Formation of superconducting pair correlations in spherical even-even nuclei

The appearance of like nucleon pair correlations in the ground state of spherical even-even nuclei is considered within a special Bogoliubov transformation. It is confirmed that in closed subshell nuclei superconducting pair correlations start to form if the coupling constant $G$ exceeds a certain threshold value. Rough upper and lower estimates are obtained for the threshold value. It is shown, that superconducting correlations exist in open subshell nuclei at any positive $G$. In this case, nucleon pairs are distributed over all subshells participating in the pairing interaction.