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To observe supernova relic neutrino events, 13 tons of gadolinium sulfate octahydrate (Gd2(SO4)3·8H2O), corresponding to 0.01% Gd solution, was dissolved in the Super-Kamiokande water Cherenkov detector in 2020. The aim is to improve the detection efficiency of neutrons from inverse β decay involving electron antineutrinos. However, Gd3+ ions can be excited by the Cherenkov light from cosmic muons, and the subsequent emission at 312 nm is a possible background (BG) source for Cherenkov signal detection. In this work, we constructed an experimental setup based on time-resolved laser-induced luminescence spectroscopy to investigate the emission characteristics of Gd3+ ions in water. The excitation laser wavelength was tuned in the range of 245–255 nm, and large resonant peaks were observed at 246.2 nm and 252.3 nm with measured emission lifetimes of around 3 ms. Good linearity was observed between Gd concentration and emission intensity for these two wavelengths, indicating that our setup is useful for remote monitoring of Gd concentration. According to the simulation results using our spectroscopic data and reference values, the Gd3+ emission BG rate from cosmic muons is expected to be 10−1 counts/μs or less, which seems small but not negligible.

Numerous particle physics experiments utilize gadolinium (Gd), a rare earth element with the most significant neutron capture cross-section among all elements, to detect anti-neutrinos via inverse beta decays or to remove neutron-induced background events. For example, to load Gd into water Cherenkov detectors, Gd2(SO4)3 · 8H2O is dissolved and rare event search experiments are required to screen for radioactive impurities in Gd2(SO4)3 · 8H2O before dissolution. This study developed a new method to rapidly measure the radium-226 (226Ra) concentration in Gd2(SO4)3 · 8H2O. This method requires only 3 days to measure a batch of samples, as opposed to the usual method using high-purity germanium detectors, which takes approximately 20 days after arrival. The detection limit for the measurement of 226Ra concentration in Gd2(SO4)3 · 8H2O is 0.43 mBq/kg. This method has been already used for Gd2(SO4)3 · 8H2O screening at the Super-Kamiokande Gd (SK-Gd) project, and it can be applied to future experiments.

Abstract This paper investigates the removal of radon from purified and ambient airs by Ag-zeolite. Ag-zeolite is known to have very high performance in terms of airborne radon removal. The dependence on zeolite type and silver content of the performance of radon removal was evaluated. The performance of radon removal by a single pass and radon emanation were also evaluated. In addition, the adsorption performance due to zeolite solidification and the change in adsorption performance due to moisture in the gas were evaluated in order to investigate properties that should be considered for future practical use. These properties will be applicable to the development of air purification systems for large-volume ultra-low-radioactivity experiments.

Abstract The scintillation decay time constant of a ZnWO4 crystal irradiated with α-particles from 241Am was precisely investigated, and was found to depend on the incident direction of the α-particles on the crystal. The longest decay time constant (24.3 ± 0.6 μs) was obtained on the surface perpendicular to the b-axis of the crystal (surface B). On surfaces A and C, the decay constants were 20.0 and 21.3 ± 0.2 μs, respectively. The scintillation yield of ZnWO4 was also anisotropic and depended on the incident direction of the heavy particles. The maximum yield was achieved on surface B, suggesting a correlation between the light yield and the scintillation decay time constant of ZnWO4 crystals.

Radon is a common background source for underground astroparticle physics experiments and its removal is essential. In this study, we fabricated several prototype silver-ion exchanged zeolites, measured their radon adsorption properties, and evaluated their applicability for radon removal in underground experiments. As a result, we succeeded in producing a prototype silver-ion exchanged zeolite that showed better radon adsorption properties than preceding studies.

Neutron tagging is a fundamental technique for electron anti-neutrino detection via the inverse beta decay channel. A reported discrepancy in neutron detection efficiency between observational data and simulation predictions prompted an investigation into neutron capture modeling in Geant4. The study revealed that an overestimation of the thermal motion of hydrogen atoms in Geant4 impacts the fraction of captured nuclei. By manually modifying the Geant4 implementation, the simulation results align with calculations based on evaluated nuclear data and show good agreement with observables derived from the SK-Gd data.

Radioactivity from radon is a major threat for high-precision low-energy physics experiments like the ones in the Kamioka mine. We developed a new high-sensitivity radon monitoring system and conducted systematic radon concentration measurements for the first time in Kamioka. The system consists of portable radon detectors with a capacity of 1 L and new electronics based on a Raspberry Pi [Raspberry Pi documentation. (Available at: https://www.raspberrypi.org/documentation/)]. These radon detectors measure the radon in the air with electrostatic collection and a PIN photo-diode. We measured the absolute humidity dependence of the 1-L radon detector for air as $C_{F} (A_{H}) = (12.86 \\pm 0.40) - (1.66 \\pm 0.19) \\sqrt{A_{H}}$$\\mathrm{(counts/day)/(Bq/m^3)}$. The background level of the 1-L radon detector is $0.65\\pm0.15$ (stat.) counts/day. This corresponds to a detection limit of $\\sim0.4$ Bq/m$^3$ in a one-day measurement. Data were collected for a period of more than one year with twenty-one 1-L radon detectors in the Kamioka mine. They indicate seasonal and day–night variations in radon concentration within the mine. These results also allow us to confirm the stability of the new Raspberry Pi electronics.

We propose a VBR (voltage-boost rectifier) circuit based on the 0.18-μm Si CMOS (complementary metal oxide semiconductor) technology, which is designed for vibrational energy harvesters utilizing environmental vibrations. The VBR employs a single-end Dickson type charge-pump topology, and the circuit would be realized as a monolithic chip. The evaluation results obtained by multi-physics simulations on a circuit simulator reveal that the proposed circuit can deliver boosted DC voltage from the input of sub-threshold AC voltage.

This paper reports the development and detailed properties of about 13 metric tons of gadolinium sulfate octahydrate, $\\rm Gd_2(\\rm SO_4)_3\\cdot \\rm 8H_2O$, which has been dissolved into Super-Kamiokande (SK) in the summer of 2020. We evaluate the impact of radioactive impurities in $\\rm Gd_2(\\rm SO_4)_3\\cdot \\rm 8H_2O$ 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 $\\rm Gd_2(\\rm SO_4)_3\\cdot \\rm 8H_2O$. 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 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 $\\rm Gd_2(\\rm SO_4)_3\\cdot \\rm 8H_2O$ 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 including the Laboratorio Subterráneo 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 $\\rm Gd_2(\\rm SO_4)_3\\cdot \\rm 8H_2O$ contained one order of magnitude more 228Ra 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 228Ra for the next stage of gadolinium loading to SK, a new process has been successfully established.

A new ultra-low background high-purity germanium (HPGe) detector has been installed at the Kamioka underground experimental site. The background count rate in the energy range from 40 keV to 2700 keV is about 25% lower than that of the first HPGe detector installed in 2016, which has the same detector specification and similar shielding geometry. This paper describes the shielding configuration, including the cleaning of the material surface, the comparison of calibration data and simulation, the time variation of the background spectra, the sample measurement procedure, and some results of the radioactivity in the selected samples.