Second gadolinium loading to Super-Kamiokande.

The first loading of gadolinium (Gd) into Super-Kamiokande in 2020 was successful, and the neutron capture efficiency on Gd reached 50%. To further increase the Gd neutron capture efficiency to 75%, 26.1 tons of Gd 2 (SO 4) 3 ⋅ 8H 2 O was additionally loaded into Super-Kamiokande (SK) from May 31 to July 4, 2022. As the amount of loaded Gd 2 (SO 4) 3 ⋅ 8H 2 O was doubled compared to the first loading, the capacity of the powder dissolving system was doubled. We also developed new batches of gadolinium sulfate with even further reduced radioactive impurities. In addition, a more efficient screening method was devised and implemented to evaluate these new batches of Gd 2 (SO 4) 3 ⋅ 8H 2 O. Following the second loading, the Gd concentration in SK was measured to be 333. 5 ± 2. 5 ppm via an Atomic Absorption Spectrometer (AAS). From the mean neutron capture time constant of neutrons from an Am/Be calibration source, the Gd concentration was independently measured to be 332.7 ± 6.8(sys.) ± 1.1(stat.) ppm, consistent with the AAS result. Furthermore, during the loading the Gd concentration was monitored continually using the capture time constant of each spallation neutron produced by cosmic-ray muons, and the final neutron capture efficiency was shown to become 1.5 times higher than that of the first loaded phase, as expected. [Display omitted] • We conducted additional loading of 26.1 tons of Gd 2 (SO 4) 3 ⋅ 8H 2 O to Super-Kmaioknade in 2022. • Developed new batches of gadolinium sulfate with reduced radioactive impurities. • Gd concentration in SK measured to be 333. 5 ± 2. 5 ppm by an AAS. • Gd concentration derived from the neutron capture time is consistent with AAS. • Demonstrated a 1.5 times higher neutron capture efficiency as expected. [ABSTRACT FROM AUTHOR]