β Decay of ¹⁰⁴Nb Verified with Total Absorption Spectroscopy

The Modular Total Absorption Spectrometer was used to measure the β decay of ¹⁰⁴Nb produced at the CAlifornium Rare Isotope Breeder Upgrade facility at Argonne National Laboratory. We report an updated half life of 5.9(3) s in disagreement with the previous measurements. Additionally, we provide evidence that the tentative, low spin assignment for the long-lived ¹⁰⁴Nb is inaccurate. The absence of ground-state-to-ground-state feeding suggests that long-lived ¹⁰⁴Nb is more likely high spin (≥2). The deduced β-strength pattern shows preferential feeding to high-lying states consistent with the Pandemonium effect and the aforementioned higher ground-state spin assignment. The newly measured β-feeding intensity increases the excess of reactor anti-neutrinos observed by Daya Bay in the 8 to 8.5 MeV anti-neutrino energy region from 24.4% to 28.5%.