3-BODY RESONANCES IN HE-6, LI-6, AND BE-6, AND THE SOFT DIPOLE MODE PROBLEM OF NEUTRON HALO NUCLEI

Using the complex scaling method, the low-lying three-body resonances of He-6, Li-6, and Be-6 are investigated in a parameter-free microscopic three-cluster model. In He-6 a 2+, in Li-6 a 2+ and a 1+, and in Be-6 the 0+ ground state and a 2+ excited state is found. The other experimentally known 2+ state of Li-6 cannot be localized by our present method. We have found no indication for the existence of the predicted 1- soft dipole state in He-6. We argue that the sequential decay mode of He-6 through the resonant states of its two-body subsystem can lead to peaks in the excitation function. This process can explain the experimental results in the case of Li-11, too. We propose an experimental analysis, which can decide between the soft dipole mode and the sequential decay mode.