Abstract
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 91°µÍø 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%.
