Spectroscopy of 39,41Si and the border of the N=28 island of inversion

The structure of the very neutron-rich nuclei 39Si and 41Si has been investigated via in-beam γ-ray spectroscopy and few-nucleon knockout from radioactive beams. The observation of low-lying states in 39Si is a clear evidence for a drastic lowering of the intruder neutron 3/2− state when going from Z=20 to Z=14. The energy of the only γ-ray transition (672(14) keV) observed in 41Si is significantly lower than that of the first excited state in 47Ca (2014 keV) suggesting a deformed ground state for 41Si. These results are consistent with large-scale shell-model calculations implying that strong proton-neutron correlations are the main reason for the lowering of the intruder configurations over the normal ones.