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First direct lifetime measurement of the 2+1 state in 72,74Zn: new evidence for shape transition between N = 40 and N = 42 close to Z = 28

We report here the first direct lifetime measurement of the 2+1 state in 72,74Zn. The neutron-rich beam was produced by in-flight fragmentation of 76Ge at the Grand Accélérateur National d'Ions Lourds and separated with the LISE spectrometer. The 2+1 state was excited by inelastic scattering and knock-out reaction on a CD2 target and its lifetime was measured by the recoil-distance Dopplershift method with the Köln plunger device combined with the EXOGAM detectors. The lifetimes of the 2+1 states in 72,74Zn were determined to be 17.9(18) and 27.0(24) ps, which correspond to reduced transition probabilities B(E2; 2+1 \rightarrow 0+) = 385(39) and 370(33) e2fm4, respectively. These values support the idea of a systematic maximum of collectivity at N = 42 for Zn, Ge and Se nuclei. In addition, the available systematics in the neighboring nuclei point towards a transition from a spherical oscillator at N = 40 to complete γ-softness at N = 42.