N = 32 Shell Evolution in Calcium Isotopes: Evidence from Macroscopic to Microscopic Models

Abstract

The well-established N=32 subshell closure in calcium serves as an important benchmark for nuclear structure evolution toward neutron-rich nuclei. Recent charge-radius measurements of potassium isotopes [Nat. Phys. 17, 439-443 (2021)] reveal no corresponding signature of enhanced stability at the same neutron number. Motivated by this discrepancy, we investigate the structural evolution of even-even Ca isotopes from N=22 to 36 using macroscopic, relativistic mean-field (RMF), and large-scale shell-model approaches, with the KB3G, GXPF1A, GXPF1B, and FPD6 interactions in the KSHELL framework. The macroscopic analysis, evaluated against seven theoretical frameworks and NNDC data, reveals a coherent change in neutron structure at N=32, most clearly in the shell-gap energy $ΔE$ and the three-point binding-energy filter, corroborated by the charge-radius filter. The RMF spectrum with NL3* shows a clear $\nu2p_{3/2}$-$\nu2p_{1/2}$ separation consistent with this signature. KB3G and FPD6 reproduce the N=32 enhancement of $E(2_1^+)$ and reduction of $R_{4/2}$, whereas GXPF1A and GXPF1B give a weaker N=32 signature than N=34 signature, the reverse of the experimental pattern, showing that the two subshells depend differently on the monopole interaction. The deformation parameter and B(E2) values have been obtained with above four interactions and are in good agreement with the experimental results.