Experimental study of 84Ga β decay: Evidence for a rapid onset of collectivity in the vicinity of 78Ni
Résumé
γ-rays following the β and β-n decays of the very neutron rich 3184Ga53 produced by the photofission of 238U have been studied at the newly built ISOL facility of IPN Orsay: ALTO. Two activities were observed and assigned to two β-decaying states: 84Gag, Iπ=(0-) and 84Gam, Iπ=(3-,4-). Excitation energies of the 21+ and 41+ states of 3284Ge52 were measured at E(21+)=624.3 keV and E(41+)=1670.1 keV. Comparison with HFB + GCM calculations allows to establish the collective character of this nucleus. The excitation energy of the 1/21+ state in 3283Ga51 known to carry a large part of the neutron 3s1/2 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 78Ni as a fragile and easily polarized doubly-magic core.