Measurement of the cosmogenic neutron yield in Super-Kamiokande with gadolinium loaded water

Cosmic-ray muons that enter the Super-Kamiokande detector cause hadronic showers due to spallation in water, producing neutrons and radioactive isotopes. These are a major background source for studies of MeV-scale neutrinos and searches for rare events. In 2020, gadolinium was introduced into the ultra-pure water in the Super-Kamiokande detector to improve the detection efficiency of neutrons. In this study, the cosmogenic neutron yield was measured using data acquired during the period after the gadolinium loading. The yield was found to be $(2.76±{}0.02(stat)±{}0.19(syst))×{}{10}^{{-}4} {{{μ}}}^{{-}1} {g}^{{-}1} {cm}²$ at an average muon energy 259 GeV at the Super-Kamiokande detector.