A First Glimpse at the Shell Structure beyond $^{54}$Ca: Spectroscopy of $^{55}$K, $^{55}$Ca, and $^{57}$Ca

TL;DR

This study uses in-beam gamma-ray spectroscopy to explore the shell structure beyond calcium-54, revealing new excited states in calcium and potassium isotopes and indicating a transition from single-particle to collective behavior.

Contribution

First experimental identification of excited states in $^{55}$K, $^{55}$Ca, and $^{57}$Ca, providing insights into shell evolution beyond $^{54}$Ca.

Findings

Excited states in $^{55}$K, $^{55}$Ca, and $^{57}$Ca were observed for the first time.

Results align with advanced theoretical models for level energies and lifetimes.

Evidence of a transition from single-particle to collective excitations at $N=37$ in calcium isotopes.

Abstract

States in the $N=35$ and 37 isotopes $^{55,57}$Ca have been populated by direct proton-induced nucleon removal reactions from $^{56,58}$Sc and $^{56}$Ca beams at the RIBF. In addition, the $(p,2p)$ quasi-free single-proton removal reaction from $^{56}$Ca was studied. Excited states in $^{55}$K, $^{55}$Ca, and $^{57}$Ca were established for the first time via in-beam $\gamma$-ray spectroscopy. Results for the proton and neutron removal reactions from $^{56}$Ca to states in $^{55}$K and $^{55}$Ca for the level energies, excited state lifetimes, and exclusive cross sections agree well with state-of-the-art theoretical calculations using different approaches. The observation of a short-lived state in $^{57}$Ca suggests a transition in the calcium isotopic chain from single-particle dominated states at $N=35$ to collective excitations at $N=37$.