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E2 rotational invariants of 01+ and 21+ states for 106Cd: The emergence of collective rotation

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Physics Letters B
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The collective structure of 106Cd is elucidated by multi-step Coulomb excitation of a 3.849 MeV/A beam of 106Cd on a 1.1 mg/cm2 208Pb target using GRETINA-CHICO2 at ATLAS which yielded 14 E2 matrix elements. The nucleus 106Cd is a prime example of emergent collectivity that possesses a simple structure: it is free of complexity caused by shape coexistence and has a small but collectively active number of valence nucleons. This work follows in a long and currently active quest to answer the fundamental question of the origin of nuclear collectivity and deformation, notably in the cadmium isotopes. The results are discussed in terms of phenomenological models, the shell model, and Kumar-Cline sums of E2 matrix elements. The ⟨0+2 ||E2||2+1 ⟩ matrix element is determined for the first time, providing a total, converged measure of the electric quadrupole strength, ⟨Q2⟩, of the first-excited 2+1 state relative to the 0+1 ground state. Strong evidence for triaxial shapes in weakly collective nuclei is indicated; collective vibrations are excluded. This is contrary to the only other cadmium result of this kind in 114Cd by C. Fahlander et al. (1988) [38], which is complicated by low-lying shape coexistence near midshell.