Mass measurements of neutron-rich nuclei near N=70

The astrophysical origin for the chemical elements between the first and second r-process peaks is a matter of intense debate, with a number of nucleosynthesis processes at explosive stellar environments possibly contributing to their production. Modeling neutron-capture processes that would produce these elements requires reliable data on the trends of neutron separation energies of neutron-rich isotopes which are highly unstable and not readily accessible by experiment. This work describes the first application of an experimental technique, time-of-flight-magnetic-rigidity (ToF-Bρ), that is well-suited to measure masses of nuclei with very short half-lives in beams with relatively low intensities. The two-neutron separation energy deduced from the measured masses exhibits a smooth trend consistent with theoretical predictions within the range of experimental uncertainty, indicating that there is no sudden shape transition in these isotopes as hinted at by previous data. The successful application of the ToF-Bρ technique to isotopes with $Z>28$ at the NSCL S800 spectrograph gives hope for a comprehensive program of mass measurements for isotopes relevant to r-process models with the same device at FRIB.