Pairing anti-halo effect

We discuss pairing correlations in weakly bound neutron rich nuclei, by using the coordinate-space Hartree-Fock-Bogolyubov approach which allows to take into account the coupling to particle continuum. We show that the additional pairing binding energy acts against a development of an infinite rms radius that characterizes standard l = 0 mean-field eigenfunctions in the limit of vanishing binding energy. As a result, neutron radii of even-N nuclei do not diverge in the limit of vanishing Fermi energy. Only the broken-pair ground states of odd-N nuclei can exhibit diverging neutron radii, provided an l = 0 (or 1) quasiparticle state appears near the Fermi surface. We also show that the pairing-increased (although not infinite) rms radii of even-N nuclei result from the coupling to low-lying l = 0 continuum, which is always available for virtual pair excitations, independently of what are the angular momenta of least-bound single-particle levels. (C) 2000 Published by Elsevier Science B.V.