Doctoral Degrees (Physics)

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Browsing Doctoral Degrees (Physics) by browse.metadata.advisor "Cowley, A. A."

Now showing 1 - 5 of 5
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    Analyzing power and cross section distributions of the 12C (p,pα)8Be cluster knockout reaction at an incident energy of 100 MeV
    (Stellenbosch : University of Stellenbosch, 2010-03) Mabiala, Justin; Cowley, A. A.; Fortsch, S. V.; University of Stellenbosch. Faculty of Science. Dept. of Physics.
    ENGLISH ABSTRACT: The (p, pα) reaction on 12C was investigated experimentally using polarized incident protons of 100 MeV. Coincident data, which were obtained at ten quasifree angle pairs for proton angles ranging from 25◦ to 110◦, were analyzed in terms of the distorted-wave impulse approximation (DWIA). Calculated energy-sharing cross section and analyzing power distributions reproduce the data reasonably well. The observed agreement allows the extraction of distorted momentum distributions from experimental data. These distributions are very consistent over a wide range of angle pairs at which cross section energy-sharing distributions vary considerably. Since measurements of analyzing powers were made, spin-orbit distortions were included in the DWIA calculations. The effects of spin-orbit distortions were found to be very small near zero recoil momentum and did not destroy the validity of the factorization approximation where the two-body p-α cross section enters as a multiplicative factor in the three-body (p, pα) cross section expression. Spectroscopic factors derived from the data are fairly consistent with the trend of the theoretical predictions. Analyzing power data also follow the trend of free p-4He scattering data, and comparisons with DWIA predictions are in reasonable agreement. The theory reproduces also very well analyzing power angular distributions of the projectile-cluster two-body scattering at large angular momentum of the residual nucleus. This indicates that a quasifree knockout mechanism dominates the reaction. The two-body interaction response between the projectile and the α cluster was found to resemble the scattering of protons from a free α particle to a remarkable degree, the present results strongly imply the existence of preformed α clusters in 12C.
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    Near-target and other heavy residues in the interaction of ¹²C and ¹⁶O with ¹⁰³Rh
    (Stellenbosch : University of Stellenbosch, 2004-04) Buthelezi, Edith Zinhle; Cowley, A. A.; Steyn, G. F.; Van der Walt, T. N.; University of Stellenbosch. Faculty of Science. Dept. of Physics.
    ENGLISH ABSTRACT: This study forms part of a larger investigation which has as a primary objective the development of a comprehensive theoretical description of all the processes which contribute to the continuum in the interaction of 12C and 16O with nuclei. Previous investigations of 12C and 16O induced reactions on targets with mass close to A = 100 have shown that the experimental excitation functions and recoil range distributions of heavy residues can be reproduced satisfactorily by means of a theoretical model which takes relatively few dominant reaction mechanisms into account. These include the complete fusion of the projectile with the target, the incomplete fusion of break-up α-type fragments (i.e. single α particles, 8Be fragments and for the 16O induced reactions also 12C fragments) with the target and single-nucleon transfer at incident energies above about 15 MeV/nucleon. The mean-field interaction is mainly responsible for these interactions. The thermalization of the intermediate excited nuclei produced in this first stage of the reaction is described by an intranuclear interaction cascade, during which pre-equilibrium emission of particles and clusters may occur, followed by evaporation after statistical equilibrium has been attained. The model also included the probability that break-up α particles may escape with a large fraction of their initial energy after only a few interactions with individual target nucleons following their initial incomplete fusion. The theory also predicted an enhanced isobaric yield for residues with mass similar or near to that of the target. The subsequent analysis of the emission spectra of intermediate mass fragments in these reactions, however, indicated that two additional aspects need to be considered as well in order to reproduce the experimental data. The first is that the projectile may lose a substantial amount of energy in an initial-state interaction before breaking up, which can be described as a friction dissipative process. The second is that several other incomplete fusion channels of “non-α-cluster”- type fragments should also be included in a more complete description of these reactions as their contributions are not negligible. The present study has two main objectives. Firstly, to investigate the isobaric yield in the neartarget mass region by measuring production cross sections for 103Pd, 103mRh and 103Ru. Previous studies only provided data for 103Ag, which constitute only a few percent of the A = 103 isobaric yield. The new data constitute more than 80% of the A = 103 isobaric yield, which provide experimental confirmation of the enhanced isobaric yield in the near-target mass region. The second objective is to perform extensive new calculations of the excitation functions and recoil ranges in order to investigate the predictive power of the extended model in a priori calculations for the entire available data set.
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    Nuclear medium effects on analyzing power investigated with a proton knockout reaction
    (Stellenbosch : Stellenbosch University, 2001-12) Neveling, Retief; Cowley, A. A.; Stellenbosch University. Faculty of Science. Dept. of Physics.
    ENGLISH ABSTRACT: Please see full text for abstract
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    Pre-equilibrium helion emission induced by protons
    (Stellenbosch : University of Stellenbosch, 2006-12) Bezuidenhout, Jacques; Cowley, A. A.; University of Stellenbosch. Faculty of Science. Dept. of Physics.
    This thesis is devoted to a study of the 93Nb( p r ,3He) and 59Co( p r ,3He) reactions at incident energies of 100 MeV, 130 MeV and 160 MeV. Double differential cross sections and analysing power distributions were measured from a threshold of ~30 MeV up to the kinematic maximum and at scattering angles between 15º and 120º. The experimental data were compared with theoretical calculations done by combining a statistical multistep theory with a deuteron pickup mechanism in the final stage. The contribution of the first three steps towards the total double differential cross section and analysing power was assessed. The theory described the experimental double differential cross section and analysing power data reasonably well over all incident and scattered energies and for both target nuclei. As the incident energy was increased, the characteristics of the reaction mechanism also remained consistent. Thus the results supported the underlying multistep-pickup theory. The two target nuclei demonstrated similar responses, suggesting these two share the same basic reaction mechanism. The total double differential cross section for the reaction dropped with an increase in incident energy. At a fixed emission energy, near the maximum allowed value, the slope of the double differential cross section increased with an increase in incident energy, resulting in the dominance of forward peaked reactions at higher incident energies. The sensitivity of analysing power to the multistep part of the reaction was valuable to the study. Forward-peaked contributions that were associated with single step direct reactions resulted in large analysing power values at small scattering angles, and values dropped to around zero at large angles. The analysing power also appeared to decrease to values approximating zero as the incident energy was increased to 160 MeV. Furthermore, this study also confirmed the dominance of higher step mechanisms at high excitation energies, irrespective of incident energy. In general the results were in agreement with the predicted properties of the assumed reaction mechanism at all incident and emission energies, which inspires confidence that the theoretical interpretation is likely to be correct.
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    Relativistic plane wave description of spin transfer observables for proton knockout reactions
    (Stellenbosch : Stellenbosch University, 2001-03) Wyngaardt, Shaun Metzler; Cowley, A. A.; Hillhouse, G. C.; Stellenbosch University. Faculty of Science. Dept. of Physics.
    ENGLISH ABSTRACT: In this dissertation we set out to develop the first relativistic model for calculating complete sets of (p, 2p) spin transfer observables. In addition to this a new technique has been developed which allows us to evaluate the transition amplitude, which is used to calculate the scattering observables for the reaction directly. The influence of various modiuin-modificd parameters ()1I the scattered wave functions anr] NN interact ion lH-IVC' })('('11 invest igatcd DlIC t,C) (I,llIlJigllitic's surrounding the nNN coupling we have included both pseudosoalar and pseudovector coupling into the nucleon-nucleon interaction model. Furthermore we have included two different kinematic prescriptions to obtain the effective NN laboratory kinetic energy and center of mass scattering angle, which are used to obtain the NN scattering amplitudes. The aim of this study is to investigate the effects of the various model parameters on complete sets of scattering observables. Our investigation has shown that although the analyzing power is not very sensitive to nuclear medium effects, and the various other spin transfer observables such as Dnn should provide valuable insight. Further refinements of the model would be to include nuclear distortions as well as the IA2 model of the NN interaction.