The ⁶He+⁶He and α+⁸He cluster states in ¹²Be via α-inelastic scattering

Cluster structures in the neutron-rich nucleus ¹²Be were experimentally investigated via α-inelastic scattering. Excited states in the ¹²Be nucleus were populated by a ¹²Be(αα') reaction at 60 A MeV in the inverse kinematics, and identified by measuring a ⁶He+⁶He and α+⁸He breakup channels in coincidence. The differential cross section and the angular correlations between the decay particles were obtained for each excitation energy at 1020 MeV for ⁶He+⁶He and at 919 MeV for α+⁸He, 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 ⁶He+⁶He channel. This result support the recent theoretical result by the generalized two-center cluster model. Several negative-parity excited states were observed in the α+⁸He 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 ¹²Be.