Magic numbers at gamma = 30o : level structure and shape in 78Ge and adjacent nuclei

by William B Walters (University of Maryland)

Wednesday 23 May 2018, 14:30 → 15:30 Europe/Zurich

An irregular sequence of levels connected by Delta_J = 1transitions has been identified in 78Ge46 that populate the 2_2^+ level with the unique feature of the absence of Delta_J = 2 transitions. Decays within this band proceed strictly through Delta_J = 1 transitions, unlike similar sequences in neighboring 32Ge and Se nuclei. Above the 2_2^+ level, members of this sequence do not decay into the ground-state band. Moreover, the energy staggering of this sequence has the phase that would be expected for a g-rigid structure. The energies and branching ratios of many of the levels are described well by shell-model calculations. However, the calculated reduced transition probabilities for the DeltaJ = 2 in-band transitions imply that they should have been observed, in contradiction with the experiment. Within the calculations of Davydov, Filippov, and Rostovsky for rigid-triaxial rotors with gamma = 30o , there are sequences of higher-spin levels connected by strong Delta_J = 1 transitions which decay in the same manner as those observed experimentally, yet calculated at too high an excitation energy. Such features were anticipated many years ago by Larsson et al. and Ragnarsson et al. who suggested that magic numbers at gamma = 30o would be found at 26, 32, 44, and 46. To approach the uniqueness of this sequence, levels in adjacent nuclei were studied, with some attention paid to the unknown position of the single-proton g9/2 state, a feature where ISOLDE data for 81Ga50 may be important.
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