Novel Delta J=1 Sequence In Ge-78: Possible Evidence For Triaxiality

A sequence of low-energy levels in Ge-78(32)46 has been identified with spins and parity of 2(+), 3(+), 4(+), 5(+), and 6(+). Decays within this band proceed strictly through Delta J = 1 transitions, unlike similar sequences in neighboring Ge and Se nuclei. Above the 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 gamma-rigid structure. The energies and branching ratios of many of the levels arc described well by shell-model calculations. However, the calculated reduced transition probabilities for the Delta J = 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 = 30 degrees, 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 are calculated at too high an excitation energy.