Thesis (M.S.)--West Virginia University, 2007. / Title from document title page. Document formatted into pages; contains xiii, 97 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 72-74).

The sensitivity reduction or ghosting mechanism of x-ray photoconductor is studied based on Monte Carlo simulation techniques. We have calculated the sensitivity reduction for different detector operating conditions (applied electric field, x-ray spectrum and photoconductor thickness) and for different levels of carrier trapping. We have analyzed the effect of photoconductor biasing (positive or negative) on ghosting. The following effects are taken into account in modeling the ghosting phenomena: (i) recombination between trapped and oppositely charged drifting carriers, (ii) trap filling, (iii) nonuniform electric field, (iv) detrapping of trapped holes, and (v) x-ray induced trap generation. Our calculation shows that not only the recombination between trapped and oppositely charged drifting carriers but the x-ray induced trap generation is also responsible for ghosting in photoconductor based x-ray image detectors. Moreover not all the trapped carriers take part in recombination; rather only a fraction of the trapped carriers are involved in recombination. Electric field also plays an important role in ghosting calculations via the electron hole pair generation mechanism. Trap filling has also non trivial effects on ghosting. The simulation results show that the amount of ghosting strongly depends on the applied electric field. Ghosting increases with decreasing applied electric field and vice versa. It is observed that ghosting is higher at high carrier trapping level than at low trapping level. Again ghosting is more pronounced in chest radiographic detector than mammographic detector. In chest radiographic detector, carrier trapping is high due to greater thickness hence recombination and electric field effects are prominent in chest radiographic detector. Biasing dependent ghosting depends on the carrier mobility lifetime product. For positively biased detectors, ghosting is less if the mobility lifetime product of hole is higher than that of electron and vice versa for negatively biased detectors. It also appears that the use of only recombination to calculate ghosting, as believed the primary source of ghosting in some literatures, will lead to significant error in the calculation of ghosting.

En instal·lacions de radiació sincrotró, es necessiten detectors amb una bona resolució espacial i un alt ritme de contes local per tal de dur a terme experiments de difracció de raigs x en l'escala temporal del mil·lisegon. Els detectors de gas de micro-fabricació (MPGD) poden satisfer aquests requeriments gràcies a l'alta densitat d'elèctrodes que presenten. No obstant aquest tipus de detectors presenten dos grans problemes: descàrregues elèctriques destructives i un deteriorament continu durant el seu us continuat.En aquest treball presentem una nova estructura de MPGD: el Micro Reading Mesh Chamber (MRMC). És un MPGD amb un estadi d'amplificació. El detector està dividit en dues regions: la regió de deriva i la regió de multiplicació. Aquestes estan separades per una malla formada per elèctrodes de lectura. El pla de l'ànode té una capa de material resistiu a la seva superfície.El disseny del MRMC redueix els problemes que els actuals MPGD presenten. Entre d'altres avantatges, el posicionament dels elèctrodes de lectura a la malla incrementa la sensibilitat dels càtodes a la senyal. Això permet una reducció de la intensitat del camp elèctric de multiplicació. En conseqüència, la càrrega del detector i la probabilitat de descàrregues destructives es redueix.S'han realitzat diverses simulacions per tal d'avaluar i optimitzar el rendiment del detector. Degut al gran nombre de paràmetres que poden ser escollits i del gran nombre de característiques que en depenen, s'ha realitzat un treball de simulació i optimització global. En primer lloc, s'ha dissenyat un programa basat en tècniques Monte-Carlo per tal de simular el procés de detecció i poder així avaluar la resolució espacial en funció dels diferents paràmetres del detector. En segon lloc, s'han realitzat una sèrie de simulacions que involucren la simulació del moviment en tres dimensions de les càrregues i la seva multiplicació dins l'estructura de detecció. Aquests estudis inclouen la estimació de la transparència de la malla, l'estudi de les línies i els temps de deriva dels ions, i la simulació del procés d'allau. Part d'aquestes simulacions han estat verificades experimentalment. Finalment, les capacitancies entre els elèctrodes i l'ànode i entre elèctrodes s'ha simulat per tal d'avaluar el soroll electrònic i els efectes de "crosstalk".S'ha construït i provat un prototip. Els tests realitzats mostren una bona concordança amb les simulacions del guany. Així doncs, queda demostrat que el MRMC pot arribar a ritmes de contatge local >1.25·105Hz/mm2. No obstant, s'observen guspires en el detector degut a la presència de material dielèctric a la superfície dels càtodes. Degut a la superfície resistiva i la robustesa de l'estructura, el detector ha resistit les descàrregues sense rebre ningun dany. En conseqüència, per tal de que el MRMC esdevingui un detector útil, s'han de millorar tant el procediment de construcció, com les tècniques de neteja.En conclusió, es necessita més treball experimental per tal d'obtenir un detector lliure de descàrregues completament funcional pel qual les simulacions prediuen un alt ritme de comptes local (>1.25·105Hz/mm2) i una bona resolució espacial (>250?m). / At synchrotron radiation facilities, detectors with high spatial resolution and high local count rate are needed in order to perform x-ray diffraction/scattering on time resolved experiments on the sub-millisecond time scale. Micro Pattern Gas Detectors (MPGD) could provide good spatial resolutions and high count rate thanks to their small cells of amplification region. Despite their promising performance, two major problems have arisen: rare but damaging discharges and, slow but continuous deterioration (aging) during sustained irradiationHere we present a new MPGD structure: the Micro Reading Mesh Chamber (MRMC). It is a one stage amplification MPGD. It is divided in two regions: the drift/conversion region and the multiplication region. They are separated by a mesh formed by thick (25?m) pick-up strips. The anode plane has a resistive layer on it.The design of the MRMC reduces the problems that the present MPGD show. Among other advantages, placing the pick-up strips at the mesh, instead of at another readout plane, increases the signal sensitivity of the cathodes. It allows a decrease of the gain through a reduction of the multiplication field. Therefore, it reduces the charging up and the probability of voltage and rate-induced dielectric breakdown.Different simulations have been carried out to evaluate and optimize the performance of the detector. Due to the large number of detector parameters that can be chosen, and the features that depend on them, a global simulation was done. Firstly, a Monte-Carlo based program that reproduces the detection in the chamber has been developed in order to simulate the spatial resolution as a function of different parameters of the detector. Secondly, a set of studies that involve the simulation of the three dimensional movement of the charges in the chamber and its multiplication have been carried out. These models include the estimation of the mesh transparency, the study of the ion drift paths and times, and the simulation of the avalanche process. Part of these simulations has been cross-checked experimentally. Finally, the capacitances between the strips and the anode, and between strips has been simulated in order to evaluate the electronic noise and the crosstalk effects.After the simulation work, a prototype was built and tested. The tests show good agreement between the simulated and the experimental gain. This agreement on the space charge calculations demonstrates that the MRMC can deliver local count rates >1.25·105Hz/mm2. Moreover, due to the resistive layer and the strength of the structure, the detector survived without noticeable damage. However, the detector suffers from discharges because of dielectric deposits on the cathode surface. Therefore, to turn this device into a detector for routine use, it will be necessary to both improve the construction process of the mesh structure and to develop an exhaustive cleaning procedure.In conclusion, more experimental work is necessary to obtain a spark protected functional detector for which the simulations predict a high local count rate (>1.25·105Hz/mm2) and a good spatial resolution).

centenares de keV hasta unos pocos MeV), presenta un extraordinario potencial para comprender los procesos más extremos que tienen lugar en el Universo, como las explosiones estelares o los aceleradores de partículas. No obstante, a pesar de los enormes esfuerzos realizados por los observatorios de rayos gamma (en el pasado y en la actualidad), se requiere una mejora en la sensibilidad instrumental para aprovechar el enorme potencial científico contenido en este rango energético. Durante las dos últimas décadas se han buscado formas de mejorar la sensibilidad de los instrumentos, incrementando la eficiencia de los detectores y reduciendo el ruido instrumental (mediante sofisticados mecanismos de blindaje y técnicas de análisis). Con este objetivo, un enorme esfuerzo en innovación instrumental (construcción de prototipos y estudios numéricos) está siendo realizado por una comunidad creciente de científicos que se enfrentan al reto de preparar la próxima generación de telescopios de rayos gamma. En particular, son especialmente notables los avances logrados en los últimos años en el campo de la focalización de rayos gamma mediante lentes de difracción. Conceptualmente, una lente de rayos gamma reduciría drásticamente el ruido instrumental ya que concentra los fotones en un detector de pequeñas dimensiones (el ruido es proporcional al volumen del instrumento). Una lente de difracción, para observaciones en astrofísica nuclear, no es sólo un concepto teórico, sino una realidad, gracias principalmente al proyecto CLAIRE. Asimismo, el desarrollo de la tecnología para la focalización de rayos gamma ha incentivado el desarrollo de las diferentes tecnologías de detección. Un detector apropiado para el plano focal de una lente gamma, debe disponer de capacidad de imagen, proporcionar espectroscopia de alta resolución y medir la polarización de los fotones incidentes. El trabajo presentado en esta tesis comprende tanto la óptica de focalización como el detector del plano focal. Con respecto a la óptica, se presenta el ensayo realizado con el prototipo CLAIRE, mediante el cual se ha confirmado los principios de una lente de difracción. En cuanto al plano focal, esta investigación se ha desarrollado principalmente en el marco de estudio de las misiones espaciales GRI (2007) y DUAL (2010), propuestas a la ESA dentro del programa “Cosmic Vision 2015-2025”. En el marco de la misión GRI, se presenta una configuración para el detector del plano focal basado en detectores pixelados de Cd(Zn)Te, al tiempo que se investiga y desarrolla un primer prototipo de detector pixelado de CdTe. Cabe destacar que el sistema de detección propuesto fue registrado con éxito mediante una patente europea, y está siendo desarrollado para su aplicación en medicina nuclear. En relación a la propuesta DUAL, se presenta un estudio del ruido instrumental obtenido mediante simulaciones numéricas con el fin de precisar la sensibilidad del instrumento (basado en detectores de Germanio) propuesto en esta misión. Más allá de las tecnología consideradas en GRI y DUAL, una amplia variedad de detectores pueden ser explorados, bien para el plano focal de una lente de difracción o bien como sistemas de detección por sí mismos. En este sentido, se ha ampliado el espectro de tecnologías y se ha incluido un estudio sobre detectores basados en xenón líquido. En esta tesis se ha realizado un trabajo de investigación y desarrollo con tecnologías vanguardistas propuestas para la próxima generación de telescopios de rayos gamma. Esta instrumentación debe enfrentarse al reto de alcanzar la sensibilidad requerida para dar respuesta a las cuestiones aun no resultas por la astrofísica de rayos gamma en el rango de energía de las transiciones nucleares. / few MeV) has an extraordinary potential for understanding the evolving and violent Universe. In spite of the strong efforts accomplished by past and current instruments, in order to perform observations in this energy range, an improvement in sensitivity over present technologies is needed to take full advantage of the scientific potential contained in this energy range. In order to achieve higher sensitivities, γ-ray astronomy has been looking over the last decades for new ways to increase the efficiency of its instruments while reducing the background noise. With the objective of reducing or avoiding as much background as possible (through shielding mechanisms and data analysis techniques), a strong effort in innovation and design (build-up of prototypes and numerical simulations studies) is being conducted by a community facing the challenge of preparing the next generation of γ-ray telescopes. In particular, the progress achieved during the last decade on focusing optics based on Laue lenses is especially remarkable. Conceptually, a focusing telescope will reduce drastically the background noise by concentrating γ-rays onto a small size detector. Focusing γ-rays with a Laue lens is not just a theoretical concept, but a reality, mainly thanks to the development of a first prototype of Laue lens for nuclear astrophysics accomplished as part of the CLAIRE project. Moreover, the development of focusing optics during these years has also encouraged the development of new detector technologies. The focal plane detector of a focusing telescope should provide imaging capabilities, perform high-resolution spectroscopy and measure the polarization of the incident photons in order to achieve the ambitious scientific goals. The research presented in this thesis covers both main areas of a γ-rays telescope: focusing optics and focal plane detector. As far as the optics is concerned, a test of the lens CLAIRE was performed in order to confirm the principles of a Laue diffraction lens. Concerning the focal plane detector, theoretical and experimental studies with new detector technologies have been carried out. Our main research has evolved in the framework of two mission concept studies -GRI (2007) and DUAL (2010)- submitted to the ESA Calls for a Medium-size mission opportunity within the Cosmic Vision 2015-2025 program. As far as the GRI mission is concerned, a focal plane detector configuration based on Cd(Zn)Te pixelated detectors is proposed, whilst development and testing of a detector prototype is accomplished. It is noteworthy that the detector configuration was successfully registered under a European Patent and is being considered for applications in the field of nuclear medicine. Regarding the DUAL mission, simulations of the expected space radiation environment and the resulting detector activation were carried out in order to estimate the performances of the all-sky Compton telescope of DUAL (based on Germanium-strip detectors). The results show that DUAL could achieve, after two years of operation, a continuum sensitivity one order of magnitude better than any past and current observatory in the MeV energy range and up to a factor 30 of improvement with its Laue lens. Beyond the detector technology proposed for GRI and DUAL, a wide variety of technologies could be explored for the focal plane of a γ-ray lens mission as well as for a stand-alone detector. In this thesis a focal plane detector based on liquid xenon is also considered. This work faces the challenges of the next generation of γ-ray telescopes, where high performance γ-ray detectors are necessary to achieve the required sensitivity in order to answer several hot scientific topics of Gamma-ray astrophysics in the energy range of nuclear transitions.

In this work, a new electrochemical detection method was developed with the ability to determine a wide range of inorganic and organic species at, trace levels. In brief, the detection method takes advantage of all possible electrochemical reactions that may occur during scanning of the electrode potential. Changes in the detector response are mainly the result of inhibition of oxygen adsorption and hydrogen adsorption, alteration of electrical properties of the double layer, or redox processes of the adsorbate. Various electrochemical techniques were examined in the measurements; i.e. cyclic voltammetry, pulse amperometric detection, and square wave voltammetry. In those electrochemical techniques, the detection was carried out in a stripping mode after accumulation of analytes on the electrode surface. The smallest discernable signal is associated with about 0.1% surface coverage, which corresponds to the adsorption of about 10-18 mol of analyte on a ultramicroelectrode 5-[mu]m in radius. The response time of the detector to the concentration change in most cases is less than 1s. Electrochemical conditioning of the working electrode is sufficient to ensure a stable response for a period of several hours. It appears that square wave and cyclic voltammetry techniques are more suitable for the detection method. The linear dynamic range of the calibration curve depends on the characteristic of the analyte-electrode bond and redox processes of the analyte, which may occur at the electrode surface. For instance, for strongly adsorbing molecules the linear dynamic range extends over two orders of magnitude from about 10 -7 M to 10-5 M and for electroactive compounds from about 10-8 M to 10-4 M. In general, the relative standard deviation for replicate determinations was lower than 5%. Moreover, in these analyses, removal of oxygen from the analyzed solutions is not required.

The sensitivity reduction or ghosting mechanism of x-ray photoconductor is studied based on Monte Carlo simulation techniques. We have calculated the sensitivity reduction for different detector operating conditions (applied electric field, x-ray spectrum and photoconductor thickness) and for different levels of carrier trapping. We have analyzed the effect of photoconductor biasing (positive or negative) on ghosting. The following effects are taken into account in modeling the ghosting phenomena: (i) recombination between trapped and oppositely charged drifting carriers, (ii) trap filling, (iii) nonuniform electric field, (iv) detrapping of trapped holes, and (v) x-ray induced trap generation. Our calculation shows that not only the recombination between trapped and oppositely charged drifting carriers but the x-ray induced trap generation is also responsible for ghosting in photoconductor based x-ray image detectors. Moreover not all the trapped carriers take part in recombination; rather only a fraction of the trapped carriers are involved in recombination. Electric field also plays an important role in ghosting calculations via the electron hole pair generation mechanism. Trap filling has also non trivial effects on ghosting. The simulation results show that the amount of ghosting strongly depends on the applied electric field. Ghosting increases with decreasing applied electric field and vice versa. It is observed that ghosting is higher at high carrier trapping level than at low trapping level. Again ghosting is more pronounced in chest radiographic detector than mammographic detector. In chest radiographic detector, carrier trapping is high due to greater thickness hence recombination and electric field effects are prominent in chest radiographic detector. Biasing dependent ghosting depends on the carrier mobility lifetime product. For positively biased detectors, ghosting is less if the mobility lifetime product of hole is higher than that of electron and vice versa for negatively biased detectors. It also appears that the use of only recombination to calculate ghosting, as believed the primary source of ghosting in some literatures, will lead to significant error in the calculation of ghosting.

The Mg-modified lead titanate [(Pb1-xMgx)TiO3,abbreviated to PMT] thin films were deposited on Pt/SiO2/Si substrates by spin coating with sol-gel processing in this thesis. 1,3 propanediol was used as solvent to minimize the number of cycles of spin coating and drying processes to obtain the desired thickness of thin film. By changing the Mg content (0~10 mol%) and the heating temperature (500~800¢J), the effects of various processing parameters on the thin films growth are studied. The effects of various Mg contents on the response of pyroelectric IR detector devices are studied also. Experimental results reveal that the Mg contents will influence strongly on grain size, dielectricity, ferroelectricity and pyroelectricity of PMT thin films. With the increase of Mg content, the grain size of PMT thin film decrease slightly, and the lattice structure of PMT thin film will change from tetragonal to cubic when the Mg content over 8 mole%. The relative dielectric constant of PMT thin film increases from 25 up to 91. The tan

The object of the present research is to design and fabricate metal-dielectric thin film multilayer structures that make use of the nonlinear optical (NLO) response of Ag for efficient nonlinear absorption for sensor protection. These structures employ structural resonances to overcome the challenges of reflection and absorption that limit access to this large NLO response. The research consists of three parts: first, we present a comprehensive analysis of the contributions to the nonlinear optical response of Ag. Second, we present a systematic investigation of the linear optical properties of Metal-Dielectric Photonic Band-Gap (MDPBG) structures, including optimization of the structure for a particular transmittance spectrum. Third, we study the linear and nonlinear optical properties of Induced Transmission Filters (ITFs). Each of these parts includes experimental results backed by modeling and simulation.

Thesis (Ph. D.)--State University of New York at Binghamton, Chemistry Department, 2001. / Includes bibliographical references (leaves 209-237).

Thesis (M.S.)--California Polytechnic State University, 2009. / Title from PDF title page; viewed on January 13, 2010. Major professor: William Ahlgren, Ph.D. "Presented to the Electrical Engineering faculty of California Polytechnic State University, San Luis Obispo." "In partial fulfillment of the requirements for the degree [of] Master of Science in Electrical Engineering." "December 2009." Includes bibliographical references (p. 81-87).