Browsing by Author "Hillhouse, G. C. (Gregory Campbell)"

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    Nuclear reactions with polarized spin-1/2 beams
    (Stellenbosch : Stellenbosch University, 1990-03) Hillhouse, G. C. (Gregory Campbell); De Kock, P. R.; Stellenbosch University. Faculty of Science. Department of Physics
    ENGLISH ABSTRACT: The first part of the thesis deals with the subject of polarization in nuclear reactions. The case where the incoming beam is polarized, is emphasized, and the theory is developed for the simple and illustrative cases of spin-t on spin-t scattering as well as spin-t on spin zero scattering. Finally, the polarization observables (or alternatively, the Wolfenstein-parameters) are introduced, their physical meanings and the ways in which they are measured, are discussed. In the second part we investigate the sensitivity of a complete set of spin-t on spin zero elastic scattering observables [ ~ , P, Q J to lelativistic and nonrelativistic predictions. The calculational approach is based on a simple relativistic extension of the first-order KMT optical potential within the context of a Dirac equation description. A formulation of this problem in terms of momentum space integral equations displaying an identifiable nonrelativistic sector is described and applied. The computer code WIZARD 1 is used in this respect and applied to the case of elastic scattering of polarized protons with energies of 135 MeV and 155 MeV on 24Mg, 2SSi and 325 targets. These results clearly demonstrate that the relativistic predictions are in far better agreement with the measured values than are the nonrelativistic results. We then propose the type of future measurements that can be made at the National Accelerator Centre, once the polarized-ion source has been installed. We also advise the implementations of relativistic calculations of the scattering observables.
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    Relativistic descriptions of polarization transfer observables for quasielastic proton scattering
    (Stellenbosch : Stellenbosch University, 1999-03) Hillhouse, G. C. (Gregory Campbell); De Kock, P. R.; Stellenbosch University. Faculty of Science. Department of Physics.
    ENGLISH ABSTRACT: This thesis is devoted to the development of relativistic Dirac-based models for describing complete sets of quasielastic (p,p') and (p, ii) polarization transfer observables at medium energies. The original relativistic-plane-wave-impulse-approximation (RPWIA) model of Horowitz and Murdock is modified to include the phenomenological Horowitz-Love-Franey meson-exchange (HLF) model for the NN amplitudes, and new HLF parameter sets are generated between 80 and 200 MeV. Medium effects are incorporated by replacing free nucleon masses in the Dirac plane waves with more refined effective projectile and target nucleon masses. For a 4°Ca target at a fixed momentum transfer of 1.97 fm-1, and incident energies between 135 and 300 MeV, the sensitivity of complete sets of quasielastic (P,p') and (P, ii) polarization transfer observables is investigated with respect to nuclear medium effects, ambiguities in 1rNN coupling, exchange contributions to NN amplitudes, and spin-orbit distortions. It is seen that, (1) compared to (p,p 1 ) scattering, the (p, ii) polarization transfer observables are more sensitive to pseudoscalar versus pseudovector forms of the 1rNN coupling, (2) as the incident proton energy is lowered, nuclear medium effects and spin-orbit distortions become more important, (3) nuclear medium effects are extremely sensitive to the type of pion coupling, (4) contrary to the original RPWIA, exchange contributions cannot be neglected at energies as high as 500 MeV. For an optimal study of nuclear medium effects, this investigation stresses the urgent need for measurements of complete sets of quasielastic polarization transfer observables for both (p,p') and (p, n) reactions at energies lower than 200 MeV. Comparison of RPWIA predictions with the small amount of available data yields an inconsistent picture: The (P, p') data favour a pseudoscalar coupling for the pion, whereas the limited (P, n) data suggest a pseudovector form. Our poor treatment of distortions is considered to be the main source for this inconsistency. The issue of distortion effects on polarization transfer observables is addressed by developing the theoretical framework for the relativistic distorted wave impulse approximation. As an additional improvement over the RPWIA, models of nuclear structure (relativistic Fermi-gas model, relativistic mean-field approximation, and local-density-approximation) are developed, whereby the nuclear structure information is contained in a large set of nuclear respon