Dilute 2α+t cluster structure in 11B

The cluster structures of the excited states in 11B are studied by analyzing the isoscalar monopole and quadrupole strengths in the 11B(d,d′) reaction at Ed=200 MeV. The excitation strengths are compared with the predictions by the shell-model and antisymmetrized molecular-dynamics (AMD) calculations. It is found that the large monopole strength for the state at Ex=8.56 MeV is well described by the AMD calculation and is an evidence for a developed 2α+t cluster structure.