Two-neutron transfer probabilities and spatial-localization effects at the drip line

Abstract

We examine Cr isotopes at the drip line, where surface effects related to the existence of a weakly bound $s1/2$ state are known to be important and tightly connected with the pairing phenomenon (anti-halo effect). For these weakly-bound isotopes, we evaluate the ground state to ground state two-neutron transfer probabilities within a mean-field-based approach. An important part of the discussion is devoted to the analysis of several procedures that can be employed to constrain the parameters of a phenomenological pairing interaction. The parameters are first adjusted to reproduce the experimental gaps evaluated with the five-point formula. This choice has however some consequences on the evolution of the pairing correlations along the isotopic chains and, in particular, at shell closures. Other procedures are then followed (adjustment on a theoretical pairing gap at mid-shell and on the two-neutron separation energies). For the transfer probabilities, we discuss the effects associated to different choices of the spatial localization of the pairing interaction. We indicate that the analysis of pair-transfer reactions for such cases (where the last bound state is a low-$l$ state in a weakly bound nucleus) may improve our understanding of two aspects: the spatial distribution of pairing correlations in nuclei and the general problem of the persistence of pairing at the drip lines.