New cluster states in 12Be

The description of light exotic nucleus such as 12Be is a very interesting challenge for experiments [1,2,3,4] as well as theoretical calculations [5,6,7]. In the shell-model approach, the structure of the ground state and of the low-lying states is linked to the understanding of breakdown of the N = 8 shell closure. Besides, 4+, 6+ and 8+ molecular-band members have been identified by M. Freer et a,l. [1] although not confirmed in a recent experiment [2]. The 0+ and 2+ members have been identified at 10.9 MeV and 11.3 MeV by A. Saito et. al. [3]. The complexity of the 12Be states justifies their study with sophisticated models such as microscopic cluster models [5,6,7] and the no-core shell model [8] We present here new calculations of 12Be cluster states within the Generator Coordinate Method (GCM) framework. We use an extended two-cluster model which enables us to describe the following channels: 8He(0+, 2+)+α, and 6He(0+, 2+)+ 6He(0+, 2+). It represents a significant improvement of a previous work [5] where only channels involving the ground states of 6He and 8He were considered. Our GCM calculations reproduce well the band of Freer et al. [1]. The 02+ isomeric state and the 11- state are also well described. We suggest new bands based on these two states and a new positive parity band. As a complementary work, we perform no-core shell-model calculations with the code Antoine [8]. We mainly focus on the 01+ , 21+ and 41+ states in order to investigate the possible existence of a band based on the ground state. Calculations until excitation of 8hΩ with two different high-precision NN interactions are performed. We find that the 01+ and 21+ states have similar structure. Furthermore, our results confirm clearly the breakdown of the N = 8 shell closure. Nevertheless, the 4+ states appears to have quite a different structure. We cannot then confirm a band based on the ground state.(author)