京都大学 / 新制・課程博士 / 博士(理学) / 甲第23018号 / 理博第4695号 / 新制||理||1673(附属図書館) / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)准教授 河上 哲生, 教授 田上 高広, 教授 下林 典正 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

Cytoplasmic dynein is a ubiquitous minus-end directed motor protein that is essential for a variety of cellular processes ranging from cargo transport to spindle and chromosome positioning. Specifically, in fission yeast during meiotic prophase, the fused nucleus follows the spindle pole body in oscillatory movements from one cell pole to the other. The three molecular players that are essential to this process are: (i) the motor protein dynein, which powers the movement of the nucleus, (ii) microtubules, which provide the tracts for the movement and (iii) Num1, the anchor protein of dynein at the cortex. Dyneins that are localized to the anchor protein at the cortex and simultaneously bound to the microtubule emanating from the spindle pole body, pull on that microtubule leading to the movement of the nucleus. The spindle pole body, by virtue of its movement establishes a leading and a trailing side. Previous work by Vogel et al. has elucidated the mechanism of these oscillations as that of asymmetric distribution of dynein between the leading and trailing sides. This differential distribution is a result of the load-dependent detachment of dynein preferentially from the trailing microtubules. This self-organization model for dynein, however, requires a continuous redistribution of dynein from the trailing to the leading side. In addition, dyneins need to be bound to the anchor protein to be able to produce force on the microtubules. Anchored dyneins are responsible for many other important processes in the cell such as spindle alignment and orientation, spindle separation and rotation. So we set out to elucidate the mechanism of redistribution of dynein as well as the targeting mechanism of dynein from the cytoplasm to cortical anchoring sites where they can produce pulling force on microtubules. By employing single-molecule observation using highly inclined laminated optical sheet (HILO) microscopy and tracking of fluorescently-tagged dyneins using a custom software, we were able to show that dyneins redistributed in the cytoplasm of fission yeast by simple diffusion. We also observed that dynein bound first to the microtubule and not directly to the anchor protein Num1. In addition, we were able to capture unbinding events of single dyneins from the microtubule to the cytoplasm. Surprisingly, dynein bound to the microtubule exhibited diffusive behaviour. The switch from diffusive to directed movement required to power nuclear oscillations occurred when dynein bound to its cortical anchor Num1. In summary, dynein employs a two-step targeting mechanism from the cytoplasm to the cortical anchoring sites, with the attachment to the microtubule acting as the intermediate step.

At the onset of cell division microtubules growing from spindle pole bodies (SPB) interact with each other to form the mitotic spindle enabling proper chromosome positioning and segregation. However, the exact mechanism of microtubule dynamics and microtubule associated proteins (MAPs) underlying spindle assembly is still not well understood. We developed an in vivo method to observe spindle assembly in the fission yeast Schizosaccharomyces pombe by inducing depolymerization of already formed and grown spindles by subjecting the cells to low temperatures, followed by subsequent repolymerization at a permissive temperature. We observed that microtubules pivot, i.e., perform angular movement around the SPB in a random manner, exploring the intranuclear space. Eventually microtubules extending from opposite SPBs come into contact and establish an antiparallel connection thus reassembling the spindle. Mutant approaches revealed that deletion of ase1 and klp5 did not prevent spindle reassembly, however introduced aberrations during the spindle formation. Amazingly, cut7p showed direct colocalization with microtubule overlap during spindle reassembly. Abrogation of cut7p led to inability to form a functional spindle. Thus, cut7p is the main regulator of spindle formation in fission yeast. None of the mutant strains affected microtubule pivoting, confirming that microtubule pivoting is a random movement unrelated to MAPs.

In this project, different fission models in the nuclear reaction code TALYS have been compared to GEF version 2020/1.2. The data included in the comparison are mass yield distributions, average prompt neutron energies per fragment mass, and average multiplicities of both neutrons and γ-rays per fragment mass. The reaction studied in the first part of the project is 1 keV neutron-induced fission of 235U. In the second part of the study, a variety of different nuclei and different incident energies were included in comparisons, but a limitation was set to only include neutron-induced fission. The results from the comparison suggested that TALYS fymodel 2 and 3 were less consistent with GEF than fymodel 4. For the comparisons with experimental data, fymodel 4 also performed better overall. TALYS fymodel 2 and 3 make use of implemented partial versions of GEF to produce fission fragment distributions, while fymodel 4 takes fission fragment distribution data from separate yieldfiles. A database of these yieldfiles with 737 different nuclei and 10 energy levels was produced, to be implemented in future versions of TALYS. The energy levels were chosen to get a range of energies that can be accurately interpolated between. This method of using TALYS fymodel 4 with a yieldfile from GEF consistently showed a strong agreement with GEF version 2020/1.2 for the mass yield distributions and the neutron multiplicities. The γ-ray multiplicities and the neutron energies show a slightly weaker agreement, and TALYS gives consistently smaller values than GEF for these quantities.

In the past decade, research in the field of singlet fission, the process in which one high energy singlet fission exciton forms two lower energy triplet excitons, has seen a resurgence as a process that has the potential to improve solar energy conversion efficiency and contribute to a push for renewable energy. While an impressive motivation, there is still much progress in terms of understanding the physics of the process as well as improving molecular design for actual applications that needs to be made before this motivation can be fully realized. Two significant current hurdles in this field are the extraction of the newly formed triplet excitons from their entangled triplet pair state before recombination, and the lack of stable chromophores with viable energetics for singlet fission and high triplet energies for application purposes. Over the past five years, we have addressed these issues with targeted molecular design. Only a couple of studies have successfully separated the triplet pair state in intramolecular singlet fission systems. We create an intramolecular singlet fission system, a PDI-pentacene-pentacene-PDI tetramer, in which a charge transfer state is utilized to separate an electronically entangled triplet pair. We have also shown that singlet fission can be controlled as well as actually induced in chromophores by employing molecular contortion to tune the energetics. With this work, we have contributed to the motivation of using singlet fission in real-life applications.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第19920号 / 理博第4220号 / 新制||理||1606(附属図書館) / 33006 / 京都大学大学院理学研究科生物科学専攻 / (主査)教授 古市 剛史, 教授 湯本 貴和, 教授 平井 啓久 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第21192号 / エネ博第366号 / 新制||エネ||72(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 野平 俊之, 教授 萩原 理加, 教授 佐川 尚 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

No description available.

1. A quantitive understanding of the nucleosynthesis process requires the knowledge of the production rates, the masses and the ?-decay characteristics of exotic neutron-rich nu- clei. Nuclear fission is a suitable method of producing such nuclei with masses from 60 - 150. Neutron-rich nuclei close to the r-process path can be produced via photo-fission at the Rossendorf superconducting linear accelerator of high brilliance and low emittance (ELBE) or by means of nuclear reactions at relativistic energies (for example at GSI). If the fission prod- ucts are identified and also their charge numbers are obtained, it will be principally possible to investigate their structure by means of beta-gamma spectroscopy. 2. For the purpose of fission-fragment detection a double time-of-flight (TOF) spectrometer has been developed. The key component of the TOF spectrometer is a TOF detector consisting of multichannel-plate (MCP) detectors with a position-sensitive readout, a foil for secondary electron (SE) production and an electrostatic mirror. The fission fragments are detected by measuring the SEs impinging on the position-sensitive anode after emission from the foil, ac- celeration and deflection by the electrostatic mirror. 3. In the first part of the work, special attention is paid to the relevant methods of building a spectrometer of such type. The functionality of the different detector components is proven in detail. A unique method for the determination of the SE foil thickness with ?-particles is pre- sented. Values for the mirror transmission and scattering are deduced. A dedicated SIMION 3D simulation showed that introducing serpentine like wires with pitch distance of 1 mm is capable of providing transparency of more than 90% without significant impact on the time resolution. 4. Since the performance of the MCP detectors is crucial, optimised schemes for their high- voltage supplies have been implemented successfully. Further enhancement of the setup was achieved by introducing surface-mount device (SMD) elements for signal decoupling, positioned close to the detector surface. Thus, we succeeded in avoiding signal deterioration coming from the additional capacitances and inductivities caused by extra cable lengths. Because the MCP signal decoupling takes place by means of rings with not well-defined impedance, impedance- matching problems arise, causing signal ringing and distortion. An approach towards solving this problem was to build a special fast, wide-band transimpedance amplifier. Using its circuit mounted close to the detectors, a significant reduction of the signal ringing was observed while maintaining the rise time of the detector signal. In order to process the multichannel-plate de- tector signals optimally, a new state-of-the-art constant-fraction discriminator (CFD) based on the amplitude and rise time compensated (ARC) technique with very low threshold capabilities and optimised walk properties has been developed and incorporated into the setup. 5. In our first laboratory test measurements conducted with an ?-particle source, we demonstrated ability of the setup to resolve pattern images placed directly in front of the MCP detector or reflected by the electrostatic mirror. The obtained position resolution for the second case is in the order of 2 mm. We showed that the detection efficiency of the system for ions like He is less than 30 %. This is mainly due to the low number of the electrons liberated from the SE foil. In a setup consisting of two mirror MCP detectors, we could successfully observe the TOF spectrum of a mixed (226Ra, 222Rn, 210Po, 218Po, 214Po) ?-source and found a good agreement with a SRIM simulation. 6. Measurements performed at the FZ Dresden-Rossendorf 5 MV tandem accelerator en- abled us to learn more about the response of the TOF detectors to various beams of heavy ions. The first in-beam experiments clearly showed that the applied setup consisting of two mirror detectors is capable of resolving different 35Cl beam charge states. In a combination with the specially designed wide-band amplifier and dedicated CFDs based on the ARC technique, we managed to achieve an in-beam time resolution of 170 ps per TOF detector. Measurements with ions of Z > 30 resulted in detection efficiencies of greater than 90%. At foil accelerating potentials approximately two times larger than the mirror deflection voltage, most of the SEs gain enough energy to pass through the electrostatic mirror without being deflected towards the MCP surface. Thus, an abrupt drop of the efficiency curve was observed - the “transparent” mode of the mirror. 7. Properties of electrons ejected from thin foils from heavy ions have been also investigated. From the MCP pulse-height-distribution spectra, a number for the forward-emitted SEs ejected by 35Cl beam was deduced. A method for obtaining widths of the SE energy distributions from the drop of the efficiency curve for various ions has been proposed. Assuming that the efficiency curve as a function of the accelerating voltage follows an error function, its standard deviation gives the standard deviation of the SE energy distribution. Another method based on the TOF technique for reconstructing the secondary electron velocity and energy distribution was also invented. It was found that the resulting mean SE velocity closely approaches the one of the beam ions. This phenomenon was attributed to the so-called “convoy” electrons. 8. The obtained position resolution for beams like 35Cl, 79Br and 107Ag at stable detection efficiency was better than 1.8 mm. It was demonstrated that with increasing the foil accelerat- ing voltage, the position resolution improves due to the minimised SE angular spread. Such a mode of operation was favoured until the mirror “semi-transparency” regime was reached, after which increasing further the accelerating potential could lead to a position resolution worsen- ing. An explanation of the fact could be the deterioration of the anode timing signals or some defocusing effects arising from the mirror wires field at high accelerating voltages. 9. Testing photo-fission experiments were performed at the bremsstrahlung facility at the ELBE accelerator. For the first time a spectrometer of this kind was successfully employed for bremsstrahlung-induced photo-fission measurements. The setup consisted of two mirror detectors (first arm) and a 80 mm diameter MCP detector (second arm) with a 238U target positioned in between. TOF measurements with two bremsstrahlung end-point energies of 12.9 and 16.0 MeV were carried out. A clear cut separation of the TOF peaks for the medium- mass and heavy fission fragments was observed. At these experimental test runs, we did not aim at one-by-one fission fragment mass resolution, since this may be the purpose of a more specific experiment utilising a much thinner fissile source than the one applied here (minimum straggling of the fragments inside the target is required) and considerably better statistics. It was possible to estimate the photo-fission production rate for the two measuring cases and to compare the obtained results with data from other measurements.

Covariant density functional theory (CDFT) is a modern theoretical tool for the description of nuclear structure physics. Here different physical properties of the ground and excited states in atomic nuclei have been investigated within the CDFT framework employing three major classes of the state-of-the-art covariant energy density functionals. The global performance of CEDFs for even-even nuclei are investigated and the systematic theoretical uncertainties are estimated within the set of four CEDFs in known regions of the nuclear chart and their propagation towards the neutron drip line. Large-scale axial relativistic Hartree-Bogoliubov (RHB) calculations are performed for even-even nuclei to calculate different ground state observabvles. The predictions for the two-neutron drip line are also compared in a systematic way with the non-relativistic results. CDFT has been applied for systematic study of extremely deformed, rotating N ∼ Z nuclei of the A ∼ 40 mass region. At spin zero such structures are located at high energies which prevents their experimental observation. The rotation acts as a tool to bring these exotic shapes down to the yrast line so that their observation could become possible with a future generation detectors such as GRETA or AGATA. The major physical observables of such structures, the underlying single-particle structure and the spins at which they become yrast or near yrast are defined. The search for the fingerprints of clusterization and molecular structures is performed and the configurations with such features are discussed. CDFT has been applied to study fission barriers of superheavy nuclei and related systematic theoretical uncertainties in the predictions of inner fission barrier heights in superheavy elements. Systematic uncertainties are substantial in superheavy elements and their behavior as a function of proton and neutron numbers contains a large random component. The benchmarking of the functionals to the experimental data on fission barriers in the actinides allows reduction of the systematic theoretical uncertainties for the inner fission barriers of unknown superheavy elements. However, even then they on average increase when moving away from the region where benchmarking has been performed.