DESCRIPTION OF DRIP-LINE NUCLEI WITHIN THE RELATIVISTIC MEAN FIELD PLUS BCS APPROACH

Abstract

Recently, it has been demonstrated, considering Ni and Ca isotopes as prototypes, that the relativistic mean-field plus BCS (RMF+BCS) approach wherein the single particle continuum corresponding to the RMF is replaced by a set of discrete positive energy states for the calculation of pairing energy provides a good approximation to the full relativistic Hartree–Bogoliubov (RHB) description of the ground state properties of drip-line neutron rich nuclei. The applicability of the RMF+BCS approach even for the drip-line nuclei is essentially due to the fact that the main contribution to the pairing correlations for the neutron rich nuclei is provided by the low-lying resonant states, in addition to the contributions coming from the states close to the Fermi surface. In order to show the general validity of this approach we present the results of our detailed calculations for the ground state properties of the chains of isotopes of O, Ca, Ni, Zr, Sn and Pb nuclei. The TMA force parameter set has been used for the effective mean-field Lagrangian with nonlinear terms for the sigma and omega mesons. Further, to check the validity of our treatment for different mean-field descriptions, calculations have also been carried out for the NL-SH force parametrization usually employed for the description of drip-line nuclei. Comprehensive results for the two neutron separation energy, rms radii, single particle pairing gaps and pairing energies etc. are presented. In particular, the Ca isotopes are found to exhibit distinct features near the neutron drip line whereby it is found that further addition of neutrons causes a rapid increase in the neutron rms radius with almost no increase in the binding energy, indicating the occurrence of halos. This is mainly caused by the pairing correlations and results in the existence of bound states of extremely neutron rich exotic nuclei. Similar characteristics, though less pronounced, are also exhibited by neutron rich Zr isotopes. A comparison of these results with the available experimental data and with the recent continuum relativistic Hartree–Bogoliubov (RCHB) calculations amply demonstrates the validity and usefulness of this fast RMF+BCS approach for the description of nuclei including those near the drip-lines.