Ti: sapphire femtosecond laser ablation of dielectrics (fused silica and BK7 glass) and metals (Cu, Fe, Al) is presented. Results of laser -induced breakdown experiments in fused silica and BK7 glass employing 130 fs -1.7 ps, 790 nm laser pulses are reported. The fluence ablation threshold does not follow the scaling of 4>th ~ ^/2 when pulses are shorter than 1 ps. Single-shot and multi-shot (130 fs pulse) ablation of selected materials are investigated with laser wavelengths of 395 nm, 790 nm, and 1300 nm. The ablation threshold is almost independent of the laser wavelength. The surface morphologies in metals after ultrashort pulse ablation are very different from dielectrics and semiconductors. The roughness of the ablated surface depends on the thermal properties of the metal target. The preliminary TEM result from Cu single crystal that was irradiated by single laser pulses shows few defects in the center region of the ablated crater. Single-shot ablation of single-crystal Fe induces much different surface features than on selected samples of poly-crystal Fe metal. / Thesis / Master of Engineering (ME)

<p>This report concerns the applicability of the Multi - Level iterative scheme to the neutron diffusion problem. In the process of this study the iterative scheme is analyzed in modal form. The results of this analysis are used to derive the Sinqle Mode Extrapolation scheme and give mathematical support to the Multi-Level theory. A program was written to test this theory and the results of test cases are discussed.</p> / Master of Engineering (ME)

<p>Two-phase gas-liquid flow in pipes is of great practical importance in the nuclear and petroleum industries, etc. Currently, in thermalhydraulic analysis which requires modelling of a two-phase flow medium, such as boiling liquids, condensing vapors, etc., a variety of computer codes are employed to solve the time dependent mass and momentum equations in different parts of the flow system. To improve the numerical simulations, more detailed experimental knowledge of two-phase flow is required. Most instrumentation available today provides some of the required information but have various disadvantages ranging from poor sensitivity to geometrical limitations.</p> <p>This work focuses on the design of a High Speed Ultrasonic Pulse-Echo system for two-phase flow parameter measurement. The ultrasonic system utilizes four ultrasonic (10 MHz frequency) transducers positioned in the axial direction on the surface of a 2.1 cm diameter cylindrical pipe, operating in pulse-echo mode. Two of the transducers are placed on the top of the pipe and the other two on the bottom of the pipe to measure the liquid level or film thickness. The liquid and gas superficial velocities studied in this work ranged from U_ls=0.019 m/s to U_Is=0.14 m/s and from U_gs=0.24 m/s to U_gs=4.33 m/s respectively, at standard conditions (1 bar, 20°C). This system was designed to acquire the data in real time 170 frames/s (A-scans/s) and the sampling rate was 2μs displaying the data at every 6msec with a spatial resolution of 0.0071 mm.</p> <p>The theoretical basis for obtaining liquid level measurements, which can be used to determine the two-phase flow parameters, such as two-phase flow pattern characterization, void fraction measurements, plug, bubble and slug velocities as well as wave amplitude and frequency for a stratified wavy and annular flow regime are discussed. The Ultrasonic System hardware and analysis software is also presented.</p> <p>The High Speed Ultrasonic Pulse-Echo System can characterize accurately each different flow regime and flow regime map can be established where the flow regime map obtained in the present work is in good agreement with the previous theoretical and experimental works. The High Speed Ultrasonic Pulse-Echo System also proved to give accurate results for the determination of the liquid level within ± 1.5% and the time averaged liquid level measurements performed in the present work agree within ±10% with the theoretical models. Time averaged void fraction measurements for a stratified smooth flow, stratified wavy, plug flow and annular flow qualitatively agree with the theoretical predictions. Liquid plug and bubble velocities measurements for gas superficial velocities lower than 0.48 m/s qualitatively and quantitatively agree with the correlations, while for higher gas superficial velocities it was proved that there is only qualitative agreement between the results due to the inability of the correlations to predict accurately. For slug velocity there is only qualitative agreement with the correlations, where the discrepancy is likely due to limitations in the correlations. Finally, wave height and frequency for a stratified wavy and annular flow were determined.</p> / Master of Applied Science (MASc)

<p>The precise knowledge of the Fast Fission Ratio is of considerable importance in Reactor Physics due to its effect on the overall reactivity of a nuclear reactor. Calculations obtained with the codes WIMS and LATREP disagreed by as much as 3% with experiments performed in the Zed-II critical facility of the Chalk River Nuclear Laboratories. It was felt that this variation might be due to the coarsity of the energy mesh since the energy range where Uranium (238) fission occurs (.8 to 10 MeV) was covered by only a few energy groups.</p> <p>Two multigroup cross-section libraries having respectively 100 and 46 groups were therefore generated with SUPERTOG. Values for the Fast Fission Ratio were then calculated using first the one dimensional transport code ANISN and the Monte Carlo code MORSE. The 28 element fuel bundle geometry was used and the thermal fission was inputted to the codes as a fix source leaving to the codes the calculation of the activities in the upper groups of the multigroup structure (above 500 KeV). The cross-section data was obtained from the ENDF/B-IV library produced by the Brookhaven National Laboratory, U.S.A.</p> <p>It was found that the WIMS energy structure with six groups above 500 KeV offered a sufficiently small energy mesh. In ANISN both the order of the angular quadrature (SN) and of the Legendre approximation to the scattering cross-sections (PN) were investigated. It was found that as SN increases the Fast Fission Ratio distribution across the fuel bundle flattens, approaching the distribution measured experimentally in Zed-II, while as PN increases the overall value of the Fast Fission Ratio increases leaving the distribution relatively unaffected.</p> <p>Cases where the coolant has been received and replaced by air have also been investigated. This simulates what would be happening to the Fast Fission Ratio in the event of a total loss of coolant accident (LOCA). It was found that the Fast Fission Ratio would increase by about 15%. This represents a substantial positive contribution to the reactivity of the reactor.</p> <p>Geometry effects were also investigated using the code MORSE. In this code the full two-dimensional pin distribution of the fuel bundle could be represented as opposed to the one dimensional smeared annuli model which had to be used in ANISN. However, it was found that this did not improve the results since a 3% decrease was observed in the absolute value of the Fast Fission Ratio while its distribution became slightly steeper than what was measured experimentally.</p> <p>Two lattice pitches were also investigated, namely 24 and 28 em. It was found that the tighter pitch led to an increase in the Fast Fission Ratio of the order of 5% without significant effect on the distribution.</p> <p>The results obtained for the estimation of the Fast Fission Ratio with these Reactor Physics codes do not agree to better than 5% with the values determined experimentally. However, if one considers the experimental errors and the fact that the cross-sections are not known to better accuracies than a few percent, especially for Uranium (238) inelastic scattering, the results obtained are quite justifiable.</p> / Master of Engineering (ME)

The increasing trend towards the use of activation analysis as an analytic means of identifying elements and determining their relative abundance demands more accurate values of reaction cross-sections. Accurate values of cross-sections are also important in the investigation of possible shell effects in reaction mechanisms. Many activation cross-sections for reactions produced by 14 MeV neutrons have been measured because of the relative ease with which this energy of neutrons can be produced. Low voltage accelerators are capable of producing 14. MeV neutrons by the 3H( d,n)4He reaction. Although many people(1-5) have measured the (n,2n) reaction cross-sections for a multitude of stable isotopes, their values differ considerably for a particular range or they report large uncertainties in their measured values. By using improved counting and flux monitoring techniques, an attempt will be made to more accurately determine the values of the (n,2n ) reaction cross-sections for 71Ga, 106Cd, and 138Ba. The values for the (n,2n) reaction cross-sections for these three isotopes are determined by the simultaneous activation of the isotope in question and two copper disks. Each sample is sandwiched between the two foils during activation. Since the 63Cu(n,2n) 62Cu reaction cross-section for 14 MeV neutrons has been accurately measured, the activity produced in the copper disks is used in the determination of the (n,2n) reaction cross-sections. The purpose of this thesis is to more accurately determine *he values of the (n,2n) reaction cross-sections for 71Ga, 106Cd, and 138Ba by improved techniques.

This thesis describes the results and analysis of Seebeck coefficient and electrical conductivity measurements on "activated", molybdenum-oxide, thin film hydrogensensing elements. The work was undertaken in an effort to produce data that would be useful in obtaining an understanding of the electronic processes which cause large electrical conductivity changes in molybdenum-oxide upon exposure to small concentrations of hydrogen in air. The results are analyzed using a simple band model of a semiconductor. This model is shown to give reasonable results at high temperatures; however, a departure from the usual .! model is required in order to explain the results at lower temperatures. Evidence is presented concerning the basic nature of the charge carriers, and a possible defect model is suggested. Similar measurements were also performed on single crystals, and the results are shown to be in reasonable agreement with those obtained on thin film samples.

The objective of this study was to build preliminary devices . which could be used to measure electron reflection coefficient values using crossed electric and magnetic fields. The technique for using crossed fields to measure reflection coefficients was developed at Air Force Cambridge Research Laboratories. The reflection coefficient is the ratio of the number of electrons in a reflected beam to the number of electrons in an incident beam striking a surface. The work described herein was a part of a project authored by AFCRL under contract AF19 (628)-5940; Project 8605 (with Western Kentucky University being the contractor) which was entitled. "An Investigation of the Reflection of Slow Electrons from Surfaces."1 Earlier researchers working on the problem of measuring reflection coefficients generally had used large electron optical systems with target collection chambers.2 Since their findings varied so considerably (Figure 1) it was decided to try a new approach. The results shown are curves describing the values obtained when electrons bombarded tungsten metal with energies ranging between .1 and 3.5 electron volts. Basically the devices discussed within this paper consist of two parallel electrodes to provide a uniform electric field and an electron gun to inject electrons with known energy levels into the field perpendicular to the electrodes. Each device utilized a Faraday cup to collect the electrons after the application of an external uniform magnetic field which bent the originally straight electron beam into a trochoidal path.

The purpose of this thesis was to construct a time-of-flight spectrometer using plastic scintillators coupled to photomultiplier tubes as the primary detectors. A time-of-flight spectrometer with at least 2.0 nanoseconds of time resolution was desired.

Zirconium dioxide, or zirconia, is seemingly an ideal refractory oxide, having a high melting point (2680°C), low thermal expansion and also a considerable resistance to most forms of chemical attack. Early researchers soon found that zirconia, in the pure form, was actually of very limited use as a refractory because of sudden and disruptive volume changes accompanying a change in crystal structure which occurs at a temperature of about 1100 o C. It was discovered that this crystal structure change could be suppressed by the addition of one of a number of alkaline earth or rare earth oxides. These additive oxides have been found to form solid solutions having a stable crystal structure for this purpose over a wide temperature range. Zirconia containing additives is termed stabilized zirconia. Stabilized zirconia may possess rather unusual electrical conduction characteristics, depending upon the particular additive oxide. In many cases, including that of yttria stabilized zirconia, a large almost purely ionic conductivity exists over a wide temperature range. An early application for this material was in the Nernst glower, which was popular as an incandescent light source around 1900, and which survives today as a source in some infrared spectrephotometers. The Nernst glower was simply a filament of yttria stabilized zirconia which could be heated to incandescence in air by passing a current, either ac or dc, through the filament. The glower was at once recognized to be an unusual ionic conductor in that there was apparently little or no electrolysis, even after extended periods of operation on dc. The chemical stability of the conducting glower material was first satisfactorily explained by Wagner (1) who found the charge carriers to be primarily oxygen ions. For operation in air the supply of oxygen at the cathode was found to be continuously replenished from the atmosphere .leaving the composition of the ceramic essentially unchanged. Several investigators have found that when a conducting sample of yttria stabilized zirconia is deprived of atmospheric oxygen, either by flushing with an inert gas or by evacuating the test chamber, immediate changes in the material are apparent (2, 3, 4). The color of the ceramic changes from white to black, with the color change proceeding from the negative electrode to the positive electrode. The color change is accompanied by the appearance of a multiplicity of cracks in the sample which suggests a volume change, perhaps as the result of a change in crystal structure. The character of the conductivity changes rapidly from ionic to metallic as evidenced by the disappearance of the large negative temperature coefficient of resistivity associated with ionic conductivity. The original color, although not the mechanical integrity, of a "converted" sample can be restored by firing in an oxidizing atmosphere. This behavior, in conjunction with the knowledge of the conduction mechanism, is a good indication that electrolytic conversion in inert atmospheres is basically a reduction process. It is .the purpose of this project to investigate the mechanism of electrolytic reduction in a vacuum, and to determine, if possible, the composition and structure of yttria stabilized zirconia which has been so reduced.

The reported usage of the palladium-silver alloy films as resistors in the electronics industry has been in encapsulated form. By mounting the resistor inside vacuum electron devices, there is the possibility of eliminating the encapsulation. A literature search has revealed no reported efforts of experiments with unencapsulated resistors and certainly no reports of the palladium-silver alloy resistor in vacuum tubes or other high vacuum environments. Therefore, the research reported in this thesis will concentrate on the environmental effects on resistivity of palladium-silver alloy films in high vacuum.