Monopole excitations and E0 transitions in even--even nuclei

The existing theoretical ideas about the structure and properties of nuclear monopole states and E0 transitions are analyzed on the basis of the most recent compilation of experimental data. The main modes forming not only the low-lying 0+ states but also giant monopole resonances are identified in light, magic, spherical, deformed, and transitional even--even nuclei. Typical results are given of calculations obtained in collective and microscopic models, as well as results based on qualitative considerations about the nature of these states deduced from their systematic behavior. The intensity of E0 transitions is characterized by ''single-particle'' estimates and energy-weighted sum rules. Particular attention is devoted to anomalies that are manifested in the decay properties of the monopole states and to their physical interpretation. The results of a self-consistent approach to pairing correlations in nuclei are presented in detail. The possibility of experimental investigation of monopole states excited by the decay of a bound muon is discussed