  Pairing rotations and pairing vibrations are collective modes associated with
a field, the pair field, which changes the number of particles by two.
Consequently, they can be studied at profit with the help of two-particle
transfer reactions on superfluid and in normal nuclei, respectively. The advent
of exotic beams has opened, for the first time, the possibility to carry out
such studies in medium heavy nuclei, within the same isotopic chain. In the
case studied in the present paper that of the Sn-isotopes (essentially from
closed (Z=N=50) to closed (Z=50,N=82) shells). The static and dynamic
off-diagonal, long range order phase coherence in gauge space displayed by
pairing rotations and vibrations respectively, leads to coherent states which
behave almost classically. Consequently, these modes are amenable to an
accurate nuclear structure description in terms of simple models containing the
right physics, in particular BCS plus QRPA and HF mean field plus RPA
respectively. The associated two- nucleon transfer spectroscopic amplitudes
predicted by such model calculations can thus be viewed as essentially "exact".
This fact, together with the availability of optical potentials for the
different real and virtual channels involved in the reactions considered,
namely (A+2)Sn+p, (A+1)Sn+d and (A)Sn+t, allows for the calculation of the
associated absolute cross sections without, arguably, free parameters. The
numerical predictions of the absolute differential cross sections, obtained
making use of the above mentioned nuclear structure and optical potential
inputs, within the framework of second order DWBA, taking into account
simultaneous, successive and non-orthogonality contributions provide, within
experimental errors an overall account of the experimental findings for all of
the measured (A+2)Sn(p,t)(A)Sn(gs) reactions, for which absolute cross sections
have been reported to date.
