Development of ultra-pure gadolinium sulfate for the Super-Kamiokande gadolinium project

This paper reports the development and detailed properties of about 13 metric tons of gadolinium sulfate octahydrate, Gd-2(SO4)(3) middot 8H(2)O, which has been dissolved into Super-Kamiokande (SK) in the summer of 2020. We evaluate the impact of radioactive im-purities in Gd-2(SO4)(3) middot 8H(2)O on diffuse supernova neutrino background searches and solar neutrino observation and confirm the need to reduce radioactive and fluorescent impurities by about three orders of magnitude from commercially available high-purity Gd-2(SO4)(3) middot 8H(2)O. In order to produce ultra-high-purity Gd-2(SO4)(3) middot 8H(2)O, we have developed a method to remove impurities from gadolinium oxide, Gd2O3, consisting of acid dissolution, solvent extraction, and pH control processes, followed by a high-purity sulfation process. All of the produced ultra-high-purity Gd-2(SO4)(3) middot 8H(2)O is assayed by inductively coupled plasma mass spectrometry and high-purity germanium detectors to evaluate its quality. Because of the long measurement time of high-purity germanium detectors, we have employed several underground laboratories for making parallel measurements includ-ing the Laboratorio Subterr & aacute;neo de Canfranc in Spain, Boulby in the UK, and Kamioka in Japan. In the first half of production, the measured batch purities were found to be consistent with the specifications. However, in the latter half, the Gd-2(SO4)(3) middot 8H(2)O contained one order of magnitude more Ra-228 than the budgeted mean contamination. This was correlated with the corresponding characteristics of the raw material Gd2O3, in which an intrinsically large contamination was present. Based on their modest impact on SK physics, they were nevertheless introduced into the detector. To reduce Ra-228 for the next stage of gadolinium loading to SK, a new process has been successfully established.