Power MOSFETs are often required to operate in a space radiation environment; therefore, they are susceptible to a catastrophic failure mode called single-event burnout. Single-event burnout of power MOSFETs is initiated by the passage of an energetic-heavy ion through the parasitic BJT inherent to the power-MOSFET structure. The electron-hole pairs generated by the ion support a short-lived current source which imposes a base-emitter voltage on the parasitic BJT. If a sufficient base-emitter voltage is imposed, the parasitic BJT enters second breakdown and burnout of the MOSFET occurs. A semi-analytical model has been developed to predict the energy required of the incident ion to initiate burnout. This thesis addresses the portion of this model which relates the energy of the incident ion to the base-emitter voltage imposed on the parasitic BJT. The initial base-emitter potential is determined using image-source techniques.

A free neutron will disintegrate into a proton after about fourteen and a half minutes, emitting an electron and an antineutrino. The Standard Electroweak Model epitomizes the process with a decay probability distribution function (PDF) derived by Jackson, Trieman, and Wyld. This function depends on the momenta of the daughter particles and the spin of the neutron. Each correlation between these vector quantities is characterized by a correlation coefficient. The work of this dissertation focused on two experiments investigating these coefficients, and thus, the properties of the beta-decay process for polarized neutrons The first of these, called emiT, was an experiment to measure the coefficient (D) that characterizes the triple product between these observables; a product which changes sign under time reversal. The experiment has a four-fold symmetric design to improve statistics over previous measurements of D while mitigating systematics, and has been completed with a result pending. My contribution, in addition to some assembly of the apparatus, consisted of performing an analysis of the data and investigating overall systematics. These systematic issues ultimately precluded the use of the analysis method I employed, however, I was able to give an account of the size of the systematic effects, and other methods have been developed to analyze the data while evading these effects The other experiment, aCORN, will measure the a coefficient, the correlation between the electron and antineutrino momenta. The apparatus for this experiment is now being constructed. The a coefficient is one of the least-precisely known of the correlation coefficients. There have been only three previous attempts to measure it, and all of those experiments relied on the same method, which possesses a systematic limitation in the precision at which a can be measured that is not easy to overcome. aCORN relies on a completely new idea, not limited by the same systematics, which promises to improve this precision significantly. My charge for aCORN was to assemble and test the backscatter-suppressing electron spectrometer to be used, and currently it is the only component which is sufficiently completed and ready for the experimental run / acase@tulane.edu

The material uniformity of cadmium zinc telluride (CZT) crystals in gamma-ray imaging detectors is examined using several existing techniques and a new technique called thermally stimulated current (TSC) imaging that has been developed for this dissertation. The TSC imaging model, simulations, and experimental demonstrations are presented here for the first time. CZT radiation detectors are used in nuclear medicine as well as other medical, industrial, national security, and scientific applications; however, the scarcity and cost of high-quality CZT materials have hindered the use of CZT in these applications. Understanding CZT's material properties and their effects on detector performance should be helpful in developing crystal growth methods that have improved yield of useful detector material. Data obtained from CZT samples using infrared transmission, electron microprobe, X-ray diffraction, and electron backscatter diffraction (EBSD) mapping methods are used to understand their crystal structure. Data obtained from these samples using both TSC imaging and conventional leakage current measurements while these samples were operated as pixelated detector arrays are used to understand their charge transport properties. Collimated gamma-ray mapping was used to understand the detector performance properties of these samples. Correlations among these spatially mapped data are investigated. Contrary to the suggestions of other researchers, it is found that leakage current is not inversely correlated with detector performance. Detector performance in these samples is well correlated with their crystal structure. High-angle grain boundaries are shown to trap charge carriers, and estimates of the locations of these boundaries are derived from the gamma-ray mapping data. EBSD distinguishes itself from X-ray diffraction methods in identifying the locations and types of grain boundaries intersecting the sample surface. Using the new TSC imaging method, evidence is obtained showing a higher density of a particular trap near incommensurate boundaries in a CZT sample. Other researchers have indicated that an electron trap associated with dislocations is present in CZT. Their observation is consistent with a conclusion drawn from these TSC imaging data that due to higher densities of dislocations near incommensurate grain boundaries these boundaries host electron traps while {111} twin boundaries do not.

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.

Semiconductor-based gamma-ray-imaging detectors are under development for use in high-resolution nuclear medicine imaging applications. These detectors, based on cadmium zinc telluride, hold great promise for delivering improved spatial resolution and detection efficiency over current methods. This dissertation presents work done on three fronts, all directed toward enhancing the practicality of these imaging devices. Electronic readout systems were built to produce gamma-ray images from the raw signals generated by the imagers. Mathematical models were developed to describe the detection process in detail. Finally, a method was developed for recovering the energy spectrum of the original source by using maximum-likelihood estimation techniques. Two electronics systems were built to read out signals from the imaging detectors. The first system takes signals from a 48 x 48-pixel array at 500 k samples per second. Pulse-height histograms are formed for each pixel in the detector, all in real time. A second system was built to read out four 64 x 64 arrays at 4 million pixels per second. This system is based on digital signal processors and flexible software, making it easily adaptable to new imaging tasks. A mathematical model of the detection process was developed as a tool for evaluating possible detector designs. One part of the model describes how the mobile charge carriers, which are released when a gamma ray is absorbed in a photoelectric interaction, induce signals in a readout circuit. Induced signals follow a "near-field effect," wherein only carriers moving close to a pixel electrode produce significant signal. Detector pixels having lateral dimensions that are small compared to the detector thickness will develop a signal primarily due to a single carrier type. This effect is confirmed experimentally in time-resolved measurements and with pulse-height spectra. The second part of the model is a simulation of scattering processes that take place when a gamma ray is absorbed within the detector volume. A separate simulation predicts the spreading of charge carriers due to diffusion and electrostatic forces. The models are used in a technique to improve the energy resolution of the detectors by estimation of the source spectrum using the Expectation-Maximization algorithm.

Research has been done for the calibration of a thermal infrared imaging system for the measurement of transmission line conductor sample emissivities. The spectral response of the imaging system was from 8 to 12 micrometers. A laboratory set-up was designed and built for this analysis. The laboratory equipment consisted of a FLIR 1000a thermal infrared imaging system, a stainless steel black-body source with dual emissivity front surface coatings, an Elexor Industries PL1000 data acquisition and control system, and an IBM personal computer AT with imaging board and imaging software. The emissivities of the conductor samples were obtained through the analysis of thermal infrared images of each conductor with a blackbody cavity and low emissivity source simultaneously imaged as references. This analysis gave statistical mean grey levels for each conductor sample and references. From these mean grey levels the emissivity of the conductor samples were computed. Nine transmission line conductor sample emissivities where measured to an average accuracy of 17.5%. The emissivities ranged from .451 to .959.

The combination of excellent space-based remote sensing, and image reconstruction techniques, as well as improvements in numerical modeling, help enhance our understanding of particle transport in solar flares. We conduct a rigorous analysis of flare hard X-ray emission using the unprecedented spectral and spatial resolution of the RHESSI telescope data in order to better understand the spectral properties of the emitting electron population in solar flares. We complete our study with a forward-fit to the data using a Fokker-Planck kinetic code, to numerically model the particle transport in phase-space in realistic magnetic geometries and for different particle injection profiles.

Satellite observations have shown that fluxes of relativistic electrons in the earth's radiation belts can vary by orders of magnitude during periods of high solar activity. Understanding the dynamic behavior of these particles is very important because they can disrupt wireless communication, impair space exploration and affect GPS navigation. We use two numerical methods to simulate the variations of energetic particles in the radiation belts. First, we develop a radial diffusion model with time-dependent boundary conditions and a Kp-dependent electron lifetime model. Using this model, we simulate a series of high-speed-stream declining-phase magnetic storm events. The results are consistent with spacecraft observations and show that radial diffusion can propagate the enhancement of phase space density from the outer boundary into the center of the outer radiation belt. The second part of the work adapts Nunn's Vlasov Hybrid Simulation method to an existing MHD-Particle simulation code, resulting in an efficient new method to calculate phase space density of energetic particles. We use the 1995 January storm event as a test case. Good agreement is obtained between the simulation results and measured phase space densities for this event. Simulating the dynamics of the radiation belts is one important part of global space weather modeling. The advance in radiation belt modeling can help us to better understand the physics behind these interesting and important phenomena.

A model for simulation of laser produced plasmas that couples absorbed energy, plasma dynamics and radiation pressure was developed. The unusual experimental result of spectral narrowing of reflected laser pulses was attributed to deceleration of the critical (reflective) electron density surface due to the radiation pressure. A mechanism different from the Doppler effect that can cause spectral shifts was identified; a plasma reflectivity that changes rapidly in time can act as a frequency filter when the reflected laser pulse is chirped. The plasma expansion length and temperature that were calculated with the model was used to estimate the X-ray emission from the plasma with a simple scheme involving Kirchhoffs radiation law and Planck's black body radiation law.

A setup for the realization of dual-phase experiments using xenon as the active medium in a radiation detector has been built. The setup consists of a gas purification system capable of achieving a purity of the gas in the ppb level and a chamber system consisting of an ionization chamber containing the sensitive elements and a cooling component used to reach cryogenic temperatures inside the chamber in the range of liquid xenon temperature. The main goal of the dual-phase experiments is the operation of gas electron multipliers (GEM) in a cryogenic environment similar to the conditions found in experiments aimed to detect the most promising candidate for dark matter, i.e. the lightest supersymmetric particle known as neutralino or WIMPS.