$\boldsymbol{N=32}$ shell closure below calcium: Low-lying structure of $^{50}$Ar

TL;DR

This study investigates the low-lying excited states of the neutron-rich isotope $^{50}$Ar with gamma-ray spectroscopy, confirming known states, discovering new ones, and comparing results with shell-model and ab initio calculations.

Contribution

It provides new experimental data on $^{50}$Ar's excited states, including five previously unobserved states and a candidate for a 3$^-$ state, enhancing understanding of shell closure below calcium.

Findings

Confirmed energies of two known transitions.

Discovered five new excited states.

Suggested assignments of new states as 2$^+$ or 3$^-$.

Abstract

Low-lying excited states in the $N=32$ isotope $^{50}$Ar were investigated by in-beam $\gamma$-ray spectroscopy following proton- and neutron-knockout, multi-nucleon removal, and proton inelastic scattering at the RIKEN Radioactive Isotope Beam Factory. The energies of the two previously reported transitions have been confirmed, and five additional states are presented for the first time, including a candidate for a 3$^-$ state. The level scheme built using $\gamma\gamma$ coincidences was compared to shell-model calculations in the $sd-pf$ model space, and to ab initio predictions based on chiral two- and three-nucleon interactions. Theoretical proton- and neutron-knockout cross sections suggest that two of the new transitions correspond to $2^+$ states, while the previously proposed $4^+$ state could also correspond to a $2^+$ state.