$\beta$-decay studies of $A=107$ fission products with the Modular Total Absorption Spectrometer (MTAS)

Determination of the feeding intensities in $\beta$-decay of fission products is of high importance to address the reactor antineutrino anomaly and model the reactor decay heat. $\beta$-decay measurements with high-resolution but low-efficiency detectors may suffer from the Pandemonium effect, which leads to the misinterpretation of the feeding to high excited levels as the feeding to low-lying levels. Modular Total Absorption Spectrometer (MTAS), which has almost $99\%$ gamma detection efficiency, is an ideal spectrometer to determine the true $\beta$ feeding intensities free from Pandemonium effect. MTAS has been utilized to measure the beta decay pattern of several fission products that are high-priority contributors to reactor decay heat and antineutrino spectrum. In this talk, we will present some preliminary results of $A=107$ decays measured at CARIBU (ANL) in March, 2020. The $\beta$-branching of $^{107}$Mo, which is absent in current nuclear dataset, is determined experimentally. We found the $\gamma$ energy of $^{107}$Tc deposit in MTAS is increasing to about $\sim 2.2$ MeV, while the number calculated using current nuclear data is only $\sim 0.8$ MeV. This suggests a large shift of the antineutrino spectrum of $^{107}$Tc towards lower energy.