Search for 28Si cluster states through the 12C+16O radiative capture, S. Courtin, IPHC and University of Strasbourg, France − The nucleus 28Si shows at energies below ~8 MeV a remarkable coexistence of various structures: a ground state oblate band, a prolate and an octupolar band with band heads at 6.69 (0+) and 6.88 (3-) MeV, respectively. At energies above, the recent and most advanced cluster model calculations of Kanada-En'yo et al. predict the existence of various cluster bands  with alpha+24Mg and 12C+16O configurations. In the present work, the radiative capture (RC) 12C+16O -> 28Si has been used to search for such 12C+16O 'molecular' structures.

The 12C+16O RC reaction has been studied at 5 bombarding energies between Elab= 15.4 and 21.4 MeV, i.e. around the Coulomb barrier (CB) which is located at ~18.4 MeV. The RC experiment has been performed at the Triumf laboratory (Vancouver, Canada) using the Dragon 0° spectrometer to detect and select the compound nucleus 28Si recoils in coincidence with the associated BGO gamma-array. The chosen energies correspond to a 28Si excitation energy region (23.4 to 25.8 MeV) where resonances have been previously observed in the fusion-evaporation channels of the 12C +16O reaction.


The most remarkable result of our experiment is the previously unobserved strong RC decay path to 28Si 'doorway' states at energies between 10 and 12 MeV. In our contribution we would like to discuss the two main features of our results: - The selective feeding of T=1states between 10 and 12 MeV which can be connected to the isospin selectivity of gamma transitions in the N=Z 28Si nucleus. At the lowest bombarding energy, the feeding of 1+,T=1 states around 11 MeV is indeed particularly strong. These observations are probably correlated with the 12C+16O RC results obtained in the same excitation energy region by Sandorfi et al. and at higher energy by Harakeh et al. These authors have both interpreted their results in terms of an RC feeding of the GDR built on 28Si excited states.  The selective feeding of some lower members of the 'deformed' prolate and octupolar bands which can probably be interpreted as the decay of cluster states in the entrance channel.  Surprisingly enough, it turns out that the main features of the measured gamma-decay spectra are highly sensitive to the entrance-channel angular momenta.  Thus in the excitation energy range covered, spin-parity assignments between 1- and 6+ can be proposed. In this context, a possible scenario is the feeding through radiative capture near CB of a 28Si cluster bands with states of positive and negative parity.