First Gadolinium Loading to Super-Kamiokande

TL;DR

This paper reports the first successful loading of gadolinium into Super-Kamiokande, enhancing neutron detection for neutrino research, with detailed procedures, system modifications, and calibration results demonstrating uniform Gd distribution and effective neutron capture.

Contribution

It introduces the first gadolinium loading process in Super-Kamiokande, including system modifications, loading procedures, and calibration confirming uniform Gd distribution and improved neutron detection.

Findings

Gd was uniformly distributed within 35 days.

Neutron capture time measured at 115 microseconds.

Gd concentration achieved was 111 ppm, matching expectations.

Abstract

In order to improve Super-Kamiokande's neutron detection efficiency and to thereby increase its sensitivity to the diffuse supernova neutrino background flux, 13 tons of $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ (gadolinium sulfate octahydrate) was dissolved into the detector's otherwise ultrapure water from July 14 to August 17, 2020, marking the start of the SK-Gd phase of operations. During the loading, water was continuously recirculated at a rate of 60 m$^3$/h, extracting water from the top of the detector and mixing it with concentrated $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ solution to create a 0.02% solution of the Gd compound before injecting it into the bottom of the detector. A clear boundary between the Gd-loaded and pure water was maintained through the loading, enabling monitoring of the loading itself and the spatial uniformity of the Gd concentration over the 35 days it took to reach the top of the detector. During the subsequent commissioning the recirculation rate was increased to 120 m$^3$/h, resulting in a constant and uniform distribution of Gd throughout the detector and water transparency equivalent to that of previous pure-water operation periods. Using an Am-Be neutron calibration source the mean neutron capture time was measured to be $115\pm1$ $\mu$s, which corresponds to a Gd concentration of $111\pm2$ ppm, as expected for this level of Gd loading. This paper describes changes made to the water circulation system for this detector upgrade, the Gd loading procedure, detector commissioning, and the first neutron calibration measurements in SK-Gd.