Mass measurements of neutron-rich nuclei near the N=20 and 28 shell closures

Mass measurements of very neutron-rich nuclei near the N=20 and 28 shell closures are presented. Seven masses have been determined for the first time and the precision of 36 masses has been significantly improved. These results are used to investigate the evolution of the odd-even staggering of binding energies with neutron number. Special attention is paid to the evolution of the N=28 shell closure as the neutron dripline is approached. Changes in shell structure are observed around N=28 for the P and S isotopes but not for Si. This may be interpreted as a persistence of the shell closure at N=28 or as the result of a very sudden onset in deformation at 42Si.