Two related lines of research in experimental electron density determination are reported in this thesis. In the first case, the well-proven and popular multipole modeling technique is applied to three high resolution, single-crystal X-ray diffraction data sets. The preliminary part of this thesis (Chapters 2-5) deals with the theoretical aspects of the multipole model, and also some of the theoretical and practical aspects of data collection and reduction. Chapter 6 reports an experimental charge density determination of a nitrogen ylide. Chapter 7 contains details of the treatment of data from a large, pendant-arm macrocyclic complex of nickel, while Chapter 8 reports the characteristics of the experimentally determined charge density for a substituted acetylene molecule which exhibits interesting intramolecular interactions. The charge densities for all three cases are analysed using Bader's Theory of Atoms in Molecules. The latter part of this thesis deals with more novel ways of treating experimental data. Chapter 9 gives a thorough review of the literature on the application of Maximum Entropy techniques to image reconstruction in general and charge density determination in particular, followed in Chapter 10 by an application to diffraction data from the cubic phase of acetylene. The novel approach of removing core scattering from the data is developed and gives improved results. Chapter 11 reviews some aspects of fermion density matrices and their relationship to electron density functions and X-ray scattering, followed in Chapter 12 by results from the density matrix refinement method applied to diffraction data from formamide. Particular emphasis is placed upon basis set effects, idempotency and various N-representable approximations to the experimentally determined density matrix.

In 1789 a German chemist, Martin Heinrich Klaproth, announced the discovery of a new element: uranium; few years later, the head of father of the modern chemistry, Antoine Lavoisier, was swept away by guillotine: a new era was destined to be opened, either where energy would have been produced in large scale by nuclear processes delivering hundreds of times the energy of chemical processes or where a mass of people, revolutionary or not, would have been melted down into a couple of seconds. After a quite long time, on the 2nd December 1942, the first nuclear reactor has been put into operation by Enrico Fermi in Chicago; few years later, came also the dark side utilization of fissile materials in Hiroshima and Nagasaki. Since those moments, three power plants generations succeeded, until the current one which is the generation IV of nuclear reactors. The latter has the goal of generating electricity in a safe manner, for the core is designed to provide an effective passive cooling of the decay heat. Amid generation IV of nuclear power plants, the Gas Turbine – Modular Helium Reactor, designed by General Atomics, is the only core with an energy conversion efficiency of 50%; the above consideration, coupled to construction and operation costs lower than ordinary Light Water Reactors, renders the Gas Turbine – Modular Helium reactor rather unequaled. In the present studies we investigated the possibility to operate the GT-MHR with two types of fuels: LWRs waste and thorium; since thorium is made of only fertile 232Th, we tried to mix it with pure 233U, 235U or 239Pu; ex post facto, only uranium isotopes allow the reactor operation, that induced us to examine the possibility to use a mixture of uranium, enriched 20% in 235U, and thorium. We performed all calculations by the MCNP and MCB codes, which allowed to model the reactor in a very detailed threedimensional geometry and to describe the nuclides transmutation in a continuous energy approach; finally, we completed our studies by verifying the influence of the major nuclear data libraries, JEFF, JENDL and ENDF/B, on the obtained results. / <p>QC 20100922</p>

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A comprehensive computer package for the calculation and simulation of charged-particle dynamics in electromagnetic fields has been developed and tested. The program provides a user-friendly and flexible interface for visualizing particle dynamics using phase space diagrams, which are essential for complete understanding of a beam optics system. The program performs an accurate finite difference computation of a user-defined boundary value problem (in the form of a grid) followed by a high-order Runge-Kutta numerical integration of the equations of motion to evaluate the particle dynamics within the field. The program is unique in its combination of these flexible finite calculation techniques with the parallel processing of particle ensembles in order to display phase space diagrams. / After extensive testing, the program has been used to design a low emittance ion source and an ion beam deceleration system for high-efficiency ion collection. The program has also been used to analyze a radiofrequency quadrupole collisional focusing system using ion mobility concepts.

Thesis (PhD)-- University of South Australia, 1997

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