Dipole γ-ray emission in fusion heavy-ion reactions: beam energy dependence

We investigated the prompt dipole γ‐ray emission, related with entrance channel charge asymmetry effects, in the 40Ar + 92Zr and 36Ar + 96Zr fusion reactions at Elab= 15 and 16 MeV/nucleon, respectively, with the aim to probe its evolution with incident energy. These reactions populate, through entrance channels having different charge asymmetries, the 132Ce compound nucleus at an average excitation energy of 304 MeV with identical spin distribution. Fusionlike events were selected by detecting high‐energy γ‐rays in coincidence with evaporation residues. By studying the differential γ‐ray multiplicity spectra of the considered reactions, it was shown that the dipole γ‐ray intensity increases by ∼14% for the more charge asymmetric system. This result, associated with those reported for the 32,36S + 100,96Mo reaction pair at lower beam energies, implies a “rise and fall” trend of the prompt dipole γ‐ray emission in the studied beam energy range with a maximum value at 9 MeV/nucleon