Nature of the pygmy dipole resonance in 140Ce studied in (alpha, alpha' gamma) experiments.

A concentration of electric-dipole excitations below the particle threshold, which is frequently denoted as the pygmy dipole resonance, has been studied in the semimagic nucleus 140Ce in (alpha, alpha' gamma) experiments at E alpha = 136 MeV. The technique of alpha-gamma coincidence experiments allows the separation of E1 excitations from states of other multipolarities in the same energy region and provides an excellent energy resolution to allow a detailed analysis for each state. The experimental results show that the PDR splits into two parts with different nuclear structure: one part which is excited in (alpha, alpha' gamma) as well as (gamma, gamma') experiments and one part which is excited only in (gamma, gamma').

Microscopic nature of the pygmy dipole resonance: the stable Ca isotopes.

The electric dipole strength distribution in 44Ca has been measured up to 10 MeV in high resolution photon scattering experiments for the first time. The data obtained have been compared to earlier measurements on (40,48)Ca in order to view the evolution of the electric pygmy dipole resonance (PDR). Calculations that were performed within the framework of the microscopic extended theory of finite Fermi systems, which adds contributions of the quasiparticle-phonon coupling to random phase approximation calculations, give a qualitative agreement with the experimental data for all three isotopes. We have shown that it is necessary to include this coupling to describe the PDR.