  For studying the collective pairing excitations of nuclei, the two-nucleon
transfer reactions in superfluid nuclei (the pairing gap of the ground state is
not zero), in particular, the (p,t) reaction, are of the greatest interest. A
simple model of monopole pairing excitations in superfluid nuclei on the basis
of the semiclassical time-dependent Hartree-Fock-Bogolyubov theory in the limit
of small amplitudes is considered. Using the anomalous density response
function, the monopole pairing mode in the energy region of double pairing gap
and the variation of the pairing gap associated with this mode are found. The
ratio of the spectroscopic factor for the excitation of monopole pairing
vibrations in the (p,t) reaction in even superfluid nuclei to the spectroscopic
factor for the transfer of two neutrons to the ground state of the daughter
nucleus (the relative spectroscopic factor) is estimated. For this, it is
assumed that the relative spectroscopic factor is proportional to the pairing
gap variation associated with the monopole pairing vibrations in agreement with
the corresponding quantum expression. Numerical estimate of the relative
spectroscopic factors for superfluid nuclei of the rare-earth and actinide
regions shows that the spectroscopic factor for the two-neutron transfer,
leading to the excitation of the monopole pairing vibrations, does not exceed
several percent of the spectroscopic factor for the transfer of two neutrons to
the ground state. This semiclassical estimate is in agreement with the
experimental rations of the cross sections for the excitation of 0+states in
the energy region of double pairing gap to the cross sections for the
excitation of the ground states for superfluid nuclei of the rare-earth and
actinide regions.
