1071 |
The inhomogeneities of (Pb,Bi)2223 superconducting tapes and their detectionVan Leeuwen, Suzanne January 1999 (has links)
No description available.
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1072 |
Dimensionally confined semiconductorsNayak, Rekha R. January 1998 (has links)
No description available.
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1073 |
A Mixed Methods Study of Class Size and Group Configuration in Online Graduate Course DiscussionsQiu, Mingzhu 01 September 2010 (has links)
Class size has long been recognized as a factor affecting achievement in face-to-face contexts. However, few studies have examined the effects of class size in online courses, or the effects of dividing an online class into smaller discussion groups. The current study examined the relationship between class size and the use of grouping strategies on note reading, note writing, and collaborative discussions in online graduate-level courses. This mixed-methods study analyzed tracking logs from 25 graduate-level online courses using Web Knowledge Forum (25 instructors and 341 students) and interviews from 10 instructors and 12 graduate students with diverse backgrounds. The quantitative and qualitative data analyses were designed to complement each other. Findings suggested 13 to 15 as an optimal class size and four to five as an ideal subgroup size. Not surprisingly, the results revealed that, as class size increased, the total notes that participants read increased significantly. However, as class size increased, the percentage of course notes that students read decreased significantly (i.e., students were reading a smaller proportion of the course notes). In larger classes, participants were more likely to experience information overload and students were more selective in the notes that they read. A significant positive correlation was found between class size and total notes written. Students’ note size and grade-level score were negatively correlated with class size. The data also suggest that the overload effects of large classes can be minimized by dividing students into small groups for discussion purposes. Interviewees felt that the use of small groups in large classes benefited their collaborative discussions. The preceding results underscore the importance of using small discussion groups when class sizes are large. The research concludes with a list of pedagogical recommendations and suggests new software features that may help enhance learning in online courses.
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THE ROLE OF COBALT-60 SOURCE IN INTENSITY MODULATED RADIATION THERAPY: FROM MODELING FINITE SOURCES TO TREATMENT PLANNING AND CONFORMAL DOSE DELIVERYDhanesar, SANDEEP 23 August 2013 (has links)
Cobalt-60 (Co-60) units played an integral role in radiation therapy from the mid-1950s to the 1970s. Although they continue to be used to treat cancer in some parts of the world, their role has been significantly reduced due to the invention of medical linear accelerators. A number of groups have indicated a strong potential for Co-60 units in modern radiation therapy. The Medical Physics group at the Cancer Center of the Southeastern Ontario and Queen’s University has shown the feasibility of Intensity Modulated Radiation Therapy (IMRT) via simple conformal treatment planning and dose delivery using a Co-60 unit.
In this thesis, initial Co-60 tomotherapy planning investigations on simple uniform phantoms are extended to actual clinical cases based on patient CT data. The planning is based on radiation dose data from a clinical Co-60 unit fitted with a multileaf collimator (MLC) and modeled in the EGSnrc Monte Carlo system. An in house treatment planning program is used to calculate IMRT dose distributions. Conformal delivery in a single slice on a uniform phantom based on sequentially delivered pencil beams is verified by Gafchromic film. Volumetric dose distributions for Co-60 serial tomotherapy are then generated for typical clinical sites that had been treated at our clinic by conventional 6MV IMRT using Varian Eclipse treatment plans. The Co-60 treatment plans are compared with the clinical IMRT plans using conventional matrices such as dose volume histograms (DVH). Dose delivery based on simultaneously opened MLC leaves is also explored and a novel MLC segmentation method is proposed. In order to increase efficiency of dose calculations, a novel convolution based fluence model for treatment planning is also proposed.
The ion chamber measurements showed that the Monte Carlo modeling of the beam data under the MIMiC MLC is accurate. The film measurements from the uniform phantom irradiations confirm that IMRT plans from our in-house treatment planning system are deliverable. Comparing the Co-60 dose distributions and DVHs to the IMRT plans from the clinic indicates that Co-60 is able to provide similar dose conformality to targets and dose sparing to critical organs. The results of the novel MLC segmentation algorithm and the photon fluence model proposed in this work compared well with the Monte Carlo calculations.
In summary, the investigations presented in this thesis confirm that Co-60 tomotherapy is indeed capable of providing state-of-the-art conformal dose delivery. We have shown that the perceived beam limitations often identified with Co 60 (e.g., lower penetration, source size artifacts under small field collimation, and larger penumbra) are negligible when using intensity modulated techniques. / Thesis (Ph.D, Physics, Engineering Physics and Astronomy) -- Queen's University, 2013-08-22 11:34:14.861
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1075 |
THE ROLE OF COBALT-60 SOURCE IN INTENSITY MODULATED RADIATION THERAPY: FROM MODELING FINITE SOURCES TO TREATMENT PLANNING AND CONFORMAL DOSE DELIVERYDhanesar, SANDEEP 23 August 2013 (has links)
Cobalt-60 (Co-60) units played an integral role in radiation therapy from the mid-1950s to the 1970s. Although they continue to be used to treat cancer in some parts of the world, their role has been significantly reduced due to the invention of medical linear accelerators. A number of groups have indicated a strong potential for Co-60 units in modern radiation therapy. The Medical Physics group at the Cancer Center of the Southeastern Ontario and Queen’s University has shown the feasibility of Intensity Modulated Radiation Therapy (IMRT) via simple conformal treatment planning and dose delivery using a Co-60 unit.
In this thesis, initial Co-60 tomotherapy planning investigations on simple uniform phantoms are extended to actual clinical cases based on patient CT data. The planning is based on radiation dose data from a clinical Co-60 unit fitted with a multileaf collimator (MLC) and modeled in the EGSnrc Monte Carlo system. An in house treatment planning program is used to calculate IMRT dose distributions. Conformal delivery in a single slice on a uniform phantom based on sequentially delivered pencil beams is verified by Gafchromic film. Volumetric dose distributions for Co-60 serial tomotherapy are then generated for typical clinical sites that had been treated at our clinic by conventional 6MV IMRT using Varian Eclipse treatment plans. The Co-60 treatment plans are compared with the clinical IMRT plans using conventional matrices such as dose volume histograms (DVH). Dose delivery based on simultaneously opened MLC leaves is also explored and a novel MLC segmentation method is proposed. In order to increase efficiency of dose calculations, a novel convolution based fluence model for treatment planning is also proposed.
The ion chamber measurements showed that the Monte Carlo modeling of the beam data under the MIMiC MLC is accurate. The film measurements from the uniform phantom irradiations confirm that IMRT plans from our in-house treatment planning system are deliverable. Comparing the Co-60 dose distributions and DVHs to the IMRT plans from the clinic indicates that Co-60 is able to provide similar dose conformality to targets and dose sparing to critical organs. The results of the novel MLC segmentation algorithm and the photon fluence model proposed in this work compared well with the Monte Carlo calculations.
In summary, the investigations presented in this thesis confirm that Co-60 tomotherapy is indeed capable of providing state-of-the-art conformal dose delivery. We have shown that the perceived beam limitations often identified with Co 60 (e.g., lower penetration, source size artifacts under small field collimation, and larger penumbra) are negligible when using intensity modulated techniques. / Thesis (Ph.D, Physics, Engineering Physics and Astronomy) -- Queen's University, 2013-08-22 11:34:14.861
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Surface Winds Affect the Movement of Water Currents and Entrained Zooplankton in a Depth Specific MannerBarth, Lauren Emily 24 June 2014 (has links)
We deployed depth-specific drifters in the western and eastern parts of the South Arm basin of Lake Opeongo and collected zooplankton samples at west and east fixed stations and at additional up- and downwind locations at three depths of the epilimnion under a range of wind conditions. Water currents had highest association with the immediate wind direction and the direction they travelled was dependent on wind strength. Along the main west-east fetch large zooplankton resided high in the epilimnion and were transported eastwards by strong surface currents where they accumulated. Small zooplankton were more uniformly distributed with depth and their accumulation patterns and transport mechanisms are less clear. Along shorter fetches oriented off-angle with the main one accumulations of zooplankton occurred at all downwind locations under heavy winds although the patterns are more variable and complex. These downwind accumulations likely create high quality habitat for warm water fish.
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Corporate Risk Disclosure: A Content Analysis of Swedish Interim ReportsKhaledi, Soheila January 2014 (has links)
The aim of this research is to examine the determinants of the level of corporate risk disclosure (CRD) in the interim reports of Swedish non-financial companies. A quantitative research approach is used, the sample data of which consist of 166 firms with 4,849 interim reports over a 10-year period. By utilizing the notion of risk and its definition, I have distinguished three categories of risk, namely risk as uncertainty, risk as threat and risk as opportunity. A systematic content analysis is conducted with the use of a software program, which is specifically designed for this purpose. The number of sentences that contain keywords related to the three risk categories is counted as the total CRD score, which is transformed to the disclosure index. I have examined the impact of firms’ characteristics and corporate governance mechanisms on the level of CRD based on agency theory. The ordinary least squares regression method with control for fixed year effects is used to analyse the data, which show that firm size and audit committee have a positive relationship with the level of corporate risk disclosure. The result demonstrates also that there is a negative relationship between family ownership and the level of CRD, and an insignificant relationship between leverage and the level of CRD.
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Exposure to tailings dust, the characterization thereof and the evaluation of current control measures / H. FourieFourie, Hanlie January 2007 (has links)
Background:
Tailings dam workers are exposed to tailings dust that has a potential health risk due to the
element contents in the dust. The extent of exposure of tailings dam workers is unknown.
Consequently, the elements, level of exposure, the potential adverse effects to health,
compliance to legislative requirements and the efficiency of the current control measures were
investigated.
Design and Method:
During the trials, 69 personal dust samples were collected during three full production shifts
from tailings dam workers in three different tailings dam environments. The field filters were
weighed to calculate the total dust concentrations. Bulk samples and field filters were analysed
using the 32-element and particle size distribution scans. The between-dam, between-days and
between-group variances were determined to assess the changes in exposure levels.
Results:
The tailings dam workers were not overexposed to tailings dust. Mean exposures were below
the OEL for PNOC, measuring 0,124 mg/m3 (TD 1) 0,366 mg/m3 (TD 2) and 2,956 mg/m3 (TD
3). Three outliers in the data were present in TD 3 and exceeded the action level for PNOC. A
major part of the particle size distribution consisted of inhalable sizes (>I0 μm). Cyclone
tailings dams have smaller particle sizes than spigot tailings dams, though 85% of the particles
in all tailings dams were larger than 10 μm. Some elements identified in the dust have potential
respiratory health effects. No significant differences existed between the three tailings dams
regarding tailings dust concentrations (p=0,527 on 5% significant level and p=0,292 on 10%
significant level). There was, however, a significant difference on a 5% significant level between
the days (p=0,003 and p=0,006). The workers on the cyclone tailings dams had smaller
exposures than workers on the spigot tailings dams, and the pipeline labourers were more
exposed than the mudguards.
The tailings dams did comply with legislative control measures. However, washing facilities for
the cleaning of overalls due to elements in the dust, such as nickel, chromium and manganese
need to be considered. Workers need to be educated in work practices and procedures,
personal hygiene and symptoms that can be experienced after exposure. Engineering control
measures such as water sprayers covering the entire tailings dam, the use of fixed or temporary
windscreens in the area where work is done and covering of the roads on the tailings dam with
gravel or water spray can be considered as dust suppressant controls.
Conclusion:
The study met the issues set out in chapter 1, hypotheses were rejected and accepted and
future studies were suggested. / Thesis (M.Sc. (Occupational Hygiene))--North-West University, Potchefstroom Campus, 2007.
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1079 |
Ion selectivity and membrane potential effects of two scorpion pore-forming peptides / D. ElgarElgar, Dale January 2005 (has links)
Parabutoporin (PP) and opistoporin 1 (OP1) are cation, a-helical antimicrobial peptides isolated
from the southern African scorpion species, Parabuthus schlechteri and Opistophthalmus carinatus,
respectively. Along with their antimicrobial action against bacteria and fungi, these peptides show
pore-forming properties in the membranes of mammalian cells. Pore-formation and ion selectivity in
cardiac myocytes were investigated by measuring the whole cell leak current by means of the patch
clamp technique. Pore-formation was observed as the induction of leak currents. Ion selectivity of
the pores was indicated by the shift of the reversal potential (E,,,) upon substitution of intra (K' with
CS' and CI- with aspartate) and extracellular (Na' with NMDG') ions. Results were compared with
the effect of gramicidin A used as a positive control for monovalent cation selective pores. PP and
OP I induced a fluctuating leak current and indicate non-selectivity of PP and OP1-induced pores.
An osmotic protection assay to determine estimated pore size was performed on the cardiac
myocytes. PP and OP1-induced pores had an estimate pore size of 1.38-1.78 nm in diameter.
The effect of PP and OP1 on the membrane potential (MP) of a neuroblastoma cell line and cardiac
myocytes was investigated. TMRM was used to mark the MP fluorescently and a confocal
microscope used to record the data digitally. The resting membrane potential (RMP) of the
neuroblastoma cells was calculated at -38.3 f 1.9 mV. PP (0.5 uM) and OP1 (0.5-1 uM) depolarized
the entire cell uniformly to a MP of -1 1.9 k 3.9 mV and -9.4 k 1.9 mV, respectively. This occurred
after 20-30 min of peptide exposure. In the case of the cardiac myocytes depolarization was induced
to -39.7 f 8.4 mV and -32.6 f 5.2 mV by 0.5-1 uM PP and 1.5-2.5 uM OPl, respectively. / Thesis (M.Sc. (Physiology))--North-West University, Potchefstroom Campus, 2006.
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A study on indium joints for low-temperature microelectronics interconnectionsCheng, Xiaojin January 2011 (has links)
For microelectronics used in the low-temperature applications, the understanding of their reliability and performance has become an important research subject characterised as electronics to serve under the severe or extreme service conditions. Along with the impact from the increased miniaturization of devices, the various properties and the relevant thermo-mechanical response of the interconnection materials to temperature excursion at micro-scale become a critical factor which can affect the reliable performance of microelectronics in various applications. Pure indium as an excellent interconnection material has been used in pixellated detector systems, which are required to be functional at cryogenic temperatures. This thesis presents an extensive investigation into the thermo-mechanical properties of indium joints as a function of microstructure, strain (loading histories-dependent) and temperature (service condition-sensitive), specifically in the areas as follows: (i) the interfacial reactions and evolution between indium and substrate during the reflow process (liquid-solid) and thermal aging (solid-solid) stages by taking low-temperature cycling into account; (ii) determination of the effects of joint thickness and the types of substrate (e.g. Cu or Ni) on the mechanical properties of indium joints, and the stress- and temperature-dependent creep behaviour of indium joints; (iii) the establishment of a constitutive relationship for indium interconnects under a wide range of homologous temperature changes that was subsequently implemented into an FE model to allow the analysis of the evolution of thermally-induced stresses and strains associated with a hybrid pixel detector.
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