A survey of isoscalar monopole strengths and its fine structure on nuclei across the periodic table
Date
2021
Authors
Bahini, Armand
Journal Title
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Abstract
Giant Resonances (GRs) are considered to be high frequency shape
vibrations of the nucleus. This nuclear excitation mode exists in different
forms, which can be classified according to the numerous modes that the
protons and neutrons can oscillate, with or without specific orientation
selections of their spin. The characteristic energies, strengths and widths of
Giant Resonances are well known, with some GRs such as the Isovector
Giant Dipole Resonance (IVGDR) first discovered as far back as before the
World War II.
Since the new millennium it became apparent that the Isoscalar Giant
Quadrupole Resonance (ISGQR) peak exhibits a fine structure that is
independent of probe. Further it has since been established that the IVGDR
also exhibits such fine structure. This fine structure as an additional GRs
observable has been shown to be a useful tool to determine the damping
mechanism of different shape vibration.
Until recently, there has not been experimental data available to comment on
the possibility of the existence of fine structure in the Giant Monopole
Resonance (GMR). This thesis is dedicated to the investigation of the
Isoscalar Giant Monopole Resonance (ISGMR). The ISGMR was excited in
nuclei across the periodic table by using inelastic α-particle scattering
measurements acquired with a Eα = 200 MeV beam at θLab = 0◦ and 4◦
. The
high energy-resolution K600 magnetic spectrometer at iThemba LABS was
used to detect the scattered alpha particles and an experimental
energy-resolution of ∼ 70 keV (FWHM) was achieved. This enabled the fine
structure in the excitation energy region of the ISGMR to be investigated.
Alpha-particle has been proven to be the best probe to investigate the ISGMR
ii
and as such, it has been widely used for nuclei across the periodic table to
access the properties of the ISGMR as well as its strength distribution. For
that, experiments are conducted at extreme forward angles including 0◦ and
this is since the L = 0 Multipole peaks at that angle while all other have
much lower yields. The multipole Decomposition Analysis (MDA) technique
was employed in numerous studies to extract the E0 strength distribution in
nuclei. However, due to the limitations in angular acceptance and resolution,
the E0 strength distributions in the present project was determined via the
Difference-of-Spectrum (DoS) method.
E0 strength distributions in 208Pb, 120Sn, 90Zr, 58Ni, 28Si and 24Mg were
determined and compared to previously published results from (low energy
resolution) experiments at the Research Center for Nuclear Physics (RCNP)
and Texas A&M University (TAMU). Overall, reasonable agreement was
obtained. In order to gain insight into the ISGMR damping mechanism, the
extraction of characteristic energy scales from the fine structure observed in
the E0 strength distributions was performed using the technique of
Continuous Wavelet Transform (CWT). The results were compared with
theoretical predictions from the Phonon-Phonon Coupling (PPC) and
Quasi-particle Random Phase Approximation (QRPA) models. In general,
most of the extracted experimental scales were reproduced by the models.
Hence, the present analysis suggests that the fine structure of ISGMR in
medium to heavy mass nuclei across the periodic table arises from coupling
to collective phonons and the non-harmonicity owing to interactions among
phonons while it is explained by three factors in the studied light nuclei:
deformation splitting of the levels, E0/E2 coupling, and Landau
fragmentation.
Furthermore, the Fluctuation Analysis technique was employed to extract
spin- and parity-dependent level densities of J
π = 0+ in the ISGMR region.
The extracted experimental level densities were compared with the
phenomenological Back-Shifted Fermi Gas (BSFG) and the microscopic
Hartree-Fock-Bogoliubov (HFB) and Hartree-Fock-BCS (HF-BCS) model
predictions. This will be important for further studies combining level
density data for J
π = 0+, 1
−, 1
+, 2
− and 2+ states obtained from high
resolution studies of other giant resonances, in order to test the spin and
parity dependence of level densities.
Description
A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the Faculty of Science, School of Physics, University of the Witwatersrand, Johannesburg, 2021