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978-3-8439-0990-7, Reihe Physik
Nuclear Structure Studies in the A ~ 100 Mass Region including g-Factor Measurements using Nuclear Deorientation of Recoils into Vacuum
272 Seiten, Dissertation Universität Köln (2012), Softcover, A5
The mass region around A~100 is known for the rich variety of nuclear phenomena. In particular, the onset of deformation shows very different character in neighboring isotopic chains. The mechanisms behind the emergence of collectivity are still an open question. The widely accepted view is that low-energy excitations in near-spherical nuclei are of collective nature and can be described as quantized surface vibrations. Regarding the excitation energies, predictions in an anharmonic vibrator model show good agreement for some Ru, Pd and Cd nuclei. However, a comparison of available data on transition strengths and decay branchings is not as convincing. Recently, systematic deviations in the Cd isotopes stirred a debate about the validity of the vibrational picture for excited states in these nuclei.
In order to extent the study of the existence or absence of vibrational patterns, experiments on weakly collective nuclei in the A~100 mass region were performed and analyzed within the scope of this thesis. An experiment performed on the nucleus 100Pd provided first results on lifetimes of the groundstate-band states up to the 12+ state and of a 5- state. Furthermore, an experiment on its even-even isotonic neighbor 98Ru supplied data on spins and multipole mixing ratios enabling a more detailed study of the low-spin excitations in this nucleus.
Lifetime measurements performed on the Ru isotopes 96Ru, 98Ru, and 104Ru using the plunger setup at Yale University, USA, improved the precision and solved discrepancies between earlier measurements. This series of experiments was also analyzed with the intent to study in very detail the deorientation effect that is based on the hyperfine interaction between the nuclear and the electronic spin. A procedure to account for the effect within the lifetime analysis was developed. In addition, the time-dependence of the attenuation of the angular correlations was investigated and a technique was formulated that allowed the extraction of absolute values of g factors. The approach of the simultaneous measurement of lifetimes and g factors promises success but needs further investigations regarding its feasibility limits and its application to radioactive ion beam experiments.
The investigated nuclei were compared to predictions from shell model calculations and collective models. In addition, systematics of essential nuclear quantities and their evolution in the mass region were discussed.