Signature splitting and non-axiality in the nuclei ¹⁶⁴Ta and ¹⁶⁵Ta
Doctoral Thesis
2001
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University of Cape Town
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Abstract
High-spin states in two deformed nuclei (¹⁶⁴Ta and ¹⁶⁵Ta) have been studied in two separate experiments with the AFRODITE escape suppressed spectrometer array at the National Accelerator Centre, near Cape Town. For both of these nuclei, definitive rotational band structures up to high spins have been established for the first time. Excited states in ¹⁶⁵Ta (Z = 73, N = 92) were populated in both the first experiment, ¹⁴²Nd(²⁷Al,4n) ¹⁶⁵Ta, and the second experiment, ¹⁴¹Pr(²⁸Si,4n) ¹⁶⁵Ta. The yrast rotational decay sequence, identified up to spin 53/2⁻, has been assigned the π[514]9 /2⁻ configuration. A comparison of the experimental results with predictions of Cranking Shell Model (CSM) calculations reveal an unexpectedly large signature splitting, for a high-Ω configuration, for the yrast band. Further discrepancies between theory and experiment are observed for the bandcrossing frequency and the signature splitting of the B(Ml)/ B(E2) ratios. The possibility that these discrepancies are associated with a substantial deviation from an axially symmetric shape is explored. In addition a weaker excited band, possibly associated with the π[402]5/2⁺ configuration, has also been identified. High-spin states in ¹⁶⁴Ta ( Z = 73, N = 91) were populated in the first experiment. The yrast band, identified up to spin 21⁻, has been assigned the configuration π[514]9/2⁻X v[660]1/2⁺. Anomalous signature splitting of the yrast band is observed at low spins. The splitting is discussed in relation to the systematic trends and its implications for the nuclear shape are considered. Previous attempts to explain the phenomenon are reviewed. The 11⁻ level was found to be isomeric, both in this nucleus and in its N = 91 isotone ¹⁶²Lu, a by-product of the first experiment. This is the first time that a nanosecond-range isomer has been found in the light tantalum or lutetium nuclei. A less intense excited band was also identified, and was tentatively associated with a four-quasiparticle configuration.
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Roux, D. 2001. Signature splitting and non-axiality in the nuclei ¹⁶⁴Ta and ¹⁶⁵Ta. University of Cape Town.