Philosophiae Doctor - PhD / Thoron Rn-220 is a radioactive gas with a half-life of 55.6 s. It has been identified as a possible health concern in specific places such as monazite processing plants and (rare-earth) mines. The short half-life of Rn-220 makes Rn-220 calibration sources and chambers less common than for the isotope Rn-222. There are many Rn-220 standard sources and chambers that are widely described in the literature and used for different applications and calibration. However, some of these chambers and sources are not easy to set up in typical nuclear environmental laboratories. In this project, we developed a Rn-220 standard source using a thorium nitrate solution (Th(NO3)4.6H2O). The solution was split into a large volume which was used in a Marinelli beaker to characterize its strength using a Hyper Pure Germanium (HPGe) detector, and a smaller volume of around 30 ml which was poured into a small bottle. The Rn-220 is extracted by bubbling air through the solution in the small bottle using an aerator. Gamma rays from the solution were measured simultaneously using a 76.2 mm × 76.2 mm NaI(Tl) detector. The gamma rays were measured for 66 hours. The accumulated spectra were thereafter analysed using an Excel spreadsheet where the counts in the Tl-208(2614 keV) peak were extracted and used to obtain the percentage of Rn-220 pumped out of the solution in the small bottle.

Philosophiae Doctor - PhD / Low- and medium-spin bands of 187Os have been studied using the AFRODITE array, following the 186W(4He,3n)187Os reaction at a beam energy of 37 MeV. The measurements of γ − γ coincidences, angular distribution ratios (RAD), polarization and γ-intensities were performed using eleven High Purity Germanium (HPGe) clover detectors. In the current work, all the previously known bands have been significantly extended and five new bands have been added to the level scheme. The observed bands are interpreted within the cranked shell model (CSM), cranked Nilsson-StrutinskyBogoliubov (CNSB) formalism and Quasiparticle-plus-Triaxial-Rotor (QTR) model. Systematic comparison of bands with the neighbouring isotopes has also been made. Comparison of the models with experimental data shows good agreement. The configurations of some of the previously observed bands have been modified. Most importantly, the coupling of 2+ γ band to the 11/2+[615] neutron configuration is observed for the first time.

京都大学 / 0048 / 新制・課程博士 / 博士(人間・環境学) / 甲第19817号 / 人博第788号 / 新制||人||189(附属図書館) / 27||人博||788(吉田南総合図書館) / 32853 / 京都大学大学院人間・環境学研究科相関環境学専攻 / (主査)教授 内本 喜晴, 教授 田部 勢津久, 教授 吉田 寿雄 / 学位規則第4条第1項該当 / Doctor of Human and Environmental Studies / Kyoto University / DFAM

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Neutron-rich rare-earth nuclei around the maximum of collectivity are predicted to exist with an extremely stable intrinsic configuration in their ground-state structure. Due to the high degree of axial symmetry and large deformation, these nuclei are also excellent candidates for having long-lived high-K isomers. The present work concerns a study of the structure of the yrast bands and a search for isomers in the neutron-rich 68Er isotopes. Excited states of 68Er isotopes were populated via multi-nucleon transfer reactions. A 859 MeV 136Xe-beam was used to bombard a 170Er-target. The experimental setup consisted of the high-resolution γ-ray spectrometer AGATA coupled to the heavy-ion magnetic spectrometer PRISMA. The experiment collected 2 TB of useful data corresponding to 3 days of effective beam time. Beam-like fragments were identified by the PRISMA specrometer placed at the grazing angle of 44 degrees. PRISMA allows for Z, A and q identification as well as TOF and velocity vector determination. This is required for the Doppler correction of the emitted γ rays detected in time coincidence with AGATA. A good Z and very clean A separation has been achieved in PRISMA. Making use of two-body kinematics, Doppler corrected γ-ray spectra for target-like fragments were obtained as well. Due to the novel techniques of PSA and γ-ray tracking, AGATA provided high-quality γ-ray spectra for both beam-like xenon and target-like erbium isotopes. Known yrast bands and isomeric states in neutron-rich erbium isotopes were observed. A candidate for the decay of an isomeric state with Eγ=184 keV  in 173Er, which has no previously known excited states, was identified.

During lithium-ion battery electrode fabrication, slurry drying conditions influence the resulting microstructure of electrodes. It has been found that the drying conditions can result in non-uniform cathode microstructures and material distributions. Accelerated drying, for example, is widely assumed to cause the binder in an electrode to migrate within the slurry, which can contribute to adhesion failure, and ultimately capacity fade and reduced battery life. While there are some conflicting studies regarding the aspects of accelerated drying that cause binder migration, there is not a widely used standard metric for measuring the gradient of binder across the thickness of an electrode. In this work, the vertical heterogeneity of electrodes, as measured using energy-dispersive X-ray spectroscopy (EDX), is correlated with different drying methods and rates. An improved metric for measuring the binder gradient in electrodes is proposed. For the electrodes in this study, binder migration is minimally affected by the drying method and the normalized binder gradient does not increase with increased drying rate. The results are compared to a drying physics model, and it is shown that further development of current models that predict binder gradient as a function of drying rate will need to be modified to more fully capture the physics of slurry drying.

Scanning Electron Microscopy combined with Energy Dispersive X-ray Spectroscopy (SEM-EDS) is a widely applied elemental microanalysis method. The integration of silicon drift detectors (SDDs) has notably enhanced EDS performance, enabling precise elemental identification due to its large sensitive area and low output capacitance.  Accurate simulations of SDDs can provide insights that enable the design and optimization of future models without the need for costly and time-consuming experimental iterations. Moreover, the current model-based quantification methods for EDS applications have reached their maximum predictive accuracy. As such, creating a more accurate simulation model could help achieve a higher level of precision in these quantification models, which would be immensely valuable for all EDS applications.  With this objective in mind, a simulation framework for modeling SDDs in EDS was developed based on Geant4, Allpix Squared, and COMSOL Multiphysics. The simulation encompasses the entire physics pipeline, including characteristic X-ray emission from the target sample and its absorption in the detector. The generated charge carriers within the detector are propagated through the internal electric field of the SDD, and their individual charge contribution is measured to simulate EDS spectra. The simulated model was compared to existing literature and in-house experimental measurements, showing strong agreement in the case of a well-tuned SDD. Limitations of the simulation framework are discussed, and further research to enhance accuracy and speed is explored.

The waters of the Indian Ocean southeast of Africa are a crucial junction for surface and deep ocean processes that serve as vital controls on Earth’s climate system. At the surface, the Agulhas Current, its retroflection, and Agulhas Leakage transfer water from the Indian and South Atlantic. The addition of this heat and salt to the Atlantic Basin helps drive the Meridional Overturning Circulation and the formation of deep water in the North Atlantic Ocean. On the timescales of centuries, the Meridional Overturning Circulation ultimately returns this water back to the Indian Ocean in the form of North Atlantic Deep Water. Proxy reconstructions show that the vigor of ocean overturning is immensely important to the global climate system, driving changes in atmospheric CO₂ concentrations and temperature and precipitation patterns across the planet. I use x-ray fluorescence core scanning, sediment provenance techniques, and core images from International Ocean Discovery Program Site U1474, located in the Natal Valley of the southwest Indian Ocean, to investigate past changes in the Agulhas Current and North Atlantic Deep Water. 40K/40Ar provenance ages measured on the clay fraction of sediment from Site U1474 indicate that, despite its great distance from the core site, the Zambezi is the most important factor influencing the deposition of terrigenous sediment in the Natal Valley. We present these results in a quantitative way, reinforcing the conclusions of previous studies. However, a comparison to newly available proxy records influenced by current speed and hydroclimate suggests that the strength of the Agulhas does not have a major influence on terrigenous sediment sources, at least at the headwaters of the Agulhas Current. Instead, I suggest that low-latitude hydrologic processes driven by zonal and meridional temperature gradients in conjunction with sea level are responsible for sediment source variability. In core photos, I found evidence for deep water variability in the Natal Valley in the form of millimeter-to-centimeter scale layers of olive-green sediment. To an overwhelming extent, these layers are formed during glacial periods, especially at their termination. I complement observations at Site U1474 with published proxy data for oxygen concentrations and measurements of total organic carbon percent in the Natal Valley and by extending our search for these green layers to core sites around the world. With these data, it is possible to confidently connect these layers to organic carbon concentrations in the sediment, reduction-oxidation processes in sediments following burial, and the local concentration of dissolved oxygen in the deep water. There are comparable fluctuations in the abundances of green layers in core sites in the path of North Atlantic Deep Water during glacial cycles, where more frequent and more intense green layer formation is driven by higher bottom water oxygen concentrations. Peaks in the abundance of green layers approximately 250 ka and 900 thousand years ago coincide with global scale excursions toward isotopically light benthic carbon isotopes. Connecting the green layers to the release of isotopically light organic carbon from sediments leads me to propose that long-observed fluctuations in the carbon cycle may be attributable to deep ocean oxygenation.

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