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Shell evolution in the vicinity of $^{68}$Ni has recently attracted many theoretical and experimental investigations. By now it has been clearly established that the presumed subshell closure at N=40 is not very pronounced. While the intruder character of the $1g_{9/2}$ and $2d_{5/2}$ neutron orbital induces collectivity by pair excitations from the $fp$ shell into the $g_{9/2}$ orbital, the parity change hinders quadrupole excitations and therefore mimics the properties of a doubly magic nucleus in $^{68}$Ni, i.e. a high 2$^+_1$ energy and a low B(E2; 2$^+$ $\to$ 0$^+$) value. Adding valence nucleons to the N=40 open shell leads to a rapid increase of collectivity, with an interplay of both collective and single-particle degrees of freedom. Such rapid changes indicate underlying complex effects and make this region ideal for testing theoretical calculations.
While measurements of B(E2; 2$^+$ $\to$ 0$^+$) values are useful to investigate the evolution of collectivity along isotopic chains, even more insight into the collective behavior can be gained by measuring lifetimes of higher-lying states. Almost all stable Zn isotopes present an anomalously low B(E2; 4$^+$ $\to$ 2$^+$)/B(E2; 2$^+$ $\to$ 0$^+$) ratio of 1 or less, which is normally observed only around closed shells. Coulomb excitation studies at REX-ISOLDE ($^{74,76}$Zn [1]) as well as a DSAM lifetime measurement in $^{70}$Zn [2] suggested an important increase of collectivity of the 4$^+$ state for heavy Zn isotopes with a maximum at N=40. However, a recent RDDS lifetime measurement performed with AGATA Demonstrator in Legnaro [3] yielded lifetimes of the 4$^+$ states in $^{70-74}$Zn that are considerably longer and correspond again to B(E2; 4$^+$ $\to$ 2$^+$)/B(E2; 2$^+$ $\to$ 0$^+$) ratio lower than 1. This has been confirmed by the results of another RDDS measurement performed at GANIL [4] for $^{70,72}$Zn.
The ISOLDE facility finished in 2016 the first phase of a major upgrade in terms of the energy of post-accelerated exotic beams, bringing it up from 3 MeV/u to 5.5 MeV/u. The increased beam energy strongly enhances the probability of multi-step Coulomb excitation, giving experimental access to new excited states and bringing in-depth information on their structure. The very first HIE-ISOLDE experiment in October 2015 and its continuation in 2016 have been dedicated to the study of the evolution of the nuclear structure along the zinc isotopic chain. The results discriminate between the two experimental values of B(E2; 4$^+$ $\to$ 2$^+$) in $^{74}$Zn, and the B(E2; 4$^+$ $\to$ 2$^+$) value in $^{78}$Zn has been obtained for the first time.
[1] J. Van de Walle et al., Phys. Rev. C79 (2009) 014309
[2] D. Muecher et al., Phys. Rev. C79 (2009) 054310
[3] C. Louchart et al, Phys. Rev. C87 (2013) 054302
[4] I. Celikovic et al, Acta Phys.Pol. B44, 375 (2013)
