Weakly bounds1∕2neutrons in the many-body pair correlation of neutron drip line nuclei

Abstract

With a simplified model in the Hartree-Fock-Bogoliubov (HFB) approximation, the behavior of weakly bound ${s}_{1∕2}$ neutrons in the many-body pair correlation is studied by solving the HFB equation in coordinate space with the correct asymptotic boundary conditions. It is shown that in one-neutron pickup reactions on the even-even neutron-drip-line nuclei, which contain loosely bound ${s}_{1∕2}$ neutrons, the strength of the ${s}_{1∕2}$ neutron can appear both at a discrete state and in the low-energy continuum spectra, with comparable strength. When there is no weakly bound discrete state, the continuum spectra may exhibit a sharp peak just above ${E}_{x}=\ensuremath{\mid}\ensuremath{\lambda}\ensuremath{\mid}$, which originates from the resonantlike behavior of the upper component of the HFB radial wave function, ${u}_{s1∕2}({E}_{qp},r)$. This resonantlike behavior may be directly observed as an $s$-wave resonance close to ${E}_{x}=\ensuremath{\mid}\ensuremath{\lambda}\ensuremath{\mid}$ in neutron-scattering experiments on those nuclei. It is also shown that a very large root-mean-square radius of loosely bound ${s}_{1∕2}$ neutrons may appear also in the presence of many-body pair correlation, since the effective pair gap in weakly bound neutron orbits with low $\ensuremath{\ell}$ values is much reduced.