Ground state properties of manganese isotopes across the N=28 shell closure

The first optical study of the N=28 shell closure in manganese is reported. Mean-square charge radii and quadrupole moments, obtained for ground and isomeric states in 50-56Mn, are extracted using new calculations of atomic factors. The charge radii show a well defined shell closure at the magic number. The behaviour of the charge radii is strikingly different to that of the neutron separation energies where no shell effect can be observed. The nuclear parameters can be successfully described by large scale shell model calculations using the GXPF1A interaction.