Shell Structure Evolution in Nuclei:. New Paradigm

Shell structure evolution in nuclei situated at the extremes of neutron and proton excess are investigated using in-beam gamma spectroscopy techniques with radioactive beams at GANIL. A selection of results obtained very recently is presented: i) The reduced transition probabilities {B(E2;0}_1^ + -> 2^ + ) of the neutron-rich ⁷⁴Zn and ⁷⁰Ni nuclei have been measured using Coulomb excitation at intermediate energy. An unexpected large proton core polarization has been found in ⁷⁰Ni and interpreted as being due to the monopole interaction between the neutron g_9/2 and protons f_7/2 and f_5/2 spin-orbit partner orbitals. ii) Two proton knock-out reactions has been performed in order to study the most neutron-rich nuclei at the N=28 shell closure. Gamma rays spectra and momentum distribution have been obtained for ⁴²Si and neighboring nuclei. Evidences has been found for a deformed structure for ⁴2Si and for the disappearance of the spherical N=28 shell effect. iii) The in-beam gamma spectroscopy of ³⁶Ca performed using neutron knock-out reactions revealed that N=16 is as large sub-shell closure as large as Z=16 in ³⁶S. The uniquely large excitation energy difference of the first 2⁺ state in these mirror nuclei turns out to be a consequence of the relatively pure neutron (in ³⁶Ca) or proton (in ³⁶S) 1p(d_3/2)-1h(s_1/2) nature.