Dynamical dipole excitation in the fission of a Ca 40 + Sm 152 composite system

The excitation of the dynamical dipole mode was explored in the formation of a heavy composite system with mass A≃ 190, by investigating its fission channel. The composite system was produced through the charge asymmetric reaction, Ca40 + Sm152, and the nearly charge symmetric one, Ca48 +Sm144, at Elab = 11 and 10.1 MeV/nucleon, respectively. High-energy γ rays and light charged particles were detected in coincidence with the two fission fragments by means of the MEDEA multidetector array coupled to two parallel plate avalanche counters. The kinetic energy spectra of the light charged particles measured at different angles were used to infer the average excitation energy, the average mass, and the average charge of the produced nuclei whereas the time of flight and the emission angle of the fragments were employed to reconstruct the fission dynamics. The study of the γ-ray spectra and angular distributions for the selected fission events, allowed us to establish (i) the excitation of the giant dipole resonance in the composite system of both reactions and (ii) the excitation of the dynamical dipole mode in the dinucleus of the charge asymmetric reaction by isolating its prompt γ radiation through the difference technique. The present results on the dynamical dipole mode were compared with the experimental findings for the evaporation channel of the Ca40 + Sm152 reaction and moreover with calculations based on a collective bremsstrahlung analysis of the reaction dynamics. Interesting hints connecting the dynamical dipole γ radiation with the superheavy element quest are given.