Prompt dipole γ-ray emission: a new cooling mechanism in fusion heavy-ion reactions

We investigated the prompt dipole γ‐ray emission, related with entrance channel charge asymmetry effects, in the 40Ar + 92Zr and 36Ar + 96Zr fusion‐evaporation reactions at Elab= 15.1 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 same compound nucleus at an average excitation energy of 304 MeV with identical spin distribution. It was shown that the dipole γ‐ray intensity in the bremsstrahlung subtracted linearized spectra 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 with beam energy with a maximum value at Elab= 9 MeV/nucleon.