Browsing by Author "Van Niekerk, David Douglas"
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- ItemApplication of the relativistic random-phase and distorted wave impulse approximations to quasielastic proton-nucleus scattering(Stellenbosch : University of Stellenbosch, 2010-12) Van Niekerk, David Douglas; Van der Ventel, B. I. S.; Hillhouse, G. C.; University of Stellenbosch. Faculty of Science. Dept. of Physics.ENGLISH ABSTRACT: In this dissertation a fully relativistic model for polarized inclusive quasielastic proton-nucleus scattering is developed. Using a standard relativistic impulse approximation (RIA) treatment of quasielastic scattering and a two-body SPVAT form of the current operator, it is shown how the behaviour of projectile and target can be decoupled. Subsequently, different models for projectile and target can be adopted and combined to examine a variety of relativistic effects. The most simplistic model of the target is provided by a mean-field nuclear matter approximation to the relativistic meson-nucleon model, quantum hadrodynamics (QHD). Here relativistic effects manifest as an effective mass, which is lower than the free mass, of the constituent nucleons. This model is improved upon by including many-body correlations through medium-modification of meson propagators in the relativistic random-phase approximation (RPA). Since it is generally accepted that the strong nuclear force and the extended range of the nuclear potential lead to distortion effects on the projectile and ejectile (seen as a modulation of the wave functions), our formalism is geared towards the use of relativistic distorted waves (RDWIA). The distorted waves are written as partial wave expansions and are solutions to the Dirac equation with potentials. The inclusion of distortions, however, greatly increases the computational burden and we show how a number of analytical and numerical techniques can be used to facilitate the process of calculation. It is also shown how the standard relativistic plane wave treatment (RPWIA) can, instead, be easily employed to obtain a baseline for determining the impact of distortions. A calculation is performed for the reaction 40Ca(!p, !p !) at a beam energy of 500 MeV. Here it is found that the effect of correlations on the RPWIA calculation can be seen as a quenching of the cross section that is expected to become more pronounced at lower energies or for higher density targets. A RDWIA calculation shows additional reduction and if target correlations are included this effect is enhanced. To our knowledge this is the first calculation that attempts to include both these effects (RPA and RDWIA) in the context of quasielastic proton-nucleus scattering.
- ItemThe production of hypernuclei via the weak interaction(Stellenbosch : University of Stellenbosch, 2007-03) Van Niekerk, David Douglas; Van der Ventel, B. I. S.; Hillhouse, G. C.; University of Stellenbosch. Faculty of Science. Dept. of Physics.In this thesis we present a relativistic formalism for the description of hypernuclei production via the weak interaction. It will be shown that the cross section can be written as the contraction of a leptonic and hadronic tensor if we model the interaction as a quasifree process. The hadronic tensor is written in a model-independent way in terms of thirteen nuclear structure functions. A Born term model is used to describe the underlying elementary hyperon production process. The bound state wave functions of the hyperon and nucleon are calculated within a relativistic mean-field approximation. Together with the relativistic kinematics a fully relativistic framework for experimental predictions is constructed and a specific cross section calculation is discussed.