Precision mass measurements of Fe 67 and Co 69,70: Nuclear structure toward N=40 and impact on r -process reaction rates

Accurate mass measurements of neutron-rich iron and cobalt isotopes Fe-67 and Co-69,Co-70 have been realized with the JYFLTRAP double Penning-trap mass spectrometer. With novel ion-manipulation techniques, the masses of the Co-69,Co-70 ground states and the 1/2(-) isomer in Co-69 have been extracted for the first time. The measurements remove ambiguities in the previous mass values and yield a smoother trend on the mass surface, extending it beyond N = 40. The moderate N = 40 subshell gap has been found to weaken below Ni-68, a region known for shape coexistence and increased collectivity. The excitation energy for the 1/2(-) intruder state in Co-69 has been determined for the first time and is compared to large-scale shell-model calculations. The new mass values also reduce significantly mass-related uncertainties for the astrophysical rapid neutron-capture process calculations.