Level structures of 56,58Ca cast doubt on a doubly magic 60Ca

Gamma decays were observed in Ca-56 and Ca-58 following quasi-free one-proton knockout reactions from Sc-57,Sc-59 beams at approximate to 200MeV/nucleon. For Ca-56, a gamma ray transition was measured to be 1456(12) keV, while for Ca-58 an indication for a transition was observed at 1115(34) keV. Both transitions were tentatively assigned as the 2(1)(+) -> 0(gs)(+) decays, and were compared to results from ab initioand 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 Ca-60 and potentially drives the dripline of Ca isotopes to Ca-70 or even beyond. (c) 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3.

Automated summary

Gamma decays in Ca-56 and Ca-58 were observed following quasi-free one-proton knockout reactions. The transitions were tentatively assigned as the 2(1)(+) -> 0(gs)(+) decays and compared to results from ab initio and conventional shell-model approaches. A new nuclear shell above the N=34 shell was confirmed, indicating a potential extension of the dripline of Ca isotopes.