We investigated two types of stellar explosions, recurrent novae (RNe) and Type Ia supernovae (SNe Ia). SNe Ia are the most useful distance markers in astrophysics, but we do not know the identity of their progenitor systems. RNe are good progenitor candidates that consist of a white dwarf (WD) that accretes material from a companion star. The material builds on the surface of the WD until a runaway thermonuclear eruption is triggered, which ejects the accreted material and causes the system to brighten dramatically. We studied the demographics of the nova population and concluded that approximately 25% of classical novae are actually RNe for which only one eruption has been discovered. Importantly, this means that there are enough RNe in our galaxy to provide a significant fraction of the SNe Ia. We present a list of good RN candidates; for one such system, V2487 Ophiuchi, we sought and found a previous eruption in the astronomical plate archives. <br><br> We examined two known RNe in detail. T Pyxidis has a unique shell; we used observations of the shell and central star to produce a new model for the long-term evolution of the system, which will never become a supernova. U Scorpii erupted in 2010 as predicted. We led a worldwide collaboration of astronomers that discovered the eruption and comprehensively observed it from start to finish. We discovered three new phenomena and were able to make the best-yet measurement of the amount of mass ejected during the eruption. <br><br> We searched the centers of nearby SN Ia remnants looking for ex-companion stars left behind after the WD exploded centuries ago. For one remnant, SNR 0509-67.5, we can definitively state that there are no ex-companion stars in the center of the remnant and therefore the system must have consisted of two WDs that collided to form the SN Ia. The other nearby remnants have possible ex-companion stars; more observations are needed to determine which, if any, are the true ex-companions. Some large fraction of the SNe Ia must come from double-WD systems, but there is a possibility that RNe provide a significant fraction as well.

We have investigated the morphology and electronic structure of two basic classes of systems: metal oxide surfaces that catalyze the formation of environmentally persistent free radicals (EPFRs) from aromatic precursors, and Au and Cu nanoparticles that may be suitable catalysts for the catalytic oxidation of CO or hydrogenation of CO2. First, we examine the adsorption behavior of phenol on rutile TiO2(110) and ultrathin films of alumina prepared on a NiAl(110) substrate. Electron paramagnetic resonance studies show that exposure of both γ-alumina and titania powder to phenol at 250°C results in the formation of persistent phenoxyl radicals. EELS studies of phenol dosed on single-crystal titania and alumina show that phenol adsorbed at elevated temperature demonstrates a significantly narrower HOMO-LUMO gap than molecular phenol in the gas phase or physisorbed molecular phenol. Ultraviolet photoelectron spectroscopy shows direct evidence of charge transfer from high-temperature adsorbed phenol to electronic states of TiO2(110) usually associated with the accumulation of charge at surface oxygen vacancies, providing direct evidence of a frequently-hypothesized radical formation mechanism. Second, we deposit and characterize Au nanoparticles on the self-assembled hexagonal boron nitride nanomesh prepared on a Rh(111) substrate. STM studies show that at all levels of coverage, Au clusters almost always remain confined to the nanomesh pores and are restricted in size to < 3 nm diameter. XPS studies suggest that the resulting Au clusters are negatively charged, and for < 1 ML Au coverage, the electronic properties of most of the clusters formed are dominated by final-state effects that arise from the reduced dimensionality of the smallest clusters (one or two Au layers). A similar morphology for Au deposited on ZnO(10-10) has been previously observed; however, we find that the Au-ZnO interaction instead results in positively charged clusters. Cu on ZnO(10-10) grows as three-dimensional clusters even at very small coverage and shows positive charging similar to Au. It is clear that catalytically relevant properties of supported metal nanoclusters are strongly influenced by interactions with the support, even if the cluster morphology is identical for particles on various different substrates.

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energy spectra from matched electron beams on <span class=SpellE>Elekta</span> radiotherapy accelerators at Mary Bird Perkins Cancer Center.</p> <p class=MsoNormal style='line-height:normal'><o:p> </o:p></p> <p clas

<p> The origin and mass composition of ultra-high energy cosmic rays (UHECRs) are among the mysteries of particle astrophysics. Since UHECRs are mostly or entirely charged particles, they will be deflected in cosmic magnetic fields on their way to Earth. UHECR deflections in the Galactic magnetic fields (GMF) should be considered to identify their origin and nature. </p> <p> The calculated Galactic magnetic deflections of UHECRs from a source strongly depend on the GMF model. I use the recent GMF model of Jansson and Farrar (JF12), a 35-parameter model which includes coherent, striated and random components and is constrained by WMAP7 Galactic synchrotron emission map and more than forty thousand extragalactic rotation measures. I develop a new method of implementing a random field using a Kolmogorov spectrum and scale it with the JF12 random component strengths. </p> <p> Simulated UHECRs are propagated through the GMF using the publicly available propagation code <i>CRT</i>. These particles are simulated from Centaurus A, which is the nearest AGN to Earth. I identify the expected arrival direction loci of UHECRs in the JF12 GMF model for different rigidities (energy divided by charge) between 2 and 100 EV (1 EV ≡ 10¹⁸ V) to determine whether Centaurus A could be a significant source of the UHECR excess reported by Pierre Auger Observatory. I calculate the excess of event arrival directions, with respect to isotropic expectations, for cases of pure protons, pure iron, and a mixture. </p> <p> I also develop a method for identifying the mass composition of UHECRs under specific source and GMF hypotheses. Comparing Auger observations with simulations of different rigidities leads to the identification of the charges of the events. Consistent simulated rigidities are found for each Auger event based on their overlap probabilities. The charge of each event is the measured energy divided by the rigidity of the most consistent simulation. </p> <p> Including a Kolmogorov random field component to the JF12 regular component does not significantly change the results of propagation at high rigidities whereas at low rigidities it smears the arrival distributions of events to such an extent that deflection studies can become difficult. </p>

Purpose: The quality of a computed tomography (CT) image and the dose delivered depend upon the acquisition parameters used to acquire the CT scan. Current, voltage and pitch are acquisition parameters that affect the image quality. The purpose of this study was to determine the influence of current, voltage and pitch on physicians ability to characterize small, solid nodules with low-dose computed tomography. Methods: A database of lung scans with various acquisition parameters was compiled. A torso phantom and acrylic beads were used to simulate the lungs and nodules within the lungs. Several sizes of acrylic sphere were used to simulate different size nodules. An image visualization software was used to display the images for physicians and to assess. The physicians assessments were compared to known objects. The reliability of CTDI estimates reported by the CT acquisition software was verified. Results: The diameter that physicians measured for the sphere became closer to the actual diameter of the sphere as the sphere size, tube current, and kVp increased. The pitch did not affect the physicians measurement of sphere size for the larger sphere as much as it did for the smaller spheres. Conclusion: We concluded that physicians are still able to judge size and shape of nodules accurately using low-dose CT. The 80 kV tube voltage proved to be an ineffective voltage for screening for lung cancer. Between the machines used there was not a substantial difference in perceived image quality when a current of 50 mA or higher was used. Based on this work, a low-dose protocol of 120 kV, 50 mA, and a pitch of 1.4 is recommended to balance patient dose and acceptable image quality.

<b>Purpose:</b> To perform a secondary dose calculation for intensity modulated radiotherapy (IMRT) or volumetric modulated arc therapy (VMAT) plans to a point on or off axis within 2% using open field data. </br> <b>Methods:</b> An independent dose calculation algorithm has been developed for complex fields with multiple segments. The algorithm subdivides dose into the contributions from each opposing leaf pair for a given multileaf collimator (MLC) configuration. Leaf pair dose is determined by drawing four rectangular fields based on leaf positions, which are symmetric about the point of calculation. Superposition of these fields yields the dose from the leaf pair to the point. VMAT plans are approximated by a static MLC configuration at four degree intervals. The algorithm requires standard open field data (e.g., head and phantom scatter factors, Scps and tissue phantom ratios, TPRs), and the MLC control point information. Calculations were done with additional measured small field output factors down to a 1.5x1.5-cm² field. Algorithm doses to the isocenter or center of the planning target volume (PTV) were compared with heterogeneous Pinnacle calculations of a series of prostate, head and neck, and chest wall treatment plans. Delivery techniques included fixed gantry IMRT and VMAT. </br> <b>Results:</b> Good agreement was obtained between doses calculated by the algorithm and the Pinnacle3 treatment planning system. Percent errors were -0.2% ± 3.8% (mean and 95% confidence interval) for algorithm calculations. Systematic offsets were observed as a function of calculation site, with prostate doses being underestimated and chest wall doses being overestimated. Errors are likely the result of patient geometry deviations from the infinite slab, flat phantom assumption of monitor unit calculations. </br> <b>Conclusion:</b> Results demonstrate that clinically acceptable agreement is obtained using this method. Further improvement could be made with more accurate heterogeneity correction factors and/or a better estimation of small field output factors.

Earth occultation provides a means of monitoring gamma-ray sources over the entire sky. This technique has been demonstrated with the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) active from 1991 to 2000, and is now being used with the Gamma-ray Burst Monitor (GBM) instrument on the Fermi mission. Although this approach provides a powerful wide field monitoring capability, the BATSE and GBM implementations of this technique have so far limited the analysis to a catalog of previously identified sources. In this dissertation, an indirect imaging method is described that permits searching for unknown sources by applying the Differential Filter Technique to archival BATSE data. Using this technique, occultation steps are transformed into gaussian-like features with amplitudes corresponding to the intensities measured from each pixel in the sky. With approximately 85% of the sky occulted every spacecraft orbit, the about 51 day precession period of the CGRO orbit makes it possible to perform an all-sky survey. By comparing both significance and the shape of the source of interest, we can finalize both known and unknown sources with spatial resolution to less than 0.5 degree. A reanalysis of the BATSE data has been performed, making it possible to complete the BATSE catalog of high energy sources, to address long-standing questions of systematic effects in the BATSE results, and to compare the new BATSE results to the current Fermi/GBM catalog in order to perform a study over 22 years of variations in the hard X-ray/low energy gamma-ray sky.

Purpose: To evaluate dosimetric differences of copper inserts compared to lead-alloy inserts for electron beam therapy. Methods: Copper inserts were manufactured by .decimal, Inc. and matching lead-alloy, Cerrobend®, inserts were constructed in-house for 32 square field sizes (2x2 to 20x20 cm&178;) for five applicator sizes (6x6 to 25x25 cm&178;). Percent depth-dose and off-axis relative dose profiles were measured using an electron diode in water for the copper and Cerrobend® inserts for a subset of insert sizes (6x6, 10x10, 25x25 cm&178;) and energies (6, 12, 20 MeV) at 100 and 110 cm source-to-surface distances (SSD). Dose outputs were measured for all field size-insert combinations and available energies (6-20 MeV) at 100 cm SSD and for a smaller subset at 110 cm SSD. Using these data, 2D planar absolute dose distributions were generated and compared. Criteria for agreement were ±2% of maximum dose or 1 mm distance-to-agreement for 99% of points. Results: A gamma analysis of the beam dosimetry showed 94 of 96 combinations of insert size, applicator, energy, and SSD were within the 2%/1 mm criteria. Failures were found for combinations of small field sizes in large applicators at 20 MeV and 100-cm SSD. Copper inserts showed less bremsstrahlung production due to coppers lower atomic number compared to Cerrobend® (greatest difference was 2.5% at 20 MeV). This effect was most prominent at the highest energies for combinations of large applicators with small field sizes. Also, more electrons scattered from the collimator edge of copper compared to Cerrobend®, resulting in an increased dose at the field edge for copper at shallow depths (greatest increase was 1% at 20 MeV). Conclusions: Inserts for field sizes ≥6x6 cm&178; at any energy, or for small fields (≤4x4 cm&178;) at energies <20 MeV, showed dosimetric differences less than 2%/1 mm for more than 99% of points. All areas of comparison criteria failures were from lower out-of-field dose from copper inserts due to a reduction in bremsstrahlung production, a dosimetric difference which is clinically beneficial in reducing dose to healthy tissue outside of the planned treatment volume. All field size-applicator size-energy combinations passed 3%/1 mm criteria for 100% of points.

Introduction: In order to maintain uniform standards in the accuracy of fractionated radiation therapy, quantification of the delivered dose per fraction accuracy is required. The pupose of this study was to investigate the feasibility of a transit dosimetry method using the electronic portal imaging device (EPID) for dose delivery error detection and prevention. Methods and Materials: In the proposed method, 2D predicted transit images were generated for comparison with online images acquired during treatment. Predicted transit images were generated by convolving through-air EPID measurements of each field with pixel-specific kernels selected from a library of pre-calculated Monte Carlo pencil kernels of various radiological thicknesses. The kernel used for each pixel was selected based on the calculated radiological thickness of the patient along the line joining the pixel to the virtual source. The accuracy of the technique was evaluated in homogeneous plastic water phantoms, a heterogeneous cylindrical phantom, and an anthropomorphic head phantom. Gamma analysis was used to quantify the accuracy of the technique for the various cases. Results: In the comparison between the measured and predicted images, an average of 99.4% of the points in passed a 3%/ 3 mm gamma for the homogeneous plastic water phantoms. Points for the heterogeneous cylindrical phantom analysis had a 94.6% passing rate. For the anthropomorphic head phantom, an average of 98.3% and 96.6% of points passed the 5%/3mm and 3%/3mm gamma criteria, respectively for all field sizes. Failures occurred typically at points when object thickness was changing rapidly or at boundaries between materials, and at the edges of large fields. Discussion: The results suggested that the proposed transit dosimetry method is a feasible approach to in vivo dose monitoring. The gamma analysis passing rates are within the accuracy needed for transit dosimetry. Future research efforts should include evaluation of the method for more complex treatment techniques and assessment of the sensitivity to changes in EPID or linac hardware, as well as characterization of any dependency the method may have on image ghosting or lag, gantry angle, or long-term stability.

Complex correlated electron materials (CEMs), such as transition-metal oxides with exotic properties and novel functionalities, present immense opportunities and formidable challenges in condensed matter physics, materials science, and engineering. These systems are characterized by a multitude of competing ground states that result from the close coupling between charge, lattice, orbital, and spin degrees of freedom, which can be tuned by chemical substitution, strain induction, or by the application of external stimulus (e.g. pressure, temperature, electric, or magnetic fields). Interest in CEMs is fueled by the richness of their novel properties (e.g. Colossal Magnetoresistance (CMR), Quantum Criticality, and High Temperature Superconductivity), the complexity of the underlying physics, and the promise of technological applications. In this work, elastic and inelastic neutron scattering is used to study two prototypes of CEMs: ruthenates and manganites. Our work on ruthenates has revealed the magnetic structure and dimensionality of the order parameter in the bilayered ruthenate Sr3(Ru1-xMnx)2O7 for manganese (Mn) concentrations of 12.5 and 16%. Results indicate 1) an unusual E-type antiferromagnetic structure with moments aligned along the c-direction exhibiting only single-bilayer (5-6 Å) ferromagnetic correlations along the c-direction, and 2) that Mn concentration is responsible for the stabilization of the long-range E-type AFM ordering observed along the basal plane. The investigation on the evolution of long- and short-range charge-ordered (CO), ferromagnetic, and antiferromagnetic correlations in single crystals of Pr1−xCaxMnO3 for various hole-doping concentrations has 1) provided direct evidence of magnetic phase separation and 2) revealed a critical doping concentration close to (x = 0.35) that divides the inhomogeneous from homogenous CO state. Preliminary studies of spin and lattice excitations in the ferromagnetic insulating phase of La1-xCaxMnO3 indicate an anomalous softening and broadening of the magnons near the zone boundary, where a longitudinal optical phonon is present, indicating that magnon-phonon coupling could play a critical role in the behavior of the spin wave dispersion of these systems. The work presented in this thesis should be a forward step towards the understanding of the nature of the competing interactions present in these CEMs, which result in the emergence of exotic phenomena and novel functionalities.