Masters Degrees (Physics)

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

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    Cross sections and analysing power energy-sharing distributions of valence (p,2p)-knockout from 208Pb with a projectile of 200MeV
    (Stellenbosch : Stellenbosch University, 2000-12) Bezuidenhout, Jacques; Cowley, A. A.; Arendse, G. J.; Stellenbosch University. Faculty of Science. Dept. of Physics.
    ENGLISH ABSTRACT: The aim of this work was to study the 208Pb(p,2p)207Tl quasi-free knockout process. The experimental data were measured at the National Accelerator Centre using incident polarised protons of 200 MeV. The two scattered particles, from the knockout reaction, were detected in coincidence and their energies were determined using a magnetic spectrometer and a solid state detector telescope. Cross section and analysing power energy distributions were extracted from the experimental measurements and these were compared with theoretical values for the Distorted Wave Impulse Approximation. The theoretical cross-section calculations predict the experimental cross-section distribution well for all combinations of distorting potentials and bound states that were investigated, both with regard to shape, as well as absolute magnitude. However the theoretical analysing power distributions did not agree with the experimental quantities. Therefore it is not clear whether the analysing power is a useful tool to extract information on the specifics of the quasi-free reaction mechanism. The spectroscopic factors were found to be consistent with the results obtained in previous studies, thereby inspiring confidence that the problem with the analysing power distribution is not ascribable to a possible deficiency in the experimental techniques exploited in this work.
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    Pre-equilibrium excitation of nuclei in ([rho], [alpha]) and ([rho], 3He) reactions induced by 120, 160 and 200 MeV protons
    (Stellenbosch : Stellenbosch University, 1994) Arendse, Gillian John; Cowley, A. A.; Stellenbosch University. Faculty of Science. Department of Physics.
    ENGLISH ABSTRACT: The study of pre-equilibrium reactions induced by medium-energy light ions have become of increasing importance in recent years as they represent the bulk of the measured spectrum. On a nuclear time scale these reactions are known to take place after a one-step process of exciting discrete states in the residual nucleus but long before the process whereby the projectile distributes its kinetic energy equally over all target nucleons to form a compound nucleus which decays statistically. In this work the systematic behaviour of the continuum of complex ejectiles was investigated as a function of incident proton energy and target mass. The continuum energy spectra for the reactions 27AI(p, X), 59Co(p,X), and 197 Au(p,X), with X=a and 3He, were measured using a five element detector telescope. Data were acquired at scattering angles between 10-100° at incident proton energies of 129 and 200 MeV, and at scattering angles between 20 - 140° for an incident proton energy of 160 MeV. Experimental angle-integrated cross sections were compared with calculated angle-integrated cross sections using the geometry dependent hybrid (GDH) model. The comparison between the experimental data and the calculations were used to quantitatively estimate the contribution of the evaporation process to the pre-equilibrium spectrum. The experimental angular distributions from the reaction (p,a) and (p,3 He) on 27 Al, 59Co, and 197 Au were compared with calculated angular distributions using the phenomenological parametrization of Kalbach. In general good agreement between the experimental data and the calculations were obtained. Minor discrepancies were observed with regard to target mass and incident proton energy dependence. It is believed that the characteristics of these discrepancies could provide guidance to the future attempts to develop a proper quantum mechanical description of the reaction mechanism of complex-ejectile production induced by energetic protons.
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    Response of NaI(T1) detectors in geophysical applications
    (Stellenbosch : Stellenbosch University, 2005-03) Titus, Nortin P.; Cowley, A. A.; Stellenbosch University. Faculty of Science. Dept. of Physics.
    ENGLISH ABSTRACT: Airborne gamma-ray spectrometry is used for exploration and environmental monitoring over large areas. Such datasets of 7-ray are acquired using large 16l NaI(T1) detector crystals mounted in an aircraft. The response of these detectors to naturally occurring radioactive isotopes is a function of detector volume, efficiency and resolution. This response is modelled using the Monte Carlo method. A photon-electron transport computer code, NIPET, is written to calculate the intrinsic efficiency and photofraction of these large prismatic crystals. The intrinsic efficiency indicates the number of incident photons that interact in some way with the detector resulting in a deposition of energy in the crystal. The photofraction, on the other hand, indicates how many of these photons that impinge on the detector are actually recorded with their full initial energy. This study presents explicity calculated photofraction values for 16l NaI(Tl) crystals. Comparison of these new values, calculated with this computer program, with that of published values for small 3" x 3" crystals shows good agreement. However, for large crystals, the calculated values differ from those in the literature by almost 15%. The study also finds that for the gamma-ray energy range between 0.2 - 3.0 MeV, these detectors have an intrinsic efficiency above 90%, whereas the photofraction values range from 80% for low energy photons to above 40% for high energy photons in the case of large crystals.
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    The Role of a direct knock-out mechanism in the inclusive (p, α) reaction
    (Stellenbosch : University of Stellenbosch, 2008-03) Van Zyl, Johannes Jacobus; Hillhouse, G. C.; Cowley, A. A.; Van der Ventel, B. I. S.
    Experimental double differential cross sections and analyzing powers for the inclusive 93 Nb(p,α) r and 93 Nb(p, 3He) r reactions have been measured at an incident energy of 160 MeV. The results of this experiment, for emission angles ranging from 15º to 60º as well as previously measured data for 93 Nb(p,α) r at 100 MeV and 59Co(p,α) r at 100 and 160 MeV, have been compared with theoretical calculations based on a direct quasi-free α- cluster knockout mechanism. A computer code has been employed which calculates triple differential cross section and analysing power values using the distorted wave impulse approximation (DWIA). Inclusive double differential cross sections and analysing powers were calculated by integrating over the solid angle of the unobserved proton. It was found that the DWIA is limited in its ability to reproduce the experimental data. Some slight agreements have been observed in the region of large emission energies and small angles. This is consistent with the findings of previous studies based on a direct multi-step analysis which indicated that the first step knock-out reaction dominates in this region.