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

TL;DR

This paper reports high-precision mass measurements of neutron-rich Fe isotopes near N=40, revealing the island of inversion's deformation and providing critical data to validate nuclear models and understand the strong interaction.

Contribution

It introduces a novel high-resolution mass spectrometry technique to measure neutron-rich Fe isotopes, including first-time measurements and discovery of a long-lived isomeric state, advancing nuclear structure understanding.

Findings

Confirmed maximum deformation at N=40 in Fe isotopes

Achieved direct mass measurements with δm/m ~ 10^{-7}

Discovered a long-lived isomeric state in ^69Fe

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 $600\,000$ 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.