https://etd.iisc.ac.in/handle/2005/44 Sun, 08 Mar 2026 04:19:38 GMT 2026-03-08T04:19:38Z http://etd.iisc.ac.in:80/bitstream/id/5611352d-baa2-4644-aefd-75a52c3ec08d/ https://etd.iisc.ac.in/handle/2005/44 https://etd.iisc.ac.in/handle/2005/4693 1H-NMR Study of Proton Glasses - Nuclear Spin Lattice Relaxation in BPxBPI(1-x) and BPxGPI(1-x) - Effects of Disorder on the Proton Group Dynamics Ramanuja, M N Mixed systems made from a combination of ferroelectric (FE) and antiferroelectric (AFE) compounds, exhibit various effects of disorder in different temperature regions. The kind of effects observed, depend on the technique and the window of observation employed. Model systems, like Potassium Ammonium dihydrogen phosphate (KADP), Rubidium Ammonium dihydrogen phosphate (RADP) and BPxBPI(1-x) , with H-bonding networks, have been well studied by dielectric techniques. These investigations have revealed disorder effects like deviations from Curie Weiss law, progressive broadening of dielectric loss curves and dispersion of dielectric constant, at sufficiently low temperatures. NMR studies in such systems are meager and mainly members of the KDP family, like Rubidium ammonium dihydrogen phosphate (RADP) and arsenate (RADA) have been investigated using mainly 2H and 87Rb NMR. On the other hand, proton NMR has been much less used, and our focus is to exploit its power/potential to study 1H group dynamics in the presence (and absence) of disorder in condensed matter systems. This thesis describes the results of proton NMR investigations in two mixed systems of ferroelectric and antiferroelectric compounds namely, (i) Betaine phosphate (BP, AFE) and Betaine phosphite (BPI, FE) and (ii) Betaine phosphate and Glycine phosphite (GPI, FE). The aim of the study is to obtain information on 1H group dynamics (activation energies and pre-exponential factors) and the effects of micro-spatial disorder. The former system is shown to exhibit orientational glass behavior by extensive dielectric investigations. BP-GPI system is synthesized for the first time and our proton NMR investigation has exhibited interesting effects of disorder like deviation from expected BPP behavior. Further, both systems have exhibited quantum tunneling effects, revealing a gradual transition from classical regime to quantum regime. Biexponential magnetization recovery at low temperatures has also been observed indicating the existence of disorder. A combination of AFE and FE compounds of this type form a mixed system, over a broad range of compositions, in which the long-range electric order is suppressed owing to frustration effects. Such systems have been treated as dipolar analogues of spin glasses and are known as ‘orientation glasses’ (OG), ‘proton glasses’ (PG) or ‘pseudo-spin glasses. Although the frustrated condensed matter system is crystalline in nature, there is an underlying microstructural randomness due to local fluctuations of the composition which usually results in static lattice strains, which are called random fields. It has been shown that these random fields can also have a pronounced effect on the spin lattice relaxation time as observed in NMR experiments. Depending on the relative concentration and temperature, the mixed system exhibits a range of states (x-T phase diagram) like FE, OG, coexisting OG and AFE, and AFE. These mixed systems exhibit various kinds of effects of disorder in different temperature regimes which depend upon the technique and window of observation. For e.g., using dielectric spectroscopy we can study the behavior of the electric dipoles during various phases and the effects of frustration seen as dispersion of dielectric constants and broadening of loss curves etc. Through quadrupole perturbed NMR study of systems containing nuclei like 87Rb or 2H, we learn about site-specific inhomogeneities and distribution of EFG in the system. Proton NMR study in the mixed systems, though not much used so far, is a powerful technique to shed light on the dynamics, disorder and Quantum tunneling effects. Our proton SLR time measurements have been carried out at two Larmor frequencies of 23.3 MHz and 11.4 MHz, in the temperature range of 300 K to 4 K and the results are presented in this thesis, which is divided into four chapters https://etd.iisc.ac.in/handle/2005/4693 https://etd.iisc.ac.in/handle/2005/4484 3D Dosimetry using optical tomography and electronic portal images Manjappa, Rakesh The primary goal of this thesis is to develop techniques to quantify the radiation dose distributions used in radiotherapy for cancer treatment. It aims at developing a physically profound calculation model for the transit dosimetry by a detailed characterization of the radiation interaction with tissues and the fluence measurements recorded in electronic portal imaging device (EPID). Radiotherapy has undergone great advances with developments such as Intensity Modulated Radiotherapy (IMRT), Volumetric modulated Arc therapy (VMAT), Radiosurgery, CyberKnife, and advances in Brachytherapy. These newer methods help in precisely administering radiation dose to patients as decided by the treatment planning system (TPS), a computer system that takes input from patient CT, medical physicist, oncologist, medical dosimetrist and physician. The radio-therapy treatment system depends on 3D dosimetry for pre-treatment quality assurance. The polymer gel dosimeters are used for estimating the 3D dose distribution using a treatment plan decided by the radiation treatment plan (RTS) before the patient undergoes radiation exposure. Gel phantoms are prepared using monomers to be tissue equivalent radiologically. The optical computed tomography has been used to scan the gel dosimeters. It was observed that upon irradiation, the monomers in gel get polymerized. Calibration measurements with varying levels of radiation exposure show that, optical density and refractive index increase with radiation dose. The optical density increased from (0.01 to 0.06) mm1 and the refractive index increased from (1.34 to 1.37) for gel irradiated from (0.5 to 25) Gy dose. The SEM imaging of calibration gels show that the particle size increases from 20nm to 400nm on radiation exposure. The exposure of radiation to tissue causes an increase in refractive index, thereby bending the light traversing through the tissue, resulting in deterioration in image quality. The solution for this is to immerse the dosimeter in a refractive index matching liquid. However, an exact match is seldom achieved. The refraction of light passing through a dose region results in artefacts in the reconstructed images. These refraction errors are dependent on the scanning geometry and collection optics. The refraction arises primarily due to (1) the refractive index mismatch between the surrounding medium and the dosimeter which results in distortions of dose regions and (2) the refractive index changes caused by radiation dose in the dosimeter itself that result in streaking, and quantitative errors. In order to account for these effects and correct the distortions we used ray path modelling of light traversed through the dosimeter. Exact path length of the ray in a discretized grid was obtained by using ray tracing methods. Rayline errors perturb the system when rays confront a radiation induced RI gradient region. This is more signi ficant in 3D as the ray get deviated and does not reach the detector plane. We extended this study to 3D, used a prototype cone-beam scanning system to collect the projection images. We developed a fully 3D image reconstruction algorithm, algebraic reconstruction technique-refraction correction (ART-rc) that corrects for the refractive index mismatches present in a gel dosimeter scanner not only at the boundary, but also for any rayline refraction due to multiple dose regions inside the dosimeter. In this study, simulation and experimental studies have been carried out to reconstruct a 3D dose volume using 2D CCD measurements taken for various views. Radiation dose absorbed at a tissue voxel can be calculated from kernels which incorporate the effects of all the interactions with matter using Monte Carlo based techniques. We studied pencil beam and point kernel based methods. Radiological depth calculation using ray tracing technique was used for path length calculations in a inhomogeneous phantom/patient volume. This is integrated with collapsed-cone convolution superposition algorithm to arrive at the complete dose-distribution. Dose reconstruction results using Monte Carlo and collapsed cone methods are presented. The EPID image was corrected with scatter factor measurements. The corrections improved the dose quanti cation from 88.9% to 96.5%. The resulting dose Monitor Unit (MU) values matches well with that from TPS computation. Per eld EPID uence, calculated from segment wise portal images acquired using step and shoot technique of the IMRT prostate eld is validated. The main ndings of this study are: 1 We have demonstrated that gel dosimeters can be used to verify dose pro les delivered using Co-60 telecobalt machines, linear accelerators, IMRT, VMAT and Brachytherapy. 2 Refraction e ects deteriorate dose readout and induce errors in quantifying dose. These can be overcome by using ray tracing method that calculate exact pathlength accounting for refraction. 3 Boundary mismatch can be overcome by using exact matching liquid, but interior refractive index changes induced by radiation can be accounted for using our ray modelling scheme. 4 The Monte Carlo modelling of polarized light propagation in a multi-layered turbid medium is extended to include multiparticle distribution of scatterers and also with embedded absorbing/ scattering inhomogeneities. 5 Fluence measurements acquired using EPID along with appropriate scatter factor corrections were found to match with those calculated by treatment planning system (TPS). In conjunction with collapsed cone convolution/superposition method, it can be used to compute 3D dose distributions. https://etd.iisc.ac.in/handle/2005/4484 https://etd.iisc.ac.in/handle/2005/1547 3D Image Reconstruction Using Optical Phase Retrieval And Cone-Beam Tomography Hemanth, T Tue, 22 Nov 2011 00:00:00 GMT https://etd.iisc.ac.in/handle/2005/1547 2011-11-22T00:00:00Z https://etd.iisc.ac.in/handle/2005/8148 A Neumerica study of laminar compressible boundary-layer problems Krishnaswami, R This thesis presents some investigations on certain problems in laminar compressible boundary-layer theory. It consists of an introductory chapter and three main chapters. A brief survey of the literature relevant to the problems considered and contributions made by the author are given at the beginning of each chapter. Chapter I is an introduction to the compressible boundary-layer theory in general. Here, several numerical techniques have also been discussed with special reference to the two-point boundary-value problems. Further, the application of the similarity variable on partial differential equations is briefly discussed. Chapter II: Axisymmetric Laminar Boundary-Layer Flow Problems Here, the role of different parameters characterizing the flow, namely, the mass injection, magnetic field, time-dependent free stream velocity and transverse curvature has been studied under three sub-divisions: Part A, Part B and Part C. Part A: The axisymmetric compressible boundary-layer flow with an applied magnetic field has been studied, with massive blowing. The difficulty arising due to massive blowing has been overcome by employing the efficient numerical method of quasilinearization in combination with finite-difference scheme. In the analysis, a realistic gas model has been employed. Further, the effect of high acceleration on the axisymmetric compressible boundary-layer flow with variable gas properties has been studied in the presence of an applied magnetic field. The difficulty that usually arises due to large acceleration is also overcome by using the above method of quasilinearization in combination with finite-difference scheme. Part B: The axisymmetric compressible boundary-layer flow with time-dependent free stream velocity and wall temperature has been studied with an applied magnetic field, variable gas properties and (time-dependent) surface mass transfer (injection and suction). Assuming two time-dependent free stream velocity distributions (constantly accelerated free stream and fluctuating free stream with a steady mean), solutions were obtained by the method of an implicit finite-difference scheme. Part C: The simultaneous effect of large injection in the presence of transverse curvature of an axisymmetric compressible boundary-layer flow has been studied with variable gas properties. The difficulty arising due to large injection in the presence of transverse curvature has been eliminated by the method of quasilinearization in combination with finite-difference scheme. Further, the effect of an applied magnetic field on the axisymmetric compressible boundary-layer flow with transverse curvature effect has also been examined. Thus, Chapter II deals with MHD compressible boundary-layer flow on the axisymmetric body with large surface mass injection, unsteady free stream velocity and transverse curvature effects. Chapter III: Three-Dimensional Stagnation-Point Boundary-Layer Flow There are two sub-divisions, namely Part A and Part B. Part A: The effect of massive blowing on the compressible laminar boundary-layer flow over a general three-dimensional stagnation-point body has been studied, for nodal and saddle point flows. In the nodal point flow, solutions were obtained using the method of quasilinearization with finite-difference scheme. In the saddle-point region, many methods including the above failed to work due to the reverse flow nature of one of the velocity components. This difficulty here is overcome by applying the method of parametric differentiation in combination with finite-difference scheme. Part B: The effect of large surface mass transfer (injection and suction) on the three-dimensional compressible stagnation-point boundary-layer flow with its second-order boundary-layer effects arising due to the curvatures of the body, boundary-layer displacements, vorticity interaction, velocity slip and temperature jump, has been investigated for nodal and saddle point flows with variable properties and cold and hot wall conditions. After solving the first-order boundary-layer equations as has been done in Part A, the second-order boundary-layer equations have been solved by the method of an implicit finite-difference scheme. Thus, Chapter III deals with three-dimensional stagnation-point boundary-layer flow with first and second-order effects and large mass transfer. Chapter IV: Non-Similar Compressible Boundary-Layer Flows There are three sub-divisions, namely Part A, Part B and Part C to study the non-similar behavior of the compressible boundary-layer flows. Part A: The axisymmetric and two-dimensional compressible non-similar boundary-layer flows were studied from the origin of the streamwise coordinate to the exact point of separation. The difficulties arising at the starting point of the streamwise coordinate and at the exact point of separation have been overcome by employing the efficient method of quasilinearization in combination with finite-difference scheme. Part B: The non-similar nature of the three-dimensional boundary-layer flow over a yawed cylinder has been studied from the starting point of the streamwise coordinate to the exact point of separation. The difficulties arising at the starting point of the streamwise coordinate and at the point of separation are overcome by using the same method as in Part A. Part C: For the sake of completeness, the non-similar boundary-layer flow over a flat plate was also discussed. Here, the shock wave propagation behind the moving boundary-layer over a perforated wall has been analyzed with variable properties using the implicit finite-difference scheme. The resulting algebraic equations have been solved using the tridiagonal matrix elimination method. Thus, in Chapter IV, the non-similar compressible boundary-layer flows have been analyzed over the two-dimensional and axisymmetric bodies, a yawed cylinder and a flat plate. The books and original papers referred to in the text of the thesis are enlisted at the end of each chapter. Figures and tables relevant to each chapter are presented at the end of the chapter. The thesis is partly based on the following paper: Hypersonic stagnation-point boundary layers with massive blowing in the presence of a magnetic field (with G. Nath), The Physics of Fluids, Vol. 22, No. 9, Sept. 1979, pp. 1631–1658. Papers based on the remaining work reported in the thesis will be communicated for publication shortly. https://etd.iisc.ac.in/handle/2005/8148