M.Phil. (Chemistry) / This project concerns the study of , specifically, the incomplete fusion mechanism. The nuclear reaction 7Li + 176Yb at 50 MeV was therefore carried out using the AFRODITE and DIAMANT facility of iThemba LABS. A 7Li nuclide is considered suitable for the breakup fusion (incomplete fusion) reaction because of its well developed cluster structure of an -particle and triton which are weakly bound in this nucleus. One of the breakup fragments may be captured by the target while the other escapes at the beam velocity. Light charged-particles (alpha, tritons, deuterons and protons) were detected with the DIAMANT (CsI) array in co-incidence with gammarays detected by the AFRODITE (HPGe) spectrometer. The light particle detection in co-incidence with gamma detection was an important innovation that allowed exclusivity in the reconstruction of the mechanism by which specific residues were produced. Off-line data processing was used to produce charged-particle-gated gamma-gamma coincidence matrices which were analysed with the RADWARE software package. The level scheme exclusive to a particular channel for the production of the 178Hf was extracted. The relative cross-section for the various reaction channels could also therefore be extracted. In particular, the intensity ratios of gamma transitions as function of spin for proton to triton-gated matrices populating the 178Hf isotope were extracted. Insights could be developed into the incomplete fusion reaction mechanisms initiated by the breakup of the incident 7Li projectile.

No description available.

The model investigated, exhibits and defines in mathematical expressions all the laws of common mechanical behavior of an inelastic solid. As a result, it excels the more common models discussed in the introduction not only in completeness of behavior, but also in possibilities of application In a practical sense, it is of interest to note the similarity between the stress-strain curves of the model and those of some aluminum alloys, Duralumin, magnesium and copperâ ¢ The creep and strain recovery curve of Fig. (20) is in close agreement with that of real metals discussed by Nadai (l5). This seems to indicate that the model will be useful in giving mathematical equations for the behavior of these metals under different test conditions. / Master of Science

In this thesis, a joint experimental and theoretical study of the dynamics of atom- molecule collisions will be presented. The focus of this study will be conducted towards the precise, quantitative theoretical description of the collision dynamics in terms of the vectors <strong>k</strong>, <strong>k'</strong>, <strong>j</strong>, and <strong>j'</strong> (the incoming and outgoing relative momenta associated with the collision, and the initial and final rotational angular momentum of the target diatom respectively) that define the collision, and on the experimental measurement of these vector correlations. Chapter 1 is introductory, providing an overview of the field of reaction dynamics, and the experimental and theoretical methods that exist to treat the collisions of atoms and molecules. This work focusses on the collisions of the spherically symmetric rare gas atoms Ar and He with the open-shell heteronuclear diatomic radicals NO and OH. In particular, the fully quantum state-to-state resolved differential cross-sections for the collisions of NO(X) with Ar (reflecting the <strong>k</strong> - <strong>k'</strong> vector correlation), and the collisional cross-sections for the depolarisation of the rotational angular momenta of the NO(A) and OH(A) radicals (reflecting the <strong>j</strong> - <strong>j'</strong> vector correlation) have been determined experimentally and theoretically, and the results have been discussed and interpreted in terms of the mechanistic aspects of the collision dynamics, and the features of the potential energy surface that give rise to these. In Chapter 2, the atom-molecule systems that constitute the subject of this work will be introduced in detail. The close-coupled quantum mechanical and quasi-classical trajectory scattering calculations performed as part of this work will be discussed in greater detail, providing a greater insight into molecular scattering theory. The explicit calculation of the quantities of interest (most significantly the differential cross-section, and the tensor/depolarisation cross-sections) will be presented for the quasi-classical and quantum cases, offering the most transparent definitions of these quantities. Finally the mathematical description of the spatial probability distribution of a single vector, a pair of correlated vectors, and three correlated vectors is described in detail, including a discussion of the quantum mechanical nature of the vectors in question. Chapter 3 describes the experimental measurement of the differential cross-sections for the collisions of NO(X) with Ar. A hexapole was used to select uniquely those NO molecules in the |&Omega; = 0.5; j = 0.5, f> quantum state, allowing full experimental quantum state-to-state selection for the first time. A crossed molecular beam apparatus with (1+1') resonantly enhanced multi-photon ionisation detection coupled with velocity mapped ion- imaging was employed to measure the differential cross-section, and the details of the experimental set-up are provided. The accurate extraction of the true, centre of mass frame differential cross-section from the laboratory frame information yielded by the experiment is something of an involved process, and much of this Chapter will be concerned with the development of a Monte Carlo method to achieve this end. In Chapter 4, the experimental and theoretical fully quantum state-to-state resolved differential cross-sections for the collisions of NO(X) with Ar are presented, having been measured for the first time. Full resolution of the initial parity of the rotational wave- function of the NO molecule has enabled the observation of parity dependent structures within the differential cross-section, and the origin of these structures has been investi- gated, employing quasi-classical, quantum mechanical and semi-classical methods in order to elucidate the mechanism by which they arise. Chapter 5 introduces the measurement of the collisional depolarisation of the rotational angular momentum of the diatom. Rate constants for the collisional depolarisation of <strong>j</strong> were measured by monitoring the time dependence of the amplitude of Zeeman and hyperfine quantum beats in the (1+1) laser induced fluorescence decays of an ensemble of NO(A) or OH(A) radicals in the presence of a series of background pressures of a collision partner. The creation and subsequent evolution of the polarisation of <strong>j</strong> induced by the absorption of polarised laser light is described, and the magnitude of this polarisation is linked to the amplitude of the quantum beat in the laser induced fluorescence decay. The extraction of the depolarisation cross-sections from the raw experimental data is discussed, and a Monte Carlo simulation of the experiment is described to account for any additional unwanted experimental factors that may contribute to the loss of polarisation of <strong>j</strong>. A formalism is also introduced that makes use of the tensor opacities to recover spin- rotation conserving and spin-rotation changing open-shell rotational energy transfer and depolarisation cross-sections from the intrinsically closed shell quasi-classical trajectory scattering calculations. In Chapter 6, the experimentally determined collisional depolarisation cross-sections for the collisions of NO(A) with He/Ar, and of OH(A) with Ar at collision energies of 39 meV/757meV are presented along with their theoretical counterparts. The relative magnitudes of the cross-sections are rationalised in terms of the potential energy surface over which the collision takes place, and the importance of spin-rotation conserving and spin-rotation changing transitions in the depolarisation process is assessed. A detailed study of the ensemble of quasi-classical trajectories is performed to determine the character of the various atom-molecule collisions, and to identify which conditions lead to the most efficient depolarisation of <strong>j</strong>. The relative importance of the potential energy surface and the collision kinematics is also assessed at this point. The results presented in this thesis thus investigate two complementary expressions of the collision dynamics, the <strong>k</strong> - <strong>k'</strong> and <strong>j</strong> - <strong>j'</strong> vector correlations, and encompass a variety of collision partners exhibiting vastly differing collision characteristics. As such, this work serves as an illustrative overview of atom-molecule scattering dynamics, containing both experimental and theoretical reflections of the collision dynamics, and relating this information back to the fundamentals of scattering theory.

The current study addresses the stability of shock waves propagating through dissipative media, analogous to both granular media and molecular gases undergoing endothermic reactions. In order to investigate the stability, a simple molecular dynamics model was developed to observe shock waves and their structures with the inclusion of energy dissipation. For this, an Event Driven Molecular Dynamics model was implemented in a 2D environment, where a molecule is represented by a disk. The simulations addressed the formation of a shock wave in a gas by the sudden acceleration of a piston. Inelastic collisions were assumed to occur only if an impact velocity threshold is surpassed, representing the activation energy of the dissipative reactions. Parametric studies were conducted for this molecular model, by varying the strength of the shock wave, the activation threshold and the degree of inelasticity in the collisions. The resulting simulations showed that a shock structure does indeed become unstable with the presence of dissipative collisions. This instability manifests itself in the form of distinctive high density non-uniformities behind the shock wave, which take the form of convective rolls. The spacing and size of this ``finger-like" unstable pattern was shown to be dependent on the degree of inelasticity, the activation energy, and the strength of the driving piston. The mechanism responsible for the instability was addressed by studying the time evolution of the material undergoing the shock wave compression and further relaxation. It is found that the gas develops the instability on the same time scales as the clustering instability in homogeneous gases, first observed by Goldhirsch and Zanetti in granular gases. This confirmed that the clustering instability is the dominant mechanism.

Inelastic collision processes driven by anistropic interactions are investigated below 1 K. Three distinct experiments are presented. First, for the atomic species antimony (Sb), rapid relaxation is observed in collisions with \(^4He\). We identify the relatively large spin-orbit coupling as the primary mechanism which distorts the electrostatic potential to introduce significant anisotropy to the ground \(^4S_{3/2}\) state. The collisions are too rapid for the experiment to fix a specific value, but an upper bound is determined, with the elastic-to-inelastic collision ratio \(\gamma \leq 9.1 x 10^2\). In the second experiment, inelastic \(\mathcal{m}_J\)-changing and \(J\)-changing transition rates of aluminum (Al) are measured for collisions with \(^3He\). The experiment employs a clean method using a single pump/probe laser to measure the steady-state magnetic sublevel population resulting from the competition of optical pumping and inelastic collisions. The collision ratio \(\gamma\) is measured for both \(\mathcal{m}_J\)- and \(J\)-changing processes as a function of magnetic field and found to be in agreement with the theoretically calculated dependence, giving support to the theory of suppressed Zeeman relaxation in spherical \(^2P_{1/2}\) states [1]. In the third experiment, very rapid atom-atom relaxation is observed for the trapped lanthanide rare-earth atoms erbium (Er) and thulium (Tm). Both are nominally nonspherical \((L \neq 0)\) atoms that were previously observed to have strongly suppressed electronic interaction anisotropy in collisions with helium \((\gamma > 10^4-10^5, [2,3])\). No suppression is observed in collisions between these atoms \((\gamma \lesssim 10)\), which likely implies that evaporative cooling them in a magnetic trap will be impossible. Taken together, these studies reveal more of the role of electrostatic anisotropy in cold atomic collisions. / Physics

Inelastic collisions between dipolar molecules, assumed to be trapped in a static electric field at cold (> 10−3K) temperatures, are investigated and compared with elastic collisions. For molecules with a Λ-doublet energy-level structure, a dipole moment arises because of the existence of two nearly degenerate states of opposite parity, and the collision of two such dipoles can be solved entirely analytically in the energy range of interest. Cross sections and rate constants are found to satisfy simple, universal formulas. In contrast, for molecules in a Σ electronic ground state, the static electric field induces a dipole moment in one of three rotational sublevels. Collisions between two rotor dipoles are calculated numerically; the results scale simply with molecule mass, rotational constant, dipole moment, and field strength. It might be expected that any particles interacting only under the influence of the dipole-dipole interaction would show similar behavior; however, the most important and general result of this research is that at cold temperatures inelastic rate constants and cross sections for dipoles depend strongly upon the internal structure of the molecules. The most prominent difference between the Λ-doublet and rotor molecules is variation of the inelastic cross section with applied field strength. For Λ-doublet dipoles, cross sections decrease with increasing field strength. For rotor dipoles, cross sections increase proportionally with the square of field strength. Furthermore, the rate constants of the two types of molecules depend very differently on the angular orientations of the dipoles in the electric field.

The current study addresses the stability of shock waves propagating through dissipative media, analogous to both granular media and molecular gases undergoing endothermic reactions. In order to investigate the stability, a simple molecular dynamics model was developed to observe shock waves and their structures with the inclusion of energy dissipation. For this, an Event Driven Molecular Dynamics model was implemented in a 2D environment, where a molecule is represented by a disk. The simulations addressed the formation of a shock wave in a gas by the sudden acceleration of a piston. Inelastic collisions were assumed to occur only if an impact velocity threshold is surpassed, representing the activation energy of the dissipative reactions. Parametric studies were conducted for this molecular model, by varying the strength of the shock wave, the activation threshold and the degree of inelasticity in the collisions. The resulting simulations showed that a shock structure does indeed become unstable with the presence of dissipative collisions. This instability manifests itself in the form of distinctive high density non-uniformities behind the shock wave, which take the form of convective rolls. The spacing and size of this ``finger-like" unstable pattern was shown to be dependent on the degree of inelasticity, the activation energy, and the strength of the driving piston. The mechanism responsible for the instability was addressed by studying the time evolution of the material undergoing the shock wave compression and further relaxation. It is found that the gas develops the instability on the same time scales as the clustering instability in homogeneous gases, first observed by Goldhirsch and Zanetti in granular gases. This confirmed that the clustering instability is the dominant mechanism.

Typescript / Copies of three papers (2 published), co-authored by the author, in back / Bibliography: leaves 179-186 / ix, 186 leaves : ill ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Physics and Mathematical Physics, 1988

Thesis (M. Sc.)--University of Adelaide, Dept of Physics, 1986. / Includes bibliographical references (leaves 103-105).