β-decay studies at the N=28 shell closure: indications for a weakening of the spin-orbit force far from stability ?

A β -decay experiment on nuclei in the region of N=28 has been performed at the GANIL facility. New measured half-lives for the Si isotopes from N=25 to N=28 are reported and discussed in the light of the deformation occuring in this region. Comparison with QRPA calculations suggests that a weakening of the spin-orbit force occurs for the very neutron-rich Si isotopes.