Shape evolution of krypton isotopes calculated with axially deformed relativistic Hartree-Bogoliubov approach

Abstract

We perform a systematic study of the structure and properties of the krypton isotopic chain including both even-even and odd-$A$ nuclei based on the axially deformed relativistic Hartree-Bogoliubov approach. Five effective interactions of three families of covariant density functionals, i.e., PC-L3R, DD-PCX, DD-PC1, DD-MEX, and DD-ME2, are employed to calculate potential energy surfaces of krypton isotopes. $^{74,75}$Kr and $^{90,91,92}$Kr are determined as typical candidates of shape coexistence. The potential surfaces originating from the PC-L3R, DD-PCX, and DD-MEX interactions exhibit an abrupt shape transition from oblate to prolate for $^{73\text{-}74}$Kr, whereas DD-PC1 and DD-ME2 preserve an oblate ground-state shape. Such discrepancies are attributed to the occupations of single-particle levels at the vicinity of the Fermi surface described by these functionals. Moreover, the comparison between spherical and deformed calculations verifies the indispensability of deformation degrees of freedom in this region. The consideration of deformation effects improves the description of two-neutron separation energies, of which its evolution clearly demonstrates the $N=50$ and $82$ shell closures. Interestingly, PC-L3R predicts a more extended two-neutron drip line up to $^{132}$Kr, in agreement with the NL3* and PC-PK1 nonlinear effective interactions, whereas other functionals estimate a rather short isotopic chain up to $^{119}$Kr. This anomalous extension implies a significant softening or even collapse of the traditional $N=82$ shell closure near the neutron-rich drip line, highlighting the need for future studies based on triaxial deformation and beyond-mean-field correlations in this nuclear region.