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Positron beam study of carbon foil and titanium dioxide nanotubes, andproposing a design of a lifetime positron beam based on secondaryelectrons emission from carbon foilYang, Bin, 杨彬 January 2012 (has links)
Secondary electron (SE) emission from thin carbon foils induced by 1-20 keV
positrons has been investigated over a range of nominal foil thicknesses from 1.0 to 5.0 μg/cm2. The measurement of SEs was carried out in forward geometry using a microchannel plate as a detector. The SE yield γ has been measured as a function of beam energy and compared with some Monte Carlo simulation results. We also present in this thesis the material parameter Λ=? / (dE / dx) and the emitted SE energy spectra. Forincident positron energy of 5 keV or higher, the distribution is found to be characterizedby the Sickafus form, AEm and m is close to 1. For low energy incident positrons however, another form, Bexp(E / t) , is proposed for describing the SE distribution. The maximum scattering angle for SEs emitted from 5.0 μg/cm2 is found to be around 60°.
Measurements of energy loss and energy loss straggling for 1-10 keV positrons
passing through thin carbon foils of different thicknesses ranging from 1.0 to 5.0 μg/cm2 are present in this thesis. The stopping power dE / dx and positron transmission coefficient have also been investigated as a function of incident positron energy and foil thickness. Particularly, our experimental results are compared with those from Monte Carlo simulation and theory with a view to providing a way to determine the real thickness of carbon foil. The ratio of the energy loss straggling to the foil thickness seems to have a linear relation with the beam energy. The transmitted positrons after passing through 5.0 μg/cm2 C-foil have a small scattering angle which is less than 10°.
Titanium dioxide nanotube arrays fabricated by anodization of titanium foil and
annealed at different temperatures were studied using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and positron annihilation spectroscopy (PAS). The crystallization process and morphological change of the nanotubes have been discussed. It is found that anatase (101) only appeared on the walls of the nanotube. The atomic concentration of fluoride and the ratio of Ti/O decreased when the annealing temperature increased. Vacancy type defects were found to diffuse toward the surface when the samples were annealed at 200°C and 400°C and healing of vacancies occurred at 600°C. In addition, the fluoride may form some complexes with vacancies on the surface hence lowering the value of the S parameter.
A new design of the lifetime positron beam based on the SE emission is proposed.
The observed experimental results and simulation results make it possible to construct a new type of high resolution (<250ps) lifetime positron beam. In addition, how rastering of the positron beam can be used to accurately locate the position of the C-foil is also shown; a feature that will be of value in setting up a positron lifetime system. Such a lifetime system is expected to be much simpler in construction than existing beam based lifetime spectroscopy systems. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy
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Rehabilitation of poorly detailed RC structures using CFRP materialsKim, InSung 22 July 2015 (has links)
In many reinforced concrete structures built in the 1970's and earlier, bottom beam reinforcement is not continuous and if a column support is lost due to terrorist attack or other unexpected action, the structure could be vulnerable to progressive collapse. The use of CFRP material may provide a solution for rehabilitating such structures. CFRP materials cannot develop full tensile capacity unless they are properly anchored to the reinforced concrete structure. The intent of this study is to find an effective method of anchoring CFRP material to a reinforced concrete beam so that the ultimate tensile strength of the CFRP is realized. In this study, ten reinforced concrete beams rehabilitated using different configurations of anchors were tested to assess the effectiveness of the anchors. Both CFRP anchors and CFRP U-wraps were investigated. The rehabilitated beams were loaded until failure of the CFRP material or anchor occurred. Different failure modes, strengths and deformation capacities of the rehabilitated beams were observed depending on the configurations of anchors. The maximum capacity was obtained using a combination of CFRP anchors and U-wraps. / text
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A model for multi-wave beam-plasma interactionEvstatiev, Evstati Georgiev 28 August 2008 (has links)
Not available / text
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Behavior of hollow, rectangular concrete piers subjected to biaxial bendingSanta Maria, Raul Hernan 28 August 2008 (has links)
Not available / text
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Interaction of intense laser fields with carbon nanotubesHsu, Han 28 August 2008 (has links)
Not available / text
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Magnetic control of supersonic beams : magnetic slowing to isotope separationChavez, Isaac 03 January 2011 (has links)
General control of atoms and molecules has long been a goal for atomic physicists and physical chemists. Techniques such as laser cooling have been a huge breakthrough in studying ultra cold atoms and BECs. Although laser cooling has been a remarkable tool, it is limited to small group of atoms on the periodic table. A general technique to control and manipulate the entire periodic table has been out of reach until now. In this thesis I describe two methods of general control of atoms in the contexts of stopping supersonic beams and of isotope separation. Both these methods take advantage of high flux supersonic beams and the fact that every atom has a magnetic moment in the ground state or a long-lived excited state which can be manipulated using magnetic field gradients.
The first method uses a series of pulsed electomagnetic coils to slow and stop a supersonic beam of paramagnetic atoms and molecules. We have demonstrated the slowing of metastable neon and molecular oxygen using 64 coils from 446.5 m/s to 55.8 m/s for metastable neon, and from 389 m/s to 83 m/s for molecular oxygen respectively.
The second method is a novel and efficient approach to isotope separation which utilizes the concept of Maxwell's Demon. We call this technique Single-Photon Atomic Sorting as it is closely related to Single-Photon Cooling, a cooling technique developed in our laboratory. Our method uses a laser beam to change the magnetic moment to mass ratio in such a way that the desired isotopes are guided through a multi-pole magnetic field and collected. We show simulation results for various test cases which highlight the general applicability of this method. / text
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Kinetics and dynamics of adsorption on single crystal semiconductor and metal surfacesReeves, Christopher Thomas 04 April 2011 (has links)
Not available / text
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Laser chemical vapor deposition of millimeter scale three-dimensional shapesShaarawi, Mohammed Saad, 1973- 06 April 2011 (has links)
Not available / text
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Strain energy capacity of reinforced and prestressed concrete beams唐嘉鴻, Tang, Ka-hung, William. January 1990 (has links)
published_or_final_version / abstract / toc / Civil and Structural Engineering / Master / Master of Philosophy
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Trajectory calculation in an electrostatic positron beam using a reformulated extended charge density model馮德操, Fung, Russell. January 1998 (has links)
published_or_final_version / Physics / Master / Master of Philosophy
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