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|Title:||Studies of 20 < A < 30 Nucleosynthesis in AGB Stars and Novae|
|Department:||Physics and Astronomy|
|Abstract:||<p> In this thesis, a variety of topics are investigated. Part I discusses asymptotic giant branch (AGB) stars. We review their evolution and their contribution to the galactic chemical evolution. We particularly pay attention to the nucleosynthesis in different layers of the AGB stars, and discuss diverse chains of reactions that can happen under different circumstances. </p> <p> Out of many of such reactions, three are the subjects of our special attention. The 23Na(p,α)20Ne, 23Na(p, γ)24Mg and 26YAl(p, γ)27Si reactions are important reactions that are part of the NeNa and MgAl cycles. Their reaction rates used to be uncertain by orders of magnitude, and thus have been subjects of investigation. Recently, there has been new experimental information released on these reactions. In this project, we have used this new information, and have calculated the new reaction rates for those reactions. The results show less uncertainty range in all three reaction rates compared to the prior measurements. </p> <p> We then have used these new less uncertain rates to calculate the AGB yields of hydrogen through to 62Ni. However, these reaction rates only affect the yields of Ne to Si isotopes noticeably, which are presented in Appendix A. Dr. Karakas has calculated the AGB yields by computing stellar evolution and nucleosynthesis models for a 6 M (symbol) AGB star with three different metallicities (Z = 0.02, 0.004 and 0.008) using the new reaction rates. The results show that the changes in the yields due to individually using the updated 23Na(p, γ)24Mg or 23Na(p,α)20Ne reaction rate are noticeable for some isotopes. However, these new reaction rates result in completely opposite changes in most of the yields; moreover, the updated 26gAl(p, γ)27Si reaction rate has no effect on any of the stellar yields except on the yield of 28 Si obtained by the Z = 0.02 model. Thus, by using all three new reaction rates simultaneously in the nucleosynthesis network, we only see major changes for a few isotopes, e.g. significant destruction of 20Ne and considerable production of 23 Na, 24Mg and 28Si. There is no noticeable effect on any of the remaining AGB yields. </p> <p> Part II of this project discusses the significance of studying the nuclear structure of 26Si and 30S, which are not yet well understood. We discuss classical novae and their nucleosynthesis. We pay attention to some reactions, whose rates are still uncertain, e.g. the 25 Al(p, γ)26 Si, and 29 P(p, γ)30S reactions. To lower the uncertainty range in such reaction rates, the structure of 26Si and 30S should be better understood. </p> <p> We have carried out an experiment at Wright Nuclear Structure Laboratory (WNSL) at Yale University to be able to determine whether or not further studies of the structure of 26Si and 308 can be pursued by the (12C,6He) reaction mechanism. We investigated the 20 NeC2C,6He)26 Si and 12C(24 Mg,6He)30 S reactions. The time for collecting the data for the whole experiment was only about five days. Taking into consideration the number of experiments that were done in five days, some of them resulted in low statistics. The 20 NeC2C,6He)26 Si experiment gave a null result. This is due to the fact that the target that was used was old, and the 20 Ne in that target has been diffused out. Thus, we could not determine whether the (12C,6He) reaction mechanism proves to be a good method to study the structure of 26 Si. As for the nuclear structure of 30 8, we could see the ground state and the first excited state. The time was not enough to collect enough data to be able to determine this structure; however, the (12C,6He) reaction mechanism for studying the structure of 30 S looks promising. </p>|
|Appears in Collections:||Digitized Open Access Dissertations and Theses|
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