A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, May 23, 2014. / A systematic experimental investigation was performed of the phenomenon of
fine structure, with emphasis on the region of the Isoscalar Giant Quadrupole
Resonance (ISGQR), in nuclei across stable even-even neodymium isotopes. The
200 MeV proton beams were delivered by the Separated Sector Cyclotron (SSC)
facility of iThemba Laboratory for Accelerator Based Sciences (iThemba LABS).
Measurements were made using the state-of-the-art K = 600 magnetic spectrometer,
where unique high energy-resolution ( E ≈ 42 − 48 keV FWHM) proton
inelastic scattering results were obtained on 142Nd, 144Nd, 146Nd, 148Nd and 150Nd
targets. All measurements were taken at θLab = 8◦, where the cross-section of
the ISGQR is at a maximum. An additional measurement was also made for
the 142Nd at θLab = 7◦. Nuclei with mass number A ≈ 150 and neutron number
N ≈ 90 are of special interest since they occupy that region of the nuclide
chart wherein the onset of permanent prolate deformation occurs. The stable
neodymium (Z = 60) isotopes have been chosen in the present study in order to
investigate the effects accompanying the onset of deformation on the excitation
energy spectra in the ISGQR region (9 ≤ Ex ≤ 15 MeV). The neodymium isotopes
extend from the semi-magic N = 82 nucleus (142Nd) to the permanently
deformed N = 90 (150Nd) nucleus.
In order to emphasize the ISGQR in the measured excitation energy spectra,
a Discrete Wavelet Transform (DWT) background subtraction was carried out.
This model independent method for background determination decomposes the
spectrum into various approximations and details through the application of high
pass and low pass filters. A comparison of the resonance widths extracted shows
a systematic broadening of the ISGQR ( = 3.220 MeV to 5.100 MeV), moving
from spherical 142Nd to highly deformed 150Nd nuclei as has already been observed
for the Isovector Giant Dipole Resonance (IVGDR) excited by γ-capture.
Even though it is known that the IVGDR spectacularly splits and shows a double
bump for the deformed 150Nd, no obvious splitting of the ISGQR was observed.
In order to investigate the fine structure of the ISGQR, a theoretical microscopic
calculation termed the Quasiparticle-Phonon Model (QPM) was applied to predict
excitation energy spectra for 142−146Nd targets. These calculations were
based on the one- plus two-phonon configuration. Characteristic energy scales
were extracted for the resonance region using the Continuous Wavelet Transform
(CWT) technique, on both experimental data and theoretical predictions.
Comparison of the resulting characteristic energy scales suggests the coupling to
low-lying collective vibrations as the dominant contributor to the ISGQR decay
width.
Level densities of 2+ states were extracted through the application of a fluctuation
analysis technique, for full spectra from the ground state upwards in all
five Nd targets. Comparisons are made with theoretical predictions from the
Back Shifted Fermi Gas, Hartree-Fock-BCS and Hartree-Fock-Bogoluibov models.
While there is generally an excellent agreement between experimental level
densities and theoretical predictions from the ground state up to less than 10
MeV excitation, there is a marked disagreement beyond 10 MeV in all target
nuclei. Comparison of the experimental results for the Nd isotope chain shows
a clear systematic trend in which the onset of this disagreement occurs at lower
and lower excitation energies, moving from low to high mass. For the spherical
142Nd nucleus the deviation occurs at about 9 MeV while in the case of the deformed
150Nd this occurs much earlier at about 4 MeV, all limited to a maximum
of 103 MeV−1 by the energy resolution of the present experiment.
Additionally, measurements of elastic scattering and inelastic excitation of lowlying
collective states in 144−150Nd has also been possible. Excitation energy
spectra in all targets predominantly exhibited various 2+ states, owing to the
“spin-filter” effects. A single strong 3−
1 state, together with a weak 4+
1 state were
observed in each target nucleus. Angular distributions were obtained for the
various ground and excited states by applying the optical model of elastic scattering
and Distorted Wave Born Approximation (DWBA) of inelastic scattering.
Deformation lengths δL were obtained for most of the states and these were in
good agreement with previously obtained results from the literature.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/15066 |
| Date | 30 July 2014 |
| Creators | Kureba, Chamunorwa Oscar |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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