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

Abstract

Accurate mass measurements of neutron-rich iron and cobalt isotopes Fe67 and Co69,70 have been realized with the JYFLTRAP double Penning-trap mass spectrometer. With novel ion-manipulation techniques, the masses of the Co69,70 ground states and the 1/2- isomer in Co69 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 Ni68, a region known for shape coexistence and increased collectivity. The excitation energy for the 1/2- intruder state in Co69 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.