Lowest reliable logft value for second-forbidden β transitions

The 3/2⁻→7/2⁻ 1.57-MeV β transition in ⁵⁹Fe decay is important for logft systematics because it has been considered to have the lowest logft value for any second-forbidden nonunique β transition. We have measured the intensity of the 1.57-MeV β group with an iron-free $\pi \sqrt{2}$ spectrometer. Our intensity value of 0.18 ± 0.04 per 100 decays leads to logft=11.15±0.11. This logft value strengthens the experimental bases for the "strong rule" proposed by Raman and Gove, that second-forbidden β transitions have logft≥11.0.[RADIOACTIVITY ⁵⁹Fe [from ⁵⁸Fe(n,γ)]; measured $E_{\beta }$, $I_{\beta }$. ⁵⁹Co deduced levels, Q, logft. $\pi \sqrt{2}$ spectrometer.]