Observation of radiative capture in 90Zr-induced fusion reactions

In the fusion reactions 90Zr+90Zr, 92Zr, and 94Mo the radiative fusion process, that is the deexcitation of the compound nucleus only by γ-radiation, was detected. The radiative fusion was identified in all three cases by the observation of the ground state α-decay of the compound nucleus. For the reaction 90Zr (358 MeV) + 90Zr, the γ-rays were detected in coincidence with evaporation residues. Gamma rays from radiative fusion and from particle evaporation channels were separted by establishing a time correlation between the evaporation residues and their subsequent radioactive α-decay. The γ-rays were observed in 20 NaI detectors covering a total solid angle of 90% of 4 π. This set-up allowed a determination of the γ multiplicity distribution of γ rays. The results do not support the assumption that specific direct processes associated with high energy γ rays are observed for radiative fusion. The low exciation energy of the compound nuclei, which is achievable with these heavy, symmetric systems seems to favour radiative fusion. The excitation functions for radiative fusion and for particle evaporation were reproduced by statistical model calculations