Upgrade of Super-Kamiokande with Gadolinium

TL;DR

SK-Gd introduces gadolinium sulfate into ultrapure water to enable fast neutron tagging in inverse beta decay, dramatically reducing backgrounds that limit DSNB searches. The approach is validated by the EGADS demonstrator, followed by a major tank refurbishment and a radiopure Gd production and loading workflow to ensure uniform distribution. Early SK-VI and SK-VII results show high neutron capture efficiency and stable uniformity, enabling improved DSNB sensitivity and expanding the physics program to galactic and pre-supernova neutrinos, reactor, atmospheric, accelerator neutrinos, and proton decay. The advances provide a blueprint for future detectors and offer a realistic path toward DSNB discovery with wide astrophysical and particle-physics impact.

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.

Figures (11)

• Figure 1: Depiction of an inverse beta reaction in the Gd-loaded Super-Kamiokande detector.
• Figure 2: The EGADS experimental hall in the Kamioka mine.
• Figure 3: Sealant work on the SK tank wall conducted from a floating floor
• Figure 4: Schematic diagram of SK-Gd water system. This figure is taken from Ref. 1stload:2022.
• Figure 5: Time evolution of neutron capture events on gadolinium over 35 days during the first Gd loading. The horizontal axis represents the square of the radial position [m$^2$] in the cylindrical detector, and the vertical axis indicates the height [m].