Radiative capture process at the Coulomb barrier: the resonant 12C+16O case

The 12C+12C and 12C+16O collisions are the only light heavy-ion reactions showing resonant structures in radiative capture and reaction channels at energies around the Coulomb barrier (CB). For both systems the resonances are narrow (∼ 200 keV) which corresponds to a long lifetime of the compound nucleus states which can thus be discussed in terms of molecular states in 24Mg and 28Si. Molecular bands with 12C-16O structures are predicted by theoretical models (AMD, GCM, cluster model ... ). At high excitation energies such bands have been measured in break-up experiments, for spin between 10 and 18 hω. Due to experimental limitations this method does not allow to measure the corresponding band heads near CB. In the case of heavy-ion induced capture reactions, the CB is high and the compound nucleus (CN) is therefore formed at high excitation energy where particle evaporation channels are the favoured decay modes. Nevertheless the radiative capture, i.e. the complete fusion between target and projectile with subsequent cooling solely by γ emission, is a powerful tool to study the interplay between reaction mechanism and nuclear structural effects. In the beginning of the 80s, a campaign of 12C+12C and 12C+16O radiative capture experiments around CB [2], using a large Nal γ-detector which limited the study to large energy γ-rays, evidenced in both systems a series of narrow resonances feeding the 24Mg and 28Si low lying states. For higher lying states, the question of their eventual feeding is of course important and can only be probed by measuring the complete γ decay pattern of the radiative capture process. Such measurements should highlight the degree of overlap between the entrance channel and the CN states and also between the CN states and lower lying states in the same CN nuclei. The 28Si is an interesting nucleus where oblate, prolate and octupole shapes coexist at low excitation energy. Information on the spins of the entrance channel and on the importance of its structural overlap with specific CN deformed bound or quasi bound states can be extracted from the complete spectrum of connecting γ transitions. To make a detailed study of the CB resonances in the 12C+12C and 12C+16O systems, we have performed exclusive measurements with coincident detection of the recoils and γ-rays at Triumf (Vancouver) using the 0o Dragon spectrometer and its associated BGO γ-array. Owing to this unique experimental set-up, we were able to measure the complete γ-decay scheme of the 12C(16O,γ)28Si reactions [3]. For the first time the relative strengths of all the decay branches could be extracted. We found important new decay branches to 28Si states located just above the a threshold. The observed decay flux will be compared to different simulations and in particular to a scenario where the γ flux is statistically fragmented in the radiative capture channel and another scenario using cluster model calculations of the γ-transition between molecular states [4]. The results of the 12C(16O,γ)28Si experiment will be detailed, discussed in terms of interplay between statistical and structural effects, and compared to similar results in the 12C+12C case [5].(author)