On the mechanisms of superfluidity in atomic nuclei

A system of equations is obtained for the Cooper gap in nuclei. The system takes two mechanisms of superfluidity into account in an approximation quadratic in the phonon- production amplitude : a Bardeen- Cooper- Schrieffer (BCS) type mechanism and a quasiparticle- phonon mechanism. These equations are solved for 120 Sn in a realistic approximation. If the simple procedures proposed are used to determine the new particle- particle interaction and to estimate the average effect, then the contribution of the quasiparticle- phonon mechanism to the observed width of the pairing gap is 26% and the BCS-type contribution is 74%. This means that at least in semimagic nuclei pairing is of a mixed nature - it is due to the two indicated mechanisms, the first being mainly a surface mechanism and the second mainly a volume mechanism.