Extended Pairing Model for Heavy Nuclei
  We study binding energies in three isotopic chains ($^{100-130}$Sn, $^{152-181}$Yb, and $^{181-202}$Pb) using the extended pairing model with Nilsson single-particle energies. The exactly solvable nature of the model means that the pairing strength G(A) required to reproduce the experimental binding energies can be determined uniquely. The valence space consists of the neutron single-particle levels between two closed shells corresponding to the magic numbers 50-82 and 82-126. In all three isotopic chains, log(G(A)) has a smooth quadratic behavior for even as well as odd nucleon numbers A; log(G(A)) for even and odd A are very similar. Remarkably, G(A) for all the Pb isotopes can be also described by a two parameter expression that is inversely proportional to the dimensionality of the model space. 
V. G. Gueorguiev, Feng Pan, and J. P. Draayer