No description available.

No description available.

No description available.

This dissertation focuses on the characterization of hydrogen implanted 3C-SiC and helium implanted 6H-SiC and the interaction of silver and palladium with the SiC. The main technique used for the analysis of the implanted SiC was transmission electron microscopy (TEM), although scanning electron microscopy (SEM) and nanoindentation hardness testing were also used. Both H and He implantations were done at an ion energy of 100 keV and the total dose for both species was 1017 ions/cm2. Specimens were annealed at 1000 °C for 20 min in an inert atmosphere. The observed depth of ion damage agreed closely with that predicted by TRIM 2008. The damaged region in the He implanted specimens had a high density of small bubbles but no cracks were observed. Severe cracking was observed along the damaged region in the H implanted specimens. A second phase hexagonal (4H-SiC) was detected in two grains in the 3C-SiC and some grown in voids were also observed. The wettability of SiC by a 2:1 by volume mixture of Ag and Pd is extremely low but is significantly increased through the addition of Si to the mixture. The Si containing metal mixture was found to migrate along the grain boundaries of polycrystalline 3CSiC while sealing the point of entry. Pd attacks SiC and severe etching was observed on the surface of 6H-SiC. An epitaxially orientated Pd2Si inclusion was observed in the 6H-SiC. No effect of implantation damage on the interaction of Pd and Ag with SiC was observed. Pd etched the surface of the He implanted 6H-SiC down through the damaged region with no evidence of implantation damage left after 67 hours of annealing and thus no conclusions could be drawn. The metal mixture interacted with the non implanted surface of the H implanted 3C-SiC after dissolving the Si substrate and therefore the effect of the ion damage on the interaction could not be investigated. Nanoindentation hardness measurements showed a marked increase in hardness of He implanted 6H-SiC annealed for 20 min at 1000 °C over that of as implanted and virgin material. There was also a large decrease in hardness corresponding to the depth of the ion damage.

Part I An efficient ionizer designed for the ionization of a low intensity molecular beam has been developed. The ionizer will have an efficiency for an argon beam at room temperature and 0.8 amperes plate electron current of greater than 5.3% and for an helium 4 beam at room temperature and 0.8 amperes plate electron current of greater than 0.12%. The electron bombardment ionization occurs between two flat plates both of which are at 200 volts potential with respect to the cathode. The two plates are at a 2.5° angle to each other so that the ions once formed experience a field gradient produced by this angle which accelerates them out the open side of the ionizer 90° to the direction of the incident neutral beam. The electrons were emitted from 6, 0.030 x 0.004 inches, thoriated tungsten ribbon filaments heated by d.c. power. Successful activation of the filaments, however, was not achieved and so they were used as pure tungsten filaments. The ionizer ran at between 1900° Kelvin and 2600°Kelvin for about 8 hours without significant distortion, filament sag, or appreciable outgassing. Part II The design of a source of doubly charged helium ions was carried out to provide an helium beam of twice the terminal energy of the Van de Graafe Accelerator. The unit, to be installed in the top terminal of the Van de Graaffe, employs a radio frequency ion source followed by a double focusing magnet. The magnet has plane parallel pole pieces with a fringing field that produces focusing in a vertical plane. The unit is designed so that at 2.5 kilovolts extraction voltage a magnetic field of 3060 gauss is required to bend the He⁺⁺ through an angle of 90° and bring them to a focus at 5.8 cm. from the exit face of the magnet. The beam acceptance angle of the magnet is 10° at a source distance from the entrance face of 5 cm. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

An experimental investigation of the velocity of sound in liquid helium at temperatures near 1° K was abandoned (at an early stage)^when the results of Whitney and Chase(16) became available. The original investigation, which had been motivated by the idea that the peak in the attenuation-temperature curve near 1° K is related to a corresponding feature in the velocity-temperature curve, evolved into a search for mathematical relations connecting the temperature variations of velocity and attenuation. Classical dispersion theory is reviewed, with emphasis on its physical and mathematical foundations. Applications of the theory in the fields of electrical network design, dielectric dispersion and ultrasonics are described in order to illustrate the use and the limitations of the Kramers-Kronig relations. The possibility of extending the scope of the Kramers-Kronig relations, by using variables other than frequency, is discussed. A brief sketch of the quantum mechanical basis of linear response theory is given in support of a proposal to use reciprocal temperature as a variable in dispersion-type relations. An attempt was made to apply dispersion relations in reciprocal temperature to the problem of ultrasonic propagation in liquid helium and a comparison of the predicted and the experimentally observed attenuations at the λ point was found to give encouraging results. The interferometer, to have been used in the proposed experiment is described. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

An atomic beam type of apparatus designed to produce a polarized He³ beam with an intensity of approximately one microampere and 90% polarization is described. He³ is a monatomic gas consisting of atoms with zero electronic magnetic moment and nuclear spin of ½. As no initial molecular dissociation is required, an intense, supersonic neutral beam can be produced with a miniature Laval nozzle cooled to liquid He⁴ temperatures. The velocity distribution of the particles in the Laval beam and the trajectories of these particles in a radially symmetric hexapole magnet have been computed. Sufficient separation of the two beams, consisting of particles in the two possible nuclear spin states, is achieved with a magnet in which the diameter of the gap between the pole pieces increases from 3mms at the entrance to 6mm in 15 cms and is then constant for 35cms. After ionization the nuclear polarization of the singly ionized particles depends upon the magnetic field strength at the position of the ion. Theoretical calculations show that field strengths of 6000 gauss at both the ionizer and target (in reaction studies) are sufficient to give 90% nuclear polarization. For an input gas temperature and pressure of 2.2°K and 15 mm.Hg. the Laval nozzle (throat diameter, 0.2 mm) has been designed to produce a supersonic beam of Mach number 4 with an intensity of 6.5x10¹⁵ particles/sec at the magnet entrance. Assuming 40% transmission through the magnet (one half of the beam being removed by the polarization process) and an ionization efficiency of 0.25%, the resulting ion beam intensity is 6.5x10¹² ions per second or approximately one microampere. The low temperature atomic beam source has been tested at liquid nitrogen temperature with a He⁴ beam. The measured beam intensity of 9x10¹³ particles/cm²/sec at the magnet exit, 76 cms from the nozzle, agrees favourably with the calculated intensity of 1.2x10¹⁴ atoms/ cm² sec under these operating conditions. The measured field gradient of 70,000 gauss/cm. near the pole tips of the hexapole splitting magnet is more than required for separating the atoms in the two nuclear spin states. A pulsed nuclear magnetic resonance method for measuring the nuclear polarization of the neutral He^ beam prior to ionization is described. The angular distribution and polarization of the protons produced by the D(He³,p)He⁴ reaction with an incident, polarized He³ beam of 150 Kev bombarding energy has been calculated, This angular distribution is isotropic. In the plane of the reaction, the proton polarization, which may be measured by a second scattering experiment, is -2/3 the incident He³ polarization. The angular distribution and polarization of the protons from this reaction has been calculated for the case of polarized and unpolarized He³ beams incident on a polarized deuteron target. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The cross section for the photodisintegration of a three body nucleus, helium-3, has been accurately measured for the first time. The He³( ɣ,p)D reaction cross section has been measured at gamma ray energies of 6.14, 6.97 and 7.08 Mev and found to be 0.104 ± .007, 0.298 ± .050 and 0.308±.024 milli-barns respectively. The photodisintegration cross section agrees with that calculated from the D(p,ɣ)He³ reaction using the principle of detailed balance to within the experimental errors. This is an accurate verification of this principle as applied to a direct nuclear-photon interaction. The photodisintegration reaction was observed in a cylindrical gridded ionization chamber using a helium-3, methane, and argon gas mixture. The F¹⁹(p, αɣ)016 reactions at proton bombarding energies of 873.5 kev and 935 kev were used as the source of gamma rays of well defined energy. The energy resolution of the chamber was adequately good to give a reasonable separation of the 642 kev photodisintegration reaction product peak from the electron background and the 765 kev peak from the He³(n,p)T reaction. It was necessary to reduce the tritium contamination in the helium-3 gas in order to reduce the background arising from the beta decay of the tritium; purification to less than 1 part tritium in 10¹⁰ parts helium was achieved. Proposed measurements of the photodisintegration cross section at higher gamma ray energies using ionization chambers and other types of detectors are discussed. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

This thesis describes an investigation of flow phenomena of liquid helium II associated with temperature and pressure gradients. The experiments were done primarily in the range of heat flow critical with respect to mutual friction and with respect to turbulence in the normal fluid. The results are analysed on a phenomenological basis due to Gorter and Mellink,(12) and are interpreted by making use of the model put forward by Hall and Vinen. (14) This model assumes that the superfluid can develop vortex lines which interact with the thermal excitations of the normal fluid. A conclusion is reached, that a considerable amount of energy is stored in non-thermal motion in this vorticity. Friction in the normal fluid is reviewed in the light of the work of Staas, et al. (8) An experiment to measure the attenuation of secound sound at very low frequencies is described. A few suggestions for further investigation are tendered. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A theoretical study of the reaction D(pγ)He³ is made in an attempt to explain the experimental data for the reaction obtained by Fowler et al. (1949), Wilkinson (1952), Griffiths and Warren (1955) and Griffiths, Larson and Robertson (1961). The angular distribution of the emitted gamma radiation, measured with respect to the incident proton beam, is predominantly proportional to sin²θ. Measurements of the polarization of the radiation by Wilkinson (1952) indicate that the sin²θ component is electric dipole radiation. In addition there is a small, possibly isotropic, component. The proportion of the total yield coming from the smaller 'isotropic' component is 0.035 at a proton energy of 1 Mev, and this proportion increases with decreasing proton energy. The sin²θ component has been interpreted by Griffiths and Warren as coming from an electric dipole transition from an initial state of a P-wave proton (L = 1, L₂ =0) and ³S deuteron to the ²S ground state of He³. This interpretation is supported by the present calculations. They also suggest that the smaller 'isotropic* component could be either a magnetic dipole transition of S-wave protons to the ²S state of He³ or an electric dipole transition involving spin-orbit coupling. In this present work the cross-sections are examined for all possible channels which might conceivably contribute to the reaction. The channels considered are 1. electric dipole transitions for a. P-wave protons to the ²S state b. P-wave protons to the ⁴D state c. F-wave protons to the ⁴D state 2. electric quadrupole transitions for a. S-wave protons to the ⁴D state b. D-wave protons to the ²S state 3. the magnetic dipole transition for S-wave protons to the ²S state. Three-body wave functions are constructed, following Verde (1950) and Derrick and Blatt (1956), making use of the symmetry properties in spin space, isotypic spin space and in ordinary space. In addition to the states of total isotropic spin T = ½ considered by Derrick and Blatt the states of total isotopic spin T = 3/2 are included. The radiation matrix elements for the above channels are calculated and are expressed in terms of integrals over the three internal coordinates. These radial integrals are estimated by using very simple radial functions which are valid outside the range of the nuclear forces and which also disregard coulomb forces. The cross-sections depend on the unknown amplitudes and relative signs of the various possible symmetry states. Therefore the size, although not the angular dependance or the general energy dependance, of the cross-sections can be used only as an order-of-magnitude estimate. By comparison of the size, angular distribution and energy dependance of the calculated cross-sections with the experimental data it is shown conclusively that the sin²θ component of the radiation comes from the electric dipole transition of P-wave protons to the ²S state of He³. The smaller 'isotropic' component of the radiation comes from either (a) an electric dipole transition of P-wave protons to the ⁴D state, giving an angular distribution proportional to 1 – (1/7)cos²θ, or (b) a magnetic dipole transition of S-wave protons to the ²S state, giving an isotropic angular distribution. The observed energy dependence of the relative yield of the small component suggests the interpretation in terms of the magnetic dipole transition. The cross-sections of the other transitions examined are too small to explain the experimental results. / Science, Faculty of / Physics and Astronomy, Department of / Graduate