Triaxiality and shape coexistence in $^{72,76}$Ge: A model independent analysis.*

An exploration of the structure of Ge isotopes is important for understanding the microscopic origin of collectivity, the nature of deformation and modifications of shell structure in nuclei of the $N\sim40$ mass region. The present study focuses on the electromagnetic properties of low-lying states in $^{72,76}$Ge obtained via sub-barrier multiple Coulomb excitation with GRETINA and CHICO2. In the case of $^{72}$Ge, the extracted matrix elements agree with a shape coexistence interpretation between the $0_1^+$ and $0_2^+$ states, but require significant mixing between the $0^+$ wavefunctions as well as triaxiality in order to reproduce the data. Similarly, the invariant sum-rule analysis of the $^{76}$Ge data indicates that both the ground state and gamma bands are characterized by the same deformation parameters, with triaxiality $(\gamma \sim 30^\circ)$ being important for a complete description. A summary of these results and data highlighting the nature of gamma deformation in $^{76}$Ge - whether rigid or soft - will be presented.