Investigation of the Isoscalar Giant Monopole Resonance as a function of neutron excess in the 40,42,44,48Ca isotope chain
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Date
2020
Authors
Olorunfunmi, Sunday Daniel
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Abstract
Experiments investigating the fine structure of the IsoScalar Giant Monopole Resonance (ISGMR) of 40Ca,42Ca,44Ca and 48Ca were carried out with 200 MeV alpha inelastic scattering at θLab= 0◦±2◦andθLab= 4◦±2◦, using the high energy-resolution K600 magnetic spectrometer Zero-degree Facility and small angle mode of iThemba LABS, Cape Town, South Africa. Good energy-resolution of 85 keV (FWHM) was attained which allowed for a clear observation of fine structure in the excitation energy region of the ISGMR which lies between 10 and 22 MeV. Alpha-particle inelastic scattering at intermediate energies has been used innumerous investigations across the periodic table to extract E0 strength and determine the properties of the ISGMR. This is made possible since the L= 0multipole has a maximum at zero degrees while all other low-order multipoles have much lower yields. In the present experiment on calcium isotopes, the L=0 multipole exhibits strong oscillating structure at small scattering angles, with a deep minimum atθLab≈3.5◦, which allows the Difference of Spectra (DoS) technique to be used. Here, the background from all other low-order multipoles excited is removed by taking an angle-cut from the measured 4◦spectrum and subtracting from the measured 0◦spectrum. This is more reliable than the alternate Multipole Decomposition Analysis (MDA) technique which requires an arbitrary background to be first defined. In the present project, the Fraction E0 EWSR and correspondingE0strength distributions were extracted and compared to other measurements .In addition and for the first time, characteristic energy scales were extracted from the measured high energy-resolution E0 strength distributions. These were compared to those from state-of-the-art theoretical calculations in order to gain insight into the ISGMR damping mechanisms. The theoretical calculations considered are within the framework of the Phonon Phonon Coupling (PPC), the Quasiparticle Time Blocking Approximation (QTBA) and the Quasi-particle Random Phase Approximation (QRPA). Of all these models, the PPC best described the experimental data for all the four calcium isotopes in terms of reproducibility of the energy scales present in them as well as in terms of the amount of fine structure exhibited and the prediction of the centroid energies, compared to the other models. Level densities of the Jπ= 0+ states were extracted from experimental data for the doubly magic 48Ca and 40Ca isotopes and compared with those of phenomenological Back Shifted Fermi Gas (BSFG) model predictions and calculations of the Hartree-Fock Bogoluibov (HFB) microscopic model
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A thesis submitted to the Faculty of Science, University of the Witwatersrand, in fulfilment of the requirements for the degree of Doctor of Philosophy, 2020