Thesis--Auburn University, 2009. / Abstract. Vita. Includes bibliographic references (p.36).

University of Macau / Faculty of Science and Technology / Department of Mathematics

The response of mammalian cells to ionizing radiation has been extensively studied with single cells exposed to acute doses. Little information is, however, available for cells growing in tissues, especially for cells subjected to multiple exposures. Our aim in this thesis was therefore to use a more complex in vitro system, three dimensional spheroids grown from V79-171b Chinese hamster lung cells, to determine the role of repair, redistribution and repopulation during multifraction irradiation. Repair and redistribution effects were isolated by using spheroids under normal culture conditions of 37°C, or at 22°C where repair occurs but cell proliferation is markedly inhibited. As expected, we found that cells surviving an initial 8 Gy dose showed cell cycle dependent fluctuations in radiosensitivity when allowed to progress at 37°C before exposure to a second 8 Gy dose. Sublethal radiation damage was repaired more rapidly at 37°C than at 22°C, and was also affected by proliferation. Due, however, to the small proliferating population in the spheroid system, a large initial dose was required to produce a population with enough synchrony for the expected split dose survival fluctuations throughout the cell cycle to be observed. When two doses of 6 Gy separated by 4 hours were administered to spheroids, the subsequent cellular radiosensitivity to a third dose remained quite constant for at least 10 hours, indicating a more extended mitotic delay than observed in the two dose experiments. Mitotic delay consequently was not linear with dose, a result apparently dependent upon the fractionation scheme used, and the complexity of the multicell system. In multifraction schedules where doses of 6 Gy or 8 Gy were administered daily for 6 days, we found, as expected, that repair, redistribution and repopulation all affected cell viability. However, each effect dominated at different times throughout the experiments. The overall cytotoxicity for each 6 Gy fraction decreased with increasing fraction number, while the 8 Gy fraction survival remained fairly constant. A novel feature of our experimental design, administering each 6 Gy or 8 Gy fraction in 1-2 Gy increments, also allowed evaluation of successive responses to the clinically relevant dose of 2 Gy. Cell survival at that level fluctuated greatly due to a decreasing repair capacity, 'and an increasing effect of repopulation with fraction number. Using two radioactive Iridium sources of different activities, high dose rate fractionated exposure was compared to continuous low dose rate irradiation. Also, the linear quadratic model was used to predict the equivalent doses. We found that the model did not provide a good prediction; more repair of radiation induced damage was observed at the lower dose rates than the higher dose rates, an effect which could not be incorporated into this theoretical model. We conclude that, with fractionated radiation exposures to the spheroid system, repair, redistribution, repopulation and cell killing all contribute to the multifraction responses. Each has varying significance on each fraction. An equal effect per fraction, often implicit in radiotherapy regimens, is therefore only achieved in the fortuitous situation where repair, redistribution, repopulation and cell killing combine in different proportions to result in the same overall survival. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Because signals that are simultaneously concentrated in time and frequency are common in geophysics, it is desirable to develop a set of basis functions with these properties to perform various types of data processing. The Prolate Spheroidal Functions (PSF's) form a complete orthonormal set, and the low-order PSF's span the space of functions which are simultaneously concentrated in time and frequency. In this study, the PSF's are utilized in three different data processing problems: spectrum estimation, signal-to-noise ratio enhancement, and wavelet estimation. All three problems are related by functions which are approximately time- and bandlimited: data tapers for spectrum estimates, time- and bandlimited signals, and seismic wavelets can all be expressed as linear combinations of the low-order PSF's. Some of the results obtained by applying the PSF's to solving these problems are encouraging. In the problems of spectrum estimation and wavelet estimation in particular, the PSF's seem to extract much of the information present in the data. The application of PSF's to solving problems in geophysical data procesing should be the focus of further research in the future. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

In this thesis I present our recent work on gamma-ray searches for dark matter with the Fermi Large Area Telescope (Fermi-LAT). We have targeted dwarf spheroidal galaxies since they are very dark matter dominated systems, and we have developed a novel joint likelihood method to combine the observations of a set of targets. In the first iteration of the joint likelihood analysis, 10 dwarf spheroidal galaxies are targeted and 2 years of Fermi-LAT data is analyzed. The resulting upper limits on the dark matter annihilation cross-section range from about 10−26 cm3 s−1 for dark matter masses of 5 GeV to about 5 × 10−23 cm3 s−1 for dark matter masses of 1 TeV, depending on the annihilation channel. For the first time, dark matter models with a cross section above the canonical thermal relic cross section (∼ 3 × 10−26 cm3 s−1) are strongly disfavored by a gamma-ray experiment. In the second iteration we include 15 dwarf spheroidal galaxies in the combined analysis, employ 4 years of data and an improved calculation of the dark matter density. The obtained upper limits range from about 10−26 cm3 s−1 for dark matter masses of 2 GeV to about 10−21 cm3 s−1 for dark matter masses of 10 TeV, depending on the annihilation channel. I briefly describe some of the evidence for dark matter, the Fermi-LAT instrument and public data releases, dwarf spheroidal galaxies, likelihood analysis, and results from analyses of Fermi-LAT data. I also document some of the tests made to verify the method and to compare different analysis setups.

The behavior of bimodal suspensions of oblate spheroids undergoing simple shearing flow has been described by analyzing the frequency spectra of orientation-dependent macroscopic properties. In particular, the frequency spectra of rheo-optical transients in turbidity and angular light scattering obtained experimentally from mono- and bimodal suspensions of human and frog erythrocytes showed peaks corresponding to the dominant frequencies of the particles in the flow. The method can, in principle, be applied to multimodal dispersions to obtain information on particle geometry. / The effect of high polydispersity on the rheo-optical transients of sheared suspensions have been studied. The time-dependent behavior of such systems is neither determined by the period of rotation defined by the mean axis ratio nor by the mean period of rotation amongst particles, but by the most probable period of rotation (i.e. the value corresponding to the maximum in the distribution of periods of rotations). Rheo-optical transients in polydisperse kaolin (clay) suspensions have been experimentally obtained. Here the oscillations, in contrast to the slightly polydisperse suspensions studied previously, are highly damped. Frequency spectra obtained from these experiments have peaks near 2/T(,m) , where T(,m) is the most probable period in the ensemble. / Finally, the influence of the rotary Brownian motion on the rheo-optical transients of sheared spheroidal suspensions is considered by using simulation procedures such as those employed previously to analyze rheological properties and memory loss. The rotary Brownian motion induced two effects: damping of oscillations and a shift in the extrema of the oscillations resulting in variation in the apparent mean period of rotation. Experiments with human erythrocytes at low Brenner number confirmed such behavior. Also a system with strong rotary Brownian motion was observed for which the rheo-optical transients showed monotonic changes. / The observations made in this work contribute to a better understanding of the transient rheo-optical properties of flowing suspensions.

Dissertation (Ph.D.)--Auburn University, 2007. / Abstract. Includes bibliographic references (p.58-59).

The earth's gravity field G * at a point P in the region surrounding the earth's surface is defined as the force acting on a unit mass concentrated at P. This is a force resulting from two components: (1) G1 due to the gravitational attraction of the earth's mass, and (2) G2 due to the earth's rotation.

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