Browsing by Subject "X-ray"
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Item Electrical, optical and x-ray diagnostics of subnanosecond breakdown in gases(2005-12) Kohl, Kevin Patrick; Krompholz, Hermann G.; Mankowski, John J.; Hatfield, Lynn L.Subnanosecond gas breakdown research is important for application in ultra-wideband radar (UWB), plasma limiters, and high speed switches for use in pulsed power applications. This paper contains breakdown data taken for voltage pulses having amplitude 150 kV and risetimes on the order of 250 ps. Voltage, current, and breakdown time constants are described in this paper for volume breakdowns and surface flashovers. In addition, the x-ray emission and the luminosity were measured. All results point to the importance of runaway electrons for this discharge regime.Item Electrical, optical and x-ray diagnostics of subnanosecond breakdown in gases(Texas Tech University, 2005-12) Kohl, Kevin Patrick; Krompholz, Hermann G.; Mankowski, John J.; Hatfield, Lynn L.Subnanosecond gas breakdown research is important for application in ultra-wideband radar (UWB), plasma limiters, and high speed switches for use in pulsed power applications. This paper contains breakdown data taken for voltage pulses having amplitude 150 kV and risetimes on the order of 250 ps. Voltage, current, and breakdown time constants are described in this paper for volume breakdowns and surface flashovers. In addition, the x-ray emission and the luminosity were measured. All results point to the importance of runaway electrons for this discharge regime.Item Noninvasive material discrimination using spectral radiography and an inverse problem approach(2014-12) Gilbert, Andrew James; Deinert, Mark; McDonald, Benjamin; Biegalski, Steven; Ghattas, Omar; Schneider, ErichNoninvasive material discrimination of an arbitrary object is applicable to a wide range of fields, including medical scans, security inspections, nuclear safeguards, and nuclear material accountancy. In this work, we present an algorithmic framework to accurately determine material compositions from multi-spectral X-ray and neutron radiography. The algorithm uses an inverse problem approach and regularization, which amounts to adding information to the problem; stabilizing the solution so that accurate material estimations can be made from a problem that would otherwise be intractable. First, we show the utility of the algorithm with simulated inspections of small objects, such as baggage, for small quantities of high-atomic-numbered materials (i.e. plutonium). The algorithm shows excellent sensitivity to shielded plutonium in a scan using an X-ray detector that can bin X-rays by energy. We present here a method to adaptively weight the regularization term, obtaining an optimal solution with minimal user input. Second, we explore material discrimination with high-energy, multiple-energy X-ray. Experimental X-ray data is obtained here and accurate discrimination of steel among lower-atomic-numbered materials is shown. Accurate modeling of the inspection system physics is found to be essential for accurate material estimations with this data, especially the detector response and the scattered flux on the image plane. Third, we explore the use of neutron radiography as complementary to X-ray radiography for the inspection of nuclear material storage containers. Utility of this extra data is shown, especially in detecting a hypothetical attempt to divert material. We present a method to choose inspection system design parameters (i.e. source energy and detector thickness) a priori by using the Cramér-Rao lower bound as a measure of resulting material estimation accuracy. Finally, we present methodology to use tomography data obtained with an energy discriminating detector for direct reconstruction of material attenuation coefficients.Item Structural characterization of post-PKS enzymes involved in spinosyn biosynthesis(2014-05) Isiorho, Eta Amauche; Keatinge-Clay, Adrian Tristan; Liu, Hung-wen, 1952-Saccharopolyspora spinosa is a rare actinomycete that synthesizes the secondary metabolite spinosyn A, which is an active ingredient in several important commercial insecticides. Spinosyn aglycone formation occurs via a type I polyketide synthase. After release of the polyketide chain from the synthase, various tailoring enzymes modify the aglycone core. These unique enzyme transformations result in unusual structural characteristics found in spinosyn A. The enzymes SpnG, SpnP, SpnF and SpnL each perform a key reaction during post-PKS processing. The work presented in this dissertation focuses on the structural determination and analysis of SpnG, SpnP, SpnF and SpnL. SpnG, which naturally catalyzes the 9-OH rhamnosylation of spinosyn, is capable of adding diverse sugars to the spinosyn aglycone from TDP-hexoses, such as TDP-glucose. However, the substitution of UDP-glucose for TDP-glucose as the donor substrate is known to result in a >60,000-fold reduction in k [subscript cat]. The structure of SpnG at 1.65 Å resolution, the 1.86 Å resolution structure of SpnG bound to TDP, and the 1.70 Å resolution structure of SpnG bound to AGL were determined. The SpnG-TDP complex reveals how SpnG employs N202 to discriminate between TDP- and UDP-sugars. The SpnG-AGL complex shows that SpnG binds the acceptor substrate primarily through hydrophobic interactions and implicates H13 as the potential catalytic base. A model for how rhamnose binds in the active site was constructed to elucidate which features enable SpnG to transfer diverse hexoses. SpnP transfers forosamine from a TDP-D-forosamine donor substrate to a spinosyn pseudoaglycone acceptor substrate. The structures of SpnP and its complex with TDP were determined to 2.50 Å and 3.15 Å resolution, respectively. SpnP possesses a structural feature that has only been previously observed in a related glycosyltransferase, which employs an auxiliary protein that aids in its catalysis. This unique feature may be a used as a predictive motif of glycosyltransferases that interact with an auxiliary protein. SpnF and SpnL are two novel S-adenosyl-L-methionine dependent cyclases. Structural data was utilized in order to gain insight into the unusual cycloaddition catalyzed by the putative Diels-Alderase and Rauhut-Currierase, SpnF and SpnL, respectively. Together these structures provide valuable insights into the unusual mechanisms involved in spinosyn biosynthesis.