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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
11

Scattered neutron tomography based on a neutron transport problem

Scipolo, Vittorio 01 November 2005 (has links)
Tomography refers to the cross-sectional imaging of an object from either transmission or reflection data collected by illuminating the object from many different directions. Classical tomography fails to reconstruct the optical properties of thick scattering objects because it does not adequately account for the scattering component of the neutron beam intensity exiting the sample. We proposed a new method of computed tomography which employs an inverse problem analysis of both the transmitted and scattered images generated from a beam passing through an optically thick object. This inverse problem makes use of a computationally efficient, two-dimensional forward problem based on neutron transport theory that effectively calculates the detector readings around the edges of an object. The forward problem solution uses a Step-Characteristic (SC) code with known uncollided source per cell, zero boundary flux condition and Sn discretization for the angular dependence. The calculation of the uncollided sources is performed by using an accurate discretization scheme given properties and position of the incoming beam and beam collimator. The detector predictions are obtained considering both the collided and uncollided components of the incoming radiation. The inverse problem is referred as an optimization problem. The function to be minimized, called an objective function, is calculated as the normalized-squared error between predicted and measured data. The predicted data are calculated by assuming a uniform distribution for the optical properties of the object. The objective function depends directly on the optical properties of the object; therefore, by minimizing it, the correct property distribution can be found. The minimization of this multidimensional function is performed with the Polack Ribiere conjugate-gradient technique that makes use of the gradient of the function with respect to the cross sections of the internal cells of the domain. The forward and inverse models have been successfully tested against numerical results obtained with MCNP (Monte Carlo Neutral Particles) showing excellent agreements. The reconstructions of several objects were successful. In the case of a single intrusion, TNTs (Tomography Neutron Transport using Scattering) was always able to detect the intrusion. In the case of the double body object, TNTs was able to reconstruct partially the optical distribution. The most important defect, in terms of gradient, was correctly located and reconstructed. Difficulties were discovered in the location and reconstruction of the second defect. Nevertheless, the results are exceptional considering they were obtained by lightening the object from only one side. The use of multiple beams around the object will significantly improve the capability of TNTs since it increases the number of constraints for the minimization problem.
12

Analytical inverse model for post-event attribution of plutonium

Miller, James Christopher 15 May 2009 (has links)
An integral part of deterring nuclear terrorism is the swift attribution of any event to a particular state or organization. By quickly being able to identify the responsible party after a nuclear event, appropriate people may be held accountable for their actions. Currently, there is a system in place to determine the origin of nuclear devices and materials from post-event data; however, the system requires significant time to produce an answer within acceptable error margins. Described here is a deterministic approach derived from first principles to solve the inverse problem. The derivation starts with the basic change rate equation and ends in relationships for important nuclear concentrations and device yield. This results in a computationally efficient and timely method for producing an estimate of the material attributes. This estimate can then be used as a starting point for other more detailed methods and reduce the overall computation time of the post-event forensics. This work focused on a specific type of nuclear event: a plutonium improvised nuclear device (IND) explosion. From post-event isotopic ratios, this method determines the device’s pre-event isotopic concentrations of special nuclear material. From the original isotopic concentrations, the field of possible origins for the nuclear material is narrowed. In this scenario, knowing where the nuclear material did not originate is as important as knowing where it did. The derived methodology was tested using several cases of interest including simplified and realistic cases. For the simplistic cases, only two isotopes comprised the material being fissioned. In the realistic cases, both Weapons Grade and Reactor Grade plutonium were used to cover the spectrum of possible fissile material to be used by terrorists. The methodology performed very well over the desired energy range. Errors were under two percent from the expected values for all yields under 50 kT. In the realistic cases, competing reactions caused an increase in error; however, these stayed under five percent. As expected, with an increased yield, the error continued to rise, but these errors increased linearly. A sensitivity analysis was performed on the methodology to determine the impact of uncertainty in various physical constants. The result was that the inverse methodology is not overly sensitive to perturbations in these constants.
13

Transparency Property of One Dimensional Acoustic Wave Equations

Huang, Yin 24 July 2013 (has links)
This thesis proposes a new proof of the acoustic transparency theorem for material with a bounded variation. The theorem states that if the material properties (density, bulk modulus) is of bounded variation, the net power transmitted through the point z = 0 over a time interval [−T,T] is greater than some constant times the energy at the time zero over a spatial interval [0,Z], provided that T equals the time of travel of a wave from 0 to Z. This means the reflected energy of an input into the earth will be received. Otherwise, the reflections may not arrive at the surface. A proof gives a lower bound for material properties (density, bulk modulus) with bounded variation using sideways energy estimate. A different lower bound that works only for piecewise constant coefficients is also given. It gives a lower bound by analyzing reflections and transmissions of the waves at the jumps of the material properties. This thesis also gives an example to illustrate that the bounded variation assumption may not be necessary for the medium to be transparent. This thesis also discusses relations between the transparency property and the data of an inverse problem.
14

Determining relaxation times for porous media: Theory, measurement, and the inverse problem

Li, Yijia January 2007 (has links)
This thesis provides an introduction to and analysis of the problem of determining nuclear magnetic resonance (NMR) relaxation times of porous media by using the so-called Carr-Purcell-Meiboom-Gill (CPMG) technique. We introduce the principles of NMR, the CPMG technique and the signals produced, porous effects on the NMR relaxation times and discuss various numerical methods for the inverse problem of extracting the relaxation times from CPMG signals. The numerical methods for solving Fredholm integral equations of the first kind are sketched from a series expansion perspective. A method of using arbitrary constituent functions for improving the performance of non-negative least squares (NNLS) is developed and applied to several synthesized data sets and real experimental data sets of saturated porous glass gels. The data sets were obtained by the author of this thesis and the experimental procedure will be presented. We discuss the imperfections in the assumptions on the physical and numerical models, the numerical schemes, and the experimental results, which may lead to new research possibilities.
15

The inverse medium problem for Timoshenko beams and frames : damage detection and profile reconstruction in the time-domain

Karve, Pranav M., 1983- 03 August 2010 (has links)
We discuss a systematic methodology that leads to the reconstruction of the material profile of either single, or assemblies of one-dimensional flexural components endowed with Timoshenko-theory assumptions. The probed structures are subjected to user-specified transient excitations: we use the complete waveforms, recorded directly in the time-domain at only a few measurement stations, to drive the profile reconstruction using a partial-differential-equation-constrained optimization approach. We discuss the solution of the ensuing state, adjoint, and control problems, and the alleviation of profile multiplicity by means of either Tikhonov or Total Variation regularization. We report on numerical experiments using synthetic data that show satisfactory reconstruction of a variety of profiles, including smoothly and sharply varying profiles, as well as profiles exhibiting localized discontinuities. The method is well suited for imaging structures for condition assessment purposes, and can handle either diffusive or localized damage without need for a reference undamaged state. / text
16

Recovery of the logical gravity field by spherical regularization wavelets approximation and its numerical implementation

Shuler, Harrey Jeong 29 April 2014 (has links)
As an alternative to spherical harmonics in modeling the gravity field of the Earth, we built a multiresolution gravity model by employing spherical regularization wavelets in solving the inverse problem, i.e. downward propagation of the gravity signal to the Earth.s surface. Scale discrete Tikhonov spherical regularization scaling function and wavelet packets were used to decompose and reconstruct the signal. We recovered the local gravity anomaly using only localized gravity measurements at the observing satellite.s altitude of 300 km. When the upward continued gravity anomaly to the satellite altitude with a resolution 0.5° was used as simulated measurement inputs, our model could recover the local surface gravity anomaly at a spatial resolution of 1° with an RMS error between 1 and 10 mGal, depending on the topography of the gravity field. Our study of the effect of varying the data volume and altering the maximum degree of Legendre polynomials on the accuracy of the recovered gravity solution suggests that the short wavelength signals and the regions with high magnitude gravity gradients respond more strongly to such changes. When tested with simulated SGG measurements, i.e. the second order radial derivative of the gravity anomaly, at an altitude of 300 km with a 0.7° spatial resolution as input data, our model could obtain the gravity anomaly with an RMS error of 1 ~ 7 mGal at a surface resolution of 0.7° (< 80 km). The study of the impact of measurement noise on the recovered gravity anomaly implies that the solutions from SGG measurements are less susceptible to measurement errors than those recovered from the upward continued gravity anomaly, indicating that the SGG type mission such as GOCE would be an ideal choice for implementing our model. Our simulation results demonstrate the model.s potential in determining the local gravity field at a finer scale than could be achieved through spherical harmonics, i.e. less than 100 km, with excellent performance in edge detection. / text
17

Calibration of ultrasound scanners for surface impedance measurement

Vollmers, Antony Stanley 04 April 2005
The primary objective of this research was to investigate the feasibility of calibrating ultrasound scanners to measure surface impedance from reflection data. The method proposed uses calibration curves from known impedance interfaces. This plot, or calibration curve, may then be used, with interpolation, to relate measured grey level to impedance for the characterization of tissue specimens with unknown properties. This approach can be used independent of different medical ultrasound scanner systems to solve for reproducible tissue impedance values without offline data processing and complicated custom electronics. <p>Two medical ultrasound machines from different manufacturers were used in the experiment; a 30 MHz and a 7.5 MHz machine. The calibration curves for each machine were produced by imaging the interfaces of a vegetable oil floating over varying salt solutions. <p>To test the method, porcine liver, kidney, and spleen acoustical impedances were determined by relating measured grey levels to reflection coefficients using calibration curves and then inverting the reflection coefficients to obtain impedance values. The 30 MHz ultrasound machines calculated tissue impedances for liver, kidney, and spleen were 1.476 ± 0.020, 1.486 ± 0.020, 1.471 ± 0.020 MRayles respectively. The 7.5 MHz machines tissue impedances were 1.467 ± 0.088, 1.507 ± 0.088, and 1.457 ± 0.088 MRayles respectively for liver, kidney and spleen. The differences between the two machines are 0.61%, 1.41%, and 0.95% for the impedance of liver, kidney, and spleen tissue, respectively. If the grey level is solely used to characterize the tissue, then the differences are 45.9%, 40.3%, and 39.1% for liver, kidney, and spleen between the two machines. The results support the hypothesis that tissue impedance can be determined using calibration curves and be consistent between multiple machines.
18

Determining relaxation times for porous media: Theory, measurement, and the inverse problem

Li, Yijia January 2007 (has links)
This thesis provides an introduction to and analysis of the problem of determining nuclear magnetic resonance (NMR) relaxation times of porous media by using the so-called Carr-Purcell-Meiboom-Gill (CPMG) technique. We introduce the principles of NMR, the CPMG technique and the signals produced, porous effects on the NMR relaxation times and discuss various numerical methods for the inverse problem of extracting the relaxation times from CPMG signals. The numerical methods for solving Fredholm integral equations of the first kind are sketched from a series expansion perspective. A method of using arbitrary constituent functions for improving the performance of non-negative least squares (NNLS) is developed and applied to several synthesized data sets and real experimental data sets of saturated porous glass gels. The data sets were obtained by the author of this thesis and the experimental procedure will be presented. We discuss the imperfections in the assumptions on the physical and numerical models, the numerical schemes, and the experimental results, which may lead to new research possibilities.
19

Calibration of ultrasound scanners for surface impedance measurement

Vollmers, Antony Stanley 04 April 2005 (has links)
The primary objective of this research was to investigate the feasibility of calibrating ultrasound scanners to measure surface impedance from reflection data. The method proposed uses calibration curves from known impedance interfaces. This plot, or calibration curve, may then be used, with interpolation, to relate measured grey level to impedance for the characterization of tissue specimens with unknown properties. This approach can be used independent of different medical ultrasound scanner systems to solve for reproducible tissue impedance values without offline data processing and complicated custom electronics. <p>Two medical ultrasound machines from different manufacturers were used in the experiment; a 30 MHz and a 7.5 MHz machine. The calibration curves for each machine were produced by imaging the interfaces of a vegetable oil floating over varying salt solutions. <p>To test the method, porcine liver, kidney, and spleen acoustical impedances were determined by relating measured grey levels to reflection coefficients using calibration curves and then inverting the reflection coefficients to obtain impedance values. The 30 MHz ultrasound machines calculated tissue impedances for liver, kidney, and spleen were 1.476 ± 0.020, 1.486 ± 0.020, 1.471 ± 0.020 MRayles respectively. The 7.5 MHz machines tissue impedances were 1.467 ± 0.088, 1.507 ± 0.088, and 1.457 ± 0.088 MRayles respectively for liver, kidney and spleen. The differences between the two machines are 0.61%, 1.41%, and 0.95% for the impedance of liver, kidney, and spleen tissue, respectively. If the grey level is solely used to characterize the tissue, then the differences are 45.9%, 40.3%, and 39.1% for liver, kidney, and spleen between the two machines. The results support the hypothesis that tissue impedance can be determined using calibration curves and be consistent between multiple machines.
20

Elektrická impedanční tomografie měkkých tkání: Řešení přímé a obrácené úlohy / Electrical impedance tomography of soft tissue: Forward and inverse modelling

Pšenka, Marek January 2017 (has links)
Electrical impedance tomography of soft tissue: Forward and inverse modelling The diploma thesis builds the neccesary apparatus to formulate and solve the inverse problem of Eletrical Impedance Tomography (EIT), including strategies to remedy the ill-conditioning of the problem. The problem itself lies in determining the structure of a body of interest by driving a set of electrical currents through electrodes connected to its surface. The aim of the thesis is to investigate possible utility of this method in medical applications, namely scanning for malignancies in the female breast, by studying the interaction of tissue with the electromagnetic field and by preparing a set of correspoding numerical experiments. An approximate characterization of the method's sensitivity with respect to noise is derived based on the most basic set of such numerical experiments, which were prepared by a complete software solution called prs4D developed by the author and his advisor, while some aspects of its implementation are included in the thesis.

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