Physics of low and intermediate spin states of atomic nuclei in the mass A ~ 160 region
The nuclear landscape provides possibly the richest set of data that can be collected for quantum systems, displaying many varying features. The lack of a complete nuclear theory has resulted in a fragmentation of the theories used to describe isolated sections of observables in the landscape of nuclear excitations. A recent model proposed that a tetrahedral deformation could explain observed low-lying, negative parity bands characterized by a lack of in-band electromagnetic transitions. For this work, experimental data has been collected specifically to quantify the intensities of the "missing" in-band transitions for several nuclei in the mass 160 region (156 68Er, 160 68Er and 158 66Dy). While these in-band transitions appear suppressed, it was possible to not only observe some of these transitions, but to quantify the suppression, which indicate a consistency within the band structure between low and medium spin states. An order of magnitude difference was observed in the B(E2)/B(E1) branching ratios between the odd-spin and the even-spin octupole vibrational bands in ¹⁵⁸Dy and ¹⁶⁰Er. This difference is consistent with that reported in other nuclei in the region, which is indicative of some shared reason for this behaviour. The theoretical aspect of this work tries to explain the observed anomalies in terms of the Random Phase Approximation model. Results of calculations suggest that in the intrinsic frame, the negative signature collective modes will have significantly stronger absolute B(E1) rates than the positive signature band partners. By considering a quadrupole deformation of these vibrational states as consistent with the ground state band, the B(E2) rates can be estimated, and the theoretical value for the B(E2)/B(E1) branching ratio is shown to be qualitatively consistent with experimental observations. From this we conclude that the negative parity vibrational bands in the mass 160 region are likely low-K octupole bands, and not tetrahedrally deformed structures.
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