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>Magister Scientiae - MSc / High spin excited stats in¹⁹²Tl were studied using gamma-ray spectroscopy. The study included taking an active part in performing the experiment, understanding the experimental procedure, electronics and data acquisition in this experiment. The high spin states in ¹⁹²Tl were produced using a ³⁷Cl beam and ¹⁶⁰Gd target. The nine clovers of the AFRODITE array were placed at 90° and 135°. The target was thin, allowing the residual thallium nuclei to recoil in vacuum. Two weeks of data was acquired and analysed in this work. In addition more data was acquired in order to perform DSAM lifetime measurements, but this is beyond the scope of this work. The preliminary data analysis of the thin target data involved energy and time-gain matching calibrations as well as Doppler shift and gain drift corrections. Then the data were sorted into matrices and spectra for the actual analysis. The known level scheme of ¹⁹²Tl was extended by more than 50 new transitions placed in three new bands. A chiral partner to the yrast band in ¹⁹²92Tl was searched for but not found. But there is still a possibility such a band to be discovered, when the additional three weekends of data (including the DSAM lifetime measurements) undergo such an analysis.

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Le mode d’excitation collective exotique appelé chiralité a été étudié et des isomères de longue durée de vie ont été recherchés dans les noyaux ¹³⁶Nd et ¹³⁵Nd. Cinq paires de bandes Δ I=1 presque dégénérées et de même parité ont été identifiées à des spins élevés dans ¹³⁶Nd Les bandes observées ont été étudiées en utilisant la théorie de la fonctionnelle de densité covariante contrainte et rotation autour d'une axe incliné, et par un nouveau modèle de rotor plus particules développé pour décrire le couplage de particules dans quatre couches j à un rotor triaxial. Les propriétés de ces bandes doublet sont en bon accord avec les résultats des calculs théoriques. Par conséquent, l'existence de bandes chirales multiples (MχD) dans le noyau ¹³⁶Nd a été établie. Il s'agit de la première preuve expérimentale de l'existence de bandes chirales dans des noyaux pairs. Les cinq paires de bandes chirales constituent le plus grand ensablé de bandes chirales observé jusqu'à présent dans un seul noyau. De plus, le schéma de niveaux de ¹³⁶Nd a été considérablement étendu aux spins bas, moyens et très élevés. Les configurations possibles de toutes les bandes rotationnelles ont été attribuées á l'aide du modèle Nilsson-Strutinsky en rotation. La structure de ¹³⁶Nd a été clarifiée et les divers types d'excitations uni-particule et collectives ont été bien compris. Une nouvelle paire de bandes chirales á parité positive a été identifiée dans. Les caractéristiques des bandes soutiennent leur interprétation en termes de bandes chirales. Des bandes chirales à parité négative ayant été identifiées précédemment, la présence de multiple bandes chirales dans ¹³⁵Nd est ainsi établie. Les bandes chirales observées ont été étudiées en utilisant la théorie avec fonctionnelle de densité covariante contrainte et avec le modèle rotor plus particules; elles reproduisent fidèlement les données expérimentales, confirmant ainsi le phénomène MχD dans ce noyau. Les bandes chirales nouvellement observées dans ¹³⁵Nd représentent une étape importante dans la confirmation de l'existence du phénomène MχD dans les noyaux. La recherche d'états isométriques à longue durée de vie dans ¹³⁵Nd et ¹³⁶Nd a été aussi effectuée, mais n'a pas permis d'identifier de nouveaux isomères. Cependant, nous avons pu confirmer la présence des isomères déjà identifiés dans les noyaux ¹³⁸Nd, ¹³⁴Ce, ¹³⁶Pr, et ¹³⁷Pr dans nos données. / The exotic collective excitation mode called chirality has been investigated and long-lived isomers have been searched for in the ¹³⁶Nd and ¹³⁵Nd nuclei. Five pairs of nearly degenerate Δ I=1 bands with the same parity have been identified at high spins in ¹³⁶Nd. The observed bands were investigated by the constrained and tilted axis cranking covariant density functional theory and a new developed four single-j shells particle-rotor model. It was found that the properties of these doublet bands are in good agreement with results of the model calculations. Therefore, the multiple chiral doublets (MχD) phenomenon in the nucleus ¹³⁶Nd was confirmed. This was the first experimental evidence for the MχD bands in even-even nuclei. The five pairs of chiral doublet bands is the largest observed in a single nucleus until now. In addition, the level scheme of the ¹³⁶Nd has been extended significantly at low, medium, and very high spins. Possible configurations of all rotational bands have been assigned by using the cranked Nilsson-Strutinsky model. The band structure of ¹³⁶Nd was clarified and the various types of single-particle and collective excitations were well understood. A new pair of positive-parity chiral doublet bands has been identified in ¹³⁵Nd. The characteristics of the doublet bands support the chiral interpretation. Together with the previously reported negative-parity chiral doublet bands show the presence of MχD bands in ¹³⁵Nd. The observed doublet bands were compared with constrained covariant density functional theory and particle-rotor model calculations which nicely reproduce the experimental data, confirming the MχD phenomenon in this nucleus. The newly observed MχD bands in ¹³⁵Nd represent an important milestone in supporting the existence of MχD in nuclei. The search for long-lived isomeric states in ¹³⁵Nd and ¹³⁶Nd has also been performed. This did not lead to the identification of new isomers, but we could confirm the existence of the previously reported isomers in the nuclei ¹³⁸Nd, ¹³⁴Ce, ¹³⁶Pr, and ¹³⁷Pr in our data.

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The nucleus is a mesoscopic system that retains features from both the quantum and macroscopic worlds. A basic property of a macroscopic body is its shape. Nuclear shapes can be deduced from experimental data as they influence the excitation mode of the nucleus and hence the energies and lifetimes of its excited levels. Various short-lived nuclei were created in fusion-evaporation experiments performed at international heavy-ion accelerator facilities. The emitted γ rays and, in some experiments, also the charged particles and neutrons emitted in the reactions were detected. The studied neutron-deficient isotopes were either selected by the type and number of particles emitted in the reactions, or by using their characteristic decays. The excited states of the different isotopes were extracted from the γ-ray analyses. Spectroscopic properties, such as the lifetimes of the excited states or the angular distribution of the emitted γ rays were measured when possible. The experimentally obtained level schemes together with the other spectroscopic information were used to deduce the excitation modes and the shapes of the studied nuclei. The detector systems are described in the first chapter and in the second chapter some techniques used to extract information from the experimental data are explained. Finally, a brief theoretical overview on the nuclear models which were used to interpret the experimental results is given. / QC 20100621

New high-K isomers have been populated in Ta and W by bombarding thick targets with pulsed 238U beams at 1600 MeV. The new inelastically excited multi-quasiparticle states include Kpi = 21/2- and 29/2- 3-quasiparticle isomers in 181Ta and Kpi = 7- and (16+) isomers in 186W. The Kpi = (16+) state in 186w has t1/2≥3 ms and extends the 4-quasiparticle isomer systematics beyond the limit accessible with fusion-evaporation reactions, using stable beams and targets, for the first time. In addition, a t1/2≥1 ms 3-quasiparticle isomeric state feeding a strongly coupled rotational band has been populated in Ta by nucleon transfer. The excitation energies of the intrinsic states are compared to predictions of blocked BCS calculations. A different study of high-K states used a radioactive 14C beam at 67 MeV to investigate the high-spin structure of Os. New rotational bands built on multi-quasiparticle states with Kpi = 5-, 7-, 9-, 10+ and 15+ are observed. The first crossing of the ground-state band at I = 14 h, is interpreted as involving a high-K t-band structure and explained using a two-band mixing model. The structure at higher angular momentum is dominated by intrinsic states, that exhibit a dramatic loss of isomerism. Potential-energy-surface calculations, with Lipkin-Nogami pairing, show these configurations to be triaxial, accounting for the breakdown in K conservation. The relation between the K projection and the total angular momentum is investigated for these non-axial states.

Philosophiae Doctor - PhD / In the first experiment high spin states in 194Tl, excited through the 181Ta (18O, 5n) heavyion fusion evaporation reaction were studied using the AFRODITE array at iThemba LABS. The γ-γ coincidences, RAD ratios and linear polarization measurements were carried out and the previously known level scheme of 194Tl was significantly extended. A total of five rotational bands four of which are new were observed. A pair of rotational bands associated with the πh9/2 ⊗ νi−1 13/2 configuration at lower spins and with the πh9/2 ⊗ νi−3 13/2 configuration at higher spins was found and interpreted as the first possible chiral bands followed above the band crossing. The two 4-quasiparticle bands show exceptionally close near-degeneracy in the excitation energies. Furthermore close similarity is also found in their alignments and B(M1)/B(E2) reduced transition probability ratios. In the second experiment lifetimes in 194Tl were measured using the DSAM technique with the excited states in this nucleus populated through the 181Ta (18O, 5n) reaction. A total of 25 lifetimes and 30 reduced transition probabilities of magnetic dipole B(M1) and electric quadrupole B(E2) have been evaluated. Furthermore B(M1) and B(E2) reduced transition probabilities in Bands 1 and 4, which have been regarded as chiral candidates, were found to be close to each other and reveals strong splitting along spin values. This further supports the proposed chiral nature of these two bands.

Thesis (Ph.D.); Submitted to North Carolina State Univ., Raleigh, NC (US); 6 Apr 2005. / Published through the Information Bridge: DOE Scientific and Technical Information. "UCRL-TH-211400" Dashdorj, D. 04/06/2005. Report is also available in paper and microfiche from NTIS.