Upgrade of Super-Kamiokande with Gadolinium

TL;DR

Super-Kamiokande was upgraded with gadolinium sulfate to improve neutron detection, significantly enhancing its ability to observe supernova neutrinos and other astrophysical phenomena.

Contribution

This paper reports the successful implementation of gadolinium in Super-Kamiokande, enabling efficient neutron tagging and opening new avenues for neutrino and astrophysics research.

Findings

High neutron-tagging efficiency achieved

Reduced background levels in neutrino detection

World-leading limits set on diffuse supernova neutrino background flux

Abstract

Super-Kamiokande [SK] 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 beta decay events, the primary channel for detecting the diffuse supernova neutrino background [DSNB]. The upgrade also enhances sensitivity to galactic and pre-supernova neutrinos, as well as atmospheric neutrino interactions. To realize this capability, extensive work was performed, including the construction and operation of the EGADS 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, SK-Gd now stands at the threshold of discovering the DSNB and opens a wide range of new opportunities in astrophysics and neutrino physics.