This work presents results on the study of mechanisms and performance of negative ion drift and gas gain in gas-filled radiation detectors with electronegative fill gases. Negative ions drift with slow drift velocity and the lowest possible (thermal limit) diffusion, which leads to relaxed requirements for readout electronics without sacrificing accuracy. Slow drift velocity and low diffusion are the bases for this work because these characteristics are highly desirable for space-based detectors and for low pressure Time Projection Chambers (TPCs) used to make direction sensitive searches for Galactic Dark Matter. The present work led to two major successes: (1) Nitromethane (CH3NO2) was discovered as a new electronegative fill gas for TPCs. Nitromethane anions drift even slower (mobility = 0.032 m2·T/V ·s) than the only other known capture agent, carbon disulfide (CS2, mobility = 0.036 m2·T/V ·s), and the measured longitudinal diffusion remains at the thermodynamic (lower) limit for fields up to 7 V/cmT. Nitromethane is of particular interest for X-ray photoelectric polarimeters in the 2-10 keV energy range because of its low atomic number (Z). (2) Using a Diethorn plot, the mechanism which initiates electron avalanches in electronegative gas mixtures was accounted for. The Diethorn plot parameter Emin, is the minimum field needed to start avalanche gain in a proportional counter. Electronegative gases were found to have extremely large pressure-reduced starting fields (10-50 times larger than electron gases) which are themselves approximately independent of pressure. This can only be accounted for by a collisional ionization mechanism leading to the release of electrons from the ions and subsequent development of gain through a normal Townsend avalanche. The collision cross-section (σc) calculated from drift velocity data allows an estimate of Emin to be made which agrees with the experimental findings (5 - 25 % difference). The light output from Townsend avalanches has been proposed as an alternative readout mechanism for TPCs. The light output from negative ion avalanches was investigated using a Negative Ion Drift Chamber (NIDC) with a Gaseous Electron Multiplier (GEM)-like device as the amplification structure, viewed through a quartz window by a photomultiplier tube. This study allowed an upper limit of light output to be assigned to several negative ion gas mixtures. Finally, a Micromegas NIDC was assembled to test the performance of negative ion mixtures in this gain device. The detector was stable and operated at gas gains of ∼ 104 in an electron gas mixture (argon-isobutane). However, with the two negative ion gases studied, CS2 and CH3NO2, high voltage breakdown and microphonic noise developed before appreciable gain was obtained. / Physics

The automotive industry, has been innovating in the field of materials development in order to meet the demand for lower emissions, improved passenger safety and performance. Despite various attempts of introducing other lightweight materials (Al, Mg or polymers) in car manufacturing, steel has remained as the material of choice till date due to its excellent adaptability to systematic upgradation and optimization in its design and processing. One of the outcomes is the development of second generation high Mn TWin Induced Plasticity (TWIP) steels with excellent strength-ductility balance suitable for automotive applications. Cost effective high performance TWIP steel design is mostly based on its alloy design and advanced up and down stream processing methods (thermomechanical controlled processing (TMCP)) which can help achieve suitable microstructure to meet the property requirements. It has been observed that grain boundary migration (GBM) in austenite during high temperature TMCP stage dictates grain growth to control the final microstructure. This research work initially investigates the grain growth in Fe-30%Mn steel within a temperature regime of 1000-1200°C. Compared to conventional low Mn steel, austenite boundary mobility in Fe-30%Mn was found to be 1-2 orders of magnitude smaller. Atom probe tomography results showed no Mn segregation at austenite high angle grain boundaries (γ-HAGB) which rules out the effect of Mn solute drag on growth kinetics in Fe-30%Mn steels. Grain boundary character distribution (GBCD) study showed that the sample consists of two different population of grain boundaries. 50% of the grain boundaries are random HAGBs with high mobility. Remaining 50% are special in nature which introduce low mobility boundary/boundary segments in the global boundary network. The special boundaries are mostly in the form of Σ3 CSL boundaries or its variants like Σ9, Σ 27. These boundary/ boundary segments were introduced by the formation of annealing twins and their interactions with the random HAGBs. An attempt to investigate the effect of Mn on growth kinetics at 1200°C showed that Mn slows down growth kinetics up to 15 wt% predominantly by the formation of annealing twins. A qualitative study of the microstructures showed that as Mn concentration is increased from 1% to 15%, the annealing twin density increases resulting in Σ3 frequency to be 30%. The increased twinning frequency is attributed to the effect of Mn on lowering the stacking fault energy (SFE). Annealing twins, belonging to Σ3 CSL family, intersect the HAGBs resulting into twin induced boundary segments which possess very low mobility. In the light of this idea, slow grain growth in high Mn steel was attributed to the population of low mobility boundaries. The proposed ‘twin inhibited grain growth’ model clearly points to the low mobility boundary/boundary segments to be the rate controlling factor during grain growth in high Mn steels. The effect of carbon on grain growth in Fe-30%Mn steel showed that the presence of carbon makes the growth kinetics faster by a factor of 4 and 6 at 1200°C and 1100°C respectively. Although, atom probe tomography results indicated that in presence of carbon, Mn segregation takes place at γ-HAGBs in Fe-30%Mn steel, solute drag does not appear to play a role as it was seen that with increase in Mn content beyond 1%, the solute effect of Mn in slowing down HAGB migration becomes weak. Also, abovementioned higher mobility values are obtained from the growth kinetics of Fe-30Mn-0.5C. This once again highlights the fact that effect of Mn in slowing down grain growth is due to the low mobility of twin/twin related boundaries or boundary segments. Controlling grain growth has been commonly proposed to be accomplished through small addition (<0.1%) of microalloying elements (Nb, V and Ti) which can slow down GBM at high temperature by solute drag and at low temperature by precipitate pinning (Zener drag). This research work has also experimentally quantified the solute drag of Nb in a series of Fe- 30%Mn steels. Grain boundary mobility was estimated for various temperatures and niobium contents. An attempt was made to calculate the grain boundary mobility in presence of niobium using Cahn’s solute drag model. This calculated mobility, when used in the proposed ‘twin inhibited grain growth’ model, the predicted growth kinetics which showed very good fit with the experimentally obtained growth kinetics in case of Fe-30Mn-0.03Nb and Fe-30Mn-0.05Nb steels at 1100°C. The effect of Nb solute drag, thus captured using Cahn’s model, was shown to be slowing down only the HAGB migration in the microstructure, whilst the special boundary mobility was not affected by solute Nb. Another attempt was made through grain boundary engineering (GBE) to control grain growth in Fe-30Mn-0.5C steel. Using different TMCP schemes, GBCD was modified to produce maximum frequency of special boundary. Preliminary studies on grain growth of single step-grain boundary engineered samples did show a significant lowering of grain size compared to a no-GBE sample after grain growth. However, the effect of iterative GBE didn’t show any significant effect in controlling grain growth in spite of the fact that it increased Σ3 frequency to 64%. This probably indicates that the effect of GBE on grain growth by the formation of annealing twins/special low mobility boundaries is a complicated process which might involve twin/special boundary morphology, annihilation kinetics and formation of grain clusters in the microstructure other than the formation of immobile special triple junctions through the intersection of twins/special boundaries with the random HAGBs. / Thesis / Doctor of Philosophy (PhD)

Carer-employees are defined as individuals who provide unpaid care to a disabled / ill dependent person(s) while working full-time in the paid labour force. In Canada, there are 6.1 million carer-employees, many of which are experiencing work-life balance struggles, which may result in ill-health. To minimize negative impacts, there is interest in developing caregiver-friendly workplace policies (CFWPs) as an intervention strategy to improve CEs’ work-life balance. However, the effectiveness of CFWPs are still in their infancy and often only focus on the work dimension. One of the most critical dimensions that have not yet been assessed is the activity-travel behaviour of carer-employees, which is largely impacted by the assisted-transport demands of their care-recipient. To contribute to filling in this gap, this dissertation addresses the following objectives: 1) develop an activity-travel behaviour profile of carer-employees using sociodemographic and caregiving characteristics; 2) identify spatial locations with potentially high assisted-transport demand while suggesting new areas to improve mobility independence of care-recipients, and; 3) create and apply a mixed-methods framework that classifies the actual activity-travel behaviour of carer-employees. The purposes of all three objectives are to: contribute to closing the literature gap; visually inform decision-makers and health planners, and; efficiently develop caregiver-friendly transport policies (CFTPs). Highlighted findings show that carer-employees conducting assisted-transport have lower income and are more likely to be tired and overwhelmed than those not performing the transport task (Objective 1). In Hamilton metropolitan area, 38% of the older adult population are not within immediate reach to a vital service, and another 15% are located in potentially high assisted-transport demand areas. Suggested areas for service implementation would improve access for older adults by 18% (Objective 2). Lastly, the framework has classified and ranked three types of activity-travel behaviours (Objective 3). All of these findings have led to the discussion of a multi-pronged implementation strategy for uptake of CFTPs. / Dissertation / Doctor of Philosophy (PhD)

This thesis presents machine and statistical learning approaches for sustainable planning in infrastructure and mobility systems. First, I have developed a convolutional neural network (CNN) to predict tree failure likelihood. Such assessments have traditionally been performed manually. I conduct a visual analysis of the predictions, indicating an approach for incorporating interpretability into model selection. Benchmarking the results against those produced by state-of-the-art CNNs, I show that a relatively simple model produces better results in a computational time that is three times faster. Via this novel framework, I demonstrate the potential of machine learning to automate and consequently reduce the costs of tree failure likelihood assessments in proximity to power lines, thereby promoting sustainable infrastructure. Secondly, I examine the effects of COVID-19 on mobility, segmented by transportation type, as well as social activity such as workplaces and residential, and their interdependencies. Using time series data across five continents, I estimate a Bayesian global vector autoregression model which explains patterns in activity and mobility trends and analyze their relationship with COVID-19 spread. I expect that the model framework and outcomes will guide policymakers to adopt appropriate measures to mitigate and safely recover from future disease outbreaks.

This study addresses the extent to which participation in the military affects subsequent status attainment and mobility in America. The purpose of this research was to conduct a comprehensive examination of existing empirical research resulting in a synthesis of findings and establishing a concise summary of the state of the literature on this topic. The study is limited to the examination of existing research on male veterans in the years between 1950 and the present. Findings from sixty-four articles and seven books are presented. The background characteristics of servicemen, the promotion and retention of servicemen, and the post-service earnings and education of servicemen, in the era of World War II, Korea, Vietnam, and the All-Volunteer Force are discussed. When possible, comparisons are made between servicemen and their civilian counterparts, as well as between Anglos and African Americans. Findings indicate that men serving in the military prior to Vietnam were from somewhat higher socio-economic and educational backgrounds. The background characteristics of Anglo veterans of Vietnam and the AVF were homogenous to their civilian counterparts. Black veterans of the period from the war in Vietnam through the present have slightly higher levels of education and income prior to service as compared to their civilian counterparts. Research on promotion, retention, and military occupational assignments demonstrate that blacks are more likely to enlist and re-enlist than are Anglos. Promotions are currently achieved at approximately the same rate regardless of race. However, throughout the period examined, blacks are more likely to be trained in military occupational specialties considered to be less transferable to the civilian workforce. Findings on post-service attainment are limited to the examination of income, earnings, and education. Military service resulted in higher levels of income, earnings, and education for all veterans serving since 1950, with the notable exception of Anglo Vietnam veterans. In other words, as compared to their civilian counterparts, veterans had significantly higher incomes, earnings, and educational levels post-service. Explanations for the association between military service and social mobility, including fluctuations in enlistment standards and educational benefits during the period under investigation, are presented. / Ph. D.

Today, regular school attendance is an important factor in school success (Rothman, 2001). Research has shown a direct correlation between good attendance and student achievement (Dekalb, 1999). Poor attendance has been linked to poor academic achievement (Ziegler, 1972). With the increase in accountability for school districts in Virginia surrounding the Standards of Learning (SOL) test and the No Child Left Behind (NCLB) legislation of 2001, educators are faced with a significant challenge to reduce the rate of absenteeism to increase students' achievement in school. “Students who are absent from school receive fewer hours of instruction; they often leave education early and are more likely to become long term unemployed, homeless, caught in the poverty trap, dependent on welfare and involved in the justice system” (House of Representatives, 1996 p. 3). Researchers have sought to find factors that contribute to student non-attendance (Odell, 1923; Reid, 1999; Mitchell, 1993). This study investigates the impact of student attendance, socio-economic status and mobility on student achievement of third grade students in two Title I schools in a Southeastern Virginia School District, with grades PK-3, as determined by the Virginia Standards of Learning (SOL) English and math tests scores. / Ed. D.

Current AlGaN/GaN Heterojunction Field Effect Transistors (HFETs) make use of multiple sources, drains, and gates in parallel to maximize transconductance and effective gain while minimizing the current density through each channel. To connect the sources to a common ground, current practice prescribes the fabrication of air bridges above the gates and drains. This practice has the advantage of a low dielectric constant and low parasitic capacitance, but it is at the expense of manufacturability and robust device operation. In the study described below, the air bridges in AlGaN/GaN HFETs were replaced by a polymer supported metallization bridge with the intention of improving ease of fabrication and reliability. The DC, high frequency, and power performance for several polymer step heights were investigated. The resultant structures were functional and robust; however, their electrical performance was degraded due to high source resistance. The cause of the high source resistance was found to be thinning of the metallization at the polymer step. The effect was more pronounced for higher step heights. / Master of Science

The discovery that negatively charged aggregates of C60 fullerene are stable in aqueous environments has elicited concerns regarding the potential environmental and health effects of these aggregates. Although many previous studies have used aggregates synthesized using intermediate organic solvents, this work primarily employed an aggregate production method that more closely emulates the fate of C60 upon accidental release into the environment — extended mixing in water. The aggregates formed via this method (aqu/nC60) differ from those produced using the more common solvent exchange methods. The aqu/nC60 aggregates are heterogeneous in size (20 nm and larger) and shape (angular to round), negatively charged, and crystalline in structure — exhibiting a face centered cubic (fcc) system. Solution characteristics such as aqu/nC60 aggregate size and concentration were found to be dependant upon preparation variables such as stirring time, initial C60 concentration, and initial particle size. Additional experiments indicate that aggregate charge, structure, and stability are highly dependant upon the identity of co-solutes (NaCl, CaCl2, sodium citrate) and their concentrations. Citrate concentrations greater than 0.5 mM resulted in the formation of very small (< 20 nm) spherical aqu/nC60 particles. At moderate citrate concentrations (~ 1 mM) a more negative surface charge was observed, which may be an indication of increased nC60 stability. In contrast, high concentrations of monovalent and divalent electrolytes result in aggregation and sedimentation of nC60 out of solution. Our research describes the effect that solution composition has on aggregate formation and stability, and suggests that C60 fate and transport will be a function of solution composition. / Master of Science

<p dir="ltr">This dissertation includes two independent chapters which investigated the relations between upward social mobility (USM) and mental health in the United States. Chapter 1 was a systematized review which identified, cataloged, and synthesized the existing literature on the relations between USM and mental health in the United States. Four electronic databases were searched (i.e., PsycINFO, PubMed, Scopus, and Web of Science) and, through an exhaustive screening and data extraction process, a sample of 15 studies were reviewed. Chapter 1 used narrative synthesis (i.e., tabulation and thematic analysis) to describe and synthesize patterns across the existing literature. Notably, review findings revealed great variability in both research methodology used by studies as well as study findings. Moreover, review findings suggested that the variability observed among study findings was likely influenced by a lack of consistent operationalization of social mobility. Finally, recommendations for future research were provided.</p><p dir="ltr">Chapter 2 was an empirical study that examined moderators and mediators of the relations between USM and mental health. Based on the status-based identity framework (SBI; Destin et al., 2017), I hypothesized that USM would relate to poorer mental health, that belonging would mediate the relations between USM and mental health, and that internalized classism, interpersonal classism, and social class centrality would moderate the mediation. Data from a sample of 331 U.S. adults were collected via Prolific. While study results supported the first two hypotheses, only interpersonal classism was found to moderate the mediation. Specifically, the conditional indirect effect showed that those who experienced high and moderate levels of interpersonal classism had a negative relation between USM and belonging. Finally, study limitations and implications for research and clinical practice are discussed.</p>

京都大学 / 新制・課程博士 / 博士(社会健康医学) / 甲第25209号 / 社医博第136号 / 新制||社医||13(附属図書館) / 京都大学大学院医学研究科社会健康医学系専攻 / (主査)教授 長尾 美紀, 教授 今中 雄一, 教授 山崎 渉 / 学位規則第4条第1項該当 / Doctor of Agricultural Science / Kyoto University / DFAM