A simple superfluid model with an effective four-body interaction of the monopole pairing type is used to explain the staggering of charge radii in the isotope chains. The contributions of deformation and of particle number projection are analyzed for the Sn isotopes. Good results are obtained for the staggering parameters and neutron pairing energies.

• Received 30 June 1994

DOI:https://doi.org/10.1103/PhysRevC.50.2834