Evolution of the one-phonon mixed-symmetry state in the N=48 isotonic chain*

In collective nuclei one can find excitations that are not fully symmetric with respect to the proton-neutron degree of freedom. Those states are called mixed-symmetry states and in vibrator-like nuclei their fundamental one-phonon excitation is the $2^+_{\mathrm{ms}}$ state. Near the $N=50$ shell closure $2^+_{\mathrm{ms}}$ states are well known in the light stable nuclei of the $N=52$ isotonic chain, while the data base in the $N=48$ chain is sparse. A nuclear resonance fluorescence experiment on $^{84}$Kr was performed at the University of Stuttgart and $\gamma \gamma$ coincidence measurements after fusion evaporation reactions on $^{90}$Mo and $^{88}$Zr at the Cologne Tandem accelerator to investigate the low spin structure and collectivity of these nuclei. We will present results concerning the identification of candidates for the $2^+_{\mathrm{ms}}$ state in all three nuclei and discuss these results in the framework of the Interacting Boson Model. Finally we will compare the $N=48$ with the $N=52$ isotonic chain.