Experimental study of {sup 84}Ga {beta} decay: Evidence for a rapid onset of collectivity in the vicinity of {sup 78}Ni

{gamma}-rays following the {beta} and {beta}-n decays of the very neutron rich {sub 31}{sup 84}Ga{sub 53} produced by the photofission of {sup 238}U have been studied at the newly built ISOL facility of IPN Orsay: ALTO. Two activities were observed and assigned to two {beta}-decaying states: {sup 84}Ga{sup g}, I{sup {pi}}=(0{sup -}) and {sup 84}Ga{sup m}, I{sup {pi}}=(3{sup -},4{sup -}). Excitation energies of the 2{sub 1}{sup +} and 4{sub 1}{sup +} states of {sub 32}{sup 84}Ge{sub 52} were measured at E(2{sub 1}{sup +})=624.3 keV and E(4{sub 1}{sup +})=1670.1 keV. Comparison with HFB + GCM calculations allows to establish the collective character of this nucleus. The excitation energy of the 1/2{sub 1}{sup +} state in {sub 32}{sup 83}Ga{sub 51} known to carry a large part of the neutron 3s{sub 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 {sup 78}Ni as a fragile and easily polarized doubly-magic core.