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?