Abstract
The quadrupole collectivity of low-lying states and the anomalous behavior of the $0^+_2$ and $2^+_3$ levels in $^{72}$Ge are investigated via projectile multi-step Coulomb excitation with GRETINA and CHICO-2. A total of forty six $E2$ and $M1$ matrix elements connecting fourteen low-lying levels were determined using the least-squares search code, gosia. Evidence for triaxiality and shape coexistence, based on the model-independent shape invariants deduced from the Kumar-Cline sum rule, is presented. These are interpreted using a simple two-state mixing model as well as multistate mixing calculations carried out within the framework of the triaxial rotor model. The results represent a significant milestone towards the understanding of the unusual structure of this nucleus.
Highlights
The structure of low-lying states in even-even Ge isotopes has been the subject of intense scrutiny for many years due to the inherent challenge of interpreting their systematics as a function of mass A
For example models based on boson degrees of freedom [27], and most notably the interacting boson model [28], but to produce a first excited 0+ state usually requires extreme parameter choices fitted to the nucleus under investigation
Other models with quadrupole collectivity introduce “pair-excitations” in an ad hoc way. These approaches are motivated by the shape coexistence phenomena observed at and near closed shells
Summary
The structure of low-lying states in even-even Ge isotopes has been the subject of intense scrutiny for many years due to the inherent challenge of interpreting their systematics as a function of mass A. The quadrupole collectivity of low-lying states and the anomalous behavior of the 0+2 and 2+3 levels in 72Ge are investigated via projectile multi-step Coulomb excitation with GRETINA and CHICO-2.
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