Beta decay: Fermi vs Gamow-Teller

What you are seeing: selection rules for allowed beta decays. Fermi: $\Delta J = 0$, no parity change. Gamow-Teller: $\Delta J = 0, \pm 1$ (but not $0 \to 0$), no parity change. The Kurie plot at right is linear in $T = Q - E_e$ for any allowed transition.