Level Structures of $^{56,58}$Ca Cast Doubt on a doubly magic $^{60}$Ca

TL;DR

This study investigates the nuclear structure of calcium isotopes $^{56}$Ca and $^{58}$Ca through gamma decay measurements, revealing a new shell structure that challenges the doubly magic nature of $^{60}$Ca and suggests a more extended calcium dripline.

Contribution

The paper provides experimental gamma decay data and shell-model calculations that identify a new shell above N=34, questioning the doubly magic status of $^{60}$Ca.

Findings

Observation of gamma transitions at 1456 keV and 1115 keV in $^{56}$Ca and $^{58}$Ca

Shell-model calculations agree with experimental level energies and separation energies

Degeneracy of $0f_{5/2}$ and $0g_{9/2}$ orbitals precludes doubly magic $^{60}$Ca

Abstract

Gamma decays were observed in $^{56}$Ca and $^{58}$Ca following quasi-free one-proton knockout reactions from $^{57,59}$Sc beams at $\approx 200$ MeV/nucleon. For $^{56}$Ca, a $\gamma$ ray transition was measured to be 1456(12) keV, while for $^{58}$Ca an indication for a transition was observed at 1115(34) keV. Both transitions were tentatively assigned as the $2^+_1 \rightarrow 0^+_{gs}$ decays, and were compared to results from ab initio and conventional shell-model approaches. A shell-model calculation in a wide model space with a marginally modified effective nucleon-nucleon interaction depicts excellent agreement with experiment for $2^+_1$ level energies, two-neutron separation energies, and reaction cross sections, corroborating the formation of a new nuclear shell above the $N$ = 34 shell. Its constituents, the $0f_{5/2}$ and $0g_{9/2}$ orbitals, are almost degenerate. This degeneracy precludes the possibility for a doubly magic $^{60}$Ca and potentially drives the dripline of Ca isotopes to $^{70}$Ca or even beyond.