No description available.

No description available.

No description available.

This thesis describes the development of a novel comprehensive Monte Carlo simulation code, ARTEMIS, for the investigation of electron-hole pair transport mechanisms in a-Se x-ray imaging detectors. ARTEMIS allows for modeling of spatiotemporal carrier transport in a-Se, combining an existing Monte Carlo simulation package, PENELOPE, for simulation of x-ray and secondary electron interactions and new routines for electron-hole pair transport with three-dimensional spatiotemporal signal output considering the effects of applied electric field. The detector Swank factor, an important imaging performance metric is calculated from simulated pulse-height spectra and shown to depend on incident x-ray energy and applied electric field. Simulation results are compared to experimental measurements and are found to agree within 2%. Clinical x-ray spectra are also used to study detector performance in terms of energy weighting and electronic noise. Simulation results show energy-weighting effects are taken into account in the ARTEMIS model, where the Swank factor and DQE have a higher dependence on the high-energy incident x rays due to increased carrier yield. Electronic noise is found to widen the pulse-height spectra and degrade the Swank factor. The effect of recombination algorithms and burst models are studied. A comparison of a first-hit algorithm and a nearest-neighbor approach shows no significant difference in the simulation output while achieving reduced simulation time. The examination of the initial generation of carriers in the burst shows that the recombination efficiency of carriers is dependent on the carrier density and electric field. Finally, the spatial resolution characteristics of a flat-panel a-Se detector are studied by using the ARTEMIS model for spatial output and image generation. The modulation transfer functions are calculated from simulated detector point response functions for monoenergetic and clinical radiation qualities.

Catalase HPII of Escherichia coli is similar in sequence and structure to other catalases including the conservation of several residues on both the distal and proximal sides of the active center heme. The roles of many residues on the distal side of the heme have been well characterized. By contrast, very few residues on the proximal side of the heme or in the plane of the heme have been investigated. The primary goal of this thesis is to develop a better understanding of the role of the residues and structural features at the core of catalases and in the lateral access channel. The results demonstrate that a break in molecular symmetry does not have any functional significance. Replacing Ile274 with a Cys resulted in the heme being covalently linked to the protein through a Cys-vinyl bond which is hypersensitive to X-ray irradiation being largely degraded within seconds of exposure to the X-ray beam.

An investigation into the structures of the antigen-binding fragments (Fab) of mouse monoclonal immunoglobulins by X-ray crystallography, is presented. The family of immunoglobulins studied, Gloops 1-5, possess the ability to bind to both the peptide antigen and the parent protein: Hen Egg-white Lysozyme. The Gloop1 and Gloop4 Fabs were generated by proteolytic cleavage of the antibody, and crystallisation trials yielded crystals of uncomplexed Fabs of both species. Attempts to grow Fab:antigen complex crystals proved unsuccessful. Partial purification of the heterogeneous Gloop4 Fab was found to be essential for the success of the crystallisations. Data were collected on crystal forms of Gloop1, Gloop2 and Gloop4 on an area detector. The structures of three Gloop2 crystal forms were solved by the molecular replacement technique, using models consisting of Fab domain pairs. Two of the crystal forms were refined by molecular dynamics methods at maximum resolutions of 3.3Å and 2.8Å, and the third by rigid body methods alone at 3.5Å resolution. Analysis of the Gloop2 structures between the crystal forms and with other Fabs indicated no atypical features in the domains. The elbow bend of the Gloop2 Fabs differed by up to 7°. The relative association of variable and constant domain pairs was also seen to vary between the Fabs. The changes in domain pairing caused significant differences in the relative disposition of the complementarity determining regions (CDRs), although there was no evidence for conformational change within individual CDRs. The Gloop2 combining site is dominated by a groove of approximate dimensions 12Åx9Åx7Å, containing many aromatic side-chains and a pair of glutamic acid residues in analogous positions to a those found in a FabrHen egg-white Lysozyme complex.

The high-mass X-ray binary SMC X-1 has been observed during the interval 1986-1989 at a threshold γ-ray energy of 0.4 TeV using the University of Durham Mark III air Cerenkov telescope at Narrabri. The Cerenkov arrival time series recorded during these observations have been tested for periodicity at the contemporary pulse period of the SMC X-1 X-ray pulsar. This period analysis was performed using a new technique - introduced in the present work - which is designed for TeV γ-ray source candidates in binary systems and which simultaneously seeks evidence for pulsed TeV γ-ray emission and information on the location of the emission site within the binary. Results are also presented from the period analysis of data accumulated at Narrabri during observations of the low-mass X-ray binaries X0021.8-7221 (in the globular cluster 47 Tucanae) and GX 1+4. X-ray binaries as a class of TeV γ-ray source are considered and their properties compared. Previous models which invoke the interaction of beams of high-energy particles with material within the environment of a binary system are discussed. One of these models is adapted in the present work in order to propose a simple explanation of the narrow TeV 7-ray orbital light curve observed for the high-mass binary Centaurus X-3. The new model accounts for the γ-ray emission from Cen X-3 in terms of the collisions of ultrarelativistic protons with an accretion wake trailing the X-ray pulsar. This model, which incorporates the steering of charged particles in the magnetosphere of the massive primary star, is also applied to the Vela X-1 binary in order to investigate claims that TeV γ-ray outbursts observed during X-ray eclipse arise at particle collisions with the limb of the supergiant companion.

No description available.

Thesis (Ph. D.)--University of Washington. / Bibliography: l. 157-163.

Thesis (Ph.D.)--University of Melbourne, School of Physics, 2006. / Typescript. Includes bibliographical references (leaves 151-159).