Staggering of the Nuclear Charge Radii in a Superfluid Model with Good   Particle Number
  A simple superfluid model with an effective four body interaction of monopole pairing type is used to explain the staggering of the charge radii in the isotope chains. The contribution of deformation and of the particle number projection are analyzed for the Sn isotopes. Good results are obtained for the staggering parameters and neutron pairing energies. 
Mihai Horoi