Extended Theory of Finite Fermi Systems for magic and nonmagic nuclei

Abstract

Consideration is given to a short review of the main features, recent results, and prospects of the Extended Theory of Finite Fermi Systems (ETFFS), which has been applied in the past 15 years to collective excitations in the neutral channel for nuclei with and without pairing. The theory is an extension of the Migdal standard TFFS to include in a consistent way the single-particle continuum and more complex 1p1h ⊗ phonon or 2qp ⊗ phonon configurations beyond the RPA or QRPA ones; i.e., the theory takes into account all three known mechanisms of giant-resonance width. To the most extent, the theory was developed and applied to nuclei without pairing. A quantitative explanation of the giant-resonance widths was obtained, with the complex configurations contributing about half of the width. In addition, a large part of the observed giant-resonance gross and fine structures can be directly traced back to the specific complex configurations, and the recent results of the (α, α′) experiments in 40Ca and 58Ni could be explained. Consistent use of the Green’s function method makes it possible to include and calculate some effects which were practically unstudied earlier. These are ground-state correlations induced by complex configurations and “refined” basis effects, in particular, the second (or quasiparticle-phonon) mechanism of pairing. Both of them can be studied in current experiments. In the past five years, the ETFFS has been developed and applied actively to even—even and odd-mass nuclei with pairing. Calculations of the E1 pygmy resonance in Ca and Sn isotopes have shown that this phenomenon, which is important for (n, γ) and (γ, n) reactions, cannot be explained without allowing for complex configurations. Consideration of the single-particle continuum and the practical universality of the interaction parameters allow the ETFFS to be used for calculations of unstable nuclei. The prospects and status of the necessary development of a self-consistent ETFFS for nuclei with pairing are discussed.