Excitation of the electric pygmy dipole resonance by inelastic electron scattering⋆

Abstract

To complete earlier studies of the properties of the electric pygmy dipole resonance (PDR) obtained in various nuclear reactions, the excitation of the 1 − states in 140 Ce by (e, e ) scattering for momentum transfers q = 0.1 − 1.2 fm −1 is calculated within the plane-wave and distorted-wave Born approximations. The excited states of the nucleus are described within the Quasiparticle Random Phase Approximation (QRPA), but also within the Quasiparticle-Phonon Model (QPM) by accounting for the coupling to complex configurations. It is demonstrated that the excitation mechanism of the PDR states in (e, e ) reactions is predominantly of transversal nature for scattering angles θe ≈ 90 o − 180 o . Being thus mediated by the convection and spin nuclear currents, the (e, e ) like the (γ, γ ) reaction, may provide additional information to the one obtained from Coulomb-and hadronic excitations of the PDR in (p, p ), (α, α ), and heavy-ion scattering reactions. The calculations predict that the (e, e ) cross sections for the strongest individual PDR states are in general about three orders of magnitude smaller as compared to the one of the lowest 2 + 1 state for the studied kinematics, but that they may become dominant at extreme backward angles. PACS. 25.30.Dh Inelastic electron scattering to specific statesDedicated to Pier Francesco Bortignon

Scissors mode and effects of the low-lying E1 excitations on the dipole distributions in 175Lu

“…As done in the QPM approach [64], the addition of nonharmonic effects leads to more complex two-and three-phonon configuration mixing into the wave functions, which may be important for producing an additional fragmentation and thus reducing the dipole strength of a single excitation [65]. However, it is known that the complex configuration in the wave functions does not remarkably influence the total dipole strength [66,67]. Therefore, it can be more convenient to discuss the integral characteristics of the dipole modes.…”

Nonperturbative theory for the QED corrections to elastic electron-nucleus scattering

“…A more refined approach takes the dominant excited states explicitly into account [37]. The required transition densities are obtained from the Hartree-Fock random phase approximation (HF-RPA [38]) and from the quasiparticle phonon model (QPM [39,40]). In both prescriptions, the respective one-phonon states are calculated from the RPA equations pertaining to some nuclear model Hamiltonian.…”