Thesis (Ph. D.)--University of Chicago, 1920. / "Private edition, distributed by the University of Chicago libraries, Chicago, Illinois." "Reprinted from the Physical review, n.s., vol. XVII, no. 3, March, 1921." Also available on the Internet.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1976. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Bibliography: leaves 87-88.

Secondary electron (SE) emission from thin carbon foils induced by 1-20 keV positrons has been investigated over a range of nominal foil thicknesses from 1.0 to 5.0 μg/cm2. The measurement of SEs was carried out in forward geometry using a microchannel plate as a detector. The SE yield γ has been measured as a function of beam energy and compared with some Monte Carlo simulation results. We also present in this thesis the material parameter Λ=? / (dE / dx) and the emitted SE energy spectra. Forincident positron energy of 5 keV or higher, the distribution is found to be characterizedby the Sickafus form, AEm and m is close to 1. For low energy incident positrons however, another form, Bexp(E / t) , is proposed for describing the SE distribution. The maximum scattering angle for SEs emitted from 5.0 μg/cm2 is found to be around 60°. Measurements of energy loss and energy loss straggling for 1-10 keV positrons passing through thin carbon foils of different thicknesses ranging from 1.0 to 5.0 μg/cm2 are present in this thesis. The stopping power dE / dx and positron transmission coefficient have also been investigated as a function of incident positron energy and foil thickness. Particularly, our experimental results are compared with those from Monte Carlo simulation and theory with a view to providing a way to determine the real thickness of carbon foil. The ratio of the energy loss straggling to the foil thickness seems to have a linear relation with the beam energy. The transmitted positrons after passing through 5.0 μg/cm2 C-foil have a small scattering angle which is less than 10°. Titanium dioxide nanotube arrays fabricated by anodization of titanium foil and annealed at different temperatures were studied using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and positron annihilation spectroscopy (PAS). The crystallization process and morphological change of the nanotubes have been discussed. It is found that anatase (101) only appeared on the walls of the nanotube. The atomic concentration of fluoride and the ratio of Ti/O decreased when the annealing temperature increased. Vacancy type defects were found to diffuse toward the surface when the samples were annealed at 200°C and 400°C and healing of vacancies occurred at 600°C. In addition, the fluoride may form some complexes with vacancies on the surface hence lowering the value of the S parameter. A new design of the lifetime positron beam based on the SE emission is proposed. The observed experimental results and simulation results make it possible to construct a new type of high resolution (<250ps) lifetime positron beam. In addition, how rastering of the positron beam can be used to accurately locate the position of the C-foil is also shown; a feature that will be of value in setting up a positron lifetime system. Such a lifetime system is expected to be much simpler in construction than existing beam based lifetime spectroscopy systems. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

No description available.

Biological damage of tissue due to intermediate energy neutrons is generally known to be very important in radiobiology and radiation protection. However, there is no suitable method to determine the quality of these neutrons in particular in the working environment of mixed (n,y) radiation fields. In this thesis, an attempt is made to develop a dosimeter based on microdosimetric principles which has the capability for such a purpose. With this object the basic concepts of microdosimetry are reviewed and discussed with emphasis on their application for radiation protection and in designing of the dosimeter. Microdosimetry based on low pressure tissue-equivalent proportional counters (TEPCs) is a powerful technique for determining microscopic distributions of energy deposition and quality of ionizing radiations. However the energy deposition spectra of intermediate energy neutrons in mixed fields of fast neutrons can only be measured using TEPC in co-axial double cylindrical form by an appropriate choice for the thickness of the common tissue-equivalent (TE) dividing wall separating the inner and outer counters and by appropriate use of coincidence/anti-coincidence pulse arrangements. An analytical calculation for the response of the inner counter operating in coincidence/anti-coincidence modes with the outer counter was developed. However there will be some events, due to fast neutrons, which will contribute to the signals from intermediate energy neutrons and which cannot be removed by anti-coincidence. For these analytical corrections must be made. Also, the events associated with the dividing wall inherent in the system can contribute to the response of the inner counter and must be corrected by calculation. The calculation was possible due to the fact that recoil particles from intermediate energy neutron interactions have effective stopping powers and projected ranges which differ significantly from the continuous slowing down approximation (CSDA) values. By incorporating these the basic CSDA formulae for energy deposition spectra of neutrons could be extended down to intermediate energy neutrons of about 1 keV. A prototype co-axial double cylindrical TEPC capable of separating the component of neutrons (≤ 850 keV) in mixed (n,y) radiation fields was manufactured and tested. The thin wall dividing the inner and outer counters was fabricated from the standard A-150 TE plastic with the thickness equivalent to the range of 850 keV protons. The operational characteristics of the dosimeter were studied to determine its applicability for use in microdosimetry. The gas gain of the inner and outer TEPCs was measured at various simulated mean chord lengths and applied voltages. The results can be expressed according to Campion's equation within a given range of the electric field strength. The resolution of the inner TEPC measured at the operating voltages is in agreement with the theoretical prediction. A series of microdosimetric experiments were performed with mixed fields of 60Co gamma-rays and neutrons from the UTR-300 nuclear reactor and from 252Cf and 241Am-Be radioactive sources. Discrimination against fast neutrons of energy > 850 keV was achieved using an anti-coincidence unit specially designed for better efficiency of data acquisition. Discrimination against fast electrons due to photon interactions was also achieved. Spectra with anti-coincidence are dominated by slow protons and electrons. Their mean lineal energies are higher than those of spectra without anti-coincidence. The quality factor and dose equivalent for spectra with anti-coincidence are higher than the spectra without anticoincidence indicating the importance of intermediate energy neutrons in mixed fields. The quality factor and the corresponding dose equivalent corrected for saturation of lineal energy corresponding to 2 nm of ionization spacing is consistently higher than those derived from the absorbed dose based formulae, the biophysical implications of which are discussed. Suggestion for future developments for microdosimetry of intermediate energy neutrons in mixed fields are made and discussed.

The question of whether the small bandgaps of semiconductors play a significant role in their secondary electron emission properties is investigated by studying evaporated graphitic amorphous carbon, which has a roughly 0.5 eV bandgap, in comparison with microcrystalline graphite, which has zero bandgap. The graphitic amorphous carbon is found to have a 30% increase in its maximum secondary electron yield over that of two microcrystalline graphite samples with comparable secondary electron yields: highly oriented pyrolytic graphite and colloidal graphite. The potentially confounding influence of the vacuum level has been isolated through the measurement of the photoelectron onset energy of the materials. Other less significant materials parameters are also isolated and discussed. Based on these measurements, it is concluded the magnitude of bandgap may have an appreciable effect on the magnitude of the secondary electron yield and further studies of this effect with annealed graphitic amorphous carbon are warranted. In support of this work, a hemispherical two-grid, retarding field electron energy analyzer has been designed, constructed, and characterized for the present work. The advantages and disadvantages of the analyzer are discussed in comparison to other methods of measuring secondary electron emission. The analyzer has a resolution of ±(1.5 eV + 4% of the incident electron energy). A novel effort to derive theoretical, absolute correction factors that compensate for electron losses within the analyzer, mainly due to the grid transmission, is presented. The corrected secondary electron yield of polycrystalline gold is found to be 30% above comparable experimental studies. The corrected backscattered electron yield of polycrystalline gold is found to be 14% above comparable experimental studies. Corrected secondary yields for the microcrystalline graphite samples are found to range from 35-70% above those found in five experimental studies in the literature. The theoretical correction factors are estimated to have a 4-6% uncertainty. Reasons for the large discrepancy in yield measurements with the analyzer are discussed and thought to be due mainly to the lack of similar corrective factors in the previous studies. The supporting instrumentation is fully characterized, including a detailed error analysis.

published_or_final_version / Physics / Master / Master of Philosophy

Piezoelectric bimorph thin films may hold solutions for many future applications, such as lightweight deployable mirrors and inflatable struts. Non-contact actuation by an electron gun has shown promise in preventing issues that arise from attaching many wire leads to a thin film surface. This study investigates piezoelectric bimorph thin film response to electron gun actuation when covered with multiple spatial regions of control. Desired parameter ranges are found that will lead to predictable control under certain circumstances. Under such circumstances, film response is influenced almost solely by the primary electrons incident on the film, and secondary electrons have negligible effect. Such information is vital before attempting closed loop control of a thin-film piezoelectric mirror with multiple electrodes.

No description available.

Nanosatellites, such as Cubesats, are a rapidly growing sector of the space industry. Their popularity stems from their low development cost, short development cycle, and the widespread availability of COTS subsystems. Budget-conscious spacecraft designers are working to expand the range of missions that can be accomplished with nanosatellites, and a key area of development fueling this expansion is the creation of micropropulsion systems. One such system, originally developed at the Australian National University (ANU), is an electrothermal plasma thruster known as Pocket Rocket (PR). This device heats neutral propellant gas by exposing it to a Capacitively Coupled Plasma (CCP), then expels the heated gas to produce thrust. Significant work has gone towards understanding how PR creates and sustains a plasma and how this plasma heats the neutral gas. However, no research has been published on varying in the device's geometry. This thesis aims to observe how the size of the RF electrode affects PR operation, and to determine if it can be adjusted to improve performance. To this end, a thruster has been built which allows the geometry of the RF electrode to be easily varied. Measurements of the plasma density at the exit of this thruster with different sizes of electrode were then used to validate a Computational Fluid Dynamics (CFD) model capable of approximately reproducing experimental measurements from both this study and from the ANU team. From this CFD, the number of argon ions in the thruster was found for each geometry, since collisions between argon ions and neutrals are primarily responsible for the heating observed in the thruster. A geometry using a 10.5 mm electrode was observed to produce a 23% increase in the quantity of ions produced compared to the baseline 5 mm electrode size, and a 3.5 mm electrode appears to produce 88% more ions.