Shell structure evolution far from stability: experimental results

Abstract

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 g9/2 and protons f7/2 and f5/2 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. Evidence has been found for a deformed structure at N = 28 for Silicon, despite a relatively large Z = 14 gap. 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 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 their relatively pure neutron or proton 1p(d3/2)-1h(s1/2) nature.