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Neutron stars in mass-transferring binaries accrete hydrogen and helium rich matter from their companions. Unstable nuclear burning of this matter results in Type I X-ray bursts every few hours to days. Theoretical models propose three regimes of unstable burning on neutron star surfaces and also define the critical accretion rate above which the neutron star burns fuel stably without bursting. Currently, the theoretical value for this stability boundary is comparable to the Eddington limit whereas the observational limit is estimated to 0.3 m˙Edd. In this dissertation, we examine the boundary between stable and unstable thermonuclear burning at the surface of an accreting neutron star for a range of initial compositions. We compute different steady-state equilibrium models and use them to conduct a linear perturbation analysis. The main results of this analysis are plots of the critical accretion rates as a function of the initial helium mass fraction which were never produced before.

Future fundamental physics experiments in neutron beta-decay require highly efficient detection of protons. Many of the experiments use electrostatic acceleration of the recoil protons into large-area silicon detectors for this purpose. A 30~keV proton accelerator was designed, created, and commissioned in order to characterize silicon detectors of this type. Final construction and initial results on the performance of the accelerator are presented. A unique aspect of the work is the use of a Penning ion generator as an ion source. The accelerator produced protons with momentum resolution ~1%. The ion source produced current stably, over a range of parameters, and over long periods of time. The accelerator achieved proton rates sufficient to calibrate silicon detectors.

The organisation and dynamic behaviour of a copolyelectrolyte monolayer is discussed A linear diblock copolymer of poly(methyl methacrylate) (PMMA) and poly(4-viny ethylpyridinium bromide) (QP4VP) has been the main focus of the study, although films of both the unquaternised copolymer, PMMA-P4VP and a PMMA homopolymei have also been examined for comparative purposes. The polymers were spread or subphases of water and potassium chloride solutions of varying concentrations to determine changes in structure and dynamics with polymer surface concentration and subphase salt concentration. Monolayer behaviour has been characterised from surface pressure isotherms and the use of Brewster angle microscopy. It has been demonstrated that the shape of the isotherm is dependent on the potassium chloride concentration of the subphase. Information on the organisation of the system has been determined by neutron reflectometry. A systematic variation in organisation occurs as both polymer surface concentration and subphase potassium chloride concentration change. The polyelectrolytic QP4VP block stretches more into the subphase with increasing surface concentration or decreasing salt concentration. The results have been compared to scaling laws for polymer brushes. Dynamic behaviour has been studied by the use of surface quasi-elastic light scattering (SQELS) and resonance between the capillary and dilational waves of the system is observed. The phenomenon of mode mixing and the application of viscoelastic models to the system have also been examined. It has been discovered that an accurate description of the surface viscoelastic properties of the system could not be obtained by the use of standard viscoelastic models. Mode mixing was not observed, even in those systems where negative dilational viscosities were found.

Solvated brush layers formed by linear polystyrenes have been investigated under quiescent and solvent shear-flow conditions using neutron reflectometry. Cyclohexane and toluene were used as solvents, and the polystyrene chains were tethered by one end to macroscopically flat silicon substrates via short poly-4-vinylpyridine end- groups. The brush systems were studied using a purpose built flow reflectometry cell. The brush height was found to increase with improving thermodynamic quality of the solvent due to increasingly strong repulsive excluded volume interactions between chain segments. Model fitting of the reflectivity data revealed that the polymer volume fraction profile was well described by parabola-like functions in agreement with the predictions of self-consistent field theory. No changes in the reflectivity profiles were observed upon exposure of the brush layers to solvent flow-induced area average shear rates of up to 147 000 s(^-1). This observation has been rationalised through a number of considerations, including comparison with recent theoretical predictions. Aqueous micellar dispersions of diblock copolymers of styrene and ethylene oxide have been studied using small angle neutron scattering in the concentration range 0.05 to 6.5 weight % copolymer. The micelles formed were found to be spherically symmetric, consisting of a polystyrene core surrounded by a corona of highly stretched solvated polyethylene oxide chains. At higher concentrations, the intermicellar interaction has been interpreted using an analytic structure factor originally developed to describe macroion solutions. On subjecting the dispersions to Couette shear, no evidence for long-range ordering of the micelles was observed in the diffraction patterns.

The smuggling of special nuclear material (SNM) has long been a concern. In April 2009, President Obama declared that a terrorist acquiring a nuclear weapon was the most immediate threat to global security. The Second Line of Defense (SLD) initiative was stood up by the National Nuclear Security Administration to deter, detect, and interdict illicit trafficking of nuclear and radioactive materials across international borders and maritime shipping. The SLD initiative does not provide for the detection of SNM being carried on small, personal watercraft. Previous work examined the possibility of using active neutron detectors to induce fission in SNM and detect the response. This thesis examines the possibility of detecting SNM using passive 3He neutron detectors. Monte Carlo N-Particle (MCNP) simulations were run to determine the best detector configuration. Detecting sources at increasing depths, detecting moving sources and the effects of waves were also simulated in MCNP. Comparisons with experimental measurements showed that detectors parallel to the surface of water were best at detecting neutron sources below the surface. Additionally, stacking detectors and placing a cadmium sheet between the polyethylene blocks resulted in a greater ability to determine the height of a source by taking the ratio of count rates in the lower and upper detectors. Using this configuration, a source of strength 3.39 x 10^5 n/s could be detected to a depth of 12.00 in below the water surface. Count rates in the presence of waves did not average out to count rates taken above a flat plane of water. Detectors closer to the water performed worse than above a flat plane while detectors placed higher recorded more counts than above a flat plane. Moving sources were also simulated; sources under water, 3.00 ft from the detectors, and moving at 5.8 kts could be detected above background.

Accurate knowledge of the neutron flux distribution in a nuclear reactor has many tangible benefits. Perhaps the most important are the contributions to reactor safety. Detailed knowledge allows reactor operators to identify off-normal conditions quickly before they cause serious complications. Furthermore, reactor safety margins can be accurately quantified. As advanced reactor types are proposed, new sensor systems should be developed together with new algorithms for neutron flux reconstruction. This thesis develops neutron flux reconstruction methods for in-core sensors placed in HTRs. Sensor systems developed for current generation reactors cannot be used in HTRs. The high temperatures inside HTRs preclude the use of existing in-core sensors, and complex flux phenomena arising from the inner reflector and three-dimensional fuel block arrangements suggest that new flux reconstruction methods should be developed as well. Computer simulations were run to generate detailed in- core neutron flux distributions representative of HTRs. Next, this data was used to test two different flux reconstruction algorithms. It was found that the reconstruction algorithm based on the proper orthogonal decomposition performed better than the algorithm based on linear interpolation.

Thesis (M.S. - Hydrology and Water Resources)--University of Arizona. / Includes bibliographical references (leaves 99-101).

Thesis (M.S.)--Oregon State University, 2008. / Printout. Includes bibliographical references (leaves 97-99). Also available on the World Wide Web.

Thesis (M. Phil.)--University of Hong Kong, 2008. / Includes bibliographical references (leaf 55-58) Also available in print.