Speaker
Description
Cosmic-ray muons that penetrate the Super-Kamiokande detector generate hadron showers in water, producing unstable radioactive isotopes through spallation reactions. These isotopes are major background sources for neutrino observation at MeV scale and for the search for rare events. While Super-Kamiokande has started observation using ultra-pure water in 1996, gadolinium was loaded with 0.011wt% in 2020 aiming for the observation of diffuse supernova neutrino background for the first time. In this study, we measured ${}^9$Li isotope generated by the muon spallation. ${}^9$Li has a lifetime of about 0.26 seconds and emit an electron and a neutron with a branching ratio of 50.8%. These pairs of an electron and a neutron are difficult to distinguish from the inverse beta decay reaction caused by an electron antineutrino, and therefore become major background for DSNB searches.
In the data analysis, we selected ${}^9$Li event candidates by searching for pairs of low energy events following cosmic-ray muons. Before the gadolinium loading, the Super-Kamiokande experiment had an energy threshold of about 8 MeV for searching for the decay electrons from ${}^9$Li. In this study, the threshold was lowered to 5 MeV for the measurement by the reduction of the accidental background with the gadolinium loading. In this presentation, we will report on the measurement method and analysis status.
| Submitted on behalf of a Collaboration? | Yes |
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