Coupling of a quasi-particle to the pairing vibrations

The intermediate coupling model of odd nuclei, where a nucleon is coupled to the surface vibration of an even system, has been successfully used in explaining various nuclear properties. In this paper, it is pointed out that a similar picture is applicable to improve the BCS approximation for seniority-one states in the pairing force model by considering coupling of a quasi-particle to the pairing normal modes. It is shown that this model is, in part, equivalent to the variational approach which takes the blocking effect into account. Several models are compared numerically.