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This paper reports the development and detailed properties of about 13 tonsof gadolinium sulfate octahydrate, $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$, whichhas been dissolved into Super-Kamiokande (SK) in the summer of 2020. Weevaluate the impact of radioactive impurities in $\rm Gd_2(\rm SO_4)_3\cdot \rm8H_2O$ on DSNB searches and solar neutrino observation and confirm the need toreduce radioactive and fluorescent impurities by about three orders ofmagnitude from commercially available high-purity $\rm Gd_2(\rm SO_4)_3\cdot\rm 8H_2O$. In order to produce ultra-high-purity $\rm Gd_2(\rm SO_4)_3\cdot\rm 8H_2O$, we have developed a method to remove impurities from gadoliniumoxide, Gd$_2$O$_3$, consisting of acid dissolution, solvent extraction, and pHcontrol processes, followed by a high-purity sulfation process. All of theproduced ultra-high-purity $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ is assayed byICP-MS and HPGe detectors to evaluate its quality. Because of the longmeasurement time of HPGe detectors, we have employed several undergroundlaboratories for making parallel measurements including LSC in Spain, Boulby inthe UK, and Kamioka in Japan. In the first half of production, the measuredbatch purities were found to be consistent with the specifications. However,inthe latter half, the $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ contained one orderof magnitude more $^{228}$Ra than the budgeted mean contamination. This wascorrelated with the corresponding characteristics of the raw materialGd$_2$O$_3$, in which an intrinsically large contamination was present. Basedon their modest impact on SK physics, they were nevertheless introduced intothe detector. To reduce $^{228}$Ra for the next stage of Gd loading to SK, anew process has been successfully establised.

Development of Ultra-pure Gadolinium Sulfate for the Super-Kamiokande Gadolinium Project