Physical Sciences and Mathematics Commons^™

A Comparison Of Ionospheric Model Performance For International Space Station Orbits, David J. Broadwater

Theses and Dissertations

The Floating Potential Measurement Unit (FPMU) aboard the International Space Station (ISS) monitors the ISS charging environment and provides a unique electron density and electron temperature in-situ data source for the purpose of ionospheric model validation. An ionospheric model that accurately predicts the ISS plasma environment is desirable for ISS mission planning and situational awareness when FPMU data is unavailable. Electron densities and temperatures from four ionospheric models (International Reference Ionosphere [IRI], Coupled Thermosphere Ionosphere Plasmasphere Electrodynamics model [CTIPe], Ionospheric Forecast Model [IFM], and Global Assimilation of Ionospheric Measurements model [GAIM]) were compared to in-situ FPMU values across a range …

Electron Damage Effects On Carbon Nanotube Thin Films, Jeremy S. Best

Theses and Dissertations

This research investigated the effects of electron damage on single walled carbon nanotube (CNT) thin films. CNT thin films were irradiated by electrons with energies of 500 keV and 1 MeV to determine what damage was created in the CNT thin film structure, how it affected conductivity, and what changes were evident in the Raman spectra.

Crystal Growth And Characterization Of Tho2 And UXTh1-XO2, Jacob G. Castilow

Theses and Dissertations

Hydrothermal synthesis of ThO₂ and U_xTh_1-xO₂, and UO_x at temperatures between 670°C and 700°C has been demonstrated. Synthesis at these temperatures is 50-80°C below prior crystal growths and represents a new lower bound of successful growth. Hydrothermal synthesis represents a cost effective, environmentally friendly way of growing bulk actinide materials of optical quality. These refractory oxide single crystals offer potential applications in thorium nuclear fuel technology, wide-band-gap uranium-based direct-conversion solid state neutron detectors, and understanding how actinide fuels age with time. ThO₂ single crystals of dimensions 6.49mm x 4.89mm x 3.89 …

Three Dimensional Positron Annihilation Momentum Spectroscopy Of Lithium Tetraborate Crystals, Stefan B. Fagan-Kelly

Theses and Dissertations

Previous endeavors in positron annihilation spectroscopy (PAS) at AFIT have resulted in the design and characterization of a second-generation three-dimensional positron annihilation momentum spectroscopy system (3DPAMSS) which combines two-dimensional angular correlation of annihilation radiation (2D ACAR) and coincident Doppler broadening of annihilation radiation (CDBAR) in order to measure electron momentum distributions in samples and thus characterize material defects. The focus of this particular research is to nondestructively measure negative defects in the crystal lattice of copper-doped, silver-doped, and undoped lithium tetraborate by 3DPAMSS spectroscopy using a Na-22 source and two high-purity germanium (HPGe) position-resolvable strip detectors. Lithium tetraborate is a …

Modeling The Thermosphere As A Driven-Dissipative Thermodynamic System, William R. Frey

Theses and Dissertations

Thermospheric density impacts satellite position and lifetime through atmospheric drag. More accurate specification of thermospheric temperature, a key input to current models such as the High Accuracy Satellite Drag Model (HASDM), can decrease model density errors. Building on Burke et al. s driven-dissipative model (2009) the arithmetic mean temperature, T_1/2 , defined by Jacchia, 1977 (J77), is modeled using the magnetospheric electric field as a driver. Three methods of treating the UV contribution to T_1/2 (T_1/2UV) are tested. Two model parameters, the coupling and relaxation constants, are adjusted for 38 storms from 2002 - 2008 to …

Neutron Spectroscopy Using Lif Thin-Film Detectors, Michael A. Ford

Theses and Dissertations

A stacked array of segmented micro-structured semiconductor neutron detectors (MSNDs) has been fabricated to perform as a neutron spectrometer simultaneously capable of differentiating fast and thermal neutrons. The MSND devices consist of thin-film perforated diodes constructed from LiF powder back-filled into an etched silicon wafer. Geant4 simulations demonstrate than an eight-layer spectrometer consisting of alternating layers of MSND and hydrogenous moderator can successfully resolve neutron energies at a resolution dependent upon the number of layers and the thickness of the adjacent moderating materials. The simulated spectrometer response was compared to that obtained experimentally with mono-energetic neutrons from a D+D neutron …

Cathodoluminescence And Thermoluminescence Of Undoped Ltb And Ltb:A (A = Cu, Ag, Mn), Zachary L. Hadfield

Theses and Dissertations

The Department of Defense is interested in lithium tetraborate (Li₂B₄O₇) crystals for their possible use as a solid state neutron detection material. With large concentrations of ⁶Li and ¹⁰B, it has a high neutron capture cross section. Furthermore, the crystal fluoresces in the presence of ionizing radiation, making it an attractive candidate for a scintillating detection device. However, there is a lack of fundamental knowledge about the material characteristics, particularly with regard to its fluorescent spectrum. Cathodoluminescence measurements were conducted on undoped and doped samples of lithium tetraborate in order to characterize …

Modeling Radioactive Decay Chains With Branching Fraction Uncertainties, Timothy M. Higley

Theses and Dissertations

This thesis is a parameter study of the effects of branching fraction uncertainty from nuclear decay on isotope quantities at later times. Development of how to calculate and accurately draw random samples of branching fractions is done. Overall effects as well as isotope quantity uncertainty distributions are also explored. Isotope quantities are calculated using exponential moments methods with transmutation matrices. Uncertainty from both half-lives and branching fractions is carried through these calculations by Monte Carlo methods. Results of the study show that uncertainty from branching fractions dominates most isotopes present from neutron fission after approximately one month. Another result is …

Analysis Of A Van De Graaff Generator For Emp Direct Current Survivability Testing, Robert J. Kress

Theses and Dissertations

The direct current produced from the Van de Graaff (VDG) at the Air Force Research Laboratory (AFRL) has been measured and analyzed. The current pulse produced from the VDG is oscillatory. Experimental data show complete damping occurs after 8 oscillations and within 10^-6 seconds. The spark gap distance and circuit resistance were varied to determine if the circuit could convert to an overdamped RLC circuit in order to reduce the oscillations. The data establishes that the VDG produces at least 3 full wave Fourier frequencies of: 3, 7, and 15 MHz ± 2.0 MHz, while the first oscillation had …

Depth-Resolved Cathodoluminescence Of Tho2, Michael G. Lee

Theses and Dissertations

Single crystal thorium dioxide (ThO₂) samples were hydrothermally grown and studied using depth-resolved cathodoluminescence (CL) to characterize the surface and bulk electronic states. X-ray diffraction (XRD) measurements were collected to confirm that these crystals were ThO₂ in the fluorite structure. Understanding the chemical and structural quality of ThO₂ will aid in the fabrication of better neutron detectors as well as in the power production with thorium breeder reactors. Monte Carlo simulations predicted the expected energy-dependent electron interaction depths in the ThO₂ crystals. CL was conducted with electron energy range of 1.5 - 12 keV, a …

Multipactor Discharge In High Power Microwave Systems: Analyzing Effects And Mitigation Through Simulation In Icepic, Robert L. Lloyd

Theses and Dissertations

Single surface multipactor in high power microwave systems was investigated computationally and analytically. The research focused upon understanding the cause and parametric dependence of the multipactor process leading to suggested methods of mitigation. System damage due to reaction was also assessed. All simulations were performed using the PIC code developed by AFRL, known as ICEPIC. In recreating the susceptibility curves that define regions of multipactor growth and decay, a discrepancy was found between previous published results and those observed in the current simulation. This was attributed to previous simulations not accounting for the magnetic component in the electromagnetic radiation incident …

Passive, Low Cost Neutron Detectors For Neutron Diagnostics At The National Ignition Facility, Nathaniel C. Loyd

Theses and Dissertations

Experimental validation of neutron fluence models of fusion events at the National Ignition Facility is necessary to predict radiation damage to measurement electronics. Due to programmatic and facility limitations, traditional neutron measurement techniques are not well suited for this application. Notably, a low cost and passive measurement technique that provides a permanent record is preferred. A detector was designed using gadolinium oxide contained within an aluminum reservoir. The reservoir is secured by a thin layer of Mylar and x-ray film, and vacuum sealed in a light tight package. In the presence of a thermal neutron flux, the gadolinium atoms absorb …

Optimization Of Coronal Mass Ejection Ensemble Forecasting Using Wsa-Enlil With Coned Model, Jack A . Shepherd Iii

Theses and Dissertations

The combination of the Wang-Sheeley-Arge (WSA) coronal model, ENLIL version 2.7, and the Coned model version 1.4 was utilized to form ensemble forecast for 21 coronal mass ejections (CMEs). The Coned model was improved by adding a weight for the associated flare location to push the propagation axis towards the flare location and an additional image was used. Fifteen CMEs were used from previous research using the Coned model version 1.3 and the results were compared to determine the improvement. The model was used again with six additional CMEs and compared to the old Coned model version 1.3 to verify …

Changes To Tensile Strength And Electromagnetic Shielding Effectiveness In Neutron Irradiated Carbon Nanocomposites, James F. Shinn

Theses and Dissertations

Electromagnetic interference shielding effectiveness (EMI-SE) and tensile strength of nanocomposites containing multi-walled carbon nanotubes (MWNTs) are investigated following neutron irradiation. This nanocomposite material consists of two plies of MWNTs in an epoxy resin, and 4 plies of an E-glass substrate. EMI-SE measurements over the X-band frequency spectrum and monotonic tension tests to determine Young’s Modulus were performed before and after irradiation on the nanocomposite material. The nanocomposite and the MWNT plies were irradiated to a total fluence of 1.4 × 10¹⁴ 1 MeV (Si-eq) n/cm². The nanocomposites showed an average increase of 8 dB in shielding effectiveness …

Collisional Broadening And Shift Of D1 And D2 Spectral Lines In Atomic Alkali Vapor - Noble Gas Systems, Robert D. Loper

Theses and Dissertations

The Baranger model is used to compute collisional broadening and shift of the D1 and D2 spectral lines of M + Ng, where M = K, Rb, Cs and Ng = He, Ne, Ar, using scattering phase shift differences which are calculated from scattering matrix elements. Scattering matrix elements are calculated using the Channel Packet Method where the collisions are treated non-adiabatically and include spin-orbit and Coriolis couplings. Non-adiabatic wavepacket dynamics are determined using the split-operator method together with a unitary transformation between adiabatic and diabatic representations. Scattering phase shift differences are thermally weighted and integrated over energies ranging from …

Solving Point-Reactor Kinetics Equations Using Exponential Moment Methods, Paul M. Thelen

Theses and Dissertations

A robust method of solving the reactor point kinetic equations was designed using exponential moment methods. Although the method requires a relatively large number of calculations to complete, the accuracy ensured by each individual step calculation allows larger time steps to be used. The algorithm designed was verified to converge to the correct value as step size was reduced. Additionally, the algorithm can take steps much larger than the average neutron lifetime while maintaining some precision. An error control scheme was designed based on changes observed in the results as a function of time step size. The error control adaptively …

Study Of Laminar Flame 2-D Scalar Values At Various Fuel To Air Ratios Using An Imaging Fourier-Transform Spectrometer And 2-D Cfd Analysis, Andrew J. Westman

Theses and Dissertations

This work furthers an ongoing effort to develop imaging Fourier-transform spectrometry (IFTS) for combustion diagnostics and to validate reactive-flow computational fluid dynamics (CFD) predictions. An ideal, laminar flame produced by an ethylene-fueled (C₂H₄) Hencken burner (25.4 x 25.4 mm² burner) with N₂ co-flow was studied using a Telops infrared IFTS featuring an Indium Antimonide (InSb), 1.5 to 5.5 µm, focal-plane array imaging the scene through a Michelson interferometer. Flame equivalency ratios of Φ = 0.81, 0.91, and 1.11 were imaged on a 128 x 200 pixel array with a 0.48 mm per pixel spatial …

Discrimination Of Neutral Postures In Computer Based Work, Alanna R. Keith

Theses and Dissertations

A biometric system recognizes users based on the way they physically interact with the system. In this work, we discover a common behavior that a typist consistently displays in non-trivial computer work. We sought to demonstrate three objectives: first, compelling proof that a user can be actively recognized over the course of a lengthy task via a neutral posture struck multiple times in that task; two, a sensing concept for capturing the neutral posture, and, third, an objective method for determine the level of work performed by each typist. This thesis develops a model for hand tracking using a simple …

Pump Diode Characterization For An Unstable Diode-Pumped Alkali Laser Resonator, Chad T. Taguba

Theses and Dissertations

Measurements of wavelength tunability, spectral linewidth, minimum spot size, and M2 were made for a rubidium diode-pumped alkali laser (DPAL) containing a volume Bragg grating (VBG) for resonator characterization. The output wavelength of the pump diode was measured as a function of volume Bragg grating temperature. A linear relationship was observed that corresponded to an output wavelength range of 780.08 nm 0.01 nm to 780.49 nm 0.01 nm with a tuning rate of 6.1 pm/ C for a diode chassis temperature of 37 C. Rubidium D1 fuorescence spectra were recorded as the diode wavelength was scanned across its full range …

Depairing Current Density In Cuprate Superconductors, Manlai Liang

Theses and Dissertations

We report a study of the depairing critical current density in the electron-doped infinite-layer Sr_1-xLa_xCuO₂ and Nd_2-xCe_xCuO_4-\delta system. In this work, many new apparatuses were set up to measure the samples. Resistance-versus-temperature transition curves measured at high pulsed current densities show the classic proportionality between the transition-temperature shift and the two-thirds power of the applied current. The measurement provides an assessment of the superfluid density in the system, and a secondary route for obtaining the penetration depth purely through a transport measurements.

Nanoparticle Self-Assembly And Ultrafast Nanomagnet Switching Dynamics, Longfei Ye

Theses and Dissertations

A novel manufacturing technology that offers a low-cost alternative for creating more complex optical materials that are assembled with single-nanometer precision is demonstrated. Using the enormous magnetic field gradients present near the surface of magnetic recording media, colloidally suspended superparamagnetic nanopartilces are self-assembled into patterned microstructures.

The position and shape of these microstructures are precisely controlled by magnetic patterns on the template. The template that can be reprogrammed and reused is magnetically recorded using commercial magnetic recording technology. These microstructures consisting entirely of self-assembled magnetic nanoparticles are then transferred to flexible polymer thin films with patterns maintained. In particular, all-nanoparticle …

Analysis Of Polyethylene Glycol In The Α-Hemolysin Nanopore, David M. Dancho

Theses and Dissertations

Nanopores have been shown to be a useful analytical tool for single molecule detection. They have been used to study the composition of DNA and other molecules of interest. These pores are usually α-hemolysin which is a toxin from Staphylococcus aureus or more recently nanoscale synthetic solid state pores. Now we are beginning to look at other molecules or proteins by sending them into the nanopores and measuring a characteristic partial current blockade. In this thesis we look at polyethylene glycol (PEG) as it enters and blocks current through a single alpha hemolysin pore. We report the effects of ionic …

An Effective Field Theory Calculation Of N(P, D)Γ Cross-Section For Big Bang Nucleo-Synthesis, Rasha Adnan Kamand

Theses and Dissertations

Studying the nuclear reaction n(p,d)γ and calculating its cross-section is not only a matter of interest from theoretical particle physics point of view but also from the viewpoint of cosmology. We now know that the universe is made up of only 5% baryonic matter. So, computing the density of baryons is of particular importance to physicists in general and cosmologists in particular. Deuterium production during Big Bang Nucleo-synthesis (BBN) is very sensitive to the density of baryons, thus baryon density can be inferred from the abundance of deuterium. In order to calculate deuterium abundance one needs to use the cross-section …

Quantifying The Impact Of Various Radioactive Background Sources On Germanium-76 Zero-Neutrino-Double-Beta-Decay Experiments, Katarina Leila Mizouni

Theses and Dissertations

The goal of searching for 0vBB-decay is to probe an absolute neutrino mass scale suggested by the mass-splitting parameters observed by neutrino oscillation experiments. Furthermore, observation of 0vBB-decay is an explicit instance of lepton-number non-conservation. To detect the rare events such as 0vBB-decay, half-lives of the order of 10^25-10^27 years have to be probed. Using an active detector with a large volume, such as hundreds of kilograms of HPGe in the case of MAJORANA, and taking efficient measures to mitigate background of cosmic and primordial origins are necessary for the success of a sensitive 0vBB-decay experiment.

One focus of the …