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I have a limited understanding of nuclear reactions. I know of the decay in which a positron is emitted from a nucleus and a proton is converted back into a neutron. There is also another type of decay in which a helium atom is emitted without any electrons, known as alpha decay.

Is there any definitively known fact that would prevent a possibly undiscovered type of decay which emitted an anti-alpha particle similar to positron emission?

Also, if antimatter annihilates with matter upon contact, releasing 100% of its energy, what happens to these positrons during/after beta decay? Can we use the emission of antimatter particles to create useful energy?

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    $\begingroup$ Concerning the title question (v1): OP already mentions positrons in $\beta^+$ decay. $\endgroup$
    – Qmechanic ♦
    Commented Dec 9, 2018 at 9:43

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Baryon number, the number of nucleons minus the number of antinucleons, is conserved. So any process producing an anti-baryon like an antiproton or antineutron must produce at least another baryon. This takes energy. So the energy release from the decay must be larger than $m_pc^2 = 938.27208$ MeV. This is a lot: most decays are a few MeV, not in the hundreds. Since the nuclear binding energy is less than 9 MeV per nucleon for all stable nuclei, this makes it likely that this never happens in "normal" nuclei. Obviously, if we throw together an enormous amount of mass and energy we might get antimatter, but that is likely unlike what the question is about.

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  • $\begingroup$ When you say an enormous amount of mass and energy, does that refer to a lot of atoms being concentrated in a small area, or simply a lot of nucleons being concentrated within the same nucleus? Would some of the heavy elements we've discovered (ignoring their instant decay) have enough energy to do something like this? $\endgroup$
    – snowg
    Commented Dec 9, 2018 at 20:50
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    $\begingroup$ I was not really separating the cases. Compressing nuclei is hard (consider the mechanical properties of neutron stars) and could presumably add enough elastic energy so that when they are released and the nuclear droplet splits some of the involved nucleons could undergo reactions like $2p^+ \rightarrow 3p^+ + p^-$. Oganesson decays through an 11.6 MeV alpha partice, so it is still far below the required energy level. Smashing nuclei together is doable and heavy ion accelerators regularly reach GeV energies and do produce antimatter: physicsworld.com/a/rhic-nets-strange-antimatter $\endgroup$
    – Anders Sandberg
    Commented Dec 10, 2018 at 0:34

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