Three-centre cluster structure in 11C and 11B

Studies of the 16O(9Be,α7Be)14C, 7Li(9Be,α7Li)5He and 7Li(9Be,ααt)5He reactions at Ebeam = 70 and 55 MeV have been performed using resonant particle spectroscopy techniques. The 11C excited states decaying into α + 7Be(gs) are observed between 8.5 and 13.5 MeV. The α + 7Li(gs), α + 7Li* (4.652 MeV) and t+8Be(gs) decays of 11B excited states between 9 and 19 MeV are observed. The decay processes are used to indicate the possible three-centre 2α + 3He (2α + 3H) cluster structure of observed states. This cluster structure is more prominent in the positive-parity states, where two rotational bands with large deformations are suggested. Excitations of some of the observed T = 1/2 resonances coincide with the energies of previously measured T = 3/2 isobaric analogues of the 11Be states, indicating that these states may have mixed isospin.