Faculty of Science

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The Faculty of Science is respected within South Africa, Africa and the world arena as a knowledge-partner of note that builds on the scientific, technological and intellectual capacity of Africa and plays an active role in the development of South African society. The faculty is placed in the top 300 within the category Natural Sciences of the QS World University Ranking list.

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Browsing Faculty of Science by Subject "180Ta -- Evolution -- Statistics"

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    Statistical properties of deformed Samarium isotopes and constraining the nucleosynthesis of 180Ta
    (Stellenbosch : Stellenbosch University, 2019-12) Malatji, Kgashane Leroy; Wiedeking, Mathis; Siem, S.; Papka, Paul; Stellenbosch University. Faculty of Science. Dept. of Physics.
    ENGLISH ABSTRACT: This thesis focuses on two areas of research, i) statistical properties of well-deformed samarium isotopes and ii) constraining the nucleosynthesis of 180Ta. The rare-earth isotopic chain of samarium provides an excellent opportunity to systematically investigate the evolution of nuclear structure effects from the near spherical 154Sm isotope to the well-deformed 154Sm. As the nuclear shape changes, statistical properties such as the nuclear level density (NLD) and g-strength function (gSF) are expected to be affected. In particular resonance modes in the rare-earth region may reveal interesting features when their evolution is investigated across several nuclei in an isotopic chain. An experiment was performed at the Oslo Cyclotron Laboratory (OCL) to measure particle-g coincidences from which the NLDs and gSFs were extracted using the Oslo Method. A deuteron beam was used to populate excited states in 154,155Sm. Results from this work indicate a pronounced M1 scissors resonance at » 3 MeV in 154Sm. Surprisingly, the gSF of 154Sm appears to be more featureless. The measured integrated strength in both 154,155Sm shows nonetheless a reasonable agreement with previous measurements of other deformed samarium isotopes. Overall, the results provide complementary information to the (3He,ag)148Sm, (3He,3He’g)149Sm, (d,d’g)152Sm, (d,pg)153Sm, (p,dg)147,149,151,153Sm, (p,p’)154Sm and (a,a’g)154Sm data previously measured. A small number of naturally occurring neutron-deficient nuclides with Z > 33, referred to as p-nuclei, cannot be produced by (n,g) processes in the cosmos. Instead, these stable nuclides are thought to be produced by the photodisintegration (p-process) of both slow-neutron capture (s-process) and rapid-neutron capture (r-process) seed nuclei. However, for 180Ta, the p-process is not sufficient to explain the observed solar abundances and therefore additional processes are invoked. The 180Ta production mechanism causes controversy since calculations show that several processes, sometimes exclusively, can reproduce its observed abundance, making it a particularly interesting case to study. Since the astrophysical sites for the nucleosynthesis of 180Ta remain unknown, a combination of several processes is undeniably possible. However, the significance of individual processes cannot be clearly determined, as a result of the uncertainties on the reaction rates for 180Ta due to the unavailability of experimental data. The 180,181,182Ta NLDs and gSFs were recently measured at the OCL. From these results, the (n,g) cross sections are calculated using the nuclear reaction code TALYS and compared to previous results. Furthermore, the reaction rates are calculated and used in s-process calculations in low-mass AGB stars and p-process simulations in Type-II supernovae to investigate the nucleosynthesis of 180Ta. From this work the newly constrained reaction rates of the s-process show that in low-mass AGB stars the contribution to the production of 180Ta is negligible and only partially contributes to the production of 181Ta. The p-process nucleosynthesis of nature’s rarest stable isotope 180Ta in Type-II supernovae using newly constrained data is significant and therefore suggests the p-process to be the fundamental production mechanism of 180 Ta in the universe.