Mapping the $N=40$ Island of Inversion: Precision Mass Measurements of Neutron-rich Fe Isotopes

Abstract

Nuclear properties across the chart of nuclides are key to improving and validating our understanding of the strong interaction in nuclear physics. We present high-precision mass measurements of neutron-rich Fe isotopes performed at the TITAN facility. The multiple-reflection time-of-flight mass spectrometer (MR-ToF-MS), achieving a resolving power greater than $600000$ for the first time, enabled the measurement of ${}^{63-70}$Fe, including first-time high-precision direct measurements ($\delta m/m\sim 10^{-7}$) of ${}^{68-70}$Fe, as well as the discovery of a long-lived isomeric state in ${}^{69}$Fe. These measurements are accompanied by both mean-field and ab initio calculations using the most recent realizations which enable theoretical assignment of the spin-parities of the ${}^{69}$Fe ground and isomeric states. Together with mean-field calculations of quadrupole deformation parameters for the Fe isotope chain, these results benchmark a maximum of deformation in the $N=40$ island of inversion in Fe, and shed light on trends in level densities indicated in the newly-refined mass surface.

Comments: 9 pages, 6 figures