Abstract
The excitation of the Dynamical Dipole mode along the fusion path was investigated for the first time in the formation of a heavy compound nucleus in the A~190 mass region. The compound nucleus was formed at identical conditions of excitation energy and spin from two entrance channels having different charge asymmetry: the charge asymmetric 40 Ca + 152 Sm and the nearly charge symmetric 48 Ca + 144 Sm at Elab =11 and 10.1 MeV/nucleon, respectively. Both fusion–evaporation and fission events were studied simultaneously for the first time. The Dynamical Dipole mode excitation in the charge asymmetric channel was evidenced, in a model-independent way, by comparing the γ -ray multiplicity spectra and angular distributions of the two entrance channels with each other.
Highlights
The “Dynamical Dipole mode" (DD throughout the text), or pre-equilibrium Giant Dipole Resonance, is a collective oscillation of protons against neutrons with a dipole spatial pattern inside the atomic nucleus
The DD radiation is characterized by the following characteristics: i) a centroid energy lower than that of the compound nucleus GDR in the same mass region indicating a high deformation of the emitting source [3, 4] ii) an anisotropic angular distribution with respect to the beam axis since the oscillation is confined in the reaction plane [6] and iii) a γ yield that should depend on both the reaction dynamics and the symmetry term of the nuclear matter Equation Of State [4]
These events are characterized by emission of pre-equilibrium light particles that reduces the compound nucleus average mass, average charge and average excitation energy and cannot be discarded in the time of flight (TOF) spectrum of the reaction products because they have overlapping velocity distributions with those of the complete fusion events [23]
Summary
The “Dynamical Dipole mode" (DD throughout the text), or pre-equilibrium Giant Dipole Resonance, is a collective oscillation of protons against neutrons with a dipole spatial pattern inside the atomic nucleus It is a pre-equilibrium phenomenon, being excited along the fusion path of nuclei with a different ratio of neutrons and protons and decaying through emission of prompt γ-rays [1,2,3,4,5]. On the other hand few experimental results exist reporting on the DD γ multiplicity and on its angular distribution, that can be directly compared with calculations These elements called for more experimental efforts to disentagle the influence of each reaction parameter on the DD features and to provide severe constraints to the theoretical models. In order to understand the behavior of the DD in heavier systems than those studied before and to test its usefulness in super-heavy element production, we decided to study the DD in a composite system in the mass region A=190 [17]
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