Measurement of cosmogenic 9Li in Super-Kamiokande with gadolinium loaded water
Super-Kamiokande is a large water Cherenkov detector located approximately 1,000 m underground in Kamioka, Japan. The detector is a cylindrical tank 39.3 m in diameter and 41.4 m high, filled with about 50 kton of gadolinium loaded water. We measured 9Li 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. Cosmic-ray muons that penetrate the detector form hadron showers in water and secondary hadrons produce unstable radioisotopes through the spallation. Among those spallation products, 9Li 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 electron antineutrinos. Therefore, 9Li is one of the main background sources in the observation of diffuse supernova neutrino background. 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 pure water. We will report the analysis method and results.
