The purpose of this research was to evaluate the performance of the imaging characteristics of two versions of a cadmium zinc telluride (CZT) gamma radiation detector system called the Laboratory Radioactive Assay Tracer (LabRAT). The performance evaluation follows the National Electrical Manufacturers Association standards for pixellated detector cameras. The LabRAT detector system hardware was developed by Mosaic Imaging Technology, Inc. LabRAT is a portable nuclear medicine imaging detector system intended for small field of view applications such as small animal imaging, portable radioisotope imaging in emergency room or intensive care units, and as an instruction tool for radiology residents and physics students. The tests performed include the measurement of count rate performance, per-pixel and composite energy resolution, uniformity of detector response, extrinsic spatial resolution, linearity, and integral and differential uniformity. Prior to the performance evaluation acquisition software was developed to operate the detector, including initializing the detector, performing data acquisition and displaying images and energy spectra. One of the systems had a better composite energy resolution due to the fact that the locations of photopeak centers for the individual pixels in that detector were consistently more uniform than the locations for the other detector. The energy resolution attainable for individual pixels is good, but due to limitations in user control over tuning of individual pixels, the composite energy resolution values were higher than expected for both systems. In practice, energy windows must be applied on a per-pixel basis. Spatial uniformity is worse than for typical scintillator-based gamma cameras, while extrinsic spatial resolution is satisfactory.

Parallel molecular dynamics (MD) simulations are performed to investigate pressure-induced solid-to-solid structural phase transformations in cadmium selenide (CdSe) nanorods. The effects of the size and shape of nanorods on different aspects of structural phase transformations are studied. Simulations are based on interatomic potentials validated extensively by experiments. Simulations range from 105 to 106 atoms. These simulations are enabled by highly scalable algorithms executed on massively parallel Beowulf computing architectures. Pressure-induced structural transformations are studied using a hydrostatic pressure medium simulated by atoms interacting via Lennard-Jones potential. Four single-crystal CdSe nanorods, each 44Å in diameter but varying in length, in the range between 44Å and 600 Å, are studied independently in two sets of simulations. The first simulation is the downstroke simulation, where each rod is embedded in the pressure medium and subjected to increasing pressure during which it undergoes a forward transformation from a 4-fold coordinated wurtzite (WZ) crystal structure to a 6-fold coordinated rocksalt (RS) crystal structure. In the second so-called upstroke simulation, the pressure on the rods is decreased and a reverse transformation from 6-fold RS to a 4-fold coordinated phase is observed. The transformation pressure in the forward transformation depends on the nanorod size, with longer rods transforming at lower pressures close to the bulk transformation pressure. Spatially-resolved structural analyses, including pair-distributions, atomic-coordinations and bond-angle distributions, indicate nucleation begins at the surface of nanorods and spreads inward. The transformation results in a single RS domain, in agreement with experiments. The microscopic mechanism for transformation is observed to be the same as for bulk CdSe. A nanorod size dependency is also found in reverse structural transformations, with longer nanorods transforming more readily than smaller ones. Nucleation initiates at the center of the rod and grows outward.

A study of the effects of system boundaries on bistable front propagation in nonequilibrium reaction-diffusion systems is presented. Two model partial differential equations displaying bistable fronts, with distinct experimental motivations and mathematical structure, are examined in detail utilizing simulations and perturbation techniques. We see that propagating fronts in both models bounce, trap, pin, or oscillate at the boundary, contingent on the imposed boundary condition, initial front speed and distance from the boundary. The similarities in front boundary interactions in these two models is traced to the fact that they display the same front instability (Ising-Bloch bifurcation) that controls the speed of propagation. A simplified dynamical picture based on ordinary differential equations that captures the essential features of front motion described by the original partial differential equations, is derived and analyzed for both models. In addition to addressing experimentally important boundary effects, we establish the universality of the Ising-Bloch bifurcation. Useful analytical insights into perturbative analysis of reaction diffusion systems are also presented.

Ag nanostructures of multilayer coverages (< 30 monolayers) epitaxially self-assembled on Cu(110) and Ni(110) have been explored by scanning tunneling microscopy (STM), angle-resolved photoelectron emission spectroscopy (ARPES), and low energy electron diffraction (LEED). We have studied varied nanostructure morphologies self-assembled depending on different deposition/annealing processes and coverages, their atomic structures, growth behaviors and mechanisms, and the electronic structures of nanowires. At nominal coverages of 1.2 ML < ¦È < ~10 ML, there are two epitaxial structures on Cu(110) and Ni(110). One is a Ag(110) multilayer film, which has a superstructure with lateral periodic units of eight and three/four substrate lattice constants along [¯110] and along [001] respectively. Another is that of Ag(110) nanowires surrounded by pseudohexagonal Ag(111) monolayer. The Ag(110) nanowires are triangular in cross section. The two side surfaces are faceted and the long axis is atomically straight along [¯110]. Typical lengths are within the range of 100 ~ 5000 Å, widths 70 ~ 300 Å, side slopes 10 ~ 30º, and heights 5 ~ 60 Å. The Ag nanowires present extraordinary anisotropy with observed aspect ratios (length:width) of up to 20:1. The Ag(110) nanowires are in-registry with the substrate along [001], but not along [¯110]. At coverages of ~ 10 ML < ¦È < ~25 ML, there also exist two different nanostructures, the nanowires and a Ag(110) atomically-flat film with some pits as deep as down to the substrate and a one-dimensional quasiperiodic superstructure along [001]. There are two basic separations of the superstructural stripes: one is three lattices wide (~11 Å) and the other is two lattices wide (~7 Å). Both of nanostructures are stable at temperature up to at least 200 ¡ãC and not inter-transformable. The growth of the nanowires is driven by the elastic strain mechanism, but the growth of the atomically-flat film is driven by electronic growth mechanism originated from the electron quantum confinement in the vertical direction of the film. The ARPES of the nanowires shows dispersion in the vertical and the [¯110] directions, but no dispersion in the [001] direction because of the limited width (~ 200 Å).

High harmonic generation (HHG) is a process in which noble gas atoms excited by an intense laser field at frequency ω₁ emit radiation of higher frequencies that are odd integer multiples of ω₁. Driven by an infrared laser, high harmonic radiation can span from the optical into the extreme ultraviolet (XUV) frequency range. In this thesis, I study HHG using short laser pulses by solving the time-dependent Schrödinger equation (TDSE). In particular, I will focus on calculating and controlling the time-dependent instantaneous frequency (chirp) of high harmonics. The harmonic chirp is a direct consequence of the atom-field interaction during HHG, and its behavior in the time-frequency domain reveals the fundamental physics in the strong laser field. In addition, controlling the harmonic chirp is essential for practical applications such as when high harmonic radiation is used as a seed for soft X-ray lasers. Moreover, I will discuss how to find the ionization probability of an atom exposed to few-cycle laser pulses. This is important in order to understand the propagation effects on high harmonics as their driving pulse goes through a rapidly ionizing medium. Finally, I will consider high harmonics as a source of attosecond pulses. Such short pulses (∼10^-18 seconds) could induce and probe the dynamics of electrons in atoms and solids. In the last part of my thesis, I will examine some examples of attosecond electron dynamics.

In recent years magnetic semiconductors have attracted a great deal of attention because it is thought that they can be used to generate spin polarized current in spintronics, an emerging field where the spin degree of freedom of charge carriers is utilized in microelectronic devices. In this dissertation work, we investigate the magnetic, thermodynamic, and transport properties of the magnetic semiconductor Fe_1-XCo_xS₂ for x less than 0.14, the doping range where Insulator-to-Metal and paramagnetic-to-ferromagnetic transitions occur. We discovered that the Kondo effect is an important ingredient in the paramagnetic region of magnetic semiconductor Fe_1-XCo_xS₂. Disorder that comes with doping, coupled with the Kondo effect and the RKKY interaction between the local moments, leads to the observed features of spin clusters and the Griffiths phase. This inhomogeneous magnetic state can be used to explain the resulting physical properties, including the NFL behavior as evidenced by the increase of C/T at very low temperature in proximity to the zero-T critical point. A second system, LaSb₂, is found to have a very inhomogeneous superconducting transition at low temperatures. We have discovered that the application of pressure induces a much more homogeneous superconducting ground state in this highly layered compound.

Purpose: Segmented-field electron conformal therapy is characterized by dose heterogeneity due to unmatched penumbra of abutted fields of differing energy. The present work investigates the potential to decrease dose heterogeneity by approximately matching beam penumbra using energy-specific source-to-collimator distances (SCDs). It was hypothesized that a clinically practical, variable-SCD method that utilizes Cerrobend® custom inserts can deliver segmented-field electron conformal therapy in the energy range of 6-20 MeV with less than ±5% variation in dose spread in the abutment regions of hypothetical planning target volumes (PTVs), i.e. constrain the PTV dose to 85%-105%. Methods: A Varian 15x15-cm2 electron applicator was modified to allow energy-dependent SCDs resulting in energy-dependent air gaps. Air gaps were chosen based on theoretical calculations to approximately match penumbra for 6, 9, 12, 16, and 20 MeV beams at a depth of 1.5 cm in water. Treatment plans developed for four simulated PTVs and a single patient using the variable-SCD applicator were compared to identical plans using the current constant-SCD applicator. Dose plans for the simulated PTVs using the variable-SCD applicator with electron inserts cut with diverging edges were delivered to film in a polystyrene phantom to assess feasibility. Results: Treatment planning results in the four simulated PTVs showed that dose homogeneity in agreement with the hypothesis can be achieved using the variable-SCD applicator. Minimum dose was increased by an average of 4%, and maximum dose was decreased by an average of 4%. On average, the standard deviation of the dose decreased by 29%, and D90-10 decreased by 32%. Measured dose in the abutment regions for all four simulated targets using the modified applicator agreed well with TPS predicted dose. For the patient PTV, the variable-SCD applicator plan predicted a 14% increase in minimum dose, a 10% decrease in maximum dose, and a 22% reduction in both the standard deviation of the dose distribution and D90-10 as compared to the standard applicator plan. Conclusion: The results of this study demonstrated that dose homogeneity in segmented-field electron conformal therapy can be substantially improved by using energy-dependent SCDs to match beam penumbra.

Purpose: TomoTherapy, capable of delivering intensity-modulated, image-guided radiotherapy with a helical fan-beam, multileaf-collimated beam and detector array mounted on a CT ring, is challenging the treatment techniques commonly used in today's radiotherapy clinic. The present works investigates the potential of using TomoTherapy in lieu of electron beams for treatment of the chest wall in post-mastectomy radiotherapy (PMRT). It is hypothesized that TomoTherapy can plan dose distributions for PMRT patients, that a pre-selected radiation oncologist judges to be equal to or better than that of a conventional plan treated with electron beams. Methods: A patient database of retrospective conventional PMRT treatment plans was generated, including contoured target and critical structure region-of-interest volumes. A TomoTherapy plan was generated for five patients out of the database using the same treatment criteria as the conventional plan. The TomoTherapy plan and the conventional plan was evaluated and compared by a radiation oncologist using a simplified scoring system. Physical and radiobiological dose metrics were generated from the treatment plans to supplement the evaluation of the radiation oncologist. Results: Four of the five TomoTherapy plans were rated superior to the rival conventional electron beam treatment plan, and the other Tomotherapy plan was rated marginally superior. The TomoTherapy plan was able to spare the ipsilateral lung and heart of excessive dose as well as the conventional plan, while delivering a more homogeneous dose distribution to the target volumes. However, the TomoTherapy plan showed the contralateral breast receiving an average dose of 2.9 Gy as opposed to 0.4 Gy for the conventional electron beam plan, and a greater volume of normal tissue outside the target volumes receiving dose between 5 and 25 Gy (average percent volume was 33% for the TomoTherapy plan and 5 % for the electron beam plan). This could affect the radiation oncologist's decision to use TomoTherapy for younger patients who are at greater risk of developing radiation-induced secondary cancers. Conclusion: The study showed TomoTherapy can deliver dose distributions the radiation oncologist judges to be equal to or better than that of a conventional electron beam PMRT plan for five treatment plans.

A new technique using direct post-implant dosimetry, which does not depend explicitly on brachytherapy seed orientation or position, was explored for a prostate and a breast case. This technique, proposed by E Sajo and ML Williams (SW), uses trace amounts of positron emitters placed in the seed capsule and uses the positron emission tomography image in conjunction with a computed tomography image (PET-CT) to compute the therapeutic dose distribution in the patient. The SW technique could reduce errors in the post-implant dose computations associated with seed localization, seed shadowing and medium heterogeneity. Dose point kernels were obtained using Monte Carlo simulation for a single seed in a breast and prostate geometry. Greens functions were computed for the positron marker and therapeutic photons using Monte Carlo (MC) simulations. Various dose computation options in the MC code MCNP were compared and the best were selected for this project. A single seed was imaged for a prostate phantom and a breast phantom using a PET-CT. The image data was used to obtain dose for the annihilation photons for the experimental seeds. The Sajo-Williams mathematical method was used to compute the therapeutic dose of the seed based on the positron marker dose. The therapeutic dose computed this way was compared to the dose obtained using the Pinnacle3 treatment planning software and to an MCNP benchmark model. For the breast case the comparison showed a good agreement with Pinnacle3, but both under-predicted the dose close to the source with respect to the benchmark. For the prostate case Pinnacle3 somewhat under-predicted the values in the MCNP benchmark, and the SW method appreciably under-predicted the dose near the source. In all cases, farther away from the source where most of the dosimetric interests lie, the agreement is very good.

An electronically-collimated radiation detector (ECRD) is being developed to be used for locating radiation sources, e.g. for intraoperative localization of sentinel lymph nodes, or for public safety applications. The design emphasizes a compact, portable detector with a wide field of view. Typical probes provide either high sensitivity but no directional information when uncollimated, or directional information but poor detection efficiency when collimated. The ECRD design provides high sensitivity to the presence of radiation because it lacks physical collimation, and simultaneously provides directional information using electronic collimation. Intended to be a hand-held device, the ECRD front end comprises an array of cadmium-zinc-telluride (CZT) detectors. An incident gamma ray scatters in the primary detector; interaction of the scattered photon in a secondary detector is detected in coincidence. For each photon, Compton kinematics specifies a cone on which the source must be located. Localization is achieved by finding the intersection of many Compton-scatter cones. This paper reports on the development and evaluation of two directional algorithms for this device, a modified Compton telescope algorithm and an algorithm based on finding the intersections of rectangles circumscribing the Compton cones. The methods developed were evaluated using ideal simulated data from a point source as well as data from a Monte Carlo simulation of an ECRD device. The accuracy, precision and convergence of each directional algorithm were evaluated. It was found that for the modified Compton telescope technique, a useful field of view extending 60º from the forward direction was observed, an angular resolution better than 20º was achieved throughout the field of view, and the method converged to these values around 300 events; the results for the ideal data did not significantly differ from those using the Monte Carlo data. For the circumscribed rectangle technique, the useful field of view covered nearly the entire area in front of the detector for the ideal data but the more realistic physics of the Monte Carlo simulated data shrank the useful field of view to the region within approximately 30º of the forward direction, while the angular resolution was better than 20º and convergence was approached at approximately 50 events.