Decays of the Three Top Contributors to the Reactor νe High-Energy Spectrum, 92Rb, 96gsY, and 142Cs, Studied with Total Absorption Spectroscopy.

We report total absorption spectroscopy measurements of 92Rb, 96gsY, and 142Cs β decays, which are the most important contributors to the high energy νe spectral shape in nuclear reactors. These three β decays contribute 43% of the νe flux near 5.5 MeV emitted by nuclear reactors. This νe energy is particularly interesting due to spectral features recently observed in several experiments including the Daya Bay, Double Chooz, and RENO Collaborations. Measurements were conducted at Oak Ridge National Laboratory by means of proton-induced fission of 238U with on-line mass separation of fission fragments and the Modular Total Absorption Spectrometer. We observe a β-decay pattern that is similar to recent measurements of 92Rb, with a ground-state to ground-state β feeding of 91(3)%. We verify the 96gsY ground-state to ground-state β feeding of 95.5(20)%. Our measurements substantially modify the β-decay feedings of 142Cs, reducing the β feeding to 142Ba states below 2 MeV by 32% when compared with the latest evaluations. Our results increase the discrepancy between the observed and the expected reactor νe flux between 5 and 7 MeV, the maximum excess increases from ~10% to ~12%. [ABSTRACT FROM AUTHOR]