Beta decay studies close to the doubly-magic nucleus 78Ni

Chapter 1 Nuclear structure far from stability towards 78 Ni In the present thesis I shall describe an experiment performed last year with the PARRNe-ALTO facility in Orsay, whose aim is the study of the exotic neutron-rich nuclei in the vicinity of 78 Ni. This is part of a series of experiments on the nuclei of this poorly explored region done in Orsay and in other laboratories as well (Cern, Oak Ridge). The 78 Ni nucleus, whose structure is still unknown, has 28 protons and 50 neutrons, and therefore in the framework given by the shell model it is expected to be one of the few doubly-magic nuclei (see fig. 1.1). Those “special” nuclei are used as cores for shell-model calculations, dramatically truncating the model space, thus rendering feasible shell-model calculations in heavy nuclei. The 78 Ni nucleus is also one of the waiting-points in the astrophysical r-process; to test the rigidity of its associated gaps is of special interest also for this reason. However, the study of 78 Ni nucleus is not yet feasible and it requires the next generation of radioactive beam facilities. One can anyway derive some informations on the properties of the Z = 28 and N = 50 shell gaps by studying the more accessible close-by nuclei. As I shall show in detail in the present thesis, we have studied the -decay of the neutron rich 84 Ga isotope and of its daughters. The nucleus of interest, as many others, was produced via a photo-fission reaction induced by an 50 MeV electron beam of 10 A on a thick UCx target. The gallium atoms were selec- tively ionized with a newly developed laser ion source. The ions were separated with the PARRNe mass separator and implanted on a movable mylar tape. Two germanium detectors in close geometry were used for the detection of -rays and - coincidence measurement, and a plastic 4 for beta tagging. The final results of this experiment are the improved level schemes of the neutron-rich 83;84 Ge and 84 As isotopes. 1