Although it is known that the $N=28$ spherical shell closure erodes, the strength of the closure with decreasing proton number $Z<20$ is an open question in nuclear structure. In this region of interest, high-precision mass measurements of neutron-rich ${}^{43-45}\mathrm{Cl}$ isotopes were performed at the Low Energy Beam and Ion Trap when coupled to the National Superconducting Cyclotron Lab. The resulting mass excesses (MEs) are $\mathrm{ME}\left({}^{43}\mathrm{Cl}\right)=-24114.4\left(1.7\right)\mathrm{keV}, \mathrm{ME}\left({}^{44}\mathrm{Cl}\right)$ = −20 450.8(10.6) keV, and $\mathrm{ME}\left({}^{45}\mathrm{Cl}\right)=-18240.1\left(3.7\right)\mathrm{keV}$, and the uncertainty of these masses is improved by up to a factor of $\sim 40$ compared to the previous values reported in the 2020 Atomic Mass Evaluation. Comparison to ab initio calculations using the Valence-Space In-Medium Similarity Renormalization Group shows good agreement up to and including the closure.

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• Binding energy & masses
• Nuclear binding
• Nuclear structure & decays
• Rare & new isotopes
• 39 ≤ A ≤ 58
• Mass spectrometry
• Penning traps
• Precision measurements
• Radioactive beams