Novel ΔJ=1 Sequence in Ge78 : Possible Evidence for Triaxiali

Abstract

A sequence of low-energy levels in _{32}^{78}Ge_{46} has been identified with spins and parity of 2^{+}, 3^{+}, 4^{+}, 5^{+}, and 6^{+}. Decays within this band proceed strictly through ΔJ=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 γ-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 ΔJ=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 γ=30°, there are sequences of higher-spin levels connected by strong ΔJ=1 transitions which decay in the same manner as those observed experimentally, yet are calculated at too high an excitation energy.

Evidence for Rigid Triaxial Deformation in Ge76 from a Model-Independent Analysis

“…[32] indicate also a significant triaxiality with a dominant prolate component, but the degree of rigidity could not be determined. The potential might well be softer in this instance, as the odd-even staggering in the γ band is opposite to that seen in 76 Ge, but is in line with that reported for the other even Se and Kr isotopes of the region as well as for all other Ge isotopes with the exception of 78 Ge [21].…”

“…As mentioned above, experimental evidence for lowspin rigid-triaxial deformation was recently proposed, based on the energy pattern of the low-spin structure of 76 Ge [18], where the phase of the odd-even staggering in the γ band is consistent with DF model predictions. Since then, a similar pattern was observed in 78 Ge as well [21]. Note that the phase of the energy staggering in the DF model is opposite to that of the WJ one, and has been suggested to be a measure of the degree of stiffness or softness of the γ deformation [22].…”

An experimental view on shape coexistence in nuclei

“…Triaxiality plays an important role in the structure of Ge nuclei. Gamma bands were identified in all Ge isotopes with 66 ≤ A ≤ 78, and the level staggering observed in the γ band in 76 Ge [202], as well as the decay pattern with enhanced ∆I = 1 transitions for that in 78 Ge [203], were discussed in the context of rigid triaxial deformation. Quadrupole invariants Q 3 cos 3δ were obtained for multiple states in both ground-state and γ bands in 72 Ge [196] and 76 Ge [201].…”

Gamma-ray spectroscopy of fission fragments with state-of-the-art techniques

“…While static triaxial deformation has been firmly established at high spin (in terms of wobbling mode [209][210][211] and breaking of chiral symmetry under rotation [212,213]), the existence of stable triaxiality at low spin, including the ground state, is still a subject of great debate. Rare examples of rigid triaxiality have been found in low-lying structures of moderately neutron-rich 76,78 Ge isotopes [214,215], following transfer and deep-inelastic reactions, while most of the nuclei, for any region of the nuclear chart, are observed to exhibit vibrational modes or "softness" with respect to the triaxiality parameter γ . As early discussed by Skalski et al in Ref.…”