Speaker
Description
We measured $^9\mathrm{Li}$ isotopic nuclei produced by muon spallation using the data taken from 2020 to 2022 by the Super-Kamiokande detector with 0.011% gadolinium concentration in water. $^9\mathrm{Li}$ is a long-lived radioactive isotope with a lifetime of about 0.26 seconds. It emits an electron and a neutron at a branching ratio of 50.8%, which is difficult to distinguish from the inverse beta decay caused by anti-electron neutrinos. Therefore, $^9\mathrm{Li}$ is one of the main background sources. In this study, the energy spectrum of the electrons was measured with a threshold at 4.5 MeV which is lowered from the previous result with 7.5MeV threshold measured with pure water. We will report the measurement method and analysis status in this poster.