Structure of the excited states of 10Be in a microscopic cluster model -: art. no. 014309

The structure of the excited states of Be-10 is theoretically explored by means of the microscopic alpha+alpha+n+n four-cluster model. The two-body scattering problem of {Be-9(3/2(-),1/2(+),5/2(-))+n}+{He-6(0(+),2(+))+alpha} is solved by the microscopic R-matrix method in order to localize the resonance excited states in Be-10. The wave functions of Be-9 and He-6 are given by the microscopic alpha+alpha+n and alpha+n+n three-cluster models, respectively. Our model can reproduce not only the ground 0(+) state but also the second 0(+) state simultaneously. The second 0(+) state has a highly developed alpha+alpha (He-6+alpha) clustering and is dominated by the 2homega excitation in the shell-model configuration wherein two neutrons occupy the 1s(1/2) orbit. The ground state has a more compact structure due to the large binding energy but some weak alpha+alpha clustering still persists. Our microscopic multicluster model is applied to the resonance excited states lying above the Be-9+n threshold, and the present model gives various resonance excited states. Among these states, the 0(2)(+), 2(4)(+), and 4(2)(+) states could constitute a K-pi=0(2)(+) rotational band which has a large deformation owing to the alpha+alpha clustering. In the wave function for the second 0(+) state in Be-10, as well as for the 1/2(+) first excited state of Be-9, the motion of the valence neutrons around the two alpha particles appears to be consistent with the sigma orbit in the molecular orbital model. We find that two or three competing configurations are necessary to reproduce these two anomalous states (the 1/2(+) and 0(2)(+) states), where one configuration produces a spatially extended neutron distribution outside the Be-8 core and another has a strong core (alpha+alpha) distortion induced by the valence neutron, with the latter being responsible for lowering the energies of these two states.