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1
Yi, Hak-Chae J. « Solution of time-independent inverse problems for linear transport theory / ». Thesis, Connect to this title online ; UW restricted, 1990. http://hdl.handle.net/1773/10677.
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Zhang, Hongbin 1965. « A model radiative transfer problem ». Thesis, The University of Arizona, 1989. http://hdl.handle.net/10150/277071.
Texte intégralRésumé :
The analytical solution to a model time-dependent continuous lethargy photon transport equation is evaluated numerically to obtain a benchmark solution using the Laplace transforms coupled with the multiple collision expansion method. The benchmark solution is then used to check the accuracy of the multigroup approximation. Excellent agreement between continuous lethargy benchmarks and multigroup approximation is obtained.
3
De, Florio Mario. « Accurate Solutions of the Radiative Transfer Problem via Theory of Connections ». Master's thesis, Alma Mater Studiorum - Università di Bologna, 2019.
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In questa tesi viene presentato un nuovo approccio per risolvere una classe di Radiative Transfer Problems, utilizzando la Theory of Connections. Il metodo prevede di risolvere in modo efficiente e accurato un problema lineare con una condizione al contorno (Linear One-Point Boundary Value Problem) derivante dall'equazione integro-differenziale di Boltzmann per il Radiative Transfer tramite un'espansione di polinomi di Chebyshev e metodo Least-Squares.
L'algoritmo proposto risiede nella categoria dei metodi numerici per la soluzione delle equazioni del trasporto, ed è dimostrato essere accurato ed adatto per applicazioni nell'Atmospheric Science e Remote Sensing.
4
Douglass, Steven James. « Consistent energy treatment for radiation transport methods ». Diss., Georgia Institute of Technology, 2012. http://hdl.handle.net/1853/47612.
Texte intégralRésumé :
A condensed multigroup formulation is developed which maintains direct consistency with the continuous energy or fine-group structure, exhibiting the accuracy of the detailed energy spectrum within the coarse-group calculation. Two methods are then developed which seek to invert the condensation process turning the standard one-way condensation (from fine-group to coarse-group) into the first step of a two-way iterative process. The first method is based on the previously published Generalized Energy Condensation, which established a framework for obtaining the fine-group flux by preserving the flux energy spectrum in orthogonal energy expansion functions, but did not maintain a consistent coarse-group formulation. It is demonstrated that with a consistent extension of the GEC, a cross section recondensation scheme can be used to correct for the spectral core environment error. A more practical and efficient new method is also developed, termed the "Subgroup Decomposition (SGD) Method," which eliminates the need for expansion functions altogether, and allows the fine-group flux to be decomposed from a consistent coarse-group flux with minimal additional computation or memory requirements. In addition, a new whole-core BWR benchmark problem is generated based on operating reactor parameters in 2D and 3D, and a set of 1D benchmark problems is developed for a BWR, PWR, and VHTR core.
5
Hudiono, Yeny C. « Thermal transport properties of nanoporous zeolite thin films ». Diss., Atlanta, Ga. : Georgia Institute of Technology, 2008. http://hdl.handle.net/1853/24748.
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Thesis (Ph.D.)--Chemical Engineering, Georgia Institute of Technology, 2009.Committee Chair: Prof. Sankar Nair; Committee Co-Chair: Prof. Samuel Graham; Committee Member: Prof. Amyn S. Teja; Committee Member: Prof. Mo Li; Committee Member: Prof. Peter Ludovice.
6
Marquez, Damian Jose Ignacio. « Multilevel acceleration of neutron transport calculations ». Thesis, Atlanta, Ga. : Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/19731.
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Thesis (M.S.)--Nuclear and Radiological Engineering, Georgia Institute of Technology, 2008.Committee Chair: Stacey, Weston M.; Committee Co-Chair: de Oliveira, Cassiano R.E.; Committee Member: Hertel, Nolan; Committee Member: van Rooijen, Wilfred F.G.
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Zachreson, Matthew R. « Comparing Theory and Experiment for Analyte Transport in the First Vacuum Stage of the Inductively Coupled Plasma Mass Spectrometer ». BYU ScholarsArchive, 2015. https://scholarsarchive.byu.edu/etd/5610.
Texte intégralRésumé :
The inductively coupled plasma mass spectrometer (ICP-MS) has been used in laboratories for many years. The majority of the improvements to the instrument have been done empirically through trial and error. A few fluid models have been made, which have given a general description of the flow through the mass spectrometer interface. However, due to long mean free path effects and other factors, it is very difficult to simulate the flow details well enough to predict how changing the interface design will change the formation of the ion beam. Towards this end, Spencer et al. developed FENIX, a direct simulation Monte Carlo algorithm capable of modeling this transitional flow through the mass spectrometer interface, the transitional flow from disorganized plasma to focused ion beam. Their previous work describes how FENIX simulates the neutral ion flow. While understanding the argon flow is essential to understanding the ICP-MS, the true goal is to improve its analyte detection capabilities. In this work, we develop a model for adding analyte to FENIX and compare it to previously collected experimental data. We also calculate how much ambipolar fields, plasma sheaths, and electron-ion recombination affect the ion beam formation. We find that behind the sampling interface there is no evidence of turbulent mixing. The behavior of the analyte seems to be described simply by convection and diffusion. Also, ambipolar field effects are small and do not significantly affect ion beam formation between the sampler and skimmer cones. We also find that the plasma sheath that forms around the sampling cone does not significantly affect the analyte flow downstream from the skimmer. However, it does thermally insulate the electrons from the sampling cone, which reduces ion-electron recombination. We also develop a model for electron-ion recombination. By comparing it to experimental data, we find that significant amounts of electron-ion recombination occurs just downstream from the sampling interface.
8
Acevedo, Nancy Isabel Alvarez. « Uma formulação explícita matricial para problemas inversos de transferência radiativa em meios participantes homogêneos unidimensionais ». Universidade do Estado do Rio de Janeiro, 2006. http://www.bdtd.uerj.br/tde_busca/arquivo.php?codArquivo=372.
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Fundação Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de JaneiroA formulação explícita matricial desenvolvida nesta tese de doutorado foi proposta visando ser uma alternativa na solução de Problemas Inversos de estimativa de propriedades radiativas em meios participantes homogêneos unidimensionais usando a Equação de Transferência Radiativa para modelar a interação da radiação com o meio participante. A equação de transporte é formulada em forma matricial e o domínio angular é discretizado usando conceitos do método de ordenadas discretas e a expansão da função de fase do espalhamento anisotrópico em uma série de polinômios de Legendre. A formulação proposta consiste em uma formulação explícita para o problema inverso. Um arranjo apropriado das condições de contorno prescritas (fluxos incidentes) e dos fluxos emergentes nos contornos de uma placa permitem o cálculo direto do operador de transmissão, do operador albedo e do operador de colisão. A partir do operador de colisão calculado são obtidos os valores estimados dos coeficientes de extinção total e de espalhamento. São apresentadas as formulações para problemas em regime estacionário e em regime transiente, bem como os resultados para alguns casos-teste.
The explicit matrix formulation developed in the present thesis has been proposed as an alternative for the solution of Inverse Problems for radiative properties estimation in one-dimensional homogeneous participating media using Radiative transfer equation for the modeling of the radiation interaction with the participating medium. This transport equation is formulated in a matrix form and the angular domain is discretized using concepts of the discrete ordinates methods and the expansion of the function of phase function of anisotropic scattering in a series of Legendre polynomial. The formulation proposed consists on an explicit formulation for the inverse problem. An adequate assembly of the prescribed boundary conditions (incidents flux) and of the emerging flux at the boundaries of the slab allows the direct computation of the transmission, albedo and collision operators. From the computed collision operator estimated values for total extinction and scattering coefficients are obtained. The formulations for steady state and transient situations are presented, as well as test case results.
9
Tamasan, Alexandru Cristian. « A two dimensional inverse boundary value problem in radiation transport / ». Thesis, Connect to this title online ; UW restricted, 2002. http://hdl.handle.net/1773/5752.
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Yoshioka, Hiroki 1967. « Applications of transport theory in optical remote sensing of land surfaces ». Diss., The University of Arizona, 1999. http://hdl.handle.net/10150/284824.
Texte intégralRésumé :
A particle/radiative transport theory widely used in nuclear engineering was applied to investigate photon transport in layers of land surfaces which consist of vegetation and soil for application to optical remote sensing. A numerical simulation code has been developed for three dimensional vegetation canopies to compute reflected radiation by the canopy-soil systems. The code solves a discretized form of the linear Boltzmann transport equation using an Adaptive Weighted Diamond-Differencing and source iteration method. Sample problems demonstrate variations of reflectance spectra of vegetation canopies as a function of soil brightness and leaf area index, and also indicate a pattern of spectral variations induced by the soil brightness changes. Special attention has been paid to the variation patterns of canopy reflectances, known as vegetation isolines. Mathematical expressions of vegetation isolines, called vegetation isoline equations, are derived in terms of canopy optical properties and two parameters that characterize soil optical properties called soil line parameters. Behavior of vegetation isolines is analyzed using the derived equations as a function of leaf area index and fractional area covered by green-vegetation. The analyses show certain trends of the behavior of vegetation isolines. The vegetation isoline equations are then applied to investigate the performance of two-band vegetation indices and to estimate the effects of the soil line parameters. It is concluded that the vegetation isoline equations are useful for investigating patterns of canopy reflectance variations and the effects of these patterns on vegetation indices.
11
Walters, William Jonathan. « Development of the Adaptive Collision Source Method for Discrete Ordinates Radiation Transport ». Diss., Virginia Tech, 2015. http://hdl.handle.net/10919/52242.
Texte intégralRésumé :
A novel collision source method has been developed to solve the Linear Boltzmann Equation (LBE) more efficiently by adaptation of the angular quadrature order. The angular adaptation method is unique in that the flux from each scattering source iteration is obtained, with potentially a different quadrature order used for each. Traditionally, the flux from every iteration is combined, with the same quadrature applied to the combined flux. Since the scattering process tends to distribute the radiation more evenly over angles (i.e., make it more isotropic), the quadrature requirements generally decrease with each iteration. This method allows for an optimal use of processing power, by using a high order quadrature for the first few iterations that need it, before shifting to lower order quadratures for the remaining iterations. This is essentially an extension of the first collision source method, and is referred to as the adaptive collision source (ACS) method. The ACS methodology has been implemented in the 3-D, parallel, multigroup discrete ordinates code TITAN. This code was tested on a variety of test problems including fixed-source and eigenvalue problems. The ACS implementation in TITAN has shown a reduction in computation time by a factor of 1.5-4 on the fixed-source test problems, for the same desired level of accuracy, as compared to the standard TITAN code.Ph. D.
12
Singleterry, Robert Clay Jr. « Neutron transport associated with the galactic cosmic ray cascade ». Diss., The University of Arizona, 1993. http://hdl.handle.net/10150/186421.
Texte intégralRésumé :
Transport of low energy neutrons associated with the galactic cosmic ray cascade is analyzed in this dissertation. A benchmark quality analytical algorithm is demonstrated for use with B scRYNTRN, a computer program written by the High Energy Physics Division of N scASA Langley Research Center, which is used to design and analyze shielding against the radiation created by the cascade. B scRYNTRN uses numerical methods to solve the integral transport equations for baryons with the straight-ahead approximation, and numerical and empirical methods to generate the interaction probabilities. The straight-ahead approximation is adequate for charged particles, but not for neutrons. As N scASA Langley improves B scRYNTRN to include low energy neutrons, a benchmark quality solution is needed for comparison. The neutron transport algorithm demonstrated in this dissertation uses the closed-form Green's function solution to the galactic cosmic ray cascade transport equations to generate a source of neutrons. A basis function expansion for finite heterogeneous and semi-infinite homogeneous slabs with multiple energy groups and isotropic scattering is used to generate neutron fluxes resulting from the cascade. This method, called the F(N) method, is used to solve the neutral particle linear Boltzmann transport equation. As a demonstration of the algorithm coded in the programs M scGSLAB and M scGSEMI, neutron and ion fluxes are shown for a beam of fluorine ions at 1000 MeV per nucleon incident on semi-infinite and finite aluminum slabs. Also, to demonstrate that the shielding effectiveness against the radiation from the galactic cosmic ray cascade is not directly proportional to shield thickness, a graph of transmitted total neutron scalar flux versus slab thickness is shown. A simple model based on the nuclear liquid drop assumption is used to generate cross sections for the galactic cosmic ray cascade. The E scNDF/B V database is used to generate the total and scattering cross sections for neutrons in aluminum. As an external verification, the results from M scGSLAB and M scGSEMI were compared to A scNISN/P scC, a routinely used neutron transport code, showing excellent agreement. In an application to an aluminum shield, the F(N) method seems to generate reasonable results.
13
Roskoff, Nathan J. « Implementation and Verification of the Subgroup Decomposition Method in the TITAN 3-D Deterministic Radiation Transport Code ». Thesis, Virginia Tech, 2014. http://hdl.handle.net/10919/78127.
Texte intégralRésumé :
The subgroup decomposition method (SDM) has recently been developed as an improvement over the consistent generalized energy condensation theory for treatment of the energy variable in deterministic particle transport problems. By explicitly preserving reaction rates of the fine-group energy structure, the SDM directly couples a consistent coarse-group transport calculation with a set of fixed-source "decomposition sweeps" to provide a fine-group flux spectrum. This paper will outline the implementation of the SDM into the three-dimensional, discrete ordinates (SN) deterministic transport code TITAN. The new version of TITAN, TITAN-SDM, is tested using 1-D and 2-D benchmark problems based on the Japanese designed High Temperature Engineering Test Reactor (HTTR). In addition to accuracy, this study examines the efficiency of the SDM algorithm in a 3-D SN transport code.Master of Science
14
Connolly, Kevin John. « A coarse mesh radiation transport method for reactor analysis in three dimensional hexagonal geometry ». Diss., Georgia Institute of Technology, 2012. http://hdl.handle.net/1853/50149.
Texte intégralRésumé :
A new whole-core transport method is described for 3-D hexagonal geometry. This is an extension of a stochastic-deterministic hybrid method which has previously been shown highly accurate and efficient for eigenvalue problems. Via Monte Carlo, it determines the solution to the transport equation in sub-regions of reactor cores, such as individual fuel elements or sections thereof, and uses those solutions to compose a library of response expansion coefficients. The information acquired allows the deterministic solution procedure to arrive at the whole core solution for the eigenvalue and the explicit fuel pin fission density distribution more quickly than other transport methods. Because it solves the transport equation stochastically, complicated geometry may be modeled exactly and therefore heterogeneity even at the most detailed level does not challenge the method. In this dissertation, the method is evaluated using comparisons with full core Monte Carlo reference solutions of benchmark problems based on gas-cooled, graphite-moderated reactor core designs. Solutions are given for core eigenvalue problems, the calculation of fuel pin fission densities throughout the core, and the determination of incremental control rod worth. Using a single processor, results are found in minutes for small cores, and in no more than a few hours for a realistically large core. Typical eigenvalues calculated by the method differ from reference solutions by less than 0.1%, and pin fission density calculations have average accuracy of well within 1%, even for unrealistically challenging core configuration problems. This new method enables the accurate determination of core eigenvalues and flux shapes in hexagonal cores with efficiency far exceeding that of other transport methods.
15
Hayward, Robert M. « A coarse mesh transport method for photons and electrons in 3-D ». Diss., Georgia Institute of Technology, 2013. http://hdl.handle.net/1853/51928.
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A hybrid stochastic-deterministic method, COMET-PE, is developed for dose calculation in radiotherapy. Fast, accurate dose calculation is a key component of successful radiotherapy treatment. To calculate dose, COMET-PE solves the coupled Boltzmann Transport Equations for photons and electrons. The method uses a deterministic iteration to compose response functions that are pre-computed using Monte Carlo. Thus, COMET-PE takes advantage of Monte Carlo physics without incurring the computational costs typically required for statistical convergence. This work extends the method to 3-D problems with realistic source distributions. Additionally, the performance of the deterministic solver is improved, taking advantage of both shared-memory and distributed-memory parallelism to enhance efficiency. To verify the method’s accuracy, it is compared with the DOSXYZnrc (Monte Carlo) method using three different benchmark problems: a heterogeneous slab phantom, a water phantom, and a CT-based lung phantom. For the slab phantom, all errors are less than 1.5% of the maximum dose or less than 3% of local dose. For both the water phantom and the lung phantom, over 97% of voxels receiving greater than 10% of the maximum dose pass a 2% (relative error) / 2 mm (distance-to-agreement) test. Timing comparisons show that COMET-PE is roughly 10-30 times faster than DOSXYZnrc. Thus, the new method provides a fast, accurate alternative to Monte Carlo for dose calculation in radiotherapy treatment planning.
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DIAS, ARTUR F. « Metodo PsubN para calculos de blindagem em geometria de multiplacas ». reponame:Repositório Institucional do IPEN, 1999. http://repositorio.ipen.br:8080/xmlui/handle/123456789/10771.
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Tese (Doutoramento)
IPEN/T
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP
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Finlator, Kristian Markwart. « Comparing Cosmological Hydrodynamic Simulations with Observations of High-Redshift Galaxy Formation ». Diss., The University of Arizona, 2009. http://hdl.handle.net/10150/195788.
Texte intégralRésumé :
We use cosmological hydrodynamic simulations to study the impact of out-flows and radiative feedback on high-redshift galaxies. For outflows, we consider simulations that assume (i) no winds, (ii) a .constant-wind. model in which the mass-loading factor and outflow speed are constant, and (iii) "momentum driven" winds in which both parameters vary smoothly with mass. In order to treat radiative feedback, we develop a moment-based radiative transfer technique that operates in both post-processing and coupled radiative hydrodynamic modes. We first ask how outflows impact the broadband spectral energy distributions (SEDs) of six observed reionization-epoch galaxies. Simulations reproduce five regardless of the outflow prescription, while the sixth suggests an unusually bursty star formation history. We conclude that (i) simulations broadly account for available constraints on reionization-epoch galaxies, (ii) individual SEDs do not constrain outflows, and (iii) SED comparisons efficiently isolate objects that challenge simulations. We next study how outflows impact the galaxy mass metallicity relation (MZR). Momentum-driven outflows uniquely reproduce observations at z = 2. In this scenario, galaxies obey two equilibria: (i) The rate at which a galaxy processes gas into stars and outflows tracks its inflow rate; and (ii) The gas enrichment rate owing to star formation balances the dilution rate owing to inflows. Combining these conditions indicates that the MZR is dominated by the (instantaneous) variation of outflows with mass, with more-massive galaxies driving less gas into outflows per unit stellar mass formed. Turning to radiative feedback, we use post-processing simulations to study the topology of reionization. Reionization begins in overdensities and then .leaks. directly into voids, with filaments reionizing last owing to their high density and low emissivity. This result conflicts with previous findings that voids ionize last. We argue that it owes to the uniqely-biased emissivity field produced by our star formation prescriptions, which have previously been shown to reproduce numerous post-reionization constraints. Finally, preliminary results from coupled radiative hydrodynamic simulations indicate that reionization suppresses the star formation rate density by at most 10.20% by z = 5. This is much less than previous estimates, which we attribute to our unique reionization topology although confirmation will have to await more detailed modeling.
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MASSICANO, FELIPE. « Modelagem de um sistema de planejamento em radioterapia e medicina nuclear com o uso do código MCNP6 ». reponame:Repositório Institucional do IPEN, 2015. http://repositorio.ipen.br:8080/xmlui/handle/123456789/26371.
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Tese (Doutorado em Tecnologia Nuclear)
IPEN/T
Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP
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Ortiz, Carlos. « First Principles Calculations of Electron Transport and Structural Damage by Intense Irradiation ». Doctoral thesis, Uppsala : Acta Universitatis Upsaliensis, 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-102376.
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ANGELOCCI, LUCAS V. « Estudo de casos clínicos em radioterapia através do sistema de planejamento AMIGOBrachy ». reponame:Repositório Institucional do IPEN, 2016. http://repositorio.ipen.br:8080/xmlui/handle/123456789/26926.
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O sucesso de uma radioterapia depende do correto planejamento da dose a ser entregue ao volume alvo. Na braquiterapia, modalidade da radioterapia onde um radioisótopo selado é implantado intracavitariamente ou intersticialmente no paciente, há menos avanços em sistemas de planejamento de tratamento computacionais do que na teleterapia, amplamente mais utilizada nos serviços típicos. Porém, a braquiterapia, quando aplicável, é preferível por poupar tecidos sadios vizinhos de uma dose desnecessária. O AMIGOBrachy, um sistema de planejamento para braquiterapia de interface amigável, compatibilidade com outros sistemas comerciais em uso e integrado ao código MCNP6 (Monte Carlo N-Particle Transport Code v. 6) foi desenvolvido no Centro de Engenharia Nuclear do Instituto de Pesquisas Energéticas e Nucleares (CEN-IPEN) e atualmente está em processo de validação. Este trabalho contribuiu para este processo, avaliando três diferentes casos clínicos através do AMIGOBrachy com o formalismo do TG43 da AAPM (Associação Americana de Física Médica), protocolo que rege a dosimetria em braquiterapia, e comparando seus resultados com as distribuições de dose calculadas por outros sistemas comerciais consagrados: Varian BrachyVision TM (Varian Medical Systems; Palo Alto, CA, EUA) e Nucletron Oncentra® (Elekta; Estocolmo, Suécia). Os resultados obtidos estão dentro de uma faixa de concordância de ±10%, estando mais discrepantes em regiões muito próximas do aplicador, onde os sistemas de planejamento comerciais e o AMIGOBrachy divergem devido aos diferentes métodos de cálculo. Em pelo menos dois terços da região de interesse, porém, a dose concordou em uma faixa de ±3% para os três casos. Também foram realizadas simulações utilizando o formalismo do TG186 da AAPM, que considera heterogeneidades no tecido, para avaliar o impacto dos mesmos na dose. Em adição ao processo de validação, também foi realizado um estudo em braquiterapia oftálmica para posterior inserção de um módulo adicional ao AMIGOBrachy; para isso, um modelo de olho humano foi desenvolvido utilizando geometria UM (Unstructured Mesh), para validação com o código MCNP6, que apenas nesta versão demonstra um novo recurso capaz de simular uma geometria híbrida: parcialmente analítica, parcialmente UM. O modelo considera dez diferentes estruturas no olho humano: esclera, coroide, retina, corpo vítreo, córnea, câmara anterior, lente, nervo óptico, parede do nervo óptico, e um tumor definido de forma arbitrária crescendo da superfície externa do globo ocular em direção ao seu centro. Os resultados foram comparados com um modelo de olho puramente analítico modelado com o MCNP6 e tomado como referência. Os resultados foram satisfatórios em todas as simulações desenvolvidas, exceto para as estruturas do nervo óptico e sua parede, que devido ao seu pequeno tamanho e distância da fonte, mostraram erros relativos maiores, mas ainda menores que 10%, e não representam problema de preocupação clínica uma vez que recebem doses muito pequenas. Discutiu-se também a eficácia e problemas encontrados nessa nova capacidade do código MCNP de simular geometrias híbridas, uma vez que é recente e ainda apresenta deficiências, que tiveram que ser contornadas no presente trabalho.
Dissertação (Mestrado em Tecnologia Nuclear)
IPEN/D
Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP
21
Pattelli, Lorenzo. « Imaging light transport at the femtosecond scale ». Doctoral thesis, 2018. http://hdl.handle.net/2158/1157248.
Texte intégralRésumé :
In this thesis work we investigated light transport in scattering media from a spatio-temporal perspective. To this purpose, we have designed and developed an experimental optical-gating setup and a new Monte Carlo software library focused on the measurement and simulation of spatio-temporal evolution of light transport down to a sub-picosecond resolution. The unique properties of both these tools allowed us to unveil an array of unexplored aspects of light propagation occurring in the extremely general plane-parallel slab geometry, which are of relevance both for application and fundamental purposes due to their asymptotic nature.
Indeed, time-domain techniques are commonly considered to offer the most straightforward and powerful characterization capabilities, in that they allow to selectively address different transport regimes and directly observe their evolution. However, as we extensively demonstrate, combining the temporal information with wide-field spatial imaging capabilities offers a significant improvement, unveiling a set of irreducible information that could not be accessed from separate spatial and temporal characterizations.
22
PATTELLI, LORENZO. « Imaging light transport at the femtosecond scale : a walk on the wild side of diffusion ». Doctoral thesis, 2017. http://hdl.handle.net/2158/1087854.
Texte intégralRésumé :
In this thesis work we investigated light transport in scattering media from a spatio-temporal perspective. To this purpose, we have designed and developed an experimental optical-gating setup and a new Monte Carlo software library focused on the measurement and simulation of spatio-temporal evolution of light transport down to a sub-picosecond resolution. The unique properties of both these tools allowed us to unveil an array of unexplored aspects of light propagation occurring in the extremely general plane-parallel slab geometry, which are of relevance both for application and fundamental purposes due to their asymptotic nature.
Indeed, time-domain techniques are commonly considered to offer the most straightforward and powerful characterization capabilities, in that they allow to selectively address different transport regimes and directly observe their evolution. However, as we extensively demonstrate, combining the temporal information with wide-field spatial imaging capabilities offers a significant improvement, unveiling a set of irreducible information that could not be accessed from separate spatial and temporal characterizations.
23
Miller, Karen Ann. « An Inverse Source Location Algorithm for Radiation Portal Monitor Applications ». Thesis, 2010. http://hdl.handle.net/1969.1/ETD-TAMU-2010-05-7772.
Texte intégralRésumé :
Radiation portal monitors are being deployed at border crossings throughout the world to prevent the smuggling of nuclear and radiological materials; however, a tension exists between security and the free-flow of commerce. Delays at ports-of-entry have major economic implications, so it is imperative to minimize portal monitor screening time. We have developed an algorithm to locate a radioactive source using a distributed array of detectors, specifically for use at border crossings.
To locate the source, we formulated an optimization problem where the objective function describes the least-squares difference between the actual and predicted detector measurements. The predicted measurements are calculated by solving the 3-D deterministic neutron transport equation given an estimated source position. The source position is updated using the steepest descent method, where the gradient of the objective function with respect to the source position is calculated using adjoint transport calculations. If the objective function is smaller than a predetermined convergence criterion, then the source position has been identified.
To test the algorithm, we first verified that the 3-D forward transport solver was working correctly by comparing to the code PARTISN (Parallel Time-Dependent SN). Then, we developed a baseline scenario to represent a typical border crossing. Test cases were run for various source positions within each vehicle and convergence criteria, which showed that the algorithm performed well in situations where we have perfect knowledge of parameters such as the material properties of the vehicles. We also ran a sensitivity analysis to determine how uncertainty in various parameters-the optical thickness of the vehicles, the fill level in the gas tank, the physical size of the vehicles, and the detector efficiencies-affects the results. We found that algorithm is most sensitive to the optical thickness of the vehicles. Finally, we tested the simplifying assumption of one energy group by using measurements obtained from MCNPX (Monte Carlo N-Particle Extended). These results showed that the one-energy-group assumption will not be sufficient if the code is deployed in a real-world scenario. While this work describes the application of the algorithm to a land border crossing, it has potential for use in a wide array of nuclear security problems.