Part A of two Project Reports; Part B can be found at http://hdl.handle.net/11375/17660 / <p> The recent development of a technique for the synthesis of the molecule 5-(¹⁸F) fluoro-dopa has opened a vast field of research into the study of the intracerebral metabolism. In order to take full advantage of this new tracer compound it will be necessary to use an imaging system which will be capable of providing three dimensional information concerning the rapid changes in activity as the ¹⁸F travels through the brain. This report describes the results os investigations into a possible design for a device which would be capable of this type of dynamic imaging. The design in question employs multiwire proportional chambers (MWPC) as detectors of the .511 MeV gamma rays which result from the annihilation of the positron emitted in the decay of ¹⁸F. </p> <p> For comparison purposes, a breif review of various other types of positron tomographic systems which are presently in use or under development is presented. This review emphasises the resolution, data aquisition speed, and instalation cost for each of these designs. </p> <p> In order to eliminate the need for costly research into the various design aspects of multiwire proportional chambers for use with positron annihilation radiation, the design presented here relies exclusively on methods and designs developed and proven feasible by other groups. These are incorporated into a system to suit the present needs. </p> <p> The design presented uses two pairs of 50x50 cm² MWPC's at 180° to each other and separated by approximately 50 cm. The chambers utilize electromagnetic delay-line readout techniques for the anode and wound bi-filar cathode planes. They are filled with a "magic gas" mixture at a slightly positive pressure. The effeciency of the chambers for .511 MeV photons is increased by employing "sandwich" type converters. Signals from the detectors are transfered to a small computer where they are stored for later tomographic reconstruction off-line. </p> <p> On the basis of the working designs from which the present design has been drawn, one would expect the positron camera to have a sensitivity of better than 1000 counts/second-microcurie. The total costs of development and construction leading up to a working device suitable for clinical use, not including the cost of a dedicated computer, are estimated to be less than seventy five thousand dollars. </p> / Thesis / Master of Engineering (ME)

<p> Long-period fiber grating (LPG) sensors have been widely used as refractive index sensors due to their high sensitivity to the ambient refractive index change surrounding the fiber cladding of the LPG. Application of the LPG refractive index sensor has been found in chemical sensing and biochemical sensing, however for application of label-free dip and measure biosensors based on receptor immobilized LPG bio-sensor, the conventional fiber optic refractive index sensors are limited in the refractive index sensitivity, resolution, and operational range owing to the low sensitivity of the cladding mode effective index dependence on the ambient refractive index and the broad-spectrum feature of the LPG transmission spectrum. Low-cost, disposable fiber optic biochemical sensors with improved sensitivity, stability and resolution are needed to provide a high-sensitivity platform for immunology and DNA/aptamer biosensor. </p> <p> In this work, a novel fiber optic biosensing platform based on the LPG and the LPG in-fiber Michelson interferometer is designed and fabricated. The sensitivity and operation range enhancement is optimized by modifying the fiber cladding structure through reducing the cladding layer radius and applying a high-refractive index overlay with appropriate refractive index and thickness. The resolution of the refractive index sensor is improved by adopting the LPG in-fiber Michelson interferometer which turns the wide-spectrum feature of the LPG transmission spectrum into a narrow spectrum feature on the reflection spectrum of the interferometer. The reflection spectrum nature of the LPG in-fiber Michelson interferometer turns the sensor head into a single-end optotrode. The optotrode coated with bio-recognition film thus physically constitutes a short piece of fiber with one section of cladding reduced fiber. With single strand DNA (ssDNA) immobilized on the surface of the fiber cladding through biotin-avidin bridge, detection of the antisense DNA for the immobilized ssDNA is demonstrated. Immunoassay based on capture of target antigen by covalently immobilized antibody shows that reduction of the fiber cladding not only improve the sensitivity of the long period grating in-fiber Michelson interferometric biosensor but also improves the assay time. </p> / Thesis / Doctor of Philosophy (PhD)

Part A: The preliminary investigation of the thermal and electrical behaviour of thin metal films gives evidence, Part I, that several mechanisms are responsible for the change of resistance as the temperature increases from room temperature to 500°C. Firstly, there appears grain growth giving a characteristic decrease in resistance. Secondly, the formation of agglomerates upon the continued growth of grains; especially for the thinner Al and Cr films. This effect tends to increase the resistance and a mathematical model is proposed to explain the results qualitatively. Thirdly, the occurrence of what appeared to be an electromigration effect. This latter point provided the incentive for a study on the effects of electromigration in thin aluminum film, Part II. The results of this study are comparable to those obtained by other workers, except that the interpretation for the direction of electromigration in Al is reversed. One possible explanation for the difference in the direction of migration could be due to the interpretation of marker motion. A mathematical model is also proposed for electromigration, in which both the effects due to the applied electric field and the electrons collision with the ions have been taken into consideration. It was found that the effect due to electrons collision with the ions upon the migration of ions could be expressed in terms of an exponential function of the square of the electron to ion collision relaxation time. / Thesis / Master of Engineering (ME)

One of two project reports. Part A can be found at: http://hdl.handle.net/11375/18522 / Recent channeling-backscattering measurements of the disorder induced by heavy ion irradiation of semiconductors has indicated radiation damage far in excess of that predicted by linear transport theory. The present work extends the investigation to TEM and compares the two techniques in an annealing study of ion irradiated silicon (~ 80-200 a.m.u. ions of 15-100 keV) for low fluence (typically 3×10¹¹/cm² for TEM and 10¹²-10¹³/cm² for channeling) bombardment. In addition to showing a good correlation between the techniques, the results indicate that neither does there exist a unique relationship between lattice disordering and cascade energy density, nor that a well defined amorphous structure seems to exist. / Thesis / Master of Engineering (ME)

<p>Ytterbium-doped optical fibers have extensive applications in high-power fiber lasers, optical amplifiers, and amplified spontaneous emission light sources. In this thesis two sub-projects associated with ytterbium doped fibers are discussed.</p> <p>Numerical simulations have been used to model high-repetition rate ultrafast ytterbium-doped fiber amplifier systems assuming continuous-wave input signals under variable situations, such as one-sided and two-sided pumping. Different system configurations are also developed, such as a single-stage amplification system, a two-stage amplification system and a separated amplification system, providing alternative choices for experiments and applications. The simulation results are compared with experimental data and the simulation results from some other software. The influence of nonlinear effects in the fiber is also very briefly discussed in this thesis.</p> <p>In a second research activity, the distribution of ytterbium atoms is being investigated in a range of double-clad ytterbium-doped fibers. Using aberration-corrected electron microscopy, ytterbium atoms are directly observed from the wedge-shaped specimen, which was prepared from ytterbium-doped optical fibers by tripod polishing combined with ion milling. Challenges related to sample preparation and the interpretations of images are discussed, but the approach shows great potential to investigate the doping behaviors down to atomic scale in the fibers. The work is expected to help reveal mechanisms affecting the performance for the doped fibers, such as photodarkening which is potentially associated with clustering effects.</p> / Master of Applied Science (MASc)

The large thermal cross section of cadmium makes it ideal for many practical applications where screening of thermal neutrons is desired. For example, in non-destructive assay techniques, or for astrophysical studies of the s-process. All such applications require precise knowledge of the neutron-capture cross section on cadmium. Although there are some data on neutron-capture cross sections particularly at thermal energies and at energies relevant for astrophysics, there is very little data at most other energies. Further, the evaluated cross sections from the ENDF and JENDL databases disagree at high energies. Therefore, there is a critical need for precise knowledge of the 114Cd(n, gamma)115Cd cross section over a large range of incident neutron energies. We performed a direct measurement of the neutron-capture cross section at the Los Alamos Neutron Science Center (LANSCE) using the Detector for Advanced Neutron Capture Experiments (DANCE). A highly enriched (∼$99%), 100 mg pressed metallic pellet sample of 114Cd was used to perform the neutron-capture measurements in the range of 1 eV to 300 keV using the white neutron source available at LANSCE. Additional neutron capture data were also taken on highly enriched samples of 112Cd and 113Cd to enable careful background subtraction of even the small contaminants found in the 114Cd sample. We used a large energy sum windows around the Q-value to circumvent any complication that may arise from populating the 180 keV isomeric (T1/2 = 44.56d) state in 115Cd.

<p>Dynamic properties of type-II superconductors have been experimentally studied in zero and low magnetic fields using SQUID magnetometry and <i>I–V</i> measurements.</p><p>In zero magnetic field close to the critical temperature, the physical properties of type-II superconductors are dominated by spontaneously created vortices. In three dimensions (3D) such vortices take the form of vortex loops and in two dimensions (2D) as vortex-antivortex pairs.</p><p>The 2D vortex dynamics has been probed using mutual inductance and flux noise measurements on YBa₂Cu₃O₇ (YBCO) and MgB₂ thin films in zero and low magnetic fields. In such measurements, information about vortex correlations is obtained through a temperature dependent characteristic frequency, below (above) which the vortex movements are uncorrelated (correlated). The results obtained in zero magnetic field indicate that sample heterogeneities influence the vortex physics and hinder the divergence of the vortex-antivortex correlation length.</p><p>In low magnetic fields the vortex dynamics is strongly dependent on the applied magnetic field and a power law dependence of the characteristic frequency with respect to the magnetic field is observed. The results indicate that there is a co-existence of thermally and field generated vortices.</p><p>The <i>I–V</i> characteristics of untwinned YBCO single crystals show that only a small broadening of the transition region influences the length scale over which the vortex movements are correlated. The dynamic and static critical exponents therefore exhibit values being larger in magnitude as compared to values predicted by relevant theoretical models. The results also suggest that the copper oxide planes in YBCO decouple slightly below the mean field critical temperature and hence, the system has a crossover from 3D to 2D behaviour as the temperature is increased. </p><p>From temperature dependent DC-magnetisation measurements performed on untwinned YBCO single crystals in weak applied fields, detailed information about the critical current density and the irreversibility line is obtained.</p>

<p>Bonding techniques intended for assembling space microsystems are studied in this work. One of the largest problems in bonding pre-processed semiconductor wafers are the severe process restrictions imposed by material compatibility issues. Plasma processes have shown to be good for sensitive materials integration why the influence of different plasma parameters on the bondability of wafers is particularly studied. Conventional wet chemical and field-assisted methods are also examined. The resulting bond quality is assessed in terms of mechanical strength, homogeneity, and yield.</p><p>The effect of spaceflight environment on the reliability of wafer bonds is also investigated. Both high and low temperature annealed bonds are found to be very robust. Effects observed are that low temperature bonds are reinforced by thermal cycling in vacuum and that high temperature bonds degrade slightly by low dose γ irradiation.</p><p>Adhesion quantification is important for all bonding. Development of accurate quantification methods is considered necessary since most methods at hand are limited. This work includes the development of the blister test method. Former test structures are improved to be more practical to work with and to yield low experimental scatter. A physical stress model for the improved structure is suggested with which successful predictions of fracture for different test specimen configurations are made. The blister test method is used throughout this work to assess the strength of wafer bonds. The physics background and modelling of other common test methods are also thoroughly analysed. The methods’ practical capabilities and limitations are commented; origin and mitigation of measurement errors are discussed. It is shown that all methods can be significantly improved by small means.</p><p>Weibull statistics is introduced as a tool to characterise wafer bonds. This method is suitable to use in brittle materials design as the inherent variability in strength can be properly accounted for.</p>

Dynamic properties of type-II superconductors have been experimentally studied in zero and low magnetic fields using SQUID magnetometry and I–V measurements. In zero magnetic field close to the critical temperature, the physical properties of type-II superconductors are dominated by spontaneously created vortices. In three dimensions (3D) such vortices take the form of vortex loops and in two dimensions (2D) as vortex-antivortex pairs. The 2D vortex dynamics has been probed using mutual inductance and flux noise measurements on YBa2Cu3O7 (YBCO) and MgB2 thin films in zero and low magnetic fields. In such measurements, information about vortex correlations is obtained through a temperature dependent characteristic frequency, below (above) which the vortex movements are uncorrelated (correlated). The results obtained in zero magnetic field indicate that sample heterogeneities influence the vortex physics and hinder the divergence of the vortex-antivortex correlation length. In low magnetic fields the vortex dynamics is strongly dependent on the applied magnetic field and a power law dependence of the characteristic frequency with respect to the magnetic field is observed. The results indicate that there is a co-existence of thermally and field generated vortices. The I–V characteristics of untwinned YBCO single crystals show that only a small broadening of the transition region influences the length scale over which the vortex movements are correlated. The dynamic and static critical exponents therefore exhibit values being larger in magnitude as compared to values predicted by relevant theoretical models. The results also suggest that the copper oxide planes in YBCO decouple slightly below the mean field critical temperature and hence, the system has a crossover from 3D to 2D behaviour as the temperature is increased. From temperature dependent DC-magnetisation measurements performed on untwinned YBCO single crystals in weak applied fields, detailed information about the critical current density and the irreversibility line is obtained.

Bonding techniques intended for assembling space microsystems are studied in this work. One of the largest problems in bonding pre-processed semiconductor wafers are the severe process restrictions imposed by material compatibility issues. Plasma processes have shown to be good for sensitive materials integration why the influence of different plasma parameters on the bondability of wafers is particularly studied. Conventional wet chemical and field-assisted methods are also examined. The resulting bond quality is assessed in terms of mechanical strength, homogeneity, and yield. The effect of spaceflight environment on the reliability of wafer bonds is also investigated. Both high and low temperature annealed bonds are found to be very robust. Effects observed are that low temperature bonds are reinforced by thermal cycling in vacuum and that high temperature bonds degrade slightly by low dose γ irradiation. Adhesion quantification is important for all bonding. Development of accurate quantification methods is considered necessary since most methods at hand are limited. This work includes the development of the blister test method. Former test structures are improved to be more practical to work with and to yield low experimental scatter. A physical stress model for the improved structure is suggested with which successful predictions of fracture for different test specimen configurations are made. The blister test method is used throughout this work to assess the strength of wafer bonds. The physics background and modelling of other common test methods are also thoroughly analysed. The methods’ practical capabilities and limitations are commented; origin and mitigation of measurement errors are discussed. It is shown that all methods can be significantly improved by small means. Weibull statistics is introduced as a tool to characterise wafer bonds. This method is suitable to use in brittle materials design as the inherent variability in strength can be properly accounted for.