DREB Conference 2024

The neutron-deficient calcium isotopes have attracted considerable attention recently. Present studies are divided over the amount of proton pf-shell occupancy, ranging from an intact $Z=20$ shell closure [1] to a considerable weakening already in the vicinity of doubly-magic ${}^{40}$Ca [2,3].

Two-neutron removal, a direct reaction sensitive to the single-particle configurations and couplings of the removed neutrons in the projectile wave function, from ${}^{38}$Ca populating states of ${}^{36}$Ca was performed at the National Superconducting Cyclotron Laboratory. Inclusive and final-state exclusive cross sections along with longitudinal momentum distributions are compared to predictions combining eikonal reaction theory and shell-model two-nucleon amplitudes [4,5].

The results yield conclusive evidence for the need of sizeable proton cross-shell excitations into the pf shell already for the $0_1^{+}$ and $2_1^{+}$ states of ${}^{36}$Ca [6]. These findings furthermore enable a close reproduction of additional observables. Ultimately, a schematic modification of sd - pf shell gap is introduced serving as a proxy for the magnitude of proton cross-shell excitations.

[1] Miller et al., Nat. Phys. 15, 432 (2019).
[2] Caurier et al., Phys. Lett. B 522, 240 (2001).
[3] Dronchi et al., Phys. Rev. C 107, 034306 (2023).
[3] Tostevin et al., Phys. Rev. C 74, 064604 (2006).
[4] Simpson et al., Phys. Rev. Lett. 102, 132502 (2009).
[5] Beck et al., Phys. Rev. C 108, L061301 (2023).