Super-Kamiokande (SK) is a 50 kton water Cherenkov detector
located approximately 1 km beneath mount Ikenoyama, Gifu, Japan. While SK can reconstruct charged particle tracks over a wide energy range, the detection efficiency of neutrons is very low. Achieving efficient neutron tagging is useful in all analyses, from the observation of the diffuse supernova neutrino background for the first time, to proton decay studies and oscillation analyses.

SK gadolinium (SK-Gd) is the upgrade project to make neutron tagging efficient. After extensive studies, the SK collaboration approved the SK-Gd project on June 27 2015. In the second half of 2018 we refurbished the detector and in the first half of 2020 we added gadolinium to the SK tank for the first time. Here we briefly report the preparations that led to SK-Gd and then, more extensively, the Gd sulfate loading process.