Fine structure of the isovector giant dipole resonance in neutron-rich calcium isotopes using the (p,p') reaction at zero-degrees
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Date
2017
Authors
Latif, Mouftahou Bakary
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Abstract
In the present study, the fine structure of the IsoVector Giant Dipole Resonance
(IVGDR) of stable neutron-rich isotopes of calcium, namely 42Ca, 44Ca and 48Ca,
has been investigated in a systematic way by including readily available data of
40Ca.
Measurements were carried out at the cyclotron facility of iThemba Laboratory
for Accelerator Based Sciences (iThemb LABS) which provided a high quality
proton beam of 200 MeV, together with the state-of-the-art K600 magnetic spectrometer in zero-degree mode. Excellent energy-resolution of up to ∆E ≈ 33
keV was attained which allowed for a clear observation of fine structure in the
excitation energy region of the IVGDR which lies between 11 and 25 MeV.
Double differential cross-sections were extracted from the experimental energy
spectra and subsequently converted to equivalent photo-absorption cross-sections.
The obtained results agree well with the photo-absorption cross-section data avail
able in the literature, within limits of experimental errors. Centroid energies and
widths of the IVGDR of the calcium isotopes were extracted by fitting the ex
perimental data with both Lorentzian and Gaussian functions over the excitation
energy range of 15 - 23 MeV. The obtained values of centroids and width agree
well with the predictions of the theoretical macroscopic model.
Characteristic energy scales were extracted from the experimental spectra and
compared to those of theoretical microscopic models of the Relativistic Quasi
particle Time Blocking Approximation (RQTBA), the Relativistic Quasi-particle
Random Phase Approximation (RQRPA), the Random Phase Approximation
(RPA), the Quasi-particle RPA (QRPA), the Second RPA (SRPA) and the Con
tinuum RPA (CRPA). The RQTBA was found to best describe the experimentally
extracted energy scales.
The level density of the Jπ = 1− states was also extracted from the high energy
resolution spectra and the results show a good agreement with the theoretical
phenomenological models of the Back-shifted Fermi Gas in the excitation energy
range of 13 and 20 MeV for all four calcium isotopes.
Description
A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the Faculty of Science, University of the Witwatersrand, Johannesburg, 2017