments in a U-235-fueled pressurized water reactor harvested and used to provide energy? Simple: Pump water through the reactor, and use it to soak up the kinetic energy of fission products as well as the gamma rays and other radiation byproducts. This heats up the water. Then through a heat exchanger, the pressurized water can transfer its energy to whatever thermal power generator you desire. On Earth, this is typically a steam generator.

When a positron and an electron annihilate, you get energy in the form of gamma rays. When a proton and antiproton annihilate, you get exotic meson products that quickly decay into the gamma rays, electrons, positrons, and neutrinos as you mentioned. Keep in mind that the produced positrons and electrons are then capable of annihilating, so in the end all you wind up with are gamma rays and neutrinos.

So write off the neutrinos, but gamma rays as a means of energy transfer are perfect for our purposes. Just surround the reaction chamber in water- it's not as good a radiation shield as lead, but it's easy to get, easy to pump, and for 500keV gamma rays has a half-value distance of just 7cm. A mere half meter of water is sufficient to absorb over 99% of all gamma ray emissions.

Your antimatter reactor doesn't need to be any more complicated than a reaction chamber surrounded by circulating water, hooked up to a heat exchanger and used to drive a steam turbine, thermopile, or any other means of generating power from heat. No need for lead enclosures with active cooling, or any kind of particle deceleration trap- this is just a nuclear reactor without all the complication of dealing with inherently 'dirty' fuel.

To be clear: Generating power from antimatter isn't conceptually difficult. It's producing antimatter, storing it safely, and then tapping into the storage mechanism in a controlled manner that currently render it science-fictional as a means of energy storage.

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    $\begingroup$ Since the question is tagged hard science (and would otherwise be a dup) and specifically asks how much energy is lost to neutrinos, I think this answer needs to cover than part to be complete. $\endgroup$
    – abarnert
    Commented Sep 3, 2018 at 18:41
  • $\begingroup$ I think this is all correct, I just want to point out that the exotic mesons travel about 21m in their short lifetime. Which puts a lower size on the reactor but no conceptual hurdle. However I am curios about the neutrinos. $\endgroup$
    – mart
    Commented Sep 4, 2018 at 7:20
  • $\begingroup$ As far a reactor out in space, the designs for fission/fusion in spacecraft suggested even by contemporaries do not rely on steam to generate power, but heat exchange with something more like a Stirling engine. $\endgroup$
    – dozTK421
    Commented Dec 10, 2020 at 4:16
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Make fuel using antimatter.

Usually we assume that matter meets antimatter and both disappear in a burst of energy (as laid out in OP). But in usual circumstances for our environment there will be a great excess of matter. Suppose we use little positrons and use them to react away the electrons associated with an atom. A given atom will find itself naked of electrons, highly (positively) charged and very reactive - fuel! Some of the energy of the matter-antimatter reaction is captured by pushing normal matter to a higher energy state.

These high energy fuel molecules you have made will be easier to store than antimatter, because everything is easier to store than antimatter. You can produce your fuel in a centralized structure suited for handling antimatter rather than toting antimatter around in your trunk. Also, a wide range of reactive fuel molecules might be generated by this process, suitable for varying applications.

We have long experience with engines driven by exothermic chemical reactions. My proposition: use antimatter in very small amounts to strip electrons and produce chemical reactivity in fuel molecules. Store the fuel molecules you have made. Then when needed, allow the fuel molecules to react, giving up the energy stored in them as heat, which is then captured and used in a steam engine.

But steam. How 19th century. Can we just use the antimatter to generate electricity. Well, we just did - by evaporating electrons off of our target molecule we have given it a tremendous positive charge. Electrons will flow to it. Down a wire. This is electrical current.

This is a novel idea as far as I know. Links to back reading, prior art, pithy comments all very welcome.

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  • $\begingroup$ I don't know, maybe black holes are harder to store than antimatter? $\endgroup$
    – Totillity
    Commented Sep 3, 2018 at 18:02
  • $\begingroup$ This is very inefficent since you are essentially only getting out the first ionization energy of the element used in the fuel (24.6 eV in the case of helium) which is much much less than you would get in terms of raw anti-matter. That being said you could use a similar system but using isotopes instead of ions. Using an anti-neutron to produce unstable alpha or beta emitters $\endgroup$
    – Ummdustry
    Commented Sep 3, 2018 at 18:05
  • $\begingroup$ @Ummdustry - that is slick. Antimatter alchemy. But if you reacted away a proton that is what you would be doing. $\endgroup$
    – Willk
    Commented Sep 3, 2018 at 18:47
  • $\begingroup$ Positron cloud is also highly positivly charged. And if you use anti-hydrogen - it would not prodice radicals. Antimatter does not brake law of charge conservation. $\endgroup$
    – ksbes
    Commented Nov 14, 2019 at 7:07

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