Regularity and chaos in nuclear structure

The BSC pairing gap, obtained from nuclear masses, shows large structural effects. A periodic orbit theory for the pairing gap has been developed, and generic expressions for the pairing gap fluctuations are derived, stressing the role of regularity/chaos. Results from the theory are compared to pairing gaps obtained from nuclear masses, calculated as well as measured. The comparison provides another quality control of nuclear mass formula, and gives additional insight in the nuclear pairing phenomenon. The theory can be applied to pairing fluctuations in other finite-size Fermi systems, as ultracold atomic gases or small metallic grains