The question of how do the properties of nuclei evolve with increasing excitation energy and angular momentum is one of the current frontiers in nuclear physics. State of the art $gamma$-ray detector systems have been used to investigate this question, in a series of rare-earth nuclei with mass extit{A}$sim$158. Significant extensions to the high-spin excitation spectrum of the $N$=91, 92, 93 isotopes $^{157,158,159}$Dy have been achieved using the high-efficiency $gamma$-ray spectrometers EUROBALL and GAMMASPHERE. These nuclei were populated via weak 3$n$ or $alpha xn$ exit channels in fusion evaporation reactions. In $^{157}$Dy, the yrast band has been extended to extit{I}$^pi$=$frac{101}{2}^{+}$ (tentatively to $frac{105}{2}^{+}$) with four sideband structures (two of which are new) observed in the 35$-$50 $hbar$ spin range. In $^{158}$Dy, three bands have been extended to 42$^{+}$ (44$^{+}$), 40$^{-}$, and 41$^{-}$ (43$^{-}$), while in $^{159}$Dy the yrast band is observed to $frac{81}{2}^{+}$ ($frac{85}{2}^{+}$). A total of 84 (99) new transitions, including 2 new bands, were added to the level schemes of $^{157,158,159}$Dy. The high-spin behavior and band crossing systematics of the Dy isotopes and of the neighboring $N$=91, 92, and 93 isotones are discussed in terms of rotational alignments and shape transitions. Cranked Nilsson-Strutinsky calculations without pairing have been performed for detailed comparisons with the very high-spin states observed in $^{157}$Dy. Results on $^{157,158,159}$Dy have been published in extit{Phys. Rev. C.} Moreover, the angular-momentum induced transition from a deformed state of collective rotation to a non-collective configuration has been studied. In $^{158}$Er this transition manifests itself as favored band termination near extit{I}$approx$45$hbar$. The feeding of these band terminating states has been investigated for the first time using the GAMMASPHERE spectrometer. A large number of weakly populated states, lying at high excitation energy, that decay into these special states have been discovered. Cranked Nilsson-Strutinsky calculations suggest that these states arise from weakly collective configurations that break the $Z$=64 semi-magic core. Additionally, a new frontier of discrete-line $gamma$-ray spectroscopy at ultra-high spin has been opened in the rare-earth nucleus $^{158}$Er. Two rotational structures, displaying high moments of inertia, have been identified, which extend up to spin $sim$65$hbar$ and bypass the band-terminating states in these nuclei near extit{I}$sim$45$hbar$. Cranked Nilsson-Strutinsky calculations suggest that these structures arise from well-deformed triaxial configurations that lie in a valley of favored shell energy, which also includes the well-known triaxial strongly deformed bands in $^{161-167}$Lu. Overall, 182 (209) new transitions, including 10 new bands, were placed in the greatly augmented level scheme of $^{158}$Er, as a result of our work in this thesis. Four of the new bands are based on high$-$ extit{K} quasiparticle excitations, which provide a stringent test of the Cranked Shell Model. This enables the investigation and interpretation of many different quasiparticle configurations from their alignment properties and band crossings systematics. Results on $^{158}$Er have been published in extit{Phys. Rev. Lett.} and extit{Phys. Scr.} Finally, a local experiment, using the FSU tandem accelerator and the FSU $gamma$-ray detectors, was performed to investigate the odd-odd nucleus $^{158}$Tb. Unfortunately, no new significant results on the latter were obtained except for the tentative assignment of a new, strongly-coupled, rotational structure.} / A Dissertation submitted to the Department of Physics in partial fulfillment of the
requirements for the degree of Doctor of Philosophy. / Degree Awarded: Fall Semester, 2006. / Date of Defense: October 31, 2006. / 157158159Dysprosium, 158Erbium, 158Terbium, High-Spin, Rare-Earth, Nuclear Structure, Band Termination, Gamma-Ray / Includes bibliographical references. / Mark A. Riley, Professor Directing Dissertation; Ettore Aldrovandi, Outside Committee Member; Jorge Piekarewicz, Committee Member; Laura Reina, Committee Member; Paul M. Eugenio, Committee Member.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_168848 |
| Contributors | Pipidis, Paschalis Akis (authoraut), Riley, Mark A. (professor directing dissertation), Aldrovandi, Ettore (outside committee member), Piekarewicz, Jorge (committee member), Reina, Laura (committee member), Eugenio, Paul M. (committee member), Department of Physics (degree granting department), Florida State University (degree granting institution) |
| Publisher | Florida State University |
| Source Sets | Florida State University |
| Language | English, English |
| Detected Language | English |
| Type | Text, text |
| Format | 1 online resource, computer, application/pdf |
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