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

Abstract This paper reports the development and detailed properties of about 13 metric tons gadolinium sulfate octahydrate, $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$, which has been dissolved into Super-Kamiokande (SK) in summer 2020. We evaluate impact radioactive impurities 8H_2O$ on diffuse supernova neutrino background searches solar observation confirm need to reduce fluorescent by three orders magnitude from commercially available high-purity 8H_2O$. In order produce ultra-high-purity we have developed a method remove oxide, Gd2O3, consisting acid dissolution, solvent extraction, pH control processes, followed sulfation process. All produced is assayed inductively coupled plasma mass spectrometry germanium detectors its quality. Because long measurement time detectors, employed several underground laboratories for making parallel measurements including Laboratorio Subterráneo de Canfranc Spain, Boulby UK, Kamioka Japan. first half production, measured batch purities were found be consistent with specifications. However, latter half, contained one more 228Ra than budgeted mean contamination. was correlated corresponding characteristics raw material an intrinsically large contamination present. Based their modest SK physics, they nevertheless introduced detector. To next stage loading SK, new process successfully established.