The operator for Fermi beta decay in terms of sums over the nucleons is $$ {\cal O}(F_{\pm}) = \displaystyle\sum _{k} t_{k\pm}. $$ The matrix element is $$ B(F) =\, \vert \langle f\vert T_{\pm} \vert i\rangle\vert ^{2}, $$ where $$ T_{\pm} = \sum _{k} t_{\pm} $$ is the total isospin raising and lowering operator for total isospin constructed out of the basic nucleon isospin raising and lowering operators $$ t_{-}\vert n\rangle=\vert p\rangle \hspace{1cm} t_{-}\vert p\rangle= 0, $$ and $$ t_{+}\vert p\rangle=\vert n\rangle, \;\; t_{+}\vert n\rangle= 0. $$