First gadolinium loading to Super-Kamiokande

In order to improve Super-Kamiokande’s neutron detection efficiency and thereby increase its sensitivity the diffuse supernova neutrino background flux, 13 tons of Gd2(SO4)3?8H2O (gadolinium sulfate octahydrate) was dissolved into detector’s otherwise ultrapure water from July 14 August 17, 2020, marking start SK-Gd phase operations. During loading, continuously recirculated at a rate 60 m3/h, extracting top detector mixing it with concentrated solution create 0.02% Gd compound before injecting bottom detector. A clear boundary between Gd-loaded pure maintained through enabling monitoring loading itself spatial uniformity concentration over 35 days took reach subsequent commissioning recirculation increased 120 resulting in constant uniform distribution throughout transparency equivalent that previous pure-water operation periods. Using an Am–Be calibration source mean capture time measured be 115±1 ?s, which corresponds 111±2 ppm, as expected for this level loading. This paper describes changes made circulation system upgrade, procedure, commissioning, first measurements SK-Gd.