Browsing by Author "Ngwetsheni, Cebo"

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    Combined analysis of the low-energy enhancement of the gamma-strength function and the giant dipole resonance
    (Springer, 2019) Ngwetsheni, Cebo
    The nuclear dipole polarizability is mainly governed by the dynamics of the giant dipole resonance and has been investigated along with the effects of the low-energy enhancement of the photon strength function for nuclides in medium- and heavy-mass nuclei. Cubic-spline interpolations to both data sets show a significant reduction of the nuclear dipole polarizability for semi-magic and doubly magic nuclei, with magic numbers N = 28, 50, 82 and 126, which supports shell effects at high-excitation energies from the quasi-continuum to the giant dipole resonance. This work expands on the data analysis of our recent publication in Ngwetsheni and Orce (Phys. Lett. B 792, 335, 2019), which reveals a new spectroscopic probe to search for “old” and “new” magic numbers at high-excitation energies. New results presented in this work suggest an even higher sensitivity of the nuclear polarizability to shell effects when extrapolating the low-energy enhancement at lower gamma-ray energies
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    Continuing influence of shell effects at high-excitation energies
    (Elsevier, 2019) Ngwetsheni, Cebo; Orce, José Nicolás
    Empirical drops in ground-state nuclear polarizabilities indicate deviations from the effect of giant dipole resonances and may reveal the presence of shell effects in semi-magic nuclei with neutron magic numbers N = 50, 82 and 126. Similar drops of polarizability in the quasi-continuum of nuclei with, or close to, magic numbers N = 28, 50 and 82, could reflect the continuing influence of shell closures up to the nucleon separation energy. These findings open a new avenue to investigating magic numbers at high-excitation energies and strongly support recent large-scale shell-model calculations in the quasi- continuum region, which describe the origin of the low-energy enhancement of the photon strength function as induced paramagnetism. The nuclear-structure dependence of the photon-strength function asserts the generalized Brink-Axel hypothesis as more universal than originally expected.
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    Polarizability effects due to low-energy enhancement of the gamma-strength function
    (University of the Western Cape, 2018) Ngwetsheni, Cebo; Orce, Nico
    Physics is the study of natural phenomena. Nuclear physicists have since the discovery of the nucleus been working on understanding its dynamics. The nuclear chart, analogous to the periodic table of elements, is illustrated in Fig. 1.1 and color coded according to decay modes. Several theoretical models, based on various hypothesis, have been developed during the years in order to understand nuclear phenomena such as nucleon-nucleon (n-n) interactions, binding energies, radii, excited states, etc. Unfortunately, no-unique model is actually able to grasp all nuclear phenomena at the desired level of accuracy. Among the di erent models, we notice that two distinct hypotheses can be used to describe nuclear properties. Firstly, the independent particle shell model (IPSM) + the n-n residual interaction, which assumes that a nucleon moves independently in a potential generated by other nucleons. Secondly, the macroscopic models, where a nucleus is considered as a whole, i.e. neutrons and protons behave cooperatively and are mutually coupled to each other; highlighting the short-ranged character of the nuclear force. The liquid-drop model is an example of such macroscopic models. Re nement of these models is dependent on experimental observations that are better detailed for nuclei along the line of - stability, making up a small fraction of the known isotopes, as shown in Fig. 1.1. In practice, various techniques for studying exotic nuclei up to neutron and proton drip-lines have been devised, including the use of radioactive ion beams. However, the main challenges are the synthesization and short lived periods of these exotic nuclei resulting in insu cient data collection from which the characteristics and structural information are extracted. In general, nuclei have unique structures represented by a particular con guration as given by the shell model (SM). These structures impact a number of physical quantities, e.g. transition probabilities, cross sections and photon-strength functions. Experimental methods such as Coulomb excitation or electromagnetic radiation are used to probe these structures without invoking the nuclear force.