Shape coexistence and the N=28 shell closure far from stability

The masses of 31 neutron rich nuclei in the range A=29-47 have been measured. The precision of 19 masses has been significantly improved and 12 masses were measured for the first time. The neutron rich Cl, S and P isotopes are seen to exhibit a change in shell structure around N=28. Comparison with shell model and relativistic mean field calculations demonstrate that deformed prolate ground state configurations associated with shape coexistence are necessary to explain the experimental data. Direct evidence for shape coexistence is found in the observation of an isomer in 43S. (authors)