Study of $\boldsymbolβ$ Decay Shape Factors in First-Forbidden Transitions with $\boldsymbol{ΔI^π= 0^-}$ for Reactor Antineutrino Spectra Predictions

The electron spectra of the $β$ decays of $^{92}$Rb and $^{142}$Cs, key contributors to the reactor antineutrino spectrum, were measured at the IGISOL facility using radioactive beams of high isotopic purity. The shapes of the measured $β$ spectra were compared with various $β$ shape models, including first-forbidden correction factors for $ΔI^π= 0^-$ ground-state to ground-state transitions. Comparisons with previous experimental results are also provided. The shapes of the newly measured $β$ spectra are well reproduced employing feedings extracted from total absorption gamma spectroscopy measurements.