Collapse of the N=28 shell closure in the newly discovered Na 39 nucleus and the development of deformed halos towards the neutron dripline

Abstract

Halos and changes of nuclear magicities have been extensively investigated in exotic nuclei during past decades. The newly discovered Na39 with the neutron number N=28 provides a new platform to explore such novel phenomena near the neutron dripline of the sodium isotopic chain. We study the shell property and the possible halo structure in Na39 within the deformed relativistic Hartree-Bogoliubov theory in continuum. It is found that the lowering of 2p orbitals in the spherical limit results in the collapse of the N=28 shell closure in Na39, and a well-deformed ground state is established. The pairing correlations and the mixing of pf components driven by deformation lead to the occupation of weakly bound or continuum p-wave neutron orbitals. An oblate halo is therefore formed around the prolate core in Na39,41, making Na39 a single nucleus with the coexistence of several exotic structures, including the quenched N=28 shell closure, Borromean structure, deformed halo, and shape decoupling. The microscopic mechanisms behind the shape decoupling phenomenon and the development of halos towards dripline are revealed. © 2023 American Physical Society.