Probing proton cross-shell excitations through the two-neutron removal from 38Ca

T. Beck, A. Gade, B. A. Brown, J. A. Tostevin, D. Weisshaar, D. Bazin, K. W. Brown, R. J. Charity, P. J. Farris, S. A. Gillespie, A. M. Hill, J. Li, B. Longfellow, W. Reviol, D. Rhodes
Nuclear Experiment, Nuclear Experiment (nucl-ex)
Phys. Rev. C 108, L061301 (2023)
2024-01-15 00:00:00
Bound states of the neutron-deficient, near-dripline nucleus $^{36}$Ca were populated in two-neutron removal from the ground state of $^{38}$Ca, a direct reaction sensitive to the single-particle configurations and couplings of the removed neutrons in the projectile wave function. Final-state exclusive cross sections for the formation of $^{36}$Ca and the corresponding longitudinal momentum distributions, both determined through the combination of particle and $\gamma$-ray spectroscopy, are compared to predictions combining eikonal reaction theory and shell-model two-nucleon amplitudes from the USDB, USDC, and ZBM2 effective interactions. The final-state cross-section ratio $\sigma(2^+_1)/\sigma(0^+)$ shows particular sensitivity and is approximately reproduced only with the two-nucleon amplitudes from the ZBM2 effective interaction that includes proton cross-shell excitations into the $pf$ shell. Characterizing the proton $pf$-shell occupancy locally and schematically, an increase of the $sd$-$pf$ shell gap by $250$ keV yields an improved description of this cross-section ratio and simultaneously enables a reproduction of the $B(E2;0^+_1\to2^+_1)$ excitation strength of $^{36}$Ca. This highlights an important aspect if a new shell-model effective interaction for the region was to be developed on the quest to model the neutron-deficient Ca isotopes and surrounding nuclei whose structure is impacted by proton cross-shell excitations.
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