This'll be back to basics for many of you, but here's something I still don't get.
How can nuclear decay of an unstable atom both create and annihilate positron-electron pairs?
You have an unstable atom, say
Mg-22 (nucleon: 22, proton: 12), which decays and emits positrons and gamma rays. I get that the positrons impacting the electrons in the atom's electron shells cause annihilation of the pair. But the decay also creates positron-electron pairs. How?