Microscopic calculations of double and triple giant resonance excitations in heavy ion collisions

Abstract

We perform microscopic calculations of the inelastic cross sections for the double and triple excitation of giant resonances induced by heavy-ion probes within a semiclassical coupled-channels formalism. The channels are defined as eigenstates of a bosonic quartic Hamiltonian constructed in terms of collective random-phase approximation phonons. Therefore, they are superpositions of several multiphonon states, also with different numbers of phonons, and the spectrum is anharmonic. The inclusion of (n+1) phonon configurations affects the states whose main component is a n-phonon one and leads to an appreacible lowering of their energies. We check the effects of such further anharmonicities on the previously published results for the cross section for the double excitation of giant resonances (GR). We find that the only effect is a shift of the peaks toward lower energies, the double GR cross section being unmodified by the explicity inclusion of the three-phonon channels in the dynamical calculations. The latter provide an important contribution to the cross section in the triple GR energy region, which, however, is still smaller than the experimental available data. The inclusion of four-phonon configurations in the structure calculations does not modify the results.

Multiphonon excitations and pygmy resonances in tin isotopes

“…Differences between theoretical standard calculations and experimental results have induced us to include corrections to the harmonic approximations, such as anharmonicities in the internal Hamiltonian and nonlinearities in the external field [10][11][12]. Studies of excitation processes of heavy nuclei have been achieved by using semiclassical methods techniques.…”

“…The model has been developed for the study of multiple giant resonance excitations, and all the details can be found in Refs. [10][11][12].…”

“…Thus it seems to us appropriate to study the excitation of the PDR in the framework of the semiclassical coupled-channel approach already used for the calculation of the multiple excitation of giant resonances in heavy ion collisions [10][11][12]. Within such an approach, anharmonicities and nonlinearities are treated in a completely microscopic description based on the HF plus RPA with Skyrme interaction, and they have been found to be responsible for an appreciable increase of the inelastic scattering cross sections.…”

Microscopic nuclear form factors for the pygmy dipole resonance

“…In previous work [8,9,34] we have followed this procedure and the internal structure of the nucleus has been described by means of a many-body model (often HF plus RPA calculations were used) which yielded microscopic neutron and proton transition densities. In addition one should include the isospindependent part of the nucleon-nucleon interaction…”

Nuclear excitations as coupled one and two random-phase-approximation modes

“…As underlined in several studies, the coupling of one-particle-one-hole (1p-1h) configurations with more complex states is very important to overcome these limitations and to describe the spreading widths and the anharmonicities in the excitation spectra (see, for example, Refs. [8][9][10] and references therein). A natural extension of the RPA is the so-called second RPA (SRPA), which amounts to enlarge the space of basic elementary excitations by including two-particle-two-hole (2p-2h) configurations and by coupling them with the 1p-1h ones and among themselves.…”