Upgrade of Super-Kamiokande with Gadolinium

The Super-Kamiokande (SK) detector was upgraded through the addition of gadolinium sulfate to its ultrapure water, initiating the SK-Gd program. This development enables efficient neutron tagging via the large capture cross section of gadolinium, greatly improving the identification of inverse β decay events, the primary channel for detecting the diffuse supernova neutrino background (DSNB). The upgrade also enhances sensitivity to Galactic and presupernova neutrinos as well as atmospheric neutrino interactions. To realize this capability, extensive work was performed, including the construction and operation of the EGADS (Evaluating Gadolinium's Action on Detector Systems) demonstrator, the refurbishment of the SK tank, the development of radiopure gadolinium production methods, and the validation of the loading and uniformity of gadolinium in solution. Early SK-Gd operation has demonstrated high neutron-tagging efficiency, reduced backgrounds, and world-leading limits on the DSNB flux. With these advances, the SK-Gd project is at the threshold of discovering the DSNB and opens a wide range of new opportunities in astrophysics and neutrino physics.