Pair correlation in neutron drip line nuclei

The Hartree-Fock-Bogoliubov (HFB) equation in a simplified model is solved in coordinate space with the correct asymptotic boundary conditions, in order to study the pair correlation in nuclei close to the neutron drip line. The occupation probability obtained in the HFB approach for lower-l orbits decreases considerably already when the binding energy of the corresponding Hartree-Fock (HF) one-particle level becomes smaller. When the HF one-particle level enters into the continuum, the lower-l orbit soon becomes almost unavailable for the pair correlation of the many-body system. In contrast, the contribution to the pair correlation by high-l orbits which have one-particle resonant levels with narrow width in the HF potential can be estimated in the BCS approximation using the one-particle resonant energies, though the associated wave functions in the HFB approach are bound-state wave functions.