Theses and Dissertations from UMD
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Item NOVEL QUASI-FREESTANDING EPITAXIAL GRAPHENE ELECTRON SOURCE HETEROSTRUCTURES FOR X-RAY GENERATION(2024) Lewis, Daniel; Daniels, Kevin M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Graphene, the 2D allotrope of carbon, boasts numerous exceptional qualities like strength, flexibility, and conductivity unmatched for its scale, and amongst its lesser-known capabilities is electron emission at temperatures and electric fields too low to allow for conventional thermionic or field emission sources to function. Driven by the mechanism of Phonon-Assisted Electron Emission (PAEE), planar microstructures fabricated from quasi-freestanding epitaxial graphene (QEG) on silicon carbide have exhibited emission currents of up to 8.5 μA at temperatures and applied fields as low as 200 C and 1 kV/cm, orders of magnitude below conventional electron source requirements.These emission properties can be influenced through variations in microstructure design morphology, and performance is controllable via device temperature and applied field in the same manner as thermionic or field emission sources. As 2D planar devices, graphene microstructure electron emitters can also be encapsulated with a thermally evaporated oxide, granting electrical isolation and environmental resistance, and can even exhibit emission current enhancement under these conditions. Graphene electron emitters expressed as heterostructure material stacks could see implementation as electron emission sources in environments or devices where conventional thermionic or field emission sources can’t be supported due to thermal, power system, or physical size limitations, the presence of contaminants, or even poor vacuum containment. An explorable application could see an oxide-encapsulated graphene electron source paired with a layered interaction-emission anode to create a micron-scale vertical alignment x-ray source with no need of vacuum containment. We investigate these properties with using hydrogen-intercalated quasi-freestanding bilayer epitaxial graphene, a rare and difficult to manufacture formulation that allows the graphene to behave as if it were a freestanding structure, while still benefiting from the macro-scale mechanical strength and fabrication process compatibility afforded by its silicon carbide substrate. The quasi-freestanding nature of the graphene limits substrate phonon interactions, allowing the graphene phonon-electron interactions to dominate, in turn empowering the PAEE mechanic. Our devices benefit from an ease of interaction that is untenable for processes not employing QEG, with the speed and simplicity of fabrication being a hallmark of our investigations. We begin our exploration of how the PAEE mechanism itself can be influenced in our designs, and how process and fabrication optimizations can be leveraged for device applications. Graphene’s role in the fields of microelectronics, condensed matter physics, and materials science is still novel, and rapidly expanding, and our investigations explore a unique facet of this wonder material’s capabilities.Item A dual modality, DCE-MRI and x-ray, physical phantom for quantitative evaluation of breast imaging protocols(2010) Freed, Melanie; Badano, Aldo; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The current clinical standard for breast cancer screening is mammography. However, this technique has a low sensitivity which results in missed cancers. Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) has recently emerged as a promising technique for breast cancer diagnosis and has been reported as being superior to mammography for screening of high-risk women and evaluation of extent of disease. At the same time, low and variable specificity has been documented in the literature as well as a rising number of mastectomies possibly due to the increasing use of DCE-MRI. In this study, we developed and characterized a dual-modality, x-ray and DCE-MRI, anthropomorphic breast phantom for the quantitative assessment of breast imaging protocols. X-ray properties of the phantom were quantitatively compared with patient data, including attenuation coefficients, which matched human values to within the measurement error, and tissue structure using spatial covariance matrices of image data, which were found to be similar in size to patient data. Simulations of the phantom scatter-to-primary ratio (SPR) were produced and experimentally validated then compared with published SPR predictions for homogeneous phantoms. SPR values were as high as 85% in some areas and were heavily influenced by the heterogeneous tissue structure. MRI properties of the phantom, T1 and T2 relaxation values and tissue structure, were also quantitatively compared with patient data and found to match within two error bars. Finally, a dynamic lesion that mimics lesion border shape and washout curve shape was included in the phantom. High spatial and temporal resolution x-ray measurements of the washout curve shape were performed to determine the true contrast agent concentration as a function of time. DCE-MRI phantom measurements using a clinical imaging protocol were compared against the x-ray truth measurements. MRI signal intensity curves were shown to be less specific to lesion type than the x-ray derived contrast agent concentration curves. This phantom allows, for the first time, for quantitative evaluation of and direct comparisons between x-ray and MRI breast imaging modalities in the context of lesion detection and characterization.Item A DEEP X-RAY SURVEY OF THE LOCKMAN HOLE NORTHWEST(2005-08-26) Yang, Yuxuan; Mushotzky, Richard F; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)I present the X-ray analysis of the Chandra Large Area Synoptic X-ray Survey (CLASXS) of the Lockman Hole Northwest field. The contiguous solid angle of the survey is about 0.4 sqr degree and the flux limits are 5x10^-16 erg/cm^2/s in the 0.4-2 keV band and 3x10^-15 erg/cm^2/s in the $2-8$~keV band. The survey bridges the gap between deep pencil beam surveys, and shallower, larger area surveys, allowing a better probe of the X-ray sources that contribute most of the 2--10 keV cosmic X-ray background. A total of 525 X-ray point sources and 4 extended sources have been found. The number counts, X-ray spectra evolution, X-ray variability of the X-ray sources are presented. We show 3 of the 4 extended sources are likely galaxy clusters or galaxy groups. We report the discovery of a gravitational lensing arc associated with one of these sources. I present the spatial correlation function analysis of non-stellar X-ray point sources in the CLASXS and Chandra Deep Field North (CDFN). I calculate both redshift-space and projected correlation functions in comoving coordinates.The correlation function for the CLASXS field over scales of 3 Mpc < s < 200 Mpc can be modeled as a power-law of the form xi(s)=(s/s_0)^{-gamma}, with gamma = 1.6^{+0.4}_{-0.3} and s_0 = 8.05^{+1.4}_{-1.5} Mpc. The redshift-space correlation function for CDFN on scales of 1 Mpc <s < 100 Mpc is found to have a similar correlation length, but a shallower slope. The real-space correlation functions are derived from the projected correlation functions. By comparing the real- and redshift-space correlation functions, we are able to estimate the redshift distortion parameter beta = 0.4 +/- 0.2 at an effective redshift z = 0.94. We found the clustering does not dependence significantly on X-ray color or luminosity. A mild evolution in the clustering amplitude is found, indicating a rapid increase of bias with redshift. The typical mass of the dark matter halo derived from the bias estimates show little change with redshift. The average halo mass is found to be log(M_{halo}/M_sun}) ~ 12.4.