Isospin ladder strength in analogue beta decay ($|M_F{|}^2=T(T+1)-T_z(T_z\pm 1)$)

The Fermi transition operator is an isospin ladder operator. Acting between normalized members of one isospin multiplet gives the displayed squared matrix element. An edge transition in a $T=3/2$ multiplet has Fermi strength three, whereas the transition between the two $T=1/2$ mirror members has strength one. This fixes the vector contribution; nonzero-spin analogue transitions can also have a Gamow-Teller transition contribution that is not fixed by this counting.