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.

Figure 1. Selection-rule classifier and Kurie plot.

2 J_i1/2

2 J_f1/2

Δπ

Q (keV)1000

WHAT TO TRY

Set the spin change between initial and final state: a Fermi transition needs Delta J = 0 with no parity flip, Gamow-Teller allows Delta J = 0 or 1 (but not 0 to 0). The readout flags whether the decay is allowed and of which type.
Toggle the parity change: an allowed decay forbids it, so flipping parity forces the transition to be forbidden, suppressing the rate by orders of magnitude.
Read the Kurie plot: it is linear in electron energy and hits zero exactly at the endpoint Q. That straight line is how the neutrino mass is bounded from the spectrum shape near the endpoint.