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Item An anomalous measurement of delta m31 squared from neutrino oscillations at the Daya Bay Reactor Neutrino Experiment(Vanderbilt University. Department of Physics and Astronomy, 2013-04-22) Burroughs, Hunter; Ernst, DavidIn 2012, the collaboration overseeing the Daya Bay Reactor Neutrino Experiment announced results which determined the magnitude of the mixing angle \theta_{13} with unprecedented precision. However, no attempt was made in the collaboration’s publications to predict the value of the most relevant mass-squared difference to the observed oscillation, \delta m^2_{31}. This paper presents the results of an analysis which suggests that the Daya Bay data prefers a value of \delta m^2_{31} which is far greater than its presently recognized value. Specifically, it is found that Daya Bay predicts \delta m^2_{31} = 3.53_(-1.07)^(+.74) × 10^(-3) eV^2, where the cited uncertainties correspond to the 99% confidence bounds. This measurement excludes the most precise current measurement of \delta m^2_{31}, the MINOS result, at a 99% confidence level and is in turn excluded by the MINOS data at a 10 \sigma level. The possibility that sterile neutrino effects are the cause of this anomalous result is considered and used to suggest further work.Item Simulations of Nano-Structures in Time Dependent External Fields(Vanderbilt University. Department of Physics and Astronomy, 2013-04-22) Atkinson, Mackenzie; Varga, Kálmán, 1963-Time Dependent Density Functional Theory is used to probe the structure of matter. Coulomb explosion of small hydrocarbons driven by strong laser pulses and electron holography of molecules are studied in a theoretical framework. The spectra of the ejected protons obtained computationally is in good agreement with experimental data of Coulomb explosion. TDDFT allowed us to obtain time-dependent data, giving us a deeper understanding of the process. Our computational approach to electron holography provides 3-d reconstructions of simple molecules. Further investigation is needed in order to reconstruct larger molecules.Item Dark radiation from particle decays during big bang nucleosynthesis(Vanderbilt University. Dept. of Physics and Astronomy, 2012-04) Menestrina, Justin; Scherrer, Robert J. (Robert Joseph), 1959-Cosmic microwave background (CMB) observations suggest the possibility of an extra dark radiation component, while the current evidence from big bang nucleosynthesis (BBN) is more ambiguous. Dark radiation from a decaying particle can affect these two processes differently. Early decays add an additional radiation component to both the CMB and BBN, while late decays can alter the radiation content seen in the CMB while having a negligible effect on BBN. Here we quantify this difference and explore the intermediate regime by examining particles decaying during BBN, i.e., particle lifetimes τ_X satisfying 0.1 sec < τ_X < 1000 sec. We calculate the change in the effective number of neutrino species, Neff, as measured by the CMB, ΔN_CMB, and the change in the effective number of neutrino species as measured by BBN, ΔN_BBN, as a function of the decaying particle initial energy density and lifetime, where DNBBN is defined in terms of the number of additional two-component neutrinos needed to produce the same change in the primordial 4He abundance as our decaying particle. As expected, for short lifetimes (τ_X < 0.1 sec), the particles decay before the onset of BBN, and DNCMB = DNBBN, while for long lifetimes (τ_X >1000 sec), ΔN_BBN is dominated by the energy density of the nonrelativistic particles before they decay, so that ΔN_BBN remains nonzero and becomes independent of the particle lifetime. By varying both the particle energy density and lifetime, one can obtain any desired combination of N_BBN and ΔN_CMB, subject to the constraint that DNCMB N_BBN. We present limits on the decaying particle parameters derived from observational constraints on ΔN_CMB, and N_BBN.Item Real-time real-space density functional theory calculations of electron scattering in materials(Vanderbilt University. Dept. of Physics and Astronomy, 2012-05-04) Wyatt, Benjamin; Varga, Kálmán, 1963-Density functional theory is utilized in real-time, real-space simulations of LEED measurements and attosecond electron scattering. For LEED measurements, we find that our simulation results agree well with experimental data and other theoretical approaches. For attosecond electron scattering, we find that the wavefunction of the scattered electron is not signficantly changed by the scattering process, and the measured electron density seems to be related to the initial form of the wave-packet. However, further investigation is needed to confirm these results for different choices of initial form.Item Engineering a perfusion-enabled mechanical compressor for long-duration immobilization and microscopy of cells and small organisms(Vanderbilt University. Dept. of Physics and Astronomy, 2011-04) Jiang, Liwei; Janetopoulos, ChristopherThe study of living specimens is essential to the understanding of organismal behavior. Unfortunately, a major difficulty in the study of live organisms is that many move in and out of the field of view or focal plane during microscopy. The present work seeks to combat this considerable problem by developing a mechanical microcompressor that immobilizes living cells and small organisms for long-duration optical microscopy. The device, dubbed the "Commodore Compressor," features two key innovations: (1) the integration of a perfusion system to keep the trapped specimen alive over several hours, as well as permitting the addition of chemoattractants, drugs, and other chemicals; (2) the incorporation of an optional patterned PDMS platform to improve the efficacy of immobilization in a targeted, organism-specific manner. One application of the Commodore Compressor is in monitoring the change in protein bioluminescence intensity in many trapped Saccharomyces cerevisiae cells during synchronized cell cycles. The experiment's feasibility and key techniques have been well demonstrated, although bioluminescence cannot currently be visualized. A second application involves fluorescence imaging of the neural network development of immobilized Caenorhabditis elegans over many hours. The development of new patterned PDMS platform designs, aided by the innovative use of established techniques, has driven the present work toward accomplishing the goal, but true long-term viability remains elusive. The Commodore Compressor may be directly used or easily adapted for many other specimen types and experimental scenarios.Item Perturbation Theory for Thin Cladding Layers on Silicon Photonic Systems(Vanderbilt University. Dept. of Physics and Astronomy, 2011-04) Phare, Christopher; Weiss, Sharon M., 1977-I develop a perturbation theory for resonant frequency shifts caused by thin layer of material added to dielectric systems. This development can be used to effectively model photonic surface-sensing systems, which use this resonance shift to measure the presence of, e.g., biomolecules attached to the sensor system. Direct modeling was previously intractable due to extremely long simulation times required to account for the cladding layers. The developed theory is verified against a special-case direct simulation and by measuring the shift caused by silicon dioxide deposited on a fabricated ring-resonator sensor.Item Developing High-Brightness Electron Beam Sources for Producing Quantum Degenerate Electron Beams(Vanderbilt University. Dept. of Physics and Astronomy, 2011-04) Kohler, Jonathan; Brau, Charles A., 1938-The Pauli Exclusion Principle places a fundamental limit on the brightness of an electron beam. Developing a cathode which can reach this limit is useful for achieving maximum operation in current applications of electron beams, but also opens new areas of physics to be explored. When the phase space of the electron beam is filled to the maximum density, the electrons will experience a degeneracy pressure, similar to that which keeps a neutron star from collapsing. One promising source for a quantum degenerate beam is field emission from adsorbates on carbon nanotubes. Adsorbates have been shown to provide several orders of magnitude enhancement to emission brightness, which approaches the degeneracy limit. We have developed experiments to test various adsorbates, in order to find those which bind tightest and provide the largest enhancement in brightness. Continuing work to discover better adsorbates should soon allow for the generation of a quantum degenerate electron beam.Item Finite-Element Analysis of Low-Power Laser Heating in Gold::Vanadium Dioxide Nanocomposites(Vanderbilt University. Dept. of Physics and Astronomy, 2011-04) MacQuarrie, Evan; Haglund, Richard F., Jr., 1942-Finite element modeling was performed using COMSOL Multiphysics to study the thermal dynamics of gold::vanadium dioxide (VO$_{2}$) nanocomposites. These simulations were done to understand the data from transient absorption pump probe measurements taken over the previous two years by the author. The various parameters contributing to the dynamics of the system were systematically varied within the simulation in order to understand how the various properties of the nanocomposite affect the thermodynamics of the system. It was determined that the background temperature and the optical properties of the film make the dominant contributions to the system response. The simulation was able to predict the maximum change in transmission measured in experiment fairly well but could not predict the speed at which the system responded to the pump laser. Steps to improve upon the agreement between experiment and simulation are proposed.Item Multipole mixing ratios of transitions out of high spin gamma vibrational states in neutron rich Mo, Ru isotopes(Vanderbilt University. Dept. of Physics and Astronomy, 2010-04-27) Fenker, Benjamin; Hamilton, Joseph H.Current models of nuclear structure describe nuclear states in terms of collective vibrations and rotations of a ground state. - One such quadrupole vibrational band is the gamma-band formed by vibrations of the short sides of a deformed nucleus. - When a nucleus that is in this mode decays to its ground state, the electromagnetic radiation is predicted to be entirely quadrupole in nature. - This prediction has been well documented in a wide range of nuclei. - However, a recent study of neutron rich molybdenum and ruthenium isotopes found a few gamma-band to ground-band transitions that contained almost no quadrupole radiation. - This work utilizes the Gammasphere detector array to examine the angular correlations in the de-excitations of the secondary fission fragments of 252Cf in order to make independent measurements of these results. - This work also uses the same technique to measure the multipole mixing ratios of gamma-band to ground-band transitions originating in states with spin-parity as high as 9+. - The results demonstrate that in these nuclei, electric quadrupole radiation is strongly favored in gamma to ground band transitions as has been predicted by theory.Item Investigation of Electroosmotic Flow in Various Microfluidic Structures(Vanderbilt University. Dept. of Physics and Astronomy, 2010-04) Diggins, Patrick; Wikswo, John PeterEnclosed microfluidic devices provide excellent systems for the study of biological processes such as cell-cell, paracrine, and autocrine signaling systems. By minimizing the fluid volume within the chambers, microfluidic devices diminish the dilution of secreted products which makes the detection of the secreted products a more straightforward task. A major design problem associated with the construction of microfluidic devices for biological research is the need to provide well-controlled fluidic transportation for cells, the nutrients that the cells need, and waste removal. Most precision syringe pumps which can accurately provide low flow rates are expensive and constitute a barrier to experiment design. Electroosmotic pumps could potentially provide a valuable alternative as a low volume flow rate pumping system for many types of microfluidic devices. We have developed a poly(dimethylsiloxane) (PDMS) microfluidic device that incorporates both electroosmotic flow and pressure driven flow. The device is designed to increase the relative strength of the electroosmotic flow (EOF) component of the total flow through the use of an array of small volume parallel pumping channels which provide higher passive resistance to pressure driven flow than a larger volume single-channel EOF pump. Using a novel microfluidic instrumentation device which we call the "Micro Programmable Object Navigation Gadget" (µ-PONG), we investigate how different properties and geometries of the device affect the EOF rate. In addition, we demonstrate that fluid flow driven by a small hydraulic pressure head can be completely canceled by an user initiated EOF in the pumping channels which are incorporated into a microfluidic device. The ability to modulate the flow and to create "stop flow" conditions in microfluidic devices is also important for biological research.Item Micro Programmable Object Navigation Gadget (micro-PONG) for Studying Electroosmotic Flow in PDMS Microchannel(Vanderbilt University. Dept. of Physics and Astronomy, 2009-04-20) Rohrman, Brittany Ann; Wikswo, John PeterPrecise control of fluid delivery in analytical devices may be achieved by harnessing electroosmotic flow (EOF) in oxidized poly(dimethylsiloxane) microfluidic channels fabricated via soft lithography. However, the magnitude of this flow depends on a number of parameters, including the geometry and surface chemistry of the channel walls, and is known to attenuate over time. In order to characterize these parameters and calibrate the magnitude of EOF in our devices, we have developed a versatile automated control and measurement system called micro-PONG (Micro Programmable Object Navigation Gadget). The system detects, controls, and measures the movement of polystyrene beads caused by EOF in the channel. The micro-PONG system consists of a LabView program, a DAQ board, an external voltage switching circuit, and a camera. A computer algorithm reverses the direction of the EOF each time the bead enters one of two user-specified "goalpost" areas, thus keeping the bead trapped and moving back and forth between the goalposts (similar to a game of ping pong) for extended periods of time. From the recorded motion of the bead, the three-dimensional velocity profile of the fluid in the channel may be estimated using a novel ImageJ macro for determining motion in three dimensions. This analysis may then be used to determine the magnitude of the EOF, inertia, and response time of the system as well as the time-dependent effects of PDMS surface properties on EOF. The micro-PONG system may also be useful for other purposes that require the automated manipulation of particles or cells.Item Measurement of Energy Spectrum of Electrons Field-Emitted from Diamond Field-Emitter Arrays(Vanderbilt University. Dept. of Physics and Astronomy, 2009-04-20) Stewart, Christopher L.; Brau, Charles A., 1938-A retarding mesh analyzer was used to measure the electron energy spectrum field emitted from a single tip of a Diamond Field Emitter Array. The emission was dominated by adsorbed gas atoms and molecules on the surface which were both spatially and temporally unstable. As a result, the spectra taken had highly variant spectral features and there was an order of magnitude span in the emitted current. The spectrum from a clean surface roughly obeyed the thermal field emission model and was consistent with reported spectra from a nitrogen-doped, diamond-like carbon film.Item Reflection and rotation for three dimensional microscopy of live cells(Vanderbilt University. Dept. of Physics & Astronomy, 2008-04-18) Wright, Charlie; Wikswo, John PeterConfocal scanning laser microscopy and multiphoton microscopy provide 3D data from biological specimens, but with limited z-axis precision. Multiple microscale mirrors can be used to obtain more accurate 3D data on living cells while using classical widefield microscopy. Etched silicon wells coated with aluminum were used to obtain 3D images of budding yeast cells, with information along the z-axis provided by reflections from the angled sides of the well. To supplement measurements obtained with this method, work was also conducted on a system to allow for rotation of a cell attached to a pulled glass tip. Images of a yeast cell obtained from either method can be fit to a simple 3D surface due to the cell's roughly spheroidal shape, which should provide accurate measurements of the volume of an individual budding yeast cell as it progresses through the cell cycle.Item Influence of patient size on dose to female breast tissue during routine computed tomography scans(Vanderbilt University. Dept. of Physics & Astronomy, 2008-04-21) Scully, Peter C.; Stabin, MichaelMonte Carlo methods are well suited to the evaluation of radiation transport phenomena. The scalable phantoms employed in this project permitted rapid creation of three models of similar geometry, but different body habitus. It is difficult at this point to draw any conclusions about the relationship between patient size and radiation dose to breast tissue during CT scans of the chest. However, the results of DeMarco et al. suggest that we may find a more complicated relationship between breast size and breast dose than we anticipated. Whatever the relationship, it is important that physicians and radiologists realize how dose delivered by CT varies with breast size, so that an effective balance of risks and benefits may be obtained for all patients in diagnostic CT imaging.