Thesis (Ph. D.)--University of Washington, 1996. / Vita. Includes bibliographical references (leaves [144]-155).

Universiẗat, Diss., 1998--Essen. / Textformat: PDF.

Thesis (MSc)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: The nite size of nucleons should play an important role in the description of high density nuclear matter as found in astro-physical objects. Yet we see that the Walecka model, which is generally used to describe these systems, treats the nucleons as point particles. Here we argue that a non-commutative version of the Walecka model may be a consistent and appropriate framework to describe nite nucleon size e ects. In this framework the length scale introduced through the non-commutative parameter plays the role of the nite nucleon size. To investigate the consequences of this description, the equations of motion and energy-momentum tensor for the non-commutative Walecka model are derived. We also derived an expression for the total energy of the system, as a function of the non-commutative parameter, in a spatially non-uniform matter approximation. The non-commutative parameter, as a variable dependent on the dynamics of the system, remains to be solved self-consistently. / AFRIKAANSE OPSOMMING: Die eindige grootte van nukleone moet 'n belangrike rol speel in die beskrywing van ho e-digtheid kern materie soos gevind in astro- siese voorwerpe. Tog sien ons dat die Walecka model, wat in die algemeen gebruik word om hierdie stelsels te beskryf, die nukleone as punt deeltjies hanteer. Ons redeneer dus dat 'n nie-kommutatiewe weergawe van die Walecka model 'n konsistente en gepaste raamwerk is om die e ekte van eindige nukleon grootte te beskryf. In hierdie raamwerk speel die lengte-skaal wat ingevoer word deur die nie-kommutatiewe parameter die rol van eindige grootte vir nukleone. Om die gevolge van hierdie beskrywing te ondersoek, word die vergelykings van beweging en die energie-momentum tensor afgelei vir die nie-kommutatiewe Walecka model. Ons het ook 'n uitdrukking vir die totale energie van die stelsel, as 'n funksie van die niekommutatiewe parameter, afgelei in 'n ruimtelik nie-uniforme materie benadering. Die niekommutatiewe parameter, as 'n veranderlike afhanklik van die dinamika van die stelsel, bly steeds om self-konsistent opgelos te word.

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

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

<p>The role of the dimensionality and strain state of metallic lattices on the phase behavior of dissolved hydrogen was explored. Metallic superlattices with well defined hydrogen absorption potential on the nm scale, were utilized as test systems. The solubility isotherms of hydrogen in Fe/V(001), MoV/V(001), and Nb/W(110) superlattices were measured by a resistometric method, and the hydrogen-induced changes of the structures were measured by <i>in-situ</i> X-ray diffraction. In the V based superlattices, the long-ranged ordered bulk V hydride phase β-V₂H is absent, which is attributed to the finite-size of V lattice. The intrinsic strain-state of the hydrogen dissolving layers was found to have a strong effect on the interaction between metal and hydrogen as well as on the hydrogen-hydrogen (H-H) interaction. For low hydrogen content in the V layers, the compressive strain resulted in a strong enhancement of the H-H interaction, while a tensile strain appeared to diminish the H-H interaction. This is due to different site occupancy of hydrogen for different strain states, which depending on the relation between the symmetries of hydrogen induced global and local strain fields, gives rise to different elastic H-H interaction. Moderately strained V layers exhibited a strong attractive H-H interaction over a broad concentration range. In the concentration ranges where attractive H-H interaction was established, the hydrogen atoms appeared to be strongly correlated on a microscopic length scale. In the Nb based superlattices, the critical temperature for the α–α’ transition was found to be suppressed as a result of the clamping of the film plane by the film-substrate coupling. An exception from this could be noticed when the intrinsic compressive strain were reduced.</p>

The role of the dimensionality and strain state of metallic lattices on the phase behavior of dissolved hydrogen was explored. Metallic superlattices with well defined hydrogen absorption potential on the nm scale, were utilized as test systems. The solubility isotherms of hydrogen in Fe/V(001), MoV/V(001), and Nb/W(110) superlattices were measured by a resistometric method, and the hydrogen-induced changes of the structures were measured by in-situ X-ray diffraction. In the V based superlattices, the long-ranged ordered bulk V hydride phase β-V2H is absent, which is attributed to the finite-size of V lattice. The intrinsic strain-state of the hydrogen dissolving layers was found to have a strong effect on the interaction between metal and hydrogen as well as on the hydrogen-hydrogen (H-H) interaction. For low hydrogen content in the V layers, the compressive strain resulted in a strong enhancement of the H-H interaction, while a tensile strain appeared to diminish the H-H interaction. This is due to different site occupancy of hydrogen for different strain states, which depending on the relation between the symmetries of hydrogen induced global and local strain fields, gives rise to different elastic H-H interaction. Moderately strained V layers exhibited a strong attractive H-H interaction over a broad concentration range. In the concentration ranges where attractive H-H interaction was established, the hydrogen atoms appeared to be strongly correlated on a microscopic length scale. In the Nb based superlattices, the critical temperature for the α–α’ transition was found to be suppressed as a result of the clamping of the film plane by the film-substrate coupling. An exception from this could be noticed when the intrinsic compressive strain were reduced.

The present thesis reviews my scientific works on disordered systems from 1995 until today. They can be roughly categorized into three main classes: (1) non-interacting disordered systems, (2) the two-interacting particle problem, and (3) the interplay of disorder and many-particle interaction. A (4)th chapter is concerned with the implementation of the numerical algorithms. The structure of the thesis reflects this division. The reprints have been added at the end of these main divisions according to their context. For the convenience of the reader, I have ordered them in each chapter alphabetically according to the names of the authors. Furthermore, in each citation of my work, the starting page number in the thesis is given, e.g, Ref.\ \cite{EPR97} refers to a paper of Eckle, Punnoose and myself and can be found on page \pageref{EPR97}. Citations which do not refer to my work are numbered and are ordered in the bibliography according to the names of the authors.

Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 102-108).

Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2006. / Includes bibliographical references (leaves 53-54). Also available in electronic version.