Penning trap mass spectrometry is presented as a complementary tool to nuclear spectroscopy experiments for the study of nuclear structure in the vicinity of $N=40$, $Z=28$. High-precision mass measurements of the ${}^{63-66}\mathrm{Fe}$ and ${}^{64-67}\mathrm{Co}$ isotopes have been carried out with the Low Energy Beam and Ion Trap (LEBIT) Penning trap mass spectrometer. The newly obtained mass values for ${}^{66}\mathrm{Fe}$ and ${}^{67}\mathrm{Co}$ are presented, together with the previously reported LEBIT mass measurements in this region. In the case of ${}^{65}\mathrm{Fe}$ the existence of a new isomer is reported, and an isomer recently discovered by decay spectroscopy in ${}^{67}\mathrm{Co}$ is confirmed. Relative mass uncertainties as low as $4\times {10}^{-8}$ are obtained. All mass values are found to be in good agreement with previous experimental results with the exception of ${}^{64}\mathrm{Co}$, where a $5\sigma$ deviation is observed. Using these data the two neutron separation energies $S_{2n}$ are calculated. However, the large error bars in the mass values of the neighbor Fe and Co isotopes with $N>40$ complicate the validation of a weak subshell closure at $N=40$ for the Co isotopes or the possible reduction in the neutron shell gap in the case of the Fe isotopes, in accordance with the theoretical predictions of an onset of deformation in the region.

• Received 15 January 2010

DOI:https://doi.org/10.1103/PhysRevC.81.044318