The masses of 31 neutron-rich nuclei in the range A = 29-47 have been measu
red. The precision of 19 masses has been significantly improved and 12 mass
es 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
the observed effects arise from deformed prolate ground state configuration
s associated with shape coexistence. Evidence for shape coexistence is prov
ided by the observation of an isomer in S-43.