Verify the isospin factor $\sqrt{2}$ in (12.19). Note that ${ }^{14} \mathrm{C}$, ${ }^{14} \mathrm{~N}^{*},{ }^{14} \mathrm{O}$ form an isospin triplet, which can be viewed as $\mathrm{nn}, \mathrm{np}, \mathrm{pp}$, together with an isospin zero ${ }^{12} \mathrm{C}$ core (see Fig. 2.2). Keep in mind that for indistinguishable proton decays, we must add amplitudes, not probabilities.