Rapid onset of collectivity in the vicinity of 78Ni

gamma-rays following the B and B-n decay of the very neutron rich 84Ga produced by photo-fission of 238U have been studied at the newly built ISOL facility of IPN Orsay: ALTO. Two activities were observed and assigned to two B-decaying states: 84gGa, I = (0\^-) and 84mGa, I = (3\^-, 4\^-). Excitation energies of the 2+1 and 4+1 excited states of 84Ge 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 indicating a substantial N=50 core polarization. The excitation energy of the 1/2+1 state in 83Ga known to carry a large part of the neutron 3s1/2 strength was measured at 247.8keV. 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.Comment: 4 pages, ReVTe

The ALTO facility at IPN Orsay and study of neutron rich nuclei in the vincinity of 78^{78}Ni

International audienceThe ALTO facility at IPN Orsay is based on a linear electron accelerator (50 MeV, 10μA) dedicated to the production of neutron rich radiactive beams by photofission of a thick uranium carbide target paced on an ISOL line. The construction of the facility has ended in december 2005 and the first on-line tests have been carried out in mid-2006. The production of fission fragments after separation have been measured and are similar to the calculated predictions. Previously, using the same ISOL line, neutron rich nuclei in the vincinity of 78Ni have been produced and studied using the deuteron beam available at the Tandem accelerator in Orsay. First results on excited states in $^{83}_{32}Ge_{51}$ and $^{81}_{31}Ge_{50}$ have been obtained and discussed in the frame of the shell model