Beta Decay

Beta ($\beta$) decay is the spontaneous emission of a negatively charged, high-speed electron -- the beta particle -- from the nucleus. Despite originating inside the nucleus (where, conceptually, a neutron converts into a proton plus an emitted electron), the beta particle carries the same charge and (negligible, compared to a nucleon) mass as an ordinary electron.

In general, when a parent nucleus $^{A}_{Z}X$ undergoes beta decay, it emits an electron, $^{0}_{-1}e$, and becomes a daughter nucleus $Y$ with the SAME mass number as the parent but an atomic number ONE unit GREATER: $^{A}_{Z}X \rightarrow\ ^{A}_{Z+1}Y + \,^{0}_{-1}e$. Two concrete examples from the source: neptunium-238 decays to plutonium-238, $238$-$93$-$Np \rightarrow 238$-$94$-$Pu + 0$-$(-1)$-$e$; and plutonium-241 decays to americium-241, $241$-$94$-$Pu \rightarrow 241$-$95$-$Am + 0$-$(-1)$-$e$. …