THE 6He+6He AND α+8He CLUSTER STATES IN 12Be VIA α-INELASTIC SCATTERING

Cluster structures in the neutron-rich nucleus Be-12 were experimentally investigated via alpha-inelastic scattering. Excited states in the Be-12 nucleus were populated by a Be-12(alpha,alpha') reaction at 60 A MeV in the inverse kinematics, and identified by measuring a He-6+He-6 and alpha+He-8 breakup channels in coincidence. The differential cross section and the angular correlations between the decay particles were obtained for each excitation energy at 10-20 MeV for He-6+He-6 and at 9-19 MeV for alpha+He-8, respectively, reconstructed by the measured momentum vectors of the two helium isotopes. A multipole decomposition analysis based on the distorted-wave Born approximation was applied for the angular distribution of the inelastic scattering together with the angular correlation between the decay particles with respect to the directions of the incident beam and to the momentum transfer simultaneously. From the decomposed excitation energy spectra for J=0-4, several new excited states were identified. The 0(+) excited states were candidates of the band-head of a largely deformed rotational band. The 11.3-MeV 0(+) state was found to decay only into the He-6+He-6 channel. This result support the recent theoretical result by the generalized two-center cluster model. Several negative-parity excited states were observed in the alpha+He-8 channel. These excited states possibly forming a negative-parity rotational band, which is very closed to the positive-parity band, can be connected to the existence of the extremely developed cluster structure in Be-12.