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Air Force Institute of Technology

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Articles 991 - 1020 of 2678

Full-Text Articles in Physical Sciences and Mathematics

Spectral Analysis And Metastable Absorption Measurements Of High Pressure Capacitively And Inductively Coupled Radio-Frequency Argon Helium Discharges, Ben Eshel Jun 2013

Spectral Analysis And Metastable Absorption Measurements Of High Pressure Capacitively And Inductively Coupled Radio-Frequency Argon Helium Discharges, Ben Eshel

Theses and Dissertations

Characterization of the excited state argon population in an argon-helium discharge is of crucial importance to the development of a continuous wave Discharge Assisted Noble Gas Laser. Experiments were conducted in the Richard W. Davis Advanced Laser Facility at AFRL on Kirtland Air Force Base, Albuquerque, NM. During these tests the capability to maintain a capactively-coupled and inductively-coupled radio-frequency discharge at pressures of 300 Torr was demonstrated. Optical Emission Spectroscopy experiments were conducted in order to examine the population distribution of the p-state in argon. The population distributions for these high-pressure and high-power cases were compared to a low-pressure, low-power …

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Context-Driven Image Annotation Using Imagenet, George E. Noel, Gilbert L. Peterson May 2013

Context-Driven Image Annotation Using Imagenet, George E. Noel, Gilbert L. Peterson

Faculty Publications

Image annotation research has demonstrated success on test data for focused domains. Unfortunately, extending these techniques to the broader topics found in real world data often results in poor performance. This paper proposes a novel approach that leverages WordNet and ImageNet capabilities to annotate images based on local text and image features. Signatures generated from ImageNet images based on WordNet synonymous sets are compared using Earth Mover's Distance against the query image and used to rank order surrounding words by relevancy. The results demonstrate effective image annotation, producing higher accuracy and improved specificity over the ALIPR image annotation system. Abstract …

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Temperature-Dependent Photoluminescence Of Ge/Si And Ge 1-Ysn Y/Si, Indicating Possible Indirect-To-Direct Bandgap Transition At Lower Sn Content, Mee-Yi Ryu, Thomas R. Harris, Yung Kee Yeo, Richard T. Beeler, John Kouvetakis May 2013

Temperature-Dependent Photoluminescence Of Ge/Si And Ge 1-Ysn Y/Si, Indicating Possible Indirect-To-Direct Bandgap Transition At Lower Sn Content, Mee-Yi Ryu, Thomas R. Harris, Yung Kee Yeo, Richard T. Beeler, John Kouvetakis

Faculty Publications

Temperature (T)-dependent photoluminescence (PL) has been investigated for both p-Ge and n-Ge1-ySny films grown on Si substrates. For the p-Ge, strong direct bandgap (ED) along with weak indirect bandgap related (EID) PL at low temperatures (LTs) and strong ED PL at room temperature (RT) were observed. In contrast, for the n-Ge1-ySny, very strong dominant EID PL at LT and strong ED PL were observed at RT. This T-dependent PL study indicates that the indirect-to-direct bandgap transitions of Ge1-ySn …

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Reversible Mn Segregation At The Polar Surface Of Lithium Tetraborate, Christina L. Dugan, Robert L. Hengehold, Stephen R. Mchale, Juan A. Colon Santana, John W. Mcclory, Volodymyr T. Adamiv, Yaroslav V. Burak, Ya B. Losovyj, Peter A. Dowben Apr 2013

Reversible Mn Segregation At The Polar Surface Of Lithium Tetraborate, Christina L. Dugan, Robert L. Hengehold, Stephen R. Mchale, Juan A. Colon Santana, John W. Mcclory, Volodymyr T. Adamiv, Yaroslav V. Burak, Ya B. Losovyj, Peter A. Dowben

Faculty Publications

We find Mn surface segregation for single crystals of Mn doped Li2B4O7, nominally Li1.95Mn0.05B4O7(001), but as the temperature increases, evidence of this Mn surface segregation diminishes significantly. At room temperature, the surface photovoltaic charging is significant for this pyroelectric material but is quenched at a temperature well below that seen for the undoped Li2B4O7 samples. The suppression of surface charging in the region of 120 °C that accompanies the temperature of Mn dissolution in the bulk of Li2B4 …

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Modeling Radioactive Decay Chains With Branching Fraction Uncertainties, Timothy M. Higley Mar 2013

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 …

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Cathodoluminescence And Thermoluminescence Of Undoped Ltb And Ltb:A (A = Cu, Ag, Mn), Zachary L. Hadfield Mar 2013

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 …

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An Inverse Kinematic Approach Using Groebner Basis Theory Applied To Gait Cycle Analysis, Anum Barki Mar 2013

An Inverse Kinematic Approach Using Groebner Basis Theory Applied To Gait Cycle Analysis, Anum Barki

Theses and Dissertations

Kinematics of the human body was researched for the purposes of this study. The force protection issues of today was the motivation to research pattern recognition in the human gait cycle to identify individuals carrying a concealed load on their body. The goal of this research was to identify gait signatures of human subjects and distinguish between subjects carrying a concealed load to subjects without load. Thus, this research was focused on studying the human gait cycle as well as methods used in identifying gait signatures. The main focus herein is concerned with the movement of the lower extremities, in …

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Electron Damage Effects On Carbon Nanotube Thin Films, Jeremy S. Best Mar 2013

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.

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Electronic Characteristics Of Rare Earth Doped Gan Schottky Diodes, Aaron B. Blanning Mar 2013

Electronic Characteristics Of Rare Earth Doped Gan Schottky Diodes, Aaron B. Blanning

Theses and Dissertations

The Schottky barrier height (SBH) was measured on GaN based diodes with three different dopant types; Gd, Er and Yb. Two methods were used to determine the SBH. The first method was the Juergen Werner method to evaluate the I-V characteristics. The calculated SBH of the diodes using this method was 1.41 ± 0.20eV, 1.71 ± 0.25eV and 1.75 ± 0.28eV for the Gd-, Er- and Yb-doped diodes respectively. It was observed that using an ad-hoc effective Richardson constant value of 0.006Axcm-2xK-2 to calculate the SBH rendered results that were in no greater than 2% disagreement (neglecting …

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Passive, Low Cost Neutron Detectors For Neutron Diagnostics At The National Ignition Facility, Nathaniel C. Loyd Mar 2013

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 …

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Crystal Growth And Characterization Of Tho2 And UXTh1-XO2, Jacob G. Castilow Mar 2013

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

Theses and Dissertations

Hydrothermal synthesis of ThO2 and UxTh1-xO2, and UOx 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 …

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Three Dimensional Positron Annihilation Momentum Spectroscopy Of Lithium Tetraborate Crystals, Stefan B. Fagan-Kelly Mar 2013

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 …

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Modeling The Thermosphere As A Driven-Dissipative Thermodynamic System, William R. Frey Mar 2013

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, T1/2 , defined by Jacchia, 1977 (J77), is modeled using the magnetospheric electric field as a driver. Three methods of treating the UV contribution to T1/2 (T1/2UV) are tested. Two model parameters, the coupling and relaxation constants, are adjusted for 38 storms from 2002 - 2008 to …

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A Comparison Of Ionospheric Model Performance For International Space Station Orbits, David J. Broadwater Mar 2013

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 …

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Analysis Of A Van De Graaff Generator For Emp Direct Current Survivability Testing, Robert J. Kress Mar 2013

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 …

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Depth-Resolved Cathodoluminescence Of Tho2, Michael G. Lee Mar 2013

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 …

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Discrimination Of Neutral Postures In Computer Based Work, Alanna R. Keith Mar 2013

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 …

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Neutron Shielding Effectiveness Of Multifunctional Composite Materials, Anthony D. Marchand Mar 2013

Neutron Shielding Effectiveness Of Multifunctional Composite Materials, Anthony D. Marchand

Theses and Dissertations

Composite materials offer a greater degree of flexibility in design and engineering of specialized space vehicle shielding applications compared to aluminum. A new design for shielding materials has been developed by including specific neutron absorbers and conductive materials into the composite structure. In this research, the neutron shielding capability of two types of custom-designed nanocomposite materials are compared to that of T6061 aluminum.

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Optimization Of Coronal Mass Ejection Ensemble Forecasting Using Wsa-Enlil With Coned Model, Jack A . Shepherd Iii Mar 2013

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 …

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Collisional Broadening And Shift Of D1 And D2 Spectral Lines In Atomic Alkali Vapor - Noble Gas Systems, Robert D. Loper Mar 2013

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 …

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Solving Point-Reactor Kinetics Equations Using Exponential Moment Methods, Paul M. Thelen Mar 2013

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 …

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Investigation Into The Ratio Of System Operations And Support Costs To Life-Cycle Costs For Department Of Defense Weapon Systems, Gary L. Jones Mar 2013

Investigation Into The Ratio Of System Operations And Support Costs To Life-Cycle Costs For Department Of Defense Weapon Systems, Gary L. Jones

Theses and Dissertations

As the arsenal of weapons managed by the Department of Defense (DoD) ages, the country's leadership is forced to make decisions regarding what to do with current and projected funding to maintain military defensive and offensive capabilities. The acquisition community at many levels has stated that O&S costs are about 70 percent of the total life-cycle costs of the average system, the other 30 percent being spent in the acquisition phase. This golden ratio does not appear to come from empirical evidence. There has been little research into the actual ratio of O&S-to-acquisition costs. This research found a significant departure …

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Determining Optimal Machine Replacement Events With Periodic Inspection Intervals, Theodore C. Shiveley Mar 2013

Determining Optimal Machine Replacement Events With Periodic Inspection Intervals, Theodore C. Shiveley

Theses and Dissertations

This research will examine the optimal maintenance and replacement policies for a generic machine with periodic inspection intervals. The considered reliability models consist of a single machine that can fail during operation or else may be found to be inoperative during regularly-scheduled maintenance inspections. A distinction will be made between spontaneously-occurring failures during operation and those that are discovered during inspections. Since the elapsed time between inspections is constant, the resulting stochastic reliability process becomes non-Markovian, and thus a Semi-Markov Decision Process (SMDP) framework must be employed in order to determine the cost-optimal stationary policy consisting of repair and replace …

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Modeling Sodium Iodide Detector Response Using A Parametric Equations, Neeraj Sethi Mar 2013

Modeling Sodium Iodide Detector Response Using A Parametric Equations, Neeraj Sethi

Theses and Dissertations

This research utilizes the relationship between source location, background radiation and detector placement to quantify an expected detector response from a 3x3 Sodium Iodide gamma detector. A parametric equation is developed to calculate the detector response as a function of distance and source photon energy. Using MCNP particle current and pulse height tally functions, backscattering photons are quantified as a function of material thickness and energy distribution. Three configurations of source - detector - scattering medium were modeled in MCNP using the pulse height tally functions, integrated over a 70keV - 360keV energy window and plotted as a function of …

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Changes To Tensile Strength And Electromagnetic Shielding Effectiveness In Neutron Irradiated Carbon Nanocomposites, James F. Shinn Mar 2013

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 …

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Mobile Network Defense Interface For Cyber Defense And Situational Awareness, James C. Hannan Mar 2013

Mobile Network Defense Interface For Cyber Defense And Situational Awareness, James C. Hannan

Theses and Dissertations

Today's computer networks are under constant attack. In order to deal with this constant threat, network administrators rely on intrusion detection and prevention services (IDS) (IPS). Most IDS and IPS implement static rule sets to automatically alert administrators and resolve intrusions. Network administrators face a difficult challenge, identifying attacks against a vast number of benign network transactions. Also after a threat is identified making even the smallest policy change to the security software potentially has far-reaching and unanticipated consequences. Finally, because the administrator is primarily responding to alerts they may lose situational awareness of the network. During this research a …

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Discrimination Between Child And Adult Forms Using Radar Frequency Signature Analysis, Stephanie R. Keith Mar 2013

Discrimination Between Child And Adult Forms Using Radar Frequency Signature Analysis, Stephanie R. Keith

Theses and Dissertations

In this thesis we develop a method to discriminate between adult and child radar signatures. In particular, we examine radar data measured from behind a wall, which introduces radar signal attenuation and multipath effects. To investigate the child/adult discrimination problem in a through-wall, multipath scenario, a previously developed free-space human scattering model was expanded to incorporate multiple paths, and the effects of transmission through, and reflections from, walls and ground. The ground was modeled as a perfectly reflecting surface, while the walls were modeled as homogeneous concrete slabs. Twenty-five reflection paths were identified, involving the direct paths, as well as …

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Development Of A Spectropolarimetric Capability, Evan J. Carlson Mar 2013

Development Of A Spectropolarimetric Capability, Evan J. Carlson

Theses and Dissertations

A Bomem 200 series Fourier transform infrared spectrometer and FLIR Systems Inc InGaAs infrared (IR) camera was used to measure the degree of linear polarization (DOLP) from a metal and glass surfaces. This was accomplished by placing a wire grid polarizer in front of the aperture of the FTIR and IR camera to measure the polarized reflection from the target material. For the IR camera, an average of pixel counts was collected over the area of the target to measure to DOLP. Factors impacting the accuracy of the measurements from the FTIR are calibration, instrument self emission, and alignment error. …

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Neutron Spectroscopy Using Lif Thin-Film Detectors, Michael A. Ford Mar 2013

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 …

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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 Mar 2013

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 …

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