This thesis is about the growth of ZnO thin films for optoelectronic applications. ZnO thin films were grown using plasma assisted molecular beam epitaxy and were studied using various conventional and novel characterisation techniques. The significance of different growth variables on growth efficiency was investigated. The growth rate of ZnO films was found to be linearly dependent on the Zn flux under O-rich growth conditions. Under Zn-rich conditions, the growth rate was dependent on both atomic and molecular oxygen flux. By characterising the oxygen plasma generated using different RF power and aperture plate designs and correlating the results with the growth rates observed, it was found that atomic oxygen was the dominant growth species under all conditions. Molecular oxygen also participated in the growth process, with its importance dependent on the aperture plate design. In addition, an increase in growth temperature was found to monotonically decrease the growth rate. A growth rate of 1.4 Å/s was achieved at a growth temperature of 650 ℃ by using an oxygen flow rate of 1.6 standard cubic centimetres utilising a plasma source with a 276 hole plate operating at 400 W, and a Zn flux 1.4✕10¹⁵ atoms/cm²⋅s. Characterisation of the MBE grown thin films revealed that the qualities of ZnO thin films were dependent on the growth conditions. Experimental evidence suggested that a maximum adatom diffusion rate can be achieved under Zn-rich conditions, giving samples with the best structural quality. O-rich conditions in general led to statistical roughening which resulted in rough and irregular film surfaces. Experimental results also suggested that by increasing the atomic oxygen content and decreasing the ion content of the plasma, the excitonic emission of the ZnO thin films can possibly be improved. It was also found that the conductivity of the films can possibly be reduced by increasing the plasma ion content. By investigating the evolution of the buffer layer surface during the early stages of growth, dislocation nucleation and surface roughening were found to be important strain relief mechanisms in MBE grown ZnO thin films that affected the crystal quality. The usage of LT-buffer layers was found to improve substrate wetting, and was shown to significantly reduce dislocation propagation. Further strain reduction was achieved via the application of a 1 nm MgO buffer layer, and a significant reduction of carrier concentration and improvement in optical quality was subsequently observed. A carrier concentration of <1✕10¹⁶ cm⁻³ and a near band emission full width half maximum of 2 meV was observed for the best sample. The study of electrical characteristics using the variable magnetic field Hall effect confirmed the existence of a degenerate carrier and a bulk carrier in most MBE grown ZnO thin films. The bulk carrier mobility was measured to be ~120 - 150 cm²/Vs for most as-grown samples, comparable to the best reported value. A typical bulk carrier concentration of ~1✕10¹⁶ - 1✕10¹⁸ cm⁻³ was observed for as-grown samples. Annealing was found to increase the mobility of the bulk carrier to ~120 - 225 cm²/Vs and decrease the bulk carrier concentration by two orders of magnitude. Using time resolved photoluminescence, it was found that the radiative recombination in MBE grown ZnO thin films was dominated by excitonic processes, and followed a T³⁄² trend with temperature. A maximum radiative lifetime of 10 ns was observed for as-grown samples. The non-radiative lifetime in ZnO thin films was dominated by the Shockley-Read-Hall recombination processes. The modelling of the temperature dependence of the non-radiative lifetime suggested that an electron trap at ~0.065 eV and a hole trap at ~0.1 eV may be present in these samples. The application of time resolved photoluminescence also allowed the direct observation of carrier freeze-out in these ZnO films at low temperature.

Thin films of Teflon AF have been directly patterned by electron-beam lithography without the need for post exposure chemical development. The relationship between pattern depth and exposure dose was found to be linear over a wide range of doses. Pattern depth was also observed to be dependent on initial film thickness. Teflon AF can be directly patterned at doses similar to typical e-beam resists. High resolution features as small as ~200 nm have been resolved. FTIR measurements revealed that CF3 and fluorinated dioxole groups play a significant role in the patterning mechanism. Teflon AF films also exhibited an increase in refractive index upon exposure to the electron-beam. This property has been exploited in waveguiding applications. Waveguides in Teflon AF were patterned using direct electron beam lithography technique. Waveguides were clearly visible to the naked eye. Characterization in the visible region showed evidences of light guiding through the waveguides. However light could not cross the entire chip. Characterization in the infrared region revealed the slab mode even though individual waveguides were not detected.

There have been very few measurements of the total cross section for n-p scattering below 500 keV. In order to differentiate among NN potential models, improved cross section data between 20 and 600 keV are required. We measured the n-p and n-C total cross sections in this energy region by transmission; a collimated neutron beam was passed through CH2 and C samples and transmitted neutrons were detected by a BC-501A deuterated liquid scintillator. Cross sections were obtained by taking the ratios of normalized neutron yields with the samples in the beam and with no sample in the beam. Both better precision and larger range between 90 keV and 1.8 MeV results are presented. The parameters resulting from fitting effective range theory to the data for n-p scattering are in good agreement with parameters determined from previous fits.

Die vorliegende Arbeit beschäftigt sich mit der Oberflächenglättung und selbstorganisierten Musterbildung auf Si(100) und Ge(100) durch Beschuss mit niederenergetischen Edelgasionen (Ne, Ar, Kr, Xe). Die Untersuchungen wurden für Ionenenergien zwischen 400 eV und 2000 eV für Ioneneinfallswinkel von 0° bis 85° durchgeführt. Zudem wurde die zeitliche Entwicklung spezifischer Erosionsformen durch die Variation der Fluenz über zwei Größenordnungen analysiert. In den Experimenten finden sich deutliche Anzeichen einer Facettierung sowie einer Vergröberung der Strukturen mit zunehmender Erosionszeit. Diese Beobachtungen deuten darauf hin, dass von Beginn an gradientenabhängiges Zerstäuben und die Reflexion von Primärionen einen wesentlichen Einfluss auf die Strukturentwicklung haben. Die Ergebnisse werden im Kontext bestehender Musterbildungsmodelle diskutiert.

No description available.

<p>X-ray computed tomography (CT) is a non-destructive medical imaging technique for assessing the cross-sectional images of an object in terms of attenuation. As it is designed based on the physical processes involved in the x-ray and matter interactions, faithfully modeling the physics in the reconstruction procedure can yield accurate attenuation distribution of the scanned object. Otherwise, unrealistic physical assumptions can result in unwanted artifacts in reconstructed images. For example, the current reconstruction algorithms assume the photons emitted by the x-ray source are mono-energetic. This oversimplified physical model neglects the poly-energetic properties of the x-ray source and the nonlinear attenuations of the scanned materials, and results in the well-known beam-hardening artifacts (BHAs). The purpose of this work was to incorporate the poly-energetic nature of the x-ray spectrum and then to eliminate BHAs. By accomplishing this, I can improve the image quality, enable the quantitative reconstruction ability of the single-spectrum CT scanner, and potentially reduce unnecessary radiation dose to patients.</p><p>In this thesis, in order to obtain accurate spectrum for poly-energetic reconstruction, I first presented a novel spectral estimation technique, with which spectra across a large range of angular trajectories of the imaging field of view can be estimated with a single phantom and a single axial acquisition. The experimental results with a 16 cm diameter cylindrical phantom (composition: ultra-high-molecular-weight polyethylene [UHMWPE]) on a clinical scanner showed that the averaged absolute mean energy differences and the normalized root mean square differences with respect to the actual spectra across kVp settings (i.e., 80, 100, 120, 140) and angular trajectories were less than 0.61 keV and 3.41%, respectively</p><p>With the previous estimation of the x-ray spectra, three poly-energetic reconstruction algorithms are proposed for different clinical applications. The first algorithm (i.e., poly-energetic iterative FBP [piFBP]) can be applied to routine clinical CT exams, as the spectra of the x-ray source and the nonlinear attenuations of diverse body tissues and metal implant materials are incorporated to eliminate BHAs and to reduce metal artifacts. The simulation results showed that the variation range of the relative errors of various tissues across different phantom sizes (i.e., 16, 24, 32, and 40 cm in diameter) and kVp settings (80, 100, 120, 140) were reduced from [-7.5%, 17.5%] for conventional FBP to [-0.1%, 0.1%] for piFBP, while the noise was maintained at the same low level (about [0.3%, 1.7%]).</p><p>When iodinated contrast agents are involved and patient motions are not readily correctable (e.g., in myocardial perfusion exam), a second algorithm (i.e., poly-energetic simultaneous algebraic reconstruction technique [pSART]) can be applied to eliminate BHAs and to quantitatively determine the iodine concentrations of blood-iodine mixtures with our new technique. The phantom experiment on a clinical CT scanner indicated that the maximum absolute relative error across material inserts was reduced from 4.1% for conventional simultaneous algebraic reconstruction technique [SART] to 0.4% for pSART.</p><p>Extending the work beyond minimizing BHAs, if patient motions are correctable or negligible, a third algorithm (i.e., poly-energetic dynamic perfusion algorithm [pDP]) is developed to retrieve iodine maps of any iodine-tissue mixtures in any perfusion exams, such as breast, lung, or brain perfusion exams. The quantitative results of the simulations with a dynamic anthropomorphic thorax phantom indicated that the maximum error of iodine concentrations can be reduced from 1.1 mg/cc for conventional FBP to less than 0.1 mg/cc for pDP.</p><p>Two invention disclosure forms based on the work presented in this thesis have been submitted to Office of Licensing & Ventures of Duke University.</p> / Dissertation

Ion beam therapy (IBT) is a promising treatment option in radiotherapy. The characteristic physical and biological properties of light ion beams allow for the delivery of highly tumour conformal dose distributions. Related to the sparing of surrounding healthy tissue and nearby organs at risk, it is feasible to escalate the dose in the tumour volume to reach higher tumour control and survival rates. Remarkable clinical outcome was achieved with IBT for radio-resistant, deep-seated, static and well fixated tumour entities. Presumably, more patients could benefit from the advantages of IBT if it would be available for more frequent tumour sites. Those located in the thorax and upper abdominal region are commonly subjected to intra-fractional, respiration related motion. Different motion compensated dose delivery techniques have been developed for active field shaping with scanned pencil beams and are at least available under experimental conditions at the GSI Helmholtzzentrum für Schwerionenforschung (GSI) in Darmstadt, Germany. High standards for quality assurance are required in IBT to ensure a safe and precise dose application. Both underdosage in the tumour and overdosage in the normal tissue might endanger the treatment success. Since minor unexpected anatomical changes e.g. related to patient mispositioning, tumour shrinkage or tissue swelling could already lead to remarkable deviations between planned and delivered dose distribution, a valuable dose monitoring system is desired for IBT. So far, positron emission tomography (PET) is the only in vivo, in situ and non-invasive qualitative dose monitoring method applied under clinical conditions. It makes use of the tissue autoactivation by nuclear fragmentation reactions occurring along the beam path. Among others, +-emitting nuclides are generated and decay according to their half-life under the emission of a positron. The subsequent positron-electron annihilation creates two 511 keV photons which are emitted in opposite direction and can be detected as coincidence event by a dedicated PET scanner. The induced three-dimensional (3D) +- activity distribution in the patient can be reconstructed from the measured coincidences. Conclusions about the delivered dose distribution can be drawn indirectly from a comparison between two +-activity distributions: the measured one and an expected one generated by a Monte-Carlo simulation. This workflow has been proven to be valuable for the dose monitoring in IBT when it was applied for about 440 patients, mainly suffering from deep-seated head and neck tumours that have been treated with 12C ions at GSI. In the presence of intra-fractional target motion, the conventional 3D PET data processing will result in an inaccurate representation of the +-activity distribution in the patient. Fourdimensional, time-resolved (4D) reconstruction algorithms adapted to the special geometry of in-beam PET scanners allow to compensate for the motion related blurring artefacts. Within this thesis, a 4D maximum likelihood expectation maximization (MLEM) reconstruction algorithm has been implemented for the double-head scanner Bastei installed at GSI. The proper functionality of the algorithm and its superior performance in terms of suppressing motion related blurring artefacts compared to an already applied co-registration approach has been demonstrated by a comparative simulation study and by dedicated measurements with moving radioactive sources and irradiated targets. Dedicated phantoms mainly made up of polymethyl methacrylate (PMMA) and a motion table for regular one-dimensional (1D) motion patterns have been designed and manufactured for the experiments. Furthermore, the general applicability of the 4D MLEM algorithm for more complex motion patterns has been demonstrated by the successful reduction of motion artefacts from a measurement with rotating (two-dimensional moving) radioactive sources. For 1D cos2 and cos4 motion, it has been clearly illustrated by systematic point source measurements that the motion influence can be better compensated with the same number of motion phases if amplitudesorted instead of time-sorted phases are utilized. In any case, with an appropriate parameter selection to obtain a mean residual motion per phase of about half of the size of a PET crystal size, acceptable results have been achieved. Additionally, it has been validated that the 4D MLEM algorithm allows to reliably access the relevant parameters (particle range and lateral field position and gradients) for a dose verification in intra-fractionally moving targets even from the intrinsically low counting statistics of IBT-PET data. To evaluate the measured +-activity distribution, it should be compared to a simulated one that is expected from the moving target irradiation. Thus, a 4D version of the simulation software is required. It has to emulate the generation of +-emitters under consideration of the intra-fractional motion, their decay at motion state dependent coordinates and to create listmode data streams from the simulated coincidences. Such a revised and extended version that has been compiled for the special geometry of the Bastei PET scanner is presented within this thesis. The therapy control system provides information about the exact progress of the motion compensated dose delivery. This information and the intra-fractional target motion needs to be taken into account for simulating realistic +-activity distributions. A dedicated preclinical phantom simulation study has been performed to demonstrate the correct functionality of the 4D simulation program and the necessity of the additional, motionrelated input parameters. Different to the data evaluation for static targets, additional effort is required to avoid a potential misleading interpretation of the 4D measured and simulated +-activity distributions in the presence of deficient motion mitigation or data processing. It is presented that in the presence of treatment errors the results from the simulation might be in accordance to the measurement although the planned and delivered dose distribution are different. In contrast to that, deviations may occur between both distributions which are not related to anatomical changes but to deficient 4D data processing. Recommendations are given in this thesis to optimize the 4D IBT-PET workflow and to prevent the observer from a mis-interpretation of the dose monitoring data. In summary, the thesis contributes on a large scale to a potential future application of the IBT-PET monitoring for intra-fractionally moving target volumes by providing the required reconstruction and simulation algorithms. Systematic examinations with more realistic, multi-directional and irregular motion patterns are required for further improvements. For a final rating of the expectable benefit from a 4D IBT-PET dose monitoring, future investigations should include real treatment plans, breathing curves and 4D patient CT images.

In the past few years, some experimental investigations have been conducted to verify seismic behaviour of fiber reinforced polymer reinforced concrete (FRP-RC) beam-column joints. Those researches were mainly focused on exterior beam-column joints without lateral beams. However, lateral beams, commonly exist in buildings, can significantly improve seismic performance of the joints. Moreover, the way the longitudinal beam bars are anchored in the joint, either using headed-end or bent bars, was not adequately addressed. This study aims to fill these gaps and investigate the shear capacity of FRP-RC exterior beam-column joints confined with lateral beams, and the effect of beam reinforcement anchorage on their seismic behaviour. Six full-scale exterior beam-column joints were constructed and tested to failure under reversal cyclic loading. Test results showed that the presence of lateral beams significantly increased the shear capacity of the joints. Moreover, replacing bent bars with headed-end bars resulted in more ductile behaviour of the joints.

Traumatic brain injury (TBI) is an injury of the brain most often caused by blunt force trauma to the head and typically characterized by an increase in reactive oxygen species (ROS), inflammation, and hemorrhaging. Heme oxygenase (HO) catalyzes the breakdown of heme into carbon monoxide (CO), biliverdin which is further reduced to bilirubin, and ferrous iron. There are two active isoforms: HO-1 which is inducible and found predominantly in liver and spleen tissue; and HO-2 which is constitutive and found predominantly in the brain and testis. The metabolites of heme possess cytoprotective properties that can limit damage resulting from TBI. Our laboratory has found a selective HO-2 activator known as menadione (MD) that has been found to increase HO-2 activity by 4-fold while not affecting HO-1 in vitro. Given the higher amounts of HO-2 found in the brain and the cytoprotective properties of heme metabolites, we postulate that activation of HO-2 using menadione would mitigate further damage after TBI. The rat controlled cortical impact (CCI) model was used to simulate TBI with spontaneous locomotor activity (SLA), spontaneous alternation behaviour (SAB), and beam balance (BB) as the behavioural tasks to assess cognitive and motor function. A dose-response study (25, 50, 100, 200 μmol/kg) was performed to ascertain the effect of MD treatment on injured animals comparing to uninjured controls and injured animals treated with the vehicle (saline). We found that BB performance improved to control levels after MD treatment at 25 μmol/kg and 50 μmol/kg whereas animals treated with saline did not improve. SLA and SAB performance did not improve after treatment with MD. The findings suggest that HO-2 activation may be a viable method in mitigating further injury after TBI. / Thesis (Master, Pharmacology & Toxicology) -- Queen's University, 2014-06-27 19:33:45.645

Concrete is a very weak and brittle material in tension. It has been shown in previous researches that the addition of steel fibers to a concrete matrix can improve this behavior. The ability of fibers to control and redistribute stresses after cracking results in a number of improvements in the structural behaviour of concrete. A review of existing literature shows that the addition of steel fibers enhances concrete’s tensile resistance, crack control properties, ductility and damage tolerance. In beams, fibers can transform brittle shear response into a flexural response and promote ductility, thereby allowing for a full or partial replacement of traditional shear reinforcement. The enhanced shear capacity, ductility and damage tolerance of Steel Fiber Reinforced Concrete (SFRC) can also potentially be used to relax seismic detailing requirements in frames by partially replacing the required transverse reinforcement in the plastic hinge regions of RC beams. One of the drawbacks associated with SFRC is that the addition of steel fibers to a traditional concrete mix at high fiber contents can result in workability problems. The combined use of Self-Consolidating Concrete (SCC) and fibers can solve this problem and facilitate placement for a wider range of structural applications. Although several studies have been conducted on the behaviour of SFRC beams subjected to monotonic loading, there is limited research on the behaviour of SFRC beams under cyclic or reverse-cyclic loading. This thesis presents the results of an experimental and analytical study conducted on nine SFRC beam specimens tested under load reversals. The main objective of this research program was to investigate the effect of fibers on structural behaviour and to examine the ability of steel fibers to replace transverse reinforcement. The experimental and analytical results show that use of fibers results in several improvements in behaviour, including enhanced damage tolerance and post-peak ductility. The results also show that steel fibers can potentially be used to allow for a reduction of transverse reinforcement in beams, however further research is required.