Q.What is beta plus decay?

Radioactivity is the spontaneous transformation of an unstable ('parent') nucleus into a different ('daughter') nucleus, accompanied by the emission of some particle, discovered by Becquerel in 1896 and named by Marie Curie. Three distinct types occur. Alpha decay ejects a helium-4 nucleus (two protons plus two neutrons), reducing Z by 2 and A by 4, $_Z^AX\rightarrow\,_{Z-2}^{A-4}Y+\,_2^4He$ -- typical of very heavy nuclei (A > 210) whose large proton count makes electrostatic repulsion overwhelm the nuclear force's stabilising effect.

Beta decay involves one nucleon actually CHANGING TYPE: in $\beta^-$ decay a neutron converts to a proton, emitting an electron and an antineutrino ($n\rightarrow p+e^-+\bar\nu$), increasing Z by one with A unchanged; in $\beta^+$ decay a (bound) proton converts to a neutron, emitting a positron and a neutrino ($p\rightarrow n+e^++\nu$), decreasing Z by one with A unchanged -- notably, $\beta^+$ decay is only possible for a proton bound INSIDE a nucleus (which can supply the small extra energy needed from the rest of the nuclear binding energy), never for a free, isolated proton, since a free neutron's rest mass is slightly greater than a free proton's. …