Experimental study of 84Ga β decay: Evidence for a rapid onset of collectivity in the vicinity of 78Ni

γ-rays following the β and β-n decays of the very neutron rich 84 31 Ga 53 produced by the photofission of 238 U have been studied at the newly built ISOL facility of IPN Orsay: ALTO. Two activities were observed and assigned to two β-decaying states: 84 Ga g , I π = (0 - ) and 84 Ga m , I π = (3-, 4 - ). Excitation energies of the 2 + 1 and 4 + 1 states of 84 32 Ge 52 were measured at E(2 1 + ) = 624.3 keV and E(4 1 + ) = 1670.1 keV. Comparison with HFB + GCM calculations allows to establish the collective character of this nucleus. The excitation energy of the 1/2 1 + state in 83 32 Ga 51 known to carry a large part of the neutron 3s 1/2 strength was measured at 247.8 keV. Altogether these data allow to confirm the new single particle state ordering which appears immediately after the double Z = 28 and N = 50 shell closure and to designate 78 Ni as a fragile and easily polarized doubly-magic core.