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 ¹²⁰Sn 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.