Indispensability of cross-shell contributions in neutron resonance spacing

Spin- and parity-dependent nuclear level densities (NLDs) are obtained for a configuration interaction shell model using a numerically efficient spectral distribution method. The calculations are performed for 24 Na and 25,26,27 Mg nuclei using full sd - pf model space that incorporates the cross-shell excitations from the sd to the pf -shell. The obtained NLDs are then employed to determine the s-wave neutron resonance spacing (D 0 ), which is one of the crucial inputs for the predictions of astrophysical reaction rates. Although the considered nuclei are not neutron-rich, the contributions from cross-shell excitations to the pf -shell are indispensable for explaining the experimental data for D 0 , which otherwise are significantly overestimated.